Center for Pulsed Power and Power Electronics.
TTU Home Edward E. Whitacre Jr. College of Engineering P3E Home Faculty

Dr. James Dickens, P.E.

Co-Director, Center for Pulsed Power and Power Electronics
C.B. Thornton Professor of Electrical and Computer Engineering

Contact Information

Department of Electrical and Computer Engineering
Texas Tech University
Lubbock, TX 79409-3102

Phone: (806)742-1254
Fax: (806)742-1245
James.Dickens@ttu.edu

Education

Research Interests

Other Interests

Projects

Awards and Professional Societies

Publications


Journal

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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+ 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.
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.
+ 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.
+ 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.
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.
+ 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.
+ 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: 
+ 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.
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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+ 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.
+ 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.
+ 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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Publication Year:  2018
+ 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).
+ 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.
+ 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: 
+ 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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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.
+ 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.
+ 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.
+ 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.
+ 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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Publication Year:  2016
+ 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.
+ 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.
+ 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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+ 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.
+ 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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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.
+ 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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+ 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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+ 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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+ 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.
+ 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.
+ 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.
+ 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.
Publication Year:  2014
+ 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.
+ 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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+ 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.
+ 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.
+ 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.
+ 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.
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]
+ 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.
+ 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.
+ 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]
+ 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.
+ 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]
+ 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]
+ 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.
Publication Year:  2011
+ 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.
+ 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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+ 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.
Publication Year:  2010
+ 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]
+ 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
+ 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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Publication Year:  2008
+ 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.

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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.

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+ 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.
+ 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.
+ 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.
Publication Year:  2006
+ 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.
+ 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.
+ 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.
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.
+ Fifth Special Issue On Pulsed Power Science And Technology
  Dickens, JC; Lehr, JM; Mankowski, J
Abstract: 
+ 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.
+ 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.
Publication Year:  2003
+ 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.
+ 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.
+ 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
+ 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.
+ 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.
+ 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.
Publication Year:  2001
+ 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.
Publication Year:  2000
+ 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.
+ 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.
Publication Year:  1999
+ 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.
+ 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.
Publication Year:  1998
+ 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:  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.

Conference Paper/Presentation

Publication Year:  2021
+ Effect Of Humidity On Charge Decay In Varying Atmospheric Gases
  Z. Cardenas; B. Esser; I. Aponte; J. Dickens; J. Mankowski; J. Stephens; D. Friesen; C. Nelson; N. Koone; D. Hattz; A. Neuber
Abstract:  Characterization of surface charge decay for varying materials "“ PTFE, Acrylic, and metals "“ of 100 mm diameter, radially symmetric bodies was performed. In a gaseous environment, charged materials will lose their charge depending on gas type, humidity, and other environmental factors. Eventually, the charge will be (1) neutralized by ions or electrons in the surrounding gas (2) distributed by conduction on the material surface or in its volume (3) or neutralized by ions or electrons in the material surface and volume. Metal and polymer samples are charged to a known potential in the tens of kilovolts range, and the decaying voltage is monitored with a non-contact probe until the potential reaches near zero. Humidity has an impact on both groups of materials. As a baseline, in air at 42% relative humidity in an enclosed chamber, a positive charge equivalent to 20 kV potential on a metal sphere of 50 mm radius decays to near zero within 25 minutes. In contrast, the same sphere negatively charged decays slightly faster within 20 minutes. Overall, the humidity has a significant effect on the charge decay time, such that an order of magnitude longer times (hours) are observed at low humidity. Effects of chamber size were also studied, drawing a connection to charge carriers in the air available for decay, with chambers of volume(s): 3.3 105 cm3 and 1.4 106 cm3, respectively.

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+ Sensitivity Of Pbx 9501 And Pbx 9502 To Milling Operations
  J. Williams; E. Weeks; R. Clark; S. Watkins; J. Dickens; J. Mankowski; J. Brinkman; A. Neuber
Abstract:  Explosive driven pulsed power performance benefits from modern polymer-bonded explosives. In practical application, high precision in the dimensional shape is required, and high machining speeds are desired. The limits of machining speed, driven mainly by the thermal response of the energetic material, are investigated. Specifically, the thermal response of PBX 9501 (95% HMX, 2.5% Estane, 2.5% BDNPA/BDNPF by weight) and PBX 9502 (95% TATB, 5% Kel F-800 by weight) under conventional milling methods is examined. The presented work focuses on face milling performed with dry machining on a CNC, remote-controlled milling machine. The primary parameters of interest are the spindle speed, feed rate, step size, and depth of cut, with additional consideration given to endmill diameter, milling technique, and the number of flutes. The temperature of the system is monitored via high-speed IR videography and near the cutting interface with a K-type thermocouple inserted into the endmill"™s through-coolant hole. Operational forces and torques are recorded by a 6-axis force sensor mounted beneath the HE samples.An empirical relationship between temperature and the milling parameters is presented. Overall, milling regimes exist outside of DOE-STD-1212-2019 for which milling temperatures remain well below the HE critical temperatures.

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+ Surface Charge Distribution Mapping Using An Electrostatic Probe
  M. LaPointe; B. Esser; I. Aponte; Z. Cardenas; J. Dickens; J. Mankowski; J. Stephens; D. Friesen; C. Nelson; N. Koone; D. Hattz; A. Neuber
Abstract:  Undesired accumulation of charge on dielectric materials causing electrostatic discharges can be an issue in pulsed power systems and electrical systems in general. As such, an understanding of surface charge distribution, charge accumulation, and decay is required. An in-house no-contact electrostatic probe designed with a 2 mm resolution was fabricated to measure and map surface charge distribution. Designed as a contactless instrument, the probe consisting of a metal rod and low leakage amplifier circuit probes the electric potential near the surface. A scan of the surface then provides the raw data, a potential distribution. The actual probe response - i.e., the surface charge to voltage transfer function, is accounted for in post-processing. This is accomplished via an Inverse Wien Filter - a technique often applied in image processing - to deconvolve the probe response from the measured data. A commercially available electrostatic probe, the Trek 341B meter with a 3455ET probe, capable of measuring +/- 20 kV was compared to the in-house probe that is designed to cover a wider range from +/- 35 kV. A resolution better than the simple probe resolution is achieved through the distribution"™s scanning voltage method and deconvolution. Applying repeated scans, surface charge decay was tracked on various polymer materials to determine the material and environment dependence; materials included were PA6, PTFE, and others. As an example of material dependence, samples charged to 20 kV at 65% humidity experience full charge decay in approximately 45 seconds for PA5 (152 mm dia.), while it took about 100 times longer for PTFE (51 mm dia.) to observe the same decay/redistribution of charge.

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Publication Year:  2020
+ A Study On The Contributing Factors Of Rf Breakdown In Atmospheric Air
  I. A. Aponte; B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  DC and RF breakdown at 3.3 MHz was studied in centimeter size gaps - 1 - 10 mm "“ with comparison to small gaps in literature and Monte Carlo simulations in atmospheric conditions. As a point of reference, DC breakdown using stainless-steel Bruce-profile electrodes were measured to compare with RF measurements. RF breakdown with a slow rising envelope ($\sim 5\ \text{mV}/\mu\mathrm{s}$) yielded approximately 80% of DC measurements (~25 kV/cm in 5 mm gap), which agrees with Monte Carlo simulations and results found in literature. Increasing the envelope rise time to greater than $1000\ \mathrm{V}/\mu \mathrm{s}$ yielded ~120% of DC measurements (~37 kV/cm in 5 mm gap). Comparisons with Monte Carlo simulations which included photon processes "“ theorized to be critical to obtaining accurate results "“ furthered understanding of the processes involved in pre-ionization before breakdown occurs.

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+ Analysis Of Experimental Multipactor Observation Signals Using Spark3D Software
  T. Sugai; Z. Shaw; J. Dickens; A. Neuber
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. Its effects range from signal degradation to the damage and destruction of microwave components. Thus, multipactor physics has been studied through theoretical analysis, numerical simulation, and experiment. Previously, we developed a direct electron observation system using an Electron Multiplier Tube (EMT) and succeeded to directly detect multipactoring electrons in the center of the broadwall of rectangular waveguides 1, 2. Here, we provide a method for evaluating the electric charge density and secondary emission yield (SEY) in waveguides. The experimentally obtained EMT signal is analyzed with the extensive usage of the numerical simulation software Spark3D. The software was utilized to analyze multipactor onset in waveguide structures, where the electric field distribution without multipactor was carefully simulated, employing high-frequency solvers. The EMT signal and the charge density were simulated for the same conditions as the experiment. As a result, a calibration line indicating the proportional relation between the EMT voltage and the charge density, which is independent of some conditions, i.e., input power and gap size, was obtained. Further, after adjusting the SEY curve imported to Spark3D, the rising shape of the experimental EMT signal pulses fit with the simulated one, and the experimental threshold power for the EMT signal generation was consistent with the simulated multipactor threshold power. Since the simulation matches the experiment in threshold power and signal shape, one expects that the charge density and SEY curve deduced from the simulation are accurate.

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+ High Power 2.85 Ghz Gan Rf Source For Direct Detection Of Multipactor Research
  B. Esser; Z. Shaw; J. C. Dickens; A. A. Neuber
Abstract:  A pulsed 2.85 GHz RF source design with high average power is presented for use as the principle exciter in an experiment to directly detect multipactor. With a rated output power of 700 W each in long pulse mode (~ 100 â–¡s), four Cree GaN HEMTs are used to achieve a maximum of 2.8 kW rated output. A fifth HEMT is used to drive the four output devices with approximately 40 W each. A free running VCO with low tuning sensitivity is used which ensures a stable frequency output despite a noisy environment. Buffering and initial amplification is provided by an LNA with a TTL RF switch used to generate the pulses. With a rise time of 35 ns, this switch is fast enough to create reasonably square pulse edges. A microwave amplifier with 45 dB of gain provides the majority of the gain in the system and the power necessary for the final GaN stages, approximately 9 W. An in-line micrometer adjusted, continuously variable attenuator between LNA and TTL switch provides level control.

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+ Multipactor Thresholds In A Planar Test Cell
  Z. C. Shaw; B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  A planar test cell was designed and implemented to observe the multipactor effect in waveguide structures. This plug and play device allows for multiple geometries to be machined and easily replaced within the test structure. A direct detection method was used to observe the multipactor effect while the upper and lower thresholds were measured for a 2.1 mm gap at 2.85 GHz. While there is an obvious lower limit to multipactor (2 kW), there was no observable upper limit even at powers over 200 kW. This is attributed to the transverse electric field distribution in the dominant TE10 mode which is not taken into account in most multipactor theoretical models.

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Publication Year:  2018
+ A 2 Kw, 2.85 Ghz Multipactor Rf Source Utilizing Depletion Mode Gan Hemts
  B. Esser; Z. Shaw; J. C. Dickens; A. A. Neuber
Abstract:  A pulsed RF source for multipactor research capable of providing 2 kW at 2.85 GHz to a multipactor test cell is described utilizing state-of-the-art GaN HEMTs from Cree/Wolfspeed and integrated GaN amplifier from Qorvo/Tri-Quint. A Mini-Circuits ZX95-2920CA+ VCO is used as the signal generator with low phase noise and modulation capable with a bandwidth of 55 MHz. An inline attenuator provides the operator power control in the range of approximately 42 to 64 dBm (16 to 2800 W). A high-speed RF switch creates a 100-microsecond pulse with a rise time of 25 ns and a typical switching time of 35 ns. A single Tri-Quint TGA2585-SM provides the majority of the gain in the system, 32 dB, and the necessary power for the final output stage (6 W). Four CGHV31500F amplifiers operating in parallel comprise the output stage providing the bulk of the power needed, 500 W each for a total of 2 kW with 12.5 dB gain each, with a single unit providing the necessary input power before splitting to the four finals (50.5 dBm, ~ 112 W). A custom PCB was designed to properly bias the GaN stages and prevent device failure due to improper bias sequencing. Custom power splitters and combiners had to be used due to the high-power levels being considered with directional couplers on the output to monitor output (forward) power and reflected (reverse) power during operation. This will provide valuable insight into characteristics of the plasma cloud generated during the multipactor event.

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+ Assessing The Influence Of Secondary Electron Emission Characteristics On Multipactor In Rectangular Waveguides
  H. K. A. Nguyen; J. Mankowski; J. C. Dickens; A. A. Neuber; R. P. Joshi
Abstract:  Multipactor in a rectangular waveguide is studied using numerical simulations. Particular attention is given to the secondary electron emission characteristics including their energy spectrum (hence velocity spread) and angular distribution. Elastically scattered, rediffused and true secondary electrons are all comprehensively included based on the Furman-Pivi model [1] for the TE10 mode. The focus is on small waveguides and lowest order resonance conditions.

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+ Cst Particle-In-Cell Modeling Of A Tunable Reflex-Triode Vircator
  D. H. Barnett; A. A. Neuber; J. C. Dickens; J. J. Mankowski
Abstract:  This study serves to describe three-dimensional particle-in-cell (PIC) simulations of a tunable reflex-triode virtual cathode oscillator (vircator). Experimental data from the compact hard-tube reflex-triode vircator developed at Texas Tech University (TTU) is used to validate simulated results. The vircator developed at TTU is capable of burst-mode operation at pulse repetition rates (PRFs) up to 100 Hz for a period of one second. A pulse energy of 158 J drives the vircator, and 600 kV (open circuit) pulse forming network (PFN) based Marx generator. The vircator is comprised of a bimodal, carbon fiber cathode and a pyrolytic graphite anode, with the ability to quickly change the distance between the anode-cathode (A-K) gap, back wall distance, and bottom plate distance between experiments. The PIC simulations have been performed using CST PIC Solver, by Dassault Systemes. The models detail virtual cathode formation and the subsequent extraction of radiated microwave power for a variety of cavity geometries. A working three-dimensional, relativistic, electromagnetic, particle-in-cell model of a vircator allows for quick, predictive results relative to building an experimental setup. The model is used to determine the necessary driving voltages, A-K gap distances, and cathode current densities to extract microwave radiation at a desired. Simulated results aid in identifying mode contributions. Voltage, current, and microwave data are presented and compared against experimental results at different operating conditions.

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+ Electrically Small Antenna Design For Transportable Ionospheric Heating
  B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  Ionospheric Heating, traditionally performed utilizing large fixed location arrays, while effective, limits research efforts to those fixed locations. As one may envision, a more compact transportable array will provide additional research opportunities at latitudes previously unexplored. For instance, closer to the equator where the Earth's magnetic field is primarily parallel to the ground. An electrically small inductively coupled antenna is considered for its suitability in such an array. A Small or Semi Loop Antenna (SLA) inductively couples to a Capacitively Loaded Loop (CLL) providing high efficiency and natural 50 $\Omega$ port matching. The CLL, of hinged petal design, consists of a large parallel plate capacitor, and half cylinder inductive sections which are hinged at the base to allow for tuning in the range of ionospheric heating (3 - 10 MHz) with a plate included angle of 0 - 16°. A prototype antenna was designed and evaluated at 1/10th scale - for ease of research efforts - with tuning range of 30 - 100 MHz including capability of tuning the coupling between SLA and CLL to achieve good source matching across the tuning range, particularly at the low and high ends. This tuning method may also be used to compensate for mutual impedance effects in array operation, improving array performance. A gain of approximately 5 dBi is observed when placed upon a sufficiently large ground plane. Tuning of the antenna prototype is achieved via stepper motor driven system to remotely adjust both the CLL and SLA angles continuously.

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+ Fundamental Study Of Atmospheric Rf Breakdown At 3.3 Mhz
  I. A. Aponte; B. Esser; Z. Shaw; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  RF breakdown 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 literature for small gaps. Paschen's famous DC breakdown study utilizing two brass spheres of radius 1 cm was recreated following the original procedure, from which the data was used to compare to a study of RF breakdown. Through testing it was found that brass as an electrode material exhibits a large standard deviation in breakdown voltage and as such stainless-steel Bruce profile uniform electric field electrodes were substituted. Steel's resistance to surface ablation provided for extremely low standard deviation of measurements and hence good repeatability. Additionally, unlike brass, steel is not contaminated through reaction with gaseous elements such as carbon, oxygen and nitrogen. Electrodes were polished to remove any field enhancements and cleaned of polishing compound such that dielectric inclusions were largely avoided.

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+ Investigation Of Gan Photoconductive Semiconductor Switches
  N. A. Wilson; J. A. Culpepper; V. Kuryatkov; M. Gaddy; J. C. Dickens; S. Nikishin; R. Ness; A. A. Neuber
Abstract:  The suitability of commercially available wide bandgap GaN material for the fabrication of photoconductive semiconductor switches, PCSS, was investigated. A variety of PCSSs were fabricated utilizing diverse GaN samples, which were shown to exhibit significantly diverse physical properties. That is, sample characterization techniques such as cathodoluminescence (CL), photoluminescence (PL), secondary ion mass spectrometry (SIMS), Current-Voltage behavior, and scanning electron microscopy (SEM) were applied to characterize the samples prior to processing.

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Publication Year:  2017
+ 160 J, 100 Hz Repetition Rate, Compact Marx Generator And High Power Microwave System
  Barnett, DH; Rainwater, K; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract:  This paper presents the electrical and mechanical hardware considerations of a compact, 160 J modular pulse forming network (PFN) Marx generator used to drive a high-power microwave (HPM) source that is a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open-circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN fashioned from five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin (c) to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin (c) allowing airlines to connect to each module chamber rather than every spark gap. Each module has two electrode inserts placed into the sealed pressure vessel contained within the module. After the Marx erects, the energy is directed into the virtual cathode oscillator (vircator) where subsequent frequency generation is manipulated through a rectangular waveguide contained within a new resonator cavity design. The new design allows the bottom wall, back wall, and anode cathode gap to be moved by two linear actuators, a linear bellows, and another linear actuator, respectively. The cavity is contained within a 10-inch circular vacuum chamber with a round stainless steel sleeve running from the back wall to the linear bellows. Contained within the round sleeve is a rectangular waveguide where the bottom wall and the cathode are housed. The anode is connected to the Marx generator via a nickel shaft that feeds through the back wall into the circular sleeve and into the rectangular waveguide. The anode made from pyrolytic graphite, remains stationary while the bottom wall, and carbon fiber velvet cathode move relative to its position. The benefit of this design is the height and depth of the cavity resonator can be controlled independently of each other while still allowing the A-K gap to be manipulated on its own.
+ Current Handling Capability And Bond Degradation Of Bond Wires Under Pulsed Conditions
  R. Rodriguez-Molina; D. Mauch; V. Meyers; A. Neuber; J. Dickens
Abstract:  Results detailing the current handling capability of Gold bond wires (1 mil diameter), and Aluminum ribbon (1mil x 10 mil) under pulsed conditions are presented. Gold wire bonds were formed through the ball-bonding technique, and the Aluminum wire was bonded through the wedge bonding technique. Both wires were bonded to gold plated bond pads. Varying peak current densities ranging from $2 \mathrm {x}10 ^{4}\mathrm {A}$ cm $^{-2}$ to $2 \mathrm {x}10 ^{7}\mathrm {A}$ cm $^{-2}$ were applied to bond wires at pulse-widths ranging from 50 ns to $500 \mu \mathrm {s}$. In addition, the effect of surrounding medium (SF6, HV epoxy, transformer oil) on the current capability was investigated. Failure modes observed were investigated with SEM imaging, and the evolution of these failure modes during current pulsing was captured with high-speeding imaging. Finally, the obtained results are compared with the experimental and theoretical results obtained from previous investigations conducted on exploding wires.

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+ Developing A High Power, Pulsed Uv Led System For Photoconductive Switching Applications
  N. A. Wilson; D. L. Mauch; V. E. Meyers; J. C. Dickens; A. A. Neuber
Abstract:  Summary form only given. A very compact system integrating a high voltage pulser (up to 250 V) and high power UV LED (365 nm wavelength) was developed for triggering SiC photoconductive semiconductor switches (PCSS). The relationships between LED drive current and forward voltage to optical power were established for currents ranging from 0 A to 100 A at pulse widths ranging from 100 ns to 5 μs. The maximum optical power observed was 25 W. An optical system composed of a parabolic reflector and short focal length lenses was used to focus the emitted light onto a lateral geometry PCSS. The observed delivery efficiency was 63% onto a target 11 mm in diameter. Using the developed system, a SiC PCSS could be successfully triggered under high impedance load conditions.

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+ Electric Field Breakdown Versus Frequency Simulated Under Atmospheric Conditions For Large Gaps
  H. K. Nguyen; A. Chowdhury; J. C. Dickens; R. P. Joshi; A. A. Neuber
Abstract:  Breakdown of air at atmospheric pressure in response to AC fields in gaps larger than 1 cm was simulated. Most previous literature concerning breakdown in this regime has focused on much smaller gaps1.

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+ Magnetic Field Diffusion Into Hollow Conductors With Walls On The Order Of The Skin Depth
  Buntin, T; Collier, L; Dickens, J; Mankowski, J; Walter, J; Neuber, A
Abstract:  Transient magnetic diffusion through conductors of thickness comparable to the skin depth is investigated. Since an analytical solution is unavailable in this case, such magnetic diffusion results must be determined via simulation or experimentation. In the experimental approach, a sinusoidal current with peak values in the range of 20-30 kA (approx. 7 kHz ringing frequency) is passed through a two turn coil generating a sinusoidally varying magnetic field. A hollow structure with metallic walls of controlled thickness is placed roughly 10 cm away from the exciting coil. The focus of this investigation is on the transient skin depth, which occurs during the first half-wave of the signal, as that is most relevant for pulsed power applications. A calibrated B-dot probe placed inside the structure facilitates measurement of the diffused field. As expected, experimental data shows that magnetic field diffusion through the wall is not instantaneous, causing a delay before the diffused field is measured inside the test structure. The impact of cracks and holes in the conductor on the speed and magnitude of the magnetic field diffusion is elucidated. Results for materials of different conductivities are compared and analyzed for the transition between thin and medium walled cases. FEM simulations are validated alongside these experimental results and used to access a larger parameter space.
+ Next Generation Ionospheric Heater Array
  B. Esser; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  An electrically small antenna (ESA) is evaluated for its potential future use in a Transportable Ionospheric Heating (TIH) array. Consisting of a Small Loop Antenna (SLA) which inductively couples to a Capacitively Loaded Loop (CLL) the antenna provides a high-Q natural match to a $50 \Omega $ source. The capacitance of the CLL may be adjusted to tune the antenna in the range of ionospheric heating of approximately 3 "“ 10 MHz. Several methods are evaluated to achieve this tuning including a horizontal sliding plate design, and a hinged petal design.

