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Journal Publications

Publication Year:  2022
+ A Continuum Approach For Multipactor Using Vlasov-Poisson Analysis
  Silvestre, L; Shaw, ZC; Sugai, T; Stephens, J; Mankowski, JJ; Dickens, J; Neuber, AA; Joshi, RP
Abstract:  Multipactor is studied based on the coupled Vlasov-Poisson equation set and applied to a parallel plate geometry. This approach can be considered complementary to the particle-in-cell (PIC) methods that have provided excellent insight into multipactor behavior. However, PIC methods have limitations in terms of 'particle noise,' which can affect electron energy distribution functions and create re-scaling issues under conditions of charge growth. Utilizing our continuum Vlasov-Poisson approach yields susceptibility curves that are in line with reports in the literature, Spark3D PIC simulations, and experimental data. Playing to the strength of the Vlasov-Poisson approach, the differences between various multipactor orders are elucidated as they are observed in phase-space, revealing electron density dynamics without requiring increased computational resources due to electron growth. The method presented is general and can be extended to multi-input excitations and higher-dimensional phase-space.
+ An Investigation Into The Surface Skidding Response Of Pbx 9501 And Pbx 9502
  S. A. Watkins, R. J. Lee, T. H. Austin, J. Mankowski, J. Brinkman, J. Dickens, A. A. Neuber
Abstract:  While polymer-bonded explosives, PBX, have reduced sensitivity to ignition from mechanical shock or heating compared to conventional explosives, the investigation of the mechanical ignition mechanisms for PBX remains vital to assessing the safety during machining and general handling. Under frictional heating, hot spot generation due to high melting point contaminants in the interface is a suspected source for increased probability of ignition. To investigate such frictional heating, samples of PBX 9501 and PBX 9502 were impacted and skidded against a tangentially moving target and thermally imaged. Temporally resolved temperature and forces were simultaneously measured with and without 300 μm silica grit at the frictional interface. A trend of increasing temperature was observed as the speed and tangential force on the sample increased. Grit particles in the frictional interface were found to act as frictional heat concentration spots owing to the grit‘s protrusion from the surface and lesser susceptibility to melting compared to the PBX. The coefficient of friction between PBX 9501 and the skidding surface was observed to be largely constant for forces and speeds at the lower end of the test range. In contrast, at high tangential speeds, the coefficient was found to be significantly lower.

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+ Announcement The Nineteenth Special Issue On High-Power Microwave And Millimeter-Wave Generation
  J. Browning; N. M. Jordan; J. Stephens; P. Zhang
Abstract:  Modern high-power microwave and millimeter-wave technologies continue to be an active focus of study with considerable interest in the scientific, industrial, defense, and academic communities. Today, these technologies find application in precision radar systems, next-generation particle accelerators, fusion devices, deep-space communications networks, electromagnetic warfare, high-bandwidth communications, microwave-assisted catalysis, and extreme depth drilling, to name a few. The ever-growing needs of these applications call for the production and utilization of electromagnetic radiation with tens of cm to sub-mm wavelength and time-averaged power on the order of tens of megawatts and peak power exceeding 1 GW. Such demands have motivated interdisciplinary innovations for both the production and delivery of high-power microwaves. Advances in modern vacuum electronic microwave and millimeter-wave sources include metamaterial and photonic bandgap interaction circuits, advances in cathode materials, geometries, and modeling capability, improved materials, treatments, and surface coatings for high-power components, and novel manufacturing techniques, such as additive manufacturing. These innovations have preceded unprecedented gains in peak and average microwave power, frequency capability and agility, and efficiency of high-power microwave systems. Complementary to microwave source development, considerable gains have been made in understanding and improving the high power capacity of microwave components, especially with regard to multipactor formation.

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+ Centralized Control Topology For Pv Farms Shading Detection And Gmpp Searching Restarting Condition
  C. A. Negri; S. Daneshvardehnavi; K. E. K. Schmitt; A. Esmaeel Nezhad; P. H. J. Nardelli; S. Bayne; M. G. Giesselmann
Abstract:  The power output of the solar panels follows a power-voltage (P-V) characteristic containing only one Global Maximum Point (GMP) in the normal conditions. However, under Partial Shading Conditions (PSC), the unbalanced irradiance in the panels creates Local Maximum Points (LMP) in the P-V curve. Standard control techniques for Maximum Power Point Tracking (MPPT) can not properly locate the GMP, stagnating in LMPs and generating losses in the energy harvesting. Specific techniques to locate the GMP are presented in the literature. However, the condition to restart the GMP is not widely discussed. The main challenges of global search algorithms are related to the restarting conditions. Avoiding unnecessary searching and providing an assertive GMP restarting condition is crucial for PV systems operation. In every GMP search, the solar inverters oscillate the power exchanged with the grid, causing frequency and voltage variations depending on the size of the PV plant. This paper proposes a novel technique that uses a centralized controller to identify the shaded inverters, creating flags that locally start the GMP searching. The solution minimizes the number of times the search is performed by providing an assertive GMP restarting condition, saving energy, and avoiding unnecessary output power oscillation. The proposed control technique was evaluated using the data of a real 150-kW solar farm containing five inverters with two MPPT trackers each.

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+ Combined First-Principles-Monte Carlo Analysis To Evaluate The Effect Of Surface Hydrogen On The Secondary Electron Yield Of Nickel
  Brown, M; Sanati, M; Joshi, RP
Abstract:  Secondary electron yield (SEY) modeling of Ni(110) surface has been carried out with and without the inclusion of wavevector-dependent harmonic corrections (which alter both the inelastic mean free path and stopping power) and is compared to available experimental data. The correction is shown to improve predictions of the inelastic electron mean free path in Ni and yield better agreement with experimental SEY data. It is found that the SEY is strongly dependent on the presence of adsorbates on surfaces. An increase of hydrogen on the surface, for example, is predicted to result in a significant enhancement in the secondary electron yield, with the positional placement of hydrogen layers on or near the Ni surface influencing the SEY. Using first-principles calculations, the permittivities work function and adsorption energy of various Ni systems have also been calculated, and have shown to compare favorably with available experimental data, and have been used in the present Monte Carlo calculations of electron transport. Published under an exclusive license by AIP Publishing.
+ Dynamic Voltage Restorer (Dvr) With A Novel Robust Control Strategy
  DaneshvarDehnavi, S; Negri, C; Bayne, S; Giesselmann, M
Abstract:  With the advancement of technology, electric equipment and loads have become more sensitive to problems related to power quality, such as voltage sag, swell, imbalances, and harmonics. To detect faults and to protect sensitive loads from these voltage distortions, a Dynamic Voltage Restorer (DVR) series compensator is among the best available cost-effective solutions. One of the main goals of the DVR is to achieve a control structure that is robust, stable, and can handle properly the disturbances (e.g., grid voltage issues, load current, and fluctuations at the DC link voltage) and model uncertainties (e.g., inverters and filter parameters). In this work, a novel framework control strategy based on Uncertainty and Disturbance Estimator (UDE) is proposed to improve the response of the DVR to properly compensate the load voltage under a variety of power quality issues, particularly the ones associated with the grid voltage disturbances. Additionally, the stability of the proposed control system is analyzed and validated using the Lyapunov stability theory. The advantages of the new control system are robustness, simplified design, good harmonic rejection, low tracking error, fast response, and sinusoidal reference tracking without the need for voltage transformations or specific frequency tuning (e.g., abc-dq0 and Proportional-Resonant). This research uses the MATLAB/Simulink software to validate the effectiveness of the proposed scheme under a diverse set of conditions with no control limitations. Moreover, the designed controller is tested under real conditions using Hardware-In-theLoop (HIL) validation with OPAL-RT real-time simulator coupled with a TI Launchpad microcontroller. The results demonstrate a good performance of the proposed control strategy for a quick transient response and a great harmonic rejection when subject to grid voltage distortions. (c) 2021 ISA. Published by Elsevier Ltd. All rights reserved.
+ Exploring The Basic Physical Mechanisms Of Cathode- And Anode-Initiated High-Voltage Surface Flashover
  W. Brooks; R. Clark; J. Young; M. Hopkins; J. Dickens; J. Stephens; A. Neuber
Abstract:  Surface flashover in vacuum imposes a substantial physical limit on modern, large-scale pulsed power. One of the ramifications is a minimum size requirement for new machines, which in itself becomes a hard barrier to the modernization and improvement of existing infrastructure. Pulsed power topologies require the physical mechanisms of both anode- and cathode-initiated flashover to be considered. Originally, the geometrical implications of field emission at the cathode triple junction (CTJ) motivated the usage of configurations that avoid electrons impinging on the insulating material. This will largely suppress the cathode-initiated flashover, which is best described by the secondary electron avalanche mechanism, gas desorption, and final breakdown in the desorbed gas. It depends on the cascade growth of a conducting plasma along the length of the insulator from the cathode. Mitigating the cathode-initiated flashover typically comes at the cost of a significant field enhancement at the anode triple junction (ATJ). In a typical implementation, the anode field may be three times higher than the cathode field for a given voltage, making the corresponding anode-initiated flashover much more common than otherwise. In the case of pulsed, anode-initiated flashover, experimental evidence suggests that charge is directly extracted from the insulator resulting in the insulator taking on a net positive charge advancing the anode potential. Along with accompanying gas desorption from the surface, the potential will then propagate from the anode toward the cathode until the effective length of the gap is sufficiently reduced to support flashover. The underlying physical mechanisms of cathode- and anode-directed flashover are discussed in light of previously gathered experimental data and recent experiments with pulsed, high-gradient, anode-initiated flashover.

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+ Lightning Current Propagation In Electrical Conduit
  W. Brooks; M. Lapointe; L. Collier; J. Mankowski; J. Dickens; D. Hattz; N. Koone; A. Neuber
Abstract:  Investigation of lightning strikes to conductors ran through long spans of rigid steel conduit was performed. An overdamped-exponential current waveform with controlled peaks and rise rates was used to inject simulated lightning strikes. The impact of the length of wire, length of conduit, grounding location/s, and load type was investigated. Breakdown of 600 V, 12 AWG, THHN insulated wire (3.23 mm OD, 2 mm conductor diameter) was observed for voltages above 45 kV. The presence of resistive loads (between wire and ground) in excess of 20 $\Omega $ or current rise times in excess of 5 kA/ $\mu \text{s}$ were found to consistently produce breakdown between wire and conduit. Practical power circuit elements such as outlets and splices were found to breakdown at voltage levels much below the wire insulation failure threshold.

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+ Optically Activated In-Waveguide Semiconductor Attenuators For The Controllable Isolation Of Ka-Band Microwaves
  Hewitt, AT; Esser, B; Joshi, RP; Mankowski, J; Dickens, J; Neuber, A; Lee, R; Stephens, J
Abstract:  Three different isolator topologies utilizing photoconductive (PC) elements are explored for their application as a controllable attenuator for a Ka-hand radar system. Network analyzer measurements are reported for each geometry in the unilluminated case, while a high-speed, high dynamic range heterodyne detection apparatus is used to measure the transient attenuation behavior of the isolators when illuminated. The electromagnetic characteristics of the illuminated isolators are demonstrated to be in good agreement with COMSOL Multiphysics simulations. Two of the isolator topologies rely on the PC element becoming highly reflective to achieve high isolation, which in turn requires high optical power and charge carrier density (similar to 10(17) cm(-3)). For the optical power available here (100 W), the first device demonstrated a peak attenuation of 53 dB, while the second device achieved only 33 dB. In the third topology, RF propagation is parallel to the major dimensions of the PC element. As a result, superior isolation is achieved with the PC element in the primarily absorbing state, associated with significantly lower carrier concentration (similar to 10(15) cm(-3)). This device achieved 63 dB of attenuation for only 3 W of optical power, demonstrating that PC technologies may be competitive with other isolator technologies with some notable advantages.
+ Striations In Moderate Pressure Dc Driven Nitrogen Glow Discharge
  Tahiyat, MM; Stephens, JC; Kolobov, VI; Farouk, TI
Abstract:  Plasma stratification has been studied for more than a century. Despite the many experimental studies reported on this topic, theoretical analyses and numerical modeling of this phenomenon have been mostly limited to rare gases. In this work, a one-dimensional fluid model with detailed kinetics of electrons and vibrationally excited molecules is employed to simulate moderate-pressure (i.e. a few Torrs) dc discharge in nitrogen in a 15.5 cm long tube of radius 0.55 cm. The model also considers ambipolar diffusion to account for the radial loss of ions and electrons to the wall. The proposed model predicts self-excited standing striations in nitrogen for a range of discharge currents. The impact of electron transport parameters and reaction rates obtained from a solution of local two-term and a multi-term Boltzmann equation on the predictions are assessed. In-depth kinetic analysis indicates that the striations result from the undulations in electron temperature caused due to the interaction between ionization and vibrational reactions. Furthermore, the vibrationally excited molecules associated with the lower energy levels are found to influence nitrogen plasma stratification and the striation pattern strongly. A balance between ionization processes and electron energy transport allows the formation of the observed standing striations. Simulations were conducted for a range of discharge current densities from similar to 0.018 to 0.080 mA cm(-2), for an operating pressure of 0.7 Torr. Parametric studies show that the striation length decreases with increasing discharge current. The predictions from the model are compared against experimental measurements and are found to agree favorably.
Publication Year:  2021
+ Apparatus For Skidding Sensitivity Testing Of Energetic Materials
  Hewitt, AT; Lee, RJ; Watkins, S; Brinkman, J; Stephens, JC; Dickens, JC; Neuber, AA
Abstract:  A remote-operated apparatus for testing the detonation sensitivity of energetic materials is detailed. Using an air ram and rotating disk, the normal force and transverse velocity of the impact plane are controlled independently, enabling the exploration of varying impact conditions over a wide parameter space. A microcontroller local to the apparatus is used to automate apparatus operation and ensure temporal alignment of the impacting ram head with the rotating disk. Calculation of the firing parameters and issuing of operational commands are handled by a remote computer and relayed to the local microcontroller for execution at the hardware level. Impact forces are taken from fast strain measurements obtained from gauges incorporated into the ram head. Infrared imaging of explosive samples provides insight into the peak thermal temperatures experienced at the sample surface during the impact event.
+ Benchmarking The Calculation Of Electrically Insulating Properties Of Complex Gas Mixtures Using A Multi-Term Boltzmann Equation Model
  M. Flynn, Max, A. Neuber, and J. C. Stephens
Abstract:  The accurate calculation of DC breakdown voltage thresholds solely from elementary electron-neutral interactions in complex gas mixtures using a multi-term Boltzmann equation (BE) kinetic model is demonstrated. SF6:N2 mixtures in the 100 Td < E/N < 400 Td field regime are explored to benchmark the model's effectiveness. A ten-term BE model is found to yield DC breakdown voltages which, on average, agree within 3% of experimental measurements. A two-term BE model is also applied in order to characterize the error introduced in all calculations by the two-term approximation. These discrepancies are largest in pure N2 where error is greater than 10% for diffusion coefficients, within 6% for particular vibrational rate coefficients, and within 5% for breakdown voltages. However, this error falls to within 1% for most parameters and breakdown voltages in mixtures with large SF6 content.

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+ Can 100% Renewable Power System Be Successfully Built?
  DaneshvarDehnavi, S; Negri, CA; Giesselmann, MG; Bayne, SB; Wollenberg, B
Abstract:  Global warming has been a critical issue in recent years. Many leaders and politicians have talked about replacing fossil fuels with Renewable Energy Resources (RES). Some of them even went further and are talking about running a country with 100% RES soon. Is that economically viable? What would be the estimated cost of such a system? In this paper, a city in west Texas with a 100 MW peak load has been assumed to operate with the wind, solar, and Battery Energy Storage System (BESS) totally disconnected from the grid for a cost evaluation. Real generation and load time series data are used to achieve an optimum combination of the installed capacity of those resources by minimizing the total overnight cost, respecting a maximum of 24 h of faults in which the system cannot provide the load during the period. A Monte Carlo simulation is applied to the previous results to evaluate the influence of faulty devices and increase the robustness of the system. The results for both cases are analyzed and compared. Finally, the total cost of install capacity will be compared with other non-renewable resources. (c) 2021 Elsevier Ltd. All rights reserved.
+ Data Needs For Modeling Low-Temperature Non-Equilibrium Plasmas: The Lxcat Project, History, Perspectives And A Tutorial
  Carbone, E; Graef, W; Hagelaar, G; Boer, D; Hopkins, MM; Stephens, JC; Yee, BT; Pancheshnyi, S; van Dijk, J; Pitchford, L
Abstract:  Technologies based on non-equilibrium, low-temperature plasmas are ubiquitous in today's society. Plasma modeling plays an essential role in their understanding, development and optimization. An accurate description of electron and ion collisions with neutrals and their transport is required to correctly describe plasma properties as a function of external parameters. LXCat is an open-access, web-based platform for storing, exchanging and manipulating data needed for modeling the electron and ion components of non-equilibrium, low-temperature plasmas. The data types supported by LXCat are electron- and ion-scattering cross-sections with neutrals (total and differential), interaction potentials, oscillator strengths, and electron- and ion-swarm/transport parameters. Online tools allow users to identify and compare the data through plotting routines, and use the data to generate swarm parameters and reaction rates with the integrated electron Boltzmann solver. In this review, the historical evolution of the project and some perspectives on its future are discussed together with a tutorial review for using data from LXCat.
+ Demonstration Of Constant-Gate-Charge Scaling To Increase The Robustness Of Silicon Carbide Power Mosfets
  J. A. Cooper; D. T. Morisette; M. Sampath; C. A. Stellman; S. B. Bayne; M. J. Westphal; C. H. Anderson; J. A. Ransom
Abstract:  We introduce the concept of constant-gate-charge scaling to increase the short-circuit withstand time of SiC power MOSFETs without increasing their ON-state resistance, gate charge, or oxide field. In gate-charge scaling, we scale the oxide thickness and gate drive voltage, keeping the oxide field constant. Short-circuit measurements on 1200 V SiC double-implanted MOSFETs (DMOSFETs) confirm that short-circuit withstand times can be increased by 2- 4× without increasing ON-resistance, simply by reducing the oxide thickness and the gate drive voltage.

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+ Doping-Less Sic P-I-N Diode: Design And Investigation
  S. Hahmady; S. Bayne
Abstract:  We introduce a novel high-voltage SiC p-i-n diode considering a charge plasma approach. This technique facilitates the formation of the anode and the cathode regions within the silicon carbide without requiring any impurity doping by taking advantage of the work-function difference between silicon carbide and metal electrodes. Utilizing the 2-D TCAD simulation, we represent the performance of the proposed doping-less silicon carbide p-i-n diode is analogous to the silicon carbide Schottky diode in terms of forward and reverse characteristics as well as temperature dependency. As opposed to the conventional (doped) silicon carbide p-i-n diode, the doping-less silicon carbide p-i-n diode holds a lower ON-state voltage drop and higher reverse saturation current. Although the doping-less silicon carbide p-i-n diode has the merits of the silicon carbide Schottky diode, but it has leverage over the corresponding counterparts by eliminating the doping and the high thermal budget fabrication processes.

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+ Fundamental Investigation Of Unipolar And Rf Corona In Atmospheric Air
  Aponte, IA; Esser, B; Dickens, JC; Mankowski, JJ; Neuber, AA
Abstract:  Unipolar (DC) and radio frequency (RF) corona at 3.3 MHz is studied at centimeter-sized gaps in a needle-plane geometry in atmospheric air at room temperature. Positive and negative corona using pure tungsten electrodes with varying tip angles revealed a lower onset voltage for the needle with the smaller included angle. The RF corona onset voltage and corresponding time delay were measured for a series of needles composed of pure tungsten or 2% lanthanated tungsten. The corona onset, established when the first instance of UV photon emission is detected via photomultiplier tube, occurred primarily during the negative half cycle of the applied RF voltage for pure tungsten needles. In contrast, with lanthanated tungsten needles, such preference was not observed. No distinguishable difference in onset voltage between pure tungsten and lanthanated tungsten was found, indicating that adding a small amount of lanthanum to tungsten has a negligible impact on the onset voltage at 3.3 MHz frequencies for electrodes at room temperature.
+ Gan-Based Pcss With High Breakdown Fields
  M. Gaddy, V. Kuryatkov, N. Wilson, A. Neuber, R. Ness, and S. Nikishin
Abstract:  The suitability of GaN PCSSs (photoconductive semiconductor switches) as high voltage switches (>50 kV) was studied using a variety of commercially available semi-insulating GaN wafers as the base material. Analysis revealed that the wafers’ physical properties were noticeably diverse, mainly depending on the producer. High Voltage PCSSs were fabricated in both vertical and lateral geometry with various contacts, ohmic (Ti/Al/Ni/Au or Ni/Au), with and without a conductive n-GaN or p-type layer grown by metal-organic chemical vapor deposition. Inductively coupled plasma (ICP) reactive ion etching (RIE) was used to form a mesa structure to reduce field enhancements allowing for a higher field to be applied before electrical breakdown. The length of the active region was also varied from a 3 mm gap spacing to a 600 µm gap spacing. The shorter gap spacing supports higher electric fields since the number of macro defects within the device’s active region is reduced. Such defects are common in hydride vapor phase epitaxy grown samples and are likely one of the chief causes for electrical breakdown at field levels below the bulk breakdown field of GaN. Finally, the switching behavior of PCSS devices was tested using a pulsed, high voltage testbed and triggered by an Nd:YAG laser. The best GaN PCSS fabricated using a 600 µm gap spacing, and a mesa structure demonstrated a breakdown field strength as high as ~260 kV/cm.

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+ Model Evaluations Of Multipactor Suppression In Rectangular Waveguides Through Grooved Surfaces And Static Magnetic Field
  Qiu, X; Saed, MA; Mankowski, JJ; Dickens, J; Neuber, A; Joshi, RP
Abstract:  Mitigation of multipactor in waveguides is of importance, and strategies have included the addition of external fields, materials engineering, or surface modifications. Here, geometry modifications of rectangular waveguide surfaces and the application of an axial magnetic field are investigated for suppressing multipactor growth. A Monte Carlo approach has been used to simulate electron dynamics. The empirical secondary electrons yield is modeled based on a modified Vaughan approach. The electric fields driving electron transport were derived from separate electromagnetic calculations to adequately include field perturbations due to the presence of surface patterns in the rectangular waveguide structure. Combinations of grooves and a DC magnetic field are shown to effectively mitigate multipactor growth at field strengths up to similar to 10(5) V/m. Finding optimal combinations for an arbitrary field and operating frequency requires further work.
+ Monte Carlo Transport Analysis To Assess Intensity Dependent Response Of A Carbon-Doped Gan Photoconductor
  Milestone, W; Guo, D; Sanati, M; Dowling, KM; Hau-Riege, S; Voss, LF; Conway, A; Joshi, RP
Abstract:  Evaluation of the photoresponse in wurtzite GaN photoconductive switches is presented based on kinetic Monte Carlo simulations. The focus is on electron transport physics and assessment of high frequency operation. The roles of GaN band structure, Pauli exclusion, and treatment of internal fields based on the fast multipole method are all comprehensively included. The implementation was validated through comparisons of velocity-field characteristics for GaN with computational results in the literature. Photocurrent widths of less than similar to 7 ps for the 1 mu m device can be expected, which translates into a 100 GHz upper bound. Photocurrent pulse compression below the laser full width at half maxima at high applied fields are predicted based on the interplay of space-charge effects and the negative differential velocity characteristics of GaN.
+ Simulations Of Hydrogen Outgassing And Sticking Coefficients At A Copper Electrode Surface: Dependencies On Temperature, Incident Angle And Energy
  Sami, SN; Sanati, M; Joshi, RP
Abstract:  Outgassing remains a pertinent issue as it typically is the first stage of possible plasma formation, and can lead to effects such as breakdown, surface flashover, and pulse shortening in high power systems. Here two pertinent aspects are probed: (i) a model-based assessment of outgassing and associated temperature-dependent rates from a copper electrode based on molecular dynamics simulations, and (ii) calculations for the sticking coefficients of hydrogen gas atoms as a function of incident energy and angle. Our results of temperature dependent diffusion coefficients for hydrogen in copper, agree well with experimental reports over a wide range from 300 to 1350 K, and show reduction in the presence of vacancies. Results also show low reflection coefficients at both high and low energies, with a maxima at around 6.5 eV. A curve fit to the data is predicted to roughly hold for a range of incident angles. Adsorption is predicted to occur for incident energies below 10 eV, with absorption dominating above 10 eV.
+ Simulations Of Hydrogen Outgassing From A Carbon Fiber Electrode
  Sami, SN; Islam, R; Khare, R; Joshi, RP
Abstract:  Outgassing remains a pertinent issue in high-power systems as it can lead to effects such as breakdown, surface flashover, and pulse shortening and is typically the first stage of deleterious plasma formation. In this context, experimental reports suggest that carbon fibers (CFs) may likely be a superior cathode material for low outgassing. Here, model-based assessments of outgassing from CFs are performed based on molecular dynamics simulations. Carbon fibers were generated based on interconnection of an array of graphene sheets resembling ladder-like structures. Our results of temperature-dependent diffusion coefficients for hydrogen in CFs are shown to exhibit Arrhenius behavior and have values smaller than copper by factors of 15.5 and 86.8 at 400 K and 1000 K, respectively. This points to even stronger improvements for operation at high temperatures, with the asymptotic diffusion constant ratio predicted to be similar to 187. With reduced outgassing, higher temperature operation, and durability, our results support CF cathodes as an excellent choice for cathode material in high-power microwave and pulsed power systems.
+ The Sensitivity Of Pbx 9502 To Drilling Operations
  R. M. Clark, J. Brinkman, and A. A. Neuber
Abstract:  Polymer-bonded explosive (PBX) 9502 (95 % TATB, 5 % Kel F-800 by weight) is dry-drilled on a CNC milling machine and its thermomechanical response to varying feed rates, drilling speeds, and peck depths with 4 mm and 5 mm diameter drill bits is investigated. The tested samples are affixed to a force sensor that enables recording temporally resolved cutting forces and torques, and a drill-embedded thermocouple yields local temperature data. From the data, an empirical relationship between temperature changes and feed per revolution is developed, which reveals reduced temperatures in higher feed per revolution regimes for PBX 9502. The observed relationship allows extrapolating to temperatures for other hole diameters, indicating increased temperature for smaller diameter drilling across the board. Additional testing was performed with PBX 9501 (95 % HMX, 2.5 % Estane®, 2.5 % BDNPA/BDNPF by weight), albeit over a reduced parameter space, which revealed the opposite behavior for the feed per revolution temperature dependence. It is concluded that both PBX 9502 and PBX 9501 can be dry-drilled efficiently beyond the limits of presently applicable US-DOE standards, where cutting interface temperatures remain far below material critical temperatures. Finally, data reveals that coolant usage in the drilling process for these materials provides a wide safety margin.

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Publication Year:  2020
+ A 2 Kw S-Band Rf Source For Multipactor Research Utilizing Gan Hemts
  Esser, B; Shaw, ZC; Dickens, JC; Neuber, AA
Abstract:  A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of similar to 100 mu s. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section-achieved with a tapered impedance transformer-placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or similar to 40 kW.
+ Analysis Of Multipactor In A Rectangular Waveguide Using Spark3D Software
  Sugai, T; Shaw, Z; Dickens, J; Neuber, A
Abstract:  Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software Spark3D. The electromagnetic wave in the simulation was a TE10 mode-2.85GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY)=1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.
+ Association Between Insulin-Like Growth Factor Binding Protein-2 And Insulin Sensitivity, Metformin And Mortality In Patients With Newly Diagnosed Type 2 Diabetes
  Kristiansen, MR; Nielsen, JS; Brandslund, I; Olsen, DA; Stidsen, JV; Nicolaisen, SK; Hjortebjerg, R; Frystyk, J
Abstract: 
+ Calculations Of Cell Transmembrane Voltage Induced By Time-Varying Magnetic Fields
  Q. Hu; R. P. Joshi; D. Miklavčič
Abstract:  Electric pulses can create pores and/or render cell membranes permeable, and this effect has been studied for decades. Applications include cell membrane permeabilization for gene electrotransfer, drug delivery, and related electrochemotherapy, as well as tissue ablation. Here, we probe the use of time-varying magnetic fields to modulate the transmembrane voltage (TMV) across cell membranes through numerical simulations. This could be a contactless, noninvasive technique. Results show that the induced TMV values exceeding the 1 V threshold for electroporation could be achieved for short duration pulsing with fast rise and fall times. The strongest response is then predicted to occur when the lateral distance between a cell and the center of a current carrying coil equals the coil radius. The induced TMV is shown to peak when the gradient in the magnetic potential is the largest. However, with the more realistic but longer microsecond pulse stimulation systems, the induced TMV is much smaller. Hence, developing shorter pulses or fast rise times is critical for achieving membrane poration based on time-varying magnetic fields. Other effects could also focus on the use of nanoparticles (including magnetic materials) for possible heating for synergistic enhancements of transport through tumor cell membranes.

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+ Causalbg: Causal Recurrent Neural Network For The Blood Glucose Inference With Iot Platform
  He, M; Gu, WX; Kong, Y; Zhang, L; Spanos, CJ; Mosalam, KM
Abstract:  Predicting blood glucose concentration facilitates timely preventive measures against health risks induced by abnormal glucose events. Advances in IoT devices, such as continuous blood glucose monitors (CGMs) have made it convenient for measurements of blood glucose in real time. However, accurate and personalized blood glucose concentration prediction is still challenging. Previous inference models yield low-inference accuracy due to the ineffective feature extraction and the limited, imbalanced personal training data. The underlying causal correlations among the blood glucose series are scarcely captured by these models. In this article, we propose CausalBG, a causal recurrent neural network (CausalRNN) deployed on an IoT platform with smartphones and CGM for the accurate and efficient individual blood glucose concentration prediction. CausalBG automatically captures the underlying causal relationships embedded in the blood glucose features through CausalRNN, and efficiently shares the limited personal data among users for the sufficient training via the multitask framework. Evaluations and case studies on 112 users demonstrate that CausalBG significantly outperforms the conventional predictive models on the blood glucose dynamics inference.
+ Commercial Gan-Based Power Electronic Systems: A Review
  Pushpakaran, BN; Subburaj, AS; Bayne, SB
Abstract:  Wide bandgap semiconductor technology is gaining widespread acceptance in the area of high-power and high-temperature power electronics. Gallium nitride (GaN) not only has a wide bandgap of 3.4 eV and all the associated superior electronic properties but also enables the development of high-mobility power devices which is critical in increasing the power density of a power electronics system. Since a commercial GaN power transistor has a lateral structure as opposed to the traditional vertical device structure, commercially available devices are rated below 1000 V breakdown voltage with a maximum value of 900 V and typical value around 650 V. The primary focus of this review will be to introduce readers to the commercially available power electronic systems developed by various manufacturers which employ GaN-based power devices and highlight their remarkable performance which surpasses existing technology. This review also includes a brief introduction on GaN technology followed by current market study showing the roadmap of integration of GaN-based power electronics in the power industry.
+ Coupled Analysis To Probe The Effect Of Angular Assignments On The Secondary Electron Yield (Sey) From Copper Electrodes
  Qiu, X; Diaz, L; Sanati, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP
Abstract:  Secondary electron emission from copper is probed utilizing Monte Carlo simulations that take account of elastic scattering based on the Mott theory and inelastic collisions based on energy-dependent energy loss functions. The loss function and stopping power were obtained through first-principles density functional theory. Angular assignment of electrons following elastic scattering or the creation of secondaries is shown to affect the energy-dependent secondary electron yield (SEY). A good match of the simulation results (with a peak SEY of similar to 180% at around 300eV and less than 10% deviation over the 0 to 1000eV energy range) to available experimental data is shown based on an energy and momentum conservation scheme. Also, the distribution of delay times for the generation of secondaries, the SEY behavior at different incident angles, the energy distribution of emergent secondaries, and their creation profiles as a function of depth are computed to provide a more complete picture of the governing mechanisms and predicted responses.
+ Evaluation Of Long-Term Reliability And Overcurrent Capabilities Of 15-Kv Sic Mosfets And 20-Kv Sic Igbts During Narrow Current Pulsed Conditions
  M. Kim; J. J. Forbes; E. A. Hirsch; J. Schrock; S. Lacouture; A. Bilbao; S. B. Bayne; H. K. O"™Brien; A. A. Ogunniyi
Abstract:  Silicon carbide (SiC) is becoming a preferred technology of choice for power dense application compared with silicon (Si). A more comprehensive analysis of the long-term pulsed power reliability of SiC is necessary so that the technology can make the transition commercially. In this article, a testbed is utilized to evaluate research grade 15-kV SiC MOSFETs and 20-kV SiC IGBTs manufactured by Wolfspeed, a Cree Company. A testbed was developed here at Texas Tech University (TTU), Lubbock, TX, USA, to test these two devices. The narrow pulse testbed's capacitor bank can be charged up to 10 kV and output square waveform pulse up to 2.0 μs. The waveform has a fullwidth at half-maximum pulse and is tested at a repetition pulse rate of three seconds. The electrical characteristics of the forward conduction and reverse breakdown of the device under test (DUT) are measured periodically during the experiment. The DUTs were pulsed at different current levels, up to 340 A (1.06 kA/cm2) for the IGBTs and 74 A (296 A/cm2) for MOSFETs, while the electrical device degradation was monitored. This work discusses the results of the long-term pulsed power reliability, failure modes, and their robustness in overcurrent operations of highpower SiC MOSFETs and IGBTs.

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+ Fundamental Study Of Dc And Rf Breakdown Of Atmospheric Air (Vol 26, 123512, 2019)
  Aponte, IA; Esser, B; Shaw, ZC; Dickens, JC; Mankowski, JJ; Neuber, AA
Abstract: 
+ Guest Editorial
  S. Bayne; R. Allen; B. Srinivasan
Abstract:  The Guest Editor team of this Special Issue of the IEEE Transactions on Plasma and Science (TPS) is very excited to present this technical issue that encompasses various topics currently being studied within the field of pulsed power science and technology (PPST). Many articles for this Special Issue came from contributors (much like past Special Issues) who participated in the 2019 IEEE Pulsed Power and Plasma Science Conference (PPPS 2019), which comprised the 22nd Biennial PPC Conference and the 46th Annual ICOPS conference. The combined conference was held in Orlando, FL, USA, in June 23"“28, 2019 (http://www.ppps2019.org/). Both PPC and ICOPS conferences continued a strong history of serving their community by providing a platform for members to give presentations and have discussions in their field of research. Although this Special Issue was prepared during the COVID-19 Pandemic, the editorial team and reviewers worked very hard to make this a very successful Special Issue.

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+ Investigation Of Lightning Attachment Risks To Small Structures Associated With The Electrogeometric Model (Egm)
  W. Brooks; D. H. Barnett; W. A. Harrison; D. Hattz; J. Mankowski; J. Dickens; A. Neuber
Abstract:  A 3-D Monte Carlo-type random walk model was constructed for the assessment of lightning attachment probabilities to small structures. The simulation assumed buildings had a negligible impact on the propagation of lightning. A purely stochastic propagation model based on a previously proposed gas and charged particle diffusion process was employed. The attachment was based on the electrogeometric model in which striking distance is determined by return stroke peak current. This model allows for hundreds of thousands of samples to be evaluated in the window of a few minutes on readily available consumer computing hardware. Using this model, it became possible to enable characterizing building protection as a probability distribution of striking distance. Such was done to provide a deeper understanding of the impact of building protection design choices than is readily available from binary testing. The model was calibrated for minimum input resolution, which is found to be insensitive to variations in step length and moderately insensitive to variations in propagation angle distribution, resulting in normalized errors of less than 15% (rms). A parametric sweep of geometric features was performed for a large (100 m $\times $ 50 m) rectangular building with catenary wire protection. For heights of less than 30 m, lightning was found to bypass protection structures and strike to the building itself at rates that were insensitive to variations in building height. The extent to which the protection may be recessed from the building's perimeter was found to have a significant impact. Variations in building aspect ratio were found to be of limited impact except for cases of extreme aspect ratio where competition with the ground appears to have resulted in much better lightning protection performance.

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+ On The Limits Of Multipactor In Rectangular Waveguides
  Shaw, ZC; Silvestre, L; Sugai, T; Esser, B; Mankowski, JJ; Dickens, JC; Neuber, AA
Abstract:  Multipactor in WR-284-like geometries is measured utilizing local and global detection techniques. To emulate conditions one may find in a waveguide filter structure while maintaining the fundamental microwave mode, a standard rectangular waveguide geometry with the reduced waveguide height set to 2.1 or 5.5mm was adopted. Two high power RF sources were used to investigate a large range of input power (few kWs to MWs): a solid state source using GaN HEMTs allowing for larger pulse widths than standard magnetrons (100 mu s as opposed to similar to 4 mu s) and a MW level S-band coaxial magnetron for the high power end. Particular interest was taken in capturing the lower and upper limits of multipactor threshold. Lower multipactor thresholds for finite pulse duration are governed by the appearance of one or more electrons in the multipactor gap during the applied pulse as well as a minimum power (electric field) level that affects a secondary electron emission yield above unity. As shown, such initial electrons(s) may easily be seeded via an external UV source illuminating the gap. However, wall collisions of excited metastable molecules may be another source of electrons, an observation based on the experiment and prior research. A multipactor upper threshold was non-existent in the experiment, even at powers over 200kW within a 2.1mm test gap, which numerically yielded a gap transit time significantly shorter than one half-period of the GHz wave. This is attributed to the electric field distribution within the waveguide structure, which results in the multipactor's spatial position moving to more favorable locations within the test gap.
+ Reverse Recovery Of 50 V Silicon Charge Plasma Pin Diode
  S. Hahmady; S. Bayne
Abstract:  In this article, a novel approach is used for the first time to design a high-voltage PIN diode without any chemical doping process of cathode and anode region. This approach favors "p" and "n" plasma region formation through various metal contacts with appropriate work-functions for anode and cathode respectively. In this study, the forward and reverse characteristics, as well as the switching performance (reverse recovery) of this novel device, charge plasma (CP) PIN diode, were compared with the Schottky diode and the conventional PIN diode using TCAD simulation.

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+ Universal Model Of Pulsed Alternators Using Ltspice
  C. A. Negri; S. D. Dehnavi; M. G. Giesselmann
Abstract:  The high power and energy density of synchronous machines turn them into reliable sources of energy for pulsed power applications. In this article, two mathematical models using both actual and normalized (per-unit) parameters have been developed and simulated in LTspice, which is a powerful and a free tool to simulate electrical/electronic circuits. In this article, an efficient solver based on the flux and current equations is presented. The model has been validated against published results with both actual and per-unit parameters. It has been observed that for models with normalized parameters, the simulation time is significantly reduced. The validated model of the machine has been used to show the effects of nonlinear loads on the voltages and currents, in particular the reaction currents in the damper winding inside of the generator. In addition, the model has been used to study the effect of the RC time constant of the load on the peak power provided by a synchronous machine used to store energy for pulsed power applications.

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+ Wind Turbine Wake Position Detection And Rotor Speed-Based Wake Steering Validation In A Wind Tunnel Wake Simulator
  Castillo, R; Bayne, S; Pol, S; Westergaard, C
Abstract:  Wind farm control has demonstrated power production improvements using yaw-based wake steering compared to individual turbine optimization. However, slower yaw actuation rates in response to rapid inflow changes lend to impracticality of yaw-based steering, as it causes time-varying downstream rotor-wake overlap, power production fluctuation, and consequent reduction. Therefore, closed-loop wake control is required to mitigate wake deflection uncertainty. To respond to rapid inflow variations, rotor speed actuated is investigated here. Furthermore, wake position information is required as feedback for closed-loop control function. For field-installed turbines, nacelle-based Light detection and ranging (LIDAR) is expected to provide this information. So far, LIDAR-derived wake position has been determined through model-based field reconstruction of scattered LIDAR data. However, this requires sophisticated, economically prohibitive LIDARs. To incorporate inexpensive, two-beam LIDAR for wake detection, a tip vortex-based approach was developed and is also presented here. These contributions can be considered as intermediate steps toward realization of a novel closed-loop wake control.
Publication Year:  2019
+ A Reflex Triode System With Multicavity Adjustment
  Barnett, DH; Rainwater, K; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract:  This paper focuses the tunability of a reflex triode virtual cathode oscillator (vircator). The vircator cathode is a bimodal carbon fiber (CF) material, while the anode is polished pyrolytic graphite. These materials have ideal operating characteristics for use within a vircator. These materials have high operating temperatures greater than 1000 K which support large current densities of similar to 200 A/cm(2). A 12-stage, 158-J pulse-forming network (PFN)-based modular Marx generator is used to drive the vircator at 350 kV, 4 kA with similar to 100-ns pulsewidth at a pulse repetition frequency up to 100 Hz. The 12 stages of the Marx are constructed from a PFN using five, 2.1 nF, high-voltage ceramic capacitors in parallel. The Marx is broken into six modules each containing two stages. The Marx modules are machined from acetyl copolymer commonly called Delrin to provide rigidity and strength. Each Marx module includes air supply lines machined directly into each block, allowing external airlines to connect to each module chamber, rather than every spark gap. After the Marx erects, the energy is used to drive the virtual cathode oscillator (vircator) where subsequent frequency generation is manipulated through a new rectangular waveguide used as the resonant cavity. The new design has three parts of the cavity that can be changed; the bottom plate, back wall, and anode-cathode gap (A-K) distance. Each of these parts moves via linear actuators, two on the bottom plate, one on the A-K gap, and linear bellows for the back wall. The square waveguide cavity is welded into a circular stainless steel sleeve and is housed within a 10-in circular vacuum chamber. The anode is stationary in the vacuum chamber and connects to the Marx generator through a nickel shaft that feeds through the back wall, circular sleeve, and the rectangular waveguide. The anode is pyrolytic graphite, while the cathode is CF. The waveguide bottom plate, back wall, and cathode move around the stationary anode. This allows the height of the resonant cavity and the back wall distance from A-K gap to be independently changed of each other.
+ Calculations Of Multipactor Growth In Rectangular Waveguides
  H. K. A. Nguyen; J. Mankowski; J. C. Dickens; A. A. Neuber; R. P. Joshi
Abstract:  Multipactor growth in rectangular waveguides is probed based on a kinetic approach. Unlike most studies relying on the Vaughan model, a probabilistic approach for random multiple secondary particle emissions is used. Spread in electron emission velocities, the angular dependence of secondary emission yields, and an external radio frequency (RF) driving field due to a TE10 mode, were all built in. The calculations predict the secondary emission yield for copper, probe the population growth dynamics, and obtain the susceptibility diagram. Despite a maximum field at the waveguide center from the RF excitation, maximum electron densities are predicted at locations symmetrically displaced from the center. The secondary electron yield (SEY) characteristics, its local maxima, and the role of oblique incident angles, collectively lead to multipactor finding its place at off-center locations.

