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

Dr. Hermann G. Krompholz

Professor, retired

Contact Information

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

Phone: (806)742-3468
Fax: (806)742-1281
Hermann.Krompholz@ttu.edu

Education

Research Interests

Other Interests

Projects

Awards and Professional Societies

Publications


Journal

Publication Year:  2012
+ A finite-difference time-domain simulation of high power microwave generated plasma at atmospheric pressures
  P. J. Ford, S. R. Beeson, H. G. Krompholz, and Andreas A. Neuber, "A finite-difference time-domain simulation of high power microwave generated plasma at atmospheric pressures," Phys. Plasmas 19, 073503 (2012), DOI:10.1063/1.4736863
Abstract:  A finite-difference algorithm was developed to calculate several RF breakdown parameters, for example, the formative delay time that is observed between the initial application of a RF field to a dielectric surface and the formation of field-induced plasma interrupting the RF power flow. The analysis is focused on the surface being exposed to a background gas pressure above 50 Torr. The finite-difference algorithm provides numerical solutions to partial differential equations with high resolution in the time domain, making it suitable for simulating the time evolving interaction of microwaves with plasma; in lieu of direct particle tracking, a macroscopic electron density is used to model growth and transport. This approach is presented as an alternative to particle-in-cell methods due to its low complexity and runtime leading to more efficient analysis for a simulation of a microsecond scale pulse. The effect and development of the plasma is modeled in the simulation using scaling laws for ionization rates, momentum transfer collision rates, and diffusion coefficients, as a function of electric field, gas type and pressure. The incorporation of plasma material into the simulation involves using the Z-transform to derive a time-domain algorithm from the complex frequency-dependent permittivity of plasma. Therefore, the effect of the developing plasma on the instantaneous microwave field is calculated. Simulation results are compared with power measurements using an apparatus designed to facilitate surface flashover across a polycarbonate boundary in a controlled N2, air, or argon environment at pressures exceeding 50 Torr.

[PDF]

Publication Year:  2011
+ Imaging of Pressure-Dependent High-Power Microwave Surface Flashover
  S. Beeson, P. Ford, J. Foster, H. Krompholz, and A. Neuber, "Imaging of Pressure-Dependent High-Power Microwave Surface Flashover," IEEE Transactions on Plasma Science 39, 2600-2601, (2011).
Abstract:  Open-shutter and intensified charge-coupled device images of high-power microwave breakdown were taken in an effort to characterize the pressure dependence of plasma development. These images were taken with a Nikon D200 and Andor iStar DH734-25U-03, respectively. With the pressures increasing from 200 mtorr to 155 torr, the plasma changes from a diffuse discharge encompassing a large volume to a multichannel structure following the electric field lines.

[PDF]

+ Investigation of the Delay Time Distribution of High Power Microwave Surface Flashover
  J. Foster, H. Krompholz, A. Neuber, "Investigation of the Delay Time Distribution of High Power Microwave Surface Flashover," Physics of Plasmas 18, 013502 (2011).
Abstract:  Characterizing and modeling the statistics associated with the initiation of gas breakdown has proven to be difficult due to a variety of rather unexplored phenomena involved. Experimental conditions for high power microwave window breakdown for pressures on the order of 100 to several 100 torr are complex: there are little to no naturally occurring free electrons in the breakdown region. The initial electron generation rate, from an external source, for example, is time dependent and so is the charge carrier amplification in the increasing radio frequency (RF) field amplitude with a rise time of 50 ns, which can be on the same order as the breakdown delay time. The probability of reaching a critical electron density within a given time period is composed of the statistical waiting time for the appearance of initiating electrons in the high-field region and the build-up of an avalanche with an inherent statistical distribution of the electron number. High power microwave breakdown and its delay time is of critical importance, since it limits the transmission through necessary windows, especially for high power, high altitude, low pressure applications. The delay time distribution of pulsed high power microwave surface flashover has been examined for nitrogen and argon as test gases for pressures ranging from 60 to 400 torr, with and without external UV illumination. A model has been developed for predicting the discharge delay time for these conditions. The results provide indications that field induced electron generation, other than standard field emission, plays a dominant role, which might be valid for other gas discharge types as well.

[PDF]

+ Rapid Formation of Dielectric Surface Flashover due to Pulsed High Power Microwave Excitation
  J. Foster, S. Beeson, H. Krompholz, A. Neuber, "Rapid Formation of Dielectric Surface Flashover due to Pulsed High Power Microwave Excitation," IEEE Transactions on Dielectrics and Electrical Insulation 18, pp. 964-970 (2011).
Abstract:  High power microwave (HPM) dielectric surface flashover can be rapidly induced by providing breakdown initiating electrons in the high field region. An experimental setup utilizing a 2.85 GHz HPM source to produce a 4.5 MW, 3 μs pulse is used for studying HPM surface flashover in various atmospheric conditions. If flashover is to occur rapidly in an HPM system, it is desirable to provide a readily available source of electrons while keeping insertion loss at a minimum. The experimental results presented in this paper utilize a continuous UV source (up to 0.3 mW/cm2) to provide photo-emitted seed electrons from the dielectric surface. Similarly, electrons were provided through the process of field emission by using metallic points deposited on the surface. Initial experiments utilizing 0.2 mm2 aluminum points with a spatial density of 25/cm2 have increased the apparent effective electric field by a factor of ~1.5 while keeping the insertion loss low (<0.01 dB). The field enhancements have sharply reduced the delay time for surface flashover. For an environment consisting of air at 2.07x104 Pa (155 Torr), for instance, the delay time is reduced from 455 ns to 101 ns. Two radioactive sources were also used in an attempt to provide seed electrons in the high field regions. Presented in this paper is a comparison of various field-enhancing geometries and how they relate to flashover development along with an analysis of time resolved imaging and an explanation of experimental results with radioactive materials.

[PDF]

Publication Year:  2010
+ Characterization of a 50 J Linear Transformer Driver
  David Matia, Hermann Krompholz, Travis Vollmer, Andreas Neuber, Michael Giesselmann, Magne Kristiansen, "Characterization of a 50 J Linear Transformer Driver", Proceedings of the 2010 IEEE International Power Modulator and High Voltage Conference, May 23-27, 2010, Atlanta, GA.
Abstract:  A detailed characterization of a 50 J linear transformer driver (LTD) stage is presented. The specific goal of the design is to achieve energy densities superior to typical Marx generators, such as a 500 J compact Marx generator previously designed and built at Texas Tech's Pulsed Power lab. Experimental and analytical techniques for determining circuit elements and especially parasitic elements were used, yielding the magnetizing, primary and secondary leakage inductances associated with the transformer, core saturation effects, parasitic capacitances, the inductance of the pulse discharge circuit, and losses in both copper and the deltamax core. The investigations into these characteristics were carried out using both sinusoidal excitation from 1 kHz to 20 Mhz, and pulsed excitation with rise times down to 5 ns. Pulse amplitudes were varied to cover both the linear and saturation regimes of the core. Distributed parasitic capacitances and the inductance of the pulse discharge circuit were estimated analytically and compared with experimental results. This work was carried out to seek an ideal arrangement of the capacitors and switches on the LTD stage and gain a better basic understanding of fast rise time pulse transformers. Adjustments to the 50 J stage are proposed based on this characterization in order to optimize a future ten stage, 500 J assembly.

[PDF]

Publication Year:  2008
+ Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere
  J.E. Chaparro, W. Justis, H.G. Krompholz, L.L. Hatfield, A. A. Neuber, Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere, IEEE Transactions on Plasma Science 36, 2505 - 2511 (2008).
Abstract:  With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

[PDF]

+ Effects of UV Illumination on Surface Flashover under Pulsed Excitation
  J. T. Krile, A. A. Neuber, H. G. Krompholz, "Effects of UV Illumination on Surface Flashover under Pulsed Excitation", to appear in IEEE Transactions on Plasma Science (2008)
Abstract:  Undesirable surface flashover of high voltage support structures can severely limit the compactness of open air high voltage systems. Only recently, increased effort has been invested in characterizing and quantifying the physical processes involved in surface flashover occurring under atmospheric conditions and under the influence of UV illumination. In this paper, a UV flash lamp and a solid-state UV source, with its much faster turn-off time, were utilized in conjunction with a high temporal resolution testing apparatus. The UV pulse, excitation voltage, discharge current, and flashover self-luminosity were measured with high temporal precision. We relate recent experiments to our experimental findings of surface flashover under atmospheric conditions gained over the past five years. A simple model that describes the observed behavior will be presented. In addition, a more advanced Monte Carlo-type code for electron collision dynamics will be utilized to further analyze the role of UV in surface flashover under atmospheric conditions.

[PDF]

+ Impact of Volume Breakdown on Surface Flashover in High Pressure SF6
  A. Neuber, J. Krile, G. Rogers, H. Krompholz, "Impact of Volume Breakdown on Surface Flashover in High Pressure SF6,", Acta Physica Polonica 115, 995-997 (2008)
Abstract:  Not Available
+ Seed Electron Production from O- Ions under High Power Microwave Excitation
  G. F. Edmiston, A. A. Neuber, H. G. Krompholz, J. T. Krile, "Seed Electron Production from O- Ions under High Power Microwave Excitation", J. Appl. Phys. 103, 063303 (2008)
Abstract:  Surface and volume breakdown formation during pulsed high-power microwave (HPM) excitation can severely limit the power densities which can be transmitted into an atmospheric medium. Recent studies in this area have focused on developing models which accurately predict flashover formation at either dielectric/air interfaces or in the gas volume directly adjacent to these interfaces. These models are typically validated through comparison with experimentally gathered data. With respect to HPM surface flashover, experiments in the S-band at 5 MW power levels have reported on the contributing factors to flashover development including the effects of gas type, pressure, and relative humidity. A Monte Carlo-type electron motion simulation code, MC, has been developed to calculate the increasing electron density during flashover formation in this case. Results from the MC code have exhibited a quantitative agreement with experimental data over a wide range of atmospheric conditions. A critical parameter to flashover development is the stochastic process involving the appearance of initiatory or “seed” electrons, as seen by the reduction in flashover delay time by approximately 10-20% in the presence of external ultraviolet illumination. While the current version of the MC code seeds the flashover location with electron densities on the order of background ion densities produced by cosmic radiation, it fails to incorporate the field-assisted collisional detachment processes which are often assumed to be the primary origin of these electrons on the time scales of interest. Investigation of these processes and development of more accurate seeding in the MC code is a key step toward predicting HPM flashover over a wide range of parameters, particularly in the presence of highly electronegative gases such as SF6 or O2, in which there is an absence of free electrons with zero applied field.

[PDF]

Publication Year:  2007
+ Contributing Factors to Window Flashover Under Pulsed High Power Microwave Excitation at High Altitude
  G. Edmiston, A. Neuber, L. McQuage, J. Krile, H. Krompholz, J. Dickens, "Contributing Factors to Window Flashover Under Pulsed High Power Microwave Excitation at High Altitude", IEEE Transactions on Dielectrics and Electrical Insulation 14, pp. 783-789 (2007)
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, Teflonreg, and high density polyethylene as a function of background pressure and gas type, air, N2, argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.

[PDF]

+ Dielectric Surface Flashover at Atmospheric Conditions with Unipolar Pulsed Voltage Excitation
  Morales, K.; Krile, J.; Neuber, A.; Krompholz, H.; IEEE Transactions on Dielectrics and Electrical Insulation, [see also IEEE Transactions on Electrical Insulation], Volume 14, Issue 4, Aug. 2007 Page(s):774 - 782
Abstract:  Not Available

[PDF]

+ IEEE Transactions on Dielectrics and Electrical Insulation
  G. Edmiston, A. Neuber, L. McQuage, J. Krile, H. Krompholz, J. Dickens, IEEE Transactions on Dielectrics and Electrical Insulation, Volume: 14 , Issue: 4, Page(s): 783 - 789
Abstract:  Not Available
+ Interface Breakdown During High-Power Microwave Transmission
  Neuber, A. A.; Edmiston, G. F.; Krile, J. T.; Krompholz, H.; Dickens, J. C.; Kristiansen, M.; IEEE Transactions on Magnetics, Volume 43, Issue 1, Part 2, Jan. 2007 Page(s):496
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

[PDF]

+ Pulsed Dielectric Surface Flashover in an SF6 Environment
  J. T. Krile, R. Vela, A. A. Neuber, and H. G. Krompholz, "Pulsed Dielectric Surface Flashover in an SF6 Environment", IEEE Transactions on Plasma Science 35, pp. 1580-1587 (2007)
Abstract:  A recently upgraded laser-triggered gas switch at Sandia National Laboratories has developed a failure mode that results in the breakdown spark tracking to the inside of the containment envelope. These breakdowns along the surface, or surface flashovers, degrade the performance of the overall switch, causing the switch to prefire in the successive shot. In the following, experimental results of pulsed surface flashover across different dielectric materials in SF6, primarily at atmospheric pressure, as well as flashover and volume breakdown in at pressures from 1.3 to 365.4 kPa are presented. In addition to fast voltage and current monitoring of the breakdown event, an increased emphasis was put on imaging the event as well as gathering optical emission spectra (~200-700 nm) from it. As much as possible, the small-scale experiments were designed to reproduce, at least partly, the conditions as they are found in the large 5-MV switch. An effort was made to determine what changes could be made to reduce the occurrence of surface flashovers, in addition to some broadly applicable conclusions on surface flashovers in an SF6 environment.

[PDF]

+ Spectral Analysis of Pulsed Volume Breakdown in SF6 at High Pressures
  Krile, J. T.; Vela, R.; Neuber, A. A.; Krompholz, H. G.; IEEE Transactions on Plasma Science, Volume 35, Issue 4, Part 3, Aug. 2007 Page(s):1163 - 1169
Abstract:  The Z-machine, which is located at Sandia National Laboratories, is currently undergoing refurbishment to increase the output drive current. Due to increased switching voltage requirements, some switch failure modes have been identified with the laser-triggered gas switch design, including envelope surface flashover. In order to improve the performance and lifetime of these switches, a basic understanding of the underlying physics of the failure mechanisms is required. A small-scale experimental setup has been constructed to approximate conditions within the switch. The possible impact of the $hbox{SF}_{6}$ volume spark between the switch electrodes on the envelope surface flashover is investigated. Measured optical spectra of the $ hbox{SF}_{6}$ volume spark over a wide pressure range, from rough vacuum to 40 psig overpressure, are analyzed regarding their potential to contribute to switch failure

[PDF]

Publication Year:  2006
+ Conduction and breakdown mechanisms in transformer oil
  M Butcher, AA Neuber, MD Cevaolls, JC Dickens, H Krompholz, "Conduction and breakdown mechanisms in transformer oil" Plasma Science, IEEE Transactions on 34 (2), 467-475
Abstract:  Not Available
+ Conduction and breakdown mechanismsin transformer oil
  M Butcher, AA Neuber, MD Cevallos, JC Dickesn, H Krompholz, "Conduction and breakdown mechanismsin transformer oil" Plasma Science, IEEE Transactions on 34 (2), 467-475
Abstract:  Not Available
+ Conduction and breakdown mechanismsin transformer oil
  Butcher, M.; Neuber, A.A.; Cevallos, M.D.; Dickens, J.C.; Krompholz, H.; IEEE Transactions on Plasma Science, Volume 34, Issue 2, Part 3, April 2006 Page(s):467 - 475
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/spl middot/10/sup -3/ cm/sup 2//V/spl middot/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.

[PDF]

+ High-Power Microwave Surface Flashover of a Gas&# 8211; Dielectric Interface at 90&# 8211; 760 torr
  G Edminston, J Krile, A Neuber, J Dickens, H Krompholz, "High-Power Microwave Surface Flashover of a Gas&# 8211; Dielectric Interface at 90&# 8211; 760 torr" Plasma Science, IEEE Transactions on 34 (5), 1782-1788
Abstract:  Not Available
+ High-Power Microwave Surface Flashover of a Gas-Dielectric Interface at 90-760 torr
  Edmiston, G.; Krile, J.; Neuber, A.; Dickens, J.; Krompholz, H.; IEEE Transactions on Plasma Science, Volume 34, Issue 5, Part 1, Oct. 2006 Page(s):1782 - 1788
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.