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+ Optical Nonlinear Absorption Properties Of 4H-Sic-Experiment And Model
  Meyers, V; Mauch, D; Dickens, J; Neuber, A
Abstract:  Intensity-dependent nonlinear light absorption in bulk 4H-SiC at the above-bandgap energy of 3.49 eV (lambda = 355 nm) is studied. Characterization and understanding of such nonlinear optical behavior in 4H-SiC forms the basis efficiency improvements and design of optoelectronic SiC devices, including photoconductive semiconductor switches. It is noted that previous research performed elsewhere had focused primarily on nonlinearities at below-bandgap energies, while little had been explored above-bandgap. In this study, absorption of short laser pulses with fluences ranging from 1 mJ/cm(2) to 30 mJ/cm(2) incident on 160 mu m-thick high purity semi-insulating 4H-SiC samples of varying recombination lifetimes is addressed. Sample bulk recombination lifetimes vary from 0.5 ns to 100 ns displaying the range of effects from growth, electron irradiation, and annealing. The effective absorption coefficient varies significantly within this range as an apparent function of bulk recombination lifetime. A four-level time-and space-dependent finite difference time domain (FDTD) model taking into account electron trapping, interband absorption, and free-carrier absorption was constructed that yielded further insight into the absorption dynamics. For instance, the importance of free carrier absorption and deep-level trapping in the nonlinear absorption behavior is elucidated.
+ Results Of A Compact Reflex Triode With Multi Cavity Adjustment
  Barnett, DH; Rainwater, K; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract:  This study focuses on achieving wide tunability of a compact reflex triode virtual cathode oscillator (vircator). The cathode is of a bimodal carbon fiber (CF) material paired with a pyrolytic graphite anode. These materials display ideal operating characteristics which include but are not limited to, long lifetime > 10(6) shots, high operating temperatures > 1000 K, and large current densities similar to 200 A/cm(2). A 12 stage, 158 J pulse forming network (PFN) based Marx generator serves to drive the vircator at 350 kV, 4 kA with similar to 100 ns pulsewidth. The operating frequency of interest is in the range of 1-6 GHz, where tunability is achieved by varying the length of the anode-cathode (A-K) gap, the length from the back wall to the A-K gap, and/or the distance from the bottom of the cavity to the A-K gap. The primary focus in this research was to increase the achievable frequencies by placing a square waveguide within a sealed vacuum tube. This allows the bottom part of the waveguide to be easily adjusted while still maintaining the waveguide integrity. The resulting microwave frequencies are shown along with the system performance.
+ Toward The Development Of An Efficient Bulk Semi-Insulating Gan Photoconductive Switch
  V. Meyers; D. Mauch; V. Kuryatkov; S. Nikishin; J. Dickens; A. Neuber; R. Ness
Abstract:  Photoconductive semiconductor switches (PCSS) made from bulk, semi-insulating GaN have been fabricated and tested under pulse-charged conditions. Switching response and photocurrent efficiency of GaN PCSSs triggered by sub-10 ns, 355 nm laser pulses is reported. It is shown that fast rise time (<;300 ns) voltage pulses can be used to charge a GaN PCSS to fields well beyond the DC breakdown field strength of GaN and improve switching performance. GaN's wide band gap, breakdown field strength, and electron mobility make it a material superior to SiC and far superior to GaAs for PCSS applications, though historically these materials have dominated PCSS research due to their relative ease of fabrication. Recent improvements to crystal quality and wafer size have allowed GaN and more recently semi-insulating GaN to play an increasing role in high-power and high-voltage solid state devices.

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Publication Year:  2016
+ A 160 J, 100 Hz Rep Rate, Compact Marx Generator For Driving And Hpm Source
  Rainwater, K; Barnett, D; Lynn, C; Dickens, J; Neuber, A; Mankowski, J
Abstract:  This paper presents the electrical and mechanical design considerations of a compact, 160 J modular pulse forming network (PFN) based Marx generator used to drive a high-power microwave (HPM) source with a time variant load at a PRF of 100 Hz. The modular Marx generator is designed to produce an open circuit output voltage of 600 kV from a 50 kV source using twelve stages. Each stage of the Marx was constructed from a PFN created with five, 2.1 nF, high voltage capacitors in parallel. Each Marx module was machined out of acetyl copolymer or Delrin to provide optimal strength, rigidity, and a dielectric constant that closely matches transformer oil. These Marx modules include air supply lines that are machined directly into each block of Delrin allowing airlines to connect to each module chamber rather than every spark gap. The spark gaps are comprised of two electrode inserts placed into the sealed pressure vessel contained within the Marx modules. The electrode inserts are of a sleeve-electrode design, which allows the user to insert the electrode into the sleeve, then the sleeve into the Marx module. The benefit of this design is the ability to adjust the electrode gap spacing without compromising the high pressure seal. Two continuous charging inductors run between each PFN and underneath the Marx modules. Due to high voltages generated by the Marx, outer field shaping rings are used to reduce the field stress across the induction coils, resulting in longer lifetime. The inductors are also of modular design allowing for individual coils to be replaced in the event of failure. Output voltage and current waveforms from a 60 Ohm water load are presented.
+ Breakdown Characteristics Of A Silicon Carbide Photoconductive Semiconductor Switch Triggered Spark Gap
  T. Ihara; D. Mauch; J. Dickens; A. Neuber
Abstract:  Summary form only given. Development of a switch capable of operation at high voltages, high repetition frequency, with long lifetime is essential for furthering pulsed power applications. Photoconductive semiconductor switches (PCSSs) possess inherent optical isolation and extremely low switching jitter (~10 ps), and have also been experimentally shown to be capable of switching high voltages (up to 50 kV) and currents with very fast rise and fall times (<; 1 ns)[1-2]. In this paper, we report the breakdown characteristics of a SiC PCSS triggered spark gap obtained via measurement of the voltage and current, and simulation of the electric field distribution. The triggered spark gap is composed of sphere-to-sphere electrodes and a field distortion electrode, which is kept at mid-potential in the center plane between the two spherical main electrodes. A normally open PCSS is connected between one of the main electrodes and the mid-plane (trigger) electrode, whose center bore diameter is varied from 5 to 15 mm. With application of the optical pulse to the PCSS, the trigger electrode is temporarily connected to the main electrode, effectively doubling the electric field between the trigger and opposite electrode, leading to main gap closure. In essence, while bulk SiC PCSS switching currents demand very high optical power input, the synergy of bulk SiC PCSS and traditional spark gaps enables the triggering of large current flows at very modest optical powers with low jitter. Overall, the obtained results reveal that incident laser energy and mid-plane electrode geometry heavily influence the breakdown characteristics of the spark gap including jitter time, and breakdown voltage.

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+ Frequency Tunability Of A Reflex-Triode Vircator Using Particle-In-Cell Modeling
  P. M. Kelly; C. F. Lynn; J. M. Parson; J. Dickens; A. Neuber; J. J. Mankowski
Abstract:  Summary form only given. The results from the development of a three-dimensional particle-in-cell (PIC) model investigating frequency tunability of a reflex-triode virtual cathode oscillator (vircator) are presented. These efforts have focused upon achieving stable frequency output at many different frequencies in the S, L, and C-bands, from a single tube, using the ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) code from the Air Force Research Laboratory (AFRL). Experimental data from the hard-tube vircator at Texas Tech University (TTU), which operates at background pressures less than 10-9 Torr and utilizes a bimodal carbon fiber cathode and pyrolytic graphite anode, is used to validate simulation results. Additionally, the vircator at TTU is capable of changing the accelerating voltage, the anode-cathode (A-K) gap distance, and the distance of the cavity backwall relative to the position of the A-K gap on the fly without breaking tube vacuum, all in an effort to achieve greater frequency tunability and output power. However, this creates a very large, time-intensive experimental parameter space and makes a simulation model attractive for exploring additional output capabilities. ICEPIC results from a large combination of A-K gap distances, backwall distances, driving voltages, and cavity diameters are presented and compared to experimental results, highlighting frequency tunability of the system from a single tube.

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+ High Electric Field Atmospheric Breakdown Of Air At High Frequency For Large Gaps
  A. R. Chowdhury; H. K. Nguyen; R. P. Joshi; J. C. Dickens; J. J. Mankowski; A. A. Neuber
Abstract:  Summary form only given. Breakdown of air at atmospheric pressure in high frequency uniform electric fields and large gaps is discussed. In the high frequency band of a few MHz to few tens of MHz, the breakdown threshold voltage is lowered from its DC value due to enhanced space charge from ions that become trapped in the gap.1 While there is some literature concerning breakdown in this frequency range, it does not consider gaps larger than 1 cm.2A fluid model is developed to simulate plasma development in a baseline 6 cm gap primarily to explore power limitations for high power, electrically small antennas, which are operated cw at MHz frequencies. The ion densities are obtained from a drift-diffusion model, though data for the ionization, electron collision, and attachment parameters were obtained from Monte Carlo simulations, while ion diffusion and drift velocities were taken from the literature. As expected, the Monte Carlo simulations reveal that the EEDF follows any change in the electric field on the picosecond timescale at atmospheric pressures, much faster than any variation due to the externally applied electric field. Results from the simulation for gap lengths varying from the 6 cm baseline and air pressures are obtained, analyzed, and also compared with available reports.3

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+ Limitations Of Bulk Sic Photoconductive Semiconductor Switches, And Evaluation Of Pin Sic Photoconductive Semiconductor Switches
  D. L. Mauch; V. E. Meyers; R. P. Joshi; A. A. Neuber; J. C. Dickens
Abstract:  A comprehensive picture of the relationship between optical fluence, optical wavelength, system load, and photocurrent efficiency (PE) in SiC photoconductive semiconductor switches (PCSSs) is presented. Variation of the optical wavelength (300-380 nm) and optical fluence (0.2-200 J m-2) was accomplished with a Nd:YAG pumped optical parametric oscillator (7 ns FWHM) and a broadband variable attenuator. The PE was found to typically be in the range of 1-2 %, depending on wavelength, and the bulk PCSS on-state voltage driven by external circuit parameters. Features of the high electric field stress behavior (> 200 kV/cm) of the bulk PCSS were captured with high fidelity in a 1D drift-diffusion model with a self-consistent Poisson solver including trap assisted tunneling, Poole-Frenkel, and barrier lowering with enhanced tunneling effects. In addition, trap to band impact ionization as well as Coulombic and repulsive trapping potentials were included.

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+ Optical Nonlinear Absorption Characterization Of Bulk Semi-Insulating 4H-Sic At And Above The Band Edge
  V. Meyers; D. Mauch; J. MaÅ„kowski; J. Dickens; R. Joshi; A. Neuber
Abstract:  Nonlinearity of optical absorption in semi-insulating bulk 4H-SiC has been investigated. Of interest was the optical bleaching behavior of 4H-SiC at and just above the band edge in the range 3.11-3.33 eV (wavelength 380-355 nm). Results of experiments on 200 μm and 490 μm thickness samples indicate partial bleaching in the optical fluence range from 70 W/cm2 to 1.8 kW/cm2, and the absorption coefficient was found to vary by approximately 10% within this range. These experimental findings are supported by simulation results obtained from a first order semi-empirical rate based model linking excitation-induced change in density of states with the absorption coefficient over the range of tested power densities. As expected, this effect scales with photon energy. Characterization of 4H-SiC absorption behavior under varying fluence will aid in design optimization of a Photoconductive Semiconductor Switch (PCSS).

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+ Particle-In-Cell Modeling Of A Reflex-Triode Vircator Using Icepic
  P. M. Kelly; J. Dickens; A. Neuber; J. J. Mankowski
Abstract:  This study describes a three-dimensional particle-in-cell (PIC) simulation for a reflex-triode virtual cathode oscillator (vircator). In particular, these efforts have focused upon the development of a robust, consistent model using the ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) code from the Air Force Research Laboratory (AFRL). The vircator operates at background pressures less than 10-9 Torr and utilizes a bimodal carbon fiber cathode and pyrolytic graphite anode. Experimental data from the hard-tube vircator at Texas Tech University (TTU) is used to validate simulation results. A working, three-dimensional model of a reflex-triode vircator allows for better understanding of the physical processes responsible for microwave generation and thus enables the development of a more efficient and more customizable system. Simulation results detail the virtual cathode formation and the subsequent extraction of radiated microwave energy. Rather than relying on a non-directional isotropic radiation pattern for the radiated power, the true effective radiated power (ERP) from a three-dimensional, frequency-dependent radiation pattern has been extracted from the ICEPIC model. Furthermore, contributions from higher-order modes, particularly in the upper C-band regime, lead to frequency hopping and decreased microwave output power. Simulated results aid in identifying mode contributions and developing schemes to minimize contributions from undesirable modes. ICEPIC results are presented and compared against experimental results at several different operating conditions.

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+ Pulsed Characterization Of A Uv Led For Pulsed Power Applications
  N. A. Wilson; D. L. Mauch; J. C. Dickens; A. A. Neuber
Abstract:  Summary form only given. The electrical and optical characteristics of a high power UV LED (365 nm wavelength) were evaluated under pulsed operating conditions for pulsed power applications. Measurements were made over varying pulse width (30 ns-100 μs), current (0 A-250 A), repetition rate (single shot -1 MHz), and temperature (23° C-80° C). Diagnostics used included a calibrated photodiode operating in the linear regime for transient optical power measurements, a grating / high speed ICCD based spectrograph for transient spectral analysis, and multiple 10:1 standard oscilloscope probes configured differentially for electrical measurements. A red shift was observed in the output spectrum of the LED with increasing temperature and increasing pulse-width. LED forward voltage was observed to increase linearly with increasing current (≈ 3.5 V-5.2 V) and decrease with increasing pulse-width. The peak optical power observed was > 13 W and a maximum efficiency of 22 % was observed. The evaluated LED and auxiliary hardware were successfully used as the optical trigger source for a SiC photoconductive semiconductor switch (PCSS) under high impedance conditions.

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+ Reflex Triode Vircator Design For Extended Frequency Tuning Capabilities
  C. Lynn; D. Barnett; K. Rainwater; A. Neuber; J. Dickens; J. Mankowski
Abstract:  A stand-alone vircator based high power microwave system has been designed as an effects test source. The goal of this system is to cover the frequencies from 4.0 GHz to 6.0 GHz. To date a working prototype has been manufactured and field tested. The current system is capable of producing microwaves at frequencies of 4.06, 4.27, 4.45, 5.83, 5.95 and 6.06 GHz at field levels in excess of 100 kV/m, measured at 3 meters. Additionally, the source and all subsystems are capable of burst mode operation for a duration of 2 s at 500 Hz pulse repetition frequency (PRF). The wide range of tunability was achieved by varying the A-K gap as well as the location of the A-K gap inside the cavity. The A-K gap is varied by a bellows sealed linear actuator which is attached to the cathode. The position of the A-K gap within the cavity is altered by moving a liner and microwave reflector (which form a cavity) within the main vacuum tube. However, the experimentally observed frequencies obtained with the current system leaves a gap from 4.45 to 5.6 GHz. In order for the test system to produce microwaves within this frequency range, a new tube is under development. The cavity of the new tube has a smaller cavity which should push the resonant frequencies into the desired range (from 4.45 GHz to 5.6 GHz). This presentation discusses the experimental results obtained with the new tube and compares that with the previously built and tested source.

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+ The Path To A Transportable Ionospheric Heater
  A. A. Neuber; D. L. Mauch; V. E. Meyers; B. Esser; R. P. Joshi; J. C. Dickens; J. J. Mankowski; T. M. Antonsen
Abstract:  A transportable ionospheric heater, TIH, research design is presented that will enable plasma studies of the ionosphere in latitudes that are presently inaccessible by fixed installations such as HAARP (High Frequency Active Auroral Research Program). The equatorial latitude with close to zero vertical magnetic fields is especially of interest for basic plasma physics studies as well as rf communication enhancement. To achieve a power level in the ionosphere of at least 70 dBW ERP in a footprint significantly smaller than HAARP the radiated power needs to be substantially increased. This minimum ERP is achievable in a 4 × 4 antenna array with 370 kW input power per element with about 25 m by 25 m footprint vs. HAARP's equivalent 365 m by 365 m (360 antenna elements total, 10 kW maximum per antenna). Maximum ERP, up to 95 dBW, may be achieved with the TIH on a 115 m by 70 m platform, a factor 17 reduced size from HAARP. Tunable, Electrically Small Antennas, ESAs are employed to overcome the maximum power limitations of the HAARP dipole based antennas. This demands a step-up from 10 kW to several 100 kW cw power in the 3 to 10 MHz band, which is required to effectively heat the ionosphere. Driving the ESAs necessitates a tunable rf source in the same power and frequency regime, where a more traditional rf tube or all solid state approach may be pursued. The focus of the driver related research has been on photoconductive solid state switching, PCSS, in a direct drive mode that incorporates the driver into the antenna itself. A full size ESA operating at 9.5 to 10 MHz has been demonstrated at 500 W cw power levels and ~ 90% efficiency, driven by a single SiC switch mimicking the full power PCSS operation. The challenges and physics limitations of scaling the switch, the tunable ESA antenna design, as well as their coupling are presented. The significant progress made towards a transportable ionospheric heater as it relates to the physics of the PCSS switching efficiency, electrical breakdown in the MHz regime in large gaps, lower power experiments, and numerical simulations is discussed.

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Publication Year:  2015
+ 1 Khz Rep-Rate Operation Of A Spark-Gap Switched Gyromagnetic Nonlinear Transmission Line Array
  Johnson, J; Reale, D; Barnett, D; Garcia, R; Cravey, W; Parson, J; Neuber, A; Dickens, J; Mankowski, J
Abstract:  This paper details the design, fabrication and performance of a coaxial ferrimagnetic nonlinear transmission line (NLTL), four element array, high power microwave (HPM) system operated at a 1 kHz repetition rate. Prime power is delivered from an 802L TDK Lambda power supply which charges a 5.2 nF capacitor bank up to -40 kV. The capacitors are discharged through a center pin trigatron spark gap. The trigger generator is optically isolated and battery powered for noise immunity and portability. It produces a 20 kV positive polarity pulse with a 20 ns risetime. The high dV/dt (1 kV/ns) is desirable to reduce jitter inherent to spark-gap switching. After the spark-gap switch has closed, the pulse is split four ways. The four pulses propagate through four adjustable delay lines for synchronization of the individual outputs. The four delay lines connect directly into four 76 cm NLTLs with NiZn ferrites where SF6 is the insulating dielectric. Each NLTL is terminated into a custom fabricated, Rexolite-filled, TEM horn antenna via a zipper balun. Lastly, a LabVIEW based control system automates the whole system using a National Instruments cRIO controller. Experimental observations will include in-line D-dot measurements of voltage waveforms and radiated D-dot field measurements.
+ A 2D Finite Difference Simulation To Investigate The High Voltage Blocking Characteristics Of 4H-Sic Photoconductive Semiconductor Switches
  Shaver, J; Mauch, D; Joshi, R; Mankowski, J; Dickens, J; Neuber, A
Abstract:  4H-SiC Photoconductive Semiconductor Switches (PCSSs) have shown significant promise for use in pulsed power related switch applications. This simulation uses the finite difference method, parallelized using a NVIDIA graphical processing unit and the CUDA framework, to solve the system of partial differential equations that model the semiconductor physics involved in the high voltage blocking state of the photoconductive switch. By taking into consideration material properties (mid-band gap trap energy level and concentration, etc.), we are able to gain an understanding of how changes in these parameters affect the space-charge-limited (SCL) currents observed in the high voltage blocking state. This subsequently allows for a fundamental understanding of the parameters controlling the high voltage switching capability of photoconductive switches. Results of the simulation are presented.
+ A Modular, High Rep-Rate, Fast-Risetime, Optically-Isolated, Pulse Trigger Generator
  D. H. Barnett; J. M. Parson; C. F. Lynn; P. M. Kelly; J. C. Dickens; A. A. Neuber; J. J. Mankowski
Abstract:  The paper presents the design and operational characteristics of an optically isolated, compact, modular pulse generator for spark-gap triggering applications. The pulse trigger generator is capable of operating at pulse repetition frequencies (PRFs) > 1 kHz for short bursts with variable pulse magnitudes and risetimes. The trigger generator utilizes a transformer, magnetic switch and an IGBT primary switch. It has been successfully used to trigger a trigatron-driven 10-stage, Marx generator driving a high power load. For portability and noise immunity, the trigger generator is optically isolated from its low voltage control and powered via a lithium ion polymer battery pack. A constant current dc-dc power supply charges the high voltage circuitry of the trigger generator and enables continuous operation with two modules. For operation, a large capacitor is initially charged and used as a buffer energy source. The intermediate charge storage in conjunction with a command-triggered MOSFET, provides the ability to quickly re-charge a trigger capacitor between each pulse of the burst. Circuit topology, experimental data, including voltage and current waveforms, and jitter of the overall system are discussed at various PRFs.

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+ A Wavelet Approach To Far-Field Signal Reconstruction Of Transient Electric Fields
  Eldridge-Looker, K; Fierro, A; Dickens, J; Neuber, A
Abstract:  Large antenna sizes present a severe limitation in the generation of low frequency signals. To alleviate this issue, a series of high frequency sources (and thus, smaller antenna sizes) may be combined to produce a lower frequency output in the far-field. This reconstruction technique applies to signals of arbitrary shapes in addition to frequency lowering. Due to the large parameter space, wavelet decomposition theory is coupled with particle swarm optimization to define appropriate time shifts and amplitude adjustments to the high frequency sources (wavelets) in order to synthesize an output signal with the desired frequency or shape. Fifteen individual high frequency wavelets (f = 1 GHz) are utilized in this application to produce a final output signal in the far-field of 600 MHz. The radiation of a single wavelet is simulated using a broadband Chebyshev TEM horn antenna in order to perform far-field constructive and destructive interference analysis of all 15 wavelet signals. Simulation of the TEM horn antenna shows reasonable wavelet signal fidelity with minimal reflections from the antenna aperture as the pulse is propagated to the far-field. Examination of the far-field electric field enables accurate temporal depiction of the reconstructed signal from the 15 pulsed sources at any given point in space. By adjusting the source array geometry, the location of the desired signal (determined by the particle swarm optimization) can be narrowed down to a single location. Overall, combining particle swarm optimization, wavelet decomposition theory, and electromagnetic wave propagation enables the accurate reconstruction of far-field temporal electric fields from the combination of 15 wavelet sources and verification of the desired signal location.
+ Analysis Of A Tunable Electrically Small Antenna
  Esser, B; Beeson, S; Dickens, J; Mankowski, J; Neuber, A
Abstract:  A tunable, metamaterial-inspired, electrically small antenna topology is evaluated for a possible future use as the principle radiating element in a mobile Ionospheric Heating (MIH) system. The RF source signal is fed via a 50 Omega coaxial cable into a small semi-loop antenna (SLA). This inductively couples to a capacitively loaded loop (CLL) providing a natural 50 Omega match to the source. The resonant frequency of the antenna can be adjusted by varying the capacitance of the CLL via inserting a large permittivity dielectric. A simplified circuit model is used to show that the resonant frequency can be tuned between 40 - 100 MHz. Also, the maximum power handling capabilities achievable with this antenna topology at frequencies relevant to ionospheric heating (similar to 10 MHz and below) are estimated.
+ Analysis Of Cathode Emission Phenomena: Effects Of Barrier Thinning, Field Enhancements And Local Heating
  Majzoobi, A; Joshi, RP; Neuber, A; Dickens, J
Abstract:  Intense electron emission from cathodes that provide very high current densities (several kA/cm(2)) is necessary for a various pulsed power applications. This contribution presents a quantitative analyses of the following processes and inherent physics: (a) Local field enhancements at micro-protrusions, (b) role of ion/ions near the emitting surface in lowering and thinning the potential barrier which increases emission. (c) localized heating at cathode tips that could produce hot-electrons and hot-phonons, ultimately leading to localized melting. Temperatures are predicted to possibly reach the cathode melting point on the nanosecond time scales. This is in keeping with the explosive emission phenomenon that is well known.
+ Burst Mode Operation Of A High Peak Power High Pulse Repetition Rate Capacitor Charging Power Supply
  Holt, SL; Lynn, CF; Parson, JM; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract:  Fast capacitor charging is a power electronics application with unique challenges requiring both low voltage, high current operation and high voltage, low current operation from the same system at different points in the charge cycle. Such dynamically differing operating points create distinct challenges in obtaining high efficiency throughout a charge cycle. In addition, the power supply must be protected from negative voltage swings when attached to a high rep-rate power modulator. This paper presents the design and testing of a rapid capacitor charger designed for high rep-rate command charging of a Marx generator. The output stage uses a large inductor to delay discontinuous conduction during the early portion of the charge cycle and greatly improve efficiency. The design goals and tradeoffs will be discussed and simulation results will be compared to experimental data.
+ Characterization Of Intra-Bandgap Defect States Through Leakage Current Analysis For Optimization Of 4H-Sic Photoconductive Switches
  Thomas, D; Mauch, D; Dickens, J; Neuber, A
Abstract:  A method of characterizing mid-bandgap defect states in high purity semi-insulating 4H-SiC through leakage current analysis for optimization of SiC photoconductive switches is presented. The method utilizes two custom IV curve tracer systems to measure leakage currents through the material under various voltage/current conditions. The first system is used under low current conditions and is capable of measurements from 0 to 45 kV at currents ranging from 0 to 3 mA with pA resolution. A second system measures the transient discharge of a charged capacitor bank through the material. Due to power dissipation concerns, the second system is used for currents higher than 0.1 mA. Voltage/current measurements in this region (>0.1 mA) are of interest due to the information concerning defect states near the conduction band. These shallow defect states are detrimental to switching performance while offering little benefit to voltage hold-off. From the combined data of these two systems, characteristics of the defect states are extracted and presented. We further elucidate the effect of contact annealing temperature on shallow trap levels in electron-beam irradiated material (2*10(18) 1/cm(2)).
+ Characterization Of The Optical Properties Of Gan:Fe For High Voltage Photoconductive Switch Applications
  Meyers, V; Mauch, D; Mankowski, J; Dickens, J; Neuber, A
Abstract:  The optical properties of bulk semi-insulating GaN:Fe are obtained to assess its future suitability as a high voltage photoconductive semiconductor switch (PCSS). The material properties of GaN: Fe hold significant promise to improve devices for pulsed power and other applications. Growth techniques of bulk GaN: Fe, which have hitherto been largely insufficient for commercial applications, are nearing the point that anticipatory characterization research is warranted. In this paper, the optical constants of bulk GaN: Fe (refractive index, absorption coefficient, and off-state dielectric function) were determined by optical reflection/transmission analysis. The results of this analysis are compared with a similar treatment of bulk 4H-SiC as well as possible elements of PCSS housing: Sylgard 184 elastomer, and EFI 20003/50013 electrical potting epoxy. The data presented provide foundational material characterization to enable assessment of the feasibility of GaN: Fe as a practical high voltage PCSS material. Beyond basic materials research, these properties inform design optimization in PCSS construction and implementation.
+ Evaluation Of Gan:Fe As A High Voltage Photoconductive Semiconductor Switch For Pulsed Power Applications
  Mauch, D; Dickens, J; Kuryatkov, V; Meyers, V; Ness, R; Nikishin, S; Neuber, A
Abstract:  Semi-insulating Gallium Nitride is evaluated as a candidate material for use as a high voltage photoconductive semiconductor switch (PCSS) for pulsed power applications. The GaN: Fe samples used for this investigation were commercially available, bulk, semi-insulating samples measuring 10 mm x 10 mm x 475 mu m. Their optical and crystallographic properties were determined utilizing cathodoluminesence, photoluminescence, RHEED, as well as microwave reflection techniques for carrier lifetime studies. Experimental results are presented elucidating the potential of GaN: Fe sustaining high potential differences in both lateral and vertical geometry devices. For instance, electric field hold-off exceeding 100 kV/cm was observed in lateral geometry with mm sized gaps. In addition, a process for the homo-epitaxial growth of GaN: Si was developed in order to facilitate the fabrication of high quality ohmic contacts. Lastly, experimental results evaluating the on-state performance and photocurrent efficiency of a GaN: Fe based PCSS are presented.
+ Gas Selection And Flow Optimization For A 1 Khz Trigatron Spark Gap
  W. H. Cravey; D. V. Reale; R. S. Garcia; J. M. Johnson; A. A. Neuber; J. C. Dickens; J. J. Mankowski
Abstract:  This paper discusses the design of a gas flow system for a 1 kHz repetition rate trigatron based spark gap. The system requires a flow rate high enough to clear the gas from the gap in under 1 ms while also maintaining the required operating pressure. The gap is a trigatron based design with an alumina insulated tungsten trigger pin. A 20 kV positive polarity pulse, with a 20 ns risetime, is applied to trigger the main gap which is charged to -40 kV. A brass guard ring is employed to shield the walls of the containment structure from contaminants in order to achieve a longer operational lifetime. Dry air and Nitrogen gasses are tested at various pressures and flow rates. Design considerations, such as inlet and outlet sizing and gap geometry, are taken into account to ensure a high air flow is achieved between the electrodes.