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+ Cost And Availability Optimization Of Wind Energy With Distributed Energy Resources Of A Microgrid
  Ramabhotla, S; Bayne, SB
Abstract:  A microgrid, an emerging technology in the electric power systems, has various benefits due to the implementation of distributed energy sources along with the loads. A microgrid utilizing the wind energy, solar energy, combined heat and power, natural gas generator, diesel generator, and battery storage is considered in an islanded mode of operation. The economic dispatch optimization is implemented using a reduced gradient algorithm to optimize the Operation and Maintenance cost in the islanded mode of the microgrid. The cost of each energy source is evaluated for every hour of the day using MATLAB code. Then, the availability of each energy source in the microgrid is evaluated. The results obtained are validated by comparing the Operation and Maintenance cost and the availability of each energy source in the microgrid. The optimal solution is achieved by considering the change in wind forecast and battery energy storage profiles.
+ Demonstration Of A Tritiated Nitroxide Nuclear Battery
  Russo, J; Litz, M; Ray, W; Bayne, S; Rosen, GM; Cho, H; Yu, J; Bigio, DI; Thomas, C; Alam, TR
Abstract:  Unattended, compact, terrestrial and space sensors require sources that have high energy and power densities to continuously operate for 3 to 99 years depending on application. Currently, chemical sources cannot fully satisfy these applications, especially in solid state form. Betavoltaic (beta V) nuclear batteries using beta(-)-emitting radioisotopes possess energy densities 1000 times greater than conventional chemical sources. Their power density is a function of fl flux saturation point relative to the planar (2D) configuration, beta(-) emission range, and the semiconductor converter, the betavoltaic (beta V) cell, properties. The figure of merit is the beta (beta(-))-flux surface power density (P-beta- in mu W-n per cm(2) footprint), where an optimal portion of incident beta particles penetrates the surrounding semiconductor depletion region. Tritiated nitroxides are favorable radioisotope sources with the potential to have the highest specific activity (A(m) in Ci/g) and P-beta- for an organic compound in solid form. The goal of this research is to demonstrate a tritiated nitroxide nuclear battery using the planar (2D) coupling configuration. The reproducible tritiation procedure produced stable product with a A(m) of approximately 635 Ci/g, which was 70% of the theoretical A(m). For the nuclear battery demonstration, the tritiated nitroxide, dissolved in methanol, was deposited on a 4H-SiC beta V and InGaP photovoltaic (PV) cell using a dispensing apparatus and micropipette. Both devices' characteristics were measured beforehand using a controlled electron beam source to approximate the surface radioactivity from the deposited radioisotope. The maximum power point (MPP) of the 4H-SiC and InGaP were 7.77 nW/cm(2) and 1.63 nW/cm(2) with 100 mCi and 67 mCi, respectively. The power and total efficiency were lower than expected due to partial solvent evaporation and droplet thickness. Numerical models using MCNP6 Monte Carlo code were used to simulate an optimal nuclear battery prototype. The models' accuracy was confirmed with the device calibration curves and a previous metal tritide model based on empirical results. Based on optimal model results, the tritiated nitroxide saturation layer thickness (D-0.99) and P-beta-(D-0.99) were 10 mu m and 558 nW/cm(2), respectively, using a 4H-SiC.
+ Direct Observation Of Electrons In Microwave Vacuum Components
  Shaw, ZC; Garcia, A; Powell, M; Dickens, JC; Mankowski, JJ; Neuber, AA
Abstract:  Apparatus which is used to directly observe electrons in microwave vacuum components was designed and implemented into a WR-284 like waveguide operated at 2.85 GHz with up to approx. 1 MW power. To generate desired electric field levels for driving secondary emission, the waveguide structure is manipulated by reducing the test section height to 6 mm from the standard WR-284 rectangular waveguide height of 34 mm. Both test and standard sections were operated in the dominant TE10 mode. A 1 mm aperture was cut into the broadside wall of the waveguide section enabling a portion of electrons in the waveguide to enter a properly biased electron multiplier tube mounted atop of the test section. Waveforms are presented showing the direct measurement of electrons, providing a local detection method with nanosecond temporal resolution. Future work will incorporate the test setup for multipactor studies. Published under license by AIP Publishing.
+ Fast Sic Switching Limits For Pulsed Power Applications
  L. Collier; T. Kajiwara; J. Dickens; J. Mankowski; A. Neuber
Abstract:  Solid-state semiconductor switches are emerging as an attractive choice for the fast switching of compact, repetitive, and pulsed power systems. In particular, the high voltage and fast switching capabilities of SiC MOSFETs are well suited for many applications when appropriately gated. For instance, the turn-on and turn-off characteristics of such devices are strongly dependent on the gate driving circuitry. Traditional commercial gate drivers, typically utilizing push-pull or totempole driving topologies, are often not well suited for fast, high current switching with rise times on the order of 10-20 ns, as the driving performance is highly dependent on the combined RLC characteristics of the driving circuitry and the switching device. The proposed gate drive topology utilizes a current-carrying inductor to rapidly charge the MOSFET gate-source capacitance. A high-voltage inductive kick generates the necessary potential to drive the inductor current into the gate through the parasitic gate impedance. As the energy stored in the drive inductor is continuously variable, it can be adjusted such that the gate voltage settles to a lower value, typically 20 V, after the initial kick to prevent excessive gate-source overvoltage. With an inductive drive current of ~23 A, a peak dI/dt of 25 kA μs-1 was achieved for the tested bare SiC MOSFET die. Additionally, a peak dI/dt of 13 kA μs-1 was achieved with the TO-247 packaged device.

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+ Fundamental Study Of Dc And Rf Breakdown Of Atmospheric Air
  Aponte, IA; Esser, B; Shaw, ZC; Dickens, JC; Mankowski, JJ; Neuber, AA
Abstract:  Radio frequency (RF) breakdown in air at a frequency of particular relevance to ionospheric heating-3.3 MHz, close to the low end of the applicable frequency range-is studied at centimeter-sized gap distances and compared to the literature for small gaps. To establish a reference point, Paschen's early DC breakdown study utilizing two brass spheres of 1 cm radius was replicated following the original procedure and subsequently extended to examine RF breakdown. Various electrode combinations were tested with brass cathodes creating the highest variance in the datasets among DC tests. The greatest variation in RF breakdown arose when either electrode was brass. Gap distances of 1-10 mm were tested for both DC and RF with the slow-rise time (5 mV/mu s) RF breakdown occurring at approximately 80% of the DC breakdown value, a value corroborated by Monte Carlo breakdown simulations. Pushing the envelope rise time of the applied RF voltage into the microsecond regime yielded an RF voltage of roughly 20% above the DC breakdown value accompanied by a distinct increase in breakdown amplitude fluctuations. Illuminating the gap electrodes with deep ultraviolet (280 nm and below) minimized the breakdown amplitude fluctuations due to photoemission at the electrodes as expected. Finally, to address the conditions found in real-world geometries with sharp corners or protrusions, RF corona behavior utilizing tungsten needles above a ground plane is measured. The obtained results help us define the operation limits of high-power antennas at 1-10 MHz frequencies.
+ Geometry Tuning Of An Electrically Small Antenna For Ionospheric Heating
  Esser, B; Mankowski, JJ; Dickens, JC; Neuber, AA
Abstract:  A modification of a previously introduced electrically small antenna is presented with tuning methods for continuous band coverage for ionospheric heating (3-10 MHz). Consisting of a small loop antenna inductively coupled to a capacitively loaded loop (CLL), the design may be tuned 50% of the center of the band by simply adjusting the capacitance of the CLL. Abandoning the use of lossy materials for tuning such as solid dielectrics or ferrites, the antenna is greater than 80% efficient across its tuning range. A tenth scale prototype with electromechanical geometry tuning is tested for frequency range and tuning capability especially at the low-frequency end where port reflection losses tend to dominate. Tuning of the small loop antenna-CLL coupling is used to mitigate this matching issue, which was demonstrated on the physical antenna model. Experimentally, a tuning range of 33.5-117.5 MHz is achieved with low reflection achievable across the range.
+ Laser-Driven Semiconductor Switch For Generating Nanosecond Pulses From A Megawatt Gyrotron
  Picard, JF; Schaub, SC; Rosenzweig, G; Stephens, JC; Shapiro, MA; Temkin, RJ
Abstract:  A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 mu s, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures. Published under license by AIP Publishing.
+ Magnetic Field Diffusion In Medium-Walled Conductors
  L. Collier; T. Buntin; J. Dickens; J. Mankowski; J. Walter; A. Neuber
Abstract:  The diffusion of transient magnetic fields through the walls of a hollow conductive shell is an important phenomenon of interest throughout a variety of pulsed power applications. Basic solutions do exist for cylindrical geometries in the limiting case that the skin depth is much larger than the wall thickness; however, in many pulsed applications, the transient skin depth is often similar to the conductor thickness. As the underlying thin-wall assumption begins to breakdown, the production of complex eddy current distributions in the conductor walls results in deviation from these simplified analytical solutions of the diffused field. Precise calculation of these current distributions is essential for many applications including inductive shielding and magnetic field diagnostics near conductors. Electromagnetic simulations using the finite-element method provide a more accurate picture of the diffusion process in this regime. A high magnetic field testbed facilitates measurement of the diffused fields in order to verify simulation accuracy. The effect of material conductivity, wall thickness, and conductor geometry on the diffusion process is examined.

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+ Model Evaluations Of Surface Modification By Energetic Incident Carbon Atoms On Graphene Coated Copper Electrodes
  Qiu, X; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  Thin nanoscale coating of metal electrodes by graphene promises to be a useful approach for suppressing the secondary electron yield and potential multipactor. Recent calculations showed reductions by as much as 50% for graphene over copper electrodes for energies below 125 eV, with results in good agreement with experimental data. Here, the resistance to possible degradation of this structure, in response to incoming atomic projectiles, is gauged based on molecular dynamics simulations. Our results for surface irradiation by carbon atoms (as an example) on nanoscale graphene coatings indicate a defect threshold of about 35 eV, lower surface damage for thicker layers, negligible sputtering, and defects less than 6 angstrom in dimension for energies up to 300 eV. The electrode structure is shown to be robust with better resistance to damage than metal alone. Published under license by AIP Publishing.
+ Modeling Of Power Supplies For Power Modulators With Ltspice
  M. Giesselmann; V. Roy
Abstract:  This paper is showing simulations of power supplies for repetitive power modulators using LTspice. We are presenting power supplies with single-phase and three-phase AC input and the effects of the rectifier stages on AC input current waveforms in both cases. We are also presenting the resulting power factor including circuits for electronic power factor correction. LTspice models for electronic power factor correction using cycle-by-cycle switching as well as time-averaged models are presented. The time-averaged models replace the PWM (pulse width modulation) switch with a transformer model with variable duty cycle. They are valid for a range of DC-DC converters. We are extending the time-averaged model for operation in continuous (CCM) and discontinuous (DCM) conduction mode and presenting results that show the fidelity of these models by comparison to cycle-by-cycle results. We are providing the detailed mathematical derivation for the DCM model expansion in the appendix. Time-averaged models do not have to keep track of thousands of steep switching transitions and can run orders of magnitude faster than cycle-by-cycle switch-mode models. Results using the time-averaged model are presented for a power supply with primary (wave-shaping) and cascaded secondary feedback control that regulates the output voltage of the converter.

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+ Nonlinear Analysis Of A Simple Amplitude-Phase Motion Equation For Power-Electronics-Based Power System
  He, MZ; He, W; Hu, JB; Yuan, XM; Zhan, M
Abstract:  With large-scale application of a large number of renewable energy sources, such as wind turbines, photovoltaics, and various power electronic equipment, the power electric system is becoming gradually more power-electronics-based, whose dynamical behavior becomes much complicated, compared to that of traditional power system. The recent developed theory of amplitude-phase motion equation provides a new framework for the general dynamic analysis of such a system. Based on this theory, we study a simple amplitude-phase motion equation, i.e., a single power-electronics device connected to an infinite-large system, but consider its nonlinear effect. With extensive and intensive theoretical analysis and numerical simulation, we find that basically the system shows some similarity with the classical second-order swing equation for a synchronous generator connected to an infinite bus, such as the two types of bifurcation including the saddle-node bifurcation and homoclinic bifurcation, and the dynamical behavior of stable fixed point, stable limit cycle, and their coexistence. In addition, we find that the Hopf bifurcation is possible, but for negative damping only. All these findings are expected to be helpful for further study of power-electronics-based power system, featured with nonlinearity of high-dimensional dynamic systems involved with not only a large timescale but also large space scale.
+ Numerical Evaluations Of Membrane Poration By Shockwave Induced Multiple Nanobubble Collapse In Presence Of Electric Fields For Transport Through Cells
  Hu, Q; Zhang, L; Joshi, RP
Abstract:  Synergistic applications of an electric field combined with nanojet-based mechanical pressure, have recently been shown to help create larger pores and provide control of the aspect ratio in biological membranes. The nanojets are formed by the collapse of nanobubbles in the vicinity of biomembranes upon being subjected to external shockwaves. Here we analyze the effects produced by the collapse of multiple nanobubbles in the presence of an electric field. Our simulations, based on molecular dynamics, show that not only would multiple nanobubbles make it possible to create larger pores, but also increase the pore density on the surface of biological cells. Both aspects could aid in the transport of drugs and genes for bio-medical applications. (C) 2019 Author(s).
+ Numerical Studies Into The Parameter Space Conducive To "Lock-On" In A Gan Photoconductive Switch For High Power Applications
  A. R. Chowdhury; S. Nikishin; J. Dickens; A. Neuber; R. P. Joshi; R. Ness
Abstract:  Time-dependent photocurrent response in semi-insulating GaN is simulated with a focus on the "Lock-On" phenomenon. A one-dimensional, time-dependent model based on drift-diffusion theory is used. The model was first tested for GaAs and shown to yield good agreement with data, before applying it to GaN simulations. The main findings are that compensated GaN with deeper traps nearer the midgap at higher densities, and/or multiple levels around the mid-gap would aid in driving the PCSS towards Lock-On. The initial average threshold field for Lock-On in GaN is predicted to be around 150 kV/cm, though this would be strongly dependent on the trap parameters of a sample.

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+ Obituary: Lynn Lamar Hatfield
  F. Hegeler; A. Neuber; C. Hatfield
Abstract:  Recounts the career and contributions of Lynn LaMar Hatfield.

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+ Optimal Dual Frames For Probabilistic Erasures
  Li, DW; Leng, JS; He, M
Abstract:  Assume that a frame is given for encoding in a communication system. J. Leng et al. investigated its dual frames for signal decoding which minimize the maximal error when the probabilistic erasures occur in the transmission process. In this paper, we present a new sufficient condition such that the canonical dual is the unique probability optimal dual for erasures. The determination conditions in our results reduces the computational complexity. We also give a necessary and sufficient condition under which a kind of alternative dual frames are probability optimal dual, and study the relation between the optimal duals of equivalent frames. Moreover, we present several examples which show that the general optimal dual frame is not a probability optimal dual frame and compare the reconstruction effects when the general optimal dual frames, the new probability optimal, and the existing probability optimal dual frames are used for decoding.
+ Probing Changes In Secondary Electron Yield From Copper Electrodes Due To Surface Defects And Changes In Crystal Orientation
  Nguyen, HKA; Sanati, M; Joshi, RP
Abstract:  There is considerable interest in mitigating secondary electron emission (SEE) from surfaces and electrodes produced by incident electrons, due to the deleterious effects of SEE in vacuum electron devices, accelerators, and other technologies. Since surface conditions are known to affect SEE, here the role played by crystal orientation and a vacancy (which is a simple example of a surface defect) is probed through Monte Carlo simulations. The effect of the lattice imperfection on the frequency-dependent permittivity, which then influences inelastic energy losses, mean free paths, and secondary generation profiles, is obtained on the basis of density-functional theory. The Monte Carlo simulations are in good agreement with previous experimental reports. The results indicate that the secondary electron yield for pure copper is the highest for the 110 orientation and the lowest for the 111 case, with a relatively higher differential predicted between a single vacancy and ideal copper for the 111 orientation. The results underscore the benefit of annealing or reducing inhomogeneities through laser or charged particle beam surface treatments.
+ Skeletal Muscle Sarcoplasmic Reticulum Ca2 Uptake Preferentially Use Glycogenolytic Derived Atp
  Kristiansen, MR; Jensen, R; Nielsen, J; Ortenblad, N
Abstract: 
+ Subterahertz Photonic Crystal Klystron Amplifier
  Stephens, JC; Rosenzweig, G; Shapiro, MA; Temkin, RJ; Tucek, JC; Kreischer, KE
Abstract:  This Letter reports the successful experimental demonstration of amplification of subterahertz radiation in a klystron with photonic crystal cavities. The klystron has six cavities, with each cavity having a series of oversized photonic crystal cells made up of a 5 x 3 array of square posts. The center post is removed from each cell to form a highly oversized (0.8 mm similar to lambda/4) beam tunnel, with power coupling from cell to cell through the tunnel. The pulsed electron beam is operated at 23.5 kV, 330 mA in a 0.5 T solenoidal field. At 93.7 GHz, a small-signal gain of 26 dB and a saturated output power of 30 W are obtained. Experimental results are in very good agreement with the predictions of a particle-in-cell code. The successful achievement of high gain operation of a photonic crystal klystron amplifier is promising for the future extension of klystron operation well into the terahertz frequency region.
+ The Inhibitory Effects Of Two Distinct Inhibitors On Glycogen Phoshorylase Activity And Contractile Function In Chemically Skinned Skinned Single Fibers
  Jensen, R; Nielsen, J; Kristiansen, MR; Ortenblad, N
Abstract: 
+ Validation Of A Novel Tumor Mutation Burden Assay Using A 130 Gene Tumor Only Targeted Sequencing Panel Covering Less Than 0.25 Megabases
  Joshi, RP; Nabet, BY; Yang, S; Costa, H; Stehr, H
Abstract: 
Publication Year:  2018
+ A Multi-Term, Multi-Harmonic Boltzmann Equation Model For Kinetic Behavior In Intense Microwave And Terahertz Excited Low Temperature Plasmas
  Stephens, JC
Abstract:  A multi-term (MT), multi-harmonic (MH) decomposition of the Boltzmann equation (BE) is developed to describe electron kinetic behavior in microwave and THz excited low temperature plasmas. In the decomposition of the BE, velocity distribution functions retain an arbitrary time dependence enabling the prediction of electron kinetic behavior from an arbitrary initial condition to a steady-state periodic solution. By exploiting the time-periodic nature of the electron swarm, the MTMH-BE model is not restricted to numerically resolving the electric field cycle. The MTMH-BE model is validated via the Reid ramp model gas and the ionization model gas of Lucas and Salee. Following successful validation, the MTMH-BE model is utilized to elucidate the basic electron kinetic behavior in air at atmospheric pressure. Namely, the error associated with the effective field approximation (EFA) is explored, where it is demonstrated that for atmospheric pressure air, given a microwave frequency of 1 GHz, the EFA may result in more than a factor of two errors in the time-averaged ionization rate. In the second part of this study, the MTMH-BE model is demonstrated as a basic modeling tool for low temperature plasmas. First, the MTMH-BE model is utilized to calculate electron heating profiles from a cold initial condition. The MTMH-BE model is demonstrated to be in excellent agreement with strictly time-dependent kinetic models, including a time-dependent MTBE model and a Monte Carlo collision model. To highlight the advantage of this work, the MTMH-BE model is used to predict the formative delay time of 95 GHz high power microwave induced breakdown. In this example, the numerical time step utilized in the MTMH-BE model is approximately six orders of magnitude larger than is possible using a strictly time-dependent MT-BE model. Overall, the MTMH-BE model presents a powerful pathway to modeling temporal kinetic behavior in microwave and THz excited low temperature plasmas.
+ Analysis Of A Dual Shock-Wave And Ultrashort Electric Pulsing Strategy For Electro-Manipulation Of Membrane Nanopores
  Hu, Q; Hossain, S; Joshi, RP
Abstract:  Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Shock waves are known to permeabilize cell membranes as well. Here we focus on and analyze the synergistic effects of both inputs in concert based on molecular dynamics simulations. Our results show that shockwaves could be used for pretreating cell membranes for electroporation. The dual strategy would either reduce the external voltage requirements (leading to more compact external circuitry) or help create larger pores. Furthermore, shockwaves could form pores at any desired membrane site location, and suitable combinations of nanojets and electric pulses would help control the aspect ratio and size as desired.
+ Assessing Lock-On Physics In Semi-Insulating Gaas And Inp Photoconductive Switches Triggered By Subbandgap Excitation
  A. R. Chowdhury; R. Ness; R. P. Joshi
Abstract:  The time-dependent photocurrent response in semi-insulating GaAs and InP was studied based on 1-D, time-dependent simulations with a focus on the Lock-On phenomenon. The results underscore the role of trap-to-band impact ionization from deep traps in rapid charge creation and its subsequent propagation much like a streamer. The numerical results compare well with the actual data. The main findings are that deeper traps nearer the valence band at higher densities, materials with larger high-field drift velocity, and cathode-side illumination would all aid in attaining Lock-On. These could be useful guidelines for producing Lock-On in new materials such as GaN for high-power applications.

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+ Assessing The Role Of Trap-To-Band Impact Ionization And Hole Transport On The Dark Currents Of 4H-Sic Photoconductive Switches Containing Deep Defects (Vol 120, 245705, 2016)
  Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP
Abstract: 
+ Calculations Of Secondary Electron Yield Of Graphene Coated Copper For Vacuum Electronic Applications
  Nguyen, HKA; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  The suppression of secondary electron yield (SEY) which can possibly lead to multipactor is an important goal for several applications. Though some techniques have focused on geometric modifications to lower the SEY, the use of graphene coatings as thin as a few monolayers is a promising new development that deserves attention either as a standalone technique or in concert with geometric alterations. Here we report on Monte Carlo based numerical studies of SEY on graphene coated copper with comparisons to recent experimental data. Our predicted values are generally in good agreement with reported measurements. Suppression of the secondary electron yield by as much as 50 percent (over copper) with graphene coating is predicted at energies below 125 eV, and bodes well for multipactor suppression in radio frequency applications. (c) 2018 Author(s).
+ Congestion Risk-Aware Unit Commitment With Significant Wind Power Generation
  Abedi, S; He, M; Obadina, D
Abstract:  Large-scale and ubiquitous penetration of wind power generation to power systems necessitates more conservative provision of system reliability by ensuring adequately committed reserve and observance of transmission constraints. In addition, wind power curtailment due to the technical limitations of system operations, such as transmission congestion, should be efficiently mitigated. To this aim, this paper presents a congestion risk-aware unit commitment formulation in a two-settlement market environment. The uncertainty impact of multicorrelated wind power and contingencies on the risk of transmission congestion for each line, called the Line Transfer Margins (LTM), is incorporated using basic statistical data on the nodal wind power forecast and probability of credible line-outages across the system. The LTMs, formulated free of any distributional assumptions, collectively provide a measure for transmission reserves, which effectively mitigate the likelihood of transmission congestion, reserve undeliverability, and wind power curtailment in the real-time economic dispatch. The effectiveness of the proposed approach is verified through comparative case studies on IEEE RTS-96 for various wind power and LTM scenarios.
+ Developing Safe Lathing Parameters For Pbx 9501
  Woodrum, RB; Barnett, DH; Dickens, JC; Neuber, AA
Abstract:  This paper presents the work performed on dry-lathing PBX 9501 to gather and analyze cutting force as well as temperature data during the machining process. The data is compared to present USA federal-regulation-constrained machining limits of high explosives. The effects of machining parameters depth of cut, surface meters per minute, and feed per revolution on cutting force and cutting interface were evaluated. Cutting tools of tip radius 0.013cm and 0.127cm were tested to determine the effect of the tool shape on the machining process. Empirically, a pronounced dependence of the maximum tool temperature on the depth of cut and surface meters per minute was found, while the dependence on the feed per revolution was found much weaker. It is elucidated that rapid, shallow cuts optimize machining time for a billet of PBX 9501 while minimizing temperature increase and cutting force.
+ Guest Editorial Special Issue On Pulsed Power Science And Technology
  S. Bayne; B. Novac; H. O"™Brien; H. Li
Abstract:  This special issue of the IEEE TRANSACTIONS ON PLASMA SCIENCE (TPS) mainly contains works presented at the 21st IEEE International Pulsed Power Conference (PPC) held in Brighton, U.K., between June 18"“22, 2017 (http://ece-events.unm.edu/ppc2017/). This conference marks an important step, as it was located outside of the USA for the first time. The PPC is well known for serving the Pulsed Power and Plasma Science communities as the principal forum for the professionals worldwide, where the most important technical presentations and discussions are taking place; the Brighton conference was no exception. The 21st PPC was extremely successful, with 470 quality papers submitted as equal contributions from the Americas, Europe, and Asia.

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+ High Power Long Pulse Microwave Generation From A Metamaterial Structure With Reverse Symmetry
  Lu, XY; Stephens, JC; Mastovsky, I; Shapiro, MA; Temkin, RJ
Abstract:  Experimental operation of a high power microwave source with a metamaterial (MTM) structure is reported at power levels to 2.9 MW at 2.4 GHz in full 1 mu s pulses. The MTM structure is formed by a waveguide that is below cutoff for TM modes. The waveguide is loaded by two axial copper plates machined with complementary split ring resonators, allowing two backward wave modes to propagate in the S-Band. A pulsed electron beam of up to 490 kV, 84A travels down the center of the waveguide, midway between the plates. The electron beam is generated by a Pierce gun and is focused by a lens into a solenoidal magnetic field. The MTM plates are mechanically identical but are placed in the waveguide with reverse symmetry. Theory indicates that both Cherenkov and Cherenkov-cyclotron beam-wave interactions can occur. High power microwave generation was studied by varying the operating parameters over a wide range, including the electron beam voltage, the lens magnetic field, and the solenoidal field. Frequency tuning with a magnetic field and beam voltage was studied to discriminate between operation in the Cherenkov mode and the Cherenkov-cyclotron mode. Both modes were observed, but pulses above 1 MW of output power were only seen in the Cherenkov-cyclotron mode. A pair of steering coils was installed prior to the interaction space to initiate the cyclotron motion of the electron beam and thus encourage the Cherenkov-cyclotron high power mode. This successfully increased the output power from 2.5 MW to 2.9 MW (450 kV, 74 A, 9% efficiency). (C) 2018 Author(s).
+ Lock-On Physics In Semi-Insulating Gaas: Combination Of Trap-To-Band Impact Ionization, Moving Electric Fields And Photon Recycling
  Chowdhury, AR; Dickens, JC; Neuber, AA; Ness, R; Joshi, RP
Abstract:  The time-dependent photoconductive current response of semi-insulating GaAs is probed based on one-dimensional simulations, with a focus on the lock-on phenomenon. Our results capture most of the experimental observations. It is shown that trap-to-band impact ionization fuels local field enhancements, and photon recycling also plays an important role in pushing the device towards lock-on above a 3.5 kV/cm threshold field. The results compare well with actual data in terms of the magnitudes, the rise times, and the oscillatory behavior seen at higher currents. Moving multiple domains are predicted, and the response shown depended on the location of the photoexcitation spot relative to the electrodes. Published by AIP Publishing.
+ Monte Carlo Analysis Of Field-Dependent Electron Avalanche Coefficients In Nitrogen At Atmospheric Pressure (Vol 24, 124501, 2017)
  Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP
Abstract: 
+ Numerical Modelling On Partial Discharge In Hvdc Xlpe Cable
  He, M; Hao, M; Chen, G; Chen, X; Li, WP; Zhang, C; Wang, HT; Zhou, MY; Lei, XZ
Abstract:  PurposeHigh voltage direct current (HVDC) cable is an important part in the electric power transmission and distribution systems. However, very little research has been carried out on partial discharge under direct current (DC) conditions. Niemeyer's model has been widely used under alternating current (AC) conditions. This paper aims to intend to modify the Niemeyer's model considering both electric field and charge dynamics under DC conditions, and therefore proposes a numerical model describing partial discharge characteristics in HVDC cable. Design/methodology/approachThis paper intends to understand partial discharge characteristics under DC conditions through numerical modelling. Niemeyer's model that has been widely used under AC conditions has been modified, taking both electric field and charge dynamics under DC conditions into consideration. The effects of loading level or current through the conductor, cavity location and material properties on partial discharges have also been studied. FindingsElectrical conductivity is important in determining the characteristics of partial discharge under DC conditions and discharges tend to happen in short when the cavity field exceeds the inception level under the parameter values studied in the paper. Research limitations/implicationsBuilding the numerical model is the purpose of the paper, and there is lack in experiment and the comparison between the simulation results and experiment. Practical implicationsThe proposed model provides the numerical model describing partial discharge in HVDC cable and helps understand the partial discharge mechanism under DC voltage. Originality/valueTo the best of the author's knowledge, this paper is a very early research on the numerical modelling work on partial discharge under DC voltage.
+ Numerical Study Of Influential Factors On Partial Discharges In Hvdc Xlpe Cables
  He, M; Hao, M; Chen, G; Li, WP; Zhang, C; Chen, X; Wang, HT; Zhou, MY; Lei, XZ
Abstract:  Purpose For the dramatically developed high voltage direct current (HVDC) power transmission, HVDC cables play a vital role in the power transmission across seas and connections with renewable power sources. However, the condition monitoring of HVDC cables is still a challenging research topic. This paper aims to understand the influence of external factors, namely, current, cavity location and material properties, on partial discharge (PD) characteristics in HVDC cable in a numerical way referring to the refined Niemeyer's model. Design/methodology/approach The influences of the three external factors are studied by a proposed numerical model for DC PDs based on the modification of a conventional PD model for AC voltage via a finite element analysis method. Findings The external factors can influence the discharge magnitude and discharge repetition rate via affecting the electrical conductivity of the material: DC PD is more frequent and with higher discharge magnitude when the cavity is closer to the conductor or the current through the conductor is higher. Both DC PD repetition rate and average discharge magnitude in long-term aged material are lower than virgin material. The effect of discharge on insulation degradation becomes decreasingly significant. Research limitations/implications The current work is based on the numerical modelling of DC PDs. Further experimental validations and comparisons are essential for improving the model. Practical implications The studies of the influence factors for PDs under HVDC voltage can benefit the research and practical power transmission on DC PDs, contributing the design and test of DC PDs in HVDC cables, exploring the understandings of the DC PDs' mechanism. Originality/value This paper, to the best of author's knowledge, first studies the influence factors on DC PDs based on the numerical modelling work.
+ Prediction Of Tumor Mutation Burden In Lung Adenocarcinoma Using A 130 Gene Targeted Sequencing Panel Covering 0.23 Megabases
  Joshi, RP; Yang, S; Costa, H; Stehr, H
Abstract: 
+ Rapid Kv-Switching Single-Source Dual-Energy Ct Ex Vivo Renal Calculi Characterization Using A Multiparametric Approach: Refining Parameters On An Expanded Dataset
  Kriegshauser, JS; Paden, RG; He, M; Humphreys, MR; Zell, SI; Fu, YL; Wu, T; Sugi, MD; Silva, AC
Abstract:  Purpose: We aimed to determine the best algorithms for renal stone composition characterization using rapid kV-switching single-source dual-energy computed tomography (rsDECT) and a multiparametric approach after dataset expansion and refinement of variables. Methods: rsDECT scans (80 and 140 kVp) were performed on 38 ex vivo 5- to 10-mm renal stones composed of uric acid (UA; n = 21), struvite (STR; n = 5), cystine (CYS; n = 5), and calcium oxalate monohydrate (COM; n = 7). Measurements were obtained for 17 variables: mean Hounsfield units (HU) at 11 monochromatic keV levels, effective Z, 2 iodine-water material basis pairs, and 3 mean monochromatic keV ratios (40/140, 70/120, 70/140). Analysis included using 5 multiparametric algorithms: Support Vector Machine, RandomTree, Artificial Neural Network, Naive Bayes Tree, and Decision Tree (C4.5). Results: Separating UA from non-UA stones was 100% accurate using multiple methods. For non-UA stones, using a 70-keV mean cutoff value of 694 HU had 100% accuracy for distinguishing COM from non-COM (CYS, STR) stones. The best result for distinguishing all 3 non-UA subtypes was obtained using RandomTree (15/17, 88%). Conclusions: For stones 5 mm or larger, multiple methods can distinguish UA from non-UA and COM from non-COM stones with 100% accuracy. Thus, the choice for analysis is per the user's preference. The best model for separating all three non-UA subtypes was 88% accurate, although with considerable individual overlap between CYS and STR stones. Larger, more diverse datasets, including in vivo data and technical improvements in material separation, may offer more guidance in distinguishing non-UA stone subtypes in the clinical setting.
+ Single-Pulse Avalanche Mode Operation Of 10-Kv/10-A Sic Mosfet
  Kelley, MD; Pushpakaran, BN; Bilbao, AV; Schrock, JA; Bayne, SB
Abstract:  The high-voltage silicon carbide MOSFET is a state-of-the-art solution for increasing power density and efficiency in power electronics; nonetheless, a full-scope of failure modes during extreme operating condition has not been established. Past efforts evaluated short-circuit capability of 10-kV silicon carbide MOSFET, however, in this manuscript, the single-pulse avalanche mode operation of a research-grade 10-kV/10-A silicon carbide MOSFET is explored for the first time. A decoupled undamped inductive circuit was selected for evaluation, and avalanche energy was increased until catastrophic failure occurred. The maximum tolerable avalanche energy was measured to be 2.84 J corresponding to an energy density of 8.8 &cm(-2). This result was compared with 1.2 kV silicon carbide MOSFETs to evaluate device robustness. Post failure analysis included: estimation of junction temperature, scanning electron microscopy, and focused ion beam cut. Peak junction temperature of 1010 degrees C was estimated using a thermal RC model and measurement results suggested gate degradation as the primary mechanism responsible for device destruction. Microscopy of the device validated gate failure which occurred at, or beneath, the gate metallization. A narrow cavity with-in the failure region was discovered during failure analysis and is hypothesized to have protruded the epitaxial region of the semiconductor.
+ Structural, Morphological, Optical And Electrical Properties Of Bulk (0001) Gan:Fe Wafers
  Gaddy, M; Kuryatkov, V; Meyers, V; Mauch, D; Dickens, J; Neuber, A; Nikishin, S
Abstract:  Characterization of three vendor's bulk semi-insulating GaN:Fe wafers, grown by either hydride vapor phase epitaxy or the ammonothermal method, was performed using: scanning electron microscopy, secondary ion mass spectroscopy, high resolution X-ray diffraction, cathodoluminescence, photoluminescence, and high voltage testing. Although the Fe doping level is significantly different for each growth method, both are promising for the fabrication of PCSS devices operating in the lock-on mode.
+ Tunable, Electrically Small, Inductively Coupled Antenna For Transportable Ionospheric Heating
  B. Esser; D. Mauch; J. Dickens; J. Mankowski; A. Neuber
Abstract:  An electrically small antenna is evaluated for use as the principle radiating element in a mobile ionospheric heating array. Consisting of a small loop antenna inductively coupled to a capacitively loaded loop, the electrically small antenna provides high efficiency with the capability of being tuned within the range of ionospheric heating. At a factor 60 smaller in area than a High-Frequency Active Auroral Research Program element, this antenna provides a compact, efficient radiating element for mobile ionospheric heating. A prototype antenna at 10 MHz was built to study large-scale feasibility and possible use with photoconductive semiconductor switch-based drivers. Based on the experimental study, the design has been extrapolated to a small 6 × 4 array of antennas. At a total power input of 16.1 MW this array is predicted to provide 3.6-GW effective radiated power typically required for ionospheric heating. Array cross talk is addressed, including effects upon individual antenna port parameters. Tuning within the range of ionospheric heating, 3"“10 MHz, is made possible without the use of lossy dielectrics through a large capacitive area suited to tune the antenna. Considerations for high power operation across the band are provided including a method of driving the antenna with a simple switcher requiring no radio frequency cabling. Source matching may be improved via adjustment of the coupling between small loop antenna and capacitively loaded loop improving |S11| from -1 to -21 dB at 3 MHz.

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+ Verifiable Outsourcing Computation For Matrix Multiplication With Improved Efficiency And Applicability
  Zhang, SM; Li, HW; Dai, YS; Li, J; He, M; Lu, RX
Abstract:  In recent years, the rapid development of Internet of Things (IoT) and big data shows the huge demand of outsourcing computing in cloud computing to assist some clients with low ability devices to fulfill massive data processing. In the mean time, considering the uncertainty of the Internet devices, we should persist security and high-efficiency. Uploaded data and returned results should be protected from attack of adversary, which guarantees the security, while efficiency requires low overhead of clients to finish the whole calculation procedure. In this paper, we main focus on the public verifiable outsourcing scheme on matrix multiplication, which can be applied in many IoT scenes, such as path planning and aggregation operation in Internet of Vehicles and smart grids, respectively. Specially, this paper presents schemes for two different functions of matrix multiplication, which strengthens the applicability. Moreover, security analysis and performance evaluation in this paper properly present the superiority of this paper.
Publication Year:  2017
+ A Software Controllable Modular Rf Signal Generator With Multichannel Transmission Capabilities
  Shaw, Z; Feilner, W; Esser, B; Dickens, JC; Neuber, AA
Abstract:  A software controllable system which generates and transmits user defined RF signals is discussed. The system is implemented with multiple, modular transmitting channels that allow the user to easily replace parts such as amplifiers or antennas. Each channel is comprised of a data pattern generator (DPG), a digital to analog converter (DAC), a power amplifier, and a transmitting antenna. All channels are controlled through a host PC and synchronized through a master clock signal provided to each DAC by an external clock source. Signals to be transmitted are generated through the DPG control software on the PC or can be created by the user in a numerical computing environment. Three experiments are discussed using a two-and four-channel antenna array incorporating Chebyshev tapered TEM horn antennas. Transmitting distinct sets of nonperiodic bipolar impulses through each of the antennas in the array enabled synthesizing a sinusoidal signal of specific frequency in free space. Opposite to the standard phased array approach, each antenna radiates a distinctly different signal rather than the same signal simply phase shifted. The presented approach may be employed as a physical layer of encryption dependent on the position of the receiving antenna. Published by AIP Publishing.
+ An Open Circuit Voltage Decay System For Performing Injection Dependent Lifetime Spectroscopy
  Lacouture, S; Schrock, J; Hirsch, E; Bayne, S; O'Brien, H; Ogunniyi, AA
Abstract:  Of all of the material parameters associated with a semiconductor, the carrier lifetime is by far the most complex and dynamic, being a function of the dominant recombination mechanism, the equilibrium number of carriers, the perturbations in carriers (e.g., carrier injection), and the temperature, to name the most prominent variables. The carrier lifetime is one of the most important parameters in bipolar devices, greatly affecting conductivity modulation, on-state voltage, and reverse recovery. Carrier lifetime is also a useful metric for device fabrication process control and material quality. As it is such a dynamic quantity, carrier lifetime cannot be quoted in a general range such as mobility; it must be measured. The following describes a stand-alone, wide-injection range open circuit voltage decay system with unique lifetime extraction algorithms. The system is initially used along with various lifetime spectroscopy techniques to extract fundamental recombination parameters from a commercial high-voltage PIN diode. Published by AIP Publishing.
+ Analysis Of Intensity Dependent Near-Bandedge Absorption In Semi-Insulating 4H-Sic For Photoconductive Switch Applications
  Meyers, V; Chowdhury, AR; Mauch, D; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  We report on the intensity-dependent behavior of the absorption coefficient (alpha) in semiinsulating 4H-SiC material. Data from as-received samples show a monotonic decrease in a with incident energy density, with a pronounced change in slope at around 10 mJ cm(-2). Annealed samples, on the other hand, exhibit an experimental trend of increasing alpha with intensity. Qualitative explanation of the observed behavior is presented that probes the possible role of spontaneous and stimulated emission for as-received samples. With annealing, trap related recombination is strongly reduced leading to higher carrier densities and increased free-carrier absorption with incident intensity. The role of band-filling and permittivity changes are shown to be inconsequential, while changes in internal fields could contribute to decreases in absorption.
+ Comparative Evaluation Of Transmembrane Ion Transport Due To Monopolar And Bipolar Nanosecond, High-Intensity Electroporation Pulses Based On Full Three-Dimensional Analyses
  Hu, Q; Joshi, RP
Abstract:  Electric pulse driven membrane poration finds applications in the fields of biomedical engineering and drug/gene delivery. Here we focus on nanosecond, high-intensity electroporation and probe the role of pulse shape (e.g., monopolar-vs-bipolar), multiple electrode scenarios, and serial-versus-simultaneous pulsing, based on a three-dimensional time-dependent continuum model in a systematic fashion. Our results indicate that monopolar pulsing always leads to higher and stronger cellular uptake. This prediction is in agreement with experimental reports and observations. It is also demonstrated that multipronged electrode configurations influence and increase the degree of cellular uptake. Published by AIP Publishing.
+ Effect Of Thermal Gradients Created By Electromagnetic Fields On Cell-Membrane Electroporation Probed By Molecular-Dynamics Simulations
  Song, J; Garner, AL; Joshi, RP
Abstract:  The use of nanosecond-duration-pulsed voltages with high-intensity electric fields (similar to 100 kV/cm) is a promising development with many biomedical applications. Electroporation occurs in this regime, and has been attributed to the high fields. However, here we focus on temperature gradients. Our numerical simulations based on molecular dynamics predict the formation of nanopores and water nanowires, but only in the presence of a temperature gradient. Our results suggest a far greater role of temperature gradients in enhancing biophysical responses, including possible neural stimulation by infrared lasers.
+ Electrothermal Simulation-Based Comparison Of 4H-Sic P-I-N, Schottky, And Jbs Diodes Under High Current Density Pulsed Operation
  B. N. Pushpakaran; S. B. Bayne; A. A. Ogunniyi
Abstract:  Pulsed power applications are characterized by very high instantaneous power due to the high voltage and current involved. Power diodes used as a closing switch in pulsed power circuits must be able to withstand the high current operation well above the continuous device rating, for a transient duration. Due to the superior electrothermal properties of wide bandgap Silicon Carbide (SiC) material, it is feasible to develop high Blocking Voltage (BV) Schottky and Junction Barrier Schottky (JBS) diodes besides p-i-n rectifiers. In order to evaluate the device performance under high current density pulsed operation, 2-D models of SiC p-i-n, Schottky, and JBS diodes rated for 3.3-kV BV and 100 A/cm2 current density were developed using Silvaco ATLAS TCAD software. The diode structures were simulated electrothermally to study the device behavior and compare the performance under high current density pulsed operation. The power dissipation and the lattice temperature profile of the SiC diodes were analyzed to compare the magnitude of heat loss and formation of thermal hot spot in the diode structure to predict the suitability of the device for pulsed power applications.

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+ Evaluation Of High Field And/Or Local Heating Based Material Degradation Of Nanoscale Metal Emitter Tips: A Molecular Dynamics Analysis
  Zhang, Z; Giesselmann, M; Mankowski, J; Dickens, J; Neuber, A; Joshi, RP
Abstract:  A molecular dynamics ( MD) model is used to study the potential for mass ejection from a metal nanoprotrusion, driven by high fields and temperature increases. Three- dimensional calculations of the electric fields surrounding the metal emitter are used to obtain the Maxwell stress on the metal. This surface loading is coupled into MD simulations. Our results show that mass ejection from the nanotip is possible and indicate that both larger aspect ratios and higher local temperatures will drive the instability. Hence it is predicted that in a nonuniform distribution of emitters, the longer and thinner sites will suffer the most damage, which is generally in keeping with the trends of a recent experimental report ( Parson et al 2014 IEEE Trans. Plasma Sci. 42 3982). A possible hypothesis for mass ejection in the absence of a distinct nanoprotrusion is also discussed.
+ Global Stability Of The Sync With Amplitude Effects
  Liu, S; Zou, W; He, MZ; Kurths, J; Zhan, M
Abstract:  Synchronization (or sync) is a basic problem in nature and engineering. One decade ago, the problem of the size of the sync (as a global geometric quantity) was first proposed by Wiley, Strogatz, and Girvan (WSG). In studying a ring of N coupled identical phase oscillators with each interacting equally with its d nearest neighbors on either side, they found that the sync state becomes globally stable when d/N is greater than a critical constant [D. A. Wiley, S. H. Strogatz, and M. Girvan, Chaos, 16 (2006) 015103]. Based on our previous results on the local stability of splay states in a ring of coupled Landau-Stuart oscillators, in this paper we further study the problem of global stability of the sync with amplitude effects, and we find that for any sufficiently large N the condition for the critical coupling range d(c) satisfying global stability of sync now becomes d(c) alpha N-gamma (gamma = 2/3), whereas when the coupling strength becomes weaker, the condition gradually changes to d(c) similar or equal to 0.3405N coincidental with the former result of the WSG in coupled phase oscillators. These relations are indicative of the favorable effect of amplitude for global stability of the sync. All these findings are based on rigorous theoretical analysis and have been well verified by numerical simulation; they are expected to be of potential significance for theory as well as application of synchronization in networked oscillatory systems.
+ Model Predictions For Atmospheric Air Breakdown By Radio-Frequency Excitation In Large Gaps
  Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  The behavior of the breakdown electric field versus frequency (DC to 100 MHz) for different gap lengths has been studied numerically at atmospheric pressure. Unlike previous reports, the focus here is on much larger gap lengths in the 1-5 cm range. A numerical analysis, with transport coefficients obtained from Monte Carlo calculations, is used to ascertain the electric field thresholds at which the growth and extinction of the electron population over time are balanced. Our analysis is indicative of a U-shaped frequency dependence, lower breakdown fields with increasing gap lengths, and trends qualitatively similar to the frequency-dependent field behavior for microgaps. The low frequency value of similar to 34 kV/cm for a 1 cm gap approaches the reported DC Paschen limit. Published by AIP Publishing.
+ Monte Carlo Analysis Of Field-Dependent Electron Avalanche Coefficients In Nitrogen At Atmospheric Pressure
  Nguyen, HK; Mankowski, J; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  Calculations of electron impact ionization of nitrogen gas at atmospheric pressure are presented based on the kinetic Monte Carlo technique. The emphasis is on energy partitioning between primary and secondary electrons, and three different energy sharing schemes have been evaluated. The ionization behavior is based on Wannier's classical treatment. Our Monte Carlo results for the field-dependent drift velocities match the available experimental data. More interestingly, the field-dependent first Townsend coefficient predicted by the Monte Carlo calculations is shown to be in close agreement with reported data for E/N values ranging as high as 4000 Td, only when a random assignment of excess energies between the primary and secondary particles is used. Published by AIP Publishing.
+ Nonlinear Uv Absorption Properties Of Bulk 4H-Sic
  Meyers, V; Mauch, D; Dickens, J; Neuber, A
Abstract:  The intensity-dependent light absorption in bulk high-purity semi-insulating 4H-SiC at above band gap photon energies has been studied. In particular, 3.49 eV (355 nm) UV absorption of 160 lmthick samples of varying recombination lifetimes in the intensity range of 1 mJ/cm(2) -30 mJ/cm(2) is addressed. The effective absorption coefficient was found to vary up to 30% within this range. Assuming deep level trapping, interband absorption, and free carrier absorption as dominant processes, a four energy level model reproduces the experimentally observed absorption behavior. While nonlinearities in the optical absorption behavior of SiC have been studied previously as function of wavelength alpha(lambda), temperature alpha(T) and, to a very limited extent, at below bandgap optical intensities, the presented elucidates the UV intensity-dependent nonlinear absorption behavior, alpha(I), of SiC at above bandgap photon energies. Published by AIP Publishing.
+ Performance Analysis Of An All Solid-State Linear Transformer Driver
  L. Collier; J. Dickens; J. Mankowski; A. Neuber
Abstract:  The performance of an all solid-state linear transformer driver (LTD) is evaluated based on experimentally verified behavior of a single stage. The single-stage LTD utilizes a low-profile design with robust thyristor switches and high-energy-density mica capacitors to minimize overall system inductance. Subnanosecond jitter is achieved with simultaneous thyristor triggering. The stage is magnetically coupled to a secondary winding through a central nanocrystalline core. A dc current source, decoupled with a large inductance, actively resets the core between pulses. The overall result is a low-impedance (<;1 Ω per stage) pulse generator that rivals the performance of traditional Marx systems with the improved reliability, increased lifetime, and fast rep-rate capabilities of solid-state switches. The stage is tested with charging voltages up to 8 kV into various loads and compared with simulations based on an analog behavioral thyristor switch model previously developed at Texas Tech University. The simulation is expanded into a full-scale, multistage LTD simulation and compared with a previously constructed Marx generator.