[PDF]

+ Monte Carlo simulation of HPM window breakdown at atmospheric conditions
  John T. Krile, Andreas A. Neuber, Hermann G. Krompholz, and Thomas L. Gibson, Monte Carlo simulation of HPM window breakdown at atmospheric conditions. Applied Physics Letters vol. 89, 201501 (2006).
Abstract:  Not Available
+ Phenomenology of subnanosecond gas discharges at pressures below one atmosphere
  HG Krompholz, LL Hatfield, AA Neuber, KP Kohl, JE Chaparro, HY Ryu, "Phenomenology of subnanosecond gas discharges at pressures below one atmosphere" Plasma Science, IEEE Transactions on 34 (3), 927-936
Abstract:  Not Available
+ Phenomenology of subnanosecond gas discharges at pressures below one atmosphere
  Krompholz, H.G.; Hatfield, L.L.; Neuber, A.A.; Kohl, K.P.; Chaparro, J.E.; Han-Yong Ryu; IEEE Transactions on Plasma Science, Volume 34, Issue 3, Part 3, June 2006 Page(s):927 -
Abstract:  Volume breakdown and surface flashover in quasi-homogeneous applied fields in 10/sup -5/ to 600 torr argon and dry air are investigated, using voltage pulses with 150 ps risetime, <1ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of about 1mm. The arrangement on the other side of the gap is symmetrical. Diagnostics include fast capacitive voltage dividers, for determination of voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics use a scintillator-photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics include use of a streak camera to get information on the discharge channel geometry and dynamics, and temporally resolved measurements with photomultipliers. Breakdown delay times are on the order of 100-400 ps, with minima occurring in the range of several 10torr. X-ray emission extends to pressures >100 torr, indicating the role of runaway electrons during breakdown. Maximum X-ray emission coincides with shortest breakdown delay times at several 10 torr. Simple modeling using the average force equation and cross sections for momentum transfer and ionization supports the experimental results

[PDF]

+ Pulsed dielectric surface flashover in nitrogen at atmospheric conditions
  Morales, K.P.; Krile, J.T.; Neuber, A.A.; Krompholz, H.G.; IEEE Transactions on Dielectrics and Electrical Insulation, Volume 13, Issue 4, Aug. 2006 Page(s):803 - 809
Abstract:  Dielectric flashover along insulators in vacuum has been comprehensively researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of an atypical electrode geometry, humidity, and ultraviolet (UV) illumination. Previous research has shown distinct discharge behavior in air and nitrogen environments for an electrode geometry in which the applied electric field lines curve above the dielectric surface. It was concluded that the discharge development path, whether along the electric field lines or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the discharge’s production of UV radiation in an oxygen rich environment. Thus, experiments were conducted in a nitrogen environment employing UV surface illumination in order to observe the affects on the flashover spark behavior. From the experimental data, it can be ascertained that UV illumination and intensity play a significant role in the discharge development path. Based on these results an explanation of the physical mechanisms primarily involved in unipolar surface flashover will be presented. Additional experiments regarding the effects of humidity on the discharge behavior will be discussed as well.

[PDF]

+ Pulsed dielectric surface flashover in nitrogen at atmospheric conditions
  KP Morales, JT Krile, AA Neuber, HG Krompholz, "Pulsed dielectric surface flashover in nitrogen at atmospheric conditions" Dielectrics and Electical Insulation, IEEE Transactions on 13 (4), 803-809
Abstract:  Not Available
+ Similarities Of Dielectric Surface Flashover at Atmospheric Conditions for Pulsed Unipolar and RF Excitation
  J. Krile, G. Edmiston, K. Morales, A. Neuber, H. Krompholz, and M. Kristiansen, Similarities Of Dielectric Surface Flashover at Atmospheric Conditions for Pulsed Unipolar and RF Excitation, Laser Physics in Special Issue "Plasma, Beams, and Lasers" Dedicated to Professor Gennady A. Mesyats on his 70th Birth Anniversary, vol. 16, pp. 194-201, 2006.
Abstract:  Not Available
+ Similarities of dielectric surface flashover under atmospheric conditions for pulsed unipolar and RF excitation
  J Krile, G Edmiston, K Morales, A Neuber, H Krompholz, M Kristiansen, "Similarities of dielectric surface flashover under atmospheric conditions for pulsed unipolar and RF excitation" Laser Physics 16 (1), 194-201
Abstract:  Not Available
Publication Year:  2005
+ DC and pulsed dielectric suface flashover at atmospheric pressure
  JT Krile, AA Neuber, JC Dickens, HG Krompholz, "DC and pulsed dielectric suface flashover at atmospheric pressure" Plasma Science, IEEE Transactions on 33 (4), 1149-1154
Abstract:  Not Available
+ DC and Pulsed Dielectric Surface Flashover at Atmospheric Pressure
  Krile, J.T.; Neuber, A.A.; Dickens, J.C.; Krompholz, H.G.; IEEE Transactions on Plasma Science, Volume 33, Issue 4, Part 1, Aug. 2005 Page(s):1149 - 1154
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.

[PDF]

+ Imaging of dielectric surface flashover in atmospheric conditions
  Krile, J.; Neuber, A.; Dickens, J.; Krompholz, H.; IEEE Transactions on Plasma Science, Volume 33, Issue 2, Part 1, Apr 2005 Page(s):270 - 271
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.

[PDF]

+ Imaging of negative polarity dc breakdown streamer expansion in transformer oil due to variations in background pressure
  Cevallos, M.D.; Butcher, M.; Dickens, J.; Neuber, A.; Krompholz, H.; IEEE Transactions on Plasma Science, Volume 33, Issue 2, Part 1, April 2005 Page(s):494 - 495
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.

[PDF]

+ Microbubble-based model analysis of liquid breakdown initiation by a submicrosecond pulse
  J. Qian, R. P. Joshi, J. Kolb, and K. H. Schoenbach, J. Dickens, A. Neuber, M. Butcher, M. Cevallos, and H. Krompholz, E. Schamiloglu and J. Gaudet, "Microbubble-based model analysis of liquid breakdown initiation by a submicrosecond pulse," J. Appl. Phys. 97, 113304, 2005.
Abstract:  Not Available
Publication Year:  2004
+ DC flashover of a dielectric surface in atmospheric conditions
  Krile, J.T.; Neuber, A.A.; Dickens, J.C.; Krompholz, H.G.; IEEE Transactions on Plasma Science, Volume 32, Issue 5, Part 1, Oct. 2004 Page(s):1828 - 1834
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.

[PDF]

+ High Power Microwave Breakdown of Gas-Dielectric Interface at 90 to 760 Torr
  G Edmistion, A Neuber, H Krompholz, J Dickens, "High Power Microwave Breakdown of Gas-Dielectric Interface at 90 to 760 Torr" APS Meeting Abstracts 1, 1011
Abstract:  Not Available
+ Research issues in developing compact pulsed power for high peak power applications on mobile platforms
  Gaudet, J.A.; Barker, R.J.; Buchenauer, C.J.; Christodoulou, C.; Dickens, J.; Gundersen, M.A.; Joshi, R.P.; Krompholz, H.G.; Kolb, J.F.; Kuthi, A.; Laroussi, M.; Neuber, A.; Nunnally, W.; Schamiloglu, E.; Schoenbach, K.H.; Tyo, J.S.; Vidmar, R.J.; Proceedings of the IEEE on Pulsed Power, Volume 92, Issue 7, July 2004 Page(s):1144 - 1165
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

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+ Unipolar Surface Flashover
  J Krile, A Neuber, H Krompholz, J Dickens, "Unipolar Surface Flashover" APS Meeting Abstracts 1, 1013
Abstract:  Not Available
Publication Year:  2003
+ Flashover across a dielectric surface at atmospheric pressure
  A Neuber, J Krile, J Dickens, H Krompholz, "Flashover across a dielectric surface at atmospheric pressure" APS Texas Sections Fall Meeting Abstracts 1, 1001
Abstract:  Not Available
+ Phenomenology of conduction and breakdown in transformer oil
  M Butcher, M Cevallos, M Haustien, A Neuber, J Dickens, H Krompholz, "Phenomenology of conduction and breakdown in transformer oil" Electrical Insulation and Dielectric Phenomena, 2003. Annual Report
Abstract:  Not Available
Publication Year:  2002
+ Field enhanced microwave breakdown in a plasma limiter
  Mankowski, J.J.; Hemmert, D.; Neuber, A.; Krompholz, H.; IEEE Transactions on Plasma Science, Volume 30, Issue 1, Part 1, Feb. 2002 Page(s):102 - 103
Abstract:  A new type of plasma limiter is being developed which can turn on in less than 1 ns. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Images were taken of the gap region in argon at several pressures in order to investigate the role of the tip region

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+ Gas breakdown in the subnanosecond regime with voltages below 15 kV
  Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; Hemmert, D.; Short, B.; Mankowski, J.; Brown, M.D.J.; Altgilbers, L.L.; IEEE Transactions on Plasma Science, Volume 30, Issue 5, Part 1, Oct. 2002 Page(s):1916 - 1921
Abstract:  Gaseous breakdown in the subnanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics, and for plasma limiters to protect electronic devices from high power microwave radiation. Previous investigations of subnanosecond breakdown were mainly limited to high-pressure gases or liquids, with voltages in excess of 100 kV. In this paper, we investigate subnanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries in argon, with a needle radius <0.5 /spl mu/m. The coaxial setup allows current and voltage measurements with temporal resolutions down to 80 ps. Voltages of 7.5 kV (which are doubled at the open gap before breakdown) produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip and the same amplitude, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 10/sup 2/ and 10/sup 4/ Pa. At lower pressure, a longer delay time (8 ns at 6 Pa) is observed. We expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

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Publication Year:  2000
+ Dielectric/Gas Interface Breakdown Caused by High Power Microwaves
  D. Hemmert, A. Neuber, H. Krompholz, L.L Hatfield, and M. Kristiansen: Dielectric/Gas Interface Breakdown Caused by High Power Microwaves. Proceedings of the 13th International Conference on High-Power Particle Beams, June 25-30, 2000, Nagaoka, Japan, invited.
Abstract:  Not Available
+ High-power microwave window breakdown under vacuum and atmospheric conditions
  D Hemmert, AA Neuber, JC Dickens, H Krompholz, LL Hatfield, M Kristiansen, "High-power microwave window breakdown under vacuum and atmospheric conditions" Proceedings of SPIE 4031, 90
Abstract:  Not Available
+ Microwave magnetic field effects on high-power microwave window breakdown
  Hemmert, D.; Neuber, A.A.; Dickens, J.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 28, Issue 3, June 2000 Page(s):472 - 477
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 with 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 dc 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

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+ Optical diagnostics on helical flux compression generators
  Neuber, A.A.; Dickens, J.C.; Krompholz, H.; Schmidt, M.F.C.; Baird, J.; Worsey, P.N.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 28, Issue 5, Oct. 2000 Page(s):1445 - 1450
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 winding 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, X-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 he 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

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+ The role of outgassing in surface flashover under vacuum
  Neuber, A.A.; Butcher, M.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 28, Issue 5, Oct. 2000 Page(s):1593 - 1598
Abstract:  Results of high-speed electrical and optical diagnostics are used as a basis to discuss a new surface flashover model. Outgassing, caused by electron stimulated desorption, is found to play a crucial role in the temporal flashover development. Dielectric unipolar surface flashover under vacuum is experimentally characterized by a three-phase development, which covers a current range from 10-4 A to 100 A. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification reaching currents in the Ampere level within typically 100 ns. The final phase is characterized by a fast current rise up to the impedance-limited current on the order of 100 A. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. This is supported by time-resolved spectroscopy that reveals the existence of excited atomic hydrogen and ionic carbon before the final phase. The feedback mechanism toward a self-sustained discharge is due to space charge leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as outgassing rate and gas density buildup above the surface, are determined by fitting calculated results to experimental data. The significance of outgassing is also discussed with a view to microwave surface flashover

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Publication Year:  1999
+ Dielectric surface flashover in vacuum at 100 K
  A Neuber, M Butcher, LL Hatfield, M Kristiansen, H Krompholz, "Dielectric surface flashover in vacuum at 100 K" Dielectrics and Electrical Insulation, IEEE Transactions on 6 (4), 512-515
Abstract:  Not Available
+ Dielectric surface flashover in vacuum at 100 K
  Neuber, A.; Butcher, M.; Hatfield, L.L.; Kristiansen, M.; Krompholz, H.; IEEE Transactions on Dielectrics and Electrical Insulation, [see also IEEE Transactions on Electrical Insulation], Volume 6, Issue 4, Aug. 1999 Page(s):512 - 515
Abstract:  Cryogenic components in high power electrical systems and in power electronics gain more and more importance. The behavior of insulators for cryogenic conditions, however, is virtually unknown. In a fast coaxial setup, dielectric test sample and electrodes in vacuum are cooled to <100 K and flashover is characterized using fast electrical and optical diagnostics. Three consecutive development stages for flashover in self-breakdown mode with a gap distance of 0.5 cm can be distinguished: (1) a fast current rise to mA amplitudes within ~2 ns, probably associated with field emission, followed by, (2) a slow current rise to ~5 to 10 A amplitude with duration of 40 ns to 1 μs, associated with secondary emission avalanche saturation, and (3) a transition to a rapid gaseous ionization above the sample caused by electron induced outgassing, leading to impedance-limited current amplitudes of ⩽300 A. Phase (1) shows a higher final current at lower temperature, which is probably due to a higher initial velocity of the secondary electrons, the duration of phase (2) is a decreasing function of breakdown voltage and only slightly dependent on temperature, which points to a weak temperature dependence of the outgassing process. Flashover potentials show a slight increase at lower temperature

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+ Electric current in dc surface flashover in vacuum
  A Neuber, M Butcher, LL Hatfield, H Krompholz, "Electric current in dc surface flashover in vacuum" Journal of applied physics 85, 3084
Abstract:  Not Available
+ Electric Current in DC Surface Flashover in Vacuum
  A. Neuber, M. Butcher, L. L. Hatfield, and H. Krompholz: Electric Current in DC Surface Flashover in Vacuum. J. Appl. Phys., vol. 15, 3084-3091 (1999)
Abstract:  Not Available
+ Imaging of high-power microwave-induced surface flashover
  Neuber, A.; Hemmert, D.; Dickens, J.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 27, Issue 1, Feb. 1999 Page(s):138 - 139
Abstract:  Using two gated intensified digital charge-coupled device cameras, one sensitive in the near infrared to ultraviolet region and one in the soft X-ray region, the temporal development of high-power microwave-induced surface flashover across a vacuum/dielectric interface has been imaged. The emission of X-ray radiation from the interface is caused by field emitted electrons accelerated in the high electromagnetic field impacting the solid. This generation of bremsstrahlung terminates at the moment of full flashover development that is indicated by the optical light emission. A rising plasma density above the dielectric surface due to electron induced outgassing triggers this behavior

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+ Initiation of High Power Microwave Dielectric Interface Breakdown
  A. Neuber, D. Hemmert, H. Krompholz, L. Hatfield, and M. Kristiansen: Initiation of High Power Microwave Dielectric Interface Breakdown. J. Appl. Phys., vol. 86, 1724-1728 (1999).
Abstract:  Not Available
Publication Year:  1998
+ Window breakdown caused by high-power microwaves
  Neuber, A.; Dickens, J.; Hemmert, D.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 26, Issue 3, June 1998 Page(s):296 -
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-band 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 0.21-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 DC flashover across insulators, and similar methods to increase the flashover field are expected to be applicable