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+ Implementation Of A Battery Management And Protection System For High Power Pulsed Applications
  Cordero, E; Holt, S; Dickens, J; Neuber, A; Mankowski, J
Abstract:  High power pulsed systems often require high current 3-phase electrical service or large generators to meet prime power requirements, limiting the portability of the systems. A high power battery system offers a useful alternative for mobile applications. Of all battery chemistries, Lithium-ion polymer (LiPo) has become popular in consumer electronics due to its high energy density, low self-discharge rate and lack of memory. Unfortunately, the chemistry still has limitations. Overcharging or overheating of a LiPo cell may result in ignition and over-discharge can destroy the cell. To prevent these hazards, overcharge and undercharge conditions must be monitored at the cell level rather than the battery level because the charge and discharge efficiency varies from cell to cell causing cell voltages within a battery to diverge during normal operation. A battery management system designed to monitor and maintain a large battery designed for pulsed power applications is presented in this poster. The design of this battery management system is presented and its implementation in a multi-cell, high voltage battery capable of high current pulsed operation. A hazard analysis of high power batteries and the implemented safety system is also provided.
+ Initial Testing Of A Reflex Triode Vircator With Adjustable A-K Gap And Cavity Reflector
  Lynn, CF; Parson, JM; Kelly, P; Barnett, DH; Neuber, AA; Dickens, JC; Mankowski, J
Abstract:  Virtual cathode oscillators (vircators) can be easily tuned by altering the current density of the electron beam. This can be accomplished by changing the voltage applied to the a-k gap, or by physically changing the gap separation. Previous results with an adjustable a-k gap vircator achieved discrete tuning at frequencies of 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz. To achieve better tunability an adjustable cavity reflector was incorporated into the vircator design. Initial testing of the new vircator design has shown increased output power, as well as the ability to greatly increase the number of frequencies that can be tuned. Sweeping the A-K gap with the cavity reflector in two positions yielded 10 different frequencies of operation. Also sweeping the cavity reflector with the a-k gap set at 8.0mm yielded another unique frequency. Currently 11 unique frequencies have been identified, and the peak field measured from this vircator increased from similar to 40 kV/m to similar to 69 kV/m (> 70% increase in field). This paper illustrates the design of the vircator and presents some of the microwave data obtained. Additionally, a summary of all of the frequencies obtained and the peak field measured are included.
+ Repetitive Operation Of A Megawatt Class Reflex-Triode Vircator
  J. M. Parson; C. F. Lynn; D. H. Barnett; S. L. Holt; P. M. Kelly; J. C. Dickens; A. A. Neuber; J. J. Mankowski
Abstract:  Repetitive operation of high-power microwave (HPM) sources is primarily restricted by thermal properties of anode and cathode materials. Note that even in single shot operation, vircators generally inject undesired plasma from the anode/cathode surfaces under high current densities, leading to excessive heating of electrode materials, vacuum degradation and impedance collapse that may ultimately lead to cut-off of microwave power. Heating of the anode and cathode materials during repetitive operation intensifies these problems and will cause accelerated erosion and/or permanent damage of the anode and cathode surfaces, especially when metallic electrodes are used. Hence, the vircator herein utilizes a carbon fiber cathode and a pyrolytic graphite anode. This paper elucidates the heating of the anode and cathode, and their spectrally resolved black-body radiation properties during repetitive operation.

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+ Solid State Linear Transformer Driver (Ltd) Development For Hpm Sources
  Collier, L; Walls, MB; Dickens, J; Mankowski, J; Neuber, A
Abstract:  The Marx pulse generator topology has been widely used in pulsed power applications(1,2). Another pulse generator topology, the linear transformer driver (LTD), has been developed3 that may serve as a viable alternative to the Marx generator. LTDs utilize inductively added stages to achieve high voltages and currents. Unlike a Marx generator, each stage in an LTD features multiple bricks, all of which are ground referenced and allow current to be distributed amongst an arbitrary number of switches. This allows for LTDs that utilize solid-state switches, potentially resulting in more compact and reliable pulse generators. A solid-state, >10 kA peak current, multiple-stage LTD is developed. The generator's performance will be analyzed for viability as a replacement for driving a high power microwave generator.
+ System Integration Of A Coaxial Ferrimagnetic Nonlinear Transmission Line Array
  R. S. Garcia; D. V. Reale; J. M. Johnson; W. H. Cravey; A. A. Neuber; J. C. Dickens; J. J. Mankowski
Abstract:  This paper describes the system integration of a four element, nonlinear transmission (NLTL) array. The HPM system components include a negative polarity 40 kV capacitor charging supply, a trigatron spark gap, a solid-state trigger generator, four delay lines with biasing coils, four coaxial ferrimagnetic NLTLs with biasing coils, eight dc current supplies, and four TEM horn antennas. A National Instruments cRIO FPGA based controller is used to interface the capacitor charging supply, the biasing coil power supplies and the trigger generator to a laptop based GUI. In order to minimize impact from EMI, all lines were properly shielded and the data acquisition equipment was located a sufficient distance away from the NLTL array. The GUI consists of options for controlling the current output on all 8 power supplies for biasing purposes, as well as pulse options that include repetitive pulsing based on the number of pulses or for a specific length of time. Pulse duration for the capacitor charger, the command charge, and the trigger can also be adjusted. An emergency stop button is included for safe shutdown of the trigger generator and all power supplies. A diagnostics setup is outlined describing the placement of inline coaxial D-Dot probes and a high voltage probe for laboratory testing.

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Publication Year:  2014
+ 10 Kv, 44 Ns Pulse Generator For 1 Khz Trigatron Reprate Operation Of Nltl
  J. M. Johnson; J. M. Parson; D. V. Reale; A. A. Neuber; J. J. Mankowski; J. C. Dickens
Abstract:  This paper presents a design and implementation of a 1 kHz, 10 kV, positively pulsed trigger generator with an average risetime of 44 ns for use in a coaxial ferrimagnetic Nonlinear Transmission Line (NLTL) High Power Microwave (HPM) system. The pulse trigger generator is based on two stacked 4 kV current-controlled n-type thyristors in a low inductance package capable of driving inductive loads with risetimes less than 200 ns. However, by implementing a magnetic switch this relatively is slow risetime is sharpened to tens of nanoseconds. The stacked boards allow for voltage addition at the output while maintaining the fast riestime.

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+ A Battery Powered 80 Kva Capacitor Charger
  S. Holt; E. Cordero; J. Mankowski; J. Dickens; A. Neuber; M. Scott; S. Calico
Abstract:  The design of a battery powered, rapid capacitor charger is discussed. The charger design is capable of an average output power of 80 kVA, for a burst time of roughly 5 seconds. Cool down time is required between bursts. A Lithium Ion Polymer (LiPo) battery pack with a nominal voltage of 355 V and fully charged open circuit DC voltage of 385 V will be used to power the rapid capacitor charger. The general topology of the charger is as follows. An IGBT H-bridge inverter uses pulse width modulation to create an AC waveform that is stepped up to a maximum of 58kV by a 1:170 ratio transformer. This high voltage output of the transformer is rectified and used to charge the capacitor bank.

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+ A Metamaterial-Inspired Electrically Small Antenna For Operation At 2 To 20 Mhz
  Gatewood, P; Neuber, A; Dickens, J; Mankowski, J
Abstract:  A metamaterial-inspired, efficient, electrically small antenna (ESA) is designed for the 2 to 20 MHz range, intended for use in a mobile Ionospheric Heating (MIH) system. The ESA consists of two main parts: a small semi-loop antenna (SLA), and an outer capacitively loaded loop (CLL). To increase the capacitance of the CLL and reduce the size of the ESA, a high permittivity dielectric is utilized. All designs were simulated in Ansoft HFSS with particular attention on radiation efficiency and bandwidth. The design aims for these ESAs are a high overall efficiency at dimensions much smaller than a wavelength.
+ A Modular High Power Battery System For Pulsed Power Applications
  E. Cordero; S. Holt; J. Dickens; A. Neuber; J. Mankowski; S. Calico; M. Scott
Abstract:  This paper presents the design of a scalable, high power battery system for pulsed power operations. The battery system is modular in design, with each module containing four Lithium Ion Polymer (LiPo) cells and a custom designed cell management board that actively monitors the voltage and temperature of each cell and also provides cell balancing functionality. The system is designed to be scalable by adding up to 25 modules in a series configuration. While the battery management system should be compatible with any lithium ion cells, this implementation uses 8 Ah capacity dual-core LiPo cells, rated for a 150 C discharge rate; allowing for a peak current output of 1,200 A. With 25 modules (96 LiPo cells) the system would have an open circuit voltage of 385 V and be capable of providing up to 1,200 A at 355 V for a peak output power of 420 kW. Special attention has been placed on safety features including overvoltage, undervoltage and temperature monitoring of every cell in the system. The charging/balancing system is capable of automatically shutting down if any of the voltages or temperatures exceeds established limits. The management circuitry is designed to have a low off-state power draw in order to maximize battery life when the system is not in use.

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+ A Wavelet Approach To High Power Microwaves
  Eldridge, K; Fierro, A; Dickens, J; Neuber, A
Abstract:  Wavelet decomposition and reconstruction are utilized to synthesize a high power microwave (HPM) signal at a frequency below the frequency of the employed radiating sources. Employing a larger number (on the order of ten) of smaller sources that produce short radiating pulses combined with appropriate amplitude scaling and shifting of the individual pulses enables the generation of a single waveform of longer duration. We describe the mathematical approach to the wavelet synthesis and give examples. For instance, an array of 10 sources, each producing a 0.5 ns pulse can be adjusted to generate a sinusoidal wave with a period of approximately 2 ns. The results of low power experiments are discussed in detail to demonstrate the practical feasibility of the wavelet approach.
+ Carrier Lifetime Measurement Of Silicon Carbide For Photoconductive Switch Applications Using An Ir Probe Laser
  White, C; Mauch, D; Thomas, D; Dickens, J
Abstract:  A system for measuring the recombination lifetime of high purity, semi-insulating (HPSI) 4H-SiC through transient free carrier absorption (FCA) for optimization of SiC photoconductive semiconductor switches (PCSS) is presented. The system measures the transient absorption of a continuous, low-power (<5mW) 1550 nm infrared probe laser. Free carriers were generated with both above and below bandgap illumination from the harmonics of a Nd:YAG laser (532 nm, 355 nm, and 266 nm - 10ns FWHM), and the carrier lifetime was numerically calculated from the absorption transient. High spatial resolution (similar to 10 um) was attained through the use of a high-precision, three-axis stage. The carrier lifetime measurements of various regions of several SiC PCSSs over varying levels of photo-excitation are presented.
+ Characterization Of Mid-Bandgap Defect States In 4H-Sic For Optimization Of Sic Photoconductive Semiconductor Switches
  Thomas, D; Mauch, D; White, C; Neuber, A; Dickens, J
Abstract:  A method of characterizing mid-bandgap defect states in high purity semi-insulating 4H-SiC through leakage current analysis for optimization of SiC photoconductive semiconductor switches, PCSS, is presented. The method utilizes two custom IV curve tracer systems to measure leakage currents through the material under various voltage/current conditions. The first system is used under low current conditions and is capable of measurements from 0 to 45 kV at currents ranging from 0 to 3 mA with pA resolution. While voltage/current measurements in the region >0.1 mA are of primary interest for quantifying defect states near the conduction band, standard IV measurements become difficult due to excessive power dissipation in the PCSS. Hence, a second system operating in transient mode is used for currents higher than 0.1 mA. This system measures the transient discharge of a charged capacitor through the PCSS, allowing for high current measurements while subjecting the material to high power dissipation for only a short period of time (milliseconds). It is the goal to extract from the combined data of these two systems characteristics of the defect states (concentration, energy level).
+ Design And Operation Of A Fast-Risetime, 500 Hz, 24 Kv, Optically-Isolated Pulse Trigger Generator
  D. H. Barnett; J. Parson; C. Lynn; P. Kelly; M. Taylor; J. Dickens; A. Neuber; J. Mankowski; S. Calico; M. Scott
Abstract:  This paper presents the design and performance characteristics of an optically isolated, trigger generator for spark gap applications that is capable of operating at over 500 Hz rep rate. The pulsed trigger generator is designed to achieve fast risetimes on the order of 1kV/ns to cause breakdown between the trigatron trigger pin and the opposite electrode. The pulsed trigger generator is designed to trigger a 10-stage, 500 kV, 42 J Marx generator. The system is capable of delivering a 4"“25 kV pulse with a 10/90 risetime of 20 ns. A board was fabricated to control a capacitor charger and high voltage thyristor via fiber optic inputs. The capacitor charger, a 60 W constant current dc-dc power supply with variable voltage control, is capable of charging 1 nF in less than 0.3 ms. The system is battery-powered using a 33 V lithium polymer battery pack for isolated operation in noisy environments. For burst mode operation, a larger 250 nF buffer capacitor, which is initially charged, is connected through a 100 kΩ resistor to the trigger capacitor, which is varied between 1 and 40 nF. The value of the trigger capacitor is adjusted to provide optimum performance for a given transformer and magnetic switch. System modeling results along with experimental data are discussed including the reliability and performance of the overall system including jitter at 500 Hz operation.

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+ Frequency Tuning A Reflex Triode Vircator From 1.5 To 5.9 Ghz
  C. F. Lynn; J. Parson; P. Kelly; M. Taylor; D. Barnett; A. Neuber; J. Dickens; J. Mankowski; S. Calico; M. Scott
Abstract:  Vircators (virtual cathode oscillators) are easily frequency tunable by simply altering the electron beam current density through either adjusting the accelerating voltage or varying the anode cathode gap (A-K gap). It is noted that vircators typically exhibit frequency hopping, an often undesired feature. Here we address the frequency hopping problem by introducing a rectangular shaped carbon fiber cathode with the width of the cathode less than one quarter of a wavelength in the direction of microwave propagation. Operating the vircator with the rectangular cathode revealed a match with a resonant condition from back wall reflections, which explains the observed dominant microwave frequency selection. That is, the vircator emitted stably during 50 ns operation at 3.89 GHz or 2.07 GHz with a 3 dB bandwidth of 37.5 MHz or 93.7 MHz, at A-K gap spacing of 8 mm, and 12 mm respectively. To further investigate frequency tuning, the A-K gap was swept from 3 mm to 17 mm while keeping a constant 5.1 cm diameter circular cathode. The resulting frequencies of operation were 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz at A-K gap spacing of 17mm, 14 mm, 7 mm, 5.75 mm, and 3 mm respectively. This demonstrates that a tuning range of 4.4 GHz should be achievable for a practical vircator system.

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+ Measurement And Simulation Of Pulsed Plasma Development At Medium Pressure In A Non-Uniform Field
  A. Fierro; J. Dickens; A. Neuber
Abstract:  The development of plasma in a medium pressure (50 torr) environment in nitrogen was studied by simulation and measurement under the influence of non-uniform, pulsed electric fields. A GPU-accelerated, 3-dimensional particle-in-cell (PIC)/Monte Carlo Collision (MCC) simulation code was written utilizing the CUDA platform to simulate pulsed plasma development in a nitrogen environment and uncover the transient plasma characteristics in detail. The simulation provides significant speed-up over the CPU equivalent implementations. Experimentally, a needle-protrusion (1.5 mm in length, 200 μm tip radius) opposite a brass ground plane with the distance between needle-tip and wall held at 1.5 mm provided a non-homogeneous field. Excitation of the needle-plane gap was achieved with a ~100 ns rise-time high-voltage pulser with a peak voltage of 30 kV. Diagnostics included time-resolved nanosecond gated imaging for light intensity measurements and high speed electrical probes for timing. A time series of the plasma formation captured with a 5 ns camera gate revealed a mostly uniform expanding plasma cloud from the needle tip.

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+ Operation Of A 500 Kv, 4 Ka Marx Generator At 500 Hz Rep-Rate
  Taylor, MB; Kelly, PM; Parson, JM; Lynn, C; Dickens, JC; Neuber, AA; Mankowski, JJ; Bragg, JWB; Calico, S; Scott, M
Abstract:  A 42 J, 10-stage pulse forming network (PFN) Marx generator capable of producing a 500 kV, 50 ns full-width-half-max (FWHM), similar to 5 ns rise time pulse into an open load at a rep-rate of 500 Hz has been designed for use as a pulsed power source for a reflex triode virtual cathode oscillator (vircator). Rayleigh PFNs are used in place of discrete capacitors for each stage of the 10-stage Marx generator. Effort was taken to minimize parasitic inductance such that the quality of the pulse shape is maintained as much as possible. In order to rep-rate the Marx generator, a trigatron-based triggering scheme is used to initiate erection of the Marx generator. A 20 ns risetime, 24 kV solid-state pulse trigger generator capable of operating at high repetition rates is used to drive the trigatron. The required charge rate for a 500 Hz pulse repetition frequency (PRF) for the Marx generator is 24 kW. Repetitive operation requires additional design considerations that would be irrelevant to single pulse firing. Pressurized air is jetted across the spark gaps by means of built-in gas manifolds to remove remaining ionized gas between each pulse and prevent premature erection during the subsequent charging cycle. The built-in gas manifolds were designed using a hydrodynamic simulation to ensure equal flow rate across each of the spark gaps and equal pressure along the length of the tube chamber.
+ Overview Of High Voltage 4H-Sic Photoconductive Semiconductor Switch Efforts At Texas Tech University
  D. Mauch; C. White; D. Thomas; A. Neuber; J. Dickens
Abstract:  Recent advances and the current state-of-the-art for high speed 4H-SiC photoconductive semiconductor switches (PCSS) developed at Texas Tech University are summarized. A performance comparison of multiple generations of switch designs is also presented. These devices have experimentally demonstrated the capability of blocking DC electric fields up to 705 kV/cm (<;0.1 mA leakage current), rise times of 0.63 ns (20/80), and switching 20 kV at 250 A with a di/dt of 75 kA/us at a burst repetition frequency of 65 MHz. Findings and optimizations pertaining to device geometry, sub-contact doping, contact thickness, triggering wavelength, and electron irradiation are presented. Device modeling and experimental results investigating current issues with device lifetime are presented as well.

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+ Performance Of St707 Getter Material In A Rep-Rated High Power Microwave Sealed-Tube Vircator Under Uhv Conditions
  Kelly, PM; Parson, JM; Lynn, C; Taylor, M; Dickens, JC; Neuber, A; Mankowski, J; Calico, S; Scott, M
Abstract:  This study focuses on the use of St707 non-evaporable getter (NEG) material in a high power microwave (HPM) sealed-tube virtual cathode oscillator (vircator) operated at repetition rates up to 200 Hz. High-current pulsed operation releases gases trapped within the bulk materials and gas monolayers on material surfaces, leading to localized plasma production between the A-K gap. This potentially leads to gap closure, shortens the duration of microwave production, and spoils low vacuum levels. A single current pulse may increase the chamber pressure to the low 10(-6) Torr range from an initial background pressure in the low 10(-9) Torr range, desorbing approximately 10(14) particles. For single pulse operation, it is sufficient to maintain this background pressure (10(-9) Torr) with a small ion pump rated at 20 L/s. However, at 200 Hz operation, a pumping speed of 1,000 L/s is needed to evacuate 10(14) particles from the vircator volume between pulses. Rather than adding a bulky pump system with the required high pump speed, St707 getter material (70% Zr, 24.6% V, 5.4% Fe) has been used to maintain ultra-high vacuum (UHV) conditions during rep-rate operation. Characteristics of the St707 NEG pump in a repetitive, high-current pulse environment are presented. Diagnostic results obtained with an inverted magnetron cold cathode gauge for absolute pressure are utilized to analyze NEG performance in detail.
+ Radiation From Sic Pcss Driven Gyromagnetic Nonlinear Transmission Line High Power Microwave Source
  D. V. Reale; D. Mauch; J. M. Johnson; A. A. Neuber; J. C. Dickens; J. J. Mankowski
Abstract:  An all solid-state high power microwave (HPM) source is constructed using a photoconductive semiconductor switch (PCSS) based HV pulse generator to drive a sulfur hexaflouride (SF6) insulated coaxial ferrimagnetic nonlinear transmission line (NLTL) which feeds a TEM horn antenna. The PCSS was fabricated from high purity semi-insulating (HPSI) 4H-SiC and is illuminated with 2 mJ from a frequency tripled Nd:YAG laser at 355 nm with a 7 ns FWHM. Fixed fiber optic delay lines are utilized to generate a burst of four optical pulses from a single solid-state laser source for rep-rate operation. The input to the NLTL is an adjustable pulse from 3 kV to 6 kV with sub-ns rise time and the resulting output of the NLTL is radiation in the L-band to S-band regime with RF power from 100"“200 kW depending on charge voltage. A Rexolite® insulated zipper transition, in which the inner conductor of the coax is gradually exposed, is used at the output of the NLTL to transition from a coaxial structure to parallel plate structure in order to feed a free space TEM horn. The zipper transition and feed section of the TEM horn are potted using Sylgard® 184 silicone elastomer to prevent breakdown at the antenna feed and maintain a homogenous dielectric constant for the transition region. Radiated waveforms are presented for several charge voltages and bias conditions.