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+ Pulsed Characterization Of A Uv Led For Pulsed Power Applications On A Silicon Carbide Photoconductive Semiconductor Switch
  Wilson, N; Mauch, D; Meyers, V; Feathers, S; Dickens, J; Neuber, A
Abstract:  The electrical and optical characteristics of a high-power UV light emitting diode (LED) (365 nm wavelength) were evaluated under pulsed operating conditions at current amplitudes several orders of magnitude beyond the LED's manufacturer specifications. Geared towards triggering of photoconductive semiconductor switches (PCSSs) for pulsed power applications, measurements were made over varying pulse widths (25 ns-100 mu s), current (0 A-250 A), and repetition rates (single shot-5 MHz). The LED forward voltage was observed to increase linearly with increasing current (similar to 3.5 V-53 V) and decrease with increasing pulse widths. The peak optical power observed was >30 W, and a maximum system efficiency of 23% was achieved. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a 4H-SiC PCSS. The lowest measured on-resistance of SiC was approximately 67 k Omega. Published by AIP Publishing.
+ Quantifying Risk Of Wind Power Ramps In Ercot
  Zhao, J; Abedi, S; He, M; Du, PW; Sharma, S; Blevins, B
Abstract:  Hourly wind power ramps in ERCOT are studied by applying extreme value theory. Mean excess plot reveals that the tail behavior of large hourly wind power ramps indeed follows a generalized Pareto distribution. The location, shape, and scale parameters of generalized Pareto distribution are then determined by using mean excess plot and the least square technique, from which risk measures including aquantile value at risk and conditional value at risk are calculated.
+ Single-Pulse Avalanche Mode Robustness Of Commercial 1200 V/80 Mω Sic Mosfets
  M. D. Kelley; B. N. Pushpakaran; S. B. Bayne
Abstract:  Commercialization of 1200-V silicon carbide (SiC) MOSFET has enabled power electronic design with improved efficiency as well as increased power density. High-voltage spikes induced in applications such as solenoid control, solid-state transformer, boost converter, and flyback converter can drive the MOSFET into avalanche mode operation due to high di/dt coupled with parasitic inductance. Avalanche mode operation is characterized by high-power dissipation within the device due to the high voltage and current crossover. This study focuses on the evaluation of two commercially available SiC MOSFETs from different manufacturers, each rated for 1200 V with an ON-state resistance of 80 mΩ, during unclamped inductive switching (UIS) mode operation. To determine device reliability, a decoupled UIS testbed was developed to evaluate the avalanche energy robustness at 22 °C and 125 °C during two specific conditions: high current and low energy, and low current and high energy. The SiC MOSFETs were evaluated using a load inductance of 1.42, 5.1, 10.5, and 15.8 mH to understand the effect of current and avalanche energy on device failure. To correlate the experimental results with the failure mechanism, estimated junction temperature and static device characteristics are presented; additionally, MOSFETs were decapsulated to examine the failure sites on the semiconductor die.

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+ The Path To A Transportable Ionospheric Heater-Tuning Methods
  B. Esser; S. R. Beeson; J. C. Dickens; J. J. Mankowski; T. M. Antonsen; A. A. Neuber
Abstract:  A tunable electrically small antenna (ESA) designed to be naturally resonant at 100 MHz is evaluated for its range of tuning and feasibility for use in a mobile ionospheric heating (MIH) setup. The overarching goal is to match the ionospheric heating performance of the 180 element array at the high frequency active auroral research program (HAARP), which occupies approximately 1.2 × 105 m2 of land in Gakona, Alaska. While each HAARP crossed dipole element occupies 440 m2 of land and is tunable in the range of 2.7-10 MHz using automatic matching networks, the presented ESA approach is aimed toward enabling the fabrication of a transportable MIH array platform capable of high continuous wave (cw) power, albeit with a linear dimension five to ten times smaller than that of an equivalent dipole antenna. It is elucidated that the capacitively tuned ESA is continuously tunable to a frequency about 50% lower than that of the ESA's base frequency, albeit the resonant antenna structure carries a fractional bandwidth of merely 1%-2%.

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+ Theory Of Linear And Nonlinear Gain In A Gyroamplifier Using A Confocal Waveguide
  A. V. Soane; M. A. Shapiro; J. C. Stephens; R. J. Temkin
Abstract:  The linear and nonlinear theory of a gyroamplifier using a confocal waveguide is presented. A quasi-optical approach to describing the modes of a confocal waveguide is derived. Both the equations of motion and the mode excitation equation are derived in detail. The confocal waveguide circuit has the advantage of reducing mode competition, but the lack of azimuthal symmetry presents challenges in calculating the gain. In the linear regime, the gain calculated using the exact form factor for the confocal waveguide agrees with an azimuthally averaged form factor. A beamlet code, including velocity spread effects, has been written to calculate the linear and nonlinear (saturated) gain. It has been successfully benchmarked against the MAGY code for azimuthally symmetric cases. For the confocal waveguide, the beamlet code shows that the saturated gain is reduced when compared with results obtained using an azimuthally averaged form factor. The beamlet code derived here extends the capabilities of nonlinear gyroamplifier theory to configurations that lack azimuthal symmetry.

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+ Towards Kilohertz Electrochemical Capacitors For Filtering And Pulse Energy Harvesting
  Fan, ZY; Islam, N; Bayne, SB
Abstract:  Electrochemical capacitors (ECs) are slow devices with charging and discharging rates limited below 1 Hz. They run at direct current and function as power source, but cannot afford the role of a conventional capacitor for current ripple filtering or pulse energy harvesting. Recently, developing ultrafast ECs that work at hundreds to kilohertz (kHz) frequency scope have attracted great interests, with the aim to replace the traditional aluminum electrolytic capacitors (AECs) that have bulky size and large equivalent series resistance. Compact kHz ECs would produce huge impacts on power design, power electronics and environmental pulse energy harvesting. Towards such a goal, the electrode material and its nanostructure are the keys to boost the response frequency of an EC from below 1 Hz to above 1 kHz. In this Review, we summarize guidelines on the electrode nanostructure design for kHz response, discuss the various carbonaceous materials and other highly conductive materials based electrode structures for kHz ECs. The configurations of higher voltage kHz ECs, and their dimension advantage over AECs are critically evaluated, followed with the outlook on the further study and development in this promising area.
Publication Year:  2016
+ A 500 A Device Characterizer Utilizing A Pulsed-Linear Amplifier
  Lacouture, S; Bayne, S
Abstract:  With the advent of modern power semiconductor switching elements, the envelope defining high power is an ever increasing quantity. Characterization of these semiconductor power devices generally falls into two categories: switching, or transient characteristics, and static, or DC characteristics. With the increasing native voltage and current levels that modern power devices are capable of handling, characterization equipment meant to extract quasi-static IV curves has not kept pace, often leaving researchers with no other option than to construct ad hoc curve tracers from disparate pieces of equipment. In this paper, a dedicated 10 V, 500 A curve tracer was designed and constructed for use with state of the art high power semiconductor switching and control elements. The characterizer is a physically small, pulsed power system at the heart of which is a relatively high power linear amplifier operating in a switched manner in order to deliver well defined square voltage pulses. These actively shaped pulses are used to obtain device's quasi-static DC characteristics accurately without causing any damage to the device tested. Voltage and current waveforms from each pulse are recorded simultaneously by two separate high-speed analog to digital converters and averaged over a specified interval to obtain points in the reconstructed IV graph. (C) 2016 AIP Publishing LLC.
+ A Take On Arbitrary Transient Electric Field Reconstruction Using Wavelet Decomposition Theory Coupled With Particle Swarm Optimization
  K. Eldridge; A. Fierro; J. Dickens; A. Neuber
Abstract:  Destructive and constructive interference of multiple time-shifted and amplitude-adjusted higher frequency signals (wavelet signals) is exploited in order to reproduce a desired signal at a given point in the far-field regime of radiating antennas. The number of individual wavelets is intentionally kept small in keeping with a realistic antenna array size, where each antenna would emit wavelets at conceivably very high power levels. Wavelet decomposition theory is coupled with particle swarm optimization to determine the necessary time shifts and amplitude adjustments of the wavelet signals. In this application, the reconstructed signal can be specified by a desired frequency or arbitrary shape. A pyramidal horn antenna array is used in the analysis of the far-field propagation of the wavelet signals due to its relatively large bandwidth and known analytical electric field solutions. It is found that when the wavelet signals are appropriately superpositioned and added in the far field, the desired signal may be reconstructed with the quality of reconstruction mostly governed by the intentionally low number of wavelets. The reconstructed signal is solely found on the centerline while the signal drastically changes off the centerline or at distances too close or too far from the antenna array.

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+ A Variable Resistance Thyristor-Type Switch Modeling Technique
  Walls, MB; Fierro, A; Dickens, J; Mankowski, J; Neuber, A
Abstract:  An accurate model for simulating the transient turn-ON performance of thyristor-type switches is desirable for the development of solid-state pulse generators. The existing thyristor models are not suitable since they are either impractical to implement in an SPICE simulator, do not accurately model transient turn-ON performance, or are dependent on external circuit parameters that may not be known during the design phase, such as the load and discharge capacitance. An empirical model is developed and presented for Silicon Power's CCS SC 14N40 thyristor. The process of model determination is detailed, and waveforms obtained from the experiment and an SPICE circuit simulation that implements the thyristor model are discussed.
+ Analysis Of Equivalent Circuit Of The Utility Scale Battery For Wind Integration
  A. S. Subburaj; S. B. Bayne; M. G. Giesselmann; M. A. Harral
Abstract:  This paper provides steady-state and transient analysis of the equivalent circuit of the 1 MWh battery tied to the grid for wind integration. It also discusses the installation of a 1 MWh battery system at Reese Technology Center (RTC) in Lubbock, Texas. The research involves deploying energy storage devices for application with wind turbine model to understand the transient behavior of the system under three phase fault conditions. A 1 MW/1 MWh battery storage system at the RTC is connected to the South Plains Electric Cooperative (SPEC) grid. The batteries are used for energy storage and for mitigation of transient conditions grid dynamics. In this paper, the equivalent circuit of the 1 MWh battery is modeled in PSCAD and analyzed for its charge and discharge characteristics under transient fault conditions when it is tied to the grid for wind integration.

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+ Assessing The Role Of Trap-To-Band Impact Ionization And Hole Transport On The Dark Currents Of 4H-Sic Photoconductive Switches Containing Deep Defects
  Chowdhury, AR; Dickens, JC; Neuber, AA; Joshi, RP
Abstract:  Simulation studies of the electrical response characteristics of 4H-SiC switches containing traps are reported in the absence of photoexcitation. The focus is on trap-to-band impact ionization and the role of hole injection from the anode. Simulations show that hole-initiated ionization can be more important than the electron-initiated process. The results also underscore the role of hole injection at the high applied voltages. Our one-dimensional, time-dependent model yielded reasonable agreement with measured current-voltage data spanning over three orders of magnitude, but only when impact ionization was taken into account. Finally, the simulations predicted undulations in the device conduction current density with respect to time, due to the dynamic interplay between impact ionization, spatial electric field values, and occupancies of the trap levels. Published by AIP Publishing.
+ Characteristics Of A Four Element Gyromagnetic Nonlinear Transmission Line Array High Power Microwave Source
  Johnson, JM; Reale, DV; Krile, JT; Garcia, RS; Cravey, WH; Neuber, AA; Dickens, JC; Mankowski, JJ
Abstract:  In this paper, a solid-state four element array gyromagnetic nonlinear transmission line high power microwave system is presented as well as a detailed description of its subsystems and general output capabilities. This frequency agile S-band source is easily adjusted from 2-4 GHz by way of a DC driven biasing magnetic field and is capable of generating electric fields of 7.8 kV/m at 10 m correlating to 4.2 MW of RF power with pulse repetition frequencies up to 1 kHz. Beam steering of the array at angles of +/- 16.7 degrees is also demonstrated, and the associated general radiation pattern is detailed. Published by AIP Publishing.
+ Characterization Of An N-Type 4-Kv Gto For Pulsed Power Applications
  T. Flack; C. Hettler; S. Bayne
Abstract:  This paper details the experimental evaluation and simulation of a 4-kV n-type gate turn-OFF thyristor (GTO) designed for pulsed power applications. The primary criteria of evaluation are rate of current rise (dI/dt), turn-ON delay time (TD), and resistance of the device during turn-ON transients [RON(t)]. The device under test (DuT) is an n-type asymmetric-blocking GTO manufactured by Silicon Power (Part No. 14N40A10) with a rated dc blocking voltage of 4 kV. A test circuit was specifically designed to minimize stray inductance in order to capitalize on the dI/dt capabilities of the DuT. Experimental data collected from resistance measurements are used to develop a single-switch approximate model for use in simulation. The results of dI/dt experiments provide a profile of DuT dI/dt operation beyond rated values; specifically dI/dt values >70 kA/μs were readily achieved. The turn-ON delay time of the DuT is also characterized and determined to be ~225 ns on average.

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+ Contact Extensions Over A High-K Dielectric Layer For Surface Field Mitigation In High Power 4H–Sic Photoconductive Switches
  A. R. Chowdhury; D. Mauch; R. P. Joshi; A. A. Neuber; J. Dickens
Abstract:  We focus on a simulation study to probe the mitigation of electric fields, especially at the edges of metal contacts to SiC-based photoconductive switches. Field reduction becomes germane given that field-induced failures near contacts have been reported. A dual strategy of extending metal contacts to effectively spread the electric field over a larger distance and to employ HfO2 as a high-k dielectric, is discussed. Simulation results show that peak electric fields can be lowered by up to ~67% relative to a standard design. Finally, our calculations predict that the internal temperature rise for a ~7-ns laser pulse and applied voltages around 20 kV (typical experimental values) would also be effectively controlled.

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+ Device-To-Device Communications For Energy Management: A Smart Grid Case
  Cao, Y; Jiang, T; He, M; Zhang, JS
Abstract:  The transmission of simultaneous and latency-sensitive data puts forth a significant challenge for the smart grid communications. In this paper, we investigate the application of device-to-device (D2D) communications for the energy management in the electric distribution network. Specifically, we develop a D2D-assisted relaying framework to exploit the spatial diversity and the differentiated data rate requirements, which improves the spectral efficiency, especially for the scenarios that there are faults in the electric distribution network. We study the data transmission scheduling problem under the proposed D2D-assisted relaying framework, aiming to minimize the overall information loss rate, while taking into account the uncertainties in the communication latency. To this end, we first cast the data transmission scheduling problem as a two-stage stochastic programming problem and derive the solution. Then, we develop a real-time distributed data transmission scheduling scheme based on the sample path realizations. Extensive simulation results show significant performance improvement by using the proposed D2D-assisted relaying framework compared with two baseline frameworks for a variety of different cases.
+ Discrete Photon Implementation For Plasma Simulations
  Fierro, A; Stephens, J; Beeson, S; Dickens, J; Neuber, A
Abstract:  The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photoionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell/Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region. (C) 2016 AIP Publishing LLC.
+ Ex Vivo Renal Stone Characterization With Single-Source Dual-Energy Computed Tomography: A Multiparametric Approach
  Kriegshauser, JS; Silva, AC; Paden, RG; He, M; Humphreys, MR; Zell, SI; Fu, YL; Wu, T
Abstract:  Rationale and Objectives: We aimed to investigate a multiparametric approach using single-source dual-energy computed tomography (ssDECT) for the characterization of renal stones. Materials and Methods: ssDECT scans were performed at 80 and 140 kVp on 32 ex vivo kidney stones of 3-10 mm in a phantom. True composition was determined by infrared spectroscopy to be uric acid (UA; n = 14), struvite (n = 7), cystine (n = 7), or calcium oxalate monohydrate (n = 4). Measurements were obtained for up to 52 variables, including mean density at 11 monochromatic keV levels, effective Z, and multiple material basis pairs. The data were analyzed with five multiparametric algorithms. After omitting 8 stones smaller than 5 mm, the remaining 24-stone dataset was similarly analyzed. Both stone datasets were also analyzed with a subset of 14 commonly used variables in the same fashion. Results: For the 32-stone dataset, the best method for distinguishing UA from non-UA stones was 97% accurate, and for distinguishing the non-UA subtypes was 72% accurate. For the 24-stone dataset, the best method for distinguishing UA from non-UA stones was 100% accurate, and for distinguishing the non-UA subtypes was 75% accurate. Conclusion: Multiparametric ssDECT methods can distinguish UA from non-UA stones of 5 mm or larger with 100% accuracy. The best model to distinguish the non-UA renal stone subtypes was 75% accurate. Further refinement of this multiparametric approach may increase the diagnostic accuracy of separating non-UA subtypes and assist in the development of a clinical paradigm for in vivo use.
+ Failure Analysis Of 1200-V/150-A Sic Mosfet Under Repetitive Pulsed Overcurrent Conditions
  J. A. Schrock; B. N. Pushpakaran; A. V. Bilbao; W. B. Ray; E. A. Hirsch; M. D. Kelley; S. L. Holt; S. B. Bayne
Abstract:  SiC MOSFETs are a leading option for increasing the power density of power electronics; however, for these devices to supersede the Si insulated-gate bipolar transistor, their characteristics have to be further understood. Two SiC vertically oriented planar gate D-MOSFETs rated for 1200 V/150 A were repetitively subjected to pulsed overcurrent conditions to evaluate their failure mode due to this common source of electrical stress. This research supplements recent work that demonstrated the long term reliability of these same devices [1]. Using an RLC pulse-ring-down test bed, these devices hard-switched 600 A peak current pulses, corresponding to a current density of 1500 A/cm2. Throughout testing, static characteristics of the devices such as BVDSS, RDS (on), and VGS(th) were measured with a high power device analyzer. The experimental results indicated that a conductive path was formed through the gate oxide; TCAD simulations revealed localized heating at the SiC/SiO2 interface as a result of the extreme high current density present in the device's JFET region. However, the high peak currents and repetition rates required to produce the conductive path through the gate oxide demonstrate the robustness of SiC MOSFETs under the pulsed overcurrent conditions common in power electronic applications.

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+ Failure Modes Of 15-Kv Sic Sgto Thyristors During Repetitive Extreme Pulsed Overcurrent Conditions
  J. A. Schrock; E. A. Hirsch; S. Lacouture; M. D. Kelley; A. V. Bilbao; W. B. Ray; S. B. Bayne; M. Giesselmann; H. O'Brien; A. Ogunniyi
Abstract:  SiC SGTO thyristors are an advanced solution for increasing the power density of medium voltage power electronics. However, for these devices to replace Si thyristor technology in industrial applications their characteristics and failure modes must be understood. This letter presents the failure modes of two 15-kV SiC SGTO thyristors during repetitive overcurrent conditions. The devices were evaluated with 2-kA (3.85 kA/cm2) square pulses of 100 μs duration using a pulse forming network. Throughout testing, each devices' static characteristics were analyzed for signs of degradation; upon degradation, testing was ceased and the physical failure mode was determined through imaging with a scanning electron microscope (SEM) in conjunction with a focused ion beam. The electrical results demonstrate the failure modes of both SiC SGTO thyristors during pulsed overcurrents electrically manifested themselves as a conductive path through the gate-anode junction and an increased device on-state voltage. SEM imaging revealed one SiC thyristor formed an approximately 10-μm wide cylindrical void, and the second SiC thyristor formed an approximately 200-μm long crack. However, the experimental results demonstrate these 15-kV SiC SGTO thyristors' robust ability to repetitively switch at extreme high current density for tens of thousands of cycles.

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+ Field Distortion By A Single Cavity In Hvdc Xlpe Cable Under Steady State
  He, M; Chen, G; Lewin, PL
Abstract:  Electric field distribution in high voltage direct current (HVDC) cross-linked polyethylene (XLPE) cable at steady state is researched in this study. Under the condition that no cavity exists, equations describing the critical temperature difference for field reversal, and electric field distribution at steady state are proposed. When the cable with cavity is studied via the finite element analysis method, 2D and 3D model can be two possible choices. The comparison between these two models provides the reference for model selection so that the problem can be solved correctly. To study field enhancement by a single air-filled cavity at steady state, some possibly influenced factors are considered, including the cavity location, cavity size, cavity shape, temperature difference and material type. The equation describing field enhancement is given in the study.
+ Gan Technology For Power Electronic Applications: A Review
  Flack, TJ; Pushpakaran, BN; Bayne, SB
Abstract:  Power semiconductor devices based on silicon (Si) are quickly approaching their limits, set by fundamental material properties. In order to address these limitations, new materials for use in devices must be investigated. Wide bandgap materials, such as silicon carbide (SiC) and gallium nitride (GaN) have suitable properties for power electronic applications; however, fabrication of practical devices from these materials may be challenging. SiC technology has matured to point of commercialized devices, whereas GaN requires further research to realize full material potential. This review covers fundamental material properties of GaN as they relate to Si and SiC. This is followed by a discussion of the contemporary issues involved with bulk GaN substrates and their fabrication and a brief overview of how devices are fabricated, both on native GaN substrate material and non-native substrate material. An overview of current device structures, which are being analyzed for use in power switching applications, is then provided; both vertical and lateral device structures are considered. Finally, a brief discussion of prototypes currently employing GaN devices is given.
+ Impact Of Silicon Carbide Semiconductor Technology In Photovoltaic Energy System
  Pushpakaran, BN; Subburaj, AS; Bayne, SB; Mookken, J
Abstract:  The increased awareness of the significance of solar energy has led to intensified research in the areas of solar energy harvesting. To increase the cost effectiveness of the generation of solar power, silicon carbide (SiC) power devices are playing a major role in the power electronics technology due to its superior material properties compared to Silicon (Si). The photovoltaic (PV) inverter is a major component in the solar energy conversion system whose performance relies on the efficient design of power electronics. In order to obtain maximum power from the solar panels, the power loss in the energy conversion system must be minimized by proper selection of semiconductor devices and thereby minimizing the number of power electronic components used. The necessity to reduce the overall switch mass and volume have led to the development of advanced high-power, high-temperature semiconductor materials such as SiC. The performance improvements are based on superior material properties of SiC, such as: bandgap of 3.26 eV, critical breakdown field of 2-4 MV/cm, thermal conductivity of 4.9 W/(cm K), and a saturated drift velocity of 2 x 10(7) cm/s. The aforementioned properties of SiC highlight the importance of silicon carbide semiconductor technology. Some of the limitations of the technology include higher device cost due to emerging technology, and need for high-temperature packaging techniques. Future research includes methods to reduce manufacturing cost, packaging issues, and also face challenges to increase the performance and reliability of SiC devices. The focus of the paper is to discuss the role of SiC semiconductor based power electronics technology in PV energy conversion system. The comparisons and analysis of various PV inverter system prototypes imply that the application of SiC power semiconductor devices in a PV energy system can help eliminate several issues which are at present due to the material limitations of silicon. (C) 2015 Elsevier Ltd. All rights reserved.
+ Investigation Of A Stripline Transmission Line Structure For Gyromagnetic Nonlinear Transmission Line High Power Microwave Sources
  Reale, DV; Parson, JM; Neuber, AA; Dickens, JC; Mankowski, JJ
Abstract:  A stripline gyromagnetic nonlinear transmission line (NLTL) was constructed out of yttrium iron garnet ferrite and tested at charge voltages of 35 kV-55 kV with bias fields ranging from 10 kA/m to 20 kA/m. Typically, high power gyromagnetic NLTLs are constructed in a coaxial geometry. While this approach has many advantages, including a uniform transverse electromagnetic (TEM) mode, simple interconnection between components, and the ability to use oil or pressurized gas as an insulator, the coaxial implementation suffers from complexity of construction, especially when using a solid insulator. By moving to a simpler transmission line geometry, NLTLs can be constructed more easily and arrayed on a single substrate. This work represents a first step in exploring the suitability of various transmission line structures, such as microstrips and coplanar waveguides. The resulting high power microwave (HPM) source operates in ultra high frequency (UHF) band with an average bandwidth of 40.1% and peak rf power from 2 MW to 12.7 MW. (C) 2016 AIP Publishing LLC.
+ Numerical Assessment Of Secondary Electron Emission On The Performance Of Rising-Sun Magnetrons With Axial Output
  A. Majzoobi; R. P. Joshi; A. A. Neuber; J. C. Dickens
Abstract:  Particle-in-cell simulations are performed to analyze the role of secondary electron emission (SEE) on the efficiency, the output power and the leakage currents of 12-cavity, 12-cathode Rising-Sun magnetrons with diffraction output. The simulation results seem to indicate that the role of SEE would be fairly negligible. Small changes are predicted, linked to deviations in the starting trajectories of secondary electrons following their generation and the lower fraction of electrons in clusters with a synchronized rotational velocity. Overall, a peak power output of about 2.48 GW is predicted at a magnetic field of 0.45 T, with efficiencies as high as 75%. Furthermore, deviations in the output power with SEE are predicted to occur at shorter times, but would not be an issue for pulses greater than 25 ns in duration.

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+ Physics-Based Simulation Of 4H-Sic Dmosfet Structure Under Inductive Switching
  Pushpakaran, BN; Bayne, SB; Ogunniyi, AA
Abstract:  The integration of high power silicon carbide (SiC) Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) in today's power systems drives the demand for deeper understanding of the device switching characteristics by way of device simulation. Applications like motor drive require power MOSFETs to drive highly inductive loads which increase the switching power loss by extending the voltage and current crossover, a situation which gets exacerbated by the presence of parasitic inductance. A 2D model of a 1200 V 4H-SiC vertical DMOSFET half-cell was developed using a commercially available TCAD software package to investigate the electro-thermal switching characteristics using clamped inductive switching circuit for ON state drain current density values up to at an ambient lattice temperature of 300 K. Device physics-based models were included to account for carrier mobility, carrier generation and recombination, impact ionization and lattice heating. In order to analyze the areas of localized lattice heating, the lattice temperature distribution was monitored during simulation. The clamped inductive switching circuit simulations were performed with and without the addition of parasitic electrode inductance to observe the difference in switching energy loss.
+ Stochastic Optimization-Based Economic Dispatch And Interruptible Load Management With Increased Wind Penetration
  Yang, L; He, M; Vittal, V; Zhang, JS
Abstract:  In this paper, stochastic optimization of economic dispatch (ED) and interruptible load management is investigated using short-term distributional forecast of wind farm generation. Specifically, using the statistical information of wind farm generation extracted from historical data, a Markov chainbased distributional forecast model for wind farm generation is developed in a rigorous optimization framework, in which the diurnal nonstationarity and the seasonality of wind generation are accounted for by constructing multiple finite-state Markov chains for each epoch of 3 h and for each individual month. Based on this distributional forecast model, the joint optimization of ED and interruptible load management is cast as a stochastic optimization problem. Additionally, a robust ED is formulated using an uncertainty set constructed based on the proposed distributional forecast, aiming to minimize the system cost for worst cases. The proposed stochastic ED is compared with four other ED schemes: 1) the robust ED; 2) deterministic ED using the persistence wind generation forecast model; 3) scenario-based stochastic ED; and 4) deterministic ED, in which perfect wind generation forecasts are used. Numerical studies, using the IEEE Reliability Test System-1996 and realistic wind measurement data from an actual wind farm, demonstrate the significant benefits obtained by leveraging the Markov chain-based distributional forecast and the interruptible load management.
+ Suitability Of N-On Recessed Implanted Gate Vertical-Channel Sic Jfets For Optically Triggered 1200 V Solid-State Circuit Breakers
  Veliadis, V; Steiner, B; Lawson, K; Bayne, SB; Urciuoli, D; Ha, HC
Abstract:  A requirement for the commercialization of power SiC transistors is their long term reliable operation under the hard-switching conditions and high temperatures encountered in the field. Normally ON 1200 V vertical-channel implanted-gate SiC junction field effect transistors (JFETs), designed for high-power bidirectional (four quadrant) solid-state-circuit-breaker (SSCB) applications, were repetitively pulse hard switched at 150 degrees C from a 1200 V blocking state to an ON-state current of 115 A, which is in excess of 13 times the JFET's 250-W/cm(2) rated current at 150 degrees C. The JFETs were fabricated in seven photolithographic levels with a single masked ion-implantation forming the p+ gates and guard rings, and with no epitaxial regrowth. The pulsed testing was performed using a low inductance RLC circuit. In this circuit, energy initially stored in a capacitor is discharged in a load resistor through the JFET under test. The JFET hard-switch stressing included over 2.4 million 1200 V/115-A hard-switch events at 150 degrees C and at a repetition rate of 10 Hz. The peak energies and powers dissipated by the JFET at each hard-switch event were 73.2 mJ and 68.2 kW, respectively. The current rise rate was 166 A/mu s and the pulse full width at half maximum (FWHM) was 1.8 mu s. After over 2.4 million hard-switch events at 150 degrees C, the JFET blocking voltage characteristics remained unchanged while the ON-state conduction slightly improved, which indicate reliable operation. An optically triggered SSCB, based on these rugged JFET, is proposed.
Publication Year:  2015
+ A Frequency Stable Vacuum-Sealed Tube High-Power Microwave Vircator Operated At 500 Hz
  J. M. Parson; C. F. Lynn; M. C. Scott; S. E. Calico; J. C. Dickens; A. A. Neuber; J. J. Mankowski
Abstract:  Operation of repetitive high-power microwave (HPM) sources is predominantly limited by thermal properties of anode and cathode materials. This letter presents a reflex-triode virtual cathode oscillator (vircator) capable of operating at 500 Hz at current densities between 100-200 A/cm2 for multiple burst durations of 1-2 s. Stable vircator operation under such a thermally punishing environment is facilitated by the use of a thin pyrolytic graphite anode. The results presented focus on two anode-cathode (A-K) gap spacings: 11 and 21 mm, which produce stable microwave radiation at 4.6 and 1.6 GHz, respectively. Characteristic voltage, current, and microwave waveforms in conjunction with short-time Fourier transforms, frequency spectrographs, and HPM power density data for 1000 and 500 pulses at 1.6 and 4.6 GHz, respectively, are presented.

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+ A High Power Microwave Triggered Rf Opening Switch
  Beeson, S; Dickens, J; Neuber, A
Abstract:  A 4-port S-band waveguide structure was designed and fabricated such that a signal of any amplitude (less than 1 MW) can be switched from a normally closed state, <0.5 dB insertion loss (IL), to an open state >30 dB IL by initiating plasma in a gas cell situated at the junction of this waveguide and one propagating a megawatt level magnetron pulse. The 90/10 switching time is as low as 20 ns with a delay of similar to 30 ns between the onset of the high power microwave pulse and the initial drop of the signal. Two ports of this device are for the high power triggering pulse while the other two ports are for the triggered signal in a Moreno-like coupler configuration. In order to maintain high isolation, these two sets of waveguides are rotated 90 degrees from each other with a TE111 resonator/plasma cell located at the intersection. This manuscript describes the design and optimization of this structure using COMSOL 4.4 at the design frequency of 2.85 GHz, comparison of simulated scattering parameters with measured cold tests (testing without plasma), and finally the temporal waveforms of this device being used to successfully switch a low power CW signal from 2W to <5 mW on a sub-microsecond timescale. (C) 2015 AIP Publishing LLC.
+ An Exposition On Friedmann Cosmology With Negative Energy Densities
  Nemiroff, RJ; Joshi, R; Patla, BR
Abstract:  How would negative energy density affect a classic Friedmann cosmology? Although never measured and possibly unphysical, certain realizations of quantum field theories leaves the door open for such a possibility. In this paper we analyze the evolution of a universe comprising varying amounts of negative energy forms. Negative energy components have negative normalized energy densities, Omega < 0. They include negative phantom energy with an equation of state parameter w < -1, negative cosmological constant: w = -1, negative domain walls: w = -2/3, negative cosmic strings: w = -1/3, negative mass: w = 0, negative radiation: w = 1/3 and negative ultralight: w > 1/3. Assuming that such energy forms generate pressure like perfect fluids, the attractive or repulsive nature of negative energy components are reviewed. The Friedmann equation is satisfied only when negative energy forms are coupled to a greater magnitude of positive energy forms or positive curvature. We show that the solutions exhibit cyclic evolution with bounces and turnovers. The future and fate of such universes in terms of curvature, temperature, acceleration, and energy density are reviewed. The end states are dubbed big crunch, big void, or big rip and further qualified as warped,curved, or flat, hot versus cold, accelerating versus decelerating versus coasting. A universe that ends by contracting to zero energy density is termed big poof. Which contracting universes bounce in expansion and which expanding universes turnover into contraction are also reviewed.
+ Analysis Of High Field Effects On The Steady-State Current-Voltage Response Of Semi-Insulating 4H-Sic For Photoconductive Switch Applications
  Tiskumara, R; Joshi, RP; Mauch, D; Dickens, JC; Neuber, AA
Abstract:  A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC is carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, are comprehensively included. Results of our model match the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters are also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples can be discounted up to applied fields as high as similar to 275 kV/cm. (C) 2015 AIP Publishing LLC.
+ Anode Materials For High-Average-Power Operation In Vacuum At Gigawatt Instantaneous Power Levels
  C. F. Lynn; J. M. Parson; M. C. Scott; S. E. Calico; J. C. Dickens; A. A. Neuber; J. J. Mankowski
Abstract:  The thermal behavior of several electrically conducting solids under high incident electron fluence in high vacuum was evaluated. At electron energies of up to ~200 keV, the depth-dose relationship for electron penetration into the materials was considered, and the resulting energy deposition profile from the surface was revealed to extend to a maximum of ~175 μm below the surface depending on the anode material. Black body radiation is considered as the major mechanism that balances the power deposited in the material on the timescales of interest. Comparing the radiated power density at the sublimation temperature for different materials, metallic/nonmetallic, revealed that pyrolytic graphite anodes may radiate over 20 times more power than metallic anodes before failure due to sublimation. In addition, transparent pyrolytic graphite anodes (with a thickness on the order of several tens of micrometer) potentially radiate up to 40 times that of metallic anodes, since heating by the electron beam is approximately uniform throughout the thickness of the material, thus radiation is emitted from both sides. Experimental results obtained from titanium and pyrolytic graphite anodes validate the thermal analysis.

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+ Asymmetric Conduction In Biological Nanopores Created By High-Intensity, Nanosecond Pulsing: Inference On Internal Charge Lining The Membrane Based On A Model Study
  Joshi, RP; Qiu, H
Abstract:  Nanosecond, high-intensity electric pulses have been reported to open rectifying pores in biological cell membranes. The present goal is to qualitatively understand and analyze the experimental current-voltage (I-V) data. Here, nanopore transport is probed using a numerical method and on the basis of an analytical model. Our results show that geometric asymmetry in the nanopore would not yield asymmetry in the I-V characteristics. However, positive surface charge lining the pore could produce characteristics that compare well with data from patch-clamp measurements, and a value of similar to 0.02 C/m(2) is predicted from the numerical calculations. (C) 2015 AIP Publishing LLC.
+ Development And Testing Of An Active High Voltage Saturation Probe For Characterization Of Ultra-High Voltage Silicon Carbide Semiconductor Devices
  Bilbao, AV; Schrock, JA; Ray, WB; Kelley, MD; Holt, SL; Giesselmann, MG; Bayne, SB
Abstract:  Obtaining accurate collector to emitter voltage measurements when characterizing high voltage silicon carbide (SiC) devices requires the ability to measure voltages in the range of zero to 10 V while the device is in the on-state and the ability to withstand ultra-high voltages while the device is in the off-state. This paper presents a specialized voltage probe capable of accurately measuring the aforementioned range. A comparison is made between the proposed probe and other commonly used high voltage probe alternatives in relation to high voltage SiC device testing. Testing of the probe was performed to ensure linearity, high accuracy, and high bandwidth. (C) 2015 AIP Publishing LLC.
+ Evaluating The Performance Of A Carbon-Epoxy Capillary Cathode And Carbon Fiber Cathode In A Sealed-Tube Vircator Under Uhv Conditions
  E. Rocha; P. M. Kelly; J. M. Parson; C. F. Lynn; J. C. Dickens; A. A. Neuber; J. J. Mankowski; T. Queller; J. Gleizer; Y. E. Krasik.
Abstract:  This paper evaluates the performance of a bimodal carbon fiber cathode and a carbon-epoxy multicapillary cathode operating within a reflex-triode sealed-tube virtual cathode oscillator (vircator). The experimental results reveal that both cathodes exhibit similar emission behavior, although with some significant operational differences. An eight-stage 84-J pulseforming network-based Marx generator serves to drive both cathodes at 250 kV and 3-4 kA with a ~70-ns pulsewidth. Both cathodes undergo conditioning over 10000 pulses to determine gas evolution as well as electrical changes over time. Gas evolution of both cathodes is observed using a residual gas analyzer to determine individual gas constituents. A comparison of diode voltage, diode current, RF output, and outgassing data for both cathodes during vircator operation over 10000 pulses is presented to quantify cathode performance in a sealed-tube vircator. Changes in cathode surface morphology, from virgin to postmortem, are discussed. Data for various anode-cathode gap distances, from 3 to 15 mm, are presented. The evolution of voltage and current inputs to the vircator is discussed.

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+ Evaluation Of A Pulsed Ultraviolet Light-Emitting Diode For Triggering Photoconductive Semiconductor Switches
  D. Mauch; C. Hettler; W. W. Sullivan; A. A. Neuber; J. Dickens
Abstract:  The power output, forward voltage, conversion efficiency, and spectral characteristics of a 365 nm ultraviolet light-emitting diode (LED) were measured for applications of triggering wide-bandgap photoconductive switches for pulsed power applications. Pulsed currents through the LED ranged from 125 mA up to 2.2 A at widths from 10 μs up to several seconds. Using time-resolved electroluminescence spectroscopy, peak emission was observed to occur at 368.5 nm for short pulses with a red-shift to 371.8 nm for pulses 8 s in duration. A peak light output of 4.1 W was measured for short pulses (<;50 μs) of 2.12 A, corresponding to six times the rated output specification. The LED was used to trigger a high-voltage photoconductive semiconductor switch (PCSS) at voltages up to 6 kV into a high-impedance load. The 365 nm LED is a promising candidate for optical triggering of PCSS devices.

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+ Feasibility Of Ultra-Low Radiation Dose Reduction For Renal Stone Ct Using Model-Based Iterative Reconstruction: Prospective Pilot Study
  Kriegshauser, JS; Naidu, SG; Paden, RG; He, M; Wu, Q; Hara, AK
Abstract:  Purpose: To compare dose, reader assessments, and noise between ultra-low-dose (ULD) and low-dose (LD) renal stone computed tomography (CT) using multiple reconstruction methods. Materials and methods: Prospective study of 19 patients having noncontrast LD and ULD renal stone CT reconstructed using filtered back projection, adaptive statistical iterative reconstruction (ASIR), and model-based iterative reconstruction (MBIR). Dose, noise measurements, and subjective image assessments were recorded. Results: ULD volume CT dose index was 61% less. Noise was 50% less with ULD MBIR than with LD ASIR (P<.001); no other significant differences existed. Conclusion: ULD MBIR is comparable to our standard of care, LD ASIR, with 61% lower dose and 50% less noise. (C) 2015 Elsevier Inc. All rights reserved.
+ High Power Lateral Silicon Carbide Photoconductive Semiconductor Switches And Investigation Of Degradation Mechanisms
  D. Mauch; W. Sullivan; A. Bullick; A. Neuber; J. Dickens
Abstract:  Several generations of high power, lateral, linear mode, intrinsically triggered 4H-SiC photoconductive semiconductor switch designs and their performance are presented. These switches were fabricated from high purity semi-insulating 4H-SiC samples measuring 12.7 mm × 12.7 mm × 0.36 mm and were able to block dc electric fields up to 370 kV/cm with leakage currents less than 10 μA without failure. Switching voltages and current s up to 26 kV and 450 A were achieved with these devices and ON-state resistances of 2 Ω were achieved with 1 mJ of 355 nm laser energy (7 ns FWHM). After fewer than 100 high power switching cycles, these devices exhibited cracks near the metal/SiC interface. Experimental and simulation results investigating the root cause of this failure mechanism are also presented. These results strongly suggest that a transient spike in the magnitude of the electric field at the metal/SiC interface during both switch closing and opening is the dominant cause of the observed cracking.

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+ High-Mobility Stable 1200-V, 150-A 4H-Sic Dmosfet Long-Term Reliability Analysis Under High Current Density Transient Conditions
  J. A. Schrock; W. B. Ray II; K. Lawson; A. Bilbao; S. B. Bayne; S. L. Holt; L. Cheng; J. W. Palmour; C. Scozzie
Abstract:  For SiC DMOSFETs to obtain widespread usage in power electronics their long-term operational ability to handle the stressful transient current and high temperatures common in power electronics needs to be further verified. To determine the long-term reliability of a single 4H-SiC DMOSFET, the effects of extreme high current density were evaluated. The 4H-SiC DMOSFET has an active conducting area of 40 mm2, and is rated for 1200 V and 150 A. The device was electrically stressed by hards-witching transient currents in excess of four times the given rating (>600 A) corresponding to a current density of 1500 A/cm2. Periodically throughout testing, several device characteristics including RDS(on) and VG S(th) were measured. After 500 000 switching cycles, the device showed a 6.77% decrease in RDS (on), and only a 132-mV decreased in VG S(th). Additionally, the dc characteristics of the device were analyzed from 25 to 150 °C and revealed a 200-mV increase in on-state voltage drop at 20 A and a 2-V reduction in VG S(th) at 150 °C. These results show this SiC DMOSFET has robust long-term reliability in high-power applications that are susceptible to pulse over currents, such as pulsed power modulators and hard-switched power electronics.