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Publication Year:  1997
+ Dielectric Surface Flashover in a Simulated Low Earth Orbit Environment
  F. Hegeler, H. Krompholz, L.L. Hatfield, M. Kristiansen, “Dielectric Surface Flashover in a Simulated Low Earth Orbit Environment”, IEEE Transactions on Plasma Science, 25 (1997) 300
Abstract:  Not Available
Publication Year:  1994
+ Plasma Development in the Early Phase of Dielectric Surface Flashover
  G. Masten, T. Mueller, F. Hegeler, H. Krompholz, "Plasma Development in the Early Phase of Dielectric Surface Flashover", IEEE Transactions on Plasma Science, 22 (1994) 1034
Abstract:  Not Available
Publication Year:  1993
+ Current, Luminosity, and X-ray Emission in the Early Phase of Dielectric Surface Flashover in Vacuum
  F. Hegeler, G. Masten, H. Krompholz, and L.L Hatfield, "Current, Luminosity, and X-ray Emission in the Early Phase of Dielectric Surface Flashover in Vacuum", IEEE Transactions on Plasma Science, 21 (1993) 223
Abstract:  Not Available
Publication Year:  1992
+ Magnetic field effects on vacuum insulator flashover
  M. Lehr, R. Korzekwa, H. Krompholz, M. Kristiansen, "Magnetic field effects on vacuum insulator flashover", J. Appl. Phys. 71 (1992) 389
Abstract:  Not Available
Publication Year:  1991
+ Expansion of Hydrogen Arcs Driven by Oscillating Currents
  T.G. Engel, M. Kristiansen, H. Krompholz, "Expansion of Hydrogen Arcs Driven by Oscillating Currents", IEEE Transactions on Plasma Science 19 (1991) 959
Abstract:  Not Available
+ The Design and Calibration of a Very Fast Current Probe for Short Pulse Measurements
  S. Calico, M. Crawford, M. Kristiansen, H. Krompholz, "The Design and Calibration of a Very Fast Current Probe for Short Pulse Measurements", Rev. Sci. Instrum 62 (1991) 1511
Abstract:  Not Available
Publication Year:  1990
+ The Influence of Magnetic Fields on Dielectric Surface Flashover
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "The Influence of Magnetic Fields on Dielectric Surface Flashover", IEEE Transactions on Electron Devices, 38 (1990) 745
Abstract:  Not Available
Publication Year:  1989
+ Inhibiting Surface Flashover for Space Conditions Using Magnetic Fields
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "Inhibiting Surface Flashover for Space Conditions Using Magnetic Fields", IEEE Trans. on Plasma Science, 17 (1989) 612
Abstract:  Not Available
+ Methods of Increasing the Surface Flashover Potential in Vacuum
  L.L. Hatfield, E.R. Boerwinkle, G. Leiker, H. Krompholz, R. Korzekwa, M. Lehr, M. Kristiansen, "Methods of Increasing the Surface Flashover Potential in Vacuum", IEEE Trans. on Electrical Insulation, 24 (1989) 985
Abstract:  Not Available
Publication Year:  1988
+ Current-Voltage Characteristics of a High Current Pulsed Discharge in Air
  H. Akiyama, M. Kristiansen, H. Krompholz, B. Maas, "Current-Voltage Characteristics of a High Current Pulsed Discharge in Air", IEEE Trans. on Plasma Science 16 (1988) 312
Abstract:  Not Available
Publication Year:  1985
+ A New Design Concept for Field Distortion Trigger Spark Gaps
  . Schaefer, B. Pashaie, P. Williams, K. Schoenbach, H. Krompholz, "A New Design Concept for Field Distortion Trigger Spark Gaps", J. Appl. Phys. 57 (1985) 2507
Abstract:  Not Available
+ An Electron Beam Controlled Diffuse Discharge Switch
  K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, H. Harjes, D. Skaggs, "An Electron Beam Controlled Diffuse Discharge Switch", J. Appl. Phys. 57 (1985) 1618
Abstract:  Not Available
Publication Year:  1984
+ Electron Beam Tetrode for Multiple, Submicrosecond Pulse Operation
  H. Harjes, K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, D. Skaggs, "Electron Beam Tetrode for Multiple, Submicrosecond Pulse Operation", Rev. Sci. Instrum. 55 (1984) 1684
Abstract:  Not Available
+ Investigations of E-Beam Controlled Diffuse Discharges
  K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, H. Harjes, D. Skaggs, "Investigations of E-Beam Controlled Diffuse Discharges" in "Gaseous Dielectrics IV", ed. L. G. Christophorou, Pergamon Press 1984, p. 246
Abstract:  Not Available
+ Nanosecond Current Probe for High Voltage Experiments
  H. Krompholz, J. Doggett, K. Schoenbach, J. Gahl, C. Harjes, G. Schaefer, M. Kristiansen, "Nanosecond Current Probe for High Voltage Experiments", Rev. Sci. Instrum. 55 (1984) 127
Abstract:  Not Available
+ Pulsed Hollow-Cathode Discharge with Nanosecond Risetime
  G. Schaefer, P. Husoy, K. Schoenbach, H. Krompholz, "Pulsed Hollow-Cathode Discharge with Nanosecond Risetime", IEEE Trans. Plasma Sci., vol. PS-12 (1984) 271
Abstract:  Not Available
+ The Use of Attachers in Electron-Beam Sustained Discharge Switches - Theoretical Considerations
  G. Schaefer, K. Schoenbach, H. Krompholz, M. Kristiansen, A.H. Guenther, "The Use of Attachers in Electron-Beam Sustained Discharge Switches - Theoretical Considerations", Laser and Particle Beams 2 (1984) 273
Abstract:  Not Available

Conference Paper/Presentation

Publication Year:  2011
+ Delay Time Distribution of High Power Microwave Surface Flashover
  J. Foster, H. Krompholz, A. Neuber, "Delay Time Distribution of High Power Microwave Surface Flashover," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  Breakdown phenomena in a high power microwave (HPM) system present unique obstacles for the further development of HPM technology. The non-uniformity of a high frequency electric field and the statistics associated with breakdown in general along with the stochastic nature of naturally occurring electron generating mechanisms introduce significant challenges for predicting and preventing breakdown occurrences within a HPM system. An experiment consisting of an S-band multi-megawatt HPM pulse is used for observing an alternating field induced plasma sheath across a dielectric surface. In order to minimize experimental deviations, a continuous UV lamp is used to provide a constant source of initiatory electrons through the process of photoemission. This reduces the waiting time for flashover initiating electrons to appear, however, primarily due to avalanche statistics, variations are still observed. A statistical model that uses an exponential distribution sampling procedure was developed to predict the surface flashover delay times for a variety of conditions. A supporting experiment that uses a continuous UV lamp and a DC electric field is used for measuring low current due to photoemission from the dielectric window. An explanation of the model describing these phenomena is presented along with a comparison of current measurements from the supporting experiment.

[PDF]

Publication Year:  2010
+ An Investigation of Pulsed High Power Microwave Surface Flashover Initiation in Atmospheric Conditions
  J. Foster, M. Thomas, J. Krile, H. Krompholz, A. Neuber, "An Investigation of Pulsed High Power Microwave Surface Flashover Initiation in Atmospheric Conditions," 37th IEEE International Conference on Plasma Science, June 20th - 24th, Norfolk, VA, (2010).
Abstract:  The production of high power microwaves (HPM) in a vacuum environment for the purpose of radiating into atmosphere requires the use of a dielectric interface to separate the vacuum and atmospheric sides of the radiating structure. For high power to pressure ratios the interface will exhibit surface flashover on the atmospheric side, thus limiting the transmission of microwave power. An experimental setup that utilizes a magnetron operating at 2.85 GHz to produce a 4.5 MW, 3μs pulse propagating in the TE10 mode along with an atmospheric test chamber enables investigating HPM surface flashover phenomena in the presence of various atmospheric conditions. One of the principle parameters measured is the delay time between application of the microwave pulse (50 ns rise time) and the sharp drop in transmitted power due to the flashover plasma formation. Several methods of delay time reduction have been employed to gain a better understanding of the source of breakdown initiatory electrons. For an environment composed of air at, for instance, 155 torr a delay time of 600 ns is observed. Illuminating the dielectric surface with continuous UV radiation reduces the average delay to about 380 ns. An even more distinct reduction in delay time was observed when electric field enhancement was introduced to the window surface via vapor deposition of sub-mm metallic points on the dielectric. These metallic points have proven to reduce the delay time to ~150 ns while increasing the global effective electric field by a factor of ~1.5. This presentation will include an overview of a variety of methods for investigating flashover initiation, including UV radiation and the application of an external DC electric field, as well as the introduction of field enhancing metallic points on the dielectric surface. An analysis of flashover behavior at atmospheric pressures (60-155 torr) in air, argon, and nitrogen will also be given along with an estimation of field enhancement- - factors for various geometries.

[PDF]

+ Compact Magnetically Insulated Transmission Line Oscillator
  Vasiliy Smirnov, Magne Kristiansen, John Mankowski, James Dickens, Andreas Neuber, Lynn Hatfield, Hermann Krompholz, John Walter, "Compact Magnetically Insulated Transmission Line Oscillator", ICOPS 2010, The 37th IEEE International Conference on Plasma Science, June 20 - 24, 2010, Norfolk, Virginia, USA, Page 251.
Abstract:  Not Available
+ High Power Microwave Surface Flashover Seed Electron Production Methods
  M. Thomas, J. Foster, H. Krompholz, A. Neuber, "High Power Microwave Surface Flashover Seed Electron Production Methods," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Surface flashover imposes a fundamental limitation to the magnitude of high power microwaves which can be radiated from the vacuum environment of the source into atmospheric conditions. Providing seed electrons through various methods allows for initiatory conditions to be more closely controlled and the delay time variations to be reduced so that developmental mechanisms can be more closely examined. The experiment uses a coaxial magnetron capable of producing a ~4.5 MW, 3 μs pulse, at 2.85 GHz propagating in the TE10 mode. The pulse rise time measured at the window is reduced using a spark gap pulse steepening technique. The fast rise time pulse propagates through the dielectric into an atmospheric test chamber where various conditions such as gas pressure, type of gas, UV illumination, and charged particle creation by radioactive sources can be controlled. Previous research has shown the significant impacts of UV radiation on the delay time averages and statistical distributions. Surface distributed seeding sources and volume distributed sources will be discussed while the primary focus of this paper will address use of alpha radiation as an ionizing agent. Thus far, a reduction in average delay time by as much as 60% has been achieved at sub-microsecond time scales, which also significantly affected the width of the statistical distributions of the delay time. Alpha particles have a short penetration distance in air which makes them a good candidate for study since the number of electron-ion pars created along the path is large. Analysis of the alpha particles influences will be discussed along with a statistical analysis of breakdown delay in the presence of ionization.

[PDF]

+ High Power Microwave Surface Flashover Using a Monte Carlo Code
  J. Krile, J. Foster, M. Thomas, H. Krompholz, A. Neuber, "High Power Microwave Surface Flashover Using a Monte Carlo Code," AFOSR Counter High Power Microwaves Meeting, July 29th - 30th, Albuquerque, NM, (2010).
Abstract:  Not Available
+ Influence of Seed Electrons on High Power Microwave Window Flashover
  J. Foster, S. Beeson, M. Thomas, J. Krile, H. Krompholz, A. Neuber, "Influence of Seed Electrons on High Power Microwave Window Flashover," AFOSR Counter High Power Microwaves Meeting, July 29th - 30th, Albuquerque, NM, (2010).
Abstract:  Not Available
+ Microwave Surface Flashover Using Metallic Initiators
  J. Foster, M. Thomas, H. Krompholz, A. Neuber, "Microwave Surface Flashover Using Metallic Initiators," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Not Available
+ Pulsed Atmospheric Breakdown
  G. Laity, H. Krompholz, A. Neuber, L. Hatfield, K. Frank, A. Fierro, J. Dickens, M. Kristiansen, "Pulsed Atmospheric Breakdown," AFOSR Counter High Power Microwaves Meeting, July 29th - 30th, Albuquerque, NM, (2010).
Abstract:  Not Available
+ Pulsed Breakdown and Flashover at Atmospheric Pressure
  A. Neuber, H. Krompholz, J. Dickens, J. Krile, J. Foster, S. Beeson, M. Thomas, G. Laity, A. Fierro, "Pulsed Breakdown and Flashover at Atmospheric Pressure," AFOSR Counter High Power Microwaves Meeting, July 29th - 30th, Albuquerque, NM, (2010).
Abstract:  Not Available
Publication Year:  2009
+ Impact of DC Electric Fields on HPM Induced Surface Flashover
  M. Thomas, J. Foster, H. Krompholz, A. Neuber, "Impact of DC Electric Fields on HPM Induced Surface Flashover," 36th IEEE International Conference on Plasma Science, May 31st - June 5th, San Diego, CA, (2009)
Abstract:  Summary form only given. The introduction of a high voltage DC electrode into an experimental setup used for observing high power microwave surface flashover has shown to significantly vary the total delay time for this type of breakdown. The experiment utilizes an S-band magnetron operating at 2.85 GHz to produce a 4 MW, 3 mus pulse. A plasma switch mounted in a WR-284 waveguide reduces the 10-90% rise time of the pulse to ~50 ns, and it reflects the pulse towards a dielectric window to induce surface flashover. A wire electrode charged to plusmn20 kV is inserted into the center of the dielectric window and oriented perpendicularly to the major electric field component of the TE10 microwave mode. The DC field from the electrode influences charge carriers in the flashover region, forcing potential breakdown initiating charged particles away from or towards the surface, dependent on polarity and particle charge sign. Initial tests were conducted in pure N2 at a pressure of 125 torr. The low probability of negative ions (stable negative N2 ions do not exist) appearing in the volume simplifies the interpretation of the experimental results by allowing for the existence of primarily electrons and positively charged ions. A significant increase (~50%) in the average total delay time for the case of a positively charged electrode has been observed. An increase in the average statistical delay was also observed as well as a decrease in the presumed formative delay for both voltage polarities. The apparent electron production rate in N2 was estimated to be 2 e/mus and 8 e/mus under HPM pulse application in the case of positive and negative DC voltages, respectively. Results of further tests conducted in Argon and Krypton-85 in Argon balance are presented along with a statistical analysis of measured delay times.

[PDF]

+ The Influence of a DC Electric Field on High Power Microwave Window Flashover in Air and N2 Environments
  J. Foster, M. Thomas, H. Krompholz, A. Neuber, "The Influence of a DC Electric Field on High Power Microwave Window Flashover in Air and N2 Environments," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 480-483, Washington, DC, June 2009.
Abstract:  Observed delay times for high power microwave surface flashover are influenced significantly by the presence of a DC electric field. The experimental setup to investigate theses influences is comprised of an S-band magnetron operating at 2.85 GHz with a pulse rise time shortening switch assembly that produces a 50 ns rise time at a ~ 2.5 MW power level. A wire electrode charged to ±20 kV is inserted into the dielectric interface perpendicular to the electric field of the TE10 mode to provide a DC electric field in the flashover region. Tests have been conducted in pure N2 at 125 torr in order to provide an environment composed of primarily electrons and positive ions. The average measured delay of window flashover with a DC field pointing into the dielectric has been observed to increase by ~50%. Additionally, effective emission rates of seed electrons initiating breakdown have shown a decrease from 14 e/¿s to 2 e/¿s, indicating the removal of charged species from the high microwave field region due to charge drift in the applied DC field. An overview of the experimental setup is given along with a statistical analysis of delay times measured in Air as well as N2. The open question of where seed electrons originate from and the quantification of the primary processes involved will be addressed.