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+ Reduction Of High Power Microwave Breakdown Delay Times Using Multiple Passes Through A Te111 Resonator
  S. Beeson; J. Dickens; A. Neuber; S. Lin
Abstract:  Summary form only given. The delay time between the rising edge of a high power microwave (HPM) pulse and plasma formation in a gas at close to atmospheric pressure is studied for a multi-pass geometry. A fundamental lower limit of the breakdown delay time is the formative delay time, defined as the time it takes for the exponentially rising electron density to reach the critical electron density for which the EM wave begins to reflect and attenuate; which is on the order of 10's to 100's of nanoseconds for the conditions of interest. The goal of this research was to surpass this limit by making two passes through the generated plasma with the HPM pulse. That is, an external structure is utilized to time delay the HPM pulse on itself for a second pass through a gas cell.

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+ Rep-Rate Operation Of A ~200 Kv Sealed-Tube Reflex-Triode Vircator At ~200 A/Cm2
  J. M. Parson; J. -W. B. Bragg; M. Taylor; D. Barnett; P. Kelly; C. F. Lynn; S. Holt; J. C. Dickens; A. A. Neuber; J. J. Mankowski
Abstract:  Thermal limitations of anode and cathode materials have shown to negatively impact operation of cold-cathode high-power microwave (HPM) sources. High pulse-repetition-frequency (PRF) operation of these devices exacerbates the problems already experienced in single shot mode where cold-cathode devices, specifically carbon fiber cathodes, are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes impedance collapse of the anode-cathode (A-K) gap. Hence, for frequency stable, repetitive operation, cold-cathode HPM devices require the use of thermally robust electrode materials and ultra-clean surfaces, leading to repeatable tube operation. This study focuses on burst-mode operation of an HPM sealed tube reflex-triode virtual-cathode-oscillator (vircator) for PRFs greater than 100 Hz. The vircator is driven by a 54 J, ~200 kV Marx generator with an approximate pulse width of 50 ns FWHM, and the vircator chamber has an empty volume of approximately 5 L with background pressures in the low 10-9 Torr. The anode materials studied include grade-1 titanium (TiG1), nickel 201L (Ni201L), and stainless steel 316L (SS316L); all in combination with a carbon fiber cathode. Empirically observed outgassing characteristics in conjunction with anode thermal modeling are presented under single-shot and rep-rate conditions. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques were employed to investigate anode and cathode surface integrities before and after vircator operation.

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+ Rep-Rate Operation Of A 300 Kv, High-Power Microwave Sealed-Tube Vircator
  J. M. Parson; C. F. Lynn; J. -W. B. Bragg; P. Kelly; M. Taylor; D. Barnett; S. Holt; J. C. Dickens; A. A. Neuber; J. J. Mankowski; M. C. Scott; S. E. Calico
Abstract:  Thermal limitations of anode materials are known to impose limits on rep-rate operation of cold-cathode high-power microwave (HPM) sources. This study focuses on performance of pyrolytic graphite (PG) anodes at a 500 Hz burst-mode operation in a reflex-triode virtual-cathode-oscillator (vircator). In most experiments, a 42 J, 300 kV pulse forming network (PFN) based Marx generator with an approximate pulse width of 50 ns full-width-half-max (FWHM), was utilized to drive the vircator. Rep-rated operation of the vircator exacerbates the problems already experienced in single-pulsed mode where vircators are plagued by plasma formation on the anode and cathode followed by plasma/gas expansion that causes degradation of anode materials. Hence, for frequency-stable, repetitive operation, vircators require the use of thermally robust electrode materials and ultra-clean vacuum surfaces, leading to repeatable diode operation. This contribution presents thermal modeling of anode heating and experimental electrical behavior of vircator rep-rate operation.

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Publication Year:  2013
+ A High-Power Transient Coaxial Power Combiner For Nonlinear Transmission Lines
  D. Reale; J. -. B. Bragg; N. Gonsalves; W. Sullivan; A. Neuber; S. Holt; J. Mankowski; J. Dickens
Abstract:  Summary form only given. Recent work on Coaxial Ferrimagnetic Nonlinear Transmissions Lines (NLTL) has been focused on developing an array of NLTLs for use as a solid-state High Power Microwave (HPM) source. The pulsed output of an NLTL requires a combiner that enables combining transient signals at voltage levels up to 50 kV. Existing combiner designs found in literature require resonant structures to achieve efficient power combination, an approach that fails for the short pulse transient signals generated by the NLTL. The novel coaxial combiner is an in-plane structure designed to combine two 50 Ω NLTLs into a single 25 Ω coaxial output which then uses a linear taper to transition back to 50 Ω. The combiner design was simulated using a transient Finite Element Method (FEM) model in COMSOL Multiphysics® and experimental results are compared with simulation. A four-way radial combiner based on the in-plane two-way design is simulated and the field stresses are examined to determine possible operational voltage levels for the four-way design.

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+ Bias Field Controlled Phasing Of Ferrimagnetic Coaxial Nonlinear Transmission Lines
  D. Reale; J. -. B. Bragg; S. Holt; A. Neuber; J. Mankowski; J. Dickens
Abstract:  Coaxial Ferrimagnetic Nonlinear Transmission Lines (NLTL) are utilized as all solid state High Power Microwave (HPM) sources with virtually fixed phase relationship between input voltage pulse and output rf wave. Since the output power of a single NLTL at a given frequency, and of a given size, is limited by the effect of line dimensions as well as charge voltage on operating frequency, we employ an NLTL array to achieve higher power levels. To operate in microwave generation mode the ferrite domains are aligned along the axial direction of the NLTL using a biasing magnetic field generated by a dc current driven solenoid. This biasing method also provides a means of controlling the line delay by adjusting the magnetization of the ferrites using an auxiliary delay coil. By utilizing the delay coil and the primary biasing coil together on an NLTL, the operating frequency can be locked at a given frequency while the delay of the line can be independently adjusted. The relationship between the delay coil field and the corresponding temporal delay is investigated, and two NLTLs fed by identical voltage pulse shapes are combined using the phase control method described.

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+ Current Capabilities Of A Low Inductance Marx Generator For Driving A High Power Microwave Source
  P. Kelly; C. Lynn; J. Parson; M. Taylor; J. Mankowski; A. Neuber; J. Dickens
Abstract:  The electrical characteristics and current capabilities of a low inductance Marx generator used to drive a high power microwave (HPM) source are described. The operating frequency of many HPM sources is dependent upon various characteristics of the input voltage pulse from the pulsed power generator. Texas Tech University has developed an 8 stage, 80 J Marx generator used to drive HPM sources at repetition rates up to 500 Hz. The Marx generator is constructed using pulse forming networks rather than discrete capacitors. Experimental efforts have shown stable frequency operation of a virtual cathode oscillator (Vircator) source using a low energy, PFN Marx. Efforts were taken to reduce parasitic inductance of the PFN Marx resulting in a voltage pulse with a risetime less than 25ns with 50ns FWHM. Additionally, efforts were taken to improve impedance matching between the PFN Marx and the vircator load. This article presents experimental results of an 80 J Marx at voltages ranging from 150 kV to 250 kV operating in burst mode. Voltage and current waveforms from the Marx are shown as well as some typical results of compact sealed tube vircator operation.

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+ Frequency Tunable Sealed Tube Reflex Triode Vircator
  M. Taylor; P. Kelly; C. Lynn; J. Parson; J. Mankowski; A. Neuber; J. Dickens; M. Kristiansen
Abstract:  Summary form only given. A virtual cathode oscillator (vircator) is a high power microwave device that exhibits frequency tunability which is inherent to its principle of operation. Two types of electron motion within the vircator generate microwaves (virtual cathode oscillation and reflexing electron motion). Although it is difficult to precisely predict the dominant microwave frequency of a vircator design prior to experimental observation, the oscillation frequency of the virtual cathode is approximately proportional to the plasma frequency of the electron beam as it enters the virtual cathode. Additionally, the reflexing electrons oscillate at a frequency which is inversely proportional to approximately four times the transit time of an electron through the anode-cathode gap (A-K gap). As a result, assuming space charge limited diode operation, the virtual cathode and reflexing electron oscillation frequencies, though different, are proportional to V1/2/d, where d is the gap separation of the A-K gap and V is the accelerating voltage applied to the gap. Thus vircators are tunable via adjusting the A-K gap and varying the applied accelerating voltage. Texas Tech University has developed a sealed tube vircator which radiates approximately 100 MW with an operational frequency of 4GHz. Operating at 4GHz the diode has an A-K gap of 8mm, an accelerating voltage of ~200 k V, and electron beam current of ~3.5kA. Here we present an experimental investigation of the tunability of a reflex triode vircator by varying the A-K gap from 4 mm to 10 mm and accelerating voltages ranging from 150 kV to 250 kV.

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+ High Voltage Solid Dielectric Coaxial Ferrimagnetic Nonlinear Transmission Line
  D. Reale; J. -. B. Bragg; R. Solarski; A. Neuber; S. Holt; J. Mankowski; J. Dickens
Abstract:  Previously developed coaxial ferromagnetic Nonlinear Transmission Lines (NLTL) relied solely on pressurized Sulfur Hexaflouride (SF6) as high-voltage insulating dielectric medium [1]. While the use of SF6 provides the necessary electric insulation, there are drawbacks including gas storage and pressure fittings that increase system size and add to the design complexity of the NLTLs themselves. We evaluate solid dielectric materials as an alternative. Initial attempts used a standard high voltage (HV) epoxy to pot the NLTL assembly. This method was effective at producing magnetic precession in the NLTL; however, there was a reduction in output power due to the high loss tangent of the epoxy. Sylgard 184, commonly used in solar cells, has been widely used as an HV potting material and has an order of magnitude lower loss tangent than standard HV epoxy at 1 kHz. Samples of HV epoxy and Sylgard 184 are tested in a microwave cavity resonator at 3 GHz to determine their loss tangents. The performance of an NLTL potted with Sylgard 184 is compared to that of the HV epoxy NLTL and the current SF6 insulated NLTL.

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+ Microwave Radiation From A Sic Pcss Driven Gyromagnetic Nltl
  J. -. B. Bragg; W. W. Sullivan; D. V. Reale; D. L. Mauch; S. Holt; A. A. Neuber; J. Mankowski; J. C. Dickens
Abstract:  An all solid-state high power microwave system comprised of a photoconductive semiconductor switch (PCSS) and nonlinear transmission line (NLTL) is presented. A single 50 kV 4H-Silicon Carbide (SiC) PCSS switches 7 ns (FWHM) pulses with 2 ns risetime into the gyromagnetic NLTL. The PCSS achieves sub-ohm resistance when illuminated by approximately 3 mJ of laser energy emitted from a tripled Nd:YAG laser (355 nm). Utilizing a fiber optic based optical triggering system enabled generating a train of laser pulses, and burst-mode operation with 65 MHz pulse repetition frequency was achieved. The NLTL sharpens the fast rising pulse from the PCSS and produces MW-class rf power levels at 2-4 GHz. Microwave generation is achieved through stimulation of damped gyromagnetic precession when the static and pulsed external magnetic fields interact with the magnetic moments of the nonlinear ferromagnetic material. The acting nonlinear material is a NiZn composite with dimensions 3 mm × 6 mm (ID × OD). The system is terminated with a wideband antenna consisting of a coaxial to parallel-plate conversion balun which is then immediately flared to provide a linearly-flared TEM horn antenna. The entire length of the compact HPM system measures 1.15 m. Radiation results for single shot and burst-mode operation are presented as well as demonstration of operational frequency control.

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+ Performance And Characterization Of A 20 Kv, Contact Face Illuminated, Silicon Carbide Photoconductive Semiconductor Switch For Pulsed Power Applications
  D. L. Mauch; W. W. Sullivan; A. B. Bullick; A. A. Neuber; J. C. Dickens
Abstract:  A 20 kV, lateral geometry, contact face illuminated, silicon carbide (SiC) photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS was fabricated from high purity semi-insulating, bulk 4H-SiC (12.7 mm × 12.7 mm × 0.35 mm), in a lateral geometry, (0.6 mm gap) with both the anode and cathode contacts located on the same face of the device. The device was illuminated with light from a tripled Nd:YAG laser (355 nm-7 ns FWHM) entering from the contact face. The device demonstrated sub-ohm on-state resistance for laser pulse energies in the mJ range, and micro-amp leakage currents at 20 kVdc in the off-state. Voltage hold-off and low leakage currents in the off state were achieved through high energy electron beam irradiation of the bulk material. The switch's geometry and packaging are discussed, along with experimental switching and blocking characteristics.

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+ Performance Of Neg Getter Materialin A Sealed Reflex-Triode Vircator At 225 A/Cm2
  P. Kelly; J. M. Parson; C. F. Lynn; J. J. Mankowski; J. C. Dickens; A. A. Neuber
Abstract:  Summary form only given. This study focuses on the use of non-evaporable getter (NEG) materials in a high power microwave (HPM) virtual-cathode oscillator (vircator). The vircator has a background pressure in the low 10-9 torr range with a vacuum volume of 5 L backed by a small 20 L/s ion pump. Presently, the source is driven by an 80 J, 300 kV Marx generator with a current density of 225 A/cm2 and pulse width of 100 ns. Initial current pulses increases the chamber pressure to the low 10-6 torr range desorbing approximately 1014 particles1,2. Reduction in the number of particles desorbed decreases an order of magnitude after approximately 2,000 current pulses, with no further noticeable decrease after 10,000 pulses3. The HPM system will eventually be modified to operate at 500 Hz with a pulse width between 10-20 ns. In first order approximations, a pumping speed of 2500 L/s will be needed to pump 1014 particles within 2 ms from a 5 L volume. This corresponds to a surface area of approximately 2400 cm2 for a zirconium-vanadium-iron (ZrVFe) getter material. An introduction of NEG materials into the vacuum chamber will aid in achieving repetitive operation; and preliminary results on the use of the NEG material in the vacuum environment will be presented. A residual gas analyzer (RGA) and an inverted-magnetron cold cathode vacuum sensor are used for the gas diagnostics and are rated to survive the 300°C, 72 hour vircator bake out process.

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+ Post Pulse Recovery Of Hpm Generated Plasma At Close To Atmospheric Pressure
  S. R. Beeson; J. C. Dickens; A. A. Neuber
Abstract:  Summary form only given. We study the recovery of the low temperature plasma generated by a microsecond high power microwave, HPM, pulse in different gases at pressures approaching one atmosphere. The experiment mimics the electrical breakdown at the interface between the vacuum HPM-source environment and the radiating environment (generally held at atmospheric pressure). This low temperature plasma is generated along the surface on the atmospheric side where the high Eeff/p, or reduced effective electric field with pressure, causes high levels of ionization leading to the formation of a highly conductive plasma within the duration of the microsecond pulse. With the maximum HPM pulse repetition rate being highly dependent on the relaxation of this plasma, it is the goal of this study to determine the kinematics and chemistry of this low temperature plasma as it pertains to ion, electron, and excited species densities. For this study, a 2.85 GHz magnetron operating in the TE10 mode of a WR-284 standard waveguide is used to generate a 3 MW, 3 microsecond pulse with a rise time of less than 50 ns that is incident on the dielectric window separating the two environments. The atmospheric side is enclosed in a structure that mimics an open radiation pattern similar to that of current HPM systems. To understand the post-pulse features, two custom multi-standard waveguide couplers were designed to implement and extract a low power 10 GHz CW source into the main waveguide structure while keeping a low insertion loss for the HPM pulse. The results of this power signal (max attenuation values range -10 to -40 dB) along with a 1D plane wave excited plasma model is used to infer the temporal average electron density (specifically the longitudinal integral of the surface plasma density) at a range of pressures and different gases, typically 10-400 torr for air, N2, Ar, and He. The peak electron density and loss rates are then correlated with diffusion lengths, recombination and attachment rates given in literature to ascertain the dominant plasma relaxation path and species along with a means to extrapolate the time required to relax to a nominal background electron density. For instance, the dominant electron loss process in 90 torr air is attachment with a frequency of 121 kHz and peak electron density of ~1013 cm-3 resulting in relaxation times of a few hundred microseconds while in N2, the dominant process soon after the pulse is determined to be 2-body dissociative recombination.

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+ Rapid Capacitor Charger And Battery System For Portable Pulsed Power Applications
  S. L. Holt; J. C. Dickens; J. J. Mankowski; G. Rodriquez
Abstract:  The design of a scalable, burst-mode rapid capacitor charger and a high power battery system will be discussed. The design is scalable up to 40 kW output power and operates at 50 k V. The system is powered by a Lithium Ion Polymer (LiPo) battery pack and includes a battery management system. The topology is derived from a 5 kV, 3 kJ/s battery power capacitor charger that was previously demonstrated. This smaller charger occupied a volume of 1.5 L including batteries and demonstrated a 2 kW/L average power density over a 1 s burst. This power density is five to ten times higher than commercially available systems. The performance of this previous design was limited by strict form factor restrictions (1.5 L, 6" diameter). The new design is intended to match or exceed the power density of the previous design due to reduced form factor restrictions. Electrical and mechanical design, component selection, and circuit simulations will be presented.

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+ Study Of Low-Temperature Plasma Development Utilizing A Gpu-Implemented 3D Pic/Mcc Simulation
  A. S. Fierro; G. R. Laity; S. R. Beeson; J. C. Dickens; A. A. Neuber
Abstract:  Summary form only given. A GPU-accelerated 3-dimensional PIC/MCC simulation code was developed using the CUDA environment to study the physical processes involved in the development of a low-temperature plasma. The simulation results aid in quantifying transient plasma development as it is often inaccessible experimentally in detail even with modern noninvasive techniques such as non-linear laser spectroscopy or high-speed electrical diagnostics. Hence, computational methods, such as Particle-in-Cell (PIC) and Monte Carlo Collision (MCC), provide a complementary approach to determining the mechanisms leading to plasma development. However, fully modeling the physics of the plasma development is made difficult by the number of plasma processes that must be tracked simultaneously, and only recently have computing resources provided the capability to track tens of millions of particle interactions. Furthermore, the introduction of graphics processing unit (GPU) computing provides an attractive means for economical and efficient parallelization of scientific codes through a framework such as NVIDIA CUDA. As such, a GPU-accelerated 3-dimensional PIC/MCC simulation was developed using the CUDA environment to provide characteristics during the initial stage of plasma development in atmospheric pressure nitrogen. The simulation was run on a NVIDIA GTX 580 with 3 GB of memory and 512 CUDA cores. The simulated geometry consists of two paraboloid electrodes with a gap distance of 5 millimeters with Dirichlet boundary conditions, and 22 unique electron interactions with molecular nitrogen are considered. The electrodes are excited with a step voltage pulse of several thousand volts also assuming a uniformly distributed initial electron density of 104 cm-3 in the volume. For instance, results from a 5 nanosecond simulation reveal the development of positive ion space charge channels near the anode and cathode regions. These channels appear consistent with high-speed streamer photographs captured during plasma formation. The electron energy distribution function (EEDF) indicates a non-Maxwellian velocity distribution during the application of the high electric field. Furthermore, a typical electron density near the cathode is on the order of 7 × 108 cm-3. The results from numerical simulation will be compared in detail to experimentally accessible parameters such as electron temperature and dissociation degree.

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Publication Year:  2012
+ Anode Material Outgassing At 250 A/Cm2 Current Density Under Uhv Conditions
  J. M. Parson; J. C. Dickens; A. Neuber; J. Walter; M. Kristiansen
Abstract:  Summary form only given. This presentation describes a study on gas evolution of plasma expansion in a reflex-triode virtual cathode oscillator (vircator) at ultra-high vacuum (UHV). Research has shown that explosive electron emission (EEE) processes at the cathode and ion / electron bombardment at the anode cause material erosion that produces regions of localized plasma. This plasma expansion has shown to lower gap impedance, cut off microwave performance and spoil low vacuum levels over time1. The goal of the study is to identify the gas species and their sources to better understand and limit the negative effects of plasma expansion in sealed tubes. The vacuum tube under investigation is a triode-geometry vircator with 20 cm2 cathode surface, driven by a 80 J Marx Generator with an approximate peak voltage and current, and pulse width of 200 kV and 5 kA, and 200 ns, respectively.

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+ Characterization Of Annealed Hpsi 4H-Sic For Photoconductive Semiconductor Switches
  Hettler, C; Sullivan, WW; Dickens, J
Abstract:  Annealing of high purity semi-insulating (HPSI) 4H-SiC is investigated as a method to improve bulk photoconductive semiconductor switches through recombination lifetime modification. Five samples of HPSI 4H-SiC were annealed at 1810 degrees C for lengths of time ranging from 3 to 300 minutes. The recombination lifetime of the unannealed and annealed samples was measured using a contactless microwave photoconductivity decay (MPCD) system. The MPCD system consists of a 35 GHz continuous microwave probe and a tripled Nd:YAG pulsed laser. The recombination lifetime was increased from 6 ns, as received, up to 185 ns by annealing for 300 minutes. To experimentally verify switch improvements, identical switches from unannealed and annealed material were fabricated and tested at low voltage. The unannealed device generated a 15 ns pulse with a 2 ns rise-time. The annealed device conducted for upwards of 300 ns with a comparable 2 ns rise-time. The increased recombination lifetime resulted in lower on-state resistance and increased energy transfer.
+ Comparison Of Csi Coated Carbon Velvet And Aluminum Cathodes Operated At Current Density On The Order Of 300 A/Cm2
  C. Lynn; J. Walter; A. Neuber; J. Dickens; M. Kristiansen
Abstract:  Many high power microwave devices use explosive or flashover electron emission cathodes in order to generate the electron beam and thus drive the device. These diodes are simple to operate, requiring no heater or other external systems, and are capable of generating beam currents of several kA at accelerating voltages on the order of 100s of kV. However, they generally suffer from non-uniform emission, anode heating, out-gassing, and pulse shortening due to anode and cathode plasma expansion. The ability to rep-rate such a diode is generally limited by anode heating and out-gassing which are both affected by beam uniformity. Two diodes are compared in this work. One uses a machined aluminum cathode, which is made from solid aluminum with grooves milled onto the surface. The other diode utilizes a carbon velvet cathode which is CVD coated with CsI. Time integrated scintillator images of the electron beam at the anode were taken for both the carbon velvet and aluminum cathodes. Additionally, time resolved images of the emission centers were taken for the carbon velvet cathodes. Data sets of over 1000 shots were taken with each cathode and shot to shot variation in the peak "turn-on" electric field are compared. The lifetime of the aluminum cathode was exceeded before 1000 shots, whereas the carbon velvet cathode showed no degradation in operation.

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+ Experimentation And Simulation Of High Current Density Surface Coated Electro-Explosive Fuses
  J. C. Stephens; A. A. Neuber; J. C. Dickens; M. Kristiansen
Abstract:  The primary objective of the research discussed in this paper is to develop a compact electro-explosive fuse (EEF) for a flux compression generator (FCG) power conditioning system, capable of rapidly obtaining and maintaining high impedance. It was observed that significant gains in EEF performance are introduced with the application of an insulating coating to the surface of the EEF wire. A 2 kA small scale test bed has been designed to provide a single wire EEF with similar current density (~107 A/cm2), voltage gradient (~7 kV/cm), and timescale (~8 μs) as to what is seen by and EEF utilized in a HPM generating FCG system. With the small scale test bed EEF performance data was rapidly obtained at a significantly lower cost than equivalent full scale FCG experiments. A one-dimensional finite difference model coupled with the Los Alamos National Laboratory SESAME Equation-of-State database was utilized to simulate the resistive behavior of the single wire EEFs. Further, a large scale test bed, designed to provide a similar current action as to what is provided by a FCG is used to test 18 wire EEF arrays at the 40 kA level.