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+ Improvements In The Modeling Of Wind Turbines In Power System Studies
  Nimmagadda, S; Islam, A; Bayne, SB; Sanchez, J; Caballero, LG
Abstract:  Several methods to improve the current wind turbine modeling techniques for power system studies are discussed in this paper. Some of the modeling improvements which are discussed are dynamic initialization of models, differences between wind turbine models developed in electromagnetic transient program and bulk power system simulation software, and techniques to avoid numerical instability with minimum errors. Algorithms to provide automatic initial conditions of all variables for any given load flow condition, wind speeds, and wind farm control strategies are developed in this paper. This approach is an improvement to the traditional method of providing constant initial values for given operating conditions. Several physical components such as harmonic filters, cable impedances, and complex control algorithms are either simplified or excluded in the power system models. The fault response of the power system model under extreme conditions leads to a numerically unstable condition. The instability might be a result of simplification discussed above and does not occur in the real turbine. Additional modeling techniques that can be used to achieve numerical stability, dynamic and fault response close to the actual wind turbine are discussed. These techniques are used to compensate for the simplifications made in power system models. Finally, the issue of nuisance tripping faced very frequently by the Independent System Operators, while working on the studies involving wind power plants, is addressed. Various methods to reduce the deviations and prevent nuisance tripping are implemented in the paper. (C) 2015 AIP Publishing LLC.
+ Material Selection Of A Ferrimagnetic Loaded Coaxial Delay Line For Phasing Gyromagnetic Nonlinear Transmission Lines
  Johnson, JM; Reale, DV; Cravey, WH; Garcia, RS; Barnett, DH; Neuber, AA; Dickens, JC; Mankowski, JJ
Abstract:  Implementing nonlinear transmission line (NLTL) technology in the design of a high power microwave source has the benefits of producing a comparatively small and lightweight solid-state system where the emission frequency is easily tuned. Usually, smaller in physical size, single NLTLs may produce significantly less power than its vacuum based counterparts. However, combining individual NLTL outputs electrically or in free-space is an attractive solution to achieve greater output power. This paper discusses a method for aligning a four element NLTL antenna array with coaxial geometry using easily adjustable temporal delay lines. These delay lines, sometimes referred to as pulse shock lines or pulse sharpening lines, are placed serially in front of the main NLTL line. The propagation velocity in each delay line is set by the voltage amplitude of an incident pulse as well as the magnetic field bias. Each is adjustable although for the system described in this paper, the voltage is held constant while the bias is changed through applying an external DC magnetic field of varying magnitude. Three different ferrimagnetic materials are placed in the temporal delay line to evaluate which yields the greatest range of electrical delay with the least amount of variability from consecutive shots. (C) 2015 AIP Publishing LLC.
+ Offshore Wind Energy: A Comparative Analysis Of Uk, Usa And India
  Kota, S; Bayne, SB; Nimmagadda, S
Abstract:  Offshore wind is one of the most fascinating industries in the renewable energy sector and it is experiencing a remarkable growth. Offshore wind energy generation offers an opportunity in the race to decrease the dependence on fossil fuels, reduce green house emissions, increase energy security and create employment opportunities. UK has proven success in offshore wind and has been enjoying the economic benefits of offshore wind since over a decade. Offshore wind energy is an emergent renewable energy industry in the United States. The United States is coping up with the challenges and heading up fast to catch up with the industry. India is still in its infancy stage where the policy frameworks are framed by MNRE government and getting ready with the tools to enter into the offshore market. This paper researches the current situation and trend of offshore wind industries in UK and US, from aspects of policy, grid connections, operation and maintenance and cost reduction and analyses the proper direction and pathways of the industry to India. Therefore this paper highlights the scenario as to how these three countries UK, USA and India, respectively, are enabling offshore wind, to make a vital and sizeable contribution to the low carbon economy. (C) 2014 Elsevier Ltd. All rights reserved.
+ Optically Isolated, 2 Khz Repetition Rate, 4 Kv Solid-State Pulse Trigger Generator
  Barnett, DH; Parson, JM; Lynn, CF; Kelly, PM; Taylor, M; Calico, S; Scott, MC; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract:  This paper presents the design and operation characteristics of a solid-state high voltage pulse generator. Its primary utilization is aimed at triggering a gaseous spark gap with high repeatability. Specifically, the trigger generator is designed to achieve a risetime on the order of 0.1 kV/ns to trigger the first stage, trigatron spark gap of a 10-stage, 500 kV Marx generator. The major design components are comprised of a 60 W constant current DC-DC converter for high voltage charging, a single 4 kV thyristor, a step-up pulse transformer, and magnetic switch for pulse steepening. A risetime of <30 ns and pulse magnitude of 4 kV is achieved matching the simulated performance of the design. (C) 2015 AIP Publishing LLC.
+ Optimizing Drive Parameters Of A Nanosecond, Repetitively Pulsed Microdischarge High Power 121.6 Nm Source
  Stephens, J; Fierro, A; Trienekens, D; Dickens, J; Neuber, A
Abstract:  Utilizing nanosecond high voltage pulses to drive microdischarges (MDs) at repetition rates in the vicinity of 1 MHz previously enabled increased time-averaged power deposition, peak vacuum ultraviolet (VUV) power yield, as well as time-averaged VUV power yield. Here, various pulse widths (30-250 ns), and pulse repetition rates (100 kHz-5 MHz) are utilized, and the resulting VUV yield is reported. It was observed that the use of a 50 ns pulse width, at a repetition rate of 100 kHz, provided 62 W peak VUV power and 310 mW time-averaged VUV power, with a time-averaged VUV generation efficiency of similar to 1.1%. Optimization of the driving parameters resulted in 1-2 orders of magnitude increase in peak and time-averaged power when compared to low power, dc-driven MDs.
+ Overview Of Grid Connected Renewable Energy Based Battery Projects In Usa
  Subburaj, AS; Pushpakaran, BN; Bayne, SB
Abstract:  Reliable source of energy is a topic of momentous concern in the world due to the uncertainty in conventional energy sources. The situation gets exacerbated with the impact of natural disasters on the transmission grid. The advent of cutting edge energy storage technology has provided a competent solution. Energy storage system is an integral part of a grid since it enhances the stability and performance by disengaging the energy generation source and the load especially when intermittent renewable energy sources are a part of the system. Battery technology should not only be able to demonstrate high performance but must be economically viable for project implementation. Several grid connected renewable energy based battery projects have been implemented for research and development as well as commercial application. The projects discussed in this review are considered based on the availability of information. This review paper will focus on grid connected battery projects powered by wind and solar energy generation sources. (C) 2015 Elsevier Ltd. All rights reserved.
+ Particle-In-Cell Based Parameter Study Of 12-Cavity, 12-Cathode Rising-Sun Relativistic Magnetrons For Improved Performance
  Majzoobi, A; Joshi, RP; Neuber, AA; Dickens, JC
Abstract:  Particle-in-cell simulations are performed to analyze the efficiency, output power and leakage currents in a 12-Cavity, 12-Cathode rising-sun magnetron with diffraction output (MDO). The central goal is to conduct a parameter study of a rising-sun magnetron that comprehensively incorporates performance enhancing features such as transparent cathodes, axial extraction, the use of endcaps, and cathode extensions. Our optimum results demonstrate peak output power of about 2.1 GW, with efficiencies of similar to 70% and low leakage currents at a magnetic field of 0.45 Tesla, a 400 kV bias with a single endcap, for a range of cathode extensions between 3 and 6 centimeters. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
+ Plasma Etching Of N-Type 4H-Sic For Photoconductive Semiconductor Switch Applications
  Ekinci, H; Kuryatkov, VV; Mauch, DL; Dickens, JC; Nikishin, SA
Abstract:  Photoconductive semiconductor switches (PCSS) fabricated on high-purity semi-insulating 4H-SiC substrates (000) are capable of switching high currents in compact packages with long device lifetimes. A heavily doped n-type SiC epitaxial layer of appropriate thickness is required to form low-resistance ohmic contacts with these devices. In addition, to enhance the performance of the PCSSs, the SiC surface between the ohmic contacts must be extremely smooth. We report a chlorine-based, inductively coupled plasma reactive ion-etching process yielding n-type SiC epitaxial layers with the required smoothness. The rate of etching and post-etching surface morphology were dependent on plasma conditions. We found that the surface smoothness of epitaxial layers can be improved by including BCl3 in the argon-chlorine mixture. The optimum etching process yielded very smooth surfaces (similar to 0.3 nm RMS) at a relatively high rate of etching of similar to 220 nm/min. This new fabrication approach significantly reduced the on-state resistance of the PCSS device and improved its durability of operation.
+ Reducing The Radiation Dose For Computed Tomography Colonography Using Model-Based Iterative Reconstruction
  Millerd, PJ; Paden, RG; Lund, JT; Hara, AK; Stiles, WL; He, M; Wu, Q; Johnson, CD
Abstract:  To determine whether radiation doses during computed tomography (CT) colonography (CTC) can be further reduced while maintaining image quality using model-based iterative reconstruction (MBIR). Twenty patients underwent CTC at a standard dose in supine and prone positions and at a reduced dose in the supine position. All other scan parameters (except noise index) were held constant. Acquisitions were reconstructed using 3 algorithms: filtered back projection (FBP), adaptive statistical iterative reconstruction (ASIR), and MBIR. Noise was assessed quantitatively by comparing the SD in Hounsfield units at 5 standard locations. Qualitative assessment was made by 2 experienced radiologists blinded to technique who subjectively scored image quality, noise, and sharpness (from 0 to 4). The standard-dose and reduced-dose CT dose index/dose-length product were 6.7/328 and 2.7 mGy/129 mGy-cm, respectively (60 % reduction). Measured mean noise level increased from the standard to the reduced dose (FBP, from 58.6 to 97.2; ASIR from 35.8 to 60.6; and MBIR from 16.6 to 21.9). MBIR had significantly less noise than ASIR on 2-dimensional images at both standard and reduced doses (P < .01). Radiation dose in CTC using MBIR can be reduced by 60 % while maintaining image quality and reducing image noise.
+ Robert Barker Memorial Session: Leadership In Plasma Science And Applications
  K. H. Becker; B. B. Godfrey; E. E. Kunhardt; M. Laroussi; L. D. Ludeking; A. A. Neuber; E. Schamiloglu; A. J. Woods
Abstract:  This paper summarizes the Dr. Robert J. Barker memorial session at the IEEE International Conference on Plasma Science 2014. Each section summarizes progress in a plasma research area strongly influenced by Dr. Barker's leadership: 1) plasma medicine; 2) atmospheric-pressure plasmas; 3) high-power microwaves; 4) pulsed power; and 5) numerical simulation of plasmas. He had a profound influence on these and other plasma science applications, as well as on numerous individual researchers. He will be missed greatly.

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+ Selection And Pilot Implementation Of A Mobile Image Viewer: A Case Study
  Zwart, CM; He, M; Wu, T; Demaerschalk, BM; Mitchell, JR; Hara, AK
Abstract:  Background: For health care providers, mobile image viewing increases image accessibility, which could lead to faster interpretation/consultations and improved patient outcomes. Objective: We explored the technical requirements and challenges associated with implementing a commercial mobile image viewer and conducted a small study testing the hypothesis that the mobile image viewer would provide faster image access. Methods: A total of 19 clinicians (9 radiologists, 3 surgeons, 4 neurologists, and 3 physician assistants) evaluated (1) a desktop commercial picture archiving and communication system (PACS) viewer, (2) a desktop viewer developed internally over 20 years and deployed throughout the enterprise (ENTERPRISE viewer) and (3) a commercial Food and Drug Administration class II-cleared mobile viewer compatible with Web browsers, tablets, and mobile phones. Data were collected during two separate 7-day periods, before and after mobile image viewer deployment. Data included image viewer chosen, time to view first image, technical issues, diagnostic confidence, and ease of use. Results: For 565 image-viewing events, ease of use was identical for PACS and mobile viewers (mean 3.6 for all scores of a possible 4.0), and significantly worse for the enterprise viewer (mean 2.9, P=.001). Technical issues were highest with the enterprise viewer (26%, 56/215) compared with the mobile (7%, 19/259, P=.001) and PACS (8%, 7/91, P=.003) viewers. Mean time to first image for the mobile viewer (2.4 minutes) was significantly faster than PACS (12.5 minutes, P=.001) and the enterprise viewer (4.5 minutes, P=.001). Diagnostic confidence was similar for PACS and mobile viewers and worst for enterprise viewer. Mobile image viewing increased by sixfold, from 14% (37/269, before the deployment) to 88.9% (263/296, after the deployment). Conclusions: A mobile viewer provided faster time to first image, improved technical performance, ease of use, and diagnostic confidence, compared with desktop image viewers.
+ Self-Induced Gaseous Plasma As High Power Microwave Opening Switch Medium
  Lin, S; Beeson, S; Liu, C; Dickens, J; Neuber, A
Abstract:  Self-induced gaseous plasma is evaluated as active opening switch medium for pulsed high power microwave radiation. The self-induced plasma switch is investigated for N-2 and Ar environments under pressure conditions ranging from 25 to 700Torr. A multi-pass TE111 resonator is used to significantly reduce the delay time inherently associated with plasma generation. The plasma forms under the pulsed excitation of a 4MW magnetron inside the central dielectric tube of the resonator, which isolates the inner atmospheric gas from the outer vacuum environment. The path from the power source to the load is designed such that the pulse passes through the plasma twice with a 35 ns delay between these two passes. In the first pass, initial plasma density is generated, while the second affects the transition to a highly reflective state with as much as 30 dB attenuation. Experimental data revealed that virtually zero delay time may be achieved for N-2 at 25 Torr. A two-dimensional fluid model was developed to study the plasma formation times for comparison with experimental data. The delay time predicted from this model agrees well with the experimental values in the lower pressure regime (error < 25%), however, due to filamentary plasma formation at higher pressures, simulated delay times may be underestimated by as much as 50%. (C) 2015 AIP Publishing LLC.
+ Simulation Study Of Hemt Structures With Hfo2 Cap Layer For Mitigating Inverse Piezoelectric Effect Related Device Failures
  Nagulapally, D; Joshi, RP; Pradhan, A
Abstract:  The Inverse Piezoelectric Effect (IPE) is thought to contribute to possible device failure of GaN High Electron Mobility Transistors (HEMTs). Here we focus on a simulation study to probe the possible mitigation of the IPE by reducing the internal electric fields and related elastic energy through the use of high-k materials. Inclusion of a HfO2 cap layer above the AlGaN barrier particularly with a partial mesa structure is shown to have potential advantages. Simulations reveal even greater reductions in the internal electric fields by using field plates in concert with high-k oxides. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
+ Support-Vector-Machine-Enhanced Markov Model For Short-Term Wind Power Forecast
  Yang, L; He, M; Zhang, JS; Vittal, V
Abstract:  Wind ramps introduce significant uncertainty into wind power generation. Reliable system operation, however, requires accurate detection and forecast of wind ramps, especially at high penetration levels. In this paper, to deal with the wind ramp dynamics, a support vector machine (SVM)-enhanced Markov model is developed for short-term wind power forecast, based on one key observation from the measurement data that wind ramps often occur with specific patterns. Specifically, using the historical data of the wind turbine power outputs recorded at an actual wind farm, data analytics-based finite-state Markov models are first developed to model the normal fluctuations of wind generation, while taking into account the diurnal nonstationarity and the seasonality of wind generation. Next, the forecast by the SVM is integrated cohesively into the finite-state Markov models. Based on the SVM-enhanced Markov model, both short-term distributional forecasts and point forecasts are then derived. Numerical test results, using real wind generation data traces, demonstrate the significantly improved accuracy of the proposed forecast approach.
Publication Year:  2014
+ 100-Kv High Voltage Power Supply With Bipolar Voltage Output And Adaptive Digital Control
  M. G. Giesselmann; T. T. Vollmer; W. J. Carey
Abstract:  This paper presents a 100-kV high frequency transformer/rectifier package, which is capable of a dual output polarity operation. An H-Bridge inverter drives the primary of the high voltage (HV) transformer at a frequency of 20 kHz. The inverter is driven by a Microchip dsPIC33F digital signal controller using peak current mode control with adaptive slope compensation. The HV-tank has two HV-coax output cables with a grounded shield on each cable. If the center conductor of the coax cable designated as negative output is grounded, positive voltage is obtained from the coax cable designated as positive output and vice versa. This paper provides design details and experimental results from tests of the entire system.

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+ 3-D Simulation Of Low-Temperature Plasma Development Under Pulsed Conditions
  A. S. Fierro; J. C. Dickens; A. A. Neuber
Abstract:  The development of a low-temperature plasma in a needle-protrusion to plane gap is investigated utilizing a 3-D particle-in-cell/Monte Carlo collision method implemented to run on single NVIDIA graphics processing unit. In addition to electron collisions, the model includes field detachment, photon tracking, and a drift-diffusion approximation for positive ions. The simulated geometry tracks several million electrons with 15-μm spatial resolution.

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+ A Passive Measurement Of Dissociated Atom Densities In Atmospheric Pressure Air Discharge Plasmas Using Vacuum Ultraviolet Self-Absorption Spectroscopy
  Laity, G; Fierro, A; Dickens, J; Frank, K; Neuber, A
Abstract:  We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5 x 10(17) cm(-3) and peak N atom densities of 9.9 x 10(17) cm(-3) are observed within the first similar to 1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0-132.0 nm band of the vacuum ultraviolet spectrum. (C) 2014 AIP Publishing LLC.
+ A Spatio-Temporal Analysis Approach For Short-Term Forecast Of Wind Farm Generation
  He, M; Yang, L; Zhang, JS; Vittal, V
Abstract:  In this paper, short-term forecast of wind farm generation is investigated by applying spatio-temporal analysis to extensive measurement data collected from a large wind farm where multiple classes of wind turbines are installed. Specifically, using the data of the wind turbines' power outputs recorded across two consecutive years, graph-learning based spatio-temporal analysis is carried out to characterize the statistical distribution and quantify the level crossing rate of the wind farm's aggregate power output. Built on these characterizations, finite-state Markov chains are constructed for each epoch of three hours and for each individual month, which accounts for the diurnal non-stationarity and the seasonality of wind farm generation. Short-term distributional forecasts and a point forecast are then derived by using the Markov chains and ramp trend information. The distributional forecast can be utilized to study stochastic unit commitment and economic dispatch problems via a Markovian approach. The developed Markov-chain-based distributional forecasts are compared with existing approaches based on high-order autoregressive models and Markov chains by uniform quantization, and the devised point forecasts are compared with persistence forecasts and high-order autoregressive model-based point forecasts. Numerical test results demonstrate the improved performance of the Markov chains developed by spatio-temporal analysis over existing approaches.
+ A Study Of Recent Changes In Southwest Power Pool And Electric Reliability Council Of Texas And Its Impact On The U.S. Wind Industry
  Nimmagadda, S; Islam, A; Bayne, SB; Walker, RP; Caballero, LG; Camanes, AF
Abstract:  Due to the increased penetration of renewable energy resources, there has been a lot of activity in the regional transmission organizations such as development of new standards, protocol revisions, new study requirements, changes to modeling procedures etc., in the last five years with a special focus given to wind energy. The key objective of this paper is to identify the impacts and the immediate technological and market related improvements required by the wind industry as a result of such changes in Southwest Power Pool (SPP) and the Electric Reliability Council of Texas (ERCOT). The paper documents the most important activities by following the higher-priority committees, work groups and task forces in both companies along with some of the special projects or initiatives such as sub-synchronous control interaction study, primary frequency response, hub concept and other modeling improvements related to wind energy. The paper provides an analysis of the impact of each change resulting in technology upgrades to wind turbines, modeling improvements by turbine manufacturers and policy/market changes affecting wind farm developers. Finally the paper provides recommendations regarding the requirements and capabilities which the future wind farms and wind turbines need to possess. (C) 2014 Elsevier Ltd. All rights reserved.
+ An Evaluation System For Experimental Silicon And Silicon Carbide Super Gate Turn Off Thyristors
  Bayne, S; Lacouture, S; Lawson, K; Giesselmann, M; Scozzie, CJ; O'Brien, H; Ogunniyi, AA
Abstract:  This paper describes the design and implementation of a small-scale pulsed power system specifically intended to evaluate the suitability of experimental silicon and silicon carbide high power Super Gate Turn Off thyristors for high action (500 A(2) s and above) pulsed power applications where energy is extracted from a storage element in a rapid and controlled manner. To this end, six of each type of device was placed in a controlled three phase rectifier circuit which was in turn connected to an aircraft ground power motor-generator set and subjected to testing protocols with varying power levels, while parameters such as offset firing angle were varied. (C) 2014 AIP Publishing LLC.
+ Assessing The Role Of Secondary Electron Emission On The Characteristics Of 6-Cavity Magnetrons With Transparent Cathode Through Particle-In-Cell Simulations
  Qiu, H; Prasad, S; Ludeking, L; Joshi, RP; Schamiloglu, E
Abstract:  Effects of secondary electron emission (SEE) on the performance of a 6-cavity relativistic magnetron with transparent cathodes are probed through particle-in-cell simulations. Appropriate relations for the secondary electron yield have been developed and used. For comparisons, separate simulations have been performed with-and without electron cascading. Simulation results seem to indicate SEE to be detrimental to the power output due to deviations in the starting trajectories of secondary electrons, and the reduced fraction with synchronized rotational velocity. A higher reduction in output power is predicted with electron cascading, though mode competition was not seen at the 0.65 T field. A possible solution to mitigating SEE in magnetrons for high power microwave applications would be to alter the surface properties of emitting electrodes through irradiation, which can lead to graphitic film formation. (C) 2014 AIP Publishing LLC.
+ Bias-Field Controlled Phasing And Power Combination Of Gyromagnetic Nonlinear Transmission Lines
  Reale, DV; Bragg, JWB; Gonsalves, NR; Johnson, JM; Neuber, AA; Dickens, JC; Mankowski, JJ
Abstract:  Gyromagnetic Nonlinear Transmission Lines (NLTLs) generate microwaves through the damped gyromagnetic precession of the magnetic moments in ferrimagnetic material, and are thus utilized as compact, solid-state, frequency agile, high power microwave (HPM) sources. The output frequency of a NLTL can be adjusted by control of the externally applied bias field and incident voltage pulse without physical alteration to the structure of the device. This property provides a frequency tuning capability not seen in many conventional e-beam based HPM sources. The NLTLs developed and tested are mesoband sources capable of generating MW power levels in the L, S, and C bands of the microwave spectrum. For an individual NLTL the output power at a given frequency is determined by several factors including the intrinsic properties of the ferrimagnetic material and the transmission line structure. Hence, if higher power levels are to be achieved, it is necessary to combine the outputs of multiple NLTLs. This can be accomplished in free space using antennas or in a transmission line via a power combiner. Using a bias-field controlled delay, a transient, high voltage, coaxial, three port, power combiner was designed and tested. Experimental results are compared with the results of a transient COMSOL simulation to evaluate combiner performance. (C) 2014 AIP Publishing LLC.
+ Conditioning Of Carbon Fiber Cathodes In Uhv-Sealed Tubes At 200 A/Cm2
  J. M. Parson; C. F. Lynn; J. J. Mankowski; M. Kristiansen; A. A. Neuber; J. C. Dickens
Abstract:  This paper presents a study on outgassing and electrical conditioning for three carbon fiber cathode types in a vacuum-sealed, high-power microwave virtual-cathode-oscillator (vircator) that operates in the low 10-9 torr pressure regime. The three cathode types consist of a bare bimodal fiber structure, a bare unimodal fiber structure, and a cesium-iodide coated bimodal fiber structure with identical fiber coverage of 2% by area with a surface area of ~20 cm2. The electrodes are cleaned by a 1.2 kW, argon/oxygen microwave plasma prior to complete vircator assembly, followed by a 300 °C bake-out for 72 h. Each cathode was pulsed in a single pulse operation by an 80 J, low inductance Marx generator with an approximate pulsewidth of 100 ns full-width at half-maximum for 10000 current pulses. The data presented includes individual gas constituents, high-speed intensified charge coupled device (ICCD) imaging, and voltage and current waveforms. The conditioning process resulted in a gas load reduction of ~80% overall, with the indication that the bare bimodal fiber structure performed the best as diode power increased throughout the experiment, while the power decreased for the other tested cathode types.

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+ Effect Of Bcl3 In Chlorine-Based Plasma On Etching 4H-Sic For Photoconductive Semiconductor Switch Applications
  Ekinci, H; Kuryatkov, VV; Mauch, DL; Dickens, JC; Nikishin, SA
Abstract:  Inductively coupled plasma reactive ion etching (ICP-RIE) of n-type SiC epitaxial layers grown on (000 (1) over bar) 4H-SiC semi-insulating substrates has been investigated using chlorine-based plasma. The etch rate and postetching- surface morphology have been studied as functions of the plasma composition, ICP power, RTE power, and process pressure. The authors found that the surface smoothness of the epitaxial layer was increased by introducing BCl3 into Cl-2/Ar plasma. An optimized process has been developed yielding etch rates of similar to 220 nm/min and very smooth surfaces with root mean square roughness of similar to 0.3 nm. The spatial-frequency dependence of the one dimensional power spectral density was interpreted using the surface height function h(x) including a low-frequency range, which exhibits saturation and a high-frequency range, Which exhibits scaling properties. Through this etching process, the effects of s-ubcontact doping on 4H-SiC photoconductive semiconductor switch (PCSS) performance were investigated. A PCSS was fabricated using this etching process with a 1 mu m heavily doped (1.6 x 10(18) cm(-3) n-type) epitaxial layer beneath the device contacts and compared with a PCSS fabricated with a subcontact doped layer created through laser enhanced diffusion (similar to 50 nm depth, 2.0 x 10(18) cm(-3) n-type). The PCSS with the epitaxial layer demonstrated on average a 30% reduction in minimum on-state resistance, and eliminated cracking of the bulk material when switching currents <= 38 A. (C) 2014 American Vacuum Society.
+ Emission Behavior Of Three Conditioned Carbon Fiber Cathode Types In Uhv-Sealed Tubes At 200 A/Cm2
  J. M. Parson; C. F. Lynn; J. J. Mankowski; A. A. Neuber; J. C. Dickens
Abstract:  When subjected to high electric fields in vacuum, carbon fiber cathodes produce intense electron beams suitable for high-power microwave (HPM) generation at very high current densities. However, the production mechanisms of these intense electron beams are not fully understood. This paper presents the postmortem examination of three conditioned carbon fiber cathode types. The three cathode types consist of an uncoated, bare unimodal fiber structure, a bare bimodal fiber structure, and a cesium-iodide (CsI)-coated bimodal fiber structure, all with identical fiber coverage of 2% by area. Each cathode was conditioned prior to testing by single pulse operation driven by an 80 J Marx generator for 10 000 pulses. HPM, voltage, and current waveforms of each cathode are presented. The bare bimodal cathode radiated more microwave power than the CsI-coated cathode and bare unimodal cathode. Scanning electron microscopy imagery presents evidence of two emission mechanisms: 1) explosive electron emission and 2) surface flashover, which both were found on the CsI-coated cathode. In addition, no evidence of surface flashover was found on either uncoated cathode.

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+ Evaluation Of Sic Jfet Performance During Repetitive Pulsed Switching Into An Unclamped Inductive Load
  B. N. Pushpakaran; M. Hinojosa; S. B. Bayne; V. Veliadis; D. Urciuoli; N. El-Hinnawy; P. Borodulin; S. Gupta; C. Scozzie
Abstract:  Silicon carbide (SiC) depletion mode junction field-effect transistors (JFETs) are well suited for pulsed power applications as an opening switch due to their normally ON (N-ON) nature. To assess the robustness and breakdown energy tolerance of JFETs under pulsed conditions, they must be evaluated for breakdown energy capability before failure. This is very important for circuit breaker applications due to the large voltage spikes induced during the opening of the circuit breaker while it still conducts substantial load current. These voltage spikes can drive the JFET into the breakdown voltage regime and may result in device failure if the energy dissipation is above the tolerance limit. To determine the maximum avalanche energy of the device under repetitive pulsed conditions, a N-ON SiC JFET with a nominal rating of 1200 V/13 A was driven into punchthrough breakdown using an unclamped inductive switching (UIS) circuit. The testing comprised of 4000 repetitive pulses at 25°C case temperature at a fixed gate voltage of -20 V. The drain current was increased after every 1000 pulses to increase the energy dissipated. The JFET was able to withstand 1000 pulses at a maximum energy dissipation value of 1160 mJ before failure. The JFET triode breakdown characteristics were analyzed after every 1000 pulses. The peak UIS energy of 1160 mJ corresponded to an energy density of 16.6 J/cm2 based on their active area.

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+ Fast Supercapacitors Based On Graphene-Bridged V2O3/Vox Core-Shell Nanostructure Electrodes With A Power Density Of 1 Mw Kg(-1)
  Pan, X; Ren, GF; Hoque, MNF; Bayne, S; Zhu, K; Fan, ZY
Abstract:  Transition metal oxides (TMOs), with their very large pseudocapacitance effect, hold promise for next generation high-energy-density electrochemical supercapacitors (ECs). However, the typical high resistivity of TMOs restricts the reported ECs to work at a low charge-discharge (C-D) rate of 0.1-1 V s(-1). Here, a novel vanadium oxides core/shell nanostructure-based electrode to overcome the resistivity challenge of TMOs for rapid pseudocapacitive EC design is reported. Quasi-metallic V2O3 nanocores are dispersed on graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO2 and V2O5 (called as VOx here) thin shell around V2O3 nanocore acts as the active pseudocapacitive material. With such a graphene-bridged V2O3/VOx core-shell composite as electrode material, ECs with a C-D rate as high as 50 V s(-1) is demonstrated. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg(-1) power density at an energy density of 10 Wh kg(-1). At the critical 45 degrees phase angle, these ECs have a measured frequency of 114 Hz. All these indicate the graphene-bridged V2O3/VOx core-shell structure is promising for fast EC development.
+ Global Model For Total Delay Time Distribution Of High-Power Microwave Surface Flashover
  S. R. Beeson; J. C. Dickens; A. A. Neuber
Abstract:  A global model of high-power microwave (HPM) window breakdown is elucidated. The model provides a practical approach for estimating the maximum microwave power and pulse length that can be transmitted for a given window geometry at varying background gas pressure. Based on recent experimental and modeling progress, the formative and statistical breakdown delay time contributions are included in the model. The provided details are intended to give the reader a starting point in designing an HPM system for which surface breakdown along the output window is a major concern. Spanning five orders of magnitude in power, four microwave bands, three orders of magnitude in pulsewidth, three orders of magnitude in pressure, and three different gas types, the model serves to determine the probability of breakdown for a given set of input parameters with the modest computational effort. Examples of how to use the model are given, and the results are compared with actual systems and measured experimental delay times.

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+ Graphics Processing Unit Accelerated Three-Dimensional Model For The Simulation Of Pulsed Low-Temperature Plasmas
  Fierro, A; Dickens, J; Neuber, A
Abstract:  A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 x 10(6) particles with dynamic weighting and a total mesh size larger than 10(8) cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes. (C) 2014 AIP Publishing LLC.
+ Imaging Of Explosive Emission Cathode And Anode Plasma In A Vacuum-Sealed Vircator High-Power Microwave Source At 250 A/Cm2
  J. M. Parson; J. J. Mankowski; J. C. Dickens; A. A. Neuber
Abstract:  Cold cathode operation under high current densities leads to explosive electron emission (EEE) that contributes to early A-K gap closure. Hence, inconsistent vacuum conditions and, if utilized in a high power microwave device, degradation of microwave output power are observed. The EEE centers are known to produce localized plasmas on the surface of the cathode that release and ionize the electrode material. Further, low melting point material in the anode is released due to electron bombardment accompanied by a significant surface temperature increase. Postmortem analysis has revealed particles up to 50 μm in diameter embedded in the opposite electrode. High speed ICCD imaging during A-K gap operation enabled resolving the plasma's spatial characteristics in time. Images of cathode and anode plasma during the operation of a virtual cathode oscillator at 250 A/cm2 under ultrahigh vacuum conditions are presented.

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+ Influence Of Vuv Illumination On Breakdown Mechanics: Pre-Ionization, Direct Photoionization, And Discharge Initiation
  Stephens, J; Fierro, A; Dickens, J; Neuber, A
Abstract:  A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N-2-NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4 x 10(-18) cm(2)) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N-2-NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.
+ Kilohertz Ultrafast Electrochemical Supercapacitors Based On Perpendicularly-Oriented Graphene Grown Inside Of Nickel Foam
  Ren, GF; Pan, X; Bayne, S; Fan, ZY
Abstract:  Ultrafast electrochemical supercapacitors (EC) that can work at or above kilohertz (kHz) frequency, 3-4 orders higher than traditional EC, call for a structure,with extremely low equivalent serial resistance (ESR) and a reasonably large surface area. Three-dimensional perpendicularly-oriented graphene (POG) network, grown inside of Ni foam (NF) by microwave plasma chemical vapor deposition, is reported as electrode to fabricate such ultrafast EC. The folded POG inside NF provides a large surface area, while the straight-forward and wide-open porous structure of POG ensures fast ion migration. In conjunction with the intrinsic high electronic conductivity of graphene and Ni, POG/NF electrode based ultrafast EC was demonstrated with a specific cell capacitance of 0.32 mF/cm(2) at 1 kHz, a relaxation time constant of 0.248 ms, and an ESR of 70 m Omega. A charge-discharge rate as high as 500 V/s was also measured, at which the cyclic voltammogram maintained a rectangular shape, corresponding to a single electrode capacitance of 0.83 mF/cm(2). (C) 2014 Elsevier Ltd. All rights reserved.
+ Micrometer-Resolution High Speed Imaging Of Pulsed Microdischarge Ignition
  J. C. Stephens; A. S. Fierro; J. C. Dickens; A. A. Neuber
Abstract:  The application of a short, pulsed excitation is known to allow for higher power deposition into microdischarges without the onset of instabilities. Here, a MOSFET-based high voltage pulser is used to drive a 50-torr argon microdischarge with short pulsed currents of ~75 A, with <;100-ns full-width at half-maximum, and a repetition rate of 1 MHz. With this excitation, an average power density of ~1013 W/m3 is achieved, with a peak power density~3.1014 W/m3. A high speed iCCD camera is used to observe ignition processes and confirm the absence of unstable operation. The images were taken using a 5-ns gate time with a spatial resolution of ~2.5 μm.

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+ Nanosecond, Repetitively Pulsed Microdischarge Vacuum Ultraviolet Source
  Stephens, J; Fierro, A; Walls, B; Dickens, J; Neuber, A
Abstract:  A microdischarge is driven by short pulses (80 ns FWHM) with peak current levels up to 80 A, with a repetition frequency of 1 MHz (1 pulse/mu s) allowing for similar to 550W input power. Experiments in pure argon (Ar-2*, 127 nm) and argon-hydrogen (Lyman-alpha, 121.6 nm) were conducted. Using short pulses, the argon excimer emission was not observed. Alternatively, Ar-H-2 operated at both higher power and efficiency (0.63%) whenever pulsed. Using Ar-H-2, the experiments result in an average generated vacuum ultraviolet power just above 3.4W with a peak power of 42.8 W, entirely at Lyman-alpha. (C) 2014 AIP Publishing LLC.
+ Pilot Study: Evaluation Of Dual-Energy Computed Tomography Measurement Strategies For Positron Emission Tomography Correlation In Pancreatic Adenocarcinoma
  Oldan, J; He, M; Wu, T; Silva, AC; Li, J; Mitchell, JR; Pavlicek, WM; Roarke, MC; Hara, AK
Abstract:  We sought to determine whether dual-energy computed tomography (DECT) measurements correlate with positron emission tomography (PET) standardized uptake values (SUVs) in pancreatic adenocarcinoma, and to determine the optimal DECT imaging variables and modeling strategy to produce the highest correlation with maximum SUV (SUVmax). We reviewed 25 patients with unresectable pancreatic adenocarcinoma seen at Mayo Clinic, Scottsdale, Arizona, who had PET-computed tomography (PET/CT) and enhanced DECT performed the same week between March 25, 2010 and December 9, 2011. For each examination, DECT measurements were taken using one of three methods: (1) average values of three tumor regions of interest (ROIs) (method 1); (2) one ROI in the area of highest subjective DECT enhancement (method 2); and (3) one ROI in the area corresponding to PET SUVmax (method 3). There were 133 DECT variables using method 1, and 89 using the other methods. Univariate and multivariate analysis regression models were used to identify important correlations between DECT variables and PET SUVmax. Both R (2) and adjusted R (2) were calculated for the multivariate model to compensate for the increased number of predictors. The average SUVmax was 5 (range, 1.8-12.0). Multivariate analysis of DECT imaging variables outperformed univariate analysis (r = 0.91; R (2) = 0.82; adjusted R (2) = 0.75 vs r < 0.58; adjusted R (2) < 0.34). Method 3 had the highest correlation with PET SUVmax (R (2) = 0.82), followed by method 1 (R (2) = 0.79) and method 2 (R (2) = 0.57). DECT thus has clinical potential as a surrogate for, or as a complement to, PET in patients with pancreatic adenocarcinoma.
+ Safe Operating Area And Long-Term Reliability Of 9-Kv Silicon Carbide Pnpn Super Gate Turn-Off Thyristors
  K. Lawson; S. B. Bayne; S. Lacouture; L. Cheng; H. O"™Brien; A. Ogunniyi; C. Scozzie
Abstract:  One of the major requirements for adoption of new silicon carbide (SiC) super gate turn-off thyristors (SGTOs) into high-energy applications is to verify the safe operating area and long-term reliability capabilities of these devices. In this letter, we have developed a unique high-energy testing system that can evaluate the performance limitations with respect to lifetime capabilities of the 9 kV, 1 cm $^{2}$ , SGTOs at ultrahigh pulsed current levels from 1 to 3.5 kA. The test system produces square current pulses with a user specified current amplitude and a 100- $\mu $ s pulsewidth at a maximum repetition rate of 1 shot/s (>0.1% duty cycle). A lifetime safe operating area with respect to maximum pulsed current was then established that these 1 cm $^{2}$ , 9 kV, SiC SGTOs can perform reliably without significant degradation at pulsed current levels up to 2.0 kA. At current levels above 2.0 kA shifts in the on-state voltage are observed probably due to device over-heating at such high current levels and having not enough time to fully dissipate the heat between any two shots, which results in the device rapidly deteriorating due to increased on-state losses ultimately leading to premature failure.

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+ Semiempirical Wide-Range Conductivity Model With Exploding Wire Verification
  Stephens, J; Dickens, J; Neuber, A
Abstract:  Based on well-established physical relationships, a semiempirical set of equations dictating the electrical conductivity of dense, strongly coupled, partially ionized copper is presented. With the empirical coefficients, the model is tuned to experimental conductivity data obtained from exploding wire experiments [A. W. DeSilva and J. D. Katsouros, Phys. Rev. E 57, 5945 (1998)]. The result is a wide-range conductivity model, with demonstrated accuracy from room temperature-density conditions to 0.01 g/cm(3) and 30 kK. Using magnetohydrodynamic simulation the ability to utilize the conductivity model for predictive simulations is demonstrated. A complete electrical conductivity dataset for copper has been made available to the public.
+ Simulations Of Voltage Transients Across Intracellular Mitochondrial Membranes Due To Nanosecond Electrical Pulses
  H. Qiu; S. Xiao; R. P. Joshi
Abstract:  Simulations to quantify the induction of transmembrane potentials across the mitochondrial membranes have been carried out, taking account of their irregular shape. Our results demonstrate that short (60 ns), high-intensity pulses have the capacity to create membrane potentials, while longer 600-ns pulses are not as effective. Also, the plasma membrane effects are always greater than those at the mitochondria, and that poration at the inner mitochondrial membrane is more difficult than at the outer mitochondrial membrane. In the shorter pulse range, geometric dependence is very pronounced, and so short pulses could be very effective in highly irregular shaped cells, such as neurons. Finally, bioeffects due to the longer 600-ns pulses as seen experimentally, is likely due to other (secondary) effects such as calcium inflow from the porated plasma membrane at the mitochondrial sites.

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+ System For Time-Discretized Vacuum Ultraviolet Spectroscopy Of Spark Breakdown In Air
  Ryberg, D; Fierro, A; Dickens, J; Neuber, A
Abstract:  A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 mu m. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization. (C) 2014 AIP Publishing LLC.
+ Temporally Resolved Electron Density Of A Repetitive, Nanosecond Pulsed Microdischarge
  Stephens, J; Fierro, A; Dickens, J; Neuber, A
Abstract:  Using high speed spectroscopic diagnostics, temporally resolved optical emission spectroscopy is performed on a nanosecond, repetitively pulsed microdischarge. The microdischarge operates in an argon-hydrogen gas mixture (99%/1%) to provide a Lyman-alpha vacuum ultraviolet emission. Based on the Stark broadening of the 486.1 nm, Balmer-beta hydrogen transition, the temporally resolved electron density was determined. Experimental electron density data are compared with the results of a 0D rate equation model. Peak electron density is estimated to be 5.6 . 10(15) cm(-3), corresponding to a similar to 0.25% degree of ionization. Using the approximate experimental ionization rate, the electron temperature is estimated to be similar to 3.5 eV.
+ Ultra-Low-Dose Computed Tomographic Angiography With Model-Based Iterative Reconstruction Compared With Standard-Dose Imaging After Endovascular Aneurysm Repair: A Prospective Pilot Study
  Naidu, SG; Kriegshauser, JS; Paden, RG; He, M; Wu, Q; Hara, AK
Abstract:  An ultra-low-dose radiation protocol reconstructed with model-based iterative reconstruction was compared with our standard-dose protocol. This prospective study evaluated 20 men undergoing surveillance-enhanced computed tomography after endovascular aneurysm repair. All patients underwent standard-dose and ultra-low-dose venous phase imaging; images were compared after reconstruction with filtered back projection, adaptive statistical iterative reconstruction, and model-based iterative reconstruction. Objective measures of aortic contrast attenuation and image noise were averaged. Images were subjectively assessed (1 = worst, 5 = best) for diagnostic confidence, image noise, and vessel sharpness. Aneurysm sac diameter and endoleak detection were compared. Quantitative image noise was 26% less with ultra-low-dose model-based iterative reconstruction than with standard-dose adaptive statistical iterative reconstruction and 58% less than with ultra-low-dose adaptive statistical iterative reconstruction. Average subjective noise scores were not different between ultra-low-dose model-based iterative reconstruction and standard-dose adaptive statistical iterative reconstruction (3.8 vs. 4.0, P = .25). Subjective scores for diagnostic confidence were better with standard-dose adaptive statistical iterative reconstruction than with ultra-low-dose model-based iterative reconstruction (4.4 vs. 4.0, P = .002). Vessel sharpness was decreased with ultra-low-dose model-based iterative reconstruction compared with standard-dose adaptive statistical iterative reconstruction (3.3 vs. 4.1, P < .0001). Ultra-low-dose model-based iterative reconstruction and standard-dose adaptive statistical iterative reconstruction aneurysm sac diameters were not significantly different (4.9 vs. 4.9 cm); concordance for the presence of endoleak was 100% (P < .001). Compared with a standard-dose technique, an ultra-low-dose model-based iterative reconstruction protocol provides comparable image quality and diagnostic assessment at a 73% lower radiation dose.
Publication Year:  2013
+ A Compact 45 Kv Curve Tracer With Picoampere Current Measurement Capability
  Sullivan, WW; Mauch, D; Bullick, A; Hettler, C; Neuber, A; Dickens, J
Abstract:  This paper discusses a compact high voltage curve tracer for high voltage semiconductor device characterization. The system sources up to 3 mA at up to 45 kV in dc conditions. It measures from 328 V to 60 kV with 15 V resolution and from 9.4 pA to 4 mA with 100 fA minimum resolution. Control software for the system is written in Microsoft Visual C# and features real-time measurement control and IV plotting, arc-protection and detection, an electrically isolated universal serial bus interface, and easy data exporting capabilities. The system has survived numerous catastrophic high voltage device-under-test arcing failures with no loss of measurement capability or system damage. Overall sweep times are typically under 2 min, and the curve tracer system was used to characterize the blocking performance of high voltage ceramic capacitors, high voltage silicon carbide photoconductive semiconductor switches, and high voltage coaxial cable. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794734]
+ A Multi-Timescale Scheduling Approach For Stochastic Reliability In Smart Grids With Wind Generation And Opportunistic Demand
  He, M; Murugesan, S; Zhang, JS
Abstract:  In this study, we focus on the stochastic reliability of smart grids with two classes of energy users-traditional energy users and opportunistic energy users (e. g., smart appliances or electric vehicles), and investigate the procurement of energy supply from both conventional generation (base-load and fast-start) and wind generation via multi-timescale scheduling. Specifically, in day-ahead scheduling, with the distributional information of wind generation and demand, we characterize the optimal procurement of the energy supply from base-load generation and the day-ahead price; in real-time scheduling, with the realizations of wind generation and the demand of traditional energy users, we optimize real-time price to manage opportunistic demand so as to achieve system-wise reliability and efficiency. More specifically, we consider two different models for opportunistic energy users: non-persistent and persistent, and characterize the optimal scheduling and pricing decisions for both models by exploiting various computational and optimization tools. Numerical results demonstrate that the proposed scheduling and pricing schemes can effectively manage opportunistic demand and enhance system reliability, thus have the potential to improve the penetration of wind generation.
+ All Solid-State High Power Microwave Source With High Repetition Frequency
  Bragg, JWB; Sullivan, WW; Mauch, D; Neuber, AA; Dickens, JC
Abstract:  An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm(2), 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic material's axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Omega, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed. (C) 2013 AIP Publishing LLC.
+ Automated Modular High Energy Evaluation System For Experimental Thyristor Devices
  Lacouture, S; Lawson, K; Bayne, S; Giesselmann, M; Scozzie, CJ; O'Brien, H; Ogunniyi, AA
Abstract:  A high energy, modular, completely automated test bed with integrated data acquisition and characterization systems was successfully designed in order to perform both safe operating area as well as very high volume reliability testing on experimental silicon carbide Super Gate Turn Off (SGTO) thyristors. Although the system follows a modular design philosophy, with each functional block acting as a peripheral to a main control module and can be adapted to arbitrary power and pulse width levels, for the specific SGTO devices initially evaluated it was configured to have the device discharge variable current levels of up to 6 kA into a 0.5 Omega resistive load with a relatively square pulse fixed at 100 mu s full width at half maximum delivering energy levels up to 1.8 kJ to the load. (C) 2013 AIP Publishing LLC.
+ Ferrimagnetic Nonlinear Transmission Lines As High-Power Microwave Sources
  J. -. B. Bragg; J. C. Dickens; A. A. Neuber
Abstract:  Ferrimagnetic nonlinear transmission lines (NLTLs) have the potential to fill a high-power microwave niche where compact cost-effective sources are lacking. NLTLs utilize nonlinear ferrimagnetic properties and magnetization dynamics to provide ultrafast pulse rise times (100 ps or less) and microwave signals with peak power ranging from kilowatts to hundreds of megawatts. The frequency of operation has been shown to range from 900 MHz up to 5 GHz depending on geometry and external magnetic fields. NLTLs, theoretically, can be pulsed to tens of kilohertz with little to no variance in microwave signal between shots. This paper covers recent advances in ferrimagnetic-based NLTLs, specifically effects of applied and bias magnetic fields on peak power and frequency, as well as temperature dependence.

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+ Focused Cathode Design To Reduce Anode Heating During Vircator Operation
  Lynn, CF; Dickens, JC; Neuber, AA
Abstract:  Virtual cathode oscillators, or vircators, are a type of high power microwave device which operates based on the instability of a virtual cathode, or cloud of electrons, which forms when electron current injected into the drift tube exceeds the space charge limited current within the drift tube. Anode heating by the electron beam during vircator operation ultimately limits achievable pulse lengths, repetition rates, and the duration of burst mode operation. This article discusses a novel cathode design that focuses electrons through holes in the anode, thus significantly reducing anode heating by the electrons emitted from the cathode during the first transit through the A-K gap. Reflexing electrons continue to deposit energy on the anode; however, the discussed minimization of anode heating by main beam electrons has the potential to enable higher repetition rates as well as efficiency and longer diode lifetime. A simulation study of this type of cathode design illustrates possible advantages. (C) 2013 AIP Publishing LLC.
+ High Temperature Unclamped Inductive Switching Mode Evaluation Of Sic Jfet
  B. N. Pushpakaran; M. Hinojosa; S. B. Bayne; V. Veliadis; D. Urciuoli; N. El-Hinnawy; P. Borodulin; S. Gupta; C. Scozzie
Abstract:  Silicon carbide (SiC) unipolar transistors are an efficient choice in the design of high temperature 1200 V switching power supplies and dc-dc converters. To reduce the form factor and increase the power density of the circuit, the switching frequency must be high. This intensifies the negative impact of parasitic inductance and results in high voltage spikes that can drive a switching device into breakdown, followed by rapid destruction. To study the device performance under unclamped inductive switching (UIS) conditions, a normally-ON 1200 V/13-A SiC junction field-effect transistor (JFET) is driven into punch through breakdown using a single pulse. The testing is performed using an UIS setup, in which energy initially stored in an inductor is discharged through the JFET. The testing comprises of 90 single pulses each at 25°C and 100 °C case temperatures for different gate voltages and drain current values. The peak energy and power dissipated in the JFET are 621 mJ and 16 kW, respectively, at the rated 1200 V blocking voltage and 13-A drain current. The JFET triode breakdown characteristics are unchanged after 180 single-pulse switching events indicating the robust nature of the device under extreme breakdown conditions. In addition, the 621 mJ peak UIS energy and its corresponding 8871 mJ/cm2 density dissipated in the JFET are the highest reported for any SiC power device.