[PDF]

+ Use of Radiation Sources to Provide Seed Electrons in High Power Microwave Surface Flashover
  M. Thomas, J. Foster, H. Krompholz, A. Neuber, "Use of Radiation Sources to Provide Seed Electrons in High Power Microwave Surface Flashover," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 124-128, Washington, DC, June 2009.
Abstract:  Delay times of high power microwave surface flashover are affected by radiation illuminating the dielectric. A controlled environment of pure Argon at a range of low pressures as compared to normal atmospheric pressure was used with 2 mW/cm2 UV-radiation illuminating the test window. Argon was chosen due to its relatively small number of processes involved such as inelastic electron collisions and due to the well-known cross-sections for these processes. Delay times in the presence of UV are significantly shorter than without UV illumination. The initial electron density contribution due the UV source is very roughly estimated to be ~106 cm-3. A small admixture of radioactive krypton-85 showed only marginal changes in the observed delay times, likely due to an insufficient concentration of Kr-85 producing ionization events only every few microseconds and the high energy distribution associated with the emitted electrons. A detailed discussion of experimental breakdown delay data, along with theoretical expectations and discussion of the statistically dependent mechanisms and analysis, will be given. The ultimate goal is to develop a model for HPM window breakdown in a UV environment, to describe the role of discharge initiating electrons, and to quantify breakdown at high altitudes.

[PDF]

Publication Year:  2008
+ Particle Simulation of Ultrafast Closing Switch at Sub-Atmospheric Pressures
  Jordan Chaparro, Hermann Krompholz, Andreas Neuber, Lynn Hatfield, Particle Simulation of Ultrafast Closing Switch at Sub-Atmospheric Pressures, IEEE International Power Modulator Conference, Las Vegas, NV, May 27-31, 2008
Abstract:  Previous research at Texas Tech University has been conducted on the physics governing highly over-voltaged gas breakdown resulting from ultrafast applied voltage pulses with risetimes less than 200 ps and durations less than 400 ps. Experimental results have shown that the breakdown characteristics of such events significantly differ from those observed in standard gas breakdown and a complete understanding of the physics behind ultrafast discharges is far from being clear. As a companion to experimental work, a numerical model is an attractive means of discerning more about the underlying physics behind such events. In this paper, a relativistic, Particle in cell model utilizing Monte-Carlo calculations is discussed as a way to directly simulate the experimental conditions, with similar geometry, background gas, and pulse characteristics. Diagnostic output from the simulation includes space-charge development over time, field and particle energy distributions, and particle number growth rates and spatial distributions. An overview of the structure and formulation behind the simulation code is given followed by a comparison of output data to experimental results. Specific points of interest for comparison include formative and statistical delay times, examination of inhomogeneous ionization regions in the discharge, and the behavior of high-energy particles in the runaway state.

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+ Short Rise Time High Power Microwave Induced Surface Flashover At Atmospheric Pressures
  J. Foster, G. Edmiston, J. Krile, A. Neuber, and H. Krompholz, Short Rise Time High Power Microwave Induced Surface Flashover At Atmospheric Pressures, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  High power microwave transmission is ultimately limited by window flashover at the vacuum-air dielectric boundary. While surface flashover in the presence of a vacuum has been studied in some detail, the mechanisms associated with flashover in an atmospheric environment need further investigation. For an aircraft based high power microwave system, atmospheric pressures ranging from 760 torr (sea level) to 90 torr (50,000 ft.) are of principal concern. The experimental setup uses a 2.85 GHz, 3 mus microwave pulse with a 10 to 90% rise time of approximately 600 ns from a magnetron capable of producing 5 MW. The slow rise time of the microwave pulse is sharply reduced by using a waveguide spark gap switch used for fast microwave reflection and a high power four port circulator [6]. This reflected pulse has a reduced rise time on the order of 50 ns. The shorter rise time produces a more ideal step waveform that can be more easily compared with theoretical perfect square pulse excitation. Past investigations showed that the delay time for breakdown in air increases with pressure as is expected from the right hand side of the Paschen curve as long as the electron collision frequency is much larger than the microwave frequency. Surface flashover experiments have produced similar results. At a pressure of 155 torr, for instance, the breakdown electric field strength is 6 kV/cm (power density 0.08 MW/cm2) and the overall delay time from HPM pulse application to reaching critical breakdown plasma density is 600 ns. An overview of the experimental setup is given along with a discussion of breakdown delay times as a function of pressure as well as an investigation of surface flashover in the presence of external UV (ultraviolet) illumination.

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+ Statistical and Formative Delay Times for Sub-Nanosecond Breakdown at Sub-Atmospheric Pressure
  Jordan Chaparro, Lynn Hatfield, Hermann Krompholz, Andreas Neuber, Statistical and Formative Delay Times for Sub-Nanosecond Breakdown at Sub-Atmospheric Pressure, IEEE International Power Modulator Conference, Las Vegas, NV, May 27-31, 2008
Abstract:  For subnanosecond switching, physical phenomena as well as basic breakdown data, such as delay times and breakdown voltages, are of interest. With a RADAN Pulser as source for voltage pulses with up to 180 kV amplitude and risetimes at a test gap of 180 ps, we investigate statistical and formative delays for argon and dry air at sub-atmospheric pressure, for gap widths of 1 and 11 mm. Formative times have minima between 50 and 200 torr, and range from 70 ps at 1.5 MV/cm to 200 ps at 50 kV/cm. For this range of electric fields, this dependence on pressure and applied field can be explained by the behavior of ionization coefficient and electron drift velocity for homogeneous discharges. For higher fields exhibiting a narrow ionization zone in cathode vicinity with pronounced electron runaway conditions, the experimental data agree with results of Monte-Carlo simulations. Statistical delays are about the same as formative delays at fields of 50 kV/cm, and are reduced with increasing field amplitude to less than 50 ps at 1.5 MV/cm. It appears that field emission is the major source for starting electrons, influencing the statistical delay time near the field emission threshold only.

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+ Statistics of High Power Microwave Induced Window Flashover
  A. Neuber, G. Edmiston, J. Krile, J. Foster, H. Krompholz, Statistics of High Power Microwave Induced Window Flashover, presented at the 35th IEEE International Conference on Plasma Science, June 15 - 19, 2008, Karlsruhe, Germany
Abstract:  ummary form only given as follows. For flashover in air, nitrogen, and SF6, we have previously shown that the delay time between microwave pulse application and breakdown increases with pressure in the regime where the elastic electron collision frequency is larger than the microwave frequency (roughly >10 torr for 2.85 GHz microwave frequency), which also coincides with conditions found at the right hand side of the Paschen curve. The specific window flashover geometry was carefully chosen to avoid local field enhancement. That is, no metallic parts are exposed to high fields. Hence, only the window surface itself, the gas, and interaction processes between surface and volume contribute to flashover. A Monte Carlo based electron motion code developed for the flashover conditions predicts formative flashover delay times reasonably well in the pressure regime between 100 to 600 torr (10,000 Pa to 80,000 Pa). However, the statistical delay time, that is the time interval required for the initiatory electron(s) to appear, is unaccounted for in the code. Further computational efforts investigating seed electron production via collisional detachment from, for instance, negative ions in the gas have shown that while effective at unipolar fields, collisional detachment is unlikely to contribute to the production of seed electrons at higher microwave frequencies above several GHz. Experiments show that illuminating the surface with light/photons (180 nm < lambda < 350 nm) reduces the observed statistical delay considerably indicating the importance of seed electron production from the surface. This paper will discuss the key processes of high power microwave surface flashover and present experimental flashover data along with continued investigation into the statistics of possible seed electron sources, including trace contaminates present in the gas or on the dielectric surface.

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+ Window Flashover Initiation Under Pulsed Microwave Excitation
  J. Krile, G. Edmiston, J. Dickens, H. Krompholz, and A. Neuber, Window Flashover Initiation Under Pulsed Microwave Excitation, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
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 (Gt ~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.

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Publication Year:  2007
+ Dielectric Surface Flashover at Atmospheric Conditions under High Power Microwave Excitation
  A. Neuber, J. T. Krile, G. F. Edmiston, H. G. Krompholz, "Dielectric Surface Flashover at Atmospheric Conditions under High Power Microwave Excitation", Phys. Plasmas 14, 057102 (2007) (invited).
Abstract:  Not Available
+ Effects of UV Illumination on Surface Flashover under Pulsed Unipolar Excitation
  J. T. Krile, A. A. Neuber, H. G. Krompholz, "Effects of UV Illumination on Surface Flashover under Pulsed Unipolar Excitation," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007 (invited).
Abstract:  Not Available
+ Experimental investigation of subnanosecond gas breakdown in the E/p range 10^3 to 10^5 V/cm torr
  H. Krompholz, L. Hatfield, A. Neuber, P, J. Chaparro, and W. Justis, “Experimental investigation of subnanosecond gas breakdown in the E/p range 103 to 105 V/cm torr,” in Proceedings of the 28th ICPIG, July 15-20, 2007, Prague, Czech Republic, pp. 1014-1017.
Abstract:  Not Available
+ Initiation of HPM Surface Flashover
  G. F. Edmiston, A. A. Neuber, J. T. Krile, L. M. McQuage, H. Krompholz, "Initiation of HPM Surface Flashover," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
Abstract:  Not Available
+ Pulsed Volume and Surface Discharges in an SF6 Environment
  R. Vela, J. T. Krile, A. A. Neuber, H. G. Krompholz, "Pulsed Volume and Surface Discharges in an SF6 Environment," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
Abstract:  Not Available
+ Scaling Laws for Sub-Nanosecond Breakdown in Gases with Pressures below One Atmosphere
  W. H. Justis, J. E. Chaparro, H. G. Krompholz, L. L. Hatfield, A. A. Neuber, "Scaling Laws for Sub-Nanosecond Breakdown in Gases with Pressures below One Atmosphere," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
Abstract:  Not Available
+ Seed Electron Model for Monte Carlo HPM Breakdown Model
  A. Neuber, G. Edmiston1, H. Krompholz, “Seed Electron Model for Monte Carlo HPM Breakdown Model,” in Proceedings of the 28th ICPIG, July 15-20, 2007, Prague, Czech Republic, pp. 1042-1045.
Abstract:  Not Available
+ X-Ray Emission from Sub-Nanosecond Gas Breakdown
  J. E. Chaparro, W. H. Justis, H. G. Krompholz, L. L. Hatfield, A. A. Neuber, "X-Ray Emission from Sub-Nanosecond Gas Breakdown," Proceedings of the 2007 IEEE Pulsed Power and Plasma Science Conference, pp. 1652-1655, Albuquerque, NM June 17-22, 2007.
Abstract:  Not Available
Publication Year:  2006
+ Contributing Factors to Window Flashover Under Pulsed High Power Microwave Excitation at High Altitude
  Edmiston, G.; Neuber, A.; Krile, J.; McQuage, L.; Krompholz, H.; Dickens, J.; 2006 Power Modulator Symposium, Conference Record of the 2006 Twenty-Seventh International May 2006 Page(s):389 - 392
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 technical 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 Torr to 90 Torr) 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.

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+ Dielectric Surface Flashover Research at Texas Tech University
  A. Neuber, H. Krompholz, J. Dickens, M. Kristiansen, “Dielectric Surface Flashover Research at Texas Tech University,” presented at the 1st Euro-Asian Pulsed Power Conference, Sept. 18 -22, 2006, Chengdu, China.
Abstract:  Not Available
+ Interface Breakdown During High Power Microwave Transmission
  A. Neuber, J. Krile, G. Edmiston, H. Krompholz, J. Dickens, M. Kristiansen ,“Interface Breakdown During High Power Microwave Transmission,” presented at the 13th EML Symposium, May 22-25, 2006, Berlin, Germany.
Abstract:  Not Available
+ Pulsed Unipolar Surface Flashover at Atmospheric Conditions
  Morales, K.; Krile, J.; Neuber, A.; Krompholz, H.; Dickens, J.; Power Modulator Symposium, 2006. Conference Record of the 2006 Twenty-Seventh International 14-18 May 2006 Page(s):174 - 180
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 arc 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 ~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

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+ Surface Flashover under RF and Unipolar Excitation at Atmospheric Conditions
  Krile, J.T.; Neuber, A.A.; Edmiston, G.F.; Krompholz, H.G.; Conference Record of the 2006 Twenty-Seventh International Power Modulator Symposium, 2006. 14-18 May 2006 Page(s):7 - 12
Abstract:  Flashover along insulators or insulating support structures has to be carefully addressed in the design of any DC, AC, or pulsed high voltage device. Although there is a large body of data on unipolar surface flashover in the atmosphere, which has led to empirical design rules primarily for the power distribution industry, the physics of the involved processes is widely unknown. The major limiting factor in the transmission of high power microwaves (HPM) into the atmosphere has been the vacuum-air interface. Both the unipolar and HPM surface flashover cases have been studied under vacuum conditions and have been found to have the same dominant mechanisms. Similarities between HPM window flashover on the air side and unipolar flashover are observed in an atmospheric environment as well

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+ Ultrafast gas breakdown at pressures below one atmosphere
  Krompholz, H.; Hatfield, L.; Neuber, A.; Chaparro, J.; Ryu, H.-Y.; Justis, W.; 2006 IEEE Conference on Electrical Insulation and Dielectric Phenomena, 15-18 Oct. 2006 Page(s):569 - 572
Abstract:  Gas breakdown in quasi homogeneous electric fields with amplitudes of up to 3 MV/cm is investigated. The setup consists of a RADAN 303 A pulser and pulse sheer SN 4, an impedance-matched oil-filled coaxial line with a lens-transition to a biconical line in vacuum or gas, and an axial or radial gap with a width on the order of mm, with a symmetrical arrangement on the other side of the gap. Capacitive voltage dividers allow to determine voltage across as well as conduction current through the gap, with a temporal resolution determined by the oscilloscope sampling rate of 20 GS/s and an analog bandwidth of 6 GHz. The gap capacitance charging time and voltage risetime across the gap is less than 250 ps. Previous experiments at TTU with a slightly larger risetime have shown that breakdown is governed by runaway electrons, with multi-channel formation and high ionization and light emission in a thin cathode layer only. In argon and air, time constants for the discharge development have been observed to have a minimum of around 100 ps at several 10 torr. A qualitative understanding of the observed phenomena and their dependence on gas pressure is based on explosive field emission and gaseous ionization for electron runaway conditions.

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Publication Year:  2005
+ Bubble Dynamics and Channel Formation for Cathode Initiated Discharges in Transformer Oil
  Cevallos, M.; Butcher, M.; Dickens, J.; Neuber, A.; Krompholz, H.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):1235 - 1238
Abstract:  The development of cathode initiated low density channel formation and propagation leading to self breakdown in transformer oil is investigated using high speed electrical and optical diagnostics in a coaxial system with a point/plane axial discharge at various hydrostatic pressures. A cathode initiated channel formation and propagation model based upon single bubble dynamics has been presented by Kattan [1]. Experiments based on high-speed shadowgraphy were conducted to decide if the principles that govern single bubble dynamics could be applied to cathode initiated channel formation. These experiments show bubble motion away from the cathode, with separation velocities on the order of 10's m/s. This separation is similar for single bubbles generated at the cathode and for bubble chains developing into low-density channels. Lifetimes of these channels are recorded and show good correlation with the Rayleigh model [2] used to predict lifetimes of single bubbles. Experiments at reduced hydrostatic pressure reveal a critical pressure below which low density channel expansion occurs, further corroborating the presence of a gas phase. Finally, the pressure dependence of the breakdown voltage due to the expansion of the low density channels is examined and a model for this dependence is presented. The experiments conducted confirm the presence of a gas phase channel, its correlation with single bubble dynamics, and its importance to final breakdown.

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+ Composite Shadowgraphy and Luminosity Images of Self Breakdown Discharge Channels in Transformer Oil
  Cevallos, M.; Butcher, M.; Dickens, J.; Neuber, A.; Krompholz, H.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):982 - 985
Abstract:  The physics of cathode initiated discharge formation leading to self breakdown in transformer oil is investigated using high speed electrical and optical diagnostics in a coaxial system with a point/plane axial discharge. Previous research conducted on self breakdown channel formation using high speed shadowgraphy and photography of the emitted light has shown tree-like structures for both cathode and anode initiated discharges, with characteristic differences. Cathode initiated discharges expand faster to a more "bushy" appearance, whereas anode initiated discharges show branching localized channels. So far, the spatial resolution to detect small luminous areas in pre-breakdown discharges and to determine their correlation to low density regions visible in the shadowgraphs was not sufficient in the experiments described in this paper, thus a systematic variation of exposure times, and time delays between luminosity pictures and shadowgraphy pictures has been performed. These experiments confirm that the luminosity emitted during prebreakdown events is generated from the low density regions seen in the shadowgraphy images, indicating charge amplification mechanisms in the gas phase for cathode initiated events. This statement is further supported by the dependence of both the channel dynamics and the light emission at lowered hydrostatic pressure.