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+ Fiber Optic System For High Frequency Burst Operation Of A Silicon Carbide Photoconductive Semiconductor Switch
  Mauch, D; Hettler, C; Sullivan, WW; Dickens, J
Abstract:  A fiber optic system was constructed to demonstrate high frequency operation of a silicon carbide (SiC) photoconductive semiconductor switch (PCSS). The goal was to transform a single high-energy laser pulse into a train of pulses by adding static delays into a multimode fiber bundle. The individual optical fibers comprising the fiber bundle incrementally add an additional amount of flight time to the light pulse. The end result is a train of pulses with a fixed delay between each pulse. A frequency-tripled Nd:YAG laser (10 ns FWHM) generating up to 300 mJ of light energy at 355 nm is coupled from free space into the optical fibers comprising the fiber light guide. Experimental results examining the collection and transmission efficiency, and the temporal output are presented.
+ Frequency Agility Of A Ferrite-Loaded, Nonlinear Transmission Line
  C. Simmons; J. -. B. Bragg; J. Dickens; A. Neuber
Abstract:  A nonlinear transmission line (NLTL) provides a solidstate means of generating high power, microwave pulses. The NLTLs in this study are coaxial transmission lines whose center conductor is encapsulated by ferrite beads. Operational frequency can be controlled by varying the dimensions of the ferromagnetic material, which affects azimuthal magnetic fields and material losses, or by varying the biasing field strength. This research demonstrates frequency agility of an NLTL by documenting the design, construction, and testing of three NLTLs. The NLTL will be one meter long, and to test for frequency agility, three different sizes of ferrites will be loaded onto NLTL with various biasing fields applied. Azimuthal field strengths due to an incident high voltage pulse range from 10-36 kA/m with magnetic biasing fields between 0 kA/m and 50 kA/m.

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+ Gas Evolution Of Nickel, Stainless Steel 316 And Titanium Anodes In Vacuum Sealed Tubes
  J. Parson; J. Dickens; J. Walter; A. Neuber; M. Kristiansen
Abstract:  This paper presents a study on gas evolution of three different anode materials in vacuum sealed tubes under UHV conditions. The experimental apparatus consists of a high-power microwave (HPM) virtual-cathode oscillator (vircator) driven by a 200 ns, 80 J, 225 kV low-impedance Marx Generator. Plasma expansion due to explosive electron field emission has shown to lower gap impedance, spoil consistent low vacuum levels, and cut-off microwave radiation. The anode materials compared are nickel 201L (Ni201L), stainless steel 316L (SS316L) and grade-1 titanium (TiG1); with the cathode material being aluminum. The anodes were cleaned by the following method: rough polishing followed by electro-polishing, a ten minute microwave argon / 10% oxygen plasma cleaning process (ArO2) and finally, a 72 hour in situ bake-out at 300°C. Outgassing characteristics of each anode material are presented and compared.

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+ Investigation Of Vacuum Uv Absorption During Low-Temperature Plasma Formation In N2/H2 Mixtures At Atmospheric Pressure
  G. Laity; A. Neuber; A. Fierro; L. Hatfield; J. Dickens; K. Frank
Abstract:  This paper describes recent advances in the study of self-generated emission of vacuum ultraviolet (VUV) radiation which is produced during the early time period leading to high voltage breakdown at atmospheric pressure. Previous studies of air breakdown showed the presence of 121.5 nm radiation which is spontaneously emitted by excited hydrogen atoms, HI. Since this Lyman-α line is self-absorbed, it enabled inferring various plasma parameters from recording emission spectra from 115-135 nm for species of HI and NI. For instance, measurements in H2/N2 mixtures have revealed that the highest amount of absorption via HI atoms occurs in the high field region near the anode, implying that significant H2 dissociation for radiation-trapping is occurring in this zone. Selective spatial measurements further showed that the apparent VUV emission centers (i.e. streamer heads) move away from the anode and the mechanisms leading to line broadening (i.e. Stark effect from space charge) are a function of streamer position. The presented self-absorption mechanisms are essential in quantitatively understanding the role of VUV radiation transport via absorption for photo-ionization during streamer breakdown, in which re-absorption of high energy photons is inherently a requirement.

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+ Performance And Optimization Of A 50 Kv Silicon Carbide Photoconductive Semiconductor Switch For Pulsed Power Applications
  C. Hettler; W. W. Sullivan; J. Dickens; A. Neuber
Abstract:  A 50 kV silicon carbide photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS device is fabricated from semi-insulating 4H-SiC in a newly-proposed rear-illuminated, radial switch structure. The improved structure reduces the peak electric field within the switch, extending the blocking voltage to over 50 kVdc. Electrostatic field simulations of the PCSS are presented along with experimental blocking curves. The PCSS demonstrated low on-state resistance, delivering over 27 MW of peak power into a 31 Ω load. Device modeling was performed to further optimize the switch for peak efficiency when illuminated with 355 nm light, a common laser wavelength. The switch structure was modified for peak operation at 355 nm and the experimental and theoretical results are compared.

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+ Serial Arrangement Of Ferrimagnetic Nonlinear Transmission Lines
  J. -. B. Bragg; C. Simmons; J. C. Dickens; A. A. Neuber
Abstract:  Nonlinear transmission lines (NLTLs) utilizing ferrimagnetic materials for microwave generation have been realized as a possible solid-state replacement to traditional high power microwave (HPM) sources. The nonlinearities present in the material, along with interaction between pulsed, azimuthal magnetic fields and static, axial-biasing magnetic fields provide microwave (mesoband) generation with peak powers exceeding 30 MW at 2-4 GHz center frequency with 25 kV incident pulse magnitude. Additionally, an incident pulse of several nanoseconds is sharpened to hundreds of picoseconds. This study focuses on a serial arrangement of two NLTLs with 5 ns electrical length separation. Tests with 25 kV incident voltage are performed with varying bias schemes for each NLTL structure. The lines are terminated into a 50 Ω matched load. Measurements taken before and after each NLTL provide insight to the behavior of the traveling pulse. Results regarding peak power, frequency of operation, and system delay are discussed.

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+ Shallow Incorporation Of Nitrogen In Hpsi 4H-Sic Through The Laser Enhanced Diffusion Process
  Sullivan, W; Hettler, C; Dickens, J
Abstract:  This paper investigates n-type doping of point-defect compensated high purity semi-insulating (HPSI) 4H-SiC using a pulsed laser (10 ns FWHM @ 260 nm) for the introduction of nitrogen to shallow depths. A thermal model is presented using COMSOL Multiphysics featuring nonlinear temperature dependent material properties and a volumetric heat source term that takes into account the laser absorption depth for common ultraviolet irradiating wavelengths. The temperature distribution in the material and the amount of time that the surface and near-surface regions are at high temperature determines how many laser pulses are required to dope to the desired depth, and simulation results are presented and fit to measured data. The simulations and measured data show that for shallow doping a short wavelength ultraviolet laser should be used to localize the heat at the surface so the dopant can't diffuse deep into the material. The laser enhanced diffusion process has been used to incorporate nitrogen into HPSI 4H-SiC with a measured surface concentration greater than 10(20) cm(-3) and a nonlinear thermal model was built.
+ Spatially-Resolved Spectral Observations Of Pulsed Surface Flashover In A Nitrogen Environment
  A. Fierro; G. Laity; A. Neuber; L. Hatfield; J. Dickens
Abstract:  The development of streamers during the initial stage of a pulsed atmospheric discharge, which carries a high content of vacuum UV (VUV) emission, is investigated. Due to the high spectral absorptivity of atmospheric air in the VUV regime, few experiments have been conducted that observe the wavelength range shorter than 180 nm. However, direct photoionization is believed to play an important role in streamer formation during this phase of breakdown. VUV radiation (hν > 7 eV) is energetic enough to promote step-ionization and directly ionize background gas mixtures. Utilizing a VUV-sensitive experimental apparatus, spatially-resolved photomultiplier tube (PMT) measurements were recorded showing that initial VUV emission is dependent on the inhomogeneous field distribution near the electrodes. It was revealed that further into the developing streamer the position of instantaneous VUV emission is propagating from anode to cathode away from the initial VUV emissions. From these observations, the low temperature plasma is capable of producing high energy photons during the streamer phase as well as after voltage collapse.

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+ Susceptibility Of Electro-Explosive Devices To High Pulsed Electric Fields
  D. V. Reale; J. Mankowski; J. Dickens
Abstract:  Commercially available Electro-Explosive Devices (EEDs), such as blasting caps, use electrical current to initiate a primary charge. Various detonators including bridge wire, match-type, exploding bridge wire, and slapper. The basic operating principle of the match-type device is to heat the ignition element to the ignition temperature of the primary explosive. The normal operation current profiles, both constant current and pulsed excitation, are well known, as is the ignition temperature. However, as safety and reliability are of great concern, both in the operation and storage of EEDs, the susceptibility of these devices to transient or spurious fields is of interest. The susceptibility of match-type EEDs to high pulsed electric fields is examined. A Finite Element Method (FEM) simulation is performed using COMSOL to determine the induced current in the bridgewire due to applied electric fields and the resulting Joule heating of the wire. Several situations are investigated including the EED in conductive and non-conductive media and leads open or terminated representing operational and storage conditions.

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+ The Effects Of Sub-Contact Nitrogen Doping On Silicon Carbide Photoconductive Semiconductor Switches
  Sullivan, W; Hettler, C; Dickens, J
Abstract:  Forming non-rectifying (ohmic) contacts to wide band gap semiconductors such as silicon carbide (SiC) requires a heavily doped subsurface layer to reduce the Schottky barrier height and allow efficient electron injection. Nitrogen, a common n-type dopant in SiC, was incorporated into a SiC sample using a laser enhanced diffusion process in which an impurity is incorporated into the semiconductor to very high surface concentrations (> 10(20) cm(-3)) and very shallow depths (< 200 nm) with the use of a pulsed 266 nm laser. This paper evaluates the effects of nitrogen introduced through laser enhanced diffusion on the contact formation and the efficiency of silicon carbide photoconductive switches at low and high injection levels under different biasing conditions. Nine lateral switches were fabricated on a high-purity semi-insulating 4H-SiC sample; three with no sub-contact doping, three with sub-contact doping on only one contact, and three with sub-contact doping on both contacts. Results are presented for tests under pulsed laser illumination with sub-contact doping on only the anode, only the cathode, neither, and on both of the contacts.
Publication Year:  2011
+ Compact Electro-Explosive Fuse Optimization For A Helical Flux Compression Generator
  J. C. Stephens; A. A. Neuber; J. C. Dickens; M. Kristiansen
Abstract:  This paper presents the optimization of a compact electro-explosive fuse designed for the power conditioning system to be driven by a helical flux compression generator (HFCG). An electro-explosive fuse interrupts the current flow from the HFCG through a storage inductor on a 50 to 100 ns timescale inducing a voltage large enough to close a peaking gap, which commutates the energy in the storage inductor into a 20 Ohm load at voltage levels above 200 kV. Experimental data has revealed that electro-explosive fuses with wires in closer proximity to one another have consistently produced lower pulsed voltages than fuses with larger wire spacings. This paper addresses possible factors that might contribute to this drop in performance. An electro-magnetic field solver is used to model the current redistribution in the fuse wires. The experimentally observed performance of compact fuses with varying wire spacings is presented.

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+ Comsed 2 - Recent Advances To An Explosively Driven High Power Microwave Pulsed Power System
  M. A. Elsayed; A. A. Neuber; J. W. Walter; A. J. Young; C. S. Anderson; S. L. Holt; J. Dickens; M. Kristiansen; L. L. Altgilbers
Abstract:  Continued efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have led to improvements to the design, testing, and performance of a high power microwave (HPM) system, which is sourced by Lithium-ion Polymer batteries, a polypropylene capacitor, and high energetics. An indirectly seeded two-stage helical flux compression generator (HFCG) produces electrical energies in the kilo-Joule regime into a low impedance inductive load, varying from 2 μH to 3 μH. This high current output of the explosively driven generator is conditioned with a pure silver-wire-based electro-explosive opening switch, which reaches voltage levels in excess of 300 kV into a 18 Ohm load. Upon reaching levels high enough to close an integrated peaking switch, this high voltage is sufficient to drive a reflex triode virtual cathode oscillator, also known as a Vircator, into radiation. The Vircator employed in the system has reached microwave radiation levels well over 100 Megawatts from a cavity volume of less than 5 Liters at a microwave frequency of a few GHz. The complete system is governed through a microcontroller that regulates seed and detonator charging levels as well as discharge times using built-in feedback diagnostics. The complete system aforementioned is constrained to 15 centimeter diameter and measures 183 centimeter in length with an overall volume of less than 34 Liters. No external power or vacuum pumping for the HPM tube is required. This report will discuss recent design advancements and improvements on the HPM system and its sub-components that include the compact seed source, HFCG, and the power conditioning system. Improved safety features implemented as well as novel diagnostic integration will be discussed as well.

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Publication Year:  2009
+ A Compact 5Kv Battery-Capacitor Seed Source With Rapid Capacitor Charger
  Holt, SL; Dickens, JC; McKinney, JL; Kristiansen, M
Abstract:  Many pulsed power applications have demanding system requirements. Power systems for these applications are expected to provide high energy, high pulsed power and long standby times without recharge, all in a very compact package. The different properties of batteries and capacitors make them most suitable for different uses. When selecting a prime power source for compact pulsed power systems a hybrid system often provides the optimal solution, utilizing a battery for prime energy storage during standby and a capacitor for intermediate energy storage before and between operations. This system takes advantage of the best characteristics of both sources to fulfill the system requirements. The design and testing of such a compact system is discussed. The system utilizes a solid-state converter to charge a 50 mu F polypropylene capacitor to 5 kV in under 500 ms from lithium-ion polymer (LiPo) batteries. Battery selection and testing is also covered. The battery and charger assembly occupies 1.25 L while the capacitor occupies an additional 1.4 L.
+ Electrical Conduction In Select Polymers Under Shock Loading
  Lynn, C; Neuber, A; Krile, J; Dickens, J; Kristiansen, M
Abstract:  It is known that polymers become conductive under shock loading, which can be critical to the operation of explosive driven high current/voltage devices. Hence, the propensity of several select polymers to conduct under shock loading was investigated. Four polymers, Nylon, Teflon, Polypropylene, and High Density Polyethylene, were tested under shock pressures up to similar to 22 GPa. Shock waves were generated with high explosives, and CTH, a hydrodynamic code developed at Sandia National Laboratories, was utilized to calculate pressure and temporal resolution of the shock waves. Time of arrival measurements of the shock waves were taken to correlate the hydrodynamic calculations with experimental results. A notable delay between shock front arrival and the onset of conduction is exhibited by each polymer. The delay tends to decrease with increasing pressure down to approximately 500 ns for HDPE at similar to 22 GPa under electric field strength of similar to 6.3 kV/cm. The data shows that some polymers exhibit more delay than others, thereby indicating better insulating properties under shock loading. Additionally, experiments revealed that the polymers conducted for a finite time on the microsecond time scale before recovering back to an insulating state. This recovery from a shock wave induced conducting state back to insulating state was investigated for a possible opening switch application
+ Energy Deposition Assessment And Electromagnetic Evaluation Of Electroexplosive Devices In A Pulsed Power Environment
  Parson, J; Dickens, J; Walter, J; Neuber, A
Abstract:  This paper assesses critical activation limits of electroexplosive devices (EED), such as blasting caps, which have recently found more usage in pulsed power environments with high EMI background. These devices, EEDs, can be very sensitive to low levels of energy (7-8 mJ) which make them dangerous to unintended radiation produced by compact pulsed generators. Safe operation and use of these devices are paramount when in use near devices that produce pulsed electromagnetic interference. The scope of this paper is to provide an evaluation of activation characteristics for EEDs that include energy sensitivity tests, thermodynamic modeling, and electromagnetic compatibility from pulsed electromagnetic interference. Two methods of energy deposition into the bridgewire of the EED are used in the sensitivity tests. These methods include single and periodic pulses of current that represent the adiabatic and non-adiabatic heating of the bridgewire. The heating of the bridgewire is modeled by a solution to the heat equation using COMSOL (TM) with physical geometries of the EED provided by the manufacturer.
+ High Electric Field Packaging Of Silicon Carbide Photoconductive Switches
  Hettler, C; James, C; Dickens, J
Abstract:  Photoconductive semiconductor switches (PCSS) made from semi-insulating (SI) silicon carbide (SiC) are promising candidates for high frequency, high voltage, and low jitter switching. However, existing switches fail at electric fields considerably lower than the intrinsic dielectric strength of SiC (3 MV/cm) because of the field enhancements near the electrode-semiconductor interfaces. Various geometries were identified which could reduce the electric field near the contact regions. The switches were simulated with various parameters and compared. In all cases, it was determined that a high dielectric constant (high-k) encapsulant is a crucial requirement that reduces high fields within the bulk material while inhibiting surface flashover. Assorted high-k encapsulants were evaluated and a portion was subsequently tested in the lab. The observed dielectric strength and relative permittivity of the encapsulants are presented. Pseudo switches, employing sapphire substrates, were constructed and biased to electrical breakdown. The dielectric strength of the interface between the semiconductor and the encapsulant was tested and improvements were discussed.
+ High Voltage Photoconductive Switches Using Semi-Insulating, Vanadium Doped 6H-Sic
  James, C; Hettler, C; Dickens, J
Abstract:  SiC manufacturers are continually improving the purity of their wafers, however, interband impurities, while detrimental in many applications, can be useful in the operation of photoconductive switches. Compact, high-voltage photoconductive switches were fabricated using c-plane; vanadium doped 6H-SiC obtained from II-VI, Inc. This material incorporates a large amount of interband impurities that are compensated by the vanadium amphoteric, but at present is only available as c-plane wafers. In order to avoid micropipe defects, lateral switches were fabricated to allow validation of material simulations. Low resistivity contacts were formed on the semi-insulating material and a high-voltage encapsulant increases the surface flashover potential of the switch. Material characteristics were determined and switch parameters were simulated with comparisons made to experimental data.
+ Low Jitter Triggered Spark Gap With High Pressure Gas Mixtures And Kr85
  Y. Chen; J. C. Dickens; J. W. Walter; M. Kristiansen
Abstract:  Summary form only given. A recent research effort at Texas Tech University on impulse antenna phased arrays has necessitated the need 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. 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 ~80 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. Previously, the system was successfully tested with 50 kV, 100 Hz switch operations. Gases tested include, dry air, H2, N2, and SF6, as well as H2-N2, and N2-SF6 gas mixtures. A discussion on switch operation time and switch gas temperature vs. jitter will be included in this paper. This paper will also discuss in detail the effects on switch jitter when different concentrations of Kr85 gas are introduced.

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+ Optimizing Power Conditioning Components For A Flux Compression Generator Using A Non-Explosive Testing System
  Davis, C; Neuber, A; Young, A; Walter, J; Dickens, J; Kristiansen, M
Abstract:  This paper discusses a non-explosive pulsed power device used to reproduce the output waveforms of a Flux Compression Generator (FCG) driving a High Power Microwave (HPM) source. This system optimizes the power conditioning components of a HPM source while reducing the time and resources inherent to explosively driven FCG schemes. An energy storage inductor, fuse opening switch, and a peaking gap make up the power conditioning system. This system couples large voltage pulses (similar to 300 kV), suited for HPM sources, to the load by disrupting the energy storage inductor current (similar to 40 kA). This paper will show that an optimal fuse length was experimentally searched for by varying the calculated fuse wire base length by 5, 10, and 15%. Various geometric fuse designs were examined to achieve a 45% reduction in the physical fuse length at constant wire length with acceptable performance losses. This paper will also show that the distance between the electrodes of a peaking gap can be optimized to more efficiently switch in the load. Finally results will be shown that depict the amount of microwave power produced by a vircator before fuse and peaking gap optimization.
+ Performance Of A Compact Triode Vircator And Marx Generator System
  Walter, J; Dickens, J; Kristiansen, M
Abstract:  Vircator high power microwave sources are simple, robust, and require no external magnetic field, making them desirable for use in practical compact high power microwave systems. A vircator can be driven directly from the output of a low-impedance Marx generator, eliminating the need for bulky intermediate energy storage components. A compact high power microwave system has been constructed and tested at Texas Tech University utilizing a triode geometry vircator and a compact Marx Generator. The size and performance of this system is compared to a similar system previously developed at Texas Tech. The current triode vircator is housed within a six inch diameter tube which is eleven inches in length. The Marx is contained in an oil tank that is 36 inches long x 12 inches wide x 18 inches tall. Diode voltage and current, and radiated microwave waveforms are presented.
+ Prediction Of Compact Explosively-Driven Ferroelectric Generator Performance
  Bolyard, D; Neuber, A; Krile, J; Dickens, J; Kristiansen, M
Abstract:  Explosively-driven ferroelectric generators are attractive as potential prime energy sources for one-time use pulsed power systems. While the output voltages of small ferroelectric discs have been shown to be on the order of the theoretical maximum values, scaling the ferroelectric to larger thicknesses has proven less successful. The primary limiting factor is how much of the ferroelectric material is compressed simultaneously. This is difficult to control for thicker ferroelectric discs or stacks of discs due to pressure pulse attenuation in the material and rarefaction waves shortening the pressure pulse. A hydrodynamic code system is utilized to calculate the temporally and spatially resolved pressure. The calculated pressure values are converted into voltage produced by the ferroelectric through an algorithm based on an empirical polarization-pressure hysteresis curve. The validity of the algorithm has been verified for PZT EC-64 with experimental data from a flyer-plate experiment reported in literature and our own experiments with the shock wave from the explosives more directly applied to the ferroelectric. Both calculations and experiments produced normalized output voltages, ranging from 1.4 to 3.4 kV/mm for 2.54 cm diameter discs. We will discuss how this pressure to voltage algorithm along with pressure simulations aided in the scaling of the amount of ferroelectric material in a generator, as well as in the design of new driver elements with the goal to increase the peak output voltage of a generator while keeping the generator compact. The calculated voltage output results are compared with experimental data of explosively-driven ferroelectric generators.
+ Stand-Alone, Fcg-Driven High Power Microwave System
  Young, A; Neuber, A; Elsayed, M; Walter, J; Dickens, J; Kristiansen, M; Altgilbers, LL
Abstract:  An explosively driven High Power Microwave (RPM) source has been developed that is based on the use of a Flux Compression Generator (FCG) as the primary driver. Four main components comprise the HPM system, and include a capacitor-based seed energy source, a dual-staged FCG, a power conditioning unit and an RPM diode (reflex-triode vircator). Volume constraints dictate that the entire system must fit within a tube having a 15 cm diameter, and a length no longer than 1.5 m. Additional design restrictions call for the entire system to be stand-alone (free from any external power sources). Presented here are the details of RPM system, with a description of each subcomponent and its role in the generation of RPM. Waveforms will be shown which illustrate the development of power as it commutates through each stage of the system, as well as power radiated from the diode. Analysis and comparisons will be offered that will demonstrate the advantages of an explosively driven HPM system over more conventional pulsed power devices.
+ Vuv Emission From Dielectric Surface Flashover At Atmospheric Pressure
  Rogers, G; Neuber, A; Laity, G; Dickens, J; Frank, K; Schramm, T
Abstract:  Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 180 nm at atmospheric pressures with a focus on the scheme used to excite the spark gap. The surface flashover of interest occurred on an MgF2 window (front side of window in air, backside in vacuum) imaged onto the entrance slit of a 1 m vacuum spectrograph. Emission spectra were recorded with an Andor DH740 series ICCD camera. All data was taken at atmospheric pressure with a flashover spark length of about 8 mm created by a pulser designed for a 500 ns pulse, max 50 kV output. The centerpiece of this pulser is the CCSTA14N40 thyristor by Solidtron / Silicon Power which features a rate of change current of maximum 30 kA/mu s and a hold-off voltage of up to 4 kV. A pulse transformer with Metglas (R) core was used to elevate the voltage to 50 kV with a rise time of 180 ns and a peak current of 500 A. The pulser was designed for a repetition rate of 10 Hz and is triggered by TTL pulses. Discussed in this paper, along with the measured spectra and their relation to the physics of surface flashover at atmospheric pressure, will be the design of the pulser.
Publication Year:  2008
+ A 50 Kv, 100 Hz Low Jitter Triggered Spark Gap With High Pressure Gas Mixtures
  Y. Chen; J. J. Mankowski; J. C. Dickens; J. Walter; M. Kristiansen
Abstract:  Recent research efforts at Texas Tech University on impulse antenna phased array has necessitated the need 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 (Y. Chen et al., 2007), we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper presents the impact 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 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 ~150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. This paper discusses in detail 50 k, 100 Hz switch operations with different gases. Gases tested include, dry Air, H2, N2, and SF6, as well as H2-N2, and N2-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.