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+ Leakage Current In High Dielectric Oxides: Role Of Defect-Induced Energies
  Maleeswaran, P; Nagulapally, D; Joshi, RP; Pradhan, AK
Abstract:  Model evaluations of field-dependent leakage currents in the high-k dielectric oxides HfO2 and ZrO2 are presented. The treatment includes a number of competing transport processes on an equal footing. Our simulation results show that trap-assisted-tunneling, though important, is not the most significant contributor. The combined Poole-Frenkel mechanism with modifications from inelastic trap-related processes dominates at electric fields above 1.5 MV/cm. Our model predictions compare very favorably against the reported experimental data for both HfO2 and ZrO2. Finally, results obtained from the density functional theory based CASTEP simulator are suggestive of dynamic changes in the net energy levels and the possible formation of a mini-band due to oxygen vacancies. (C) 2013 AIP Publishing LLC.
+ Material Selection Considerations For Coaxial, Ferrimagnetic-Based Nonlinear Transmission Lines
  Bragg, JWB; Dickens, JC; Neuber, AA
Abstract:  The growing need for solid-state high power microwave sources has renewed interest in nonlinear transmission lines (NLTLs). This article focuses specifically on ferrimagnetic-based NLTLs in a coaxial geometry. Achieved peak powers exceed 30 MW at 30 kV incident voltage with rf power reaching 4.8 MW peak and pulse lengths ranging from 1-5 ns. The presented NLTL operates in S-band with the capability to tune the center frequency of oscillation over the entire 2-4 GHz band and bandwidths of approximately 30%, placing the NLTL into the ultra-wideband-mesoband category of microwave sources. Several nonlinear materials were tested and the relationship between NLTL performance and material parameters is discussed. In particular, the importance of the material's ferromagnetic resonance linewidth and its relationship to microwave generation is highlighted. For a specific nonlinear material, it is shown that an optimum relation between incident pulse magnitude and static bias magnitude exists. By varying the nonlinear material's bias magnetic field, active delay control was demonstrated. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792214]
+ Online Dynamic Security Assessment With Missing Pmu Measurements: A Data Mining Approach
  He, M; Vittal, V; Zhang, JS
Abstract:  A data mining approach using ensemble decision trees (DTs) learning is proposed for online dynamic security assessment (DSA), with the objective of mitigating the impact of possibly missing PMU data. Specifically, multiple small DTs are first trained offline using a random subspace method. In particular, the developed random subspace method exploits the hierarchy of wide-area monitoring system (WAMS), the locational information of attributes, and the availability of PMU measurements, so as to improve the overall robustness of the ensemble to missing data. Then, the performance of the trained small DTs is re-checked by using new cases in near real-time. In online DSA, viable small DTs are identified in case of missing PMU data, and a boosting algorithm is employed to quantify the voting weights of viable small DTs. The security classification decision for online DSA is obtained via a weighted voting of viable small DTs. A case study using the IEEE 39-bus system demonstrates the effectiveness of the proposed approach.
+ Plasma Relaxation Mechanics Of Pulsed High Power Microwave Surface Flashover
  Beeson, S; Dickens, J; Neuber, A
Abstract:  Microwave transmission and reflection characteristics of pulsed radio frequency field generated plasmas are elucidated for air, N-2, and He environments under pressure conditions ranging from 10 to 600 torr. The pulsed, low temperature plasma is generated along the atmospheric side of the dielectric boundary between the source (under vacuum) and the radiating environment with a thickness on the order of 5 mm and a cross sectional area just smaller than that of the waveguide. Utilizing custom multi-standard waveguide couplers and a continuous low power probing source, the scattering parameters were measured before, during, and after the high power microwave pulse with emphasis on the latter. From these scattering parameters, temporal electron density estimations (specifically the longitudinal integral of the density) were calculated using a 1D plane wave-excited model for analysis of the relaxation processes associated. These relaxation characteristics ultimately determine the maximum repetition rate for many pulsed electric field applications and thus are applicable to a much larger scope in the plasma community than just those related to high power microwaves. This manuscript discusses the diagnostic setup for acquiring the power measurements along with a detailed description of the kinematic and chemical behavior of the plasma as it decays down to its undisturbed state under various gas type and pressure conditions. (C) 2013 AIP Publishing LLC.
+ Reliable Operation Of Sic Jfet Subjected To Over 2.4 Million 1200-V/115-A Hard Switching Events At 150 Degrees C
  Veliadis, V; Steiner, B; Lawson, K; Bayne, SB; Urciuoli, D; Ha, HC; El-Hinnawy, N; Gupta, S; Borodulin, P; Howell, RS; Scozzie, C
Abstract:  A requirement for the commercialization of power SiC transistors is their long-term reliable operation under hard switching conditions and high temperatures encountered in the field. Normally ON 1200-V vertical-channel implanted-gate SiC JFETs, designed for high-power bidirectional (four-quadrant) solid-state circuit breaker applications, were repetitively pulsed hard switched at 150 degrees C from a 1200-V blocking state to an ON-state current of 115 A, which is in excess of 13 times the JFET's 250-W/cm(2) rated current at 150 degrees C. The JFETs were fabricated in seven photolithographic levels with a single masked ion implantation forming the p(+) gates and guard rings and with no epitaxial regrowth. The pulsed testing was performed using a low-inductance RLC circuit. In this circuit, the energy initially stored in a capacitor is discharged in a load resistor through the JFET under test. The JFET hard switch stressing included over 2.4 million 1200-V/115-A hard switch events at 150 degrees C and at a repetition rate of 10 Hz. The peak energies and powers dissipated by the JFET at each hard switch event were 73.2 mJ and 68.2 kW, respectively. The current rise rate was 166 A/mu s, and the pulse FWHM was 1.8 mu s. After over 2.4 million hard switch events at 150 degrees C, the JFET blocking voltage characteristics remained unchanged while the ON-state current conduction slightly improved, which indicate reliable operation.
+ Robust Online Dynamic Security Assessment Using Adaptive Ensemble Decision-Tree Learning
  He, M; Zhang, JS; Vittal, V
Abstract:  Online dynamic security assessment (DSA) is examined in a data-mining framework by taking into account the operating condition (OC) variations and possible topology changes of power systems during the operating horizon. Specifically, a robust scheme is proposed based on adaptive ensemble decision tree (DT) learning. In offline training, a boosting algorithm is employed to build a classification model as a weighted voting of multiple unpruned small-height DTs. Then, the small-height DTs are periodically updated by incorporating new training cases that account for OC variations or the possible changes of system topology; the voting weights of the small-height DTs are also updated accordingly. In online DSA, the updated classification model is used to map the real-time measurements of the present OC to security classification decisions. The proposed scheme is first illustrated on the IEEE 39-bus test system, and then applied to a regional grid of the Western Electricity Coordinating Council (WECC) system. The results of case studies, using a variety of realized OCs, illustrate the effectiveness of the proposed scheme in dealing with OC variation and system topology change.
+ Simultaneous Measurement Of Nitrogen And Hydrogen Dissociation From Vacuum Ultraviolet Self-Absorption Spectroscopy In A Developing Low Temperature Plasma At Atmospheric Pressure
  Laity, G; Fierro, A; Dickens, J; Neuber, A; Frank, K
Abstract:  We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N-2/H-2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-alpha H radiation emitted within the first similar to 1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6 x 10(17) cm(-3) was observed similar to 100 ns into spark formation, with an estimated electron density of 2.65 x 10(18) cm(-3) determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8 x 10(17) cm(-3) during the same discharge period. (C) 2013 AIP Publishing LLC.
Publication Year:  2012
+ An Explosively Driven High-Power Microwave Pulsed Power System
  Elsayed, MA; Neuber, AA; Dickens, JC; Walter, JW; Kristiansen, M; Altgilbers, LL
Abstract:  The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 1. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681443]
+ Analysis Of Dislocation Scattering On Electron Mobility In Gan High Electron Mobility Transistors (Vol 93, Pg 10046, 2003)
  Joshi, RP; Sridhara, V; Jogai, B; Shah, P; del Rosario, RD
Abstract: 
+ Charged Electret Deposition For The Manipulation Of High Power Microwave Flashover Delay Times
  Stephens, J; Beeson, S; Dickens, J; Neuber, A
Abstract:  A quasi-permanent charged electret is embedded into the radiation window of a high power microwave system. It was experimentally observed that the additional electrostatic field introduced by the electret alters the delay times associated with the development of plasma at the window surface, resulting from high power microwave excitation. The magnitudes of both the statistical and formative delay times are investigated in detail for different pressures. Experimental observations are related to calculated discharge parameters using known E/p dependent properties. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767649]
+ Evaluation Of Wave Energy Generation From Buoy Heave Response Based On Linear Generator Concepts
  Stelzer, MA; Joshi, RP
Abstract:  Previous studies of linear generator power extraction from ocean waves have usually ignored the buoy heave dynamics and taken the linear generator movement to coincide with the wave motion. Here, the actual buoy motion is first determined and then used to solve the coupled hydrodynamic and electromagnetic problem of electrical power generation for both regular and irregular waves. Several buoy sizes are modeled to exploit the buoys' natural frequency in an attempt to create a greater heave response for a given sea state. Power output ranging from 75 to 375 W is predicted for the dimensions chosen. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4771693]
+ Experimental And Theoretical Evaluation Of Surface Coated Exploding Wires
  Stephens, J; Neuber, A; Kristiansen, M
Abstract:  This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 10(7) A/cm(2) on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3689855]
+ Hard-Switch Stressing Of Vertical-Channel Implanted-Gate Sic Jfets
  Lawson, K; Alvarez, G; Bayne, SB; Veliadis, V; Ha, HC; Urciuoli, D; El-Hinnawy, N; Borodulin, P; Scozzie, C
Abstract:  A requirement for the commercialization of power SiC transistors is their long-term reliable operation under the hard-switching conditions encountered in the field. Normally ON 1200-V vertical-channel implanted-gate SiC JFETs, designed for high-power bidirectional (four-quadrant) solid-state-circuit-breaker applications, were repetitively hard switched from a 150-V blocking state to an ON-state current in excess of eight times the JFET's 250-W/cm(2) rated current. The JFETs were fabricated in seven photolithographic levels with a single masked ion implantation forming the p(+) gates and guard rings and no epitaxial regrowth. The hard-switch testing was performed using an RLC circuit capable of currents in excess of 200 A with a rise time of 150 A/mu s. In this circuit, energy initially stored in the capacitor is discharged to the resistor through the JFET under test. The JFET hard-switch stressing included 1000 shots at each temperature of 25 degrees C, 50 degrees C, 100 degrees C, and 150 degrees C and at each repetition rate of 1, 5, 10, and 100 Hz for a total of 16 000 shots. Peak energies and powers dissipated by the JFET were 7.5 mJ and 9 kW, respectively. JFET conduction and blocking-voltage characteristics remain unchanged after 16 000 pulsed hard-switching events, which is indicative of reliable operation and excellent JFET suitability for nondegrading repeated bidirectional high surge-current fault isolation.
+ Investigation Into The Temperature Dependence Of Ferrimagnetic Nonlinear Transmission Lines
  J. -. B. Bragg; J. Dickens; A. Neuber
Abstract:  In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between -20°C to 150 °C, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 °C; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.

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+ Light Emission From Csi-Coated Carbon Velvet Cathodes Under Varied Conditions
  C. F. Lynn; A. A. Neuber; J. W. Walter; J. C. Dickens; M. Kristiansen
Abstract:  Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm2 under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10-9 torr levels between subsequent shots.

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+ Operation Of A Sealed-Tube-Vircator High-Power-Microwave Source
  Walter, JW; Lynn, CF; Dickens, JC; Kristiansen, M
Abstract:  The high-power-microwave (HPM) sources currently under development typically require constant pumping to maintain the high vacuum levels required for operation. This pumping is often done with either a cryo-or turbopumping system, either of which would be difficult to deploy in a compact portable system. A compact sealed-tube virtual cathode oscillator (vircator) source has been developed at Texas Tech University (TTU) that does not require a bulky external vacuum pump for operation. This device has a base vacuum pressure in the low 10(-9)-torr range compared to the majority of laboratory HPM sources having vacuum levels in the 10(-5)-10(-7)-torr range. The reduced amount of trapped gasses in the sealed-tube ultrahigh-vacuum environment has the potential to greatly impact device performance. The TTU sealed-tube vircator is useful as a testbed for studying HPM source optimization under UHV conditions. Measured operational characteristics of the tube utilizing a carbon fiber cathode and a nickel anode are presented, along with radiated microwave measurements.
+ Performance And Simulation Verification Of A Mobile Solid State Pulsed Ring Down Array
  Reale, DV; Mankowksi, J; Holt, S; Walter, J; Dickens, J; Gonsalves, NR
Abstract:  Mobile pulsed ring down arrays are of great interest due to their ability to deliver high peak power electromagnetic pulses. Global positioning system (GPS) devices are used as a means to provide position information to individual array elements. An array of solid state radiating structures was built and tested. The array test results are compared to a Monte Carlo array simulation that take into account source jitter, timing error, and distance error between array elements due to GPS measurements.
+ Role Of Electropores On Membrane Blebbing-A Model Energy-Based Analysis
  Joshi, RP; Hu, Q
Abstract:  Cell blebbing has been observed in vitro in the context of external intense electric field pulsing of biological cells. Here, we propose that this is promoted in part by the formation of electropores that influence the membrane tension. Model energy-based calculations of bleb formation are presented. It is shown that the high density of electropores (with radii around 1 nm) driven by a nanosecond voltage pulse could lead to quasi-stable 1 mu m blebs, and that the preferred site for bleb formation would predominantly be near the polar caps facing the electrodes. This is in keeping with recent experimental reports. Our study has implications to pore closing and their lifetimes, an aspect important for intracellular drug delivery. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754568]
+ Special Issue On Pulsed Power Science And Technology
  R. P. Joshi; L. M. S. Redondo; M. Crawford
Abstract:  The forty-six papers in this special issue represent state-of-art of pulsed power science and technology. Congtributed articles to this issue are provided mainly by participants from the 18th International Pulsed Power Conference, in Chicago, IL, June 19-23, 2011.

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+ The Impact Of Wire Environment On Electro-Explosive Fuse Performance
  J. Stephens; W. Mischke; A. A. Neuber
Abstract:  The environment surrounding an exploding wire is known be a controlling factor in electro-explosive fuse performance. Recent experiments have shown that the application of an insulating surface coating to the fuse wire can significantly increase the rate of impedance transition and impedance magnitude of the exploding wire. This paper discusses the performance of surface coated fuses tested in commonly used solid and gaseous media. For comparison, these experiments are compared to bare wire fuse experiments in identical environments. Previously developed exploding wire models are utilized to aid in the interpretation of the experimental fuse behavior. Differential wire voltage, voltage pulse length, and degree of post vaporization conduction (i.e., restrike) are discussed for each experiment.

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+ Theoretical And Experimental Analysis Of Breech Fed And 40-Distributed Energy Stage Plasma Arc Railguns
  R. W. Karhi; D. A. Wetz; J. J. Mankowski; M. Giesselmann
Abstract:  The design and experimental results of a 40-stage distributed energy store (DES) plasma arc railgun are presented. The railgun drives a free running hypervelocity plasma arc, one that is not pushing a payload, to velocities in excess of 10 km/s. These high velocities are of interest as they are required to successfully launch payloads into low earth orbit (LEO). The ability to launch payloads into LEO using a hypervelocity electromagnetic launcher has many financial benefits over the more conventional chemical combustion launchers. In collaboration with an Air Force Office of Scientific Research funded Multidisciplinary University Research Initiative project, the Center for Pulsed Power and Power Electronics at Texas Tech University in Lubbock, Texas has been responsible for developing and investigating a functional scale model of a multistage DES railgun to determine its effectiveness to suppress restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back emf voltage is localized to active stage regions where high gas density and low temperature inhibits breakdown. B-dot sensors positioned along the length of the launcher provide data to measure the plasma arc velocity and detect restrike, arc splitting, or additional secondary arc formation phenomena.

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Publication Year:  2011
+ A 40-Stage Synchronous Distributed Energy Railgun
  R. W. Karhi; D. A. Wetz; M. Giesselmann; J. J. Mankowski; J. P. Diehl; P. M. Kelly
Abstract:  The development process pertaining to the design, fabrication, and testing of a 40-stage free-running arc synchronous distributed energy railgun is presented. Research efforts are still ongoing to suppress the restrike phenomenon that is responsible for causing a velocity ceiling around 6 km/s to exist on plasma armature breech-fed railguns. Numerous solutions have been theorized as viable methods of restrike prevention but lack experimental verification. In collaboration on an AFOSR Multidisciplinary University Research Initiative project, the team at Texas Tech University is responsible for characterizing a functional scale model of a synchronous distributed energy railgun to investigate the effectiveness of a distributed energy scheme to suppress the plasma restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back-EMF voltage is localized to active stage regions. Synchronous operation refers to the speed of an electromagnetic wave in the LC transmission line formed by the rails and capacitors being matched to the velocity of the armature. The railgun drives a hypervelocity (8 km/s) plasma armature, with no payload, to emulate the conditions of a high-altitude microsatellite launch while relieving the financial burden of a large stored energy facility. Experimental data collected from a seven-stage prototype distributed energy system are discussed which will mimic the design and operation of the first seven stages associated with the final 40-stage system, which is currently under construction. The data collected from this prototype as well as the final 40-stage system will be analyzed for secondary arc formation in an attempt to verify the distributed energy scheme's success in suppressing restrike formation.

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+ A Dependency Graph Approach For Fault Detection And Localization Towards Secure Smart Grid
  He, M; Zhang, JS
Abstract:  Fault diagnosis in power grids is known to be challenging, due to the massive scale and spatial coupling therein. In this study, we explore multiscale network inference for fault detection and localization. Specifically, we model the phasor angles across the buses as a Markov random field (MRF), where the conditional correlation coefficients of the MRF are quantified in terms of the physical parameters of power systems. Based on the MRF model, we then study decentralized network inference for fault diagnosis, through change detection and localization in the conditional correlation matrix of the MRF. Particularly, based on the hierarchical topology of practical power systems, we devise a multiscale network inference algorithm that carries out fault detection and localization in a decentralized manner. Simulation results are used to demonstrate the effectiveness of the proposed approach.
+ Design And Evaluation Of A Compact Silicon Carbide Photoconductive Semiconductor Switch
  James, C; Hettler, C; Dickens, J
Abstract:  A high-power vertical photoconductive switch was fabricated from a high-purity semi-insulating 4H-SiC wafer. The device was fabricated from an as-grown wafer with resistivity > 10(9) Omega . cm and had a dark resistance of greater than 6 x 10(9) Omega. The switch was operated at 15 kV/cm and achieved a peak photocurrent of 14 A into a 25-Omega load. Optimization of the excitation wavelength and switch geometry using an optical parametric oscillator was studied in order to decrease the laser requirements for optical triggering. This has led to a decrease in ON-state resistance of almost two orders of magnitude for similar excitation energy levels at visible wavelengths. This work forms the basis for developing very compact high-voltage photoconductive switches.
+ Evaluation Of A Triggered 50 Kv, 100 Hz, Sub-Ns Jitter High Pressure Gas Switch With Pressure, Trigger Magnitude And Gas Temperature
  Chen, Y; Dickens, J; Mankowski, J; Kristiansen, M
Abstract:  Research efforts at Texas Tech University on impulse antenna phased array has resulted in the development of a dependable high voltage, high repetition rate switch that can minimize jitter into the ps range. To accurately synchronize a phased array to steer and preserve the risetime of a radiated pulse, the jitter can only be a fraction of this risetime. Initial testing with a similar system in [1] produced sub-ns jitter results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations using different testing parameters. The switch jitter as a function of triggering conditions is discussed, including a comprehensive evaluation of jitter as a function of operation pressure as well as trigger magnitude. Several phenomenon were observed and discussed to quantify the switch jitter with respect to operation pressure and trigger magnitude. The temperature of gas and its effects on switch jitter is also documented in this paper, with a jitter improvement of similar to 25% recorded. An empirical formula was determined as a function of the gas density, electric field of the main gap, and electric field for the trigger for the experiments conducted in this manuscript. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the switch. The hermetically sealed spark gap, with a modular design composed of copper tungsten electrodes, gas feeds, Kel-F lining, as a well as a G-10 shell is used to house the high pressure gases for the experiments. Trigatron-type triggering is provided by a solid state opening switch voltage source that supplies 75-150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode.
+ Imaging Of Pressure-Dependent High-Power Microwave Surface Flashover
  S. R. Beeson; P. J. Ford; J. Foster; H. G. Krompholz; A. A. Neuber
Abstract:  Open-shutter and intensified charge-coupled device images of high-power microwave breakdown were taken in an effort to characterize the pressure dependence of plasma development. These images were taken with a Nikon D200 and Andor iStar DH734-25U-03, respectively. With the pressures increasing from 200 mtorr to 155 torr, the plasma changes from a diffuse discharge encompassing a large volume to a multichannel structure following the electric field lines.

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+ Phased Array Pulsed Ring-Down Source Synchronization With A Gps Based Timing System
  Chen, Y; Reale, D; Dickens, J; Holt, S; Mankowski, J; Kristiansen, M
Abstract:  A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be increased to the squared of N sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the amplitude of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the operation of each of the elements of a geospatially distributed phased array to maximize the peak power delivered to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser, low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maximum power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to similar to 10 ns without any correction factors and 1-2 ns with simple averaging. With the timing and geospatial errors taken into account, the proposed concept will show usable gains at phased array operating frequencies up to several hundred MHz.
+ Phenomenology Of Streamer Propagation During Pulsed Dielectric Surface Flashover
  G. Laity; A. Neuber; A. Fierro; J. Dickens; L. Hatfield
Abstract:  There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.

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+ Spatially Resolved Vuv Spectral Imaging Of Pulsed Atmospheric Flashover
  G. R. Laity; A. S. Fierro; L. L. Hatfield; J. C. Dickens; A. A. Neuber
Abstract:  The quantitative role of self-produced vacuum-ultraviolet (VUV) light on photoionization-dominated gas discharges is currently an area of interest in the aerospace community. In this paper, we present the images of the VUV spectroscopic analysis of a pulsed atmospheric flashover, where the spatial content of emission relative to electrode geometry has been preserved. The observed spatial profile of emission is dependent on radiating species in the range of 120-125 nm and is discussed in relation to the physics of nanosecond discharges.

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+ Special Issue On Plenary And Invited Papers From Icops 2010
  R. P. Joshi; X. Lu; Y. Sakiyama
Abstract:  The nine papers in this special issue were originally presented at the 37th IEEE International Conference on Plasma Science (ICOPS), held in Norfolk, VA, from June 20 to June 24, 2010.

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+ Special Issue On Power Modulators And Repetitive Pulsed Power [Editorial]
  A. A. Neuber; B. Novac
Abstract:  It is our pleasure to present to the professional reader this "third" Special Issue on Power Modulators and Repetitive Pulsed Power of the IEEE Transactions on Dielectrics and Electrical Insulation (previous issues were published in 2007 and 2009), and the fifth such Special Issue published by an IEEE Transactions (1991-IEEE TED; 2005-IEEE TPS). Most of the papers that appear in this issue are based on contributions to the 2010 International Power Modulator and High Voltage Conference Paper/Presentation, which was held in Atlanta, GA, from May 23rd to 27th, 2011. Others heeded the "Call for Papers" that attracted many contributions in the general field of interest to this Special Issue.

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+ Theoretical Performance Of A Mobile Gps Linked Pulsed Ring Down Array
  Reale, D; Mankowski, J; Chen, Y; Walter, J; Holt, S; Dickens, J
Abstract:  The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz.
+ Transient Performance Of Sic Mosfets As A Function Of Temperature
  K. Lawson; S. B. Bayne
Abstract:  This research was completed to study the effects of extreme transient conditions on Silicon Carbide MOSFET devices. Two different transient conditions that are common in power converters were studied in this paper. The first is effects of voltage rise time, or dV/dt, on these devices. The second is the effects of current pulses with short pulse width and high peak currents. Both of these tests were conducted at temperatures of 150 °C to determine the performance of these devices in high stress environments. For both of these experiments, testing apparatus had to be designed and built to create these specific conditions.

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Publication Year:  2010
+ Capacitor Evaluation For Compact Pulsed Power
  E. J. Matthews; M. Kristiansen; A. A. Neuber
Abstract:  An evaluation of the energy density and efficiency of high-voltage capacitors, from various manufacturers, at voltages above their rated level is presented. Characteristics such as decreasing capacitance, decreasing efficiency, and increasing energy density are described. Data are taken from eight capacitors; six of which are composed of varying nonlinear ceramic materials as a dielectric, and the remaining two are wound capacitors, which were chosen to exhibit their linear characteristics. Rapid (1-100 ms) charging and discharging, similar to that of a generator operation at a repetition rate of more than 10 Hz with an ~100-ns pulse, were the conditions simulated in this particular test setup.

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+ Model Evaluation Of Changes In Electrorotation Spectra Of Biological Cells After Nspef Electroporation
  Hu, Q; Joshi, RP
Abstract:  Cellular electroporation involves electrically driven dielectric membrane breakdown, and has several bio-medical applications. Electrorotation and changes in its characteristic spectra could be a useful method for monitoring electroporation. Towards this goal, a simulation analyses is presented to probe electrorotation features arising from poration. Breakdown of both outer plasma membrane and inner organelle membranes are included based on a double-shell dielectric model. Our results demonstrate the feasibility of such monitoring, and show that anti-field rotation would be a good indicator of outer plasma membrane rupture, and the degree of poration.
+ Modeling And Simulation Of Simple Flux-Trapping Fcgs Utilizing Pspice Software
  A. Young; A. Neuber; M. Kristiansen
Abstract:  A novel modeling and simulation method for flux-trapping flux-compression generators (FT-FCGs) is presented, which utilizes PSpice circuit-simulation software to solve complex differential equations derived from circuit analysis. The primary motivation for the model development is the desire for a technique to rapidly design and prototype FT-FCGs for use as drivers in high-power microwave sources. The derivation of FT-FCG equations will be given, both in the ideal (lossless) and nonideal cases. For the nonideal case, three flux conservation coefficients are added to the equations to account for intrinsic flux loss in the circuit. Time-varying inductance curves are calculated using zero-dimensional models found in literature and adapted to fit this model. A simple FT-FCG design is used as an example to show the steps taken to complete a simulation. The same design was also fabricated and tested for comparison with predicted results from the model. A comparison of the waveforms acquired through simulation and experiment was found to result in good agreement for a given set of values for the flux conservation coefficients. A discussion of the derived equations, both lossless and nonideal, is given, as well as a discussion on the investigation of the impact of the three flux constants on the circuit. Analysis is offered on the results of this investigation, and conclusions are given on the effectiveness of this model to predict FT-FCG behavior.

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+ Pulsed Magnetic Field Excitation Sensitivity Of Match-Type Electric Blasting Caps
  Parson, J; Dickens, J; Walter, J; Neuber, AA
Abstract:  This paper presents a study on energy deposition and electromagnetic compatibility of match-type electroexplosive devices (EEDs), which recently have found more usage in pulsed power environments with high electromagnetic interference (EMI) background. The sensitivity of these devices makes them dangerous to intended and unintended radiation produced by devices commonly used in pulsed power environments. Match-type EEDs have been found to be susceptible to such low levels of energy (7-8 mJ) that safe operation of these EEDs is vital when in use near devices that produce high levels of pulsed EMI. The scope of this paper is to provide an investigation that incorporates results of similar studies to provide detonation characteristics of these EEDs. The three topics included in this study are sensitivity testing, modeling of the thermodynamic heat propagation, and electromagnetic compatibility from pulsed electromagnetic radiation. The thermodynamic joule heating of the primary explosive has been modeled by a solution to the 1D heat equation. A simple pulsed generator, Marx generator with an inductive load, was used for the electromagnetic compatibility assessment of the coupled field between the pulse generator and shorted EED. The results of the electromagnetic compatibility assessment relate the resistive, inductive, and capacitive components of the pulse generator to the area of the shorted EED. (C) 2010 American Institute of Physics. [doi:10.1063/1.3499245]
+ Simulation Of Compact Explosively Driven Ferroelectric Generators
  D. W. Bolyard; A. A. Neuber; J. T. Krile; M. Kristiansen
Abstract:  Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.

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+ Vuv Emission And Streamer Formation In Pulsed Dielectric Surface Flashover At Atmospheric Pressure
  T. G. Rogers; A. A. Neuber; K. Frank; G. R. Laity; J. C. Dickens
Abstract:  There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115-180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

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Publication Year:  2009
+ Aspects Of Lipid Membrane Bio-Responses To Subnanosecond, Ultrahigh Voltage Pulsing
  Joshi, RP; Song, J; Schoenbach, KH; Sridhara, V
Abstract:  Membrane electroporation is probably one of the best-known effects of applying external voltages to biological cells. Reports in the literature have focused on relatively long voltage pulse durations (100 ns-1 ms). Here we probe the very short (< 1 ns), but intense electric field (> 500 kV/cm) regime that is made possible by advances in pulsed power technology. Our analyses based on continuum Smoluchowski and Molecular Dynamics (MD) approaches, predict two new aspects. First, it is shown that pore formation rates would be dramatically lower than predicted by conventional theory due to their dependence on local pore area. Second, such high fields are predicted to affect membrane proteins and ion-channels, without causing electroporation in regions between the proteins. Hence, such high voltage, short duration pulsing should not be associated with electroporation alone, but rather be viewed as a novel vehicle that opens possibilities for a range of new electrically-driven bio-response phenomena.
+ Guest Editorial Special Issue For Selected Papers From The 17Th International Conference On High-Power Particle Beams
  W. Jiang; B. Weber; J. Mankowski
Abstract:  The 23 papers in this special issue were originally presented at the 17th International Conference on High-Power Particle Beams (BEAMS), held in Xi'an China, on July 7-11, 2008.

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+ Mechanisms And Predictors Of Insulator Degradation And Erosion Produced By Pulsed High-Current Surface Discharges
  T. G. Engel; M. Kristiansen
Abstract:  The mechanisms and predictors of insulator degradation and erosion by pulsed high-current surface discharges are presented and discussed. Erosion and degradation depend on the insulator material, the electrode material, the ambient gas, and the presence of UV stabilizers in, or on the surface of, the insulator. Insulator degradation is the result of material decomposition into conductive metal or carbon species and is measured by a decrease in the surface breakdown voltage. Insulator erosion is measured by the material's mass loss. The performance of a large group of ceramic, polymeric, and elastomeric materials tested with graphite and molybdenum electrodes is presented in this investigation. The insulators are exposed to repetitive 300-kA 20-mus-long surface discharges. Tests are performed in atmospheric air and pure nitrogen. Various methods to rank insulators in terms of holdoff voltage degradation, mass erosion, and holdoff voltage conditioning (HVC) using the material's thermochemical properties are presented and discussed. HVC is characterized by an initial increase in surface holdoff voltage. The ranking method developed by the authors characterizes the insulator according to the holdoff degradation resistance (HDR), mass vaporization coefficient (MVC), and HVC figures of merit calculated by the material's thermochemical properties. The investigation also shows the relationship between the HDR, MVC, and HVC figures of merit.

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+ Model Study Of Electroporation Effects On The Dielectrophoretic Response Of Spheroidal Cells
  Hu, Q; Joshi, RP; Beskok, A
Abstract:  Model-based numerical predictions of the dielectrophoretic behavior of spheroidal biological cells are carried out. Membrane electroporation has been specifically included to gauge its effect on the Clausius-Mossotti factor. It is shown that poration could be utilized as an additional flexible tool for cell separation. Furthermore, the simulation study focuses on differences in dielectrophoretic characteristics between normal and malignant cells, and that one cell type can be more prone to poration than another. These results point to the possibility of judiciously selecting electrical parameters for effective cell separation or tumor detection, especially in high conductivity media. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3173344]
+ Optimization Of A Low Jitter, 50 Kv, 100 Hz Triggered Spark Gap With High Pressure Gas Mixtures
  Chen, Y; Dickens, J; Mankowski, J; Kristiansen, M
Abstract:  Recent research efforts at Texas Tech University on impulse antenna phased array has needed to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. In [1], we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations with different gases. The effects of gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter will be investigated. Gases tested include, dry air, H-2, N-2, and SF6, as well as H-2-N-2, and N-2-SF6 gas mixtures. Switch jitter as a result of triggering conditions is discussed, also including a comprehensive evaluation of jitter as a function of formative delay in the various gases. The temperature of gas and its effects on switch jitter is also documented in this paper. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid state opening switch voltage source that supplies similar to 150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a KeI-F - PCTFE (PolyChloroTriFluoroEthylene) lining is used to house the switch and high pressure gas.
+ Optimization Of An Fcg-Based High-Power Microwave System Using Nonexplosive Pulsed Power
  C. B. Davis; A. A. Neuber; A. Young; J. Walter; J. C. Dickens; M. Kristiansen
Abstract:  This paper presents a nonexplosive pulsed-power system that replicates the output current waveform of a flux compression generator (FCG). The primary purpose of this system is to efficiently test the power conditioning components of an explosively driven HPM system, while drastically reducing the time between tests which are inherent with explosive experiments. The power conditioning system (PCS) of the HPM system includes an energy-storage inductor, an electroexplosive opening switch (fuse), and a peaking gap and serves to match the FCG output characteristics with the HPM diode load requirements. A secondary purpose of the nonexplosive test bed is to provide data points which could be directly compared with those from explosively driven experiments. For this reason, a reflex-triode virtual cathode oscillator (vircator) was connected to the output of the nonexplosive system, and the results of which were compared with similar testing done with an FCG and a compact Marx generator. Since the behavior of the fuse is known to play a critical role in the performance of the PCS, a study was performed on the effect of different fuse designs on the overall performance of the PCS. Specifically, the quality of the electrical connection between the fuse wire array and the rest of the system was tested. Fuse design experiments were conducted with the nonexplosive test bed firing into a water resistor dummy load, which showed a 13% increase in peak load voltage and more than an 11% increase in energy transfer for fuses with improved wire-electrode connection strength. Some basic rules about fuse design, as well as conclusions on the performance of the PCS when driving an HPM load, are given.

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+ Short-Pulse High-Power Microwave Surface Flashover At 3 Ghz
  J. T. Krile; L. McQuage; G. F. Edmiston; J. Walter; A. A. Neuber
Abstract:  High-power microwave (HPM)-induced surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. This paper builds on previous testing using a magnetron producing 5 MW for 4 mus at 2.85 GHz. Both the previous and current experimental setups are designed to produce a flashover on the high-pressure side of a transmission window without the influence of a triple point. The limitations of the previous experiment included a maximum power of 5 MW and a pulse rise time of 50 ns. The current HPM source is an experimental virtual cathode oscillator (vircator), the output of which has been extensively characterized. The vircator is capable of producing 50-MW peak for 100 ns with an adjustable frequency from 3 to 5 GHz and a rise time of < 4 ns. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes, respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable field geometries at the transmission window. The experimental setup permits the study of factors, including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels and flashover luminosity with subnanosecond resolution. Additional experimental results, including a detailed analysis of the flashover delay times under various conditions, are compared with data from literature and previous testing. A trend of increasing delay time with pressure is clearly observable, and Eeff/p versus p * r data fall within what has been previously observed in literature primarily for HPM volume breakdown.

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+ Special Issue For Selected Papers From The 17Th International Conference On High-Power Particle Beams
  Jiang, WH; Weber, B; Mankowski, J
Abstract: 
Publication Year:  2008
+ Breakdown Delay Times For Subnanosecond Gas Discharges At Pressures Below One Atmosphere
  J. E. Chaparro; W. Justis; H. G. Krompholz; L. L. Hatfield; A. A. Neuber
Abstract:  With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

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+ Compact Electroexplosive Fuses For Explosively Driven Pulsed Power
  D. R. McCauley; D. W. Belt; J. J. Mankowski; J. C. Dickens; A. A. Neuber; M. Kristiansen
Abstract:  Compact electroexplosive fuses (EEFs) as part of an explosively driven system are of interest for the one-time single-shot generation of high-power pulses. For instance, the transition from a very large driving current produced by an explosively driven flux compression generator (FCG), i.e., low impedance, to a large voltage spike delivered to the load, i.e., high impedance, can be done using an inductive storage system and an EEF. Typically, the EEF can be as large as, if not larger than, the current driver attached to it, thus making it one of the largest components in the system. Reduction in the size of the fuse will allow for size reductions of the entire high-power microwave (HPM) system. The goal of optimizing an EEF as an opening switch is to produce the greatest voltage multiplication possible to drive a load under physical size constraints. To optimize the fuse, several parameters are taken into account, including, but not limited to, fuse material, fuse length, fuse shape, and quenching medium. Individual optimization of these parameters will lead to complete optimization of an EEF, therefore resulting in a compact fuse capable of consistently producing maximum voltage multiplication for HPM systems.

[PDF]

+ Diagnostics Of The Start-Up Process Of An Arc Hollow Cathode
  D. Belt; J. Mankowski
Abstract:  Hall-effect thrusters scaled to power levels below 300 W are of great interest due to their compact size but still require further system optimization. A major component of these thrusters is the free electron source. The majority of the current systems utilize heated element hollow cathodes, but in the event of heater failure, the overall system becomes inoperable. For this reason, a simplistic alternate system such as an arc hollow cathode has been examined. The drawback of utilizing the arc hollow cathode is the reduction in the operational lifetime, especially when the cathode experiences multiple start-up cycles. In order to remedy this, we have developed a soft start-up and continuous operation power supply system. Utilizing this system, we were able to minimize the start-up process from the lifetime influences and examine other factors.

[PDF]

+ Effects Of Uv Illumination On Surface Flashover Under Pulsed Excitation
  J. T. Krile; A. A. Neuber; H. G. Krompholz
Abstract:  Undesirable surface flashover of high voltage support structures can severely limit the compactness of open air high voltage systems. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring under atmospheric conditions and under the influence of UV illumination. In this paper, a UV flash lamp and a solid-state UV source, with its much faster turn-off time, were utilized in conjunction with a high temporal resolution testing apparatus. The UV pulse, excitation voltage, discharge current, and flashover self-luminosity were measured with high temporal precision. We relate recent experiments to our experimental findings of surface flashover under atmospheric conditions gained over the past five years. A simple model that describes the observed behavior will be presented. In addition, a more advanced Monte Carlo-type code for electron collision dynamics will be utilized to further analyze the role of UV in surface flashover under atmospheric conditions.

[PDF]

+ Electrothermal Model Evaluation Of Grain Size And Disorder Effects On Pulsed Voltage Response Of Microstructured Zno Varistors
  Zhao, GG; Joshi, RP; Hjalmarson, HP
Abstract:  Time-dependent, two-dimensional, electrothermal simulations based on random Voronoi networks have been developed to study the internal heating, current distributions and breakdown effects in ZnO varistors in response to high-voltage pulsing. The simulations allow for dynamic predictions of internal failures and to track the progression of hot-spots and thermal stresses. The focus is on internal grain-size variations and relative disorder including micropores. Our results predict that parameters such as the hold-off voltage, internal temperature, and average dissipated energy density would be higher with more uniform grains. This uniformity is also predicted to produce lower thermal stresses and to allow for the application of longer duration pulses. It is shown that the principal failure mechanism arises from internal localized melting, while thermal stresses are well below the thresholds for cracking. Finally, detrimental effects of micropores have been quantified and shown to be in agreement with experimental trends.
+ Fast-Charging Compact Seed Source For Magnetic Flux Compression Generators
  Elsayed, M; Kristiansen, M; Neuber, A
Abstract:  Flux compression generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kiloampere range for midsized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark gaps and overvoltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS) using solid-state components for the switching scheme. The CSS developed is a system (0.005 m(3) volume and weighing 3.9 kg) capable of delivering over 360 J (similar to 12 kA) into a 5.20 mu H load with a trigger energy of microjoules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. A detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid-state components used in the system.
+ High-Current Surface Flashover In A High-Pressure $\Hbox{Sf}_{6}$ Environment
  J. Krile; A. Neuber; R. Vela
Abstract:  A recent upgrade of Sandia National Laboratories' Z-machine has exposed a possible failure mode in the 5 MV laser-triggered gas switches (LTGS). During the closure of the cascade section of the switch, the surface flashover (SF) inside the dielectric switch housing occurred sporadically, affecting subsequent closing timing and damaging the switch housing. A small-scale experiment has been constructed to mimic conditions within the LTGS and to examine the survivability of various materials exposed to high-current SFs.

[PDF]

+ Images From The Development Of A High-Power Microwave System
  T. A. Holt; A. J. Young; M. A. Elsayed; J. W. Walter; A. A. Neuber; M. Kristiansen
Abstract:  A recently developed self-contained compact single-shot high-power microwave (HPM) system was tested and characterized. The explosive-driven system utilizes a reflex triode virtual cathode oscillator (vircator) as the HPM source. An open-shutter image acquired with a digital single-lens reflex camera during operation was used to show plasma development extending beyond the anode-cathode gap of the vircator. The plasma's self-emission is due to ionized material eroded and desorbed from both the cathode and the anode.

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+ Imaging Of High-Power Microwave-Induced Surface Flashover On A Corrugated Dielectric Window
  G. F. Edmiston; J. T. Krile; A. A. Neuber
Abstract:  Dielectric window flashover is a severe pulse-shortening phenomenon limiting the power levels radiated in high power microwave (HPM) systems. This type of flashover develops in regions under high field stress coinciding with the dielectric interfaces separating the vacuum and atmospheric pressure sections of a microwave system. The formation of plasma at the exit aperture of a transmitting system can have several detrimental effects, including premature termination of the radiated pulse and/or the reflection of potentially damaging levels of radiation back toward the microwave source. Experimental studies of HPM surface flashover have been conducted under a variety of conditions in the S-band at power levels up to 5 MW with the aim of quantifying the relative impact of parameters such as gas pressure, type, and window geometry. One particular geometry variant designed with grooves perpendicular to the major electric field component at the window surface exhibited superior flashover suppression characteristics when compared with smooth window geometries. Images of HPM surface flashover evolution on this corrugated dielectric window geometry are presented.

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+ Imaging Of The Explosive Emission Cathode Plasma In A Vircator High-Power Microwave Source
  J. Walter; J. Mankowski; J. Dickens
Abstract:  Most pulsed high-power microwave sources use explosive electron emission cathodes to generate high current electron beams. In the explosive emission process, the current emitted through small field emission points becomes high enough to cause the cathode material to vaporize and form a plasma. Plasma characteristics, such as uniformity and expansion rate, will affect the performance of the microwave source. High-speed optical imaging can be used to resolve some characteristics of the plasma in time. The images of the cathode plasma during the operation of a triode-geometry virtual cathode oscillator high-power microwave source are presented for three different cathode materials.

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+ Low-Jitter Triggered Spark Gap With High-Pressure Gas Mixtures
  Y. Chen; J. J. Mankowski; J. C. Dickens; J. Walter; M. Kristiansen
Abstract:  Recent attention to impulse antenna phased array has necessitated the need to develop a reliable high-voltage high-repetition-rate switch that will operate with ultralow jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact that gases and gas mixtures have on switch performance which includes recovery rate and, in particular, jitter. A 50-Omega 1-nF pulse-forming line is charged to 30 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid-state opening switch voltage source that supplies >100-kV 10-ns rise-time pulses at a rep rate of up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high-pressure gas. The system includes a gas-mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6. Switch operations in 30 kV and 10 Hz have shown reliable subnanosecond jitter times with pure gases, including dry air, H2, N2, and with H2 - N2 and N2 - SF6 gas mixtures. The system was then modified for 50-kV 100-Hz operations with data collected for each of the pure gases. Recovery was monitored with no major problems at the 100-Hz operation, and subnanosecond jitter results for H2 , N2, and SF6 are also recorded.

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+ Secondary Arc Formation Within A Distributed Energy Railgun
  R. W. Karhi; J. J. Mankowski; J. C. Dickens; M. Kristiansen; D. A. Wetz
Abstract:  Experimental results comparing a breech-fed scheme and two distributed energy schemes for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma arc in the railgun are explored. The use of a free-running arc allows experiments to emulate the ablation and restrike phenomenon of a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma arcs within a distributed energy source railgun. Variations of switch timing, bore pressure, bore material, current amplitude, and current pulse length within each stage have been tested. These data reveal important design parameters for distributed energy railguns. The arc length, stage length, and stage trigger timing play a crucial role in distributed energy railgun performance. Failure to take these parameters into consideration will result in velocity reduction through plasma arc restrike and/or splitting.

[PDF]

+ Simulation Studies For Nonlinear-Transmission-Line Based Ultrafast Rise Times And Waveform Shaping For Pulsed-Power Applications
  Zhao, G; Joshi, RP; Rogers, S; Schamiloglu, E; Hjalmarson, HP
Abstract:  The generation of high-voltage electrical pulses with very fast rise times is important for several pulsed-power applications. Although several techniques and devices have been used to generate ultrashort rise-time pulses, most suffer from problems relating to reliability, lifetime, and power-handling capacity. Here, the concept of using nonlinear transmission lines is used for attaining ultrashort rise times and pulse sharpening. Numerical simulations are carried out using barium strontium titanate as the dielectric system. The concept is based on using the nonlinear voltage-dependent capacitance of the granular material. The presence of internal grains increases the breakdown strength and also provides for a nonlinear voltage-dependent capacitance that depends on the internal grain size. The output characteristics of transmission lines based on such nonlinear material are simulated. Our results clearly demonstrated rise-time shortening. The results were in agreement with some published experimental data.
+ Simulation Studies Of Internal Mechanisms In The Static Deflection Of A Cellulose Electroactive Paper Actuator
  Joshi, RP; Mbaye, F; Basappa, P; Jang, SD; Kim, J; Hall, JC
Abstract:  Studies of voltage-induced deflections in electroactive paper (EAPap) have been carried out. On the experimental side, measurements of bias-dependent deflections and strain, water absorption as a function of time, and relative humidity were obtained for the cellulose EAPap actuator. In addition, model simulations have also been carried out to probe and quantify the role of the various internal mechanisms responsible for the deflection. Our simulation predictions yield good agreement with the measured deflection data for the EAPap. The modeling suggests that internal ion content and its migration, water absorption leading to a nonuniform permittivity, random variations in the transverse piezoelectric-coupling coefficient d(31,i), and the modulus of elasticity all collectively contribute to the EAPap deflection electrophysics. It also appears that higher sensitivity, with a minimal bias dependence, could be achieved by deliberately adding ions during EAPap processing. (C) 2008 American Institute of Physics.
+ Utilization Of A Nonexplosive Test Bed For Flux-Compression-Generator Electroexplosive Opening Switches
  D. W. Belt; J. J. Mankowski; A. A. Neuber; J. C. Dickens; M. Kristiansen
Abstract:  Helical flux compression generators (HFCGs) of a 50-mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3-muH inductor. One way to drive a high-power microwave source with an HFCG is by power conditioning, such as an inductive energy storage system (IESS). The output performance of the IESS is contingent upon the opening switch scheme, usually an electroexplosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor-based magnetic switching scheme to generate the near-exponential rise of the HFCG. The use of the nonexplosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. The nonexplosive test bed has provided a more efficient method for electroexplosive switch development and has allowed us to expand the study of opening switches. We will also discuss the a priori calculated baseline fuse design and compare the experimental results of the gold-wire-material with the silver-wire-material baseline design. With the results presented, an accurate PSpice model applicable to our 45-kA HFCG systems will be available.

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Publication Year:  2007
+ A Bench Top Railgun With Distributed Energy Sources
  Mankowski, J; Dickens, J; Giesselmann, M; McDaniel, B; McHale, B; Kristiansen, M
Abstract:  Experimental results of a distributed energy source railgun are presented. Distributed energy source railguns were first proposed by Marshal in an asynchronous scheme and later by Parker synchronously. Both schemes employ a traveling excitation wave to push the projectile along the rail. The primary advantages of such a scheme over the common breech-fed is higher efficiency due to less energy remaining in the rail and lower rail resistive loses. Another advantage is the reduction in the probability of re-strike. However, these advantages are achieved at a cost of higher switching complexity. As a proof of principle experiment, we have constructed a bench-top solid armature railgun with distributed energy sources. Instead of a single, capacitive, breech-fed, energy source, the current is supplied by two storage capacitor banks, placed at different positions along the rail. The switching configuration, which requires a dedicated switch at each capacitor, is realized with sold state switches. The railgun is diagnosed in order to evaluate performance and to appropriately trigger the switches. In addition, experimental results are compared to simulation.
+ An Experimental Approach For Thermal Characterization Of Water-Cooled Heat Sinks Using Fourier Analysis Techniques
  Salem, TE; Bayne, SB; Porschet, D
Abstract:  As power electronic applications continue to switch higher levels of voltage and current in smaller-sized component packages, the resulting increase in power density requires efficient thermal management. This paper compares the thermal performance for operating a metal-oxide-semiconductor field-effect transistor on a water-cooled pole-arrayed heat sink versus a novel water-cooled microchannel heat sink. Details are presented on an innovative technique using Fourier analysis techniques for determining the thermal capacitance modeling parameter for the heat sinks from experimental data.
+ Cathode And Anode Optimization In A Virtual Cathode Oscillator
  Chen, Y; Mankowski, J; Walter, J; Kristiansen, M
Abstract:  We are fabricating and testing several different types of cathodes for the same vircator driven by a single shot Marx generator and pulse forming line (300 kV, 60 us, 30 0). The cathodes types, each with an emitting area of similar to 32 cm(2), include the original velvet with a new geometry, carbon fiber, pin-array, and an array of all metal cathodes. The metal cathodes are made from aluminum and oxygen free copper fashioned to similar geometries with either a chemical etch or a CNC machining process. The vircator is tested with all of these cathodes using both polarity configurations and evaluated for beam voltage, current density, microwave output, and single shot lifetime. In addition to the cathode testing, several stainless steel and tungsten anode meshes with varying transparencies (50% - 80%) are evaluated. The construction and testing of an anode fashioned from Tantalum (70% transparency) is also discussed. Electron beam uniformity of the metal cathodes is investigated with the emitted electron beam current-density distribution evaluated in both time and space. Optimization of output power using resonant effects is also examined.
+ Contributing Factors To Window Flashover Under Pulsed High Power Microwave Excitation At High Altitude
  Edmiston, G; Neuber, A; McQuage, L; Krile, J; Krompholz, H; Dickens, J
Abstract:  One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technological importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 to 90 Torr; 1 Torr=133.3 Pa) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes. Experimental data of flashover delay times across different materials, such as polycarbonate, Teflon (R), and high density polyethylene as a function of background pressure and gas type, air, N-2, argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.
+ Dielectric Surface Flashover At Atmospheric Conditions With Unipolar Pulsed Voltage Excitation
  K. Morales; J. Krile; A. Neuber; H. Krompholz
Abstract:  Dielectric surface flashover along insulators in atmospheric conditions has only been empirically characterized over the years. The underlying physics involved in atmospheric flashover has yet to be understood, where parameters such as background gas, humidity, surface roughness, and temporal characteristics of the applied voltage play a role. Understanding the fundamental physical mechanisms and the extent to which these parameters influence the discharge behavior is vital to characterizing and modeling surface flashover for various structures and conditions. A solid state high voltage pulser with an adjustable pulse width of ~500 ns at FWHM and amplitudes in excess of 30 kV was developed to replicate the non-standard temporal shape of the transient voltage observed inside a rebar enforced building during a lightning strike. Based on experimental results, the phenomenology of pulsed unipolar surface flashover is discussed, with the emphasis on the influence of external parameters (applied voltage pulse shape/risetime, environment, UV illumination, humidity, etc.) on spatial and temporal discharge channel behavior.

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+ Electro-Thermal Simulation Studies For Pulsed Voltage Induced Energy Absorption And Potential Failure In Microstructured Zno Varistors
  Zhao, G; Joshi, RP; Lakdawala, VK; Hjalmarson, HP
Abstract:  A time-dependent, two-dimensional model is used to study internal heating effects and possible device failure in ZnO varistors in response to a high-voltage pulse. The physics and qualitative trends discussed here should hold for materials with internal microstructured grain boundaries. Our analysis is based on an electro-thermal, random Voronoi network. It allows for the dynamic predictions of internal failure and to track the progression of hot-spots and thermal stresses. Results here show that application of high voltage pulses can lead to the attainment of Bi2O3 melting temperatures in the grain boundaries and an accelerated progression towards failure. Comparisons between uniform and normally distributed barrier breakdown voltage showed relatively small difference. Physically, this is shown to be associated with the applied bias regime and grain size. It is argued that reduction in grain size would help lower the maximum internal stress. This is thus a desirable feature, and would also work to enhance the hold-off voltage for a given sample size.
+ Evaluation Of 4 Mm X 4 Mm Silicon Carbide Thyristors
  O'Brien, H; Shaheen, W; Bayne, SB
Abstract:  The U. S. Army Research Laboratory (ARL) is evaluating silicon carbide thyristors to determine the extent of silicon carbide capabilities as a possible replacement for silicon in future pulsed switching applications. Individual SiC die measuring 4 mm x 4 mm were pulsed at high temperatures and varying pulse widths. At 150 degrees C, these thyristors were switched in an RLC circuit up to 3.2 kA and repetitively pulsed at 2.6 kA and 5 Hz for greater than 14,000 pulses. A pulse forming network (PFN) was designed to increase the pulse width and the action seen by the SiC devices. At ambient temperature and a peak current of 2 kA, SiC thyristors were switched in the PFN at a 50% pulse width of 40 mu s and an action of 150 A(2)s. These devices were also pulsed at narrower pulse widths in pairs to look at their behavior in parallel. One pair reached a peak total current of 6.7 kA with current sharing as good as 51% / 49%. This paper includes further data on the three aforementioned test procedures, as well as analysis of the devices' failure points.
+ Evaluation Of Advanced Si And Sic Switching Components For Army Pulsed Power Applications
  H. O'Brien; W. Shaheen; R. L. Thomas; T. Crowley; S. B. Bayne; C. J. Scozzie
Abstract:  Super gate turn-off thyristors (SGTOs) implemented in both silicon (Si) and silicon carbide (SiC) semiconductors were investigated for high-voltage, high-current pulsed power applications. Modular 80 and 400 kA switches implemented in silicon (2.0-cm2 dies) and individual SiC switch die (0.16 cm2) were evaluated. The Si 80- and 400-kA switches were demonstrated (at ambient temperature) to provide rates of current rise (10%-90% peak current) and peak currents (145-mus width) of 24 kA/mus and 92 kA; and 40 kA/mus and 400 kA, respectively. The Si 80-kA switch was repetitively pulsed 1000 times with no significant performance degradation. The SiC switch die were demonstrated to provide specific rate of current rise and current density of 49 kA/mus/cm2 and 56.1 kA/cm2, which are at least 2.5 times greater than are possible in silicon pulse switches. The SiC switches were repetitively pulsed at 5 Hz up to 99 000 times without failure and were demonstrated to operate at case temperatures up to 150 degC

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+ Interface Breakdown During High-Power Microwave Transmission
  Neuber, AA; Edmiston, GF; Krile, JT; Krompholz, H; Dickens, JC; Kristiansen, M
Abstract:  The major limiting factor in the transmission of narrowband high-power microwaves (HPM) has been the interface between vacuum-vacuum or even more severely between vacuum-air if HPM are to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum/dielectric flashover, as opposed to the mechanisms associated with dielectric/air flashover, which are not as well known. Due to the high electron collision frequencies (in the terahertz range) with the background gas molecules, established mitigation methods and concepts of vacuum/dielectric flashover will have to be re-evaluated. The primarily limiting factors of HPM transmission through a dielectric/air interface are presented based on recent experiments at 2.85 GHz. The physics of the involved mechanisms and their practical ramifications are discussed. The potential of surface roughness/geometry for flashover mitigation is addressed as well.
+ Pulsed Dielectric-Surface Flashover In An $ \Hbox{Sf}_{6}$ Environment
  J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz
Abstract:  A recently upgraded laser-triggered gas switch at Sandia National Laboratories has developed a failure mode that results in the breakdown spark tracking to the inside of the containment envelope. These breakdowns along the surface, or surface flashovers, degrade the performance of the overall switch, causing the switch to prefire in the successive shot. In the following, experimental results of pulsed surface flashover across different dielectric materials in SF6, primarily at atmospheric pressure, as well as flashover and volume breakdown in at pressures from 1.3 to 365.4 kPa are presented. In addition to fast voltage and current monitoring of the breakdown event, an increased emphasis was put on imaging the event as well as gathering optical emission spectra (~200-700 nm) from it. As much as possible, the small-scale experiments were designed to reproduce, at least partly, the conditions as they are found in the large 5-MV switch. An effort was made to determine what changes could be made to reduce the occurrence of surface flashovers, in addition to some broadly applicable conclusions on surface flashovers in an SF6 environment.

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+ Spectral Analysis Of Pulsed Volume Breakdown In $ \Hbox{Sf}_{6}$ At High Pressures
  J. T. Krile; R. Vela; A. A. Neuber; H. G. Krompholz
Abstract:  The Z-machine, which is located at Sandia National Laboratories, is currently undergoing refurbishment to increase the output drive current. Due to increased switching voltage requirements, some switch failure modes have been identified with the laser-triggered gas switch design, including envelope surface flashover. In order to improve the performance and lifetime of these switches, a basic understanding of the underlying physics of the failure mechanisms is required. A small-scale experimental setup has been constructed to approximate conditions within the switch. The possible impact of the SF6 volume spark between the switch electrodes on the envelope surface flashover is investigated. Measured optical spectra of the SF6 volume spark over a wide pressure range, from rough vacuum to 40 psig overpressure, are analyzed regarding their potential to contribute to switch failure.

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Publication Year:  2006
+ Analysis Of Polarity Effects In The Electrical Breakdown Of Liquids
  Qian, J; Joshi, RP; Schamiloglu, E; Gaudet, J; Woodworth, JR; Lehr, J
Abstract:  Electrical breakdown simulations are carried out for liquids in response to a sub-microsecond (similar to 100-200 ns) voltage pulse. This model builds on our previous analysis and focuses particularly on the polarity effect seen experimentally in point-plane geometries. The flux-corrected transport approach is used for the numerical implementation. Our model adequately explains experimental observations of pre-breakdown current fluctuations, streamer propagation and branching as well as disparities in hold-off voltage and breakdown initiation times between the anode and cathode polarities. It is demonstrated that polarity effects basically arise from the large mobility difference between electrons and ions. The higher electron mobility leads to greater charge smearing and diffusion that impacts the local electric field distributions. Non-linear couplings between the number density, electric field and charge generation rates then collectively affect the formation of ionized channels and their temporal dynamics.
+ Conduction And Breakdown Mechanisms In Transformer Oil
  Butcher, M; Neuber, AA; Cevallos, MD; Dickens, JC; Krompholz, H
Abstract:  With a fast coaxial test setup using high speed electrical and optical diagnostics, prebreakdown current pulses and shadowgraphy images are measured for direct current (dc) breakdown in Univolt 61 transformer oil. Also, dc currents across the gap are measured using a high sensitivity electrometer. The conduction and breakdown mechanisms in transformer oil as function of applied hydrostatic pressures are quantified. Together, this information provides data on the development of current flow in the system. We have identified three stages in the conduction process prior to breakdown for highly nonuniform fields. Stage 1 is characterized by a resistive current at low fields. Increasing the applied electric field lowers the effective barrier at the metal/dielectric interface allowing a tunneling mechanism to begin, leading to the rapid rise in the injection current observed in stage 2. In stage 3, at high fields, the current reaches space charge saturation with an apparent mobility of 3.10(-3) cm(2)/V.s prior to breakdown. The processes of final breakdown show a distinct polarity dependence. A strong pressure dependence of the breakdown voltage is recorded for negative needle/plane breakdown; a 50% reduction in breakdown voltage is observed when the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. Weak pressure dependence indicates the breakdown mechanism does not have a strong gaseous component. We will discuss possible links between conduction current and dc breakdown.
+ Design And Implementation Of A Flux Compression Generator Nonexplosive Test Bed For Electroexplosive Fuses
  Belt, D; Mankowski, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  Helical flux compression generators (HFCGs) of a 50 mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3 mu H inductor. By utilizing an electroexplosive fuse, a large dI/dt into a coupled load is possible. Our previous work with a nonoptimized fuse has produced similar to 100 kV into a 15 Omega load, which leads into a regime relevant for high power microwave systems. It is expected that similar to 300 kV can be achieved with the present two-stage HFCG driving an inductive storage system with electroexploding fuse. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor based, magnetic switching scheme to generate the near exponential rise of the HFCG. The varying inductance approach utilizes four stages of inductance change and is based upon a piecewise linear regression model of the HFCG wave form. The nonexplosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results. By utilizing the nonexplosive test bed, we hope to reduce the physical size of the inductive energy storage system and fuse substantially. (c) 2006 American Institute of Physics.
+ High-Power Microwave Surface Flashover Of A Gas-Dielectric Interface At 90-760 Torr
  Edmiston, G; Krile, J; Neuber, A; Dickens, J; Krompholz, H
Abstract:  The major limiting factor in the transmission of high-power microwave (HPM) has been the interface between dielectric-vacuum or, even more severely, between dielectric-air, if HPM is to be radiated into the atmosphere. Extensive studies have identified the physical mechanisms associated with vacuum-dielectric flashover, as opposed to the mechanisms associated with air-dielectric flashover, which are not as well known. Surface-flashover tests involving high field enhancement due to the presence of a triple point have shown that volume breakdown threshold (dielectric removed) is approximately 50% higher than the flashover threshold with a dielectric interface over the 90-760 torr range. In order to quantify the role of field enhancement in the flashover process independent of electron injection from metallic surfaces, the effects of the triple point are minimized by carefully choosing the geometry, and in some cases, the triple point is removed from the flashover location. Experimental results were presented, including the impact of gas pressure and the presence of UV illumination, along with temperature analysis of the developing discharge plasma and temporally resolved images of the flashover formation. These results are compared with literature data for volume breakdown in air, with discussion on the similarities and differences between the data.
+ Iminosugars As Potential Inhibitors Of Glycogenolysis: Structural Insights Into The Molecular Basis Of Glycogen Phosphorylase Inhibition
  Oikonomakos, NG; Tiraidis, C; Leonidas, DD; Zographos, SE; Kristiansen, M; Jessen, CU; Norskov-Lauritsen, L; Agius, L
Abstract:  Iminosugars DAB (5), isofagomine (9), and several N-substituted derivatives have been identified as potent inhibitors of liver glycogen phosphorylase a (IC50 = 0.4-1.2 mu M) and of basal and glucagon-stimulated glycogenolysis (IC50 = 1-3 mu M). The X-ray structures of 5, 9, and its N-3-phenylpropyl analogue 8 in complex with rabbit muscle glycogen phosphorylase (GPb) shows that iminosugars bind tightly at the catalytic site in the presence of the substrate phosphate and induce conformational changes that characterize the R-state conformation of the enzyme. Charged nitrogen N1 is within hydrogen-bonding distance with the carbonyl oxygen of His377 (5) and in ionic contact with the substrate phosphate oxygen (8 and 9). Our findings suggest that the inhibitors function as oxocarbenium ion transition-state analogues. The conformational change to the R state provides an explanation for previous findings that 5, unlike inhibitors that favor the T state, promotes phosphorylation of GPb in hepatocytes with sequential inactivation of glycogen synthase.
+ Plasma Membrane Voltage Changes During Nanosecond Pulsed Electric Field Exposure
  Frey, W; White, JA; Price, RO; Blackmore, PF; Joshi, RP; Nuccitelli, R; Beebe, SJ; Schoenbach, KH; Kolb, JF
Abstract:  The change in the membrane potential of Jurkat cells in response to nanosecond pulsed electric fields was studied for pulses with a duration of 60 ns and maximum field strengths of similar to 100 kV/cm (100 V/cell diameter). Membranes of Jurkat cells were stained with a fast voltage-sensitive dye, ANNINE-6, which has a subnanosecond voltage response time. A temporal resolution of 5 ns was achieved by the excitation of this dye with a tunable laser pulse. The laser pulse was synchronized with the applied electric field to record images at times before, during, and after exposure. When exposing the Jurkat cells to a pulse, the voltage across the membrane at the anodic pole of the cell reached values of 1.6 V after 15 ns, almost twice the voltage level generally required for electroporation. Voltages across the membrane on the side facing the cathode reached values of only 0.6 V in the same time period, indicating a strong asymmetry in conduction mechanisms in the membranes of the two opposite cell hemispheres. This small voltage drop of 0.6 - 1.6 V across the plasma membrane demonstrates that nearly the entire imposed electric field of 10 V/mu m penetrates into the interior of the cell and every organelle.
+ Similarities Of Dielectric Surface Flashover Under Atmospheric Conditions For Pulsed Unipolar And Rf Excitation
  Krile, J; Edmiston, G; Morales, K; Neuber, A; Krompholz, H; Kristiansen, M
Abstract:  Mechanisms in vacuum surface flashover caused by rf (f < 10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high-power microwave window breakdown on the high-pressure side) and unipolar surface flashover are expected in an atmospheric environment as well. Two separate experimental setups were utilized to investigate both unipolar flashover and rf window flashover under atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present, all under a similar electric field-surface geometry. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV prepulse has a distinct impact on the arc's path and a tendency to decrease the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, is discussed.
+ The Impact Of Field Enhancements And Charge Injection On The Pulsed Breakdown Strength Of Water
  Wetz, DA; Mankowski, JJ; Dickens, JC; Kristiansen, M
Abstract:  A unique theoretical model of the breakdown mechanism in water has been developed and further tested in both simulation software and experimentation. The conducted experiments test the degree to which electrode material, surface roughness, and surface area impact the dielectric strength of water. Voltage pulses with respective rise times of roughly 200 and 20 ns were applied to a water test gap producing electric fields in excess of 1.5 MV/cm. In experiments testing various electrode materials, thin film coatings of various metallic alloys and oxides were applied to Bruce-profiled stainless steel electrodes, with an effective area of 5 cm(2), through ion beam deposition. Similar Bruceprofiled stainless steel electrodes with surface roughness ranging from 0.26 to 1.96 mu m and effective areas ranging from 0.5 to 75 cm(2) were used in the study of surface roughness and area. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur.
Publication Year:  2005
+ Dc And Pulsed Dielectric Surface Flashover At Atmospheric Pressure
  Krile, JT; Neuber, AA; Dickens, JC; Krompholz, HG
Abstract:  In a wide variety of high-voltage applications surface flashover plays a major role in the system's performance and yet has not been studied in great detail for atmospheric conditions with modern diagnostic tools. Environmental conditions to be considered include pressure, humidity, and gas present in the volume surrounding the dielectric. In order to gain knowledge into the underlying process involved in dielectric surface flashover, a setup has been created to produce and closely monitor the flashover event. Surface flashover for both direct current and pulsed voltages is considered. Within the setup, parameters such as geometry, material, and temporal characteristics of the applied voltage can be altered. Current, voltage, and luminosity are measured with nanosecond to sub-nanosecond resolution. Previously measured optical emission spectra is also discussed.
+ Design And Optimization Of A Compact, Repetitive, High-Power Microwave System
  Chen, YJ; Neuber, AA; Mankowski, J; Dickens, JC; Kristiansen, M; Gale, R
Abstract:  The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate Marx generator for driving an high-power microwave (HPM) source are discussed. Benefiting from the large energy density of mica capacitors, four mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to similar to 18.5 Omega characteristic Marx impedance. Using solely inductors, similar to 1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise time. Repetitive HPM generation with the Marx directly driving a small virtual cathode oscilator (Vircator) has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode, primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results. Design and testing of a low cost, all-metal Vircator cathode will also be discussed. (c) 2005 American Institute of Physics.
+ Fast, Transient Energy Extraction From High-Frequency Ac Alternators For Use In. Electromagnetic Launch Applications
  Giesselmann, M; McHale, B; Crawford, M
Abstract:  This paper describes work toward transient energy extraction from high-frequency alternators to drive electromagnetic launchers through intelligent triggering of thyristors, or silicon controlled rectifiers (SCR). The work was done using a three-phase, 75 kVA level motor-generator set with a dc machine acting as the load. This system serves as a scale model to develop control techniques for full-scale, multimegajoule applications. With the use of a microcontroller and advanced sensors, the system is able to maintain synchronization of the SCR converter with the generator voltage despite dynamically changing frequency and voltage distortion.
+ Imaging Of Dielectric Surface Flashover In Atmospheric Conditions
  Krile, J; Neuber, A; Dickens, J; Krompholz, H
Abstract:  Using a gated intensified digital charge coupled device (ICCD) camera, the development of flashovers across a dielectric surface has been imaged in various gasses at atmospheric pressures. The arc displayed a strong tendency to develop close to the surface, as opposed to following the electric field line leading away from the surface, when oxygen is present in the environment. These findings along with spectroscopy data help to yield a better understanding of the processes involved in surface flashover.
+ Imaging Of Negative Polarity Dc Breakdown Streamer Expansion In Transformer Oil Due To Variations In Background Pressure
  Cevallos, MD; Butcher, M; Dickens, J; Neuber, A; Krompholz, H
Abstract:  The breakdown physics of transformer oil is investigated using high speed electrical and optical diagnostics. Experiments are done in self-breakdown mode utilizing a needle/plane geometry. Shadowgraphy combined with high-speed electrical diagnostics are aimed at measuring streamer expansion as a function of external pressure. Assuming a breakdown mechanism for negative needle based on bubble formation with subsequent carrier amplification in the gas phase implies a pressure dependence, which is observed in the experiments, i.e. the expansion velocity decreases with increasing pressure.
+ Microbubble-Based Model Analysis Of Liquid Breakdown Initiation By A Submicrosecond Pulse
  Qian, J; Joshi, RP; Kolb, J; Schoenbach, KH; Dickens, J; Neuber, A; Butcher, M; Cevallos, M; Krompholz, H; Schamiloglu, E; Gaudet, J
Abstract:  An electrical breakdown model for liquids in response to a submicrosecond (similar to 100 ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission. (C) 2005 American Institute of Physics.
+ The Impact Of Surface Conditioning And Area On The Pulsed Breakdown Strength Of Water
  Wetz, DA; Truman, KP; Mankowski, JJ; Kristiansen, M
Abstract:  Experimental results are presented on the degree to which electrode surface conditioning and surface area impact the dielectric strength of water. The applied pulse to the test gap reached electric field levels greater than 1.5 MV/cm with risetimes around 200 and 50 ns, respectively. The test gap is composed of 304 stainless steel electrodes machined with a Bruce profile. Electrode surface roughness ranged from 0.34 to 1.41 mu m and effective areas ranged from 0.5 to 75 cm(2). Additional results are presented on the pulsed breakdown strength of Rexolite with various surface finishes ranging from .025 to 5.715 mu m. Conclusions are made as to the effect electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. Conclusions are also made as to the impact of the surface condition of Rexolite has on its bulk breakdown strength.
+ Thermal And Electrical Evaluation Of Sic Gtos For Pulsed Power Applications
  Geil, BR; Bayne, SB; Ibitayo, D; Koebke, MG
Abstract:  For applications which require high peak current and fast rise time, silicon carbide (SiC) material is ideal because of its ability to tolerate high localized temperatures generated during switching. This research was performed to,investigate SiC devices for pulse power applications and to analyze the failure, of the devices. Seven 2 mm x 2 mm SiC gate turn-off thyristors (GTOs) manufactured by Cree, Inc., Durham, NC, were evaluated. The devices were tested at single shot and under repetitive stress using a ring-down capacitor discharge circuit. The current pulsewidth was 2 mu s with a peak current of 1.4 kA (current density of 94.6 kA/cm(2)) and a maximum di/dt of 2.36 kA/mu s. The maximum power dissipated within the devices was 240 kW. Thermal modeling of these devices was done using ANSYS to analyze the heating and cooling. A two-dimensional model was used that included the device package and bonding materials. The maximum amount of power dissipated was calculated from the 1000-A, 2-mu s pulse. No further power input was added to the model and the heat transfer was plotted on an exponential scale. It was found that heat applied to a 2-mu m-thick region of the fingers yielded a temperature greater than 800 degrees C in the device. It took 1.0E-(02) s for this heat to dissipate and for the device to return to 23 degrees C. The minimum and maximum stresses were found to be -2.83E+(09) Pa. and 4.06E(+08) Pa respectively.
Publication Year:  2004
+ Approximate Analytical Solutions For The Space-Charge-Limited Current In One-Dimensional And Two-Dimensional Cylindrical Diodes
  Chen, XP; Dickens, J; Hatfield, LL; Choi, EH; Kristiansen, M
Abstract:  In high-power microwave diode design, the space-charge-limited current is important because of its relation to the diode impedance, and the formation of the virtual cathode. Although the Langmuir-Blodgett law, as a numerical solution, is helpful, a simple functional expression would be more convenient for practical research. In this paper, a physical approximation has been introduced to analyze the nonlinear Poisson's equation in a one-dimensional (1-D) cylindrical vacuum diode. With the help of this physical approximation, a solution for the space-charge-limited current for 1-D cylindrical diodes has been investigated and developed. In addition, a comparison between our approximate result and the Langmuir-Blodgett numerical solution shows that the physical approximation method is valid in nonlinear differential equation analyses. This physical approximation can be used to analyze similar nonlinear differential equations. Also, a correction for the space-charge-limited current in a two-dimensional cylindrical diode is obtained within a limitation. (C) 2004 American Institute of Physics.
+ Dc Flashover Of A Dielectric Surface In Atmospheric Conditions
  Krile, JT; Neuber, AA; Dickens, JC; Krompholz, HG
Abstract:  Surface flashover is a major consideration in a wide variety of high-voltage applications, and yet has not been studied in great detail for atmospheric conditions, with modern diagnostic tools. Environmental conditions to be considered include pressure, humidity, and gas present in the volume surrounding the dielectric. In order to gain knowledge into the underlying process involved in dielectric surface flashover, a setup has been created to produce and closely monitor the flashover event. Within the setup parameters such as geometry, material, and temporal characteristics of the applied voltage can be altered. Current, voltage, luminosity, and optical emission spectra are measured with nanosecond to subnanosecond resolution. Spatially and temporally resolved light emission data is also gathered along the arc channel. Our fast imaging data show a distinct trend for the spark in air to closely follow the surface even if an electrical field with a strong normal component is present. This tendency is lacking in the presence of gases such as nitrogen, where the spark follows more closely the electric field lines and develops away from the surface. Further, the breakdown voltage in all measured gases decreases with increasing humidity, in some cases as much as 50% with an increase from 10% relative humidity to 90% relative humidity.
+ Expression Of Leptin Receptor Isoforms And Effects Of Leptin On The Proliferation And Hormonal Secretion In Human Pituitary Adenomas
  Kristiansen, MT; Clausen, LR; Nielsen, S; Blaabjerg, O; Ledet, T; Rasmussen, LM; Jorgensen, JOL
Abstract:  Objective: To pursue whether leptin regulates anterior pituitary cells, we studied the ex vivo expression of several isoforms of the leptin receptor (OB-R) as well as the in vitro effects of leptin administration in human pituitary adenomas. Methods: OB-R mRNA expression and in vitro response to leptin were studied in 39 pituitary macroadenomas. Results: All 4 OB-R subtypes were expressed in most adenomas. The expression was significantly more pronounced in GH-secreting adenomas as compared to non-functioning tumor cells (p < 0.05). Leptin administration in vitro did not significantly influence cell proliferation or the secretion of GH, FSH, LH or alpha-subunit. Conclusions: ( 1) Several isoforms of the OB-R, including the signal transducing full-length receptor, are expressed in most human pituitary adenomas. ( 2) This expression ex vivo is not associated with significant effects of leptin in vitro. Copyright (C) 2004 S. Karger AG, Basel.
+ Fifth Special Issue On Pulsed Power Science And Technology
  Dickens, JC; Lehr, JM; Mankowski, J
Abstract: 
+ Growth Hormone Receptor Expression And Function In Pituitary Adenomas
  Clausen, LR; Kristiansen, MT; Rasmussen, LM; Billestrup, N; Blaabjerg, O; Ledet, T; Jorgensen, JOL
Abstract:  OBJECTIVE AND DESIGN Hypopituitarism, in particular GH deficiency, is prevalent in patients with clinically nonfunctioning pituitary adenomas (NFPAs) both before and after surgery. The factors regulating the growth of pituitary adenomas in general and residual tumour tissue in particular are not fully characterized, and the effect of GH and IGF-I on human pituitary cell proliferation has not previously been reported. In NFPA tissue from 14 patients we evaluated GH receptor (GHR) expression and signal transduction, and the effect of GH and IGF-I exposure on cell proliferation and hormone secretion in vitro. MEASUREMENTS Tissue samples from 14 NFPAs were investigated. Expression of GHR in tissue samples was assessed by reverse transcription polymerase chain reaction (RT-PCR). Six tumours were immunostained with a GHR antibody. In the cell cultures, STAT5 (signal transducer and activator of transcription 5) phosphorylation was measured by Western blot analysis as an index of GHR signalling; cell proliferation was evaluated by [H-3]-thymidine incorporation and glycoprotein hormone production analysed by radioimmunoassay (RIA). RESULTS All adenomas investigated expressed the GHR, but there was no detection of STAT5 phosphorylation. Overall, GH and IGF-I administration did not significantly stimulate cell proliferation in vitro, although some individual adenomas exhibited a proliferative response to various extents. GH also did not significantly influence glycoprotein hormone secretion in vitro. CONCLUSION GH receptors are expressed in human pituitary adenoma cells but their functional role is uncertain. GH and IGF-I do not consistently influence the proliferation of cultured pituitary adenoma cells.
+ High Power Water Switches: Postbreakdown Phenomena And Dielectric Recovery
  Xiao, S; Kolb, J; Kono, S; Katsuki, S; Joshi, RP; Laroussi, M; Schoenbach, KH
Abstract:  The physical processes following the electrical breakdown of water between planar and hemispherical electrodes separated by a sub-millimeter gap have been studied using electrical and optical diagnostics. The expanding plasma column after breakdown generates first shockwaves and at a later stage a vapor bubble which expands for about 200 mus and then decays with a time constant of 1 ms. The bubble decay time determines the dielectric recovery of the switch as has been shown with pulse-probe experiments.
+ Identification, Synthesis, And Characterization Of New Glycogen Phosphorylase Inhibitors Binding To The Allosteric Amp Site
  Kristiansen, M; Andersen, B; Iversen, LF; Westergaard, N
Abstract:  Inhibition of glycogen phosphorylase (GP) has attracted considerable attention during the last five to 10 years as a means of treating the elevated hepatic glucose production seen in patients with type 2 diabetes. Several different GP inhibitors binding to various binding sites of the GP enzyme have been reported in the literature. In this paper we report on a novel class of compounds that have been identified as potent GP inhibitors. Their synthesis, mode of binding to the allosteric AMP site as well as in vitro data on GP inhibition are shown. The most potent inhibitor was found to be 4-[2,4-bis-(3-nitrobenzoylamino)phenoxylphthalic acid (4j) with an IC50 value of 74 nM. This compound together with a closely related analogue was further characterized by enzyme kinetics and in primary rat hepatocytes.
+ Magnetic Flux Compression Generators
  Neuber, AA; Dickens, JC
Abstract:  Magnetic flux compression generators offer the largest pulsed power output per unit size or weight when compared with other more conventional systems. They have found widespread use as pulsed power sources for hydrodynamics programs and high magnetic field research at national laboratories or in commercial applications, including exploration for oil and minerals and mine detection. Also, due to their nature as a true one-time-use device with superior energy density, a large portion of applications is defense related. A variety of basic magnetic flux compression generator designs have been developed and tested during the past four decades. All of them rely on the explosive-driven deformation of a system of conductors having an initial, preferably large, inductance. The most successful basic design is the helical flux compression generator which is capable of producing a high-energy output into large impedance loads, just as it is needed for a practical pulsed power source. This paper will review the advances and state of the art of primarily helical magnetic flux compression generators mainly developed as pulsed power sources and will offer new insights gained as a result of a recently completed five-year AFOSR/DoD Multidisciplinary University Research Initiative program that studied the basic physics and engineering aspects of helical flux compression generators.
+ Microwave Frequency, Determination Mechanisms In A Coaxial Vircator
  Chen, XP; Dickens, J; Mankowski, J; Hatfield, LL; Choi, EH; Kristiansen, M
Abstract:  Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. In this paper, some experiments showing different results are reported. In particular, the E-beam is observed to play an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator with different geometries has been investigated in detail. Investigation of the E-beam performance will improve understanding of the interaction, between the E-beam and microwaves, which is a key for determining the microwave frequency. These results are helpful in optimizing the design of a cylindrical diode to avoid microwave frequency shifting and mode competition.
+ Modeling Electrode-Based, Stimulation Of Muscle And Nerve By Ultrashort Electric Pulses
  Joshi, RP; Chen, F; Rogers, WR
Abstract:  Numerical simulations of electrostimulation of frog gastrocnemius muscles have been carried out for pulse durations in the nanosecond regime. There are a number of potential advantages in using ultrashort pulses for electrical stimulation, and no previous electrostimulation work in the submicrosecond regime has been reported. A time-dependent, three-dimensional analysis model was developed and implemented for two cases: 1) direct stimulation via electrode contact and 2) indirect excitation through a saline-filled bath. The simulations yielded strength-duration (S-D) curves with pulse durations as short as 5 us. Good agreement between the model, predictions and experimental measurements was obtained. For example, with direct contact, a peak current of about 30 A was predicted for the shortest pulse; the measured value was 34 A. Calculations of the S-D curves for both direct and indirect stimulation yielded a good match with the available experimental data. A time constant of 160 mus was estimated; this value is indicative of a nerve-based response. The modeling also led to a demonstration of the nonthermal nature of electrostimulation with nanosecond pulses, even with an applied voltage of 5 kV. Finally, it was shown quantitatively that inhomogeneities in the nerve geometry and size can affect the S-D curve. For contact stimulation, the greatest potential for muscle twitching occurs at boundaries and within regions that have internal nonuniformity.
+ Modeling Studies Of Cell Response To Ultrashort, High-Intensity Electric Fields-Implications For Intracellular Manipulation
  Joshi, RP; Hu, Q; Schoenbach, KH
Abstract:  The dynamics of electroporation in biological cells subjected to nanosecond, high-intensity pulses are studied based on a coupled scheme involving both the current continuity and Smoluchowski equations. A new distributed network model, that includes dynamic conductivities of cell membranes and substructures, is introduced for evaluations of transmembrane potential. It is shown that subcellular structures could be affected through nanosecond pulses, and that, despite the high field intensity, the processes remain nonthermal. As an example of selectivity, differences in cell responses between normal and malignant (Farage) tonsillar B-cells are compared and discussed. It is shown that ultrashort, high-intensity electric pulses could damage cancer cells. Finally, the model predicts that it is possible to target the inner mitochondrial membrane (i.e., selectivity at the organelle level), in keeping with recent experimental observations.
+ Nanosecond Pulsed Electric Fields Modulate Cell Function Through Intracellular Signal Transduction Mechanisms
  Beebe, SJ; Blackmore, PF; White, J; Joshi, RP; Schoenbach, KH
Abstract:  These studies describe the effects of nanosecond (10-300 ns) pulsed electric fields (nsPEF) on mammalian cell structure and function. As the pulse durations decrease, effects on the plasma membrane (PM) decrease and effects on intracellular signal transduction mechanisms increase. When nsPEF-induced PM electroporation effects occur, they are distinct from classical PM electroporation effects, suggesting unique, nsPEF-induced PM modulations. In HL-60 cells, nsPEF that are well below the threshold for PM electroporation and apoptosis induction induce effects that are similar to purinergic agonist-mediated calcium release from intracellular stores, which secondarily initiate capacitive calcium influx through store-operated calcium channels in the PM. NsPEF with durations and electric field intensities that do or do not cause PM electroporation, induce apoptosis in mammalian cells with a well-characterized phenotype typified by externalization of phosphatidylserine on the outer PM and activation of caspase proteases. Treatment of mouse fibrosarcoma tumors with nsPEF also results in apoptosis induction. When Jurkat cells were transfected by electroporation and then treated with nsPEF, green fluorescent protein expression was enhanced compared to electroporation alone. The results indicate that nsPEF activate intracellular mechanisms that can determine cell function and fate, providing an important new tool for probing signal transduction mechanisms that modulate cell structure and function and for potential therapeutic applications for cancer and gene therapy.
+ Quantification Of Ohmic And Intrinsic Flux Losses In Helical Flux Compression Generators
  Hernandez, JC; Neuber, AA; Dickens, JC; Kristiansen, M
Abstract:  Helical magnetic flux compression generators (MFCGs) are the most promising energy sources with respect to their current amplification and compactness. They are able of producing high current pulses required in many pulsed power applications with at least one order of magnitude higher energy density than capacitive storage with similar discharge characteristics. However, the main concern with MFCGs is their intrinsic flux loss that limits severely their performance and which is not yet well understood. In general, all flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed in the open literature, none of them truly quantify, starting from basic physics principles, the ohmic and intrinsic flux losses in helical MFCGs. This paper describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses, being especially easy to implement and fast to calculate. We also provide a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used to a priori estimate the MFCG performance. Further, we will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.
+ Research Issues In Developing Compact Pulsed Power For High Peak Power Applications On Mobile Platforms
  Gaudet, JA; Barker, RJ; Buchenauer, CJ; Christodoulou, C; Dickens, J; Gundersen, MA; Joshi, RP; Krompholz, HG; Kolb, JE; Kuthi, A; Laroussi, M; Neuber, A; Nunnally, W; Schamiloglu, E; Schoenbach, KH; Tyo, JS; Vidmar, RJ
Abstract:  Pulsed power is a technology that is suited to drive electrical loads requiring very large power pulses in short bursts (high-peak power). Certain applications require technology that can be deployed in small spaces under stressful environments, e.g., on a ship, vehicle, or aircraft. In 2001, the U.S. Department of Defense (DoD) launched a long-range (five-year) Multidisciplinary University Research Initiative (MURI) to study fundamental issues for compact pulsed power This research program is endeavoring to: 1) introduce new materials for use in pulsed power systems; 2) examine alternative topologies for compact pulse generation; 3) study pulsed power switches, including pseudospark switches; and 4) investigate the basic physics related to the generation of pulsed power such as the behavior of liquid dielectrics under intense electric field conditions. Furthermore, the integration of all of these building blocks is impacted by system architecture (how things are put together). This paper reviews the advances put forth to date by the researchers in this program and will assess the potential impact for future development of compact pulsed power systems.
+ Time Domain Dielectric Spectroscopy Measurements Of Hl-60 Cell Suspensions After Microsecond And Nanosecond Electrical Pulses
  Garner, AL; Chen, NY; Yang, J; Kolb, J; Swanson, RJ; Loftin, KC; Beebe, SJ; Joshi, RP; Schoenbach, KH
Abstract:  Pulsed electric fields (PEFs) above a certain voltage threshold cause electroporation for microsecond pulses and intracellular effects for submicrosecond pulses. Models describing these effects often depend on the electrical properties of the cell, which are altered by the PER We used time domain dielectric spectroscopy to provide data for these models and to measure effects on cell suspension conductivity. We applied single 50-mus and 10-ns pulses to HL-60 cells, with the voltages chosen so the pulses have approximately the same energy. For 1.1-kV/cm, 50-mus pulses, the conductivity rose within a minute after the pulse and dropped dramatically approximately 40 minute after the pulse. For 78-kV/cm, 10-ns pulses, we observed a brief delay prior to the conductivity rise and noted the same drop in conductivity after approximately 40 min. For both pulse durations, higher voltages frequently led to membrane poration followed by a gradual recovery approximately 3040 min after the pulse. For 2-kV/cm, 50-mus pulses, we observed significantly more scatter in Trypan Blue uptake measurements due to stronger effects on the cell membrane. By using a cell model, we showed that a 50-mus pulse caused a much larger rise in membrane conductivity than a 10-ns pulse of the same energy.
Publication Year:  2003
+ Analysis Of Dislocation Scattering On Electron Mobility In Gan High Electron Mobility Transistors
  Joshi, RP; Viswanadha, S; Jogai, B; Shah, P; del Rosario, RD
Abstract:  A Monte Carlo study of two-dimensional electron gas mobility in wurtzite GaN high electron mobility transistors is presented to include scattering due to edge dislocations strains. For self-consistency, numerical solution of Schrodinger, Poisson, and charge balance equations are used for the eigenfunctions. Electron mobility predictions around 1.71x10(3) cm(2)/V s, are in close agreement with the reported data. At the highest dislocation density of 10(10) cm(-2), mobility reductions of 16.8% and 8.6% are predicted for 77 and 300 K, respectively. Electron mobility behavior as a function of the channel density at different temperatures is also discussed. (C) 2003 American Institute of Physics.
+ Completely Explosive Pulsed Power Minisystem
  Talantsev, EF; Shkuratov, SI; Dickens, JC; Kristiansen, M
Abstract:  It is demonstrated that it is feasible to produce pulsed power using an autonomous completely explosive system that harnesses two physical phenomena successively: the transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnets and magnetic cumulation. (C) 2003 American Institute of Physics.
+ Currents Produced By Explosive Driven Transverse Shock Wave Ferromagnetic Source Of Primary Power In A Coaxial Single-Turn Seeding Coil Of A Magnetocumulative Generator
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  Experimental and digital simulation studies of the generation of seed currents by an ultracompact (8.66-8.75 cm(3) in volume) ferromagnetic explosive-driven generator of primary power (FMG) loaded on the coaxial single-turn seeding coil of a magnetocumulative generator (MCG) have been performed. The operation of the FMG is based on transverse shock wave demagnetization of Nd2Fe14B high-energy hard ferromagnets. The FMG is capable of producing in the coaxial seeding coil of MCG a seed current with peak amplitude I(t)(max)=3.0 kA and full width at half maximum of 60 mus. The methodology was developed for digital simulation of the seeding processes in the combined FMG/MCG system. (C) 2003 American Institute of Physics.
+ Dynamical Modeling Of Cellular Response To Short-Duration, High-Intensity Electric Fields
  Joshi, RP; Hu, Q; Schoenbach, KH
Abstract:  The interaction of ultra-short duration, high-intensity electric fields with biological cells has recently begun to generate significant interest due to the possibility for non-thermal manipulation of cellular functions. It is clear that a full understanding requires a dynamical model for both electroporation and the electrostatic potential evolution. Here, dynamical aspects related to electroporation are reviewed. The simple model used in the literature is somewhat incorrect and unphysical for a variety of reasons. Our model for the pore formation energy, E(r), includes a dependence on pore population, density, a variable surface tension, and is dynamic in nature. It is shown that membranes can survive a strong electric pulse and recover provided the pore distribution has a relatively large spread. If, however, the population consists predominantly of larger radii pores, then irreversibility can result. Physically, such a distribution could arise if pores at adjacent sites coalesce. Results show that a finite time delay exists for pore formation, and can lead to a transient overshoot of the transmembrane potential V-mem beyond 1.0 V. Pore resealing is shown to consist of an initial fast process, a 10(-4) s delay, followed by a much slower closing at a time constant of about 10(-1) s. This establishes a time-window for effective killing by a second pulse.
+ Fast, Real-Time Monitoring Of Rotating Machines Using Digital Motion Control Coprocessors
  Giesselmann, M; McHale, B; Crawford, M
Abstract:  This paper describes the use of methods and techniques that have been developed for high-performance control of relatively small and slow turning industrial machines and extend these techniques to the more demanding regime of ac machines used for kinetic energy storage and fast ac servos for military applications. In particular,this paper describes a technique for fast monitoring of the output voltage of ac generators. To accomplish fast monitoring, the (sinusoidal) ac output voltage is converted to a dc quantity that represents the instantaneous amplitude. This is achieved through the use of a rotational transformation. This transformation, also called vector rotation, can be used for very fast observation of the momentary amplitudes of all electrical machine quantities, such that averaging of the ac value (to determine 66 momentary average rms amplitude) is not necessary. The procedure is implemented by tightly integrating a digital motion control coprocessor into the memory map of a 16-bit microcontroller.
+ Improve Your Visual Inspection Program
  Stephens, JC
Abstract: 
+ Insect Chemical Ecology Research In The United States Department Of Agriculture - Agricultural Research Service
  Aldrich, JR; Bartelt, RJ; Dickens, JC; Knight, AL; Light, DM; Tumlinson, JH
Abstract:  This multi-author paper reviews current work by USDA-ARS scientists in the field of chemical ecology. Work with pheromones, the discovery and development of the codling moth kairomone, studies on insect-plant interactions and chemically mediated tritrophic plant-insect interactions have led to practical methods for control of important insect pests.
+ Longitudinal-Shock-Wave Compression Of Nd2Fe14B High-Energy Hard Ferromagnet: The Pressure-Induced Magnetic Phase Transition
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M; Baird, J
Abstract:  A study of a magnetic phase state of Nd2Fe14B high-energy hard ferromagnets subjected to longitudinal-shock-wave compression (where the shock wave propagates along magnetization vector M) has been performed. The results of the investigation show that longitudinal-shock-wave compression of Nd2Fe14B at 28-38 GPa causes a magnetic phase transition terminated by practically complete demagnetization of Nd2Fe14B. Due to this phase transition all electromagnetic energy stored in Nd2Fe14B is released and can be transformed into pulsed power. Explosive-driven autonomous sources of primary power utilizing this effect are capable of producing high-current pulses [current amplitude of 1.0 kA, full width at half maximum (FWHM) of 165 mus] and high-voltage pulses (peak voltage of 13.4 kV, FWHM of 8.2 mus). (C) 2003 American Institute of Physics.
+ Monte Carlo Calculations Of The Impulse Response In Bulk Inas For Low-Intensity Photon Avalanche Detection At Two Micron Wavelength (Vol 41, Pg 5559, 2002)
  Satyanadh, G; Joshi, R; Abedin, N
Abstract: 
+ Thermal Deformation Noise In Large Optical Systems
  Tolomeo, J; Klavins, A; Tenerelli, D; Dickens, J
Abstract:  Future large space-based telescope systems require precise optical surface quality and wave-front stability. One source of noise for very large precise optical systems is ambient thermal energy which induces statistical fluctuations in the strain energy state of the structure. We broadly model such optical systems as bending energy dominated or membrane in-plane energy dominated and derive analytical expressions for the governing parameters that determine noise magnitude. It is shown that for bending-based systems thermal noise increases as aperture is increased and as bending stiffness is decreased, while for membrane mirror systems it is the in-plane pretension level that determines the noise magnitude. The analysis is extended to numerical finite element techniques to illustrate the effects on very general large damped structures where we address the form of equivalent thermal loading density required in modeling such distributed structures. Calculations show that temporal rms deformation noise on the order of a picometer or less can be expected for apertures up to about 10 m and therefore is probably not significant. For lightweight precision aperture systems greater than 10 m, thermal noise may need to be considered in the design. (C) 2003 American Institute of Physics.
Publication Year:  2002
+ Advanced Operational Techniques And Pn-Pn-Pn Structures For High-Power Silicon Carbide Gate Turn-Off Thyristors
  Shah, PB; Geil, BR; Ervin, ME; Griffin, TE; Bayne, SB; Jones, KA; Oldham, T
Abstract:  SiC GTO thyristors may soon be the best available choice for very high-power switching. At this time, we have developed new operational techniques, growth requirements and pn-pn-pn type structures to address the issues of high on-state voltage, poor turn-off gain, and inability to reach predicted breakover voltages. We present these findings using experimental measurements and numerical simulations.
+ Compact Explosive-Driven Generator Of Primary Power Based On A Longitudinal Shock Wave Demagnetization Of Hard Ferri- And Ferromagnets
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  A new type of compact explosive-driven generator of primary power, which utilizes phenomena of a shock-wave demagnetization of hard ferri- and Mirromagnets, was developed. The shock wave initiated by high explosive, as well as accelerated flyer plate, passes along the hard ferri- or ferromagnetic body, which serves as initial energy carrier. The shock wave demagnetizes the energy-carrying element, reducing the initial magnetic flux Phi(0). In accordance with Faraday's law, this change of magnetic flux DeltaPhi(0) generates an electromotive force in, a coil wound on the energy carrier. Several types of compact generators with energy-carrying element of 10 cm(3) in volume were explored. High-voltage generators that utilize energy of BaFe12O19 hard ferrimagnets are capable of producing pulses of amplitude 5.5 kV with full width at half maximum (FWHM) of 1 mus. The generators that utilize energy of Nd2Fe14B high-energy hard ferromagnets are capable of producing pulses with amplitude more than 10 kV and FWHM about 4 mus. The high-current generators based on Nd2Fe14B produced pulses yielded 826 A and FWHM of 180 mus. The developed generator can be used as the most reliable and effective source of primary power capable of seeding magnitocumulative generators.
+ Ectopic Acth Syndrome: Discrepancy Between Somatostatin Receptor Status In Vivo And Ex Vivo, And Between Immunostaining And Gene Transcription For Pomc And Crh
  Kristiansen, MT; Rasmussen, LM; Olsen, N; Asa, SL; Jorgensen, JOL
Abstract:  Objective: To characterize somatostatin receptor status in vivo and ex vivo and immunoreactivity and gene transcription for proopiomelanocortin (POMC) and corticotropin-releasing hormone (CRH) in a case of Cushing's syndrome caused by a sporadic metastatic medullary thyroid carcinoma (MTC). Methods: In-111 octreoscan, analysis of tumorous mRNA transcripts for somatostatin receptor subtypes (SSTR) as well as for POMC and CRH. Results: The tissue was intensely positive by In-111 octreoscan but expressed only SSTR 1, 3 and 5. There was immunopositivity only for CRH, but gene transcription for both POMC and CRH was seen. Conclusions: (1) This first comparison between somatostatin receptor status in vivo and ex vivo in MTC shows a marked positive octreoscan despite absent SSTR 2 expression, and (2)this is the first report of a discrepancy between immunostaining and gene transcription for POMC and CRH. Copyright (C) 2002 S. Karger AG, Basel.
+ Extended Kalman Filter Training Of Neural Networks On A Simd Parallel Machine
  Li, SH; Wunsch, DC; O'Hair, E; Giesselmann, MG
Abstract:  The extended Kalman filter (EKF) algorithm has been shown to be advantageous for neural network trainings. However, unlike the backpropagation (BP), many matrix operations are needed for the EKF algorithm and therefore greatly increase the computational complexity. This paper presents a method to do the EKF training on a SIMD parallel machine. We use a multistream decoupled extended Kalman filter (DEKF) training algorithm which can provide efficient use of the parallel resource and more improved trained network weights. From the overall design consideration of the DEKF algorithm and the consideration of maximum usage of the parallel resource, the multistream DEKF training is realized on a MasPar SIMD parallel machine. The performance of the parallel DEKF training algorithm is studied. Comparisons are performed to investigate pattern and batch-form trainings for both EKF and BP training algorithms. (C) 2002 Elsevier Science (USA).
+ Field Enhanced Microwave Breakdown In A Plasma Limiter
  Mankowski, JJ; Hemmert, D; Krompholz, H
Abstract:  A new type of plasma limiter is being developed which can turn on in less than 1 ns. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Images were taken of the gap region in Argon at several pressures in order to investigate the role of the tip region.
+ Gas Breakdown In The Subnanosecond Regime With Voltages Below 15 Kv
  Krompholz, H; Hatfield, LL; Kristiansen, M; Hemmert, D; Short, B; Mankowski, J; Brown, MDJ; Altgilbers, LL
Abstract:  Gaseous breakdown in the subnanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics, and for plasma limiters to protect electronic devices from high power microwave radiation. Previous investigations of subnanosecond breakdown were mainly limited to high-pressure gases or liquids, with voltages in excess of 100 kV. In this paper, we investigate sulnanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries in argon, with a needle radius <0.5 mum. The coaxial setup allows current and voltage measurements with temporal resolutions down to 80 ps. Voltages of 7.5 kV (which are doubled at the open gap before breakdown) produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip and the same amplitude, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 10(2) and 10(4) Pa. At lower pressure, a longer delay time (8 ns at 6 Pa) is observed. We expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.
+ Half-Bridge Inverter Using 4H-Sic Gate Turn-Off Thyristors
  Tipton, CW; Bayne, SB; Griffin, TE; Scozzie, CJ; Geil, B; Agarwal, AK; Richmond, J
Abstract:  This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm(2)) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/mus in these devices.
+ High-Speed Optical Diagnostic Of An Exploding Wire Fuse
  Giesselmann, M; Heeren, T; Neuber, A; Walter, J; Kristiansen, M
Abstract:  Explosive flux compression generators generate hundreds of kiloamperes and voltages of a few kilovolts. A power conditioning stage is required since typically voltages in the hundreds of kilovolts range are needed. Inductive energy storage systems with an opening switch provide the necessary voltage gain. In our application, the opening switch has been implemented as an exploding wire fuse. The voltage gain, and hence the performance of the system, is greatly dependent on the opening switch. We utilized high-speed optical imaging (up to 10(7) pictures/s) to assess the performance of the exploding wire fuse.
+ Inductive Switching Of 4H-Sic Gate Turn-Off Thyristors
  Bayne, SB; Tipton, CW; Griffin, T; Scozzie, CJ; Geil, B; Agarwal, AK; Richmond, J
Abstract:  The high-temperature operation of a silicon carbide gate turn-off thyristor is evaluated for use in inductively loaded switching circuits. Compared to purely resistive load elements, inductive loads subject the switching de-ice to higher internal power dissipation. The ability of silicon carbide components to operate at elevated temperatures and high power dissipations are important factors for their use in future power conversion/control systems. In this work, a maximum current density of 540 A/cm(2) at 600 V was switched at a frequency of 2 kHz and at several case temperatures up to 150degreesC. The turn-off and turn-on characteristics of the thyristor are discussed.
+ Monte Carlo Calculation Of Electron Drift Characteristics And Avalanche Noise In Bulk Inas
  Satyanadh, G; Joshi, RP; Abedin, N; Singh, U
Abstract:  Field dependent drift velocity results are presented for electron transport in bulk indium arsenide (InAs) material based on a Monte Carlo model, which includes an analytical treatment of band-to-band impact ionization. Avalanche multiplication and related excess noise factor (F) are computed as a function of device length and applied voltage. A decrease in F with increases in device length is obtained. The results suggest an inherent utility for InAs-based single-photon avalanche detectors, particularly around the 2 mum region of interest for atmospheric remote sensing applications. (C) 2002 American Institute of Physics.
+ Monte Carlo Calculations Of The Impulse Response In Bulk Inas For Low-Intensity Photon Avalanche Detection At Two Micron Wavelength
  Satyanadh, G; Joshi, R; Abedin, N
Abstract:  Monte Carlo based calculations of the impulse photoresponse in bulk InAs p-i-n structures at the 2.0 mum wavelength are reported. The predicted results look promising with effective turn-off times of less than 5 ps, These results are much better than the response reported for GaAs photodetectors at their bandedge.
+ Shock Wave Demagnetization Of Bafe12O19 Hard Ferrimagnetics
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  A study of the effect of shock waves on the phase state of a hard ferrimagnetic material has been performed. A plane shock wave was passed along the axis of a cylindrical BaFe12O19 hard ferrite magnet. The shock wave demagnetized the cylinder, reducing the magnetic flux. This change in magnetic flux generated an electromotive force (EMF) in a coil wound around the ferrite. The value of the EMF calculated on the assumption that the ferrite was completely demagnetized by the shock wave is in good agreement with the peak EMF value obtained experimentally. (C) 2002 American Institute of Physics.
+ Simulations Of Electroporation Dynamics And Shape Deformations In Biological Cells Subjected To High Voltage Pulses
  Joshi, RP; Hu, Q; Schoenbach, KH; Beebe, SJ
Abstract:  The temporal dynamics of electroporation of cells subjected to ultrashort voltage pulses are studied based on a coupled scheme involving the Laplace, Nernst-Plank, and Smoluchowski equations. It is shown that a finite time delay exists in pore formation, and leads to a transient overshoot of the transmembrane potential V-mem beyond 1.0 V. Pore resealing is shown to consist of an initial fast process, a 10(-4) second delay, followed by a much slower closing at a time constant of about 10(-1) s. This establishes a time window for effective killing by a second pulse. The results are amply supported by our experimental data for E.-coli cells, and the time constant also matches experiments. An electromechanical analysis for analyzing cell shape changes is also presented. Our calculations show that at large fields, the spherical cell geometry can be significantly modified, and even ellipsoidal forms would be inappropriate to describe the deformation. Values of surface forces obtained are in very good agreement with the 1-10-nN/m range reported for membrane rupture. It is also demonstrated that, at least for the smaller electric fields, both the cellular surface area and volume change roughly in a quadratic manner with electric field. Finally, it is shown that the bending moments are generally quite small and can be neglected for a simpler analysis.
+ Single-Shot, Repetitive, And Lifetime High-Voltage Testing Of Capacitors
  Shkuratov, SI; Talantsev, EF; Hatfield, LL; Dickens, JC; Kristiansen, M
Abstract:  Four different types of capacitors have been tested to determine the maximum usable high voltage. Ceramic, drop-dipped film, molded-mylar tubulars, and polyester/foil capacitors of different values and different nominal voltages were tested in four modes: the single-shot mode, the repetitive mode, the lifetime dc voltage mode, and the group mode. Experiments have shown that the breakdown voltage for all types of the capacitors tested is ten to seventeen times higher than the nominal voltage. The energy stored in the capacitors for a short time under overstress conditions is from 100 to 250 times higher-their normal engery. Data are given for the limitations for single capacitors, and for two, three, and four capacitors connected in parallel.
+ Subnanosecond Corona Inception In An Ultrawideband Environment
  Mankowski, J; Dickens, J; Kristiansen, M; Lehr, J; Prather, W; Gaudet, J
Abstract:  Corona discharges in ultrawideband radiating systems can have adverse effects on performance such as reflection, phase dispersion, and significant power losses. A test-bed has been assembled to experimentally observe corona created by voltage pulses similar to ultrawideband systems. The current work involves the attenuation of an incident pulse after propagation through a self-initiated corona and relative measurements of visible light emission from the photoionization produced during streamer development. Several gas dielectrics, including ambient air, N-2, H-2, and SF6, were tested.
+ The Conductivity Of A Longitudinal-Shock-Wave-Compressed Nd2Fe14B Hard Ferromagnetics
  Talantsev, EF; Shkuratov, SI; Dickens, JC; Kristiansen, M
Abstract:  The conductivity of Nd2Fe14B hard ferromagnetic subjected to compression by a longitudinal shock wave (the shock wave propagates along the magnetization vector M) with a pressure of 35 GPa is measured. The results of the experiments show that the conductivity of the longitudinal-shock-wave-compressed Nd2Fe14B is sigma(sw) = (2.83 +/- 0.24)x 10(2) (Omega cm)(-1), which is 22 times lower than the conductivity of Nd2Fe14B under normal conditions.
+ The Current Mode Of Pulsed Power Generation In A Moving Magnet System
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M; Hernandez, JC
Abstract:  Results of an experimental study of the generation of high-current pulses in a moving magnet system based on an open ferromagnetic circuit design are presented. The magnet was accelerated with the use of a light gas gun. Experimental data are given for the output high current pulses, output voltage, and power delivered in the load for different types of pulse-generating coils. The effect of various pulse-generating windings is given. It has been shown that the Nd2Fe14B hard ferromagnetic projectile (diameter 2.54 cm and height 1.9 cm) moving with a velocity of 320 m/s is capable to produce in the pulse-generating coil a current pulse with amplitude of 1.4 kA and a full-width at half-maximum 80 mus.
+ Theoretical Treatment Of Explosive-Driven Ferroelectric Generators
  Tkach, Y; Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M; Altgilbers, LL; Tracy, PT
Abstract:  As a part of the-New World Vistas Program, a series of ultra-compact explosive-driven ferroelectric generators (EDFEGs) has been designed, constructed, and tested by Texas Tech University providing well-documented EDFEG output parameters that were used to benchmark,a theoretical model of the EDFEG developed at the Institute of Electromagnetic Research. A description of the model for the EDFEG is presented along with a brief description of the EDFEG, the experimental setup, and test procedures that were used. A comparison of the experimental and calculated results shows them to be in good agreement.
+ Thermodynamic State Of The Magnetic Flux Compression Generator Volume
  Neuber, A; Holt, T; Dickens, JC; Kristiansen, M
Abstract:  The thermodynamic state of the gas trapped in the volume of helical magnetic flux compression generators was measured using optical emission spectroscopy and fast pressure probes. Three main stages of operation are discussed: 1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, as low as 2000 K; 2) the intermediate stage during 144 mus before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV, 3) the last few mus that reveal a highly compressed gas with temperatures of about 5000 K and pressures of about 1500 bar. Most experiments were conducted in air, initially at STP, some results are given for argon and sulfur hexafluoride initially at one atmosphere. Additionally, the thermodynamic state is linked to the electrical volume breakdown threshold via simple resistance measurements that were conducted in current-free flux compression generators.
+ Transverse Shock Wave Demagnetization Of Nd2Fe14B High-Energy Hard Ferromagnetics
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  The action of transverse shock waves (the shock wave propagates across the magnetization vector M) on the magnetic phase state of a Nd2Fe14B high-energy hard ferromagnetic was investigated experimentally. The design of the ferromagnetic sample, which was made as a hollow cylinder, has made it possible to dramatically reduce the amount of the explosive that initiates a transverse shock wave in Nd2Fe14B to 1.0 g (for Nd2Fe14B samples weighing 67.5 g). The results of the experiment have shown that the transverse shock wave propagating through Nd2Fe14B causes hard ferromagnetic-to-paramagnetic phase transformation terminating by practically complete demagnetization of Nd2Fe14B. Pulse generators based on the transverse shock wave demagnetization of hollow cylindrical Nd2Fe14B samples with diameter of 25.4 mm and length of 19.1 mm are capable of producing high-voltage pulses [peak voltage of 11.3 kV, full width at half maximum (FWHM) of 4.5 mus] and high-current pulses (peak current of 1.93 kA, FWHM of 100 mus, peak power of 27.0 kW). The effect of transverse shock wave demagnetization of high-energy hard ferromagnetic, Nd2Fe14B, was detected. (C) 2002 American Institute of Physics.
+ Ultracompact Explosive-Driven High-Current Source Of Primary Power Based On Shock Wave Demagnetization Of Nd2Fe14B Hard Ferromagnetics
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  A new type of explosive driven high-current pulsed source utilizing a shock wave demagnetization of a Nd2Fe14B hard ferromagnetic energy carrier was developed. The design of the ferromagnetic energy carrier, which was made a hollow cylinder, has made it possible to reduce dramatically to 1 g the amount of the explosive providing a complete demagnetization of Nd2Fe14B energy carrier of weight 64 g. The developed generator is capable of producing high-current [up to 1.9 kA, 100 mus full width at half maximum (FWHM)] and high-power pulses (up to 42 kW, 2.8 mus FWHM). (C) 2002 American Institute of Physics.
+ Vacuum Ultraviolet Luminous Efficiency And Plasma Ion Density In Alternating Current Plasma Display Panels
  Choi, EH; Ahn, JC; Moon, MW; Jung, Y; Choi, MC; Seo, Y; Cho, G; Uhm, HS; Tachibana, K; Whang, KW; Kristiansen, M
Abstract:  The correlation between the vacuum ultraviolet (VUV) luminous efficiency and the plasma ion density has been investigated in terms of the xenon mole fraction in the neon filling gas for alternating current plasma display panels. The VUV luminous efficiency and plasma ion density are found to have strong correlation and to be saturated at xenon mole fractions greater than 7% and gas pressures of 400 Torr. (C) 2002 American Institute of Physics.
Publication Year:  2001
+ Comparative Analysis Of Regression And Artificial Neural Network Models For Wind Turbine Power Curve Estimation
  Li, SH; Wunsch, DC; O'Hair, E; Giesselmann, MG
Abstract:  This paper examines and compares regression and artificial neural network models used for the estimation of wind turbine power curves. First, characteristics of wind turbine power generation are investigated. Then, models for turbine power curve estimation using both regression and neural network methods are presented and compared. The parameter estimates for the regression model and training of the neural network are completed with the windfarm data, and the performances of the two models are studied. The regression model is shown to be function dependent, and the neural network model obtains its power curve estimation through learning. The neural network model is found to possess better performance than the regression model for turbine power curve estimation tinder complicated influence factors.
+ Diverse Effects Of Inhibition Of 3-Hydroxy-3-Methylglutaryl-Coa Reductase On The Expression Of Vcam-1 And E-Selectin In Endothelial Cells
  Rasmussen, LM; Hansen, PR; Nabipour, MT; Olesen, P; Kristiansen, MT; Ledet, T
Abstract:  The expression of monocyte adhesion molecules, such as VCAM-1 (vascular cell adhesion molecule-1) and E-selectin, on the surface of the endothelium is an important step in the initiation and progression of atherosclerotic lesions. We hypothesized that the inhibition of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in endothelial cells could influence the expression of VCAM-1 and E-selectin. Using cultured human umbilical vein endothelial cells, we found that mevastatin (0.1-1 muM) significantly reduced the expression of VCAM-1 protein in cells activated by tumour necrosis factor-alpha (TNF-alpha) for 7 h. In contrast, TNF-alpha -induced E-selectin protein expression was augmented after mevastatin treatment. Mevastatin inhibited the mRNA expression of both VCAM-1 and E-selectin in TNF-alpha -stimulated endothelial cells. The activity of the transcription factor nuclear factor-kappaB, which is known to regulate the transcription of VCAM-1 and E-selectin, was significantly reduced after incubation with mevastatin. Analysis of the time-dependent variation in the TNF-a-induced expression of E-selectin, and estimation of the rate of surface disappearance of E-selectin together with measurement of the amounts of E-selectin molecules secreted, indicated that mevastatin inhibited the surface removal of E-selectin. This is compatible with the observed increase in E-selectin expression after statin treatment. All observed effects of mevastatin were reversed by mevalonate, the product of the HMG-CoA reductase reaction. In conclusion, inhibition of HMG-CoA reductase in endothelial cells attenuates VCAM-1 expression, but increases E-selectin expression, after cytokine induction. These diverse effects are associated with changes in the transcriptional regulation of the two adhesion molecule genes and modulation of the surface removal of E-selectin.
+ Electrical Behavior Of A Simple Helical Flux Compression Generator For Code Benchmarking
  Neuber, A; Dickens, J; Cornette, JB; Jamison, K; Parkinson, ER; Giesselmann, M; Worsey, P; Baird, J; Schmidt, M; Kristiansen, M
Abstract:  A variety of basic magnetic flux compression (MFC) generator geometries have been tested during the last three decades. Though size and operating regimes differ widely, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that is solely based on physical principles and manages without heuristic factors. A simple generator was designed to address flux and current losses of the helical generator. The generator's maximum current amplitude is given as a function of the seed current and the resulting seed-current spread is compared to the output of state-of-the-art computer models. Temporally resolved current and current time derivative signals are compared as well. The detailed generator geometry is introduced in order to facilitate future computer code bench marking or development. The impact of this research on the present understanding of magnetic flux losses in helical MFC generators is briefly discussed.
+ Iminosugars: Potential Inhibitors Of Liver Glycogen Phosphorylase
  Jakobsen, P; Lundbeck, JM; Kristiansen, M; Breinholt, J; Demuth, H; Pawlas, J; Candela, MPT; Andersen, B; Westergaard, N; Lundgren, K; Asano, N
Abstract:  The first synthesis of the single isomers (3R,4R,5R); (3S,4S,5S); (3R,4R,5S) and (3S,4S,5R) of 5-hydroxymethyl-piperidine-3,4-diol from Arecolin is reported, including the synthesis of a series of N-substituted derivatives of the (3R,4R,5R)-isomer (Isofagomine). The inhibitory effect of these isomers as well as of a series of N-substituted derivatives of the (3R,4R,SR)-isomer and selected hydroxypiperidine analogues on liver glycogen phosphorylase (GP) showed that the (3R,4R,5R) configuration was essential for obtaining an inhibitory effect at submicromolar concentration. The results also showed that all three hydroxy groups should be present and could not be substituted, nor were extra OH groups allowed if sub-micromolar inhibition should be obtained. Some inhibitory effect was retained for N-substituted derivatives of Isofagagomine; however, N-substitution always resulted in a loss of activity compared to the parent compound, IC50 values ranging from 1 to 100 muM were obtained for simple alkyl, arylalkyl and benzoylmethyl substituents. Furthermore, we found that it was not enough to assure inhibitory effect to have the (R,,R) configuration. Fagomine, the (2R,3R,4R)-2-hydroxymethylpiperidine-3,4-diol analogue, showed an IC50 value of 200 muM compared to 0.7 muM for Isofagomine. In addition, Isofagomine was able to prevent basal and glucagon stimulated glycogen degradation in cultured hepatocytes with IC50 values of 2-3 muM. (C) 2001 Elsevier Science Ltd. All rights reserved.
+ Modeling Of A Compulsator And Railgun System
  Giesselmann, M; Eccleshall, D
Abstract:  A PSpice (R) model for a 4-phase, 4-pole air core compulsator was created. With minor modifications it could also model the performance of other pulse alternators. In addition, models for a railgun and switching elements have been created. The models were used to evaluate the system performance of the compulsator and an attached railgun in various configurations with diode rectifiers and SCR's. The process of self-excitation of the machine was also studied. The paper is focusing on the basic theory behind the models as well as techniques for numerical implementation, In addition generic results using normalized parameters are given.
+ Mos-Gated Thyristors (Mcts) For Repetitive High Power Switching
  Bayne, SB; Portnoy, WM; Hefner, AR
Abstract:  Certain applications for pulse power require narrow, high current pulses for their implementation. This work was performed to determine if MOS controlled thyristors (MCTs) could be used for these applications. The MCTs were tested as discharge switches in a low inductance circuit delivering 1 mus pulses at currents between roughly 3 kA and 11kA, single shot and repetitively at 1, 10, and 50 Hz, Although up to 9000 switching events could be obtained, all the devices failed at some combination of current and repetition rate, Failure was attributed to termperature increases caused by average power dissipated in the thyristor during the switching sequence, A simulation was performed to confirm that the temperature rise was sufficient to account for failure. Considerable heat sinking, and perhaps a better thermal package, would be required before the MCT could be considered for pulse power applications.
+ Pharmacological Approaches To Inhibit Endogenous Glucose Production As A Means Of Anti-Diabetic Therapy
  McCormack, JG; Westergaard, N; Kristiansen, M; Brand, CL; Lau, J
Abstract:  The inappropriate overproduction of glucose by the liver is one of the key contributors to the hyperglycaemia of the diabetic state, and thus is a logical site of intervention for novel anti-diabetic approaches. Metformin is the only currently marketed anti-hyperglycaemic drug whose action is attributed largely to its having inhibitory effects on hepatic glucose production, but its molecular site and mechanism(s) of action remain unknown, whereas the liver acting PPAR alpha agonists have their effects primarily on lipid metabolism. This review therefore rather focuses on candidate molecular targets within the liver for anti-hyperglycaemic therapy, and describes potential rate-control ling receptors and enzymes within the glucose producing pathways (glycogenolysis and gluconeogenesis). Most focus is directed towards inhibitors of the enzymes glucose-6-phosphatase, fructose-1,6-bisphosphatase and glycogen phosphorylase, and towards glucagon receptor antagonists, as these appear to be the most advanced in preclinical and clinical development, although progress with other potential targets is also described. Evidence of the anti-diabetic potential of such agents from animal studies is presented, and the relative merits of each approach are reviewed and compared. It is likely that such agents will become important additions to the therapeutic approaches to combat diabetes.
+ Theory Of The Virtual Cathode Oscillator
  Jiang, WH; Kristiansen, M
Abstract:  A one-dimensional analytical theory is developed to describe the behavior of the virtual cathode oscillator. Theoretical predictions for the microwave frequency and efficiency have been obtained. The results suggested that the feedback of the microwave field to the virtual cathode is very important for the microwave efficiency which, under optimum conditions, can be as high as 30%-40%. In addition, the dependence of the microwave efficiency on the electron-beam energy spread and the electron lifetime was determined. The threshold value of the cavity Q, which is necessary to enhance the microwave field, is also estimated. (C) 2001 American Institute of Physics.
+ Transient And Steady State Simulations Of Internal Temperature Profiles In High-Power Semi-Insulating Gaas Photoconductive Switches
  Kayasit, P; Joshi, RP; Islam, NE; Schamiloglu, E; Gaudet, J
Abstract:  Simulations have been performed to determine the internal temperature profiles of high-power GaAs photoconductive switches in the presence of a current filament. No thermal instability is predicted below a power generation density level of about 1.3x10(14) W/m(3). This prediction is in keeping with recent experimental data on photoconductive semiconductor switch devices. It is shown that this power dissipation density threshold for stability exists under both dc and transient conditions. A simple model provides qualitative support for the power density threshold, and an explanation of the filamentary current radii that have been observed experimentally. (C) 2001 American Institute of Physics.
+ Using Neural Networks To Estimate Wind Turbine Power Generation
  Li, SH; Wunsch, DC; O'Hair, EA; Giesselmann, MG
Abstract:  This paper uses data collected at Central and South West Services Fort Davis wind farm to develop a neural network based prediction of power produced by each turbine. The power generated by electric wind turbines changes rapidly because of the continuous fluctuation of wind speed and direction. It is important for the power industry to have the capability to perform this prediction for diagnostic purposes-lower-than-expected wind power may be an early indicator of a need for maintenance. In this paper, characteristics of wind power generation are first evaluated in order to establish the relative importance for the neural network. A four input neural network is developed and its performance is shown to be superior to the single parameter traditional model approach.
Publication Year:  2000
+ A Review Of Short Pulse Generator Technology
  Mankowski, J; Kristiansen, M
Abstract:  Today's ultrafast, pulse generators are capable of producing high-voltage pulses, (>1 kV), with fast, leading-edge rise times, (< 1 ns), A review of generator implementation methods is presented that includes a detailed discussion of the various circuit designs and a list of commercially available high-voltage pulse generators. All of these generators are capable of rise times less than a few ns and voltages greater than several hundred volts. Finally, a brief description of the three primary switch types, reed, spark gap, and solid state is presented.
+ Bacterial Decontamination Of Liquids With Pulsed Electric Fields
  Schoenbach, KH; Joshi, RP; Stark, RH; Dobbs, FC; Beebe, SJ
Abstract:  The effect of pulsed electric fields on the viability of microorganisms, mainly bacteria, in liquids has been studied since the 1960's. Experimental results obtained over a large range of electrical and microbiological parameters, point towards an irreversible formation of pores in the cell membrane as mechanism for lysing. The model of membrane pore formation seems to fail only for ultrashort electrical pulses, where intracellular effects, and possibly resonant effects, might dominate. This paper presents an overview of the effect of pulsed electric fields on the viability of microorganisms in liquids. In particular, the lytic effect of variations in the electrical pulse parameters, such as pulse shape, amplitude, duration, and single shot vs. repetitive operation, is described. A major application of the pulsed electric field method is 'cold' bacterial decontamination of liquid food and drinking water. The energy consumption for complete bacterial decontamination is presently 100 to 400 kj/l. A possible reduction of the required energy by utilizing intracellular electric field interactions and resonance effects is discussed.
+ Calculations Of Hole Transport Characteristics In Bulk Gasb With Comparisons To Gaas
  Damayanthi, P; Joshi, RP; McAdoo, JA
Abstract:  Field dependent drift velocity results are presented for hole transport in bulk gallium antimonide material based on a Monte Carlo model which includes energy band warping. Transient drift velocities are demonstrated to be higher than for gallium arsenide. The steady-state characteristics are also shown to be superior. The material appears to have potential for high-speed photodetection. (C) 2000 American Institute of Physics. [S0021-8979(00)03914-1].
+ Experimental And Analytical Investigation Of A Pulsed Power Conditioning System For Magnetic Flux Compression Generators
  Giesselmann, M; Heeren, T; Kristiansen, E; Kim, JG; Dickens, JC; Kristiansen, M
Abstract:  The pulsed power conditioning system (PPCS) is one of the key enabling technologies for using the energy output of a magnetic flux compression generator (MFCG), This paper shows the results of comprehensive experimental studies of an inductive energy storage system using an exploding wire fuse. The effects of metal oxide varistors (MOVs) for use as pulse-shaping devices are also presented. The experimental results are complemented by a comprehensive evaluation and interpretation of the results using the tools available in the professional version of MathCAD.
+ High-Current And High-Voltage Pulsed Testing Of Resistors
  Shkuratov, SI; Kristiansen, M; Dickens, JC; Hatfield, LL; Horrocks, E
Abstract:  Three types of resistors have been tested to determine maximum usable power at pulsed high,voltage and pulsed high current. Experiments ere carried out using high-voltage cable generators, spark-gap generators, and thyratron drivers, Pulse durations were varied from 0.7 mus to 21 mus The pulse amplitudes were varied from 1 kV to 35 kV. The peak cm rent reached was 3 kA. Metal him, carbon film, and carbon composition resistors of four different rated powers (0.25 W, 0.5 W, 1 W, and 2 W) have been tested. Data are given for the limiting pulsed power and energy for each type of resistor in nanosecond and microsecond time ranges. The experimental investigation of the threshold loading of the resistors in the high-current pulsed mode and in the high-voltage pulsed mode has shown that the process of destruction of resistors has specific features for each mode. The mechanisms of failure and destruction of resistors under the action of high-voltage and high-current pulses are discussed.
+ Kinetic And Functional Characterization Of 1,4-Dideoxy-1,4-Imino-D-Arabinitol: A Potent Inhibitor Of Glycogen Phosphorylase With Anti-Hyperglyceamic Effect In Ob/Ob Mice
  Fosgerau, K; Westergaard, N; Quistorff, B; Grunnet, N; Kristiansen, M; Lundgren, K
Abstract:  The effects of 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) were investigated on preparations of glycogen phosphorylase (GP) and in C57BL6J (ob/ob) mice by C-13 NMR in vivo. Independent of the phosphorylation state or the mammalian species or tissue from which GP was derived, DAB inhibited GP with Ki-values of approximately 400 nM, The mode of inhibition was uncompetitive or noncompetitive, with respect to glycogen and Pi, respectively. The effects of glucose and caffeine on the inhibitory effect of DAB were investigated. Taken together, these data suggest that DAB defines a novel mechanism of action. Intraperitoneal treatment with DAB (a total of 105 mg/kg in seven doses) for 210 min inhibited glucagon-stimulated glycogenolysis in obese and lean mice. Thus, liver glycogen levels were 361 +/- 19 and 228 +/- 19 mu mol glucosyl units/g with DAB plus glucagon in lean and obese mice, respectively, compared to 115 +/- 24 and 37 +/- 8 mu mol glucosyl units/g liver with glucagon only. Moreover, with glucagon only end-point blood glucose levels were at 29 +/- 2 and 17.5 +/- 2 mM in obese and lean mice, respectively, compared to 17.5 +/- 1 and 12 +/- 1 mM with glucagon plus DAB, In conclusion, DAB is a novel and potent inhibitor of GP with an apparently distinct mechanism of action. Further, DAB inhibited the hepatic glycogen breakdown in vivo and displayed an accompanying anti-hyperglycemic effect, which was most pronounced in obese mice. The data suggest that inhibition of GP may offer a therapeutic principle in Type 2 diabetes. (C) 2000 Academic Press.
+ Microwave Magnetic Field Effects On High-Power Microwave Window Breakdown
  Hemmert, D; Neuber, AA; Dickens, J; Krompholz, H; Hatfield, LL; Kristiansen, M
Abstract:  Microwave window breakdown in vacuum is investigated for an idealized geometry, where a dielectric slab is located in the center of a rectangular waveguide,vith its normal parallel to the microwave direction of propagation, An S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side, Simple trajectory calculations of secondary electrons in an RF field show a significant forward motion of electrons parallel to the direction of microwave propagation. Electrons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are driven off the surface, because of the influence of the microwave magnetic field, In agreement with the standard model of dielectric surface flashover for de conditions (saturated avalanche and electron-induced outgassing), the corresponding change in the surface charge density is expected to be proportional to the applied breakdown threshold electric field parallel to the surface.
+ Optical Diagnostics On Helical Flux Compression Generators
  Neuber, AA; Dickens, JC; Krompholz, H; Schmidt, MFC; Baird, J; Worsey, PN; Kristiansen, M
Abstract:  Explosively driven magnetic flux compression (MFC) has been object of research for more than three decades, Actual interest in the basic physical picture of flux compression has been heightened by a newly started Department of Defense (DoD) Multi-University Research Initiative. The emphasis is on helical flux compression generators comprising a hollow cylindrical metal liner filled with high explosives and at least one helical coil surrounding the liner. After the application of a seed current, magnetic flux is trapped and high current is generated by moving, i.e., expanding, the liner explosively along the finding of the helical coil, Several key factors involved in the temporal development can be addresses by optical diagnostics. 1) The uniformity of liner expansion is captured by framing camera photography and supplemented by laser illuminated high spatial and temporal resolution imaging. Also, S-ray flash photography is insensitive to possible image blur by shockwaves coming from the exploding liner, 2) The thermodynamic state of the shocked gas is assessed by spatially and temporally resolved emission spectroscopy, 3) The moving liner-coil contact point is a possible source of high electric Losses and is preferentially monitored also by emission spectroscopy, Since optical access to the region between liner and coil is not always guaranteed, optical fibers can be used to extract light from the generator. The information so gained will give, together with detailed electrical diagnostics, more insight in the physical loss mechanisms involved in MFC.
+ Pulsed Power Generation Using Open And Closed Ferromagnetic Circuits
  Shkuratov, SI; Kristiansen, M; Dickens, JC; Hatfield, LL; Martin, R
Abstract:  Results are presented of an experimental study of the generation of high-voltage and high-current pulses in generators designed as open and closed ferromagnetic circuits. Experiments were carried out using a light gas gun system, The magnetic projectiles were composed of ferromagnetic disks having 1.27- and 2.54-cm diameters, It has been shown that with velocities of the magnetic projectiles of 200-380 m/s. the peak voltage of the pulses produced by the generators reach several tens of kilovolts, peak current reaches kiloampere, and the energy delivered at the load is a few Joules, Generating modules connected in series will make it possible to produce a high-energy pulse with a peak voltage of a felv hundred kilovolts. It has been shown that a closed ferromagnetic circuit generator is capable of generating not only single high-voltage pulses, but also repetitive oscillations. Data are given for the effects on the amplitude of high-voltage pulses caused by the length and velocity of the ferromagnetic projectiles and the design of the generating unit for both high-voltage and high-current modes of pulsed power generation.
+ Rapid Decontamination Of Large Surface Areas
  Farrar, LC; Haack, DP; McGrath, SF; Dickens, JC; O'Hair, EA; Fralick, JA
Abstract:  The effectiveness of the decontamination of biological agents (spores) on surfaces by two thermal plasma systems is reported here. Using existing systems, operating at nonoptimum conditions, a steam plasma decontaminated surfaces at a maximum speed of 1.4 mph and a nitrogen plasma decontaminated at a maximum of 2.4 mph.
+ The Role Of Outgassing In Surface Flashover Under Vacuum
  Neuber, AA; Butcher, M; Krompholz, H; Hatfield, LL; Kristiansen, M
Abstract:  Results of high-speed electrical and optical diagnostics are used as a basis to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flashover development. Dielectric unipolar surface flashover under vacuum is experimentally characterized buy a three-phase development, which covers a current range from 10(-4) A to 100 A. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification reaching currents in the Ampere level within typically 100 ns, The final phase is characterized by a fast current rise up to the impedance-limited current on the order of 100 A. The development during phase tno and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the sur face. This is supported by time-resolved spectroscopy that reveals the existence of excited atomic hydrogen and ionic carbon before the final phase. The feedback mechanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as outgassing rate and gas density buildup above the surface, are determined by fitting calculated results to experimental data. The significance of outgassing is also discussed with a view to microwave surface flashover.
Publication Year:  1999
+ Dielectric Surface Flashover In Vacuum At 100 K
  Neuber, A; Butcher, M; Hatfield, LL; Kristiansen, M; Krompholz, H
Abstract:  Cryogenic components in high power electrical systems and in power electronics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielectric test sample and electrodes in vacuum are cooled to <100 K and flashover is characterized using fast electrical and optical diagnostics. Three consecutive development stages for flashover in self-breakdown mode with a gap distance of 0.5 cm can be distinguished: (1) A fast current rise to mA amplitudes within similar to 2 ns, probably associated with field emission, followed by (2) a slow current rise to similar to 5 to 10 A amplitude with duration of 40 ns to 1 mu s , associated with secondary emission avalanche saturation, and (3) a transition to a rapid gaseous ionization above the sample caused by electron induced outgassing, leading to impedance-limited current amplitudes of less than or equal to 300 A. Phase (1) shows a higher final current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreasing function of breakdown voltage and only slightly dependent on temperature, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature.
+ Efficiency Enhancement Of A Coaxial Virtual Cathode Oscillator
  Jiang, WH; Dickens, J; Kristiansen, M
Abstract:  The microwave field intensity around the virtual cathode oscillator was enhanced by using a microwave reflector in the output waveguide. The experimental results show that the microwave output power strongly depends on the position and geometry of the microwave reflector. The maximum microwave efficiency obtained was twice as large as that without field enhancement by the microwave reflector.
+ Electron Mobility And Drift Velocity Calculations For Bulk Gasb Material
  Damayanthi, P; Joshi, RP; McAdoo, JA
Abstract:  Electron mobility and drift velocity simulation results are presented for bulk GaSb based on a many-valley, anisotropic Monte Carlo model. Our mobility results at 300 K are in good agreement with available experimental data. Values close to the room temperature GaAs mobility have been predicted. Transient drift velocities are demonstrated to be higher than for GaAs and the negative differential velocity regime is shown to occur at much lower electric fields of about 1 kV/cm. The material could be useful as a low voltage oscillator or in high speed photodetection. (C) 1999 American Institute of Physics. [S0021- 8979(99)00321-7].
+ High Power Microwave Generation By A Coaxial Virtual Cathode Oscillator
  Jiang, WH; Woolverton, K; Dickens, J; Kristiansen, M
Abstract:  A new type of virtual cathode oscillator, the coaxial vircator, was studied analytically and experimentally. A one-dimensional analytical model was used to describe the steady-state behavior of the election beam and the virtual cathode,from which the diode current, the space-charge limited current, the virtual cathode position, and the estimated oscillation frequency were obtained. The experiments mere carried out with typical electron-beam parameters of 500 kV, 40 kA, and 30 ns, where pulsed microwaves of 400 MW in peak power and 2 GHz in frequency have been obtained. The energy efficiency from the electron beam to microwaves was similar to 2%. This efficiency is expected to be improved by increasing the microwave. field strength around the vircator.
+ Imaging Of High-Power Microwave-Induced Surface Flashover
  Neuber, A; Hemmert, D; Krompholz, H; Hatfield, LL; Kristiansen, M
Abstract:  Using two gated intensified digital charge-coupled device cameras, one sensitive in the near infrared to ultraviolet region and one in the soft X-ray region, the temporal development of high-power microwave-induced surface flashover across a vacuum/dielectric interface has been imaged. The emission of X-ray radiation from the interface is caused by field emitted electrons accelerated in the high electromagnetic field impacting the solid. This generation of bremsstrahlung terminates at the moment of full flashover development that is indicated by the optical light emission. A rising plasma density above the dielectric surface due to electron induced outgassing triggers this behavior.
+ Initiation Of High Power Microwave Dielectric Interface Breakdown
  Neuber, A; Hemmert, D; Krompholz, H; Hatfield, L; Kristiansen, M
Abstract:  A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown. (C) 1999 American Institute of Physics. [S0021-8979(99)09315-9].
+ Simulation Studies Of Persistent Photoconductivity And Filamentary Conduction In Opposed Contact Semi-Insulating Gaas High Power Switches
  Joshi, RP; Kayasit, P; Islam, N; Schamiloglu, E; Fleddermann, CB; Schoenberg, J
Abstract:  A self-consistent, two-dimensional, time-dependent, drift-diffusion model is developed to simulate the response of high power photoconductive switches. Effects of spatial inhomogeneities associated with the contact barrier potential are incorporated and shown to foster filamentation. Results of the dark current match the available experiment data. Persistent photoconductivity is shown to arise at a high bias even under the conditions of spatial uniformity. Filamentary currents require an inherent spatial inhomogeneity, and are more likely to occur for low optical excitation. Under strong uniform illumination, the spatial nonuniformities were quenched as a result of a polarization-induced collapse in the internal fields. However, strong electric fields resulting at the contacts create a bipolar plasma, and hence, a virtual double injection. (C) 1999 American Institute of Physics. [S0021-8979(99)05119-1].
Publication Year:  1998
+ Effects Of Transverse Doping Variations On The Transient Response Of Silicon Avalanche Shaper Devices
  Jalali, H; Joshi, RP; Gaudet, JA
Abstract:  Two-dimensional (2-D) drift-diffusion simulations were performed to study the transient response of silicon avalanche shaper (SAS) devices that are used in high-power switching and pulse sharpening applications, The role of transverse doping variations on the transient device response has been studied. Our results clearly reveal a potential for filamentary current conduction. The filamentation, however, is shown to be strongly dependent on the transverse doping characteristics, and hence in principle, could be tailored.
+ High Voltage Subnanosecond Breakdown
  Mankowski, J; Dickens, J; Kristiansen, M
Abstract:  Present-day ultra-wideband radiation sources produce Megavolt pulses at hundreds of picosecond (ps) risetimes, Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or nonexistent. In support of the design of these ultra-wideband sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and high-pressure gases. These include air, N-2, H-2, SF6, and transformer oil. Gap voltages attained were over 700 kV and gas pressures were over 150 atm (15 MPa), Breakdown times achieved were on the order of 600 ps. Electric field strengths observed for given breakdown times were higher than predicted by other investigators. An empirical fit is presented for the data obtained.
+ Window Breakdown Caused By High-Power Microwaves
  Neuber, A; Dickens, J; Hemmert, D; Krompholz, H; Hatfield, LL; Kristiansen, M
Abstract:  Physical mechanisms leading to microwave breakdown on windows are investigated for power levels on the order of 100 MW at 2.85 GHz, The test stand uses a 3-MW magnetron coupled to an S-hand traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10(-6) Pa, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries, Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and x-ray emission. All quantities are recorded with 0.2-1-ns resolution. In addition, a framing camera with gating times of 5 ns is used. The breakdown processes for the case with a purely tangential electric field is similar to de flashover across insulators, and similar methods to increase the flashover field are expected to be applicable.
Publication Year:  1997
+ Arc Erosion Behavior Of Cu-15%Nb And Cu-15%Cr In Situ Composites
  Liu, P; Bahadur, S; Verhoeven, JD; Gibson, ED; Kristiansen, M; Donaldson, A
Abstract:  The are erosion behavior of Cu-15%Nb and Cu-15%Cr in situ composites was studied for both low-energy make-and-break contact and a high-energy stationary arcing gap configuration. For low-energy make-and-break contacts, a computerized test set-up was developed, while the high-energy pulsed power stationary arcing tests were performed in the Mark VI facility at Texas Tech University, Lubbock, TX. The study dealt with variation in contact resistance for make-and-break contacts, are erosion at both energy levels, and materials response to are erosion. The surface films formed in the make-and-break operation were analyzed by an X-ray diffraction technique, and the eroded surfaces and are erosion mechanisms were studied by scanning electron microscopy. It was concluded that in low-energy contacts, oxidation was the major cause of deterioration of electrical contacts, while melting was the major failure mode in high-energy contacts. The contact resistance of Cu-15%Nb was much lower than that of Cu-15%Cr. The are erosion resistance of Cu-15%Nb and Cu-15%Cr was higher than that of the commercially used Cu-W composite in stationary are erosion tests. (C) 1997 Elsevier Science S.A.
+ Dielectric Surface Flashover In A Simulated Low Earth Orbit Environment
  Hegeler, F; Krompholz, HG; Hatfield, LL; Kristiansen, M
Abstract:  Surface flashover on insulators under UV irradiation or with a plasma background was investigated with high-speed electrical and optical sensors in order to clarify differences in the breakdown development compared to the pure vacuum case, Results with a plasma background show a more rapid development in the breakdown initiation compared to measurements in vacuum with no plasma, With a magnetic shielding technique using permanent magnets, the duration of an applied voltage pulse can be increased by a factor of 2-3 without causing flashover. UV illumination on the electrodes decreases the flashover voltage (for the de case) or the voltage pulse duration without breakdown (for the pulsed case), whereas UV illumination on the dielectric surface increases the flashover potential.
+ Dynamic Models Of 3-Phase Alternators Including Saliency Effects Using Pspice For Windows
  Giesselmann, M
Abstract:  This paper presents the dynamic model of a 3-phase synchronous machine using the graphical user interface provided by the PSpice program environment. The machine model includes the effects of the damper winding and salient poles. The paper presents results of the dynamic behavior of a salient pole synchronous machine including voltage, current, torque, and power traces. The model is organized in a hierarchical structure and includes custom symbols for the machine several functional modules like reference frame transformation modules. The custom symbols have been created within the Schematics editor of the PSpice program group. Details of the implementation as well as the theoretical equations are presented. The model that is shown in this paper is working on the free evaluation version of the PSpice software, Release 6.2a.
+ Electrical And Optical Characterization Of Crystalline Silicon Porous Silicon Heterojunctions
  Palsule, C; Liu, S; Gangopadhyay, S; Holtz, M; Lamp, D; Kristiansen, M
Abstract:  We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm(2). From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.
Publication Year:  1995
+ Hot-Electron And Thermal Effects On The Dynamic Characteristics Of Single-Transit Sic Impact-Ionization Avalanche Transit-Time Diodes
  JOSHI, RP; PATHAK, S; MCADOO, JA
Abstract:  We report our simulation results for the dynamic characteristics of single-transit SiC SiC impact-ionization avalanche transit-time (IMPATT) diodes. An iterative Monte Carlo-Crank-Nicholson technique has been used to solve the coupled electron transport-heat conduction problem. This procedure allows for accurate computation of both the device current as a function of temperature, time, and position, and the internal heat generation. The technique is quite general, has not been used before, and can be applied for the analysis of any power device. Our results show that the internal power generation profile within the SiC IMPATT device can have a very nonuniform axial distribution, The internal heating is seen to significantly degrade the device efficiency and optimum operating frequency. With thermal effects the output current values are lower and the transit time is increased. (C) 1995 American Institute of Physics.
+ Monte-Carlo Calculations Of The Temperature-Dependent And Field-Dependent Electron-Transport Parameters For 4H-Sic
  JOSHI, RP
Abstract:  Monte Carlo simulation results for the field- and temperature-dependent electronic mobilities, drift velocities, and diffusion coefficients in 4H-SiC are presented. The calculations include crystal anisotropy, and values are obtained for field orientations both parallel and transverse to the c axis of the hexagonal structures. The simulations are based on electron effective mass data that has only recently become available. Our theoretical predictions of the electron mobilities and their anisotropy ratios compare very well with available experimental data at 300 K. A room-temperature velocity of 2.7x10(7) cm/s was obtained in 4H-SiC for transport parallel to the c axis. This value is found to be larger than both 6H and 3C material. Finally, our calculations for the longitudinal and transverse diffusion coefficients at 300 K indicate that both have appreciable field dependences and exhibit a ''soft'' threshold. (C) 1995 American Institute of Physics.
+ Review Of The Mechanisms Of Electrode And Insulator Erosion And Degradation In High-Current Arc Environments
  ENGEL, TG; WESTER, SL; KRISTIANSEN, M; DONALDSON, AL
Abstract:  The mechanisms responsible for the erosion and degradation of electrode and insulator materials in high current are discharge environments are reviewed, The review represents the experimental results obtained from several studies and are from investigations into materials performance in high current closing switches such as spark gaps and surface discharge switches, Parameters of interest that affect electrode and insulator material erosion and degradation include peak current, charge transfer, mass erosion, the surface voltage holdoff recovery, and are velocity, Other parameters that have been shown to affect materials performance include the synergistics produced by certain electrode, insulator, and gas combinations, Models that describe the erosion and degradation processes are presented and compared to the experimental results,
Publication Year:  1994
+ Characteristics Of Photostrictively Produced Optoacoustic Pulses
  DHARAMSI, AN; JOSHI, RP
Abstract:  The characteristics of optoacoustic pulses produced by photostrictive means are investigated. Since such acoustic disturbances have a nonthermal origin, and can be as short as the inverse of the highest frequency phonons that couple the photostrictive deformation to the lattice, ultrashort (subpicosecond) optoacoustic pulses can be produced. The differences are discussed between cold photostrictive optoacoustic pulses and those produced by thermal means in terms of what may be called a rise time reduction factor that characterizes the relative pulsewidths of the thermal to nonthermal pulses; a criterion for picking materials that would show large rise time reduction factors is given. The strength of a photostrictive optoacoustic pulse, in the high-intensity regime, is calculated. Since the nonthermal nature of the photostrictive optoacoustic pulse is important in the production of ultrashort optoacoustic pulses, thermalization mechanisms subsequent to laser irradiation of the photostrictive material are examined. It is shown that there is a trade-off between the strength of the compression and the initial temperature that it exhibits, and an estimate of the expected initial temperature of the compression is given. The characteristics of photostrictively generated optoacoustic pulses and those produced by other means, including nonthermal electrostrictive generation, are compared.
+ Electron-Beam Controlled Switching Using Quartz And Polycrystalline Zns
  JIANG, WH; ZINSMEYER, K; LESS, M; SCHOENBACH, KH; KRISTIANSEN, M
Abstract:  Results of electron-beam controlled switching experiments with switch samples of quartz crystal and polycrystalline zinc selenide (ZnSe) are presented. For switch samples of both materials, drastic reductions of the switch resistance were induced by the electron beam. The quartz sample showed very fast temporal response (less than 1 ns) with potential applicability for current control. The ZnSe samples, on the other hand, showed longer current transients (on the order of 10 ns) with exponential development of the switch resistance after the electron beam pulse.
+ Impact Of Field-Dependent Electronic Trapping Across Coulomb Repulsive Potentials On Low-Frequency Charge Oscillations
  JOSHI, RP; SCHOENBACH, KH; RAHA, PK
Abstract:  We have performed Monte Carlo simulations to obtain the field dependence of electronic trapping across repulsive potentials in GaAs. Such repulsive centers are associated with deep level impurities having multiply charged states. Our results reveal a field-dependent maxima in the electronic capture coefficient, and the overall shape is seen to depend on the background electron density due to the effects of screening. Based on the Monte Carlo calculations, we have examined the stability of compensated semiconductors containing such repulsive centers. Our analysis indicates a potential for low frequency charge oscillations which is in keeping with available experimental data.
Publication Year:  1993
+ Insulator And Electrode Mass Erosion And Surface Voltage Holdoff Recovery For Transient, High-Current Surface Discharges
  ENGEL, TG; DICKENS, JC; KRISTIANSEN, M
Abstract:  Several polymeric insulator materials commonly used as sidewall insulators in electromagnetic accelerators are subjected to repetitive (from approximately 0.1 to 1 discharges per second), high current (from approximately 100 to 300 kA peak or approximately 100 to 300 kA/cm), transient (approximately 20 mus pulse width) surface discharges. The insulator materials tested include the thermosetting polymers G-9, G-10, and G-11 (i.e, fiberglass reinforced melamine and epoxy) and the thermoplastic polymers Lexan(TM) (i.e, polycarbonate) and Delrin(TM) (i.e, polyacetyl). Empirical scaling relationships are given that relate the total amount of insulator and electrode (i.e, molybdenum) mass erosion versus the total amount of arc energy transferred. Scaling relationships are alw given that relate the ''lifetime'' of the given polymer as a function of the initial discharge current. The ''lifetime'' of an insulator material is defined as the number of discharges required to reduce the initial surface holdoff voltage to its half-power level (i.e.,V(initial)/square-root 2) for three consecutive discharges and is a useful parameter when specifying insulator materials to be used in high power switching devices.
+ Status-Report On Project Hercules
  LOREE, D; GIESSELMANN, M; KRISTIANSEN, M; LARSON, D
Abstract:  Project Hercules is a project to improve ignitron switches which will then be used on the upgrade of Lawrence Livermore's Nova Laser for their ICF program. The goals of Hercules, which stands for High Energy Research Concerning the Ultimate Lifetime of Experimental Switches, are to lifetime test (up to 10,000 shots) prototype ignitrons or other switches with the required Nova current and coulomb parameters (300 kA, 200 C), recommend design changes, and retest the second generation switches. This report describes the design and construction of the test circuit and necessary diagnostics. The details of the design and construction of a 0.5 MJ electrolytic capacitor bank and a semi-automatic diagnostic/ control system are described. The required test run data include peak current and corresponding tube voltage for every shot, entire current and voltage waveforms every few shots, and ignitor resistance values every few shots. Additionally, the conversion of a 120 kW, 12 kV constant voltage supply to an 8 A constant current supply with the use of six SCRs and a commercial control board will be described. The final results of this project will be lifetime data at high current and high coulomb for and improvements on some of the best of the new generation of pulsed power switches.
+ Studies Of Electron-Beam Penetration And Free-Carrier Generation In Diamond Films
  JOSHI, RP; SCHOENBACH, KH; MOLINA, C; HOFER, WW
Abstract:  Experimental observations of the energy-dependent electron-beam penetration in type II-A natural diamond are reported. The experimental data are compared with results obtained from numerical Monte Carlo simulations, and the results are in very good agreement. The results also reveal that a threshold energy of about 125 keV is necessary for complete penetration for a 35 mum sample. It is found that over the 30-180 keV range, the energy dependence of the penetration depth and total path length exhibits a power-law relation. Monte Carlo simulations have also been performed to investigate the excess carrier-generation profiles within diamond for a set of incident e-beam energy distributions. The simulation results demonstrate the feasibility of tailoring the internal source function, and hence influencing the diffusion currents, the internal electric fields, and charge injection through the contacts.
Publication Year:  1992
+ Magnetic-Field Effects On Vacuum Insulator Flashover
  LEHR, M; KORZEKWA, R; KROMPHOLZ, H; KRISTIANSEN, M
Abstract:  The influence of magnetic fields (both dc and pulsed) on dielectric surface breakdown in vacuum and simulated low-earth-orbit conditions has been investigated using pulsed test voltages. Predictions from the saturated secondary electron emission avalanche breakdown model and the experimental results both show magnetic insulation effects (i.e., an increase in flashover voltage) at magnetic-field amplitudes as low as 0.1 T. The most favorable configuration for magnetic insulation is with the magnetic field oriented parallel to the insulator surface and perpendicular to the electric field. An increase in flashover voltage with increasing magnetic field is seen when the vector E x B points away from the surface, while a decrease followed by an increase in flashover voltage is seen for E x B into the surface. The magnitude of the insulation effect depends on the dielectric material, ambient pressure, surface roughness, and the presence of background plasma. Predictions from single-particle computer simulations of the secondary electron avalanche process, using nonuniform fields, point to the importance of conditions at the cathode in producing magnetic insulation effects. It was found that it is sufficient to apply the magnetic field in the cathode region only, and that significant magnetic insulation effects can be observed using small, lightweight permanent magnets. An applied magnetic field will also increase the flashover voltage in a low-density (n(e) almost-equal-to 10(4) cm-3) plasma environment. The dependence of the flashover voltage on electrode separation (gap distance) is observed to remain sublinear with the application of an insulating magnetic field. Prebreakdown luminance measurements are presented which further support the saturated secondary electron emission avalanche model.
+ Studies Of High-Field Conduction In Diamond For Electron-Beam Controlled Switching
  JOSHI, RP; KENNEDY, MK; SCHOENBACH, KH; HOFER, WW
Abstract:  Experimental studies on a vertical metal-diamond-silicon switch structure have been conducted for potential pulsed power applications. Both the dc current-voltage characteristics and the transient switching response have been measured for a range of voltages. With a 1 mum diamond film, the switch has been seen to withstand electric fields up to 1.8 MV/cm. Our results show a polarity dependence which can be associated with current injection at the asymmetric contacts. Polarity effects were also observed in the presence of e-beam excitation, and arise due to nonuniform carrier generation near the diamond-silicon interface. Our switching transients were seen to follow the shape of the e-beam for a negative bias at the silicon substrate. For positive voltage values exceeding about 80 V however, the switch is seen to go into a persistent-photocurrent mode. This effect is a result of free carrier trapping within diamond and is enhanced by the double injection process.
Publication Year:  1991
+ Estimating The Erosion And Degradation Performance Of Ceramic And Polymeric Insulator Materials In High-Current Arc Environments
  ENGEL, TG; KRISTIANSEN, M; OHAIR, E; MARX, JN
Abstract:  The rates of erosion and voltage holdoff degradation are critical parameters when selecting insulator materials that will be used in pulsed power devices such as spark gaps, surface discharge switches, and electromagnetic launchers. This investigation is concerned with modeling the erosion and holdoff degradation performance of various commercially available polymeric and ceramic insulators. The insulators are tested on a surface discharge switch at approximately 300 kA in atmospheric air. Test diagnostics include the surface voltage holdoff recovery and the eroded mass loss of the insulator and electrode materials used. The ceramic materials which were tested include several types of aluminum and magnesium silicates, several alumina and zirconia composities, and aluminum and silicon nitride. The polymeric insulators include polyvinyl chloride, low and high molecular weight polyethylene, polytetrafluoroethylene, polyamide, acetyl, polyamide-imide, and several types of glass-reinforced epoxies, melamines, and phenolics. The test results indicate that the holdoff degradation resistance and erosion rates can be qualitatively predicated by the use of merit figures which are based upon the thermo-chemical properties of the insulator. The results also show that the holdoff degradation and erosion rates can be improved for some thermoset polymers by a suitable choice of electrode material and/or by the U.V. stabilization of the insulator.
+ Expansion Of Hydrogen Arcs Driven By Oscillating Currents
  ENGEL, TG; KRISTIANSEN, M; KROMPHOLZ, H
Abstract:  Calculations of the arc channel radius using the arc current as the independent variable are presented and discussed. Previously reported experimental results for the expansion of approximately 300 to approximately 3000 A discharges in 460-torr hydrogen are compared with the theoretical radius calculations presented here. Errors introduced in the calculated channel radius are investigated by first assuming the channel conductivity to vary according to the Spitzer conductivity, and then assuming the channel conductivity to be constant and comparing the numerical results of these two methods with the experimentally measured values of channel radius. It is shown that according to Hugoniot adiabatics, the apparent relationship between the arc channel pressure and the directed energy of the shock front is given by the pressure-velocity relationship of the gas leaving the shock front instead of the more commonly used pressure-velocity relationship for the gas entering the shock front. This result is a direct consequence of the channel boundary being defined by the shock front. The scaling relationship for the channel conductivity as a function of the circuit parameters is also reported.
+ Picosecond Time-Resolved Photoluminescence Characterization Of A-Sic-H Films Prepared By Electron-Cyclotron Resonance Plasma
  GANGOPADHYAY, S; PLEIL, M; BORST, W; YOUNG, C; KRISTIANSEN, M
Abstract:  Microwave excited electron cyclotron resonance (ECR) plasmas were used to produce five a-SiC:H films of varying relative carbon content. This was achieved by adjusting the relative hydrogen gas/organic liquid source flow rates and resulted in optical gaps between 2.3eV and 3.3eV. A time-correlated single photon counting technique incorporating a picosecond laser was used to acquire the fast photoluminescence decay emitted at several wavelengths for each sample. A lifetime distribution analysis of the photoluminescence decay reveals the presence of up to four distinct components. These resolved decay times range from 80ps to over 20ns. Each component decay has its own emission spectrum. The blue component has the fastest and the red the slowest decay. We present the variation of photoluminescence lifetime distributions with deposition parameters. We also present continuous wave (cw) and absorption photoluminescence spectra.
+ Plasma Diagnostics For High-Power Ignitron Development
  LOREE, DL; GIESSELMANN, M; KRISTIANSEN, M; SHULSKI, A
Abstract:  The development of high power ignitrons with peak current ratings of up to 1000 kA and simultaneous charge transfer rates of 250-500 C is currently under way in a joint effort between Texas Tech University (TTU), Lawrence Livermore National Laboratory (LLNL) and industry. Research at TTU is concentrated on plasma diagnostics, novel anode designs, electrode placements and trigger schemes. 1,2,3 Electrical measurements as well as optical plasma studies such as high speed framing photography, Mach-Zehnder interferometry and spectroscopy have been performed. The cooperative efforts have lead to the development of a new commercial tube (Richardson Electronics NL-9000). This paper describes plasma diagnostics performed on a demountable ignitron (DIG) which provides optical access to the discharge plasma through four viewports, two of which are on opposite sides on a common optical axis. The latter pair of viewports was used to perform plasma density studies using a Mach-Zehnder interferometer. Time resolved recordings of the interference patterns, either visually or electronically, during changes in plasma behavior (such as current conduction or plasma heating) yield time resolved information about the particle density This technique was applied to the demountable ignitron during high current discharges. The light source was a 2W CW argon laser which was pulsed using a ferroelectric liquid crystal light valve. The resulting fringe patterns were recorded with a mechanical high speed camera. In the paper all experimental details, results and a theoretical evaluation are given. In addition, high speed framing photography was used to study the influence on electrode design and placement on the discharge plasma.
+ Recent Advances In High-Power Ignitron Development
  LOREE, DL; GIESSELMANN, MG; KRISTIANSEN, M; SHULSKI, AP; KIHARA, R
Abstract:  The development of high-power ignitrons with peak current ratings of up to 1000 kA and simultaneous charge transfer rates of 250-500 C is currently under way in a joint effort between Texas Tech University (TTU), Lawrence Livermore National Laboratory (LLNL), and industry. Two industrial manufacturers, Richardson Electronics, US, and English Electric Valve, UK, have participated with TTU and LLNL in three workshops to advance the state of the art in high-power ignitrons. Less than three years after the start of the program, the cooperative efforts have led to the development of a new commercial tube (Richardson Electronics NL-9000). High power testing of prototypes of this tube and other unique ignitrons was done by Kihara at LLNL. Research at TTU is concentrated on plasma diagnostics, novel anode designs, electrode placements, and trigger schemes. Electrical measurements as well as optical and microwave plasma studies, such as high-speed framing photography, Mach-Zehnder and microwave interferometry, and spectroscopy have been performed. This paper describes the advances made in high-power ignitron switching capabilities in a comparison study between conventional Size D and Size E tubes, demountable experimental tubes and the new NL-9000 (Richardson Electronics). The paper shows the differences in tube design, and the associated peak current and charge transfer capabilities and lifetime expectancies. The critical design criteria are the anode shape and placement in order to control the plasma and prevent prefires. Tube failure modes and recent studies on alternate ignitor schemes are presented. In addition, results of plasma diagnostics performed on a demountable ignitron with optical access to the discharge plasma are shown. Time resolved images of the interference patterns from an optical Mach-Zehnder interferometer were recorded with a mechanical high-speed camera and evaluated with the help of image processing.
+ Surface-Discharge Switch Design - The Critical Factor
  ENGEL, TG; KRISTIANSEN, M; BAKER, MC; HATFIELD, LL
Abstract:  Many pulsed modulator applications require high-power, low-impedance closing switches. While the surface-discharge switch (SDS) is easily integrated into most systems (e.g., using a parallel-plate or coaxial geometry) and can easily be made into a low-impedance switch (e.g., operating in the multi-channel mode) most designers prefer and use other types of switches (e.g., spark gaps, thyratrons, ignitrons, etc.). This is because the SDS suffers from poor voltage holdoff recovery (caused by decomposition of the switching dielectric) and from dielectric punch-through (caused by dielectric erosion). Thus the selection of the switching dielectric is the critical factor which must be considered by the designer if the SDS is to have a long and trouble-free lifetime. This paper reports which dielectric properties are critical to designing a long-life SDS. Theory is correlated with experiment by evaluating the performance of a large group of polymeric and ceramic dielectrics. These dielectrics are tested in a single-channel, self-commutating SDS operating at approximately 35 kV and approximately 300 kA (oscillatory discharge) with a pulse length of approximately 20-mu-s (1/4 period approximately 2-mu-s). The performance of a dielectric is characterized by its shot-to-shot breakdown voltage and by its mass erosion. Theoretically, the voltage holdoff degradation resistance (HDR) and the arc melting/erosion resistance (AMR) of a dielectric can be qualitatively predicted from its formativity and its impulsivity, respectively. The formativity and impulsivity are figures of merit calculated from the known thermophysical properties of the dielectric. The effects produced in dielectric performance by choice of electrode material (e.g., molybdenum, graphite, and copper-tungsten) and discharge repetition rate are also discussed.
+ The Design And Calibration Of A Very Fast Current Probe For Short Pulse Measurements
  CALICO, SE; CRAWFORD, MT; KRISTIANSEN, M; KROMPHOLZ, H
Abstract:  The design, construction, and calibration of a probe to measure fast rise, short pulse currents is described. This probe is a 60-degrees section of a Rogowski coil that is designed to behave as a slow wave structure. The device described has nanosecond response and the output is proportional to the excitation current for approximately 70 ns.
+ The Influence Of Magnetic-Fields On Dielectric Surface Flashover
  KORZEKWA, R; LEHR, FM; KROMPHOLZ, HG; KRISTIANSEN, M
Abstract:  The influence of low-amplitude magnetic fields, in a variety of configurations, on pulsed dielectric surface flashover has been investigated. These variations include dc magnetic fields; pulsed magnetic fields simulating conditions for magnetic self-insulation; and different environments (vacuum, ambient gas, plasma), geometries, dielectric materials, and orientations of the magnetic field. For field amplitudes of 0.3 T, typically a doubling of the flashover voltage is observed, if the E x B drift is away from the surface. For flashover in vacuum, it is sufficient to place permanent magnets in the cathode vicinity to increase the flashover voltage. The observations are consistent with the saturated surface secondary avalanche model and electron-induced gas desorption. The pulse shape of light emission during the prebreakdown phase depends on the orientation and amplitude of the magnetic field and shows the electron trajectories above the surface are altered by magnetic fields.
Publication Year:  1990
+ On The Road To Tomsk
  KRISTIANSEN, M; GUENTHER, AH; THOMPSON, JE
Abstract: 
+ Photochemistry Of 1,3-Dimethyluracil - A Novel Photochemical-Reaction Leading To 3,5-Dialkoxycarbonyl-1,2-Dihydropyridines And 3,5-Dialkoxycarbonyl-1,4-Dihydropyridines
  KRISTIANSEN, M; ERIKSEN, J; JORGENSEN, KA
Abstract: 
Publication Year:  1989
+ Design Options For Optically Restructuring Damaged Image Detectors
  IGEL, EA; KRISTIANSEN, M
Abstract: 
+ Electrode Erosion From High-Current Moving Arcs
  LEHR, FM; KRISTIANSEN, M
Abstract: 
+ Inhibiting Surface Flashover For Space Conditions Using Magnetic-Fields
  KORZEKWA, R; LEHR, FM; KROMPHOLZ, HG; KRISTIANSEN, M
Abstract: 
+ Methods Of Increasing The Surface Flashover Potential In Vacuum
  HATFIELD, LL; BOERWINKLE, ER; LEIKER, GR; KROMPHOLZ, H; KORZEKWA, R; LEHR, M; KRISTIANSEN, M
Abstract: 
+ State-Of-The-Art Insulator And Electrode Materials For Use In High-Current, High-Energy Switching
  DONALDSON, AL; ENGEL, TG; KRISTIANSEN, M
Abstract: 
+ The Performance Of New, Redesigned Ignitron Tubes In Axial Magnetic-Fields
  ADKINS, DL; GIESSELMANN, M; KRISTIANSEN, M
Abstract: 
+ The Pulsed Discharge Arc Resistance And Its Functional-Behavior
  ENGEL, TG; DONALDSON, AL; KRISTIANSEN, M
Abstract: 
+ Warm Plasma Dispersion-Relation Of The Fast Alfven-Wave For Asymmetrical Heating Current Drive
  GAHL, JM; ISHIHARA, O; HAGLER, MO; KRISTIANSEN, M
Abstract: 
Publication Year:  1988
+ A Treatment Which Improves Surface Withstand Voltage In Vacuum
  HATFIELD, LL; LEIKER, GR; KRISTIANSEN, M; COLMENARES, C; HOFER, WW; DICAPUA, MS
Abstract: 
+ Analytical And Experimental Investigations Of Reed Contact Bouncing
  MIEDZINSKI, B; KRISTIANSEN, M
Abstract: 
+ Co2-Laser-Enhanced Electron-Attachment In Externally Sustained Diffuse Gas-Discharges Containing Vinyl-Chloride
  SCHAEFER, G; GIESSELMANN, M; PASHAIE, B; KRISTIANSEN, M
Abstract: 
+ Current-Voltage Characteristics Of A High-Current Pulsed Discharge In Air
  AKIYAMA, H; KRISTIANSEN, M; KROMPHOLZ, H; MAAS, B
Abstract: 
+ Optimization Of Gas-Mixtures For Electron-Beam-Controlled Diffuse Discharge Opening Switches
  KORZEKWA, R; SCHAEFER, G; KRISTIANSEN, M
Abstract: 
Publication Year:  1987
+ Fast Alfven-Wave Propagation In A Deuterium-Hydrogen Tokamak Plasma
  AKIYAMA, H; WONG, KL; GAHL, J; KRISTIANSEN, M; HAGLER, M
Abstract: 
+ Numerical-Studies Of Minority Ion Heating Current Drive
  WONG, KL; ISHIHARA, O; GAHL, J; HAGLER, M; KRISTIANSEN, M
Abstract: 
+ Observations Of Local Cavity Resonances Of Fast Alfven Waves
  WONG, KL; HAGLER, M; KRISTIANSEN, M; ISHIHARA, O; AKIYAMA, H
Abstract: 
Publication Year:  1986
+ Analytical Studies Of An Ablation Mass Driver System
  IKUTA, K; KRISTIANSEN, M; ROSE, MF
Abstract: 
+ Charging Of Insulators In A Surface Discharge Switch
  CURRY, RD; KRISTIANSEN, M; AGARWAL, VK; HATFIELD, LL; LEIKER, GR
Abstract: 
+ Conical Liner Implosion As A Projectile Injector For Mass Drivers
  IKUTA, K; KRISTIANSEN, M
Abstract: 
+ Electrode Erosion In High-Current, High-Energy Transient Arcs
  DONALDSON, AL; KRISTIANSEN, M; WATSON, A; ZINSMEYER, K; KRISTIANSEN, E; DETHLEFSEN, R
Abstract: 
+ Experimental-Observation Of Current Generation By Asymmetrical Heating Of Ions In A Tokamak Plasma
  GAHL, J; ISHIHARA, O; WONG, KL; KRISTIANSEN, M; HAGLER, M
Abstract: 
+ Influence Of The Circuit Impedance On An Electron-Beam Controlled Diffuse Discharge With A Negative Differential Conductivity
  SCHAEFER, G; SCHOENBACH, KH; KRISTIANSEN, M; STRICKLAND, BE; KORZEKWA, RA; HUTCHESON, GZ
Abstract: 
+ Insulator Damage In High-Current Discharges
  RANON, PM; KRISTIANSEN, M; LEHR, FM; HATFIELD, LL
Abstract: 
+ Mechanism Of Electrode Surface Damage And Material Removal In High-Current Discharges
  WATSON, A; DONALDSON, AL; IKUTA, K; KRISTIANSEN, M
Abstract: 
+ Reliability Assurance In Electromagnetic Launch Technology
  KOLARIK, WJ; LANDERS, TL; KRISTIANSEN, M
Abstract: 
Publication Year:  1985
+ An Electron-Beam Controlled Diffuse Discharge Switch
  SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; KROMPHOLZ, H; HARJES, HC; SKAGGS, D
Abstract: 
+ Antenna For Unidirectional Propagation Of Fast Alfven Waves In A Tokamak
  AKIYAMA, H; GAHL, J; RATHBUN, K; KRISTIANSEN, M; HAGLER, M
Abstract: 
+ Experiment To Investigate Current Drive By Fast Alfven Waves In A Small Tokamak
  GAHL, J; ISHIHARA, O; WONG, K; HAGLER, M; KRISTIANSEN, M
Abstract: 
+ Modeling Of Self-Breakdown Voltage Statistics In High-Energy Spark Gaps
  DONALDSON, AL; HAGLER, MO; KRISTIANSEN, M; HATFIELD, LL; NESS, RM
Abstract: 
+ Wall Effects On The Propagation Of Compressional Alfven Waves In A Cylindrical Plasma With 2-Ion Species
  AKIYAMA, H; HAGLER, MO; KRISTIANSEN, M
Abstract: 
+ Wall Effects On The Propagation Of Compressional Alfvén Waves In A Cylindrical Plasma With Two-Ion Species
  H. Akiyama; M. O. Hagler; M. Kristiansen
Abstract:  The dispersion relations for the compressional Alfvén waves in a two-ion species plasma of deuterium and hydrogen are calculated for a configuration which includes a vacuum layer between the cylindrical plasma and the conducting wall. The presence of the vacuum layer strongly affects the propagation of the compressional Alfvén wave, permitting some branches to propagate and penetrate the plasmacolumn over most frequencies in the ion-cyclotron range. Basic Alfvén-wave propagation and heating experiments in two-ion species consequently should be possible using tokamak and mirror devces with minor radii smaller than the Alfvén wavelength.

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Publication Year:  1984
+ A Review Of Opening Switch Technology For Inductive Energy-Storage
  SCHOENBACH, KH; KRISTIANSEN, M; SCHAEFER, G
Abstract: 
+ Electrode Erosion Phenomena In A High-Energy Pulsed Discharge
  DONALDSON, AL; HAGLER, MO; KRISTIANSEN, M; JACKSON, G; HATFIELD, L
Abstract: 
+ Electron-Beam Tetrode For Multiple, Submicrosecond Pulse Operation
  HARJES, CH; SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; KROMPHOLZ, H; SKAGGS, D
Abstract: 
+ Nanosecond Current Probe For High-Voltage Experiments
  KROMPHOLZ, H; DOGGETT, J; SCHOENBACH, KH; GAHL, J; HARJES, C; SCHAEFER, G; KRISTIANSEN, M
Abstract: 
+ Surface Studies Of Dielectric Materials Used In Spark Gaps
  JACKSON, G; HATFIELD, L; KRISTIANSEN, M; HAGLER, M; MARX, J; DONALDSON, AL; LEIKER, G; CURRY, R; NESS, R; GORDON, L; JOHNSON, D
Abstract: 
+ The Use Of Attachers In Electron-Beam Sustained Discharge Switches - Theoretical Considerations
  SCHAEFER, G; SCHOENBACH, KH; KROMPHOLZ, H; KRISTIANSEN, M; GUENTHER, AH
Abstract: 
Publication Year:  1983
+ Pulse Flashover Of Solid Dielectrics In Vacuum
  JACKSON, GL; HATFIELD, LL; KRISTIANSEN, M; MARX, J; BOWLING, A
Abstract: 
Publication Year:  1982
+ Concepts For Optical Control Of Diffuse Discharge Opening Switches
  SCHOENBACH, KH; SCHAEFER, G; KRISTIANSEN, M; HATFIELD, LL; GUENTHER, AH
Abstract: 
+ Investigations Of Reed Switch Dynamics And Discharge Phenomena When Switching Intermediate And Heavy Loads
  MIEDZINSKI, B; KRISTIANSEN, M
Abstract: 
+ Material Studies In A High-Energy Spark Gap
  GORDON, LB; KRISTIANSEN, M; HAGLER, MO; KIRBIE, HC; NESS, RM; HATFIELD, LL; MARX, JN
Abstract: 
+ Space-Charge Effects In A Laser Fiber-Optics Triggered Multichannel Spark Gap
  HARJES, HC; KUNHARDT, EE; KRISTIANSEN, M; HATFIELD, LL; GUENTHER, AH
Abstract: 
+ Special Issue On Plasma Science
  GUENTHER, AH; KRISTIANSEN, M; ROSE, MF; THOMPSON, JE
Abstract: 
+ The Effect Of Space-Charge Induced By An Electron-Beam On Spark Gap Operation
  TZENG, YH; KUNHARDT, EE; KRISTIANSEN, M; GUENTHER, AH
Abstract: 
Publication Year:  1981
+ Investigations Of Various Probe Sheath Materials In The Texas-Tech-Tokamak
  COLEMAN, PD; BLACKWELL, BD; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1980
+ An Electron-Beam Triggered Spark Gap
  MCDONALD, K; NEWTON, M; KUNHARDT, EE; KRISTIANSEN, M; GUENTHER, AH
Abstract: 
+ An Experimental And Numerical Investigation Of Laser-Plasma Interactions
  DRUCE, R; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Electro-Optical Surface Flashover Measurements
  THOMPSON, JE; LIN, J; MIKKELSON, K; KRISTIANSEN, M
Abstract: 
+ Investigations Of Fast Insulator Surface Flashover In Vacuum
  THOMPSON, JE; LIN, J; MIKKELSON, K; KRISTIANSEN, M
Abstract: 
+ Laser Triggering Through Fiber Optics Of A Low Jitter Spark Gap
  HARJES, HC; SCHONBACH, KH; KRISTIANSEN, M; GUENTHER, AH; HATFIELD, LL
Abstract: 
+ Magnetic Probe Measurements Of Toroidal Eigenmodes In The Texas Tech Tokamak
  COLEMAN, PD; BLACKWELL, BD; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Plasma Switches
  GUENTHER, AH; KRISTIANSEN, M
Abstract: 
Publication Year:  1979
+ Beat Heating In Plasmas Using Co2-Lasers
  CHU, E; DRUCE, R; KRISTIANSEN, M; HAGLER, M; BENGTSON, R
Abstract: 
+ Electro-Optical Measurements Of Insulator Surface Flashover In Vacuum
  MIKKELSON, K; KRISTIANSEN, M; LIN, J; THOMPSON, J
Abstract: 
+ Fast-Wave Resonances Near The Ion-Cyclotron Frequency
  DOLLINGER, RE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Laser Fiber Optic Breakdown Of A Pulse Charged 90-Percent Ar-10-Percent N2 Gas Switch
  HATFIELD, LL; HARJES, HC; KRISTIANSEN, M; GUENTHER, AH; SCHONBACK, KH
Abstract: 
+ Measurements Of Toroidal Eigenmode Field Structures In The Texas Tech Tokamak
  COLEMAN, PD; BLACKWELL, BD; BECKERICH, SR; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Review Of High-Power Switch Technology
  BURKES, TR; CRAIG, JP; HAGLER, MO; KRISTIANSEN, M; PORTNOY, WM
Abstract: 
Publication Year:  1978
+ Phase Measurements Of Toroidal Eigenmodes In Texas Tech Tokamak
  KNOX, SO; COLEMAN, PD; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1977
+ Ablation Rates Of Polystyrene Microspheres In A Theta-Pinch Plasma
  SMITH, DL; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Description Of Texas Tech Tokamak
  KNOX, SO; KIRBIE, HC; CROSS, RC; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Numerical-Analysis Of High-Power Laser Propagation In Magnetized Plasmas
  DRUCE, RL; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1976
+ Laser-Heating Of Magnetized Plasmas
  KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Numerical Parameter Study Of Laser-Plasma Interaction
  DRUCE, RL; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Optical Measurement Of High Electric And Magnetic-Fields
  THOMPSON, JE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1975
+ Differential Measurement Of Fast Energy Discharge Capacitor, Inductance, And Resistance
  NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Guanosine 5'-Diphosphate 3'-Diphosphate (Ppgpp) - Positive Effector For Histidine Operon Transcription And General Signal For Amino-Acid Deficiency
  STEPHENS, JC; ARTZ, SW; AMES, BN
Abstract: 
+ Helical Antenna For Exciting Azimuthally Asymmetric Alfven Waves
  KNOX, SO; PAOLONI, FJ; KRISTIANSEN, M
Abstract: 
+ Plasma-Solid Interaction In A Theta-Pinch
  NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1974
+ Co2-Laser Heating Of A Magnetized Plasma Column
  MOLEN, GM; KRISTIANSEN, M; HAGLER, MO; BENGTSON, RD
Abstract: 
+ Simple, Multiple Arc, Dielectric Switch Applied To A Theta-Pinch
  NUNNALLY, WC; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Synthesis Of Substituted 9,10-Dihydroanthracenes By Reduction Of Anthraquinones In Hydriodic Acid
  RENAUD, RN; STEPHENS, JC
Abstract: 
Publication Year:  1973
+ Co2-Laser Beam Refraction In A Linear Discharge Plasma
  MOLEN, GM; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1972
+ Azimuthally Symmetric Unidirectional Ion-Cyclotron Wave Exciter
  HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ C02 Laser Design And Laboratory Projects
  G. M. Molen; R. H. Trotter; M. Kristiansen; M. O. Hagler
Abstract:  The design of an inexpensive CO2 laser and some associated laboratory projects are described. The laser is easily constructed by undergraduate students. The experiments are designed to familiarize students with some characteristics and applications of CO2 lasers.

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+ Co2-Laser Design And Laboratory Projects
  MOLEN, GM; HAGLER, MO; KRISTIANSEN, M; TROTTER, RH
Abstract: 
+ Fast Wave Damping At Second-Harmonic Of Ion-Cyclotron Frequency
  CATO, JE; WATSONMU.CN; HAGLER, MO; KRISTIANSEN, M
Abstract: 
+ Some Engineering Problems Of Low-Frequency Heating Of Fusion Reactors
  CATO, JE; HAGLER, MO; KRISTIANSEN, M
Abstract: 
Publication Year:  1971
+ Global Temperature Effects Of Use Of Fusion Energy And Fusion Torch
  PORTER, WA; HAGLER, MO; KRISTIANSEN, M
Abstract: 
+ Heat Shrinkable Tubing As An Inexpensive Vacuum Seal
  MOLEN, GM; ROSELAND, LG; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Traveling-Wave Antenna For Exciting Waves In A Cylindrical Anisotropic Plasma
  HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1970
+ Arc Welding Generators Used As Air Core Magnet Power Supplies
  CATO, JE; HIPP, JE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
+ Industrial Applications Of Lasers
  PECKHAM, LN; HAGLER, MO; KRISTIANSEN, M
Abstract: 
+ Inexpensive, Fast On-Off, High Power, Pulsed Rf Amplifier
  MELTON, RD; CATO, JE; KRISTIANSEN, M; HAGLER, MO
Abstract: 
Publication Year:  1969
+ Stop- And Pass-Bands For Harmonic Ion Cyclotron Waves
  KRISTIANSEN, M; DOUGAL, AA
Abstract: 
Publication Year:  1968
+ Plasma Heating And Wave Propagation At Harmonics Of Ion-Cyclotron Frequency
  KRISTIANSEN, M; DOUGAL, AA
Abstract: 
Publication Year:  1967
+ Experimental Investigation Of Harmonic Ion Cyclotron Wave Propagation And Attenuation
  KRISTIANSEN, M; DOUGAL, AA
Abstract: 
+ Inexpensive High Vacuum High Voltage Electric Feedthrough
  KRISTIANSEN, M; MELTON, JG; DOUGAL, AA
Abstract: 
Publication Year:  1966
+ Evaluation Of Faraday Shielded Stix Coils For Ion Cyclotron Resonance Heating Of Plasma
  DODGE, NB; KRISTIANSEN, M; DOUGAL, AA
Abstract: 
Publication Year:  1963
+ Hamilton'S Modified Principle Applied To Nonlinear Circuit Problems
  M. Kristiansen; L. G. Clark
Abstract:  Hamilton's Modified Principle (HMP) is used to solve two nonlinear problems. The amplitude, frequency, and stability in the limit cycle of a simple pentode oscillator are calculated in order to demonstrate the principle. A standard graphical method finds the amplitude of oscillations, and results are compared with experimental data. For the case under consideration, the HMP solution gives better agreement with the experimental results - 6.6% deviation as compared with 12.4% for the graphical solution. An approximate answer is also found for the transient behavior of a nonlinear system where the input is a step function. The result is compared with an analog computer solution and shows good agreement. The solution of a simple relay servo system is indicated in which the amplitude and frequency in the limit cycle along with its stability are obtained by application of the one method of HMP.

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