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+ High Power Microwave Breakdown Limits of Dielectric/Gas Interfaces
  A. Neuber, G. Edmiston, J. Krile, K. Morales, J. Dickens, H. Krompholz, "High Power Microwave Breakdown Limits of Dielectric/Gas Interfaces,” presented at the 2005 International COE Forum on Pulsed Power Science held on November 13 – 14 at Kumamoto, Japan. (invited)
Abstract:  Not Available
+ High Power Microwave Surface Flashover of a Gas-Dielectric Interface at 90 to 760 Tor
  Edmiston, G.; Krile, J.; Neuber, A.; Dickens, J.; Krompholz, H.; 2005 IEEEPulsed Power Conference, June 2005 Page(s):350 - 353
Abstract:  The major limiting factor in the transmission of 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 [1]. 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. We will present experimental results, including the impact of gas pressure, and discuss possible causes for the difference in the rf-breakdown field with and without the interface/metallic triple point portion.

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+ Investigation of Charge Conduction and Self-Breakdown in Transformer Oil
  Butcher, M.; Cevallos, M.; Neuber, A.; Krompholz, H.; Dickens, J.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):1143 - 1146
Abstract:  With a fast coaxial setup using a needle/plane geometry and a high sensitivity electrometer, conduction mechanisms in transformer oil at varying temperature and hydrostatic pressure are quantified. There are 3 stages in the conduction process prior to breakdown for highly nonuniform field geometries. Stage I is characterized by a resistive current at low fields. Stage II consists of a rapid rise in the injection current associated with increasing field due to a "tunneling" mechanism through the metal/dielectric interface. The transition from the resistive to tunneling stage occurs when the applied field reduces the barrier at the metal/insulator interface to a point where tunneling of charge carriers through the barrier begins. This transition point is polarity dependent. In stage III, at high fields the current reaches space charge saturation at electron mobilities >100 cm2/V*s prior to breakdown. The processes of final breakdown show distinct polarity dependence. Data for the negative needle exhibits strong pressure dependence of the breakdown voltage, which is reduced by 50% if the hydrostatic pressure is lowered from atmospheric pressure to hundreds of mtorr. Such a strong pressure dependence, at reduced hydrostatic pressure, indicates breakdown is gaseous in nature. This is supported by images of bubble/low density regions forming at the current injection point. Positive needle discharges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. A 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.

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+ Limits of High Power Microwave Transmission due to Interface Breakdown
  A. Neuber, J. Krile, G. Edmiston, H. Krompholz, J. Dickens, "Limits of High Power Microwave Transmission due to Interface Breakdown," presented (oral) at 2005 Tri-Service VED Workshop, 12-16 September 2005
Abstract:  Not Available
+ Pulsed Dielectric Surface Flashover at Atmospheric Conditions
  Morales, K.P.; Krile, J.T.; Neuber, A.A.; Krompholz, H.G.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):1147 - 1150
Abstract:  Dielectric flashover along insulators in vacuum has been sufficiently researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of various electrode geometries, humidity, and type of gas present. Previous research has shown distinct arc behavior in air and nitrogen for an electrode geometry in which the electric field lines curve above the dielectric surface. Specifically, flashover experiments in nitrogen have shown that the arc path will follow the electric field lines, not the dielectric surface. As a result, it was concluded that the arc development path, whether along the electric field line or the surface of the dielectric, is related to the oxygen content in the atmospheric background [1]. It is believed that this dependence is due to the arc's production of UV radiation in an oxygen rich environment. Further testing, in a pure nitrogen environment with UV illumination of the surface prior to the pulse application, has shown that UV plays a significant role in the arc development path. There is a near linear relationship between the percentage of liftoffs and the time delay between UV application and flashover. Additional studies have also shown a relationship between the UV intensity and the percentage of liftoffs. Based on these results we will discuss the physical mechanisms primarily involved in unipolar flashover at atmospheric pressure. Additional experimental results regarding the effects of humidity on the liftoff phenomenon will be presented as well.

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+ Similarities of Dielectric Surface Flashover at Atmospheric Conditions for Pulsed Unipolar and RF Excitation
  Krile, J.; Edmiston, G.; Neuber, A.; Dickens, J.; Krompholz, H.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):354 - 357
Abstract:  Mechanisms in vacuum flashover caused by rf (f<10 GHz) or unipolar voltages are virtually identical. Similarities between rf (representing high power microwave window breakdown on the high pressure side) and unipolar flashover are expected in an atmospheric environment as well. Our experimental setups enable studying both unipolar flashover and rf window flashover at atmospheric conditions while controlling excitation, temperature, pressure, humidity, and type of gas present. The local electric field at the flashover initiating points has been numerically calculated in detail for all test geometries. For both rf and unipolar pulsed excitation, the flashover dynamics are changed by the application of UV light to the dielectric surface. A UV pre-pulse has a distinct impact on the arc's path and a tendency to increase the hold-off electric field. The effect of humidity on the hold-off electric field for both pulsed unipolar and rf excitations, along with temporally resolved emission spectroscopy of the flashover event, will be discussed.

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+ Simulation Studies of Liquid Water Breakdown By a Sub-Microsecond Pulse
  Qian, J.; Joshi, R.P.; Kolb, J.; Schoenbach, K.H.; Dickens, J.; Neuber, A.; Cevallos, M.; Krompholz, H.; Schamiloglu, E.; Gaudet, J.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):738 - 741
Abstract:  An electrical breakdown model for liquids in response to a sub-microsecond (~ 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 micro-bubbles. Impact ionization within the micro-bubble 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 pre-breakdown 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. energy associated with a sub-microsecond pulse is too low to induce any significant heating [2]. Temperature increases of less than 6 K were predicted. Hence, bubble formation on the basis of localized liquid vaporization can effectively be ruled out. Here a general model of liquid breakdown is developed that incorporates two important features. First, the preexistence of spatially localized micro-bubbles, in equilibrium with the liquid phase, is implicitly assumed. As a result, no strong internal heating or vaporization is necessary for the creation of local low-density regions. Such bubbles are assumed to be filled with dissolved gas.

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+ Subnanosecond Breakdown in Argon at High Overvoltages
  Krompholz, H.; Hatfield, L.L.; Neuber, A.; Hemmert, D.; Kohl, K.; Chaparro, J.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):423 - 426
Abstract:  Volume breakdown and surface flashover in quasi homogeneous applied fields in 10-5 to 600 torr argon are investigated, using voltage pulses with 150 ps risetime, < 1 ns duration, and up to 150 kV amplitude into a matched load. The test system consists of a transmission line, a transition to a biconical section, and a test gap, with gap distances of one to several mm. The arrangement on the other side of the gap is symmetrical. An improved system, with oil-filled transmission lines and lens between coax and biconical section to minimize pulse distortion, is being constructed. Diagnostics include fast capacitive voltage dividers, which allow to determine voltage waveforms in the gap, and conduction current waveforms through the gap. X-ray diagnostics uses a scintillator-photomultiplier combination with different absorber foils yielding coarse spectral resolution. Optical diagnostics to obtain information about the discharge channel dynamics is in preparation. Breakdown delay times, and e-folding time constants for the conduction current during the initial breakdown phase, are on the order of 100-400 ps, with minima in the range of several 10 torr. X-ray emission extends to pressures > 100 torr, indicating the role of runaway electrons during breakdown. Maximum x-ray emission coincides with fastest current risetimes at several 10 torr, which is probably related to an efficient feedback mechanism from gaseous amplification to field enhanced electron emission from the cathode.

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Publication Year:  2004
+ Compact Pulsed Power
  M. Kristiansen, J. Dickens, H. Krompholz, M. Giesselmann, A. Neuber, J. Mankowski, L. Hatfield, “Compact Pulsed Power,“ Proceedings of the 5th International Symposium on Pulsed Power and Plasma Applications, Oct. 18-20, Chan-Won, Korea, p. 10-15, 2004. (Invited paper)
Abstract:  Not Available
+ DC and Pulsed Dielectric Surface Flashover at Atmsopheric Pressure
  J. T. Krile, A. Neuber, H. G. Krompholz, and J. C. Dickens, „DC and Pulsed Dielectric Surface Flashover at Atmsopheric Pressure,“ 2004 Power Modulator Conference, San Francisco, CA, 2004.
Abstract:  Not Available
+ Effect of temperature and pressure on DC pre-breakdown current in transformer oil
  Butcher, M.; Neuber, A.; Krompholz, H.; Dickens, J.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):258
Abstract:  Summary form only given. Any attempt to model the complex interaction of hydrodynamic and electronic processes leading to breakdown in transformer oil suffers from the lack of microscopic transport data. Also, interface processes, such as electron emission from metal electrodes immersed in liquid, are poorly understood. As a first step toward the understanding of breakdown phenomenology, the voltage-current characteristics for pre-discharge conditions are measured. An experimental setup was constructed which allows temperature variations between 10/spl deg/C and 50/spl deg/C, at pressures between 0.5 and 3 bar. DC currents ranging from a few nA with a few kV of applied voltage, to a few /spl mu/A prior to full breakdown are measured using an electrometer. Preliminary results at NTP with a tip-plane geometry indicate Ohmic behavior at low voltages, Schottky emission at intermediate voltages, and saturation due to space charge at high voltages, and allow estimates on the physical parameters governing these effects. The indicated temperature range of the measurements is associated with a variation of the viscosity of a factor of 3, where we anticipate similar relative changes for the transport of electrons. The intermediate voltage range where a Schottky emission process is assumed will be emphasized. For instance, the electron mobility, derived from experimental data to about 0.06 cm/sup 2//Vs at NTP in the intermediate voltage range should distinctly vary with changing temperature and pressure.

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+ Electrical breakdown in transformer oil
  Cevallos, M.D.; Dickens, J.C.; Neuber, A.A.; Krompholz, H.G.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):401
Abstract:  Summary form only given. The fundamental breakdown physics of transformer oils is investigated with high-speed electrical and optical diagnostics with temporal resolution down to 500 ps. Univolt 63 and Envirotemp FR3 (biodegradable) are used for this study. The system set up employs a cable discharge into a coaxial system with point/plane axial discharge and load line to providing a matched terminating impedance. Overall, the impedance of the system is matched at 50 ohms throughout with the exception of the very narrow gap region and includes a 50 ohm load resistor terminating the load line. Self breakdown is achieved by applying up to 50 kV to the charging line. Pulsed breakdown is achieved by charging a pulse forming line with a two way transient time of 300 ns, up to 100 kV. The pulse forming line is then fed into the discharge line via an oil spark gap. Transmission line type current sensors and a capacitive voltage divider with fast amplifiers/attenuators are used in order to attain a complete range of information from amplitudes of 0.1 mA to 1 kA with temporal resolutions of 300 ps. Optical measurements are performed on low level light emission using fast photo-multiplier tubes (risetime of 800 ps) spatially resolved, supplemented with high speed and spectroscopic investigations on a nanosecond timescale. Breakdown voltages at gap distances of 5 mm for pre, self, and pulsed breakdown voltages are given, where breakdown with negative needle tips show 20% higher breakdown voltages than the ones with positive needle. Velocities of propagating "tree"-structures for the pre and self-breakdown are a few km/s while pulsed breakdown velocities are several 10's of km/s, with higher velocities for a negative needle. The trees have to reach the opposite electrode before full breakdown occurs. Simultaneous optical measurements for a single breakdown event are presented, such as the luminosity in comparison to shadowgraphy images, which is necessary to describe the complex interaction of hydrodynamic phenomena (channel and tree formation and propagation) and charge carrier multiplication.

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+ Fast Volume Breakdown in Argon and Air at Low Pressures
  E. Crull, H. Krompholz, A. Neuber, and L. Hatfield, “Fast Volume Breakdown in Argon and Air at Low Pressures,” Euro Electromagnetics, Magdeburg, Germany, July 12-16, 2004.
Abstract:  Not Available
+ High Power Microwave Breakdown of a Gas-Dielectric Interface at 90 to 760 Torr
  G. Edmiston, A. Neuber, H. Krompholz, J. Dickens, "High Power Microwave Breakdown of a Gas-Dielectric Interface at 90 to 760 Torr,” presented at the 46th Annual Meeting of the Division of Plasma Physics November 15-19, 2004, Savannah, GA, Meeting ID: DPP04APS
Abstract:  Not Available
+ Light-matter interaction in transformer oil
  Namihira, T.; Wang, D.; Neuber, A.; Butcher, M.; Dickens, J.; Krompholz, H.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):448
Abstract:  Summary form only given. Considering highly stressed dielectric liquids, the role of mechanisms such as photoionization in the liquid volume or photoeffect at the cathode for the development of dielectric breakdown is investigated. We used a pulsed 300 W Xenon light source (25 mm output window, 5 degree divergence) with a broad spectral range of 200 to 1100 nm to study the impact of the light beam focused either solely on the high field region between the breakdown electrodes or including the electrodes. Typical field strengths in the electrode gap (/spl sim/4 mm gap, 3 mm tip radius,) were 15 to 25 kV/cm resulting in a DC current amplitude (without light) of up to 2 nA (apparatus resolution /spl sim/10 pA). Standard transformer oil, Univolt 61, and a biodegradable oil, Environtemp FR3 (natural ester fluid), were examined in the present work. Both oils exhibit strong optical absorption in the UV. However, Univolt 61 has its cut off wavelength at 450 nm, while bio oil easily transmits down to 350 nm. Below the cutoff wavelength, virtually all radiation is absorbed within a few mm. When pulsing the Xenon lamp at /spl sim/500 microsec no increase in DC current amplitude (increase <10 pA) could be detected for either oil. Increasing the pulse length to several seconds lead to a distinct increase in current amplitude (up to 300 pA), however, only for Univolt 61. Such an increase in current amplitude can also be achieved by raising the temperature of the dielectric liquid by external heating (/spl sim/100 pA/K). The temperature levels leading to similar current amplitudes due to heating by the Xenon lamp or external heating are comparable. Since bio oil absorbs only below 350 nm, the temperature rise due to the light irradiation was comparably smaller than in Univolt 61. Thus, any heating and increase in current were less pronounced in bio oil. For both oils, the observed behavior can be entirely explained by thermal effects. Both, photoionization and photoeffect have seemingly a minor impact on breakdown development. The detailed discussions are given in the present work.

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+ Nanosecond, optical diagnostics for liquid dielectric switches
  Kolb, J.F.; Xiao, S.; Goan, B.; Lu, X.P.; Schoenbach, K.H.; Laroussi, M.; Joshi, J.P.; Dickens, J.; Neuber, A.; Krompholz, H.; Cevallos, M.; Butcher, M.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):402
Abstract:  Summary form only given. The high dielectric strength of liquid dielectrics allows for the design of small, low inductance and consequently fast high power switches. The investigation of the streamer formation which eventually leads to electrical breakdown requires diagnostic techniques with high temporal and spatial resolution. Optical methods, such as interferometry, Schlieren photography and shadowgraphy have been used to study the development of streamers and subsequent spark channel formation and decay in a pin-plane geometry. The temporal resolution is determined by the shutter speed of a high-speed camera, and was generally on the order of 1 ns. Interferometric measurements in water under high dielectric stress allowed for the characterization of the transient electric field distribution up to the imminent breakdown. Schlieren and shadow photographs allowed us to explore the development of the discharge and the switch recovery. With the pin electrode being the cathode tree-shaped inhomogeneities expand into the gap before breakdown is initiated by the formation of a single streamer that eventually bridges a gap of 400 /spl mu/m in about 7 ns. The recovery is determined by the formation of a vapor bubble that is cleared from the gap in about 1 ms. In oil, the processes involving the interaction of hydrodynamic and electronic processes are more complex. DC breakdown in a pin-plane geometry is strongly polarity dependent. Successively growing trees are observed, which bridge a 1-mm gap after as much as 1 /spl mu/s causing large breakdown delays. For fast pulse breakdown, the observed phenomena resemble more the ones observed in water. Gaining complete information on the breakdown phenomenology in oil requires the simultaneous use of all diagnostics methods including high resolution current measurements. Of special importance is information on the propagation of gaseous channels involved in the tree formation, and measurement of the correlated light emission indicating charge carrier amplification.