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+ A Compact, Self-Contained High Power Microwave Source Based On A Reflex-Triode Vircator And Explosively Driven Pulsed Power
  Young, A; Holt, T; Elsayed, M; Walter, J; Dickens, J; Neuber, A; Kristiansen, M; Altgilbers, LL; Stults, AH
Abstract:  Single-shot High Power Microwave (HPM) systems are of particular interest in the defense industry for applications such as electronic warfare. Virtual Cathode Oscillators (vircators) are manufactured from relatively simple and inexpensive components, which make them ideal candidates in single-shot systems. The Flux Compression Generator (FCG) is an attractive driver for these systems due to its potential for high energy amplification and inherent single-shot nature. A self-contained (battery operated prime power), compact (0.038 m(3)), FCG-based power delivery system has been developed that is capable of delivering gigawatts of power to a vircator. Experiments were conducted with the delivery system connected to a resistive dummy load and then to a reflex-triode vircator. In order to optimize the performance of the vircator when driven by the power delivery system, a second experimental setup was constructed using a Marx-generator based system operating at similar voltages and rise-times. Performance measures of the delivery system when discharged into a resistive load will be presented, as well as vircator output power levels and waveforms from both experimental setups.
+ Analysis Of Mesoband Single Element Pulsed Ring-Down Antennas For Implementation In Phased Array Systems
  Belt, D; Mankowski, J; Walter, J; Dickens, J; Kristiansen, M
Abstract:  In recent years, the pulsed ring-down antenna has become of great interest due to its compact size and high power on target potential. Since these systems are fairly new in study, it is often difficult to predict the overall performance without experimental evaluation. A pulsed ring-down antenna operates by charging the single element antenna with a high potential source and then dosing a switch to develop transient wave reflections on the antenna, typical CW case analysis does not apply. For this reason, we have constructed a simulation model that allows us to predict the transient behavior of the structure. By utilizing the Comsol RF module transient analysis functions, we are able to characterize various parameters of different antennas, beginning with a dipole pulsed ring-down antenna operating around the 100 MHz range. After examining the simulated results against the experimental results for accuracy, we then moved to more complicated mesoband antenna structures. The simulation model developed within the COMSOL RF module allows us to examine various influential factors such as material losses, transient switching effects, structure capacitance, switch capacitance, and initial charging losses. With this, we are able to examine methods to improve the results in the far field such as capacitive spark gap loading and other capacitive storage methods. Utilizing the pulsed ring-down antenna model, we are able to give a better characterization of mesoband pulsed ring-down structures for implementation into a specific or multi-purpose phased array system.
+ Compact Silicon Carbide Switch For High Voltage Operation
  James, C; Hettler, C; Dickens, J; Neuber, A
Abstract:  Vanadium compensated, 6H silicon carbide (SiC) is investigated as a compact, high-power, linear-mode photoconductive semiconductor switch (PCSS) material. SiC is an attractive material due to its high resistivity, high electrical breakdown strength, and long recombination times compared to other photoconductive materials. The PCSS is designed for fast-rise time, low-jitter (sub-nanosecond) operation in a matched 50 Omega test bed. Ohmic contacts were applied by physical vapor deposition and initial tests utilized an external Nd:YAG laser trigger source. Analysis of the optical properties of Va-compensated SiC and of switch conduction resistance are presented and performance of contact material is discussed.
+ Energy Deposition And Electromagnetic Compatibility Assessment Of Electroexplosive Devices
  Parson, J; Dickens, J; Walter, J; Neuber, A
Abstract:  This paper assesses the critical activation energy required to set off electroexplosive devices (EED) at constant joule heating and rates of joule heating. Safe operation and use of these devices are of great concern in and around pulsed electromagnetic interference. Sensitivity characterization of EEDs include firing sensitivity plots, thermodynamic modeling and electromagnetic interference. Activation energy evaluation of single and periodic rectangular pulses are included to represent adiabatic and non-adiabatic bridge wire heating of the EED. The scope of this paper is to provide a short overview of sensitivity, thermodynamic, and electromagnetic compatibility of EEDs. The results provide crucial information in evaluating energy induced by pulsed electromagnetic fields of compact pulse generators.
+ High-Current Compact Fcg Seed Source Implementing Solid State Switching
  Elsayed, M; Holt, T; Young, A; Neuber, A; Dickens, J; Kristiansen, M; Altgilbers, LL; Stults, AH
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 kilo-Ampere range for mid-sized 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 over-voltage 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 35 sec), single-use Compact Seed Source (CSS) using solid state components for the switching scheme developed at the Center for Pulsed Power and Power Electronics at Texas Tech University. The CSS developed is a system (0.007-m(3) volume and weighing 13 lbs) capable of delivering over 250-J (similar to 10 kA) into a 6-mu H load with a trigger energy of micro-Joules 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. An in-depth and 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.
+ Jitter And Recovery Rate Of A 50 Kv, 100 Hz Triggered Spark Gap With High Pressure Gas Mixtures
  Yeong-Jer Chen; J. J. Mankowski; J. W. Walter; J. C. Dickens
Abstract:  Summary form only given.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 ultra low 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 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 50 kV and provides the low inductance voltage source. Triggering is provided by an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate 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. Initial testing with 30 kV, 10 Hz switch operations have shown reliable sub-ns 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 no recovery issues. Jitter data for pure gases, H2-N2, N2-SF6, and various Kr85 gas mixtures at the 50 kV, 100 Hz operation is also documented and compared in this paper.

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+ Opening Switch Utilizing Stress Induced Conduction In Polymethylmethacrylate
  Lynn, C; Krile, J; Neuber, A; Dickens, J
Abstract:  It is known that polymethylmethacrylate, PNUqA, becomes conductive under shock loading. To develop an opening switch utilizing shock induced conduction, the reversibility of this process must be studied. It is suggested in literature that changes in electrical properties begin at pressures as low a similar to 2 GPa. Applying the minimum pressure necessary for conduction is desirable in order to maximize the reversibility by limiting compression heating of the material. CTH, a hydrodynamic code written at Sandia National Laboratory, was used to design various drivers that deliver pressures in the range of similar to 2 GPa to 4 GPa to the PMMA. By utilizing the switch to trigger an RC discharge, the resistance and on-time of the switch was characterized. Experiments have shown conduction durations on the order of similar to 4 mu s. The switch was then placed into a capacitive driven inductive energy storage circuit, IES, to determine the polymer's ability to recover. This paper will present experimental data, CTH simulation results, and discuss the attained switching characteristics under varying shock pressure profiles.
+ Scaling And Improvement Of Compact Explosively-Driven Ferroelectric Generators
  Bolyard, D; Neuber, A; Krile, J; Walter, J; Dickens, J; Kristiansen, M
Abstract:  Explosively-driven ferroelectric generators are capable of producing single-shot voltage pulses of more than 100 W, while requiring no seed electrical source, being very compact, and shelf stable. Problems with ferroelectric generators are the low energy output, high dielectric constant of the ferroelectric material, low surface flashover voltage, inconsistent ferroelectric material quality, and uneven or excessive shockwave compression. Initial generator voltage waveforms show that breakdown occurred towards the end of the generator operation time. Several designs and methods have been tested and implemented to prevent surface flashover with varying results. The ferroelectric discs used for the generators were 0.4 inch thick, 1 inch diameter EC-64 PZT ceramic discs. Several six-disc generators were built and tested with resulting open-circuit voltage pulses of 80-140 kV with a FWHM of 2-4 mu s. Further improvements to the generators have been designed to prevent surface flashover, improve the explosive driver element and propagating shockwave, as well as increasing the number of discs per generator. Measured output waveforms into varying loads including direct driven antennas will be shown and discussed.
+ Shock Wave Simulation Of Ferrite-Filled Coaxial Nonlinear Transmission Lines
  Sullivan, W; Dickens, J; Kristiansen, M
Abstract:  Ferrite-filled coaxial shock lines have recently been used to significantly decrease the rise time of a high voltage pulse. This decrease can be enhanced by initially axially biasing the ferrite material with an applied external magnetic field, allowing for a faster transition from the unsaturated to the saturated state. The simulation of the ferrite material's operation, including saturation, is discussed as well as the simulation of coaxial nonlinear transmission lines. The project explores the rise time changes with variations of magnetic bias, ferrite geometry, input signal characteristics, and transmission line characteristics. Simulated waveforms are discussed for a nickel-zinc ferrite-filled coaxial line. The pulse steepening effect observed in electromagnetic shock lines occurs primarily because of an increase in phase velocity for points higher on the waveform due to the saturation of the ferrite material [1]. An incident pulse of high enough amplitude will drive the ferrite material into saturation, decreasing the relative permeability to one. This saturation front propagates through the ferrite material in the direction of the incident wave until the entire material is saturated, producing a sub-nanosecond rise time pulse. The shock line is designed for a saturated impedance of 50 Ohms to couple easily into existing systems. Pulsed operation of up to low kilohertz repetition is desired and being explored. Applications of electromagnetic shock lines include laser triggering and ultra-wideband radar generation, as well as others.
+ Window Flashover Initiation Under Pulsed Microwave Excitation
  Krile, J; Edmiston, G; Dickens, J; Krompholz, H; Neuber, A
Abstract:  Surface flashover development at the output window of high power microwave (HPM) systems presents a major limitation to the power densities and pulse lengths transmitted through these interfaces. As a result, developing a physical model accurate in predicting surface flashover initiation is of prime interest A Monte-Carlo type electron motion simulation has been developed to estimate the delay time from initial electron to flashover. Although this approach has shown reasonable agreement with experimental results, the process yielding the initial seed electron(s) was neglected in the model, primarily due to the lack of quantitative and qualitative information on seed electron production. For instance, computational efforts investigating seed electron production via collisional detachment from negative oxygen ions have shown that while effective at DC, the collisional detachment model cannot remain a likely contributor of electrons at high frequencies (> similar to 5 GHz). The key parameters impacting high power microwave surface flashover will be discussed and presented along with continued investigation into the statistics of possible seed electron sources, including trace contaminates present in the gas or on the dielectric surface.
Publication Year:  2007
+ A Synchronous Free-Running Arc Distributed Energy Railgun
  R. W. Karhi; J. J. Mankowski; J. C. Dickens
Abstract:  Summary form only given. Experimental results utilizing a distributed energy scheme and free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma armature in the railgun will be explored. The use of a free running arc allows experiments to emulate a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Diagnostics for this examination include rail and plasma current probes as well as independent Rogowski coils for each stage. The distributed system is comprised of four stages spaced uniformly along the rail length. Each stage contains a high voltage capacitor, an inductor, a phase controlled SCR, and a driver board for triggering. The high voltage capacitors used are capable of storing 50 kJ, but are typically charged to store less than 20 kJ. Fiber optic lines are integrated into the system to prevent misfire in the noisy EMI environment. Optimal switch timing is predicted by a computer simulation and tested for accuracy. The assembled railgun is 2.4 m long with a 1.7 cm times 1.7 cm bore cross section. The rails are made of machined UNS C11000 ETP copper. G-10 insulation contains the current probes and separates the Aluminum 6061-T6 outer support structure from the rails. Alumina ceramic inserts are secured by the G-10 and function to reduce in-bore ablation. A PVC chamber encloses the railgun and allows experiments to be conducted within the desired 1-10 torr environment. A plasma source at the breech reliably supplies plasma for each experimental test. The plasma injector is powered by a Marx generator to supply a 40 kV voltage and pulse length of 10 s. The coaxial injector is comprised of a tungsten rod, ceramic insulator, and stainless steel outer casing. Numerous experimental tests were conducted to investigate the dynamics of plasma armatures within a distributed energy source railgun. Variations of switch timing, bore pressure, current amplitude, and current pulse length within each stage have been tested. This data is analyzed to determine the effectiveness of a distributed energy system to suppress the plasma restrike phenomenon and increase plasma armature railgun performance.

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+ An Asynchronous Free-Running Arc, Distributed Energy Railgun
  Karhi, R; Mankowski, J; Dickens, J; Kristiansen, M
Abstract:  Experimental results comparing a breech-fed scheme and a distributed energy scheme for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma armature in the railgun will be explored. The use of a free running arc allows experiments to emulate a plasma armature railgun at high speeds (>5km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma armatures within a distributed energy source railgun. Variations of switch timing, bore pressure, current amplitude, and current pulse length within each stage have been tested. These data are analyzed to determine the effectiveness of a distributed energy system to suppress the plasma restrike phenomenon and increase plasma armature railgun performance(1).
+ Development Of An Ultra-Compact Explosively Driven Magnetic Flux Compression Generator System
  Krile, J; Holt, S; Hemmert, D; Walter, J; Dickens, J; Altgilbers, L; Stults, A
Abstract:  Explosively driven magnetic flux compression generators (MFCG) are effective high current, compact, disposable pulsed power supplies [1]. A common type of MFCG in use is a helical generator, HFCG, because it can provide very high current gain in a compact form. Successful implementation of a HFCG as a pulsed power source requires several peripheral systems including a seed current source and a high current switching mechanism. Additionally, for driving higher impedance loads, a power conditioning system is required to convert the high current, low voltage HFCG output into a more suitable voltage. HEM Technologies' currently developed system utilizes an ultra compact, seed source previously developed by HEM Technologies [2]. An explosively driven closing switch provides both the switching action and acts as a delay generator to allow for the current rise in the HFCG. The output of the HFCG is conditioned via an exploding fuse wire and spark gap pair to convert the high current output to high voltage. HEM Technologies has performed extensive modification and testing of the end-to-end system. The current and energy conversion will be presented along with typical output voltages using different fusing techniques.
+ Electrical And Optical Measurements Of Explosively Driven Plasma Jets
  Walter, J; Dickens, J; Kristiansen, M
Abstract:  Electrical and optical measurements were performed on explosively driven plasma jets. The explosive devices are in the form of a conical shaped charge. The electrical diagnostic consisted of measuring the current passed through a jet Ab en it made a connection between two metal plates. An energy storage capacitor connected to the two plates was charged to between 500 V and 10 kV, and then discharged through the jet. Also, an ICCD camera was used to capture high-speed single frame pictures of the propagating jet. Several shots were fired with variations in the capacitor voltage, the plate geometry, and the jet material. Current waveforms and optical images are presented. All experiments were performed in the explosive and high power RF experimental facility at the Center for Pulsed Power and Power Electronics at Texas Tech University.
+ Electro-Explosive Fuse Optimization For Helical Flux Compression Generator Using A Non-Explosive Test Bed
  McCauley, D; Belt, D; Mankowski, J; Dickens, J; Neuber, A; Kristiansen, M
Abstract:  Helical Flux Compression Generators (HFCG) of 50 mm form factor have been shown to produce a maximum energy deposit of 3 kJ into a 3 mu H inductor from a seed current. A large dI/dt into a coupled load is possible when an electro-explosive fuse is used. Previous work with a non-optimized fuse has produced similar to 100 kV into a 15 Omega load which leads into a regime relevant for High Power Microwave (HPM) systems. It is expected that similar to 300 kV can be achieved with the present 2 stage HFCG driving an inductive storage system with an electro-exploding fuse. In order to optimize the electro-explosive fuse design, a non-explosive test bed, which closely simulates the 45 kA HFCG output, is used. To optimize the fuse, effects of fuse material, fuse length, and fuse shape will be examined as well as the effects of various quenching materials. Additionally, to maximize the output voltage and minimize the fuse recovery time, we are optimizing the length of the fuse wire. For shorter fuse lengths, we are optimizing fuse shape as well as fuse length to find the best fuse recovery time. By optimizing the individual parameters of an electro-explosive fuse, the fuse as a whole will be optimized to produce maximum output voltage when used with an HFCG.
+ Electro-Explosive Switches For Helical Flux Compression Generators
  Bell, D; Mankowski, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  Helical Flux Compression Generators coupled with an inductive energy storage system have shown promising results as a driving source for High Power Microwave (HPM) loads. The output performance of the inductive energy storage system is contingent upon the opening switch scheme, usually an electro-explosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. By applying this fuse characterization model to an HFCG powered system, a non-optimized fuse has produced 60 kV into an HPM equivalent load with an HFCG output of 15 kA into a 3 mu H inductor. Utilization of a non-explosive HFCG test-bed has produced 36 kV into an HPM equivalent load with an output of 15 kA into a 1.3 mu H inductor. The use of a non-explosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. Prior analysis of fuse parameters has been accomplished with various materials including Silver (Au), Copper (Cu), and Aluminum (Al), but particular interest resides in the use of Gold (Ag) fuse material. We will 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 and allow the development of accurate modeling for higher current systems.
+ Evaluation Of Switch Jitter In A High Pressure Coaxial Spark Gap
  James, C; Dickens, J; Holt, S; Kristiansen, M
Abstract:  The design and jitter performance of a high-pressure, coaxial spark gap, triggered by field distortion by a center plane electrode is presented. Additional comparisons with trigatron style triggering are also presented. The switch was tested up to 100 pulses per second (pps) with a maximum switching voltage of 50 kV in nitrogen. Analysis will focus on jitter measurements taken over the full lifetime of the switch. This paper presents the results of this analysis. Specifically, switch jitter and lifetime will be evaluated as a function of switch geometry as a whole and as a function of trigger electrode geometry.
+ Explosive Opening Switch Utilizing Shockwave Induced Conduction In Pmma And Pvc
  Lynn, C; Neuber, A; Dickens, J
Abstract:  It is known that PMMA and PVC become conductive under shock compression. However, to develop an opening switch, the material's recovery from conductor back to insulator is critical. Heating must be minimized in order to optimize recovery. Two sources of heat in this switch include shock induced heating and switch loss heating, which should be balanced for optimal results. Furthermore, it is also important to determine if the observed switching behavior is due to shock unloading or intrinsic material relaxation properties. In the extreme case of recovery, bulk breakdown may become an issue. This paper presents experimental data and discusses initial results as they relate to the development of an opening switch.
+ Jitter And Recovery Rate Of A Triggered Spark Gap With High Pressure Gas Mixtures
  Y. -J. Chen; J. J. Mankowski; J. W. Walter; J. C. Dickens
Abstract:  Summary form only given. Recent attention in impulse antenna phased array has necessitated the need 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 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, gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Omega, 2.4 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 an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate 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 will also introduce a high pressure gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, and SF6.

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+ Modeling Of A Single Element Pulsed Ring-Down Antenna For Implementation In A Phased Array System
  Belt, D; Mankowski, J; Walter, J; Dickens, J
Abstract:  A pulsed ring-down phased array antenna provides substantial energy deposition in the far field region in addition to a broad range main beam with scanning capabilities. This allows remote neutralization of Improvised Explosive Devices (IEDs) at far field distances and in virtually any direction. The pulsed ring-down antenna operates by charging the single element antenna with a high potential source and closing a switch to develop transient wave reflections on the antenna which then propagate in air. The performance of a pulsed ring-down phased array is highly contingent upon the design and performance of the individual antenna elements within the array. Such factors as operating voltage, antenna capacitance, material losses, antenna geometry and closing switch conductance characteristics must be examined for optimal performance to be achieved. By utilizing the COMSOL RF module transient analysis functions, we are able to characterize the various parameters beginning with a monopole and a dipole pulsed ring-down antenna operating in the hundreds of MHz range. We have examined and compared the results achieved from the experimental setup to the simulation model in order to better characterize the individual components of the antenna. We have also examined the discrepancies between an ideal closing switch and the experimental setup closing switch, which dramatically affects the far field range of the antenna. We have examined the material properties of the antenna to improve losses and increase system capacitance allowing an increase in the number of RF cycles per antenna discharge. With the results presented, an accurate model of pulsed ring-down antennas is available and will allow future development of more complex geometries that will improve the operation of pulsed ring-down phased array.
+ Testing Of New Ferroelectric Elements Custom Engineered For Explosively Driven Ferroelectric Applications
  Holt, SL; Krile, JT; Hemmert, DJ; Hackenberger, WS; Alberta, EF; Walter, JW; Dickens, JC; Altgilbers, LL; Stults, AH
Abstract:  Explosively driven ferroelectric generators (FEGs) are reliable, compact, high voltage sources that utilize high pressures to liberate charge trapped in the crystal structure of ferroelectric materials. For the active ferroelectric element most FEG designs use commercial lead zirconate-titanate (PZT) compositions designed for either precision actuators or naval sonar transducers. However, the material properties that are important in FEG applications are not the same material properties for which these materials have been designed to maximize. FEG designs utilizing these commercial materials are performance limited by high voltage breakdown, mechanical failure and low energy densities. TRS Technologies inc. has produced a new series of ferroelectric elements designed specifically for FEG applications. HEM Technologies has performed dielectric strength and shock compression experiments on these new materials to evaluate their performance in comparison to existing commercially available materials.
+ Theoretical Pulsed Ring Down Antenna Array Performance
  Walter, J; Dickens, J; Mankowski, J; Kristiansen, M
Abstract:  High-power transient RF transmitters are currently being developed for multiple applications. One potential high-power pulsed source being considered is the pulsed ring-down source (PRDS). In a typical PRDS, electrical energy that is stored in the device structure is discharged through a switch, generating a damped ringing radiated waveform. The magnitude of the transmitted field is limited by parameters such as the overall device geometry and switch characteristics. The potential exists to increase the radiated far field power density by utilizing an array of PRDS transmitters. The performance of such an array depends on the triggering jitter of the device switches and the design of the individual radiating elements. The application of the array concept to pulsed ring-down sources using gas spark gap switches is examined. Theoretical array performance is discussed, taking into account practical switch jitter distributions.
Publication Year:  2006
+ A Flux Compression Generator Non-Explosive Test Bed For Explosive Opening Switches
  Belt, D; Mankowski, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  Helical Flux Compression Generators (HFCG) 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. Our previous work with a non-optimized fuse has produced similar to 100 kV into a 15. load, which leads into a regime relevant for High Power Microwave (HPM) systems. It is expected that similar to 300 kV can be achieved with the present 2-stage HFCG driving an inductive storage system with electro-exploding fuse. In order to optimize the electro-explosive wire fuse, we have constructed a non-explosive 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 4 stages of inductance change and is based upon a piecewise linear regression model of the HFCG waveform. The non-explosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results.
+ Characterization Of Explosively Driven Ferroelectric Generator Material
  S. L. Holt; D. J. Hemmert; J. W. Walter; J. J. Mankowski; J. C. Dickens; M. Kristiansen; L. L. Altgilbers
Abstract:  Explosively driven ferroelectric generators (FEGs) are reliable, compact, high voltage sources for driving ultra-compact high power devices. When energized by mechanical stress, they release charge from their crystal structure, acting similar to a charged high voltage capacitor. However, due to the relatively low energy of these devices, they must be well matched to their application so that the full potential of the material can be utilized. This requires proper characterization of the material's electrical output as a function of the input shockwave. HEM Technologies and Texas Tech University characterized a ferroelectric ceramic, EDO EC-64, for different pressures to determine possible loss mechanisms and its maximum output power and energy.

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+ Pressure Induced Conductivity For High Power Switching
  H. Veselka; A. Neuber; J. Dickens
Abstract:  Summary form only given. Investigations of shock induced conductivity of non-crystalline insulators and crystalline semiconductors were performed. Although we measured the insulator-to-metallic state transition time and conductivity, the focus of this investigation was on the recovery phase of the induced conductivity (i.e. metallic-to-insulator state transition). The recovery time and shock conditions were measured with high speed electrical diagnostics. The goal of this research is to determine the feasibility of using shock induced conductivity as a means of producing a high power opening switch. To minimize switch losses, the insulator-to-metallic transition time and conductance is also important, but has been more widely studied. Initial impact studies have shown that certain insulator can be conductive for 100 microseconds and recover under modest voltage less than one microsecond using a ten gram explosive charge. Various shock intensities are used in the study. The shock is produced primarily with conventional commercial explosives. In addition, the impact of sample thickness and compression duration on the induced the conductivity were also studied. The correlation between modest voltage and high voltage recovery time and shut-off current were studied

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+ Pulsed Unipolar Surface Flashover At Atmospheric Conditions
  Morales, K; Krile, J; Neuber, A; Krompholz, H; Dickens, J
Abstract:  Dielectric surface flashover along insulators in vacuum has been comprehensively researched over the years. However, the primary mechanisms involved in dielectric flashover at atmospheric pressures have yet to be as extensively analyzed with variable parameters such as electrode geometry, background gas, humidity, and temporal characteristics of the applied voltage. Understanding the fundamental physical mechanisms involved in surface flashover at atmospheric pressures is vital to characterizing and modeling the are behavior. Previous dc and unipolar excitation experiments have shown distinct arc behavior in air and nitrogen environments for an electrode geometry that produces electric field lines that curve above the dielectric surface. Specifically, flashover arcs in an air environment were observed to develop along the dielectric surface. Experiments conducted in nitrogen revealed that the arc developed along the electric field lines, above the surface of the dielectric. It was also of importance to alter the temporal characteristics of the applied voltage to simulate lightning situations and investigate the impact on the arc behavior and voltage delay times. A solid state high voltage pulser with an adjustable pulse width of similar to 500 ns at FWHM and amplitudes in excess of 30 kV was specifically developed to replicate the temporal characteristics of a voltage pulse observed when a building structure is hit by a lightning strike. Based on these results, the physical mechanisms primarily involved in pulsed unipolar surface flashover will be discussed. Additional studies regarding the effects of humidity and surface roughness on the flashover arc behavior will also be presented.
+ The Impact Of Water Conductivity, Electrode Material, And Electrode Surface Roughness On The Pulsed Breakdown Strength Of Water
  Wetz, D; Mankowski, J; McCauley, D; Dickens, J; Kristiansen, M
Abstract:  Experimental results are presented on the impact water conductivity, electrode material, and electrode surface roughness have on the dielectric strength of water. A 2 mm water gap was tested under pulsed conditions with maximum electric fields in excess of 1.5 MV/cm. Bruce profiled electrodes manufactured from various materials including Molybdenum, Aluminum, Copper, Tungsten, Nickel, Stainless Steel, and Zinc Oxide, which have work functions ranging from 4.2 eV to 5.3 eV, have been tested. The roughness average of the various surfaces was varied from 0.1 mu m to 1.96 mu m and the conductivity of the water within the gap was varied from I mu S/cm to 39 mu S/cm to determine the impact each has when tested in combination with the various electrode materials and surface roughnesses. Conclusions are made as to the effect each of these factors has on the holdoff strength of a water dielectric system.
Publication Year:  2005
+ A Compact, Repetitive, 500Kv, 500 J, Marx Generator
  A. A. Neuber; Y. J. Chen; J. C. Dickens; M. Kristiansen
Abstract:  The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate, Marx generator are discussed. While benefiting from the large energy density of mica capacitors, 4 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 ~18.5 Omega characteristic Marx impedance. Using solely inductors, ~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 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.

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+ Multi-Frequency And Multi-Mode Microwave Identification In A Vircator Research
  X. Chen; J. C. Dickens; L. L. Hatfield; J. Mankowski; M. Kristiansen
Abstract:  In virtual cathode oscillator (VIRCATOR) research, multi-frequency and multi-mode microwaves are coupled out and propagate along a waveguide. Identifying the microwave propagating modes has become a priority work for further research on microwave radiation physical mechanisms. Generally, an antenna matrix at the end of an open waveguide, which can shape the radiated microwave power distribution, is used to identify the microwave propagating modes [1-2]. Actual microwave radiation mechanisms in a VIRCATOR are complicated. Multi-propagating modes and multifrequency microwaves coexist simultaneously, which challenges the antenna matrix method. In this paper, based on microwave propagating theory in a waveguide, a new method is proposed to identify multi-frequency and multi-mode microwaves, which is partially justified by the experimental data at Texas Tech University.

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Publication Year:  2004
+ Experimental Optimization Of A Reflex Triode Virtual Cathode Oscillator
  J. J. Mankowski; X. Chen; J. C. Dickens; M. Kristiansen
Abstract:  Experimental findings on a high power Reflex Triode Virtual Cathode Oscillator (Vircator) at Texas Tech University are reported. In order to optimize performance, changes to the AK gap distance were made. The AK gap was varied from 0.5 to 1.5 cm. The type of cathode material employed was velvet. Cathode radius was 2.5 cm. The Reflex Triode Vircator is driven by a 350 kV Marx generator with a 60 ns, 20 ohm pulsed forming line. Typical peak input power is 4 GW. The output microwave power is measured in the far-field with a waveguide to coax adapter. Thus far we have observed microwave peak power output as high as 330 MW corresponding to a peak power efficiency of ~ 11 %. In an effort to provide premodulation to the vircator region, reflecting strips were added in the downstream waveguide. This technique has been employed with success by this group with a coaxial Vircator.

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Publication Year:  2003
+ Cavity Resonance Effect On A Coaxial Vircator
  Chen, XP; Dickens, J; Choi, EH; Mankowski, J; Hatfield, LL; 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. However, some experiments at TTU show that the interaction between the virtual cathode oscillation and the cavity is a key in determining the microwave frequency and propagating mode. Particularly, we observe that the E-beam plays 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 has been investigated in detail. The understanding of the E-beam characteristics is critical in optimizing the design of the cylindrical diode to avoid the microwave frequency shifting and mode competition.
+ Compact, Portable Pulsed Power: Physics And Applications
  Gundersen, M; Dickens, J; Nunnally, W
Abstract:  We describe an academic research program designed to investigate fundamental issues that will impact future pulsed power technology. The project is intended to enhance the scientific footing for pulsed power, and to address issues in the development of compact, repetitive pulsed power. These include physics issues implicit in switch development, including both solid state and gas-phase, pulse generation at various levels from extremely small for fast high field generation to >100kV, and applications including ignition of jet fuels and biological response to electric fields.
+ Conductivity Measurements Of Explosively Shocked Aluminum And Ofhc Copper Used For Armature Material In A Magnetic Flux Compression Generator
  Hemmert, D; Mankowski, J; Rasty, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  Modeling and characterization of a Magnetic Flux Compression Generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. In the studies reported here, a thin metallic strip is shocked with an explosively generated shock wave produced from a charge of Composition C-4. The shock wave is intended to simulate the shock wave and pressures produced in MFCG research currently being conducted at Texas Tech University. These pressures are estimated to be between 1 and 3 GPa. The experimental setup is arranged so that the shapes of the metallic strip and shock front are the same, as confirmed using optical fibers. This was to ensure that the test sample was shocked uniformly. The metallic test strip is pulsed with a 70 A current pulse during application of the shock wave. The current and voltage across the test sample are measured directly to determine the change in conductivity. Pressure measurements are conducted in separate tests under similar conditions using strain gauges. The results are then compared to results determined previously using a Split Hopkinson Pressure Bar apparatus (SHPB).
+ Design Criteria For Prevention Of Armature Turn-Skipping In Helical Magnetic Flux Compression Generators
  Rasty, J; Le, X; Dickens, J; Neuber, A; Kristiansen, M
Abstract:  Helical Magnetic Flux Compression Generators (MFCGs) are capable of producing ultra-high power electric pulses by trapping and compressing a seed magnetic field into a load coil via an explosive-driven armature. The efficiency of helical MFCGs is generally very low, about 10%, due to large magnetic flux losses. One of the main sources of magnetic flux loss is the turn-skipping phenomenon, in which the expanding armature fails to establish contact with every turn of the helical coil, resulting in magnetic flux loss in the skipped turns of the coil. The turn-skipping phenomenon is related to non-uniform or asymmetric expansion of the armature, as well as detonation end effects, Equations describing the turn-skipping phenomenon are developed in terms of the eccentricity of the armature with respect to the helical coil, the armature's wall thickness variations and the length of the detonation end effect. Design criteria for prevention of turn-skipping are presented in order to achieve optimum MFCG performance.
+ Efficiency Results From A Coaxial Vircator Using A Simple Feedback Technique
  Mankowski, J; Cheng, X; Dickens, J; Kristiansen, M; Choi, E
Abstract:  Experimental findings on the coaxial virtual cathode oscillator (Vircator) at Texas Tech University are reported. A major modification to the system extended the pulse forming line several meters. The pulsed power system now delivers up to 750 kV and 60 kA for at least 70 nsec. Additional modifications include diagnostics to measure the diode voltage, current, and radiated field pattern of the output microwaves. In this initial phase of experiments copper reflectors were installed within the diode to provide a simple means of microwave feedback to assist in the e-beam modulation. Thus far we have observed microwave peak power output with feedback reflectors as high as 3.5 GW corresponding to an efficiency of similar to9.5%. Measured radiated field patterns show evidence of multi-mode operation.
+ Ferromagnetic And Ferroelectric Materials As Seed Sources For Magnetic Flux Compressors
  Schoeneberg, N; Walter, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  An alternative to the use of capacitors as the seed source for flux compression generators (FCG) are permanent magnet based systems. Permanent magnets provide a larger energy-to-volume ratio given that the use of capacitors requires a power source as well as charging and firing circuitry. A recent design developed at Texas Tech University's Center for Pulsed Power and Power Electronics [1], focused on the demagnetization of a Neodymium-Iron-Boron magnet (Nd2Fe14B, B-r = 1.23 T) by a shockwave generated from high explosives. The maximum specific energy achieved with this design was only 2.3 J/kg, which may not be sufficient for effectively seeding an FCG (fast capacitors have at least 20 J/kg). The same magnets were used with an alternative design, referred to as a strip FCG, which utilizes opposing magnets to generate an initial magnetic field intensity within an air gap. The air gap exists between a central tube, filled with high explosives, and metal strips placed between the magnets a few millimeters off-axis. The detonation of the explosive causes the expansion of the central tube, subsequently compressing the flux into the strips and then into the load. The original strip FCG design [2] used oxide-barium magnets (B-r = 0.2T), which produced an estimated specific energy of 5.27 J/kg into a low inductance load. These magnets were replaced with the Nd2Fe14B (B-r = 1.23 T) magnets in order to achieve better performance, which will be analyzed with respect to the specific energy. In addition, a design utilizing a shocked ferroelectric material, PZT, as a seed current source is discussed. An explosively generated shock wave is passed through a ferroelectric material, generating a current that establishes the initial seed flux for an FCG. Preliminary tests of the ferroelectric sources indicate a possible theoretical specific energy of more than 11 J/kg or 25 J/dm(3). A comparison of the designs will be discussed with regard to the specific energy produced and the effectiveness of each to seed FCG's.
+ Investigation Of Pulse Power Thyristor Thermal Variations
  Hoffman, MG; Dickens, JC; Giesselmann, MG
Abstract:  This paper presents a study of temperature variations in a. Pulse Power Thyristor (PPT) during, and after discharge. The PPT studied was the SPT411BHT. The SPT411BHT is a 5000 V 4600 A, 125 mm thyristor made by Silicon Power Corporation. In order to determine the temperature of the PPT silicon, a 125 mm diode with identical thermal properties is placed in series with the PPT. There is a strict relationship between the forward voltage and the silicon temperature of the diode. Measurement of the forward voltage of the diode before and after discharge will accurately predict the temperature increase of the silicon. Peak discharge currents will be varied from 30 kA to 90 kA. The forward voltage measurement, circuit must be able to resolve millivolts in the presence of common mode voltages in excess of 2 W. Also, the circuit must isolate the measurement equipment from the circuit. This paper will discuss the temperature measurement concept as well as the design details of the circuit used to measure the diode forward voltage.
+ Investigation Of Ultra-High Switching Frequency To Reduce Size In Rapid Capacitor Charging
  Morris, BW; Dickens, JC
Abstract:  A compact high switching frequency rapid capacitor charging supply was designed and implemented. The challenges of topology design, component selection and implementation are discussed. The end goal is a supply capable of achieving 40kW at 40kV on the output and 100-300V input. The gate driver design is discussed with particular emphasis on gate voltage rise-time and propagation delay. Switching frequencies from 100 kHz to 1 MHz are explored. Transformer and output voltage multiplier designs are also described with expected performance parameters based upon computer simulations.
+ Nanosecond Laser-Triggered Microwave Switch
  McQuage, MM; Neuber, AA; Dickens, JC
Abstract:  The design and experimental testing of a laser-triggered microwave switch with a nanosecond activation time is described. The objectives of the project include, confirming that a nanosecond to sub-nanosecond risetime is achievable in the X-band waveguide at 9 GHz with the laser-triggered switch and to determine the minimum laser energy necessary to obtain the fastest possible risetime. A I kW pulsed X-band source with a 500 ns output pulse provides the microwave power for the system. A variable power Nd:YAG laser with a maximum 450 mJ at 532 rim, 10 ns FWHM output pulse is used in conjunction with an applied high voltage pulse to trigger the microwave switch. The microwave signal is switched with the rapid formation of plasma caused by the breakdown of a gas contained by a quartz tube inserted through a section of waveguide. The centerpiece of the waveguide system is a magic tee, which controls the direction of power flow through the system. Compared to tests in air and N-2, the best results have been obtained in Argon. Risetimes below 2 ns have been obtained using Argon at a reduced pressure of 150 Torr and a high voltage pulse of 28 kV from a spark gap. The impact of gas pressure, applied voltage pulse and applied laser pulse on the risetime of the microwave switch are discussed.
+ Optical Diagnostics Of Liquid Nitrogen Volume Pre-Breakdown Events
  Butcher, M; Neuber, A; Krompholz, H; Dickens, J
Abstract:  An increased need for compact pulsed power systems requiring new switching technologies combined with the benefits of cryogenic properties, such as higher energy density and miniaturization, has lead to increased interest in liquid nitrogen as a switching medium. High hold off voltage, low dielectric constant, and low environmental impact are further advantages of liquid nitrogen. Characterization of breakdown is investigated using high-speed (temporal resolution < 1 ns) optical and electrical diagnostics in a coaxial system with 52 Omega impedance. Experiments are done in self-breakdown mode in supercooled liquid nitrogen with a temperature near 70 K. Discharge current and voltage are determined using transmission line type current sensors and capacitive voltage dividers. Discharge luminosity is measured with photomultiplier tubes (risetimeapproximate to800 ps) that are focused on the negative electrode tips and the center of the channel. Optical investigations of breakdown and pre-breakdown events on a nanosecond time scale will provide a better understanding of the fundamental physics of breakdown formation. Detailed optical and spectroscopic diagnostics combined with high-speed electrical diagnostics are aimed at clarifying the overall breakdown mechanisms, including electronic initiation and bubble formation. The breakdown initiation/development will be discussed.
+ Physical Efficiency Limits Of Inch-Sized Helical Mfcg'S
  Neuber, AA; Hernandez, JC; Holt, TA; Dickens, JC; Kristiansen, M
Abstract:  Helical Magnetic Flux Compression Generators (MFCG) are attractive energy sources with respect to their specific energy output. A variety of one-time use applications would benefit from small inch-sized helical generators with high specific energy output. However, it is widely accepted that the generator performance deteriorates with decreasing size. Previous experimental data have shown that the increase of the ohmic resistance of the MFCG with a reduction in size is the primary cause for the observed behavior when the initial generator inductance is held constant. We will analyze the situation in more depth and quantify how much the efficiency is determined by ohmic losses and intrinsic flux losses (flux that is left behind in the conductors and lost for compression) for different generator sizes and geometries. Our simple constant diameter MFCGs exhibit more intrinsic than ohmic losses (69% compared to 16%), while our MFCGs with tapered armatures display less intrinsic and more ohmic flux losses (13% compared to 66%), however, at increased overall efficiency. We will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.
+ Physics Of Dielectric Surface Flashover At Atmospheric Pressure
  Krile, J; Neuber, A; Dickens, J; Krompholz, H
Abstract:  The limits of the applicability of dc, ac, or pulsed high voltage are determined by breakdown along insulators or insulating support structures. It is of major technical importance to predict breakdown voltages for given structures, with parameters such as geometry, material, and temporal characteristics of the applied voltage. The impact of atmospheric conditions such as humidity, pressure, temperature, and types of gas present is also important. A setup has been devised to simulate and closely monitor flashover across various gap distances and insulator geometries at atmospheric conditions at different humidities. Current, voltage, luminosity, and optical emission spectra were measured with nanosecond to sub-nanosecond resolution. Spatially and temporally resolved light emission data yielded quantitative information about the charge carrier amplification, the location of this amplification, and its role in the formation of streamers.
+ Pre-Breakdon Current Behavior In Dc Volume Breakdown In Transformer Oil
  Butcher, M; Neuber, A; Krompholz, H; Dickens, J
Abstract:  The phenomenology of pre-breakdown events in transformer oil is investigated using high-speed electrical and optical diagnostics. Data collection using a coaxial test setup terminating into a 50Omega load line to simulate a matched impedance system allows very fast risetimes. Transmission line type current sensors and capacitive voltage dividers with temporal resolution of 300 ps provide information about the discharge voltage and current. Steady, DC currents ranging from a few nA with less than 10 W of applied voltage, to a few muA prior to full breakdown are measured using an electrometer. Pre-breakdown events are measured with positive and negative charging voltages with respect to ground. Light emission from the discharge is measured using a series of fast photomultiplier tubes, (risetimes 800 ps), that observe positive and negative electrode tips and center of the channel. Preliminary results on self-breakdown (breakdown voltage +44kV) with a 2.35 mm gap show a DC (seed) current of several hundred nA with pre-breakdown spikes of a few mA immediately before final breakdown. Periodicity of the current spikes combined with a general increase in magnitude prior to full breakdown has been observed. Data collection using a negative charging line, with respect to ground with enhanced field at the cathode, indicates current spikes that are typically 25 to 50% faster than spikes using a positive charging line with enhanced field at the anode. Detailed optical diagnostics along with high-speed electrical diagnostics of the pre-breakdown phase will address the physical mechanisms initiating volume breakdown.
+ Reflex-Triode Geometry Of The Virtual-Cathode Oscillator
  Lara, MB; Mankowski, J; Dickens, J; Kristiansen, M
Abstract:  An eight-stage four-hundred kilovolt Marx bank, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a planar version of the Virtual Cathode Oscillator (Vircator). Eight 1upsilonF capacitors, charged to 50kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-300kV pulse to the diode. The design of the Planar or Reflex-Triode geometry Vircator is based upon claims of high efficiency by Didenko et al. A previously constructed TTU Vircator includes a unique E-beam source, the brush cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a Virtual Cathode. This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.
+ Self Electrical Breakdown In Biodegradable Oil
  Cevallos, MD; Dickens, JC; Neuber, AA; Haustein, MA; Krompholz, HG
Abstract:  The fundamental breakdown physics of biodegradable oil is investigated with high speed electrical and optical diagnostics with temporal resolution down to several 100ps. The set up employs a cable discharge into a coaxial system with axial discharge and load line to simulate a matched terminating impedance. A unique feed-through design creates no discontinuities in the system lines through the discharge chamber. The impedance of the system is matched at 50 ohms including a novel design for impedance matching transitions from discharge cable to coaxial system to load line allowing for a sub-nanosecond response. This paper presents results on self breakdown with voltages of up to 60 kV. Self breakdown is achieved by charging the discharge cable and load line to +/-30 kV respectively. Transmission line type current sensors and a capacitive voltage divider with fast amplifiers/attenuators are used in order to obtain a complete range of information from amplitudes of 0.1 mA to 1kA with temporal resolutions of 300 ps. Optical measurements include high speed photography and shadowgraphy. Detailed optical diagnostics along with high speed electrical diagnostics will address the mechanism initiating/assisting biodegradable oil volume breakdown.
+ Short Pulse Electric Field Sterilization Of Liquid Media
  Wetz, D; Truman, K; Dickens, J; Mankowski, J; Neuber, A
Abstract:  In recent years, researchers have investigated methods of liquid sterilization by applying pulsed high electric fields with some degree of success(1-2). The mechanism by which microorganisms are damaged has been shown to be a function of the local electric field and exposure time while independent from thermal and electrolytic effects from the applied pulse. Most published experiments have employed electrical pulses of 10's of kV and microsecond long pulse lengths. We are employing electrical pulses in the 100's of kV range with 100 nanosecond long pulse lengths. This type pulse should be more effective at killing microorganisms and minimize energy losses due to thermal processes in the liquid media.
+ Single Shot Overstressing Of High Voltage Capacitors For Compact Arkadiev-Marx Generator
  Shkuratov, SI; Talantsev, EF; Dickens, JC; Kristiansen, M
Abstract:  Five different types of high voltage capacitors have been tested to determine their maximum high voltage overstress coefficient. Chicago Condenser, Reynolds Industries, Hivolt, and TDL capacitors of capacitance 100 nF and different nominal voltages were tested in the single shot mode. Experiments have shown that the energy stored under overstress conditions in all types of the capacitors tested is 10 to 20 times more than the nominal energy.
+ Small Sized Mfcg For Driving A High Impedance Load
  Hernandez, JC; Neuber, AA; Dickens, JC; Kristiansen, M
Abstract:  End-initiated small volume magnetic flux compression generators (MFCG) have at least one order of magnitude higher energy density (by weight or volume) than capacitive energy storage with similar discharge time characteristics. Since the prime energy is built into the MFCG in form of HE, the capacitor looses even more ground if the necessary prime energy source and the charging supply are included in the weight/ volume balance. However, simple MFCGs with a single helix produce high output energy only into low inductance loads, thus producing several 100 kA of current at a voltage level of only a few 10 kV. Many pulsed power devices require less current but a considerably higher voltage level. Two approaches for achieving a higher output voltage level, both utilizing two staged MFCGs, have been reported in the open literature. The first employs a more traditional transformer coupling, the second relies on a dynamic transformer or flux-trapping scheme. Although the traditional transformer coupling has theoretically the better efficiency, we chose the latter approach for our generator design, mostly since it requires a smaller number of components. Our generator has a total length of 250 mm, a helix inner diameter of 51 mm, and is wound with Teflon insulated stranded wire of different sizes in the range from AWG 12 to AWG 22. We have presently achieved an energy gain of similar to 8 and will discuss the generator performance based on experimental current/voltage waveforms.
+ Space-Charge Limited Current For 1-D Cylindrical Diodes
  Chen, XP; Dickens, J; Choi, EH; Kristiansen, M
Abstract:  By introducing a physics approximation method into analyzing the nonlinear Poisson's equation, an analytical expression for the space-charge-limited current density for a one-dimensional (1-D) cylindrical diode has been investigated and developed. This expression is different from Child-Langmuir law for the planar diode and is simpler than Langmuir-Blodgett law for the cylindrical diode. This expression builds an explicit connection between the current density and the physical parameters, which is helpful in optimizing the design of the cylindrical vacuum diode. In addition, a comparison between our analytical result and Langmuir-Blodgett law shows that the physics approximation method is valid in nonlinear differential equation analysis and can be used in other similar cases. Applying the approximation method, we get the relativistic theory corrected current for 1-D cylindrical diodes.
+ Surface Flashover Across Ceramic Disks In Vacuum At Cryogenic Temperatures
  Keene, H; Dickens, J; Neuber, A; Krompholz, H
Abstract:  As superconducting technology becomes more viable in the marketplace, especially in high power applications, the need for a well researched high thermal conductivity electrical insulator is needed. The electrical failure mode for these types of insulators is often surface flashover at sub-atmospheric temperature and pressure. Testing of two such insulators, aluminum nitride and aluminum oxide, for this failure mode is done for two differing electrode geometries. In addition three coats of GE 7031 dielectric varnish are applied to the exposed parts of the insulator for comparison testing with non-varnished samples. In general the testing shows an increasing breakdown voltage trend with decreasing temperature. These results indicate a temperature related dependence of the secondary electron emission and electron induced outgassing, which is a component in the process of surface flashover. The addition of the varnish results in a lowered breakdown voltage. The research also covers the effect of electrode conditioning, and presents optical diagnostics of the gas species involved during breakdown.
Publication Year:  2002
+ Dependence Of Flux Losses On Mcg Helix Geometry
  A. A. Neuber; T. Holt; J. -C. Hernandez; J. C. Dickens; M. Kristiansen
Abstract:  While Magneto Cumulative Generators (MCGs) differ widely in size and operating regimes, 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 would be solely based on physical principles and manage without heuristic factors. This means, all programs known to the authors utilize an additional parameter that adjusts for the loss in flux that is currently unexplained. Experimental efforts revealed that the unexplained loss in flux becomes smaller the larger the generator volume is. Specifically, for generators with constant diameter helix and armature, the figure of merit, β, for generator performance exhibits a distinct decline with the angular frequency, ω, of the progressing contact between end-initiated armature and helix. Since ω is proportional to the square root of the ratio of initial MCG inductance to compressed volume, it becomes apparent that the generator performance is limited by size. For large generators on a meter scale, a β of approximately 0.95 has been reported (β = 1 means no flux loss at all), and own tests indicate that β drops quickly to about 0.7 to 0.6 for generator on the 10 cm scale. Still, even very small generators with about 30 mm outer diameter can have a β of around 0.6. Tapered generators, with tapered helix or tapered armature might perform differently and can be used to further assess the situation. Preliminary results for MCGs with a tapered helix show a performance that is somewhat superior to the straight helix generator.