[PDF]

+ Pulsed gas breakdown with high overvoltages in argon and air
  Crull, E.; Krompholz, H.; Neuber, A.; Hatfield, L.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):273
Abstract:  Summary form only given. Fast gas breakdown with formative times in the sub-nanosecond regime is of interest for pulsed power switching and UWB applications. Use of coaxial transmission lines with conical sections connected to a test gap enables to apply fast voltage pulses to the gap, as well as the simultaneous measurement of voltage across and current through the gap. For small pulse amplitudes, with risetimes of 400 ps, a tip-plane geometry is used, with radii of curvature of 0.5 /spl mu/m. At pulse amplitudes of 5 kV, and macroscopic field enhancements on the order of 1000, delay times between current and voltage of less than 200 ps for pressures larger than 100 torr are observed, in both argon and dry air. Corresponding current risetimes I/(dI/dt) are less than 100 ps. Using a high voltage pulser (RADAN 303B with pulse slicer SN4, risetime 150 ps at 150 kV amplitude) enables the comparison of formative times for the tip-plane geometry with those of more homogeneous field distributions in the gap.

[PDF]

+ The relevancy of environmental parameter space for electrical insulation design in aerospace vehicles
  “The relevancy of environmental parameter space for electrical insulation design in aerospace vehicles”, D. L. Schweikart, J.C. Horwath, L.C. Walko, L.L. Hatfield, H. G. Krompholz, 2004 Power Modulator Conference, San Francisco, CA, May 23-26, 2004
Abstract:  Not Available
+ Unipolar Surface Flashover
  J. Krile, A. Neuber, H. Krompholz, J. Dickens, "Unipolar Surface Flashover," presented at the 46th Annual Meeting of the Division of Plasma Physics November 15-19, 2004, Savannah, GA, Meeting ID: DPP04APS
Abstract:  Not Available
+ Voltage-current characteristic of transformer oil under high electrical stress
  Butcher, M.; Neuber, A.; Krompholz, H.; Dickens, J.; IEEE Conference Record - Abstracts. The 31st IEEE International Conference on Plasma Science, 2004. ICOPS 2004. 28 June-1 July 2004 Page(s):258
Abstract:  Summary form only given. The scattering cross-sections, transport coefficients, and details of the breakdown mechanism needed for describing current conduction in transformer oil at high applied voltages are virtually unknown. This makes it extremely difficult, if not impossible, to properly model electrical breakdown in oil. To address this issue, we have measured the V-I characteristic of transformer oil in the point-plane geometry for /spl sim/3 mm gap widths. Three regimes can be distinguished. (1) For low voltages, V <3 kV, the relationship between voltage and current is linear, exhibiting just resistive behavior within the measurement accuracy. (2) At intermediate voltages, the dependence is linear on a Fowler-Nordheim plot. (3) Between the breakdown voltage V/sub B/ /spl sim/ 30 kV and 10 kV, the current is proportional to V/sup 2/, indicating space charge limited current. Assuming a Schottky-type emission mechanism for electrons injected into the liquid, we derive an electron mobility of /spl sim/6.0/spl times/10/sup -6/ m/sup 2//Vs, which is close to values reported in the literature before. Assuming Fowler-Nordheim emission leads to 3.8/spl times/10/sup -6/ m/sup 2//Vs. The ion mobility is believed to be more than one order of magnitude lower. At this point, it is difficult to gain any more detailed information on charge transport and possible multiplication based on simple analytical methods. Hence, we applied our numerical modeling techniques already proven in analyzing high-field phenomena in polar liquids (e.g., water), to evaluate current conduction and the breakdown process in non-polar oil. By comparing our calculations with the experimental data, we hope to characterize important transport parameters, such as the electron ionization coefficient as a function of the electric field. We also present our attempts to extract details of the field-dependent non-linear processes and electrode effects close to the breakdown regime.

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Publication Year:  2003
+ Electrical Breakdown in Transformer Oil
  M. Cevallos, J. Dickens, A. Neuber H. Krompholz, “Electrical Breakdown in Transformer Oil,” Joint Fall Meeting of the Texas Sections of the APS and AAPT and Zone 13 Society of Physics Students, Lubbock, TX, Oct 2003 (abstract only published)
Abstract:  Not Available
+ Electromagnetic fields and discharges in advanced power systems
  “Electromagnetic fields and discharges in advanced power systems,”, J.F. Prewitt, R. St.John, S. Oetzel, G. Skidmore, L. Hatfield, H. Krompholz, s. Calico, S. Iden, D. Borger, J. Havey, S. Graybill, 29th IEEE International Pulsed Power Conference, Dallas, TX, June 2003,
Abstract:  Not Available
+ Flashover Across a Dielectric Surface at Atmospheric Pressure
  J. Krile, A. Neuber, J. Dickens, and H. Krompholz, “Flashover Across a Dielectric Surface at Atmospheric Pressure,” Joint Fall Meeting of the Texas Sections of the APS and AAPT and Zone 13 Society of Physics Students, Lubbock, TX, Oct 2003 (abstract only published)
Abstract:  Not Available
+ Optical diagnostics of liquid nitrogen volume pre-breakdown events
  Butcher, M.; Neuber, A.; Krompholz, H.; Dickens, J.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1029 - 1032 Vol.2
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 /spl Omega/ impedance. Experiments are done in self-breakdown mode in super-cooled 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 (risetime/spl ap/800 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.

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+ Phenomenology of conduction and breakdown in transformer oil
  Butcher, M.; Cevallos, M.; Haustein, M.; Neuber, A.; Dickens, J.; Krompholz, H.; Electrical Insulation and Dielectric Phenomena, 2003. Annual Report. Conference on 2003 Page(s):301 - 304
Abstract:  In a coaxial test apparatus enabling the measurement of voltage and current at the test gap, dc conduction and breakdown in transformer oil caused by the application of dc voltages are investigated. Current measurements cover the range from 10/sup -1/ A to 1 kA, with temporal resolutions of milliseconds at the lowest current amplitudes to sub-nanoseconds for currents larger than 10/sup -4/ A. The dc current/voltage characteristic for sub-breakdown voltage amplitudes point to the injection of charge carriers, allow us to characterize the transport mechanisms, and the influence of space charges. For voltages approaching breakdown thresholds, quasi dc-currents rising from nanoamperes to microamperes are superimposed by current pulses with amplitudes of milliamperes and above, and durations of nanoseconds. The onset of these current pulses occurs up to 10 /spl mu/s before breakdown. One of these current pulses reaches a critical amplitude causing voltage breakdown and current rise to the impedance-limited value within 2 ns. Additional optical diagnostics using photomultipliers and high-speed photography with gated microchannel plates yield information on hydrodynamic processes and charge carrier amplification mechanisms associated with the current pulses and final breakdown, such as bubble formation, as well as on the development of the spark plasma finally bridging the gap.

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+ Physical efficiency limits of inch-sized helical MFCGs
  Neuber, A.A.; Hernandez, J.-C.; Holt, T.A.; Dickens, J.C.; Kristiansen, M.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):413 - 416 Vol.1
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.

[PDF]

+ Physics of dielectric surface flashover at atmospheric pressure
  Krile, J.; Neuber, A.; Dickens, J.; Krompholz, H.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):285 - 288 Vol.1
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 subnanosecond 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.

[PDF]

+ Prebreakdown current behavior in DC volume breakdown in transformer oil
  Butcher, M.; Neuber, A.; Krompholz, H.; Dickens, J.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):289 - 292 Vol.1
Abstract:  The phenomenology of prebreakdown events in transformer oil is investigated using high-speed electrical and optical diagnostics. Data collection using a coaxial test setup terminating into a 50/spl Omega/ 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 kV of applied voltage, to a few /spl mu/A prior to full breakdown are measured using an electrometer. Prebreakdown 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 +44 kV) with a 2.35 mm gap show a DC (seed) current of several hundred nA with prebreakdown 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.

[PDF]

+ Pulsed and self electrical breakdown in biodegradable oil
  Cevallos, M.D.; Dickens, J.C.; Neuber, A.A.; Haustein, M.A.; Krompholz, H.G.; 2003. ICOPS 2003. IEEE Conference Record - Abstracts. The 30th International Conference on Plasma Science, 2-5 June 2003 Page(s):315
Abstract:  The fundamental breakdown physics of biodegradable oil is investigated with a set up that employs a cable discharge into a coaxial system with axial discharge and load line to simulate a matched terminating impedance. No discontinuities are created in the system lines when entering the discharge chamber with the implementation of a unique feed-through design. The entire impedance of the system is matched at 50 ohms. A novel design for impedance matching transitions from the discharge cable to the coaxial system to the load line allow for a sub-nanosecond response. Final results are measured on pulsed and self breakdown voltages of up to 200 kV. Self breakdown is achieved by charging the discharge cable and load line to +/- 100 kV respectively. Pulsed breakdown is achieved by charging the discharge cable and load line to +100 kV. Shorting the discharge cable generates a reflected negatively polarized pulse causing breakdown. High speed electrical and optical diagnostics have temporal resolution down to several 100 ps A complete range of information from amplitudes of 0.1 mA to 1 kA with temporal resolutions of 300 ps is achieved by using transmission line type current sensors with fast amplifiers. Capacitive voltage dividers with fast attenuators are also used. Optical measurements are performed on low level light emission using spatially resolved, fast photo-multiplier tubes (risetime of 800 ps), supplemented with high speed photography and spectroscopic investigations on a nanosecond timescale Detailed optical and spectroscopic diagnostic along with high speed electrical diagnostics will address the mechanism initiating/assisting biodegradable oil volume breakdown.

[PDF]

+ Self electrical breakdown in biodegradable oil
  Cevallos, M.D.; Dickens, J.C.; Neuber, A.A.; Haustein, M.A.; Krompholz, H.G.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1036 - 1039 Vol.2
Abstract:  The fundamental breakdown physics of biodegradable oil is investigated with high-speed electrical and optical diagnostics with temporal resolution down to several 100 ps. The set up employs a cable discharge into a coaxial system with axial discharge and load line to simulate 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 1 kA 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.

[PDF]

+ Sub-nanosecond point-plane gas breakdown in conical shaped spark gap
  “Sub-nanosecond point-plane gas breakdown in conical shaped spark gap”, J. Spears, H. Krompholz, L.L. Hatfield, 29th IEEE International Pulsed Power Conference, Dallas, TX, June 2003
Abstract:  Not Available
+ Surface flashover across ceramic disks in vacuum at cryogenic temperatures
  Keene, H.; Dickens, J.; Neuber, A.; Krompholz, H.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):305 - 308 Vol.1
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 subatmospheric 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 nonvarnished 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.

[PDF]

Publication Year:  2002
+ Electric breakdown in liquid nitrogen
  Krompholz, H.; Neuber, A.; Haustein, M.; Dickens, J.; Proceedings of 2002 IEEE 14th International Conference on Dielectric Liquids, 2002. ICDL 2002. 7-12 July 2002 Page(s):167 - 170
Abstract:  The phenomenology of breakdown in liquid nitrogen is investigated with high-speed electrical and optical diagnostics (temporal resolution down to several 100 ps). The discharge apparatus uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen. Transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps), and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. First results on self-breakdown with a gap width of 1 mm and electrodes with 5 mm radius of curvature (breakdown voltage 42 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. Detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiating/assisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.

[PDF]

+ Fast dielectric volume breakdown in liquid nitrogen
  Neuber, A.; Krompholz, H.; Haustein, M.; Dickens, J.; IEEE Conference Record - Abstracts. The 29th IEEE International Conference on Plasma Science, 2002. ICOPS 2002. 26-30 May 2002 Page(s):196
Abstract:  Summary form only given, as follows. Miniaturization of electrical components along with growing superconductor technology requires a better understanding of the phenomenology of breakdown in liquid nitrogen. It is known that the time delay between breakdown-onset and final impedance-limited arc current can occur within a few nanoseconds. For a temporal resolution down to several 100 ps, a discharge apparatus was built and tested that uses a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance. Main experiments are done in self-breakdown mode in supercooled liquid nitrogen, pulsed breakdown at high over-voltages in standard electrode geometry is investigated as well. Transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators cover an amplitude range of 0.1 mA to 1 kA with a time resolution of 300 ps, providing complete information about discharge voltage and current. The light emission is measured with fast photomultiplier tubes (risetime 800 ps), and these optical measurements will be supplemented by high-speed photography and spectroscopic investigations on a nanosecond time scale. Preliminary results on self-breakdown in the surface flashover mode with a gap width of 2 mm and electrodes with 5 mm radius of curvature (breakdown voltage /spl sim/ 60 kV) show a three-phase development: the current rises from an unknown level to several mA during 2 ns, stays approximately constant for 100 ns with superimposed ns-duration spikes, and shows a final exponential rise to the full impedance limited current amplitude during several nanoseconds. The detailed optical and spectroscopic diagnostics along with the high-speed electrical diagnostics will in particular address the physical mechanisms initiating/assisting the liquid nitrogen volume breakdown, such as bubble formation during the pre-breakdown phase.

[PDF]

+ Field enhanced microwave breakdown in gas for a plasma limiter
  Hemmert, D.; Neuber, A.; Krompholz, H.; Mankowski, J.; Saeks, D.; IEEE Conference Record - Abstracts. The 29th IEEE International Conference on Plasma Science, 2002. ICOPS 2002. 26-30 May 2002 Page(s):322
Abstract:  Summary form only given, as follows. A new type of plasma limiter is being developed capable of turnon in less than 1 nsec. The approach taken is to initiate streamer breakdown via a micron radius needle tip. Studies were conducted on breakdown with a variety of gases at pressures from 10/sup 3/ to 10/sup -2/ torr. Gases tested included dry air, neon, argon, and krypton. Studies were also conducted on dc-voltage biasing the needle and its effect on breakdown. The experimental setup uses an S-band traveling wave resonant ring (TWRR) capable of power levels up to 100 MW when coupled to a 2.85 GHz, 4 MW, magnetron. High speed diagnostics with a response on the order of 1 ns record the microwave power, luminosity, and x-rays. A high speed CCD camera with an adjustable exposure time down to 10 nsec records a snapshot of the breakdown sequence. Preliminary results exhibit a reduction in expected gas breakdown levels by over two orders of magnitude.

[PDF]

+ High Voltage, Sub Nanosecond Feedthrough Design for Liquid Breakdown Studies
  M. Cevallos, J. Dickens, A. Neuber, H. Krompholz, “High Voltage, Sub Nanosecond Feedthrough Design for Liquid Breakdown Studies,” presented at the 14th International Conference on High-Power Particle Beams, Albuquerque, NM, June 23 - 28, 2002, AIP conference Proceedings, Vol. 650, p. 73-76.
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 50. 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. ©2002 American Institute of Physics

[PDF]

+ Liquid Nitrogen As Fast High Voltage Switching Medium
  J. Dickens, A. Neuber, M. Haustein, H. Krompholz, “Liquid Nitrogen As Fast High Voltage Switching Medium,” presented at the 14th International Conference on High-Power Particle Beams, Albuquerque, NM, June 23 - 28, 2002, AIP conference Proceedings, Vol. 650, p. 95-98.
Abstract:  Not Available
+ Nanosecond pulsed breakdown for point-plane geometries at moderate voltage
  Nanosecond pulsed breakdown for point-plane geometries at moderate voltage”, H. Krompholz, L. Hatfield, M. Haustein, J. Spears, M. Kristiansen, AMEREM 2002, Annapolis, Md, June 2002
Abstract:  Not Available
Publication Year:  2001
+ Gas breakdown in the sub-nanosecond regime with voltages below 15 kV
  Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; Hemmert, D.; Short, B.; Mankowski, J.; Brown, M.; Altgilbers, L.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 1, 17-22 June 2001 Page(s):487 - 490 vol.1
Abstract:  Gaseous breakdown in the sub-nanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics and for plasma limiters to protect devices from high power microwave radiation. Previous investigations of sub-nanosecond breakdown were mainly limited to high-pressure gases or liquids, with applied voltages in excess of 100 kV. In this paper, the authors investigate possibilities to achieve sub-nanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries. The setup consists of a pulser (risetime between 400 ps to 1 ns), 50-/spl Omega/ transmission line, axial needle-plane gap with outer coaxial conductor, and a 50-/spl Omega/ load line. The needle consists of tungsten and has a radius of curvature below 0.5 /spl mu/m. The constant system impedance of 50 /spl Omega/ (except in the vicinity of the gap) and a special transmission-line-type current sensors enables current and voltage measurements with a dynamic range covering several orders of magnitude, with temporal resolution down to 80 ps. For pulse amplitudes of 1.7 kV (which are doubled at the open gap before breakdown) delay times between start of the pulse and start of a measurable current flow (amplitude > several milliamperes) have a minimum of about 8 ns, at a pressure of 50 torr in argon. Voltages of 7.5 kV produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip, at an amplitude of 7.5 kV, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 1 and 100 torr. At lower pressure, a longer delay time (8 ns at 50 mtorr) is observed. They authors expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

[PDF]

Publication Year:  2000
+ Dielectric/Gas Interface Breakdown Caused by High Power Microwave
  D. Hemmert, A. Neuber, H. Krompholz, L.L. Hatfield, and M. Kristiansen, “Dielectric/Gas Interface Breakdown Caused by High Power Microwaves," Third Directed Energy Symposium, November 1-2, 2000, Albuquerque, New Mexico.
Abstract:  Not Available
+ Fundamental Studies of a Helical Magnetic Flux Compression Generator
  A. Neuber, J. Dickens, M. Giesselmann, B. Freeman, J. Rasty, H. Krompholz, and M. Kristiansen: Fundamental Studies of a Helical Magnetic Flux Compression Generator. Proceedings of the 13th International Conference on High-Power Particle Beams, June 25-30, 2000, Nagaoka, Japan.
Abstract:  Not Available
+ Fundamental studies of a simple helical magnetic flux compression generator
  Neuber, A.; Dickens, J.; Giesselmann, M.; Freeman, B.; Rasty, J.; Krompholz, H.; Kristiansen, M.; IEEE Conference Record - Abstracts. The 27th IEEE International Conference on Plasma Science, 2000. ICOPS 2000. 4-7 June 2000 Page(s):276
Abstract:  Summary form only given, as follows. The design of a helical flux compression generator, driven by 150 g of high explosives, for basic studies is presented and experimental results are discussed in comparison to numerical simulations. Simulation of the electric current output with the commercial circuit simulator PSPICE shows that this generator conserves the magnetic flux ideally in the low current mode, <30 kA. At current amplitudes in excess of 100 kA heating and melting of the single wound helix wire, AWG 12, limit the current flow. The volume between armature and stator is spectroscopically probed with fiber optics and valuable insight into the state of the shocked and compressed gas is gained. The same fiber optic probes are used to measure the velocity of the armature-stator contact along the generator axis. This contact velocity is largely affected by armature end effects, mainly due to the pressure loss at the detonator end. Both gas temperature and contact velocity have been successfully simulated with LS-DYNA3D, a three dimensional finite element hydrodynamic code. The generator's magnetic field structure is briefly discussed and magnetic field probe measurements are presented

[PDF]

+ Helical Flux Compression Generator for Basic Research
  A. Neuber, J. Dickens, M. Giesselmann, B. Freeman, P. Worsey, H. Krompholz, and M. Kristiansen, “Helical Flux Compression Generator for Basic Research,” 12th Symposium on High Current Electronics, September 25-29, 2000, Tomsk, Russia.
Abstract:  Not Available
+ High Power Microwave Window Breakdown under Vacuum and Atmospheric Conditions
  A. Neuber, D. Hemmert, J. Dickens, H. Krompholz, L. L. Hatfield, and M. Kristiansen: High Power Microwave Window Breakdown under Vacuum and Atmospheric Conditions. Proceedings of the SPIE conference, vol. 4031, pp. 90-98, Aerosense 2000, 24-28 April 2000, Orlando Florida.
Abstract:  Not Available
+ Pressure dependence of high power microwave solid dielectric/gas interface breakdown
  Neuber, A.; Hemmert, D.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; 2000. ICOPS 2000. IEEE Conference Record - Abstracts. The 27th IEEE International Conference on Plasma Science, 4-7 June 2000 Page(s):124
Abstract:  The knowledge of the behavior of solid dielectric/gas interface breakdown caused by microwaves is crucial for developing new design methods for high power microwave windows. We investigate the physical mechanisms leading to breakdown for power levels on the order of 10 MW/cm2 at 2.85 GHz and gas pressures varying from 10-4 to 103 Torr. As an interface that is in widespread use, the focus was put on an alumina/air interface. Other gases are considered mainly for reference purposes. The high power microwaves are generated with a 4 MW magnetron having a 3.5 μs pulse width in conjunction with an S-band traveling wave resonator. This approach provides a power gain of maximum 25, sufficient to cause breakdown across the interface located in the pressure adjustable test region. The interface geometry comprises a thin ceramic alumina slab placed in the waveguide center. We ensure an almost purely tangential field and a localized breakdown by orienting the alumina slab normal to the direction of the wave propagation and making contact with two field enhancement tips placed in the middle of each waveguide broad wall

[PDF]

+ Studies on a Helical Magnetic Flux Compression Generator
  A. Neuber, J. Dickens, B. Freeman, P. Worsey, H. Krompholz, and M. Kristiansen: Studies on a Helical Magnetic Flux Compression Generator. Proceedings of the SAE Power Systems Conference, October 31-November 2, 2000, San Diego, CA.
Abstract:  Not Available
+ Sub-nanosecond gas breakdown phenomena in the voltage regime below 15 kV
  H. Krompholz, L.L. Hatfield, B. Short, M. Kristiansen, “Sub-nanosecond gas breakdown phenomena in the voltage regime below 15 kV”, EUROEM 2000, Edinburgh, UK, June 2000
Abstract:  Not Available
Publication Year:  1999
+ A theory of RF window failure
  Valfells, A.; Ang, L.K.; Lau, Y.Y.; Gilgenbach, R.M.; Kishek, R.A.; Verboncoeur, J.; Neuber, A.; Krompholz, H.; Hatfield, L.L.; Plasma Science, 1999. ICOPS '99. IEEE Conference Record - Abstracts. 1999 IEEE International Conference on Plasma Science, 20-24 June 1999 Page(s):105 (Abstract only)
Abstract:  Summary form only given, as follows. We have recently developed a novel theory of multipactor discharge on a dielectric. The main results include the susceptibility diagram and the prediction that about one percent of the RF power is deposited to the dielectric surface over a wide range of conditions. In this paper, we extend the analysis to include the effects of outgassing and the subsequent ionization by the multipactoring electrons. This is an attempt to understand the final stage of dielectric failure that is initiated by multipactor. Similarities and differences in such failures, under RF and DC conditions, are explored. Analytic theory and simulation results will be presented and compared with experiments

[PDF]

+ Characterization of the power handling capability of an S-band double disc gas cooled microwave window
  Neuber, A.; Ferguson, P.; Hendricks, K.; Hemmert, D.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Conference Record - Abstracts. 1999 IEEE International Conference on Plasma Science, 1999. ICOPS '99. 20-24 June 1999 Page(s):253
Abstract:  Summary form only given. The S-Band double disc microwave window comprises a rectangular waveguide to circular pillbox transition with two separate high purity, TiN coated alumina discs brazed into the pillbox. The geometrical dimensions are optimized for minimum electromagnetic wave reflection at a microwave frequency of 2.85 GHz in TE10 mode. The window is designed for power levels up to a few 100 MW with several microseconds pulse duration. Crucial for the power handling capability is the gas species and pressure of the gas flow applied for cooling the alumina discs

[PDF]

+ Influence of the microwave magnetic field on high power microwave window breakdown
  Hemmert, D.; Neuber, A.; Dickens, J.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Conference Record - Abstracts. 1999 IEEE International Conference on Plasma Science, 1999. ICOPS '99. 20-24 June 1999 Page(s):229
Abstract:  Summary form only given. Effects of the microwave magnetic field on window breakdown are investigated at the upstream and downstream side of a dielectric interface. Simple trajectory calculations of secondary electrons in an RF field show significant forward motion of electrons parallel to the microwave direction of propagation. The Lorentz-force due to the microwave magnetic field on high-energy secondary electrons might substantially influence the standard multipactor mechanism. As a result, the breakdown power level for the downstream side of a window would be higher than for the upstream side. This hypothesis was tested utilizing an S-band traveling wave resonant ring, powered by a 3 MW magnetron at 2.85 GHz, leading to a total power greater than 60 MW. Breakdown was studied on an interface geometry consisting of a thin alumina slab in the waveguide, oriented normal to the microwave propagation direction

[PDF]

+ Optical Diagnostics on Helical Flux Compression Generators
  A. Neuber, J. Dickens, H. Krompholz, and M. Kristiansen: Optical Diagnostics on Helical Flux Compression Generators. 12th IEEE Int. Pulsed Power Conference, Monterey, CA (1999)
Abstract:  Not Available
+ Surface flashover in liquid nitrogen
  Butcher, M.; Neuber, A.; Krompholz, H.; Hatfield, L.L.; 1999 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Volume 2, 17-20 Oct. 1999 Page(s):654 - 657 vol.2
Abstract:  The use of cryogenic high voltage components to achieve higher energy densities is limited by virtually unknown insulator characteristics for cryogenic conditions. Using a fast coaxial setup, the flashover phenomena of dielectric test samples (Lexan, Alumina) immersed in liquid nitrogen are measured with optical and electrical diagnostics with nanosecond time resolution. The flashover voltage reaches a maximum of 30 kV for a 1.75 mm gap after 3 shots, and averages to about 10 kV after conditioning. Three phases in the discharge development can be distinguished: Phase 1 is a rapid current rise to the mA-regime, with several current spikes with amplitudes of less than 5 mA and durations of typically 20 ns. This phase lasts up to several 100 ns. Phase 2 is characterized by a rapid ionization with a current rise to the impedance limited value of several 100 A of phase 3 in a few nanoseconds. Waveforms of the luminosity follow the ones of the current in general. The physical mechanisms leading to this development, and the difference to the flashover phenomena in vacuum, are discussed qualitatively

[PDF]

+ Surface flashover of dielectrics immersed in super-cooled liquid nitrogen
  Butcher, M.; Neuber, A.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 1, 27-30 June 1999 Page(s):450 - 453 vol.1
Abstract:  In a fast coaxial setup, dielectric test sample and electrodes are immersed in super-cooled liquid nitrogen with a temperature near 68 K, and the flashover development process is characterized using fast optical and electrical diagnostics with nanosecond time resolution. The measured breakdown voltage as a function of consecutive flashover shots reaches its peak around the second flashover and declines to a constant value. This voltage is initially about 50% of the volume breakdown voltage in liquid nitrogen and drops to about 20% after the sample has been conditioned. Two materials, Lexan and alumina, were tested in the system and showed quite similar breakdown voltages, but alumina exhibited much more severe surface damage. The time resolved electrical diagnostic revealed three phases in the temporal development of the current. Phase 1 is a rapid rise to the mA regime that is probably associated with field emission. Phase 2 contains several pre-breakdown spikes with current amplitudes of less than 5 mA and duration of typically 20 nanoseconds that form and collapse over a period of tens to a couple hundred nanoseconds. Finally, Phase 3, is characterized by a rapid ionization across the surface with a current rise that covers 4 orders of magnitude in a few nanoseconds

[PDF]

+ The Role of Outgassing in Surface Flashover under Vacuum
  A. Neuber, J. Dickens, H. Krompholz, and M. Kristiansen: The Role of Outgassing in Surface Flashover under Vacuum. 12th IEEE Int. Pulsed Power Conference, Monterey, CA (1999), invited.
Abstract:  Not Available
Publication Year:  1998
+ Breakdown of dielectric/vacuum interfaces caused by high power microwaves
  Neuber, A.; Dickens, J.; Hemmert, D.; Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; IEEE Conference Record - Abstracts. 1998 IEEE International on Plasma Science, 1998. 25th Anniversary. 1-4 June 1998 Page(s):205
Abstract:  Summary form only given. Physical mechanisms leading to microwave breakdown on dielectric/vacuum interfaces are investigated for power levels on the order of 100 MW at 2.85 GHz. A 3 MW magnetron with 3.5 μs pulse width, is coupled to an S-band traveling wave resonator which is kept at a pressure of 10-8 Torr. The investigation is focused on an interface geometry comprising a thin dielectric polymer slab in the waveguide, oriented vertical to the direction of wave propagation, and two field enhancement tips placed in the middle of each waveguide broad wall. This ensures an almost purely tangential field at the interface surface and a localized breakdown

[PDF]

+ Dielectric Surface Flashover in Vacuum at (100 K))
  A. Neuber, M. Butcher, H. Krompholz, and L. L. Hatfield: Dielectric Surface Flashover in Vacuum at (100 K). Proceedings of the XVIIIth International Symposium on Discharges and Insulation in Vacuum, Eindhoven (1998), Page(s): 815-818
Abstract:  Not Available
Publication Year:  1997
+ Dielectric surface flashover in vacuum at cryogenic temperature (100 K)
  Neuber, A.; Krompholz, H.; Hatfield, L.L.; 1997. IEEE 1997 Annual Report of the Conference on Electrical Insulation and Dielectric Phenomena, Volume 2, 19-22 Oct. 1997 Page(s):575 - 578
Abstract:  Recent developments in high power systems use cryogenic components, where the performance of insulators at such low temperatures is unknown. In a fast coaxial setup, electrodes and dielectric sample are cooled to less than 100 K in vacuum, and the flashover initiation processes for gap distances on the order of 1 cm are characterized using fast electrical and optical diagnostics. As reported before, two development stages can be distinguished: a first phase with slowly rising current and the presence of free electrons, with a duration of 10-50 ns, and a second stage with a fast current rise due to gaseous ionization and collision dominated electrons. First experiments comparing cooled samples and samples at room temperature do not show discernible differences for Lexan. For Alumina, however, an increase of the duration of phase 1 is observed, with a higher current amplitude at the transition from phase one to phase two. Also, the current rise in phase two is reduced. These results indicate a temperature dependence of the electron induced outgassing process which is expected to cause the transition from phase one to phase two

[PDF]

+ Window and Cavity Breakdown caused by High Power Microwaves
  A. Neuber, J. Dickens, D. Hemmert, H. Krompholz, L. L. Hatfield, and M. Kristiansen: Window and Cavity Breakdown caused by High Power Microwaves. International Workshop on High Power Microwave Generation and Pulse Shortening, Edinburgh UK (1997)
Abstract:  Not Available
+ Window and Cavity Breakdown caused by High Power Microwaves
  A. Neuber, J. Dickens, D. Hemmert, H. Krompholz, L. L. Hatfield, and M. Kristiansen: Window and Cavity Breakdown caused by High Power Microwaves. 11th IEEE Int. Pulsed Power Conf., Baltimore (1997)
Abstract:  Not Available
+ Window and Cavity Breakdown caused by High Power Microwaves
  A. Neuber, J. Dickens, D. Hemmert, H. Krompholz, L. L. Hatfield, and M. Kristiansen: Window and Cavity Breakdown caused by High Power Microwaves. IEEE International Conference on Plasma Science, San Diego (1997)
Abstract:  Not Available
Publication Year:  1996
+ Breakdown at window interfaces caused by high power microwave fields
  J.C. Dickens, J. Elliott, L.L. Hatfield, M. Kristiansen, H. Krompholz, “Breakdown at window interfaces caused by high power microwave fields”, International Symposium on Optical Science, Engineering, and Instrumentation - SPIE’s Annual Meeting, Denver, CO, August 4-9, 1996, published in SPIE proceedings Volume 2843, “Intense Microwave Pulses IV”, pg.153
Abstract:  Not Available
+ Dielectric Surface Flashover with UV and Plasma Background
  F. Hegeler, H. Krompholz, L.L. Hatfield and M. Kristiansen, "Dielectric Surface Flashover with UV and Plasma Background," Symposium on “Physics, Diagnostics, and Application of Pulsed High Energy Density Plasma as an Extreme State”, Nagoya, Japan, May 1996, Japanese National Institute for Fusion Science Proceedings Series # 26 (NIFS-PROC-26, ISSN 0915-6348), pg. 104-114
Abstract:  Not Available
Publication Year:  1995
+ Insulator Surface Breakdown in a Simulated Low Earth Orbit Environment
  F. Hegeler, H. Krompholz, L. Hatfield, M. Kristiansen, “Insulator Surface Breakdown in a Simulated Low Earth Orbit Environment”, Proc. of the 10th IEEE Pulsed Power Conf., Albuquerque, NM, July 1995, pg. 568
Abstract:  Not Available
+ Insulator Surface Flashover with UV and Plasma Background and External Magnetic Field
  F. Hegeler, H. Krompholz, L.L. Hatfield, M. Kristiansen, “Insulator Surface Flashover with UV and Plasma Background and External Magnetic Field”, Proc. of “Conference on Electrical Insulation and Dielectric Phenomena (CEIDP)”, Virginia Beach, Va, Oct. 22-25, 1995 IEEE Catalog Nr. 95CH35842), pg. 171
Abstract:  Not Available
Publication Year:  1994
+ Outgassing and Plasma Development in the Early Phase of Dielectric Surface Flashover in Vacuum
  G. Masten, T. Mueller, H. Krompholz, F. Hegeler, L.L. Hatfield, M. Kristiansen, "Outgassing and Plasma Development in the Early Phase of Dielectric Surface Flashover in Vacuum", Proc. of 10th International Conference on High Power Particle Beams ("Beams '94"), San Diego, CA, June 20-24, 1994
Abstract:  Not Available
+ The Early Phase of Dielectric Surface Flashover in a Simulated Low Earth Orbit Environment
  F. Hegeler, H. Krompholz, L.L. Hatfield, M. Kristiansen, "The Early Phase of Dielectric Surface Flashover in a Simulated Low Earth Orbit Environment", Proc. of 1994 Conf. on Electrical Insulation and Dielectric Phenomena (CEIDP), Arlington, TX, Oct. 1994, p. 594
Abstract:  Not Available
Publication Year:  1993
+ Experimental Investigation of the Early Phase of Dielectric Surface Flashover in Vacuum
  F. Hegeler, G. Masten, G. Leiker, H. Krompholz, M. Kristiansen, "Experimental Investigation of the Early Phase of Dielectric Surface Flashover in Vacuum", Proc. 9th IEEE International Pulsed Power Conference, Albuquerque, NM, USA, 1993, p. 237
Abstract:  Not Available
+ Insulator Flashover under the Influence of Externally Applied Magnetic Fields
  H. Krompholz, L.L. Hatfield, "Insulator Flashover under the Influence of Externally Applied Magnetic Fields", Proc. of ICPIG XXI (International Conference on Phenomena in Ionized Gases), Bochum, Germany, September 1993 (invited), Vol. 3, p. 179
Abstract:  Not Available
+ Real-time Detection of Outgassing and Plasma Buildup during the Early Phase of Dielectric Surface Flashover
  G. Masten, T. Mueller, F. Hegeler, H. Krompholz, L.L. Hatfield, "Real-time Detection of Outgassing and Plasma Buildup during the Early Phase of Dielectric Surface Flashover", Proc. 9th IEEE International Pulsed Power Conference, Albuquerque, NM, USA, 1993, p.241
Abstract:  Not Available
Publication Year:  1992
+ The Early Phase of Dielectric Surface Flashover
  F. Hegeler, G. Masten, H. Krompholz, L. Hatfield, "The Early Phase of Dielectric Surface Flashover", Proc. of Conference on Electrical Insulation and Dielectric Phenomena, Victoria, B.C., Canada, Oct. 18-21, 1992, p. 463
Abstract:  Not Available
+ The Early Phase of Dielectric Surface Flashover in Vacuum
  F. Hegeler, G. Masten, H. Krompholz, L. Hatfield, "The Early Phase of Dielectric Surface Flashover in Vacuum", Proc. XVth International Symposium on Discharges and Electrical Insulation in Vacuum, Darmstadt, Germany, Sep. 6-10, 1992, p. 213
Abstract:  Not Available
Publication Year:  1991
+ A simple computer simulation of magnetic inhibition of dielectric surface flashover
  M. Lehr, H. Krompholz, M. Kristiansen, "A simple computer simulation of magnetic inhibition of dielectric surface flashover", Proc. Eighth IEEE International Pulsed Power Conference, San Diego, Ca., June 1991, p. 332
Abstract:  Not Available
+ Fast electrical and optical diagnostics for the early phase of dielectric surface flashover
  F. Hegeler, G. Masten, H. Krompholz, L. Hatfield, "Fast electrical and optical diagnostics for the early phase of dielectric surface flashover", Proc. Eighth IEEE International Pulsed Power Conference, San Diego, Ca., June 1991, p. 878
Abstract:  Not Available
+ Pulsed Vacuum Diode Diagnostics at the Texas Tech University High Power Microwave Facility
  M. Crawford, S. Calico, M. Kristiansen, H. Krompholz, "Pulsed Vacuum Diode Diagnostics at the Texas Tech University High Power Microwave Facility", Proc. Eighth IEEE International Pulsed Power Conference, San Diego, Ca., June 1991, p. 602
Abstract:  Not Available
+ Simulation of the Low Earth Orbit for the Study of Pulsed Insulator Flashover and Degradation
  M. Mayerchak, M. Kristiansen, H. Krompholz, L.L. Hatfield, G. Leiker, C. Young, "Simulation of the Low Earth Orbit for the Study of Pulsed Insulator Flashover and Degradation", Proc. Eighth IEEE International Pulsed Power Conference, San Diego, Ca., June 1991, p. 1041
Abstract:  Not Available
Publication Year:  1990
+ Magnetic Field Effects on Dielectric Surface Flashover
  M. Lehr, R. Korzekwa, H. Krompholz, M. Kristiansen, "Magnetic Field Effects on Dielectric Surface Flashover", Proc. 14th International Symposium on Discharges and Electrical Insulation in Vacuum, Santa Fe, NM, Sep. 17-20, 1990, p. 347
Abstract:  Not Available
+ The Influence of Magnetic Fields on Dielectric Surface Flashover
  H. Krompholz, R. Korzekwa, M. Lehr, M. Kristiansen, "The Influence of Magnetic Fields on Dielectric Surface Flashover", Proc. 1990 Conference on Electrical Insulation and Dielectric Phenomena, Pocono Manor, PA, Oct. 28 - Nov. 1, 1990, p. 451
Abstract:  Not Available
+ The Influence of Magnetic Fields on Dielectric Surface Flashover
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "The Influence of Magnetic Fields on Dielectric Surface Flashover", Proc. of 1990 Nineteenth Power Modulator Symposium, San Diego, Ca, June 26-28, 1990, pg. 277
Abstract:  Not Available
Publication Year:  1989
+ Inhibiting Surface Flashover in Vacuum and Plasma
  L. Hatfield, H. Krompholz, "Inhibiting Surface Flashover in Vacuum and Plasma", Proc. of the 6th International Symposium on High Voltage Engineering, New Orleans, La, Aug. 1989, Vol. 2, paper 25.01, invited
Abstract:  Not Available
+ Inhibiting Surface Flashover with Magnetic Field
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "Inhibiting Surface Flashover with Magnetic Fields", IECEC '89 (24th Intersociety Energy Conversion Conference-"International Forum on Energy Engineering", August 6-11, 1989, Washington, D.C.)
Abstract:  Not Available
+ Inhibiting Surface Flashover with Magnetic Fields
  M. Lehr, R. Korzekwa, H. Krompholz, M. Kristiansen, "Inhibiting Surface Flashover with Magnetic Fields", Proc. of the 6th International Symposium on High Voltage Engineering, New Orleans, La, Aug. 1989, Vol. 3, paper 48.09
Abstract:  Not Available
+ Inhibiting Surface Flashover with Magnetic Insulation
  M. Lehr, R. Korzekwa, H. Krompholz, M. Kristiansen, "Inhibiting Surface Flashover with Magnetic Insulation", Proc. of the 7th IEEE Pulsed Power Conference, Monterey, Ca, June 1989, p. 812
Abstract:  Not Available
+ Inhibition of Dielectric Surface Flashover with Magnetic Fields for Space Conditions
  M. Lehr, R. Korzekwa, H. Krompholz, M. Kristiansen, "Inhibition of Dielectric Surface Flashover with Magnetic Fields for Space Conditions", Proc. of the 19th International Conference on Phenomena in Ionized Gases, Belgrade, Yugoslavia, July 1989, p. 86
Abstract:  Not Available
Publication Year:  1988
+ Magnetic Insulation for Dielectric Surfaces
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "Magnetic Insulation for Dielectric Surfaces", Beams '88, 7th Int. Conference on High Power Particle Beams, Karlsruhe, FRG, July 1988, vol. 2, p. 1061
Abstract:  Not Available
+ Magnetic Insulation for Space Environment
  H. Krompholz, R. Korzekwa, M. Lehr, M. Kristiansen, "Magnetic Insulation for Space Environment" SPSE conference on Space Power and Power Conditioning, Los Angeles, Ca, January 1988, pg. 341
Abstract:  Not Available
+ Magnetic Insulation of Dielectric Surfaces for Space Environment
  R. Korzekwa, M. Lehr, H. Krompholz, M. Kristiansen, "Magnetic Insulation of Dielectric Surfaces for Space Environment", 1988 IEEE International Symposium on Electrical Insulation, Boston, Ma., June 1988, p. 50
Abstract:  Not Available
+ Methods of Increasing the Surface Flashover Potential in Vacuum
  L.L. Hatfield, G.R. Leiker, E.R Boerwinkle, H. Krompholz, R. Korzekwa, M. Lehr, M. Kristiansen, "Methods of Increasing the Surface Flashover Potential in Vacuum", 13th Symposium on Discharges and Electrical Insulation in Vacuum, Paris, France, 1988, vol. 1, p. 241 (invited)
Abstract:  Not Available
Publication Year:  1986
+ Slow Wave Line-Type Pulsers
  H. Krompholz, J. Cooper, J. Doggett, K. Schoenbach, G. Schaefer, "Slow Wave Line-Type Pulsers", 17th IEEE Power Modulator Symposium, Seattle, WA, June 1986, p. 207
Abstract:  Not Available
Publication Year:  1985
+ An e-beam controlled diffuse discharge switch
  K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, D. Skaggs, E. Strickland, "An e-beam controlled diffuse discharge switch", Proc. 5th IEEE Pulsed Power Conf., Arlington, VA, USA, 1985, p. 640
Abstract:  Not Available
+ An Electron-beam Controlled Diffuse Discharge Switch
  G. Schaefer, K. Schoenbach, M. Kristiansen, H. Krompholz, "An Electron-beam Controlled Diffuse Discharge Switch", Proc. XVII International Conference on Phenomena in Ionized Gases, Budapest, Hungary, 1985, p. 626
Abstract:  Not Available
+ Analysis of Electrode Surface Damage in High Energy Spark Gaps
  A.L. Donaldson, M. Kristiansen, H. Krompholz, M. Hagler, L. Hatfield, G. Leiker, P. Predecki, G. Jackson, "Analysis of Electrode Surface Damage in High Energy Spark Gaps", Proc. 5th IEEE Pulsed Power Conf., Arlington, VA, USA, 1985, p. 457
Abstract:  Not Available
+ Arc Current, Voltage, and Resistance in a High Energy, Gas Filled Spark Gap
  B. Maas, H. Krompholz, M. Kristiansen, M. Hagler, "Arc Current, Voltage, and Resistance in a High Energy, Gas Filled Spark Gap", Proc. 5th IEEE Pulsed Power Conference, Arlington, VA, USA, 1985, p. 99
Abstract:  Not Available
+ High Current Surface Discharge Switch
  P.M. Ranon, H. Krompholz, M. Kristiansen, L.L. Hatfield, "High Current Surface Discharge Switch", Proc. 5th IEEE Pulsed Power Conf., Arlington, VA, USA, 1985, p. 276
Abstract:  Not Available
+ Transmission Line Current Sensor
  H. Krompholz, K. Schoenbach, G. Schaefer, "Transmission Line Current Sensor", IEEE Instrumentation and Measurement Technology Conference, Tampa, Florida, USA 1985, p. 224
Abstract:  Not Available
Publication Year:  1984
+ An E-Beam Controlled Diffuse Discharge Switch
  K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, H. Harjes, D. Skaggs, "An E-Beam Controlled Diffuse Discharge Switch", 16th IEEE Power Modulator Symposium, Arlington, Virginia, USA 1984, p. 152
Abstract:  Not Available
+ Investigations of E-Beam Controlled Diffuse Discharge
  K. Schoenbach, G. Schaefer, M. Kristiansen, H. Krompholz, H. Harjes, D. Skaggs, "Investigations of E-Beam Controlled Diffuse Discharges", 4th Int. Symp. on Gaseous Dielectrics, Knoxville, TN, USA, 1984
Abstract:  Not Available
+ Recovery Measurements in a Spark Gap
  C. Yeh, H. Krompholz, M. Hagler, M. Kristiansen, "Recovery Measurements in a Spark Gap", 16th IEEE Power Modulator Symposium, Arlington, Virginia, USA 1984, p. 64
Abstract:  Not Available
Publication Year:  1983
+ E-Beam Triode for Multiple Submicrosecond Pulse Operation
  H. Harjes, K. Schoenbach, G. Schaefer, H. Krompholz, M. Kristiansen, G. Leiker, L. Hatfield, "E-Beam Triode for Multiple Submicrosecond Pulse Operation", 4th IEEE Pulsed Power Conference, Albuquerque, New Mexico, USA, 1983, p. 474
Abstract:  Not Available
+ Nanosecond Current Probe for High Voltage Experiments
  H. Harjes, J. Doggett, J. Gahl, K. Zinsmeyer, H. Krompholz, K. Schoenbach, G. Schaefer, M. Kristiansen, "Nanosecond Current Probe for High Voltage Experiments", 4th IEEE Pulsed Power Conference, Albuquerque, New Mexico, USA 1983, p. 396
Abstract:  Not Available

Book

Publication Year:  2007
+ Pulsed Gas Breakdown With High Overvoltages in Argon and Air
  E. Crull, H. Krompholz, A. Neuber, and L. Hatfield, "Pulsed Gas Breakdown With High Overvoltages in Argon and Air", Ultra-Wideband Short-Pulse Electromagnetics 7, F. Sabbath, Ed., Springer, NY, 2007, Page(s) 409-414.
Abstract:  Not Available
Publication Year:  2001
+ Windows and RF Breakdown
  A. Neuber, L. Laurent, Y. Y. Lau, and H. Krompholz, “Windows and RF Breakdown,” Chapter 10 (pages 325-375) in High-Power Microwave Sources and Technologies, R. J. Barker and E. Schamiloglu, Editors, IEEE Press 2001, ISBN 0.7803-6006-0.
Abstract:  Not Available