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+ High Voltage, Sub Nanosecond Feedthrough Design For Liquid Breakdown Studies
  M. Cevallos; J. Dickens; A. Neuber; H. Krompholz
Abstract:  Experiments in self-breakdown mode and pulsed breakdown at high over-voltages in standard electrode geometries are performed for liquids to gain a better understanding of their fundamental breakdown physics. Different liquids of interest include liquids such as super-cooled liquid nitrogen, oils, glycerols and water. A typical setup employs a discharge chamber with a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance, thus providing a sub-nanosecond response. This study is focused on the feed-through design of the coaxial cable into this type of discharge chamber, with the feed-through being the critical element with respect to maximum hold-off voltage. Diverse feedthroughs were designed and simulated using Maxwell 3-D Field Simulator Version 5. Several geometrically shaped feed-through transitions were simulated, including linearly and exponentially tapered, to minimize electrostatic fields, thus ensuring that the discharge occurs in the volume of interest and not between the inner and outer conductor at the transition from the insulation of the coaxial cable to the liquid. All feedthroughs are designed to match the incoming impedance of the coaxial cable. The size of the feedthroughs will vary from liquid to liquid in order to match the coaxial cable impedance of 50Ω. The discharge chamber has two main ports where the feed-through will enter the chamber. Each feed-through is built through a flange that covers the two main ports. This allows the use of the same discharge chamber for various liquids by changing the flanges on the main ports to match the particular liquid. The feedthroughs were designed and built to withstand voltages of up to 200 kV. The feedthroughs are also fitted with transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators in order to attain a complete range of information from amplitudes of 0.1mA to 1 kA with a temporal resolution of 300 ps.

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+ Liquid Nitrogen As Fast High Voltage Switching Medium
  Dickens, J; Neuber, A; Haustein, M; Krile, J; Krompholz, H
Abstract:  Compact pulsed power systems require new switching technologies. For high voltages, liquid nitrogen seems to be a suitable switching medium, with high hold-off voltage, low dielectric constant, and no need for pressurized systems as in high pressure gas switches. The discharge behavior in liquid nitrogen, such as breakdown voltages, formative times, current rise as function of voltage, recovery, etc. are virtually unknown, however. The phenomenology of breakdown in liquid nitrogen is investigated with high speed (temporal resolution < I ns) electrical and optical diagnostics, in a coaxial system with 50-Ohm impedance. Discharge current and voltage are determined with transmission line type current sensors and capacitive voltage dividers. The discharge luminosity is measured with photomultiplier tubes. Preliminary results of self-breakdown investigations (gap I nun, breakdown voltage 44 kV, non-boiling supercooled nitrogen) show a fast (2 ns) transition from an unknown current level to several mA, a long-duration (100 ns) phase with constant current superimposed by ns-spikes, and a final fast transition to the impedance limited current during several nanoseconds. The optical measurements will be expanded toward spectroscopy and high speed photography with the aim of clarifying the overall breakdown mechanisms, including electronic initiation, bubble formation, bubble dynamics, and their role in breakdown, for different electrode geometries (different macroscopic field enhancements).
Publication Year:  2001
+ Calculation Of Air Temperature And Pressure History During The Operation Of A Flux Compression Generator
  Le, XB; Rasty, J; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  During the operation of Magnetic Flux Compression Generators (MFCG), the gas-plasma, shocked by the rapidly expanding armature, could lead to electrical arcing across the gas between the armature and the stator at locations where physical contact between the armature and stator has not yet occurred. This will result in a loss of magnetic flux and a decrease in the electrical efficiency of the MFCG. Therefore, knowledge of the ensuing gas temperature and pressure histories is necessary for identification of loss mechanisms in an effort to optimize the efficiency of MFCGs. This paper describes the procedure for estimating the air temperature and pressure histories via Finite Element (FE) simulation of the an-nature expansion and its ensuing contact with the stator in an MFCG. First, the validity of the FE model was verified by comparing deformation contours obtained from the simulations to those obtained experimentally via high-speed photography. Utilizing the pressure history data obtained from the FE results, the air temperature was theoretically calculated. The results indicate that the air pressure and temperature in an MFCG, having a compression ratio of 1.8, could be as high as 30 MPa and 4000degrees Kelvin, respectively.
+ Compact Explosive Driven Shock Wave Ferromagnetic Generators
  Shkuratov, SI; Talantsev, EF; Kristiansen, M; Dickens, J; Hernandez, JC; Neuber, A
Abstract:  The results are presented of tests with compact, explosively driven shock wave ferromagnetic generators. The shock wave from high a explosive charge is passed along the axis of a cylindrical, hard magnet. Two types of permanent magnets were used in the experiments: rare-earth NdFeB cylinders (D = 2.5 cm, L = 1.9 cm) and hard ferrite BaFe2O3 cylinders (D = 2.2 cm, L = 2.5 cm). The shock wave demagnetizes the cylinder, reducing the flux from the remnant value to zero. This change in flux generates a voltage in the winding. The current generated in the loads of the generators yielded a peak of 0.75 kA. The operation of the shock wave ferromagnetic generators was analyzed by the Maxwell 3D code. An analysis is given on the specific features of pulse generation in a system like this.
+ Conductivity Measurements Of Mfcg Armature Material Under Shock And High Strain Rates Utilizing A Split-Hopkinson Pressure Bar Apparatus
  Hemmert, D; Rasty, J; Neuber, A; Dickens, J; Le, X; Kristiansen, M
Abstract:  Modeling and characterization of a Magnetic Flux Compression Generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. A Split-Hopkinson Pressure Bar apparatus (SHPB) and current source/ differential amplifier setup was used to study shock-loading under controlled conditions while monitoring changes in resistivity in armature material samples. The SHPB apparatus was capable of producing strain rates up to the fracture limit of the samples tested. Actual fracturing of samples would not have allowed detailed analysis of thermal and mechanical effects in sample resistivity changes. Sample strain rate levels of up to 1x10(4) strain sec(-1) were achieved with the apparatus on OFHC copper and aluminum samples. This may be comparable to switching contacts under similar shock-loading and is an order of magnitude less than the expected strain rates in the MFCG. Sample resistivity showed an initial and abrupt decrease followed by a rapid increase during loading to levels twice that of virgin samples. Short and long time based resistivity monitoring and high speed framing photography allowed differentiation between changes in resistivity due to bulk material deformation, and changes due to thermal effects.
+ Effect Of Electron-Beam Pulse Length On Microwave Efficiency Of Coaxial Vircator
  Jiang, W; Woolverton, K; Dickens, J; Kristiansen, M
Abstract:  The coaxial virtual cathode oscillator (vircator) at Texas Tech University was studied by varying the pulse length of the electron beam. Previous experimental results obtained with electron beam pulse width of 25 ns have indicated that the microwave output power had not saturated before the electron beam power terminated. Therefore, it has been expected that the maximum microwave power can be increased if the electron beam pulse is lengthened. Experiments were carried out with the electron-beam pulse length of 25 ns and 70 ns, respectively, while keeping other parameters the same. The microwave outputs obtained with different electron-beam pulse lengths are compared in order to understand the effect of the pulse length on the microwave efficiency of the coaxial vircator.
+ Experimental And Numerical Investigation Of Armature/Stator Contact In Magnetic Flux Compression Generators
  Rasty, J; Le, XB; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  The efficiency of a Magnetic Flux Compression Generators (MFCG) is highly dependent on the expanding characteristics of the exploding armature and the nature of contact between the armature and the surrounding stator coil. A hydrodynamic Finite Element (FE) model was developed to simulate the expansion characteristics of the armature and its ensuing impact with the stator. The effectiveness of the FE model to simulate the explosive behavior of the armature was qualified by comparing the numerical results with experimentally measured parameters. Specifically, the radial displacement of the armature as well as the axial velocity of the armature/stator contact point were measured experimentally and compared with numerical results showing excellent agreement between the two. The results indicated that the radial and axial velocity with which the armature impacted the stator did not change through the length of the armature. However, the results showed that the velocity with which the contact point between the armature and the stator traveled along the length of the armature decreased as the explosion process went on. As expected, the axial propagation velocity of the contact point was found to be at its highest value (2.25 X detonation velocity) at the region close to the detonation end while approaching the detonation velocity at points away from the detonation end.
+ Experimental Study Of Compact Explosive Driven Shock Wave Ferroelectric Generators
  Shkuratov, SI; Kristiansen, M; Dickens, J; Neuber, A; Altgilbers, LL; Tracy, PT; Tkach, Y
Abstract:  The design of explosive driven ferroelectric generators is presented and experimental data are discussed. The active elements are lead zirconium titanate (PZT) disks with diameter D = 25 mm and thicknesses H = 2.5 mm and H = 6 mm and PZT cylinders with D = 21 mm and H = 25 mm. The high explosive charge was varied from 4.2 g to 30 g. Two different ways to initiate shock waves in the active elements were used: explosively driven flyer plates and direct action of high explosives. The data presented is for the maximum power into a resistive load.
+ Explosive Driven Ferroelectric Generators
  Tkach, Y; Shkuratov, S; Dickens, J; Kiristiansen, M; Altgilbers, LL; Tracy, PT
Abstract:  Explosive Driven Ferroelectric Generators (EDFEGs) are compact power sources that have been considered for use as seed sources for magnetocumulative generators, as well as prime power sources [1,2]. Shock waves generated by high explosives arc used to shock depolarize ferroelectric materials, winch results in a voltage pulse being delivered to a load. These generators have been experimentally investigated at Texas Tech University. Data from these experiments was used to benchmark a code developed at the Institute of Electromagnetic Research. In this paper, a description of the simulation and a comparison of the experimental and simulation results will be presented.
+ Explosive-Driven Moving Magnet Generators
  Shkuratov, SI; Kristiansen, M; Dickens, J
Abstract:  The results are presented of experimental studies of explosively driven moving magnet generators. The study was performed with the use of high explosive and propellant charge. The projectiles used were NdFeB cylinders with D = 2.5 cm and H = 1.9 cm. Data are given for amplitude of high current pulses for different designs of the devices. The experiments performed have shown that the best option for generators using a moving magnetic projectile is to utilize the energy of the gases formed as a result of burning of the explosive rather than the energy of a flyer plate accelerated under the action of burning of the explosive. The explosive of preference is not C-4, but a military propellent charge which shows a high burning velocity and a high energy of explosive gases.
+ High Current Testing Of Batteries
  Shkuratov, SI; Kristiansen, M; Dickens, J; Horrocks, E
Abstract:  Different types of commercial batteries were tested in a high-current mode to determine which is the best in portable pulsed power applications. The testing of the batteries was performed in two regimes: short time regime (one second) and long time regime (one hundred seconds). Alkaline, carbon, lithium, lithium ion, lead acid and nickel cadmium batteries of different dimensions have been tested to determine the maximum usable power for short and long time modes. The resistance of the load was varied from 1 Omega to 22 mOmega. Current-voltage characteristics are presented for sixteen different types of batteries. Data are given for maximum power per unit of weight and per unit of volume.Different types of commercial batteries were tested in a high-current mode to determine which is the best in portable pulsed power applications. The testing of the batteries was performed in two regimes: short time regime (one second) and long time regime (one hundred seconds). Alkaline, carbon, lithium, lithium ion, lead acid and nickel cadmium batteries of different dimensions have been tested to determine the maximum usable power for short and long time modes. The resistance of the load was varied from 1 Omega to 22 mOmega. Current-voltage characteristics are presented for sixteen different types of batteries. Data are given for maximum power per unit of weight and per unit of volume.
+ High Voltage Testing Of Capacitors
  Shkuratov, SI; Talantsev, EF; Kristiansen, M; Dickens, J
Abstract:  Three different types of capacitors have been tested to determine maximum usable high voltage. The capacitor testing was performed in the dynamic mode. The voltage rise varied from 200 to 400 V/sec. Disc ceramic and thin film capacitors of different value and different nominal voltages were tested. Experiments have shown that the breakdown voltage for all types of the capacitors tested is about ten times more than the nominal voltage of the capacitors. Data are given for the limiting high voltage for each kind of the capacitors. Experiments have shown that the mechanisms for the destruction of each type of capacitors have specific features.
+ Impact Of Helix Geometry On Mcg Flux Losses
  Holt, T; Neuber, A; Dickens, J; Kristiansen, M
Abstract:  Explosively driven magnetic flux compression has been the object of research for more than three decades. Recently heightened interest has been focused on the basic physical mechanisms that determine the performance of helical Magneto Cumulative Generators (MCGs). Two single-pitch helical flux compression generators of different sizes have been tested using current-voltage probes and optical diagnostics. The main parameters used to characterize the experimental performance of the flux compression generators were the flux conservation and theoretical current gain of each type of generator. Helices with constant pitch and differing separation between wires as well as wires with different insulation thickness were tested and analyzed with respect to their flux conservation and theoretical current gain. Preliminary results show that the insulation thickness plays only a minor role for a change in flux conservation due to geometry in the range from 0.01 to 0.5 min provided that no internal breakdown occurred. Additionally, the overall physical dimension of the generator was modified to allow for a substantial increase in initial inductance. The outer diameter of the generator armature was held constant at 1.5 inches and the coil diameter was varied from 2.6 to 3.5 inches (expansion ratio of 1.7 or 2.3, respectively). The results gained from the conservative expansion ratio of 1.7 were used as a base to compare to the generator performance at the more aggressive expansion ratio of 2.3. First results show that an expansion ratio of 2.3 produces viable results for a partially annealed Aluminum armature with a Gurney angle of approximately 15 degrees.
+ Parametric And Experimental Investigation Of The Edfeg
  Tkach, Y; Shkuratov, S; Dickens, J; Kristiansen, M; Altgilbers, LL; Tracy, PT
Abstract:  The objective of this effort was to investigate both parametrically and experimentally the properties of the Explosive Driven Ferroelectric Generator (EDFEG). The parametric investigation was conducted using a simulation developed by the Institute of Research (see paper P1-E31) and the experimental investigation using the explosive test facilites at Texas Tech University (see paper)OM. Both efforts were conducted synergistically in order to fully understand the characteristics of the EDFEG and to optimize its operation Results of both studies will be presented.
+ The Current Mode Of Pulsed Power Generation In Moving Magnet Systems
  Shkuratov, SI; Kristiansen, M; Dickens, J; Hernandez, JC
Abstract:  Results are presented of investigations of the generation of high-current pulses in moving magnet generators designed as an open ferromagnetic circuit. The experimental study was performed with the use of a light gas gun. The magnetic projectiles were ferromagnetic cylinders having 2.54-cm diameters. Data are given for the amplitude of high current pulses and power in the load for different designs of the devices, the effect of various windings on the pulse-generating coils, and different types of loads. It has been shown that with velocities of the magnetic projectiles of 200-300 m/s, the peak current of the pulses produced by the generators reach kiloamperes.
Publication Year:  2000
+ High Power Microwave Window Breakdown Under Vacuum And Atmospheric Conditions
  Hemmert, D; Neuber, AA; Dickens, JC; Krompholz, H; Hatfield, LL; Kristiansen, M
Abstract:  Microwave window breakdown is investigated in vacuum and atmospheric conditions. An S-band resonant ring with a frequency of 2.85 GHz and a power of 80 MW with a 4 MW magnetron as a source is used. Window breakdown on the vacuum side is simulated using a dielectric slab partially filling an evacuated waveguide. Various high-speed diagnostic methods yield a complete picture on the breakdown phenomenology, with far reaching similarities to de surface flashover. During the initiation phase, free electrons are present, which can be influenced by magnetic fields, followed by a saturated secondary electron avalanche with electron-induced outgassing. Final breakdown occurs in the desorbed gas layer above the surface. In order to simulate window breakdown on the gas-side, a segment of the resonant ring separated by two windows was filled with gas at variable pressure, and breakdown was initiated by field-enhancement tips on one of the gas-side surfaces. Threshold power densities for breakdown are measured, and first results on the phenomenology of this gas breakdown are compared with the processes of flashover in vacuum.
Publication Year:  1997
+ A Subnanosecond High Voltage Pulser For The Investigation Of Dielectric Breakdown
  Mankowski, J; Dickens, J; Kristiansen, M
Abstract:  A high voltage, sub-nanosecond pulser is designed and built for the purposes of investigating dielectric breakdown. The requirement for the pulser is a voltage pulse of several hundred kilovolts, pulsewidth less than a few nanoseconds, and a risetime <400 psec. This is achieved by using pulse sharpening techniques on the output of a 500 kV Marx bank. Originally designed to stimulate a lightning strike, the voltage waveform from the Marx generator has a slow decay time of tens of mu sec. In order to obtain a more desirable pulse, the Marx bank is modified. By removing a lumped-element resistor a higher peak output voltage with a faster risetime can be obtained. Circuit simulations have shown the capability of achieving an 800 kV output in less than 40 nsec to a charging (pulse forming) line. The 50 Omega impedance, oil-filled, pulse forming line consists of a peaking gap and pulse slicing gap. The peaking gap decreases the risetime of the applied pulse down to about 300 psec. The pulse slicing gap is included to short the voltage applied to the delay line and test chamber.
+ Evaluation Of A Russian Sos Diode For Use In A Compact Modulator System
  Dickens, J; Kristiansen, M; Giesselmann, M; Kim, JG
Abstract:  A Russian Power Modulator, which is based on a diode that has been touted as a high power solid state opening switch (SOS) was characterized and tested. The diode has characteristics similar to those of a step-recovery diode, except that the reverse current density is 10-100 times larger than in US manufactured diodes. The modulator has a DC input voltage of 600 V and produces pulse of 150 kV amplitude and 25 ns half width into a 350 Omega load. The maximum pulse repetition rate is 100 Hz. The limit is given by thermal limitations and can be increased through the use of more effective cooling methods.
+ Neurobiology Of Pheromonal Signal Processing In Insects
  Dickens, JC
Abstract: 
+ Streamer Discharges By Pulsed Power On A Spiral Transmission Line
  Akiyama, H; Nishihashi, Y; Tsukamoto, S; Sueda, T; Katsuki, S; Hagler, M; Dickens, JC; Inoue, N
Abstract:  Streamer corona discharges produced by pulsed power make enough high energy electrons to dissociate gases directly. Consequently, pulsed streamer discharges have been proposed for the removal of NOx and SO2 from flue gases, the production of ozone and the treatment of poisonous gases. It is proposed here to produce the pulsed streamer discharges on a spiral transmission line. The characteristics of the discharges are studied, comparing with results using a PSpice code. Then, the effectiveness and advantages of the spiral transmission line are discussed in view of real applications.
+ Window And Cavity 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 and in cavities 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-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10(-8) torr, 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. Other configurations, such as air filled two window setups, relevant for vacuum-air interfaces, can be investigated as well. 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 to 1 ns resolution. In addition, a framing camera with gating times of 5 ns is used. Based on the experimental results, methods to increase the power density which can be transmitted through windows, such as surface coatings and window profiles, will be investigated as well.
Publication Year:  1996
+ Breakdown At Window Interfaces Caused By High Power Microwave Fields
  Dickens, JC; Elliott, J; Hatfield, LL; Kristiansen, M; Krompholz, H
Abstract: 
+ Compact Modulator Using Inductive Energy Storage And A Solid State Opening Switch
  Dickens, JC; Bridges, J; Kristiansen, M
Abstract:  A compact modulator system using inductive energy storage and a diode as an opening switch was investigated. The system was designed around a Russian diode that has characteristics similar to those of a step-recovery diode except that the reverse current density is 10-100 times larger than in U.S. manufactured diodes. The main goal of this investigation was to characterize the Russian diode and develop an understanding of its operation for possible improvement and integration into nanosecond pulse generators. The basic modulator circuit designed uses IGBT's in the forward pumping circuit and is capable of delivering 5 kV at 400 A into the diode. Using this forward pumping circuit, diode voltage and current measurements were made at several input power levels. In addition, data obtained using this test stand was used to produce a computer simulation model of the diode for a more thorough circuit analysis.
Publication Year:  1993
+ Electrode Performance Of A 3 Electrode Triggered High Energy Spark Gap Switch
  DICKENS, JC; ENGEL, TG; KRISTIANSEN, M
Abstract: 
Publication Year:  1991
+ Green Leaf Volatiles - A Ubiquitous Chemical Signal Modifies Insect Pheromone Responses
  DICKENS, JC; BILLINGS, RF; PAYNE, TL
Abstract: 
+ Novel Nitrogen-Containing Functional-Group Mimics Of Aldehyde Insect Sex-Pheromones
  UJVARY, I; DICKENS, JC
Abstract: 
+ Orientation To Host Plant And Conspecifics By The Boll-Weevil, Anthonomus-Grandis Boh (Coleoptera, Curculionidae) Electrophysiological And Behavioral Correlations
  DICKENS, JC
Abstract: 

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Publication Year:  2017
+ Synthetization Of Signals By The Transmission And Superposition Of Bipolar Impulses In Free Space
  Shaw, Z; Feilner, W; Dickens, JC; Neuber, AA
Abstract: 
Publication Year:  2016
+ Compact Reflex Triode With Multi Cavity Adjustment
  Barnett, DH; Rainwater, K; Lynn, CF; Dickens, JC; Neuber, AA; Mankowski, JJ
Abstract: 
+ Next Generation Ionospheric Heater Antenna
  Esser, B; Dickens, J; Mankowski, J; Neuber, A
Abstract: 
Publication Year:  2015
+ Compact Reflex Triode Operation At 10 Hz Repetition Rate And Long Pulsewidths
  Rocha, E; Parson, JM; Lynn, C; Dickens, JC; Neuber, A; Mankowski, J; Queller, T; Gleizer, JZ; Krasik, YE
Abstract: 
+ Nanosecond, Pulsed Microdischarge Uv And Vuv Sources
  Stephens, J; Mauch, D; Feathers, S; Mankowski, J; Dickens, J; Neuber, A
Abstract: 
+ Photoionization Relevant Extreme Ultraviolet Emission From Developing Low Temperature Plasmas In Air
  Stephens, J; Fierro, A; Beeson, S; Dickens, J; Neuber, A
Abstract: