Center for Pulsed Power and Power Electronics.
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Dr. Andreas Neuber, P.E.

Co-Director, Center for Pulsed Power and Power Electronics
Paul Whitfield Horn Professor
AT&T Professor of Electrical and Computer Engineering

Contact Information

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

Phone: (806)834-8270
Fax: (806)742-1281
andreas.neuber@ttu.edu

Education

Research Interests

Other Interests

Projects

Awards

Professional Societies

Publications


Journal

Publication Year:  2016
+ Characteristics of a Four Element Gyromagnetic Nonlinear Transmission Line Array High Power Microwave Source
  J. Johnson, D. Reale, J. Krile, R. Garcia, W. Cravey, A. Neuber, J. Dickens, and J. Mankowski, "Characteristics of a Four Element Gyromagnetic Nonlinear Transmission Line Array High Power Microwave Source," Review of Scientific Instruments 87, 034706 (2016)
Abstract:  In this paper, a solid-state four element array gyromagnetic nonlinear transmission line high power microwave system is presented as well as a detailed description of its subsystems and general output capabilities. This frequency agile S-band source is easily adjusted from 2-4 GHz by way of a DC driven biasing magnetic field and is capable of generating electric fields of 7.8 kV/m at 10 m correlating to 4.2 MW of RF power with pulse repetition frequencies up to 1 kHz. Beam steering of the array at angles of ±16.7° is also demonstrated, and the associated general radiation pattern is detailed.

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+ Contact Extensions Over a High-k Dielectric Layer for Surface Electric Field Mitigation in High Power 4H-SiC Photoconductive Switches
  A. Chowdhury, D. Reale, R. P. Joshi, A. A. Neuber, and J. C. Dickens, "Contact Extensions Over a High-k Dielectric Layer for Surface Electric Field Mitigation in High Power 4H-SiC Photoconductive Switches." Accepted, 2016
Abstract:  We focus on a simulation study to probe the mitigation of electric fields, especially at the edges of metal contacts to SiC-based photoconductive switches. Field reduction becomes germane given that field-induced failures near contacts have been reported. A dual strategy of extending metal contacts to effectively spread the electric field over a larger distance and to employ HfO₂ as a high-k dielectric, is discussed. Simulation results show that peak electric fields can be lowered by up to ~67% relative to a standard design. Finally, our calculations predict that the internal temperature rise for a ~7-ns laser pulse and applied voltages around 20 kV (typical experimental values) would also be effectively controlled.

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+ Discrete Photon Implementation for Plasma Simulations
  A. Fierro, J. Stephens, S. Beeson, J. Dickens, and A. Neuber, "Discrete Photon Implementation for Plasma Simulations," Phys. Plasmas 23, 03506 (2016)
Abstract:  The self-produced light emission from pulsed plasma discharges and its impact on plasma development are challenging to characterize through simulation and modeling, chiefly due to the large number of radiating species and limited computer memory. Often, photo-processes, such as photo-ionization or photo-emission of electrons, are implemented through over-simplifying approximations or neglected altogether. Here, a method applicable to plasma simulations is implemented in a Particle-in-Cell /Monte Carlo Collision model, which is capable of discretely tracking photons and their corresponding wavelengths. Combined with the appropriate cross sections or quantum yields, a wavelength dependent model for photo-ionization or photo-emission may be implemented. Additionally, by resolving the wavelengths of each photon, an emission spectrum for a region of interest may be generated. Simulations for a pure nitrogen environment reveal that the calculated emission profile of the second positive system agrees well with the experimental spectrum of a pulsed, nanosecond discharge in the same spectral region.

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+ Investigation of a stripline transmission line structure for gyromagnetic nonlinear transmission line high power microwave sources
  D. V. Reale, J. M. Parson, A. A. Neuber, J. C. Dickens, and J. J. Mankowski, "Investigation of a stripline transmission line structure for gyromagnetic nonlinear transmission line high power microwave sources," Review of Scientific Instruments 87, 054704 (2016).
Abstract:  A stripline gyromagnetic nonlinear transmission line (NLTL) was constructed out of yttrium iron garnet ferrite and tested at charge voltages of 35 kV–55 kV with bias fields ranging from 10 kA/m to 20 kA/m. Typically, high power gyromagnetic NLTLs are constructed in a coaxial geometry. While this approach has many advantages, including a uniform transverse electromagnetic (TEM) mode, simple interconnection between components, and the ability to use oil or pressurized gas as an insulator, the coaxial implementation suffers from complexity of construction, especially when using a solid insulator. By moving to a simpler transmission line geometry, NLTLs can be constructed more easily and arrayed on a single substrate. This work represents a first step in exploring the suitability of various transmission line structures, such as microstrips and coplanar waveguides. The resulting high power microwave (HPM) source operates in ultra high frequency (UHF) band with an average bandwidth of 40.1% and peak rf power from 2 MW to 12.7 MW.

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+ Photoionization Capable, Extreme and Vacuum Ultraviolet Emission in Developing Low Temperature Plasmas in Air
  J. Stephens, A. Fierro, S. Beeson, G. Laity, D. Trienekens, R. P. Joshi, J. C. Dickens, and A. A. Neuber, "Photoionization Capable, Extreme and Vacuum Ultraviolet Emission in Developing Low Temperature Plasmas in Air." Plasma Sources Sci. Technol. 25 (2016) 025024 (11pp)
Abstract:  Not Available

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Publication Year:  2015
+ A Frequency Stable Vacuum-Sealed Tube High-Power Microwave Vircator Operated at 500 Hz
  J. M. Parson, C. F. Lynn, M. C. Scott, S. Calico, J. C. Dickens, A. A. Neuber, and J. J. Mankowski, "A Frequency Stable Vacuum-Sealed Tube High-Power Microwave Vircator Operated at 500 Hz," Electron Device Letters, (2015).
Abstract:  Operation of repetitive high-power microwave (HPM) sources is predominantly limited by thermal properties of anode and cathode materials. This paper presents a reflex-triode virtual cathode oscillator (vircator) capable of operating at 500 Hz at current densities between 100-200 A/cm2 for multiple burst durations of 1-2 seconds. Stable vircator operation under such a thermally punishing environment is facilitated by the use of a thin pyrolytic graphite anode. The results presented focus on two anode-cathode (A-K) gap spacings: 11 mm and 21 mm, which produce stable microwave radiation at 4.6 GHz and 1.6 GHz, respectively. Characteristic voltage, current and microwave waveforms in conjunction with short-time Fourier transforms (STFTs), frequency spectrographs and HPM power density data for 1,000 and 500 pulses at 1.6 GHz and 4.6 GHz, respectively, are presented.

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+ A high power microwave triggered RF opening switch
  S. Beeson, J. Dickens, A. Neuber, "A high power microwave triggered RF opening switch", Rev. Sci. Instrum., (2015).
Abstract:  A 4-port S-band waveguide structure was designed and fabricated such that a signal of any amplitude (less than 1 MW) can be switched from a normally closed state, <0.5 dB insertion loss (IL), to an open state >30 dB IL by initiating plasma in a gas cell situated at the junction of this waveguide and one propagating a megawatt level magnetron pulse. The 90/10 switching time is as low as 20 ns with a delay of ∼30 ns between the onset of the high power microwave pulse and the initial drop of the signal. Two ports of this device are for the high power triggering pulse while the other two ports are for the triggered signal in a Moreno-like coupler configuration. In order to maintain high isolation, these two sets of waveguides are rotated 90° from each other with a TE111 resonator/plasma cell located at the intersection. This manuscript describes the design and optimization of this structure using COMSOL 4.4 at the design frequency of 2.85 GHz, comparison of simulated scattering parameters with measured "cold tests" (testing without plasma), and finally the temporal waveforms of this device being used to successfully switch a low power CW signal from 2 W to <5 mW on a sub-microsecond timescale.

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+ A Model Study of the Role of Workfunction Variations in Cold Field Emission From Microstructures With Inclusion of Field Enhancements
  H. Qiu, R. P. Joshi, A. Neuber, and J. Dickens, "A Model Study of the Role of Workfunction Variations in Cold Field Emission From Microstructures With Inclusion of Field Enhancements," Semicond. Sci. Technol. 30, 105038 (2015).
Abstract:  An analytical study of field emission from microstructures is presented that includes position-dependent electric field enhancements, quantum corrections due to electron confinement and fluctuations of the workfunction. Our calculations, applied to a ridge microstructure, predict strong field enhancements. Though quantization lowers current densities as compared to the traditional Fowler–Nordheim process, strong field emission currents can nonetheless be expected for large emitter aspect ratios. Workfunction variations arising from changes in electric field penetration at the surface, or due to interface defects or localized screening, are shown to be important in enhancing the emission currents.

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+ Analysis of high field effects on the steady-state current-voltage response of semi-insulating 4H-SiC for photoconductive switch applications
  R. Tiskumara, R. P. Joshi, D. Mauch, J. C. Dickens, and A. A. Neuber, "Analysis of high field effects on the steady-state current-voltage response of semi-insulating 4H-SiC for photoconductive switch applications," J. Appl. Phys. 118, 095701 (2015)
Abstract:  A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC is carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, are comprehensively included. Results of our model match the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters are also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples can be discounted up to applied fields as high as ∼275 kV/cm.

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+ Anode Materials for High-Average-Power Operation in Vacuum at Gigawatt Instantaneous Power Levels
  Curtis F Lynn, Jonathan M. Parson, Michael C. Scott, Steve E. Calico, James C. Dickens, Andreas A. Neuber, and John J. Mankowski. "Anode Materials for High-Average-Power Operation in Vacuum at Gigawatt Instantaneous Power Levels." Electron Devices, IEEE Transactions on 62, no. 6 2044-2047 (2015)
Abstract:  The thermal behavior of several electrically conducting solids under high incident electron fluence in high vacuum was evaluated. At electron energies of up to ~200 keV, the depth-dose relationship for electron penetration into the materials was considered, and the resulting energy deposition profile from the surface was revealed to extend to a maximum of ~175 μm below the surface depending on the anode material. Black body radiation is considered as the major mechanism that balances the power deposited in the material on the timescales of interest. Comparing the radiated power density at the sublimation temperature for different materials, metallic/nonmetallic, revealed that pyrolytic graphite anodes may radiate over 20 times more power than metallic anodes before failure due to sublimation. In addition, transparent pyrolytic graphite anodes (with a thickness on the order of several tens of micrometer) potentially radiate up to 40 times that of metallic anodes, since heating by the electron beam is approximately uniform throughout the thickness of the material, thus radiation is emitted from both sides. Experimental results obtained from titanium and pyrolytic graphite anodes validate the thermal analysis.

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+ Evaluating the Performance of a Carbon-Epoxy Capillary Cathode and Carbon Fiber Cathode in a Sealed-Tube Vircator Under UHV Conditions
  E. Rocha, P. M. Kelly, J. M. Parson, C. F. Lynn, J. C. Dickens, A. A. Neuber, J. J. Mankowski, T. Queller, J. Gleizer, and Y. E. Krasik. "Evaluating the Performance of a Carbon-Epoxy Capillary Cathode and Carbon Fiber Cathode in a Sealed-Tube Vircator Under UHV Conditions." IEEE Trans. on Plasma Sci. 43, 2670-2675 (2015)
Abstract:  This paper evaluates the performance of a bimodal carbon fiber cathode and a carbon-epoxy multicapillary cathode operating within a reflex-triode sealed-tube virtual cathode oscillator (vircator). The experimental results reveal that both cathodes exhibit similar emission behavior, although with some significant operational differences. An eight-stage 84-J pulseforming network-based Marx generator serves to drive both cathodes at 250 kV and 3-4 kA with a ~70-ns pulsewidth. Both cathodes undergo conditioning over 10000 pulses to determine gas evolution as well as electrical changes over time. Gas evolution of both cathodes is observed using a residual gas analyzer to determine individual gas constituents. A comparison of diode voltage, diode current, RF output, and outgassing data for both cathodes during vircator operation over 10000 pulses is presented to quantify cathode performance in a sealed-tube vircator. Changes in cathode surface morphology, from virgin to postmortem, are discussed. Data for various anode-cathode gap distances, from 3 to 15 mm, are presented. The evolution of voltage and current inputs to the vircator is discussed.

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+ Evaluation of a Pulsed Ultraviolet Light-Emitting Diode for Triggering Photoconductive Semiconductor Switches
  D. Mauch, C. Hettler, W. W. Sullivan, A. A. Neuber, and J, Dickens, "Evaluation of a Pulsed Ultraviolet Light-Emitting Diode for Triggering Photoconductive Semiconductor Switches," IEEE Trans. Plasma Sci. 43, 2182-2186 (2015)
Abstract:  The power output, forward voltage, conversion efficiency, and spectral characteristics of a 365 nm ultraviolet light-emitting diode (LED) were measured for applications of triggering wide-bandgap photoconductive switches for pulsed power applications. Pulsed currents through the LED ranged from 125 mA up to 2.2 A at widths from 10 μs up to several seconds. Using time-resolved electroluminescence spectroscopy, peak emission was observed to occur at 368.5 nm for short pulses with a red-shift to 371.8 nm for pulses 8 s in duration. A peak light output of 4.1 W was measured for short pulses (<;50 μs) of 2.12 A, corresponding to six times the rated output specification. The LED was used to trigger a high-voltage photoconductive semiconductor switch (PCSS) at voltages up to 6 kV into a high-impedance load. The 365 nm LED is a promising candidate for optical triggering of PCSS devices.

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+ Heating Based Model Analysis for Explosive Emission Initiation at Metal Cathodes
  A. Majzoobi, R. P. Joshi, A. A. Neuber, and J. C. Dickens, "Heating Based Model Analysis for Explosive Emission Initiation at Metal Cathodes," AIP Advances 5, 127237, 2015.
Abstract:  This contribution presents a model analysis for the initiation of explosive emission; a phenomena that is observed at cathodesurfaces under high current densities. Here, localized heating is quantitatively evaluated on ultrashort time scales as a potential mechanism that initiates explosive emission, based on a two-temperature, relaxation time model. Our calculations demonstrate a strong production of nonequilibrium phonons, ultimately leading to localized melting. Temperatures are predicted to reach the cathode melting point over nanosecond times within the first few monolayers of the protrusion. This result is in keeping with the temporal scales observed experimentally for the initiation of explosive emission.

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+ High Power Lateral Silicon Carbide Photoconductive Semiconductor Switches and Investigation of Degradation Mechanisms
  D. Mauch, W. Sullivan III, A. Bullick, A. Neuber, J. Dickens, "High Power Lateral Silicon Carbide Photoconductive Semiconductor Switches and Investigation of Degradation Mechanisms," IEEE Trans. Plasma Sci. 43, 2021-2031, 2015.
Abstract:  Several generations of high power, lateral, linear mode, intrinsically triggered 4H-SiC photoconductive semiconductor switch designs and their performance are presented. These switches were fabricated from high purity semi-insulating 4H-SiC samples measuring 12.7 mm × 12.7 mm × 0.36 mm and were able to block dc electric fields up to 370 kV/cm with leakage currents less than 10 μA without failure. Switching voltages and current s up to 26 kV and 450 A were achieved with these devices and ON-state resistances of 2 Ω were achieved with 1 mJ of 355 nm laser energy (7 ns FWHM). After fewer than 100 high power switching cycles, these devices exhibited cracks near the metal/SiC interface. Experimental and simulation results investigating the root cause of this failure mechanism are also presented. These results strongly suggest that a transient spike in the magnitude of the electric field at the metal/SiC interface during both switch closing and opening is the dominant cause of the observed cracking.

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+ Material selection of a ferrimagnetic loaded coaxial delay line for phasing gyromagnetic nonlinear transmission lines
  J. M. Johnson, D. V. Reale, W. H. Cravey, R. S. Garcia, D. H. Barnett, A. A. Neuber, J. C. Dickens, and J. J. Mankowski. "Material selection of a ferrimagnetic loaded coaxial delay line for phasing gyromagnetic nonlinear transmission lines." Review of Scientific Instruments 86, 8, 084702 (2015)
Abstract:  Implementing nonlinear transmission line (NLTL) technology in the design of a high power microwave source has the benefits of producing a comparatively small and lightweight solid-state system where the emission frequency is easily tuned. Usually, smaller in physical size, single NLTLs may produce significantly less power than its vacuum based counterparts. However, combining individual NLTL outputs electrically or in free-space is an attractive solution to achieve greater output power. This paper discusses a method for aligning a four element NLTL antenna array with coaxial geometry using easily adjustable temporal delay lines. These delay lines, sometimes referred to as pulse shock lines or pulse sharpening lines, are placed serially in front of the main NLTL line. The propagation velocity in each delay line is set by the voltage amplitude of an incident pulse as well as the magnetic field bias. Each is adjustable although for the system described in this paper, the voltage is held constant while the bias is changed through applying an external DC magnetic field of varying magnitude. Three different ferrimagnetic materials are placed in the temporal delay line to evaluate which yields the greatest range of electrical delay with the least amount of variability from consecutive shots.

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+ Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory
  S. Lin, H. Wang, Y. Li, C. Liu, N. Zhang, W. Cui and A. Neuber, "Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory," Phys. Plasmas 22, 082114 (2015)
Abstract:  This paper presents a statistical theory for the initial onset of multipactor breakdown in coaxial transmission lines, taking both the nonuniform electric field and random electron emission velocity into account. A general numerical method is first developed to construct the joint probability density function based on the approximate equation of the electron trajectory. The nonstationary dynamics of the multipactor process on both surfaces of coaxial lines are modelled based on the probability of various impacts and their corresponding secondary emission. The resonant assumption of the classical theory on the independent double-sided and single-sided impacts is replaced by the consideration of their interaction. As a result, the time evolutions of the electron population for exponential growth and absorption on both inner and outer conductor, in response to the applied voltage above and below the multipactor breakdown level, are obtained to investigate the exact mechanism of multipactor discharge in coaxial lines. Furthermore, the multipactor threshold predictions of the presented model are compared with experimental results using measured secondary emission yield of the tested samples which shows reasonable agreement. Finally, the detailed impact scenario reveals that single-surface multipactor is more likely to occur with a higher outer to inner conductor radius ratio.

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+ Optically isolated, 2 kHz repetition rate, 4 kV solid-state pulse trigger generator
  D. H. Barnett, J. M. Parson, C. F. Lynn, P. M. Kelly, M. Taylor, S. Calico, M. C. Scott, J. C. Dickens, A. A. Neuber, and J. J. Mankowski, "Optically isolated, 2 kHz repetition rate, 4 kV solid-state pulse trigger generator," Rev. Sci. Instrum. 86, 034702 (2015).
Abstract:  This paper presents the design and operation characteristics of a solid-state high voltage pulse generator. Its primary utilization is aimed at triggering a gaseous spark gap with high repeatability. Specifically, the trigger generator is designed to achieve a risetime on the order of 0.1 kV/ns to trigger the first stage, trigatron spark gap of a 10-stage, 500 kV Marx generator. The major design components are comprised of a 60 W constant current DC-DC converter for high voltage charging, a single 4 kV thyristor, a step-up pulse transformer, and magnetic switch for pulse steepening. A risetime of <30 ns and pulse magnitude of 4 kV is achieved matching the simulated performance of the design.

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+ Optimizing drive parameters of a nanosecond, repetitively pulsed microdischarge high power 121.6 nm source
  J. Stephens, A. Fierro, D. Trienekens, J. Dickens, and A. Neuber, "Optimizing drive parameters of a nanosecond, repetitively pulsed microdischarge high power 121.6 nm source," Plasma Sources Science and Technology 24, 015013 (6pp) (2015).
Abstract:  Utilizing nanosecond high voltage pulses to drive microdischarges (MDs) at repetition rates in the vicinity of 1 MHz previously enabled increased time-averaged power deposition, peak vacuum ultraviolet (VUV) power yield, as well as time-averaged VUV power yield. Here, various pulse widths (30-250 ns), and pulse repetition rates (100 kHz-5 MHz) are utilized, and the resulting VUV yield is reported. It was observed that the use of a 50 ns pulse width, at a repetition rate of 100 kHz, provided 62 W peak VUV power and 310 mW time-averaged VUV power, with a time-averaged VUV generation efficiency of ~1.1%. Optimization of the driving parameters resulted in 1-2 orders of magnitude increase in peak and time-averaged power when compared to low power, dc-driven MDs.

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+ Particle-in-Cell Based Parameter Study of 12-Cavity, 12-Cathode Rising-Sun Relativistic Magnetrons for Improved Performance
  A. Majzoobi, R. P. Joshi, A. A. Neuber, and J. C. Dickens, "Particle-in-Cell Based Parameter Study of 12-Cavity, 12-Cathode Rising-Sun Relativistic Magnetrons for Improved Performance," AIP Advances 5, 107102 (2015).
Abstract:  Particle-in-cell simulations are performed to analyze the efficiency, output power and leakage currents in a 12-Cavity, 12-Cathode rising-sun magnetron with diffraction output (MDO). The central goal is to conduct a parameter study of a rising-sun magnetron that comprehensively incorporates performance enhancing features such as transparent cathodes, axial extraction, the use of endcaps, and cathode extensions. Our optimum results demonstrate peak output power of about 2.1 GW, with efficiencies of ∼70% and low leakage currents at a magnetic field of 0.45 Tesla, a 400 kV bias with a single endcap, for a range of cathode extensions between 3 and 6 centimeters.

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+ Robert Barker Memorial Session: Leadership in Plasma Science and Applications
  K. Becker, Kurt, B. Godfrey, E. Kunhardt, M. Laroussi, L. Ludeking, A. Neuber, E. Schamiloglu, A. Woods, Robert Barker Memorial Session: Leadership in Plasma Science and Applications," to be published in IEEE Trans Plasma Sci (2015).
Abstract:  This paper summarizes the Dr. Robert J. Barker memorial session at the IEEE International Conference on Plasma Science 2014. Each section summarizes progress in a plasma research area strongly influenced by Dr. Barker's leadership: 1) plasma medicine; 2) atmospheric-pressure plasmas; 3) high-power microwaves; 4) pulsed power; and 5) numerical simulation of plasmas. He had a profound influence on these and other plasma science applications, as well as on numerous individual researchers. He will be missed greatly.

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+ Self-Induced Gaseous Plasma as HPM Opening Switch Medium
  S. Lin, S. Beeson, C. Liu, J. Dickens, A. Neuber, "Self-Induced Gaseous Plasma as HPM Opening Switch Medium," Phys. Plasmas 22, 043509 (2015)
Abstract:  Self-induced gaseous plasma is evaluated as active opening switch medium for pulsed high power microwave radiation. The self-induced plasma switch is investigated for N2 and Ar environments under pressure conditions ranging from 25 to 700 Torr. A multi-pass TE111 resonator is used to significantly reduce the delay time inherently associated with plasma generation. The plasma forms under the pulsed excitation of a 4 MW magnetron inside the central dielectric tube of the resonator, which isolates the inner atmospheric gas from the outer vacuum environment. The path from the power source to the load is designed such that the pulse passes through the plasma twice with a 35 ns delay between these two passes. In the first pass, initial plasma density is generated, while the second affects the transition to a highly reflective state with as much as 30 dB attenuation. Experimental data revealed that virtually zero delay time may be achieved for N2 at 25 Torr. A two-dimensional fluid model was developed to study the plasma formation times for comparison with experimental data. The delay time predicted from this model agrees well with the experimental values in the lower pressure regime (error < 25%), however, due to filamentary plasma formation at higher pressures, simulated delay times may be underestimated by as much as 50%.

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+ Time-discretized Extreme and Vacuum Ultraviolet Spectroscopy of Spark Discharges in Air, N2 and O2
  D. Trienekens, J. Stephens, A. Fierro, J. Dickens, and A. Neuber "Time-discretized Extreme and Vacuum Ultraviolet Spectroscopy of Spark Discharges in Air, N2 and O2," Journal of Physics D: Applied Physics 49, 3, (2015).
Abstract:  In this paper we present time-discretized spectra of spark discharges in air, N2 and O2. In previous work, a system for temporally resolved spectral analysis of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission from spark discharges was presented, along with some initial results. As was noted in this paper, statistical variances and the lacking of an apparatus sensitivity profile limited the usability of the data obtained. We have investigated the cause of these variances and improved the setup to reduce their effect. We also investigated the apparatus sensitivity profile to correct the intensity of measured lines. Newly obtained spectra in dry air, N2 and O2 are presented. Air and N2 show high emission in the vicinity of 100 nm, where direct photoionization of molecular oxygen is possible, in the first 250 ns of the discharge. We conclude this emission originates from nitrogen, which has several intense molecular transitions in this region. This finding is confirmed by our experimental results which show the emission in this region is much lower in oxygen.

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Publication Year:  2014
+ 3-D Simulation of Low-Temperature Plasma Development Under Pulsed Conditions
  A. Fierro, J. Dickens, A. Neuber, "3-D Simulation of Low-Temperature Plasma Development Under Pulsed Conditions," IEEE Transactions on Plasma Science 42, 2864-2865, (2014).
Abstract:  The development of a low-temperature plasma in a needle-protrusion to plane gap is investigated utilizing a 3-D particle-in-cell/Monte Carlo collision method implemented to run on single NVIDIA graphics processing unit. In addition to electron collisions, the model includes field detachment, photon tracking, and a drift-diffusion approximation for positive ions. The simulated geometry tracks several million electrons with 15-μm spatial resolution.

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+ A Passive Measurement of Dissociated Atom Densities in Atmospheric Pressure Air Discharge Plasmas using Vacuum Ultraviolet Self-Absorption Spectroscopy
  G. Laity, A. Fierro, J Dickens, A Neuber, "A Passive Measurement of Dissociated Atom Densities in Atmospheric Pressure Air Discharge Plasmas using Vacuum Ultraviolet Self-Absorption Spectroscopy," Journal of Applied Physics 115, 123302, (2014).
Abstract:  We demonstrate a method for determining the dissociation degree of atmospheric pressure air discharges by measuring the self-absorption characteristics of vacuum ultraviolet radiation from O and N atoms in the plasma. The atom densities are determined by modeling the amount of radiation trapping present in the discharge, without the use of typical optical absorption diagnostic techniques which require external sources of probing radiation into the experiment. For an 8.0 mm spark discharge between needle electrodes at atmospheric pressure, typical peak O atom densities of 8.5e17 1/cm3 and peak N atom densities of 9.9e17 1/cm are observed within the first ∼1.0 mm of plasma near the anode tip by analyzing the OI and NI transitions in the 130.0-132.0 nm band of the vacuum ultraviolet spectrum.

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+ Bias-field controlled phasing and power combination of gyromagnetic nonlinear transmission lines
  D. V. Reale, J.-W. B. Bragg, N. R. Gonsalves, J. M. Johnson, A. A. Neuber, J. C. Dickens, and J. J. Mankowski, "Bias-field controlled phasing and power combination of gyromagnetic nonlinear transmission lines," Review of Scientific Instruments, 85, 054706 (2014).
Abstract:  Gyromagnetic Nonlinear Transmission Lines (NLTLs) generate microwaves through the damped gyromagnetic precession of the magnetic moments in ferrimagnetic material, and are thus utilized as compact, solid-state, frequency agile, high power microwave (HPM) sources. The output frequency of a NLTL can be adjusted by control of the externally applied bias field and incident voltage pulse without physical alteration to the structure of the device. This property provides a frequency tuning capability not seen in many conventional e-beam based HPM sources. The NLTLs developed and tested are mesoband sources capable of generating MW power levels in the L, S, and C bands of the microwave spectrum. For an individual NLTL the output power at a given frequency is determined by several factors including the intrinsic properties of the ferrimagnetic material and the transmission line structure. Hence, if higher power levels are to be achieved, it is necessary to combine the outputs of multiple NLTLs. This can be accomplished in free space using antennas or in a transmission line via a power combiner. Using a bias-field controlled delay, a transient, high voltage, coaxial, three port, power combiner was designed and tested. Experimental results are compared with the results of a transient COMSOL simulation to evaluate combiner performance.

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+ Conditioning of carbon fiber cathodes in UHV-sealed tubes at 200 A/cm²
  J. Parson, C. Lynn, J. Mankowski, M. Kristiansen, A. Neuber, J. Dickens, "Conditioning of carbon fiber cathodes in UHV-sealed tubes at 200 A/cm²," IEEE Trans. On Plasma Sci. 42, 2007-2014 (2014).
Abstract:  This paper presents a study on outgassing and electrical conditioning for three carbon fiber cathode types in a vacuum-sealed, high-power microwave virtual-cathode-oscillator (vircator) that operates in the low 1e-9 torr pressure regime. The three cathode types consist of a bare bimodal fiber structure, a bare unimodal fiber structure, and a cesium-iodide coated bimodal fiber structure with identical fiber coverage of 2% by area with a surface area of ~20 cm2. The electrodes are cleaned by a 1.2 kW, argon/oxygen microwave plasma prior to complete vircator assembly, followed by a 300 °C bake-out for 72 h. Each cathode was pulsed in a single pulse operation by an 80 J, low inductance Marx generator with an approximate pulsewidth of 100 ns full-width at half-maximum for 10000 current pulses. The data presented includes individual gas constituents, high-speed intensified charge coupled device (ICCD) imaging, and voltage and current waveforms. The conditioning process resulted in a gas load reduction of ~80% overall, with the indication that the bare bimodal fiber structure performed the best as diode power increased throughout the experiment, while the power decreased for the other tested cathode types.

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+ Emission Behavior of Three Conditioned Carbon Fiber Cathode Types in UHV-Sealed Tubes at 200 A/cm²
  J. Parson, C. Lynn, J. Mankowski, A. Neuber, and J. Dickens, "Emission Behavior of Three Conditioned Carbon Fiber Cathode Types in UHV-Sealed Tubes at 200 A/cm²," IEEE Trans. on Plasma Sci. 42, 3982 - 3988 (2014).
Abstract:  When subjected to high electric fields in vacuum, carbon fiber cathodes produce intense electron beams suitable for high-power microwave (HPM) generation at very high current densities. However, the production mechanisms of these intense electron beams are not fully understood. This paper presents the postmortem examination of three conditioned carbon fiber cathode types. The three cathode types consist of an uncoated, bare unimodal fiber structure, a bare bimodal fiber structure, and a cesium-iodide (CsI)-coated bimodal fiber structure, all with identical fiber coverage of 2% by area. Each cathode was conditioned prior to testing by single pulse operation driven by an 80 J Marx generator for 10 000 pulses. HPM, voltage, and current waveforms of each cathode are presented. The bare bimodal cathode radiated more microwave power than the CsI-coated cathode and bare unimodal cathode. Scanning electron microscopy imagery presents evidence of two emission mechanisms: 1) explosive electron emission and 2) surface flashover, which both were found on the CsI-coated cathode. In addition, no evidence of surface flashover was found on either uncoated cathode.

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+ Global Model for Total Delay Time Distribution of High Power Microwave Surface Flashover,
  S. Beeson, J. Dickens, and A. Neuber, "Global Model for Total Delay Time Distribution of High Power Microwave Surface Flashover," IEEE Trans. On Plasma Sci. 42, 3450-3457 (2014).
Abstract:  A global model of high-power microwave (HPM) window breakdown is elucidated. The model provides a practical approach for estimating the maximum microwave power and pulse length that can be transmitted for a given window geometry at varying background gas pressure. Based on recent experimental and modeling progress, the formative and statistical breakdown delay time contributions are included in the model. The provided details are intended to give the reader a starting point in designing an HPM system for which surface breakdown along the output window is a major concern. Spanning five orders of magnitude in power, four microwave bands, three orders of magnitude in pulsewidth, three orders of magnitude in pressure, and three different gas types, the model serves to determine the probability of breakdown for a given set of input parameters with the modest computational effort. Examples of how to use the model are given, and the results are compared with actual systems and measured experimental delay times.

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+ Graphics processing unit accelerated three-dimensional model for the simulation of pulsed low-temperature plasmas
  A. Fierro, J. Dickens, and A. Neuber, "Graphics processing unit accelerated three-dimensional model for the simulation of pulsed low-temperature plasmas," Physics of Plasmas 21, 123504 (2014).
Abstract:  A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10e6 particles with dynamic weighting and a total mesh size larger than 1e8 cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes.

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+ Imaging of Explosive Emission Cathode and Anode Plasma in a Vacuum-Sealed Vircator High-Power Microwave Source at 250 A/cm²
  J. Parson, J. Mankowski, J. Dickens, A. Neuber, "Imaging of Explosive Emission Cathode and Anode Plasma in a Vacuum-Sealed Vircator High-Power Microwave Source at 250 A/cm²," IEEE Trans. On Plasma Sci. 42, 2592 - 2593 (2014).
Abstract:  Cold cathode operation under high current densities leads to explosive electron emission (EEE) that contributes to early A-K gap closure. Hence, inconsistent vacuum conditions and, if utilized in a high power microwave device, degradation of microwave output power are observed. The EEE centers are known to produce localized plasmas on the surface of the cathode that release and ionize the electrode material. Further, low melting point material in the anode is released due to electron bombardment accompanied by a significant surface temperature increase. Postmortem analysis has revealed particles up to 50 μm in diameter embedded in the opposite electrode. High speed ICCD imaging during A-K gap operation enabled resolving the plasma's spatial characteristics in time. Images of cathode and anode plasma during the operation of a virtual cathode oscillator at 250 A/cm2 under ultrahigh vacuum conditions are presented.

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+ Influence of VUV illumination on breakdown mechanics: pre-ionization, direct photoionization, and discharge initiation
  J Stephens, A Fierro, J Dickens, and A Neuber, "Influence of VUV illumination on breakdown mechanics: pre-ionization, direct photoionization, and discharge initiation," Journal of Physics D: Applied Physics 47 (32), 325501 (2014).
Abstract:  A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N2-NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4e−18 cm2) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N2-NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.

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+ Micrometer-resolution high speed imaging of pulsed microdischarge ignition
  J. Stephens, A. Fierro, J. Dickens, and A. Neuber, "Micrometer-resolution high speed imaging of pulsed microdischarge ignition," IEEE Trans. On Plasma Sci. 42, 2652 - 2653 (2014).
Abstract:  The application of a short, pulsed excitation is known to allow for higher power deposition into microdischarges without the onset of instabilities. Here, a MOSFET-based high voltage pulser is used to drive a 50-torr argon microdischarge with short pulsed currents of ~75 A, with <100-ns full-width at half-maximum, and a repetition rate of 1 MHz. With this excitation, an average power density of ~1e13 W/m3 is achieved, with a peak power density~3e14 W/m3. A high speed iCCD camera is used to observe ignition processes and confirm the absence of unstable operation. The images were taken using a 5-ns gate time with a spatial resolution of ~2.5 μm.

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+ Nanosecond, repetitively pulsed microdischarge vacuum ultraviolet source
  J. Stephens, A. Fierro, B. Walls, J. Dickens, A. Neuber, "Nanosecond, repetitively pulsed microdischarge vacuum ultraviolet source," Applied Physics Letters 104 (7), 074105, (2014).
Abstract:  A microdischarge is driven by short pulses (80 ns FWHM) with peak current levels up to 80 A, with a repetition frequency of 1 MHz (1 pulse/μs) allowing for ∼550 W input power. Experiments in pure argon (Ar2*, 127 nm) and argon-hydrogen (Lyman-α, 121.6 nm) were conducted. Using short pulses, the argon excimer emission was not observed. Alternatively, Ar-H2 operated at both higher power and efficiency (0.63%) whenever pulsed. Using Ar-H2, the experiments result in an average generated vacuum ultraviolet power just above 3.4 W with a peak power of 42.8 W, entirely at Lyman-α.

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+ Piezoelectric Ignition of Nanocomposite Energetic Materials
  E. Collins, M. Pantoya, AA. Neuber, M. Daniels, "Piezoelectric Ignition of Nanocomposite Energetic Materials," Journal of Propulsion and Power 30 (1), 15-18, (2014).
Abstract:  Piezoelectric initiators are a unique form of ignition for energetic material because the current and voltage are tied together by impact loading on the crystal. This study examines the ignition response of an energetic composite composed of aluminum and molybdenum trioxide nanopowders to the arc generated from a lead zirconate and lead titanate piezocrystal. The mechanical stimuli used to activate the piezocrystal varied to assess ignition voltage, power, and delay time of aluminum-molybdenum trioxide for a range of bulk powder densities. Results show a high dielectric strength leads to faster ignition times because of the higher voltage delivered to the energetic. Ignition delay is under 0.4 ms, which is faster than observed with thermal or shock ignition. Electric ignition of composite energetic materials is a strong function of interparticle connectivity, and thus the role of bulk density on electrostatic discharge ignition sensitivity is a focus of this study. Results show that the ignition delay times are dependent on the powder bulk density with an optimum bulk density of 50%. Packing fractions and electrical conductivity were analyzed and aid in explaining the resulting ignition behavior as a function of bulk density. Read More: http://arc.aiaa.org/doi/abs/10.2514/1.B35034

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+ Semiempirical wide-range conductivity model with exploding wire verification
  J Stephens, J Dickens, and A Neuber, "Semiempirical wide-range conductivity model with exploding wire verification," Physical Review E 89 (5), 053102 (2014).
Abstract:  Based on well-established physical relationships, a semiempirical set of equations dictating the electrical conductivity of dense, strongly coupled, partially ionized copper is presented. With the empirical coefficients, the model is tuned to experimental conductivity data obtained from exploding wire experiments [A. W. DeSilva and J. D. Katsouros, Phys. Rev. E 57, 5945 (1998)]. The result is a wide-range conductivity model, with demonstrated accuracy from room temperature-density conditions to 0.01 g/cm3 and 30 kK. Using magnetohydrodynamic simulation the ability to utilize the conductivity model for predictive simulations is demonstrated. A complete electrical conductivity dataset for copper has been made available to the public.

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+ System for Time-Discretized VUV Spectroscopy of Spark Breakdown in Air
  D. Ryberg, A. Fierro, J. Dickens, and A. Neuber,"System for Time-Discretized VUV Spectroscopy of Spark Breakdown in Air," Rev. Sci. Inst. 85, 103109 (2014).
Abstract:  A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 μm. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization.

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+ Temporally resolved electron density of a repetitive, nanosecond pulsed microdischarge
  J. Stephens, A. Fierro, J. Dickens, and A. Neuber, "Temporally resolved electron density of a repetitive, nanosecond pulsed microdischarge" Journal of Physics D: Applied Physics 47, 465205 (2014).
Abstract:  Using high speed spectroscopic diagnostics, temporally resolved optical emission spectroscopy is performed on a nanosecond, repetitively pulsed microdischarge. The microdischarge operates in an argon-hydrogen gas mixture (99%/1%) to provide a Lyman-α vacuum ultraviolet emission. Based on the Stark broadening of the 486.1 nm, Balmer-β hydrogen transition, the temporally resolved electron density was determined. Experimental electron density data are compared with the results of a 0D rate equation model. Peak electron density is estimated to be 5.6e15 1 cm-3, corresponding to a ~0.25% degree of ionization. Using the approximate experimental ionization rate, the electron temperature is estimated to be ~3.5 eV.

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Publication Year:  2013
+ A compact 45 kV curve tracer with picoampere current measurement capability
  W.W. Sullivan, D. Mauch, A. Bullick, C. Hettler, A. Neuber, J. Dickens, "A compact 45 kV curve tracer with picoampere current measurement capability," Rev. Sci. Inst. 84, 034702 (2013).
Abstract:  This paper discusses a compact high voltage curve tracer for high voltage semiconductor device characterization. The system sources up to 3 mA at up to 45 kV in dc conditions. It measures from 328 V to 60 kV with 15 V resolution and from 9.4 pA to 4 mA with 100 fA minimum resolution. Control software for the system is written in Microsoft Visual C# and features real-time measurement control and IV plotting, arc-protection and detection, an electrically isolated universal serial bus interface, and easy data exporting capabilities. The system has survived numerous catastrophic high voltage device-under-test arcing failures with no loss of measurement capability or system damage. Overall sweep times are typically under 2 min, and the curve tracer system was used to characterize the blocking performance of high voltage ceramic capacitors, high voltage silicon carbide photoconductive semiconductor switches, and high voltage coaxial cable.

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+ All solid-state high power microwave source with high repetition frequency
  J.-W. Bragg, W. Sullivan, D. Mauch, A. Neuber, J. Dickens, "All solid-state high power microwave source with high repetition frequency," Review of Scientific Instruments, 84(5), 054703 - 054703-7 (2013)
Abstract:  An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm2, 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic material's axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Ω, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed.

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+ Ferrimagnetic Nonlinear Transmission Lines as High-Power Microwave Sources
  J.-W. Bragg, J. Dickens, and A. Neuber, “"Ferromagnetic Nonlinear Transmission Lines as High Power Microwave Source" IEEE Trans. on Plasma Sci. 41, 232-237, 2013.
Abstract:  Ferrimagnetic nonlinear transmission lines (NLTLs) have the potential to fill a high-power microwave niche where compact cost-effective sources are lacking. NLTLs utilize nonlinear ferrimagnetic properties and magnetization dynamics to provide ultrafast pulse rise times (100 ps or less) and microwave signals with peak power ranging from kilowatts to hundreds of megawatts. The frequency of operation has been shown to range from 900 MHz up to 5 GHz depending on geometry and external magnetic fields. NLTLs, theoretically, can be pulsed to tens of kilohertz with little to no variance in microwave signal between shots. This paper covers recent advances in ferrimagnetic-based NLTLs, specifically effects of applied and bias magnetic fields on peak power and frequency, as well as temperature dependence.

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+ Focused cathode design to reduce anode heating during vircator operation
  C. F. Lynn, J. C. Dickens, and A. A. Neuber, "Focused cathode design to reduce anode heating during vircator operation," Phys. Plasmas 20, 103113 (2013).
Abstract:  Virtual cathode oscillators, or vircators, are a type of high power microwave device which operates based on the instability of a virtual cathode, or cloud of electrons, which forms when electron current injected into the drift tube exceeds the space charge limited current within the drift tube. Anode heating by the electron beam during vircator operation ultimately limits achievable pulse lengths, repetition rates, and the duration of burst mode operation. This article discusses a novel cathode design that focuses electrons through holes in the anode, thus significantly reducing anode heating by the electrons emitted from the cathode during the first transit through the A-K gap. Reflexing electrons continue to deposit energy on the anode; however, the discussed minimization of anode heating by main beam electrons has the potential to enable higher repetition rates as well as efficiency and longer diode lifetime. A simulation study of this type of cathode design illustrates possible advantages.

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+ Material selection considerations for coaxial, ferrimagnetic-based nonlinear transmission lines
  J.-W. B. Bragg, J. C. Dickens, and A. A. Neuber, "Material selection considerations for coaxial, ferrimagnetic-based nonlinear transmission lines," J. Appl. Phys. 113, 064904, (2013).
Abstract:  The growing need for solid-state high power microwave sources has renewed interest in nonlinear transmission lines (NLTLs). This article focuses specifically on ferrimagnetic-based NLTLs in a coaxial geometry. Achieved peak powers exceed 30 MW at 30 kV incident voltage with rf power reaching 4.8 MW peak and pulse lengths ranging from 1-5 ns. The presented NLTL operates in S-band with the capability to tune the center frequency of oscillation over the entire 2-4 GHz band and bandwidths of approximately 30%, placing the NLTL into the ultra-wideband-mesoband category of microwave sources. Several nonlinear materials were tested and the relationship between NLTL performance and material parameters is discussed. In particular, the importance of the material's ferromagnetic resonance linewidth and its relationship to microwave generation is highlighted. For a specific nonlinear material, it is shown that an optimum relation between incident pulse magnitude and static bias magnitude exists. By varying the nonlinear material's bias magnetic field, active delay control was demonstrated.

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+ Plasma relaxation mechanics of pulsed high power microwave surface flashover
  S. Beeson, J. Dickens, and A. Neuber, "Plasma relaxation mechanics of pulsed high power microwave surface flashover," Physics of Plasmas 20(9), 093509 - 093509-9 (2013).
Abstract:  Microwave transmission and reflection characteristics of pulsed radio frequency field generated plasmas are elucidated for air, N2, and He environments under pressure conditions ranging from 10 to 600 torr. The pulsed, low temperature plasma is generated along the atmospheric side of the dielectric boundary between the source (under vacuum) and the radiating environment with a thickness on the order of 5 mm and a cross sectional area just smaller than that of the waveguide. Utilizing custom multi-standard waveguide couplers and a continuous low power probing source, the scattering parameters were measured before, during, and after the high power microwave pulse with emphasis on the latter. From these scattering parameters, temporal electron density estimations (specifically the longitudinal integral of the density) were calculated using a 1D plane wave-excited model for analysis of the relaxation processes associated. These relaxation characteristics ultimately determine the maximum repetition rate for many pulsed electric field applications and thus are applicable to a much larger scope in the plasma community than just those related to high power microwaves. This manuscript discusses the diagnostic setup for acquiring the power measurements along with a detailed description of the kinematic and chemical behavior of the plasma as it decays down to its undisturbed state under various gas type and pressure conditions.

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+ Simultaneous measurement of nitrogen and hydrogen dissociation from vacuum ultraviolet self-absorption spectroscopy in a developing low temperature plasma at atmospheric pressure
  G. Laity, A. Fierro, J. Dickens, A. Neuber, and K. Frank, "Simultaneous measurement of nitrogen and hydrogen dissociation from vacuum ultraviolet self-absorption spectroscopy in a developing low temperature plasma at atmospheric pressure," Appl. Phys. Letters, 102, 184104, (2013).
Abstract:  We demonstrate a method for determining the dissociation density of N and H atoms present in a developing low temperature plasma, based on the emission and self-absorption of vacuum ultraviolet radiation produced from the plasma. Spark plasmas are produced via pulsed discharge in N2/H2 mixtures at atmospheric pressure, where information on the dissociated densities of the constituent gas molecules is desired without employing invasive diagnostic techniques. By analyzing the self-absorption line profile of 121.5 nm Lyman-α H radiation emitted within the first ∼1.0 mm of plasma near the anode tip, a peak dissociated H atom concentration of 5.6e17 1/cm3 was observed ∼100 ns into spark formation, with an estimated electron density of 2.65 e18 1/cm3 determined from Stark broadening. Similarly, simultaneous line fitting of the N 120.0/124.3 nm emission profiles revealed a peak dissociated N atom concentration of 3.8e17 1/cm during the same discharge period.

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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.

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+ An Explosively Driven High Power Microwave Pulsed Power System
  M. Elsayed, A. Neuber, J. Dickens, J. Walter, M. Kristiansen, and L. Altgilbers, "An Explosively Driven High Power Microwave Pulsed Power System," Rev. Sci. Instrum. 83, 024705 (2012).
Abstract:  The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 l. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks.

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+ Design and Testing of Multi-Standard Waveguide Couplers
  S. Beeson and A. Neuber, "Design and Testing of Multi-Standard Waveguide Couplers," Rev. Sci. Instrum. 83, 034702 (2012).
Abstract:  Most applications that use waveguides are designed for a single frequency or single band of frequency, and thus the waveguide dimensions are chosen for single mode operation. In special cases where multiple frequencies across multiple bands are needed (i.e., probing the temporal response of decaying plasma using a cw source that is generated by a pulsed source), special techniques must be used in order to implement both sources into a single waveguide structure. This paper presents two types of couplers designed to implement x-band frequencies into an s-band system with a large coupling coefficient (< −10 dB) and small reflection coefficient (> −10 dB) at the design frequency of 11 GHz. Along with a discussion on the design procedure, a detailed description on the parameter optimization and initial values estimation is presented. The custom waveguide structures were tested utilizing an Agilent E8364B PNA network analyzer, and showed reasonable agreement with the simulated performance over the frequency range of interest.

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+ Electric field enhanced conductivity in strongly coupled dense metal plasma
  J. Stephens and A. Neuber, "Electric field enhanced conductivity in strongly coupled dense metal plasma," Phys. Plasmas 19, 060702 (2012).
Abstract:  Experimentation with dense metal plasma has shown that non-negligible increases in plasma conductivity are induced when a relatively low electric field (∼6 kV/cm) is applied. Existing conductivity models assume that atoms, electrons, and ions all exist in thermal equilibrium. This assumption is invalidated by the application of an appreciable electric field, where electrons are accelerated to energies comparable to the ionization potential of the surrounding atoms. Experimental data obtained from electrically exploded silver wire is compared with a finite difference hydrodynamic model that makes use of the SESAME equation-of-state database. Free electron generation through both thermal and electric field excitations, and their effect on plasma conductivity are applied and discussed.

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+ Experimental and Theoretical Evaluation of Surface Coated Exploding Wires
  J. Stephens, A. Neuber, and M. Kristiansen, "Experimental and Theoretical Evaluation of Surface Coated Exploding Wires," Phys. Plasmas 19, 032702 (2012).
Abstract:  This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 107 A/cm2 on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating.

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+ Investigation into the Temperature Dependence of Ferrimagnetic Nonlinear Transmission Lines
  J.-W. Bragg, J. Dickens, and A. Neuber, "Investigation into the Temperature Dependence of Ferrimagnetic Nonlinear Transmission Lines," IEEE Trans. Plasma Sci. 99, pp. 1-5 (2012).
Abstract:  In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between $-20 ^{circ}hbox{C}$ to 150 $^{circ}hbox{C}$, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 $^{circ}hbox{C}$; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.

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+ The Impact of Wire Environment on Electro-Explosive Fuse Performance
  J. Stephens, W. Mischke, A. Neuber, "The Impact of Wire Environment on Electro-Explosive Fuse Performance", to be published in IEEE Trans. Plasma Sci. (2012).
Abstract:  The environment surrounding an exploding wire is known be a controlling factor in electro-explosive fuse performance. Recent experiments have shown that the application of an insulating surface coating to the fuse wire can significantly increase the rate of impedance transition and impedance magnitude of the exploding wire. This paper discusses the performance of surface coated fuses tested in commonly used solid and gaseous media. For comparison, these experiments are compared to bare wire fuse experiments in identical environments. Previously developed exploding wire models are utilized to aid in the interpretation of the experimental fuse behavior. Differential wire voltage, voltage pulse length, and degree of post vaporization conduction (i.e., restrike) are discussed for each experiment.

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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.

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+ 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.

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+ Modeling statistical variations in high power microwave breakdown
  J. Krile and A. Neuber, "Modeling statistical variations in high power microwave breakdown," Appl. Phys. Lett. 98, 211502, (2011).
Abstract:  Flashover of high power microwave (HPM) vacuum isolation windows presents a serious design limitation of megawatt class HPM systems. The delay time from HPM radiation incident on the window to flashover development on the atmospheric side is critical. Previously developed modeling efforts have yielded reasonable correlation with experimentally observed average delay times while failing to capture any statistical variations. Simply preseeding the volume with an initial electron density is identified as inadequate to describe the source of initiatory electrons. The process of field assisted electron detachment is examined and shown to be a probable candidate for the initiatory electron generation.

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+ Phenomenology of Streamer Propagation during Pulsed Dielectric Surface Flashover
  G. Laity, A. Neuber, A. Fierro, L. Hatfield, "Phenomenology of Streamer Propagation during Pulsed Dielectric Surface Flashover," IEEE Transactions on Dielectrics and Electrical Insulation 18, pp. 946-953, (2011).
Abstract:  There is a growing demand for understanding the physics of surface flashover, as it relates to the breakdown of electric fields on high power systems in the aerospace community. Specifically, the quantitative role of vacuum ultraviolet (VUV) radiation which is self-produced during the initial nanoseconds of surface flashover is virtually unknown. An experiment was constructed which allows detailed electrical and optical measurements of VUV emission during the timescales in which streamers are propagating before the transition into spark discharge. Repeated surface flashover events are generated using a solid-state high voltage pulser, with breakdown recorded in a number of gases at atmospheric pressure. Streamers are photographed using fast optical imaging with 3 ns resolution. Fast voltage and current diagnostics revealed a number of distinct stages of streamer development ranging from the onset of cathode directed streamers to the sharp current rise during final voltage collapse. The emission of VUV radiation is discussed in context to the observed streamer and electrical characteristics.

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+ 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]

+ Spatially Resolved Vacuum UV Spectral Imaging of Pulsed Atmospheric Flashover
  G. Laity, A. Fierro, L. Hatfield, J. Dickens, and A. Neuber, "Spatially Resolved Vacuum UV Spectral Imaging of Pulsed Atmospheric Flashover," to be published in IEEE Transactions on Plasma Science, (2011).
Abstract:  Not Available

[PDF]

+ Statistical analysis of high power microwave breakdown surface flashover delay times in nitrogen with metallic field enhancements
  J. Foster, H. Krompholz, and A. Neuber, "Statistical analysis of high power microwave breakdown surface flashover delay times in nitrogen with metallic field enhancements," Phys. Plasmas 18, 113505, (2011).
Abstract:  The physical mechanisms that contribute to atmospheric breakdown induced by high power microwaves (HPMs) are of particular interest for the further development of high power microwave systems and related technologies. For a system in which HPM is produced in a vacuum environment for the purpose of radiating into atmosphere, it is necessary to separate the atmospheric environment from the vacuum environment with a dielectric interface. Breakdown across this interface on the atmospheric side and plasma development to densities prohibiting further microwave propagation are of special interest. In this paper, the delay time between microwave application and plasma emergence is investigated. Various external parameters, such as UV illumination or the presence of small metallic points on the surface, provide sources for electron field emission and influence the delay time which yields crucial information on the breakdown mechanisms involved. Due to the inherent statistical appearance of initial electrons and the statistics of the charge carrier amplification mechanisms, the flashover delay times deviate by as much as ±50% from the average, for the investigated case of discharges in N2 at pressures of 60–140 Torr and a microwave frequency of 2.85 GHz with 3 μs pulse duration, 50 ns pulse risetime, and MW/cm2 power densities. The statistical model described in this paper demonstrates how delay times for HPM surface flashover events can be effectively predicted for various conditions given sufficient knowledge about ionization rate coefficients as well as the production rate for breakdown initiating electrons.

[PDF]

Publication Year:  2010
+ Capacitor Evaluation for Compact Pulsed Power
  E. J. Matthews, M. Kristiansen, A. A. Neuber, "Capacitor Evaluation for Compact Pulsed Power", IEEE Transaction on Plasma Science 38, 500-508 (2010)
Abstract:  An evaluation of the energy density and efficiency of high-voltage capacitors, from various manufacturers, at voltages above their rated level is presented. Characteristics such as decreasing capacitance, decreasing efficiency, and increasing energy density are described. Data are taken from eight capacitors; six of which are composed of varying nonlinear ceramic materials as a dielectric, and the remaining two are wound capacitors, which were chosen to exhibit their linear characteristics. Rapid (1-100 ms) charging and discharging, similar to that of a generator operation at a repetition rate of more than 10 Hz with an ~100-ns pulse, were the conditions simulated in this particular test setup.

[PDF]

+ 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]

+ Modeling and Simulation of Simple Flux-Trapping FCGs Utilizing PSpice Software
  Young, A.; Neuber, A.; Kristiansen, M.; "Modeling and Simulation of Simple Flux-Trapping FCGs Utilizing PSpice Software"; Plasma Science, IEEE Transactions on Volume: 38 , Issue: 8 , Part: 1; Publication Year: 2010 , Page(s): 1794 - 1802
Abstract:  A novel modeling and simulation method for flux-trapping flux-compression generators (FT-FCGs) is presented, which utilizes PSpice circuit-simulation software to solve complex differential equations derived from circuit analysis. The primary motivation for the model development is the desire for a technique to rapidly design and prototype FT-FCGs for use as drivers in high-power microwave sources. The derivation of FT-FCG equations will be given, both in the ideal (lossless) and nonideal cases. For the nonideal case, three flux conservation coefficients are added to the equations to account for intrinsic flux loss in the circuit. Time-varying inductance curves are calculated using zero-dimensional models found in literature and adapted to fit this model. A simple FT-FCG design is used as an example to show the steps taken to complete a simulation. The same design was also fabricated and tested for comparison with predicted results from the model. A comparison of the waveforms acquired through simulation and experiment was found to result in good agreement for a given set of values for the flux conservation coefficients. A discussion of the derived equations, both lossless and nonideal, is given, as well as a discussion on the investigation of the impact of the three flux constants on the circuit. Analysis is offered on the results of this investigation, and conclusions are given on the effectiveness of this model to predict FT-FCG behavior.

[PDF]

+ Pulsed magnetic field excitation sensitivity of match-type electric blasting caps
  J. Parson, J. Dickens, J. Walter, A. Neuber, "Pulsed magnetic field excitation sensitivity of match-type electric blasting caps," Review of Scientific Instruments 81, pp. 105115-105115-7 (2010).
Abstract:  This paper presents a study on energy deposition and electromagnetic compatibility of match-type electroexplosive devices (EEDs), which recently have found more usage in pulsed power environments with high electromagnetic interference (EMI) background. The sensitivity of these devices makes them dangerous to intended and unintended radiation produced by devices commonly used in pulsed power environments. Match-type EEDs have been found to be susceptible to such low levels of energy (7-8 mJ) that safe operation of these EEDs is vital when in use near devices that produce high levels of pulsed EMI. The scope of this paper is to provide an investigation that incorporates results of similar studies to provide detonation characteristics of these EEDs. The three topics included in this study are sensitivity testing, modeling of the thermodynamic heat propagation, and electromagnetic compatibility from pulsed electromagnetic radiation. The thermodynamic joule heating of the primary explosive has been modeled by a solution to the 1D heat equation. A simple pulsed generator, Marx generator with an inductive load, was used for the electromagnetic compatibility assessment of the coupled field between the pulse generator and shorted EED. The results of the electromagnetic compatibility assessment relate the resistive, inductive, and capacitive components of the pulse generator to the area of the shorted EED.

[PDF]

+ Simulation of Compact Explosively-Driven Ferroelectric Generators
  D. Bolyard, T. Krile, A. Neuber, M. Kristiansen, "Simulation of Compact Explosively-Driven Ferroelectric Generators", IEEE Trans. On Plasma Sci. 38, 1008 - 1014 (2010)
Abstract:  Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm

[PDF]

+ System for Time Resolved Spectral Studies of Pulsed Atmospheric Discharges in the Visible to VUV Range
  G. Laity, A. Neuber, G. Rogers, K. Frank, "System for Time Resolved Spectral Studies of Pulsed Atmospheric Discharges in the Visible to VUV Range", accepted for publication to Review of Scientific Instruments, 2010.
Abstract:  Vacuum ultraviolet (VUV) emission is believed to play a major role in the development of plasma streamers in pulsed atmospheric discharges, but detection of VUV light is difficult in pulsed experiments at atmospheric pressures. Since VUV light is absorbed in most standard optical materials as well, careful attention must be given to the selection of the lens and mirror optics used in these studies. Of highest interest is the VUV emission during the initial stage of pulsed atmospheric discharges, which has a typical duration in the nanosecond regime. An experiment was designed to study this fast initial stage of VUV emission coupled with fast optical imaging of streamer propagation, both with temporal resolution on the order of nanoseconds. A repetitive solid-state high voltage pulser was constructed which produces triggered flashover discharges with low jitter and consistent pulse amplitude. VUV emission is captured utilizing both photomultiplier and intensified charge-coupled device detectors during the fast stage of streamer propagation. These results are discussed in context with the streamer formation photographed in the visible wavelength regime with 3 ns exposure time.

[PDF]

+ VUV Emission and Streamer Formation in Pulsed Dielectric Surface Flashover at Atmospheric Pressure
  G. Rogers, A. Neuber, G. Laity, K. Frank, J. Dickens, "VUV Emission and Streamer Formation in Pulsed Dielectric Surface Flashover at Atmospheric Pressure", accepted for publication in IEEE Trans. On Plasma Sci. (2010).
Abstract:  There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115–180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

[PDF]

Publication Year:  2009
+ Optimization of an FCG Based High Power Microwave System using Nonexplosive Pulsed Power
  C. Davis, A. Neuber, A. Young, J. Walter, J. Dickens, M. Kristiansen, "Optimization of an FCG Based High Power Microwave System using Nonexplosive Pulsed Power," IEEE Trans. On Plasma Science, 27, 2321- 2327 (Sept. 2009)
Abstract:  This paper presents a nonexplosive pulsed-power system that replicates the output current waveform of a flux compression generator (FCG). The primary purpose of this system is to efficiently test the power conditioning components of an explosively driven HPM system, while drastically reducing the time between tests which are inherent with explosive experiments. The power conditioning system (PCS) of the HPM system includes an energy-storage inductor, an electroexplosive opening switch (fuse), and a peaking gap and serves to match the FCG output characteristics with the HPM diode load requirements. A secondary purpose of the nonexplosive test bed is to provide data points which could be directly compared with those from explosively driven experiments. For this reason, a reflex-triode virtual cathode oscillator (vircator) was connected to the output of the nonexplosive system, and the results of which were compared with similar testing done with an FCG and a compact Marx generator. Since the behavior of the fuse is known to play a critical role in the performance of the PCS, a study was performed on the effect of different fuse designs on the overall performance of the PCS. Specifically, the quality of the electrical connection between the fuse wire array and the rest of the system was tested. Fuse design experiments were conducted with the nonexplosive test bed firing into a water resistor dummy load, which showed a 13% increase in peak load voltage and more than an 11% increase in energy transfer for fuses with improved wire-electrode connection strength. Some basic rules about fuse design, as well as conclusions on the performance of the PCS when driving an HPM load, are given.

[PDF]

+ Plasma Structures Observed in Gas Breakdown Using a 1.5-MW, 110-GHz Pulsed Gyrotron
  Y. Hidaka, E. M. Choi, I. Mastovsky, M. A. Shapiro, J. R. Sirigiri, and R. J. Temkin, G. F. Edmiston, A. A. Neuber, and Y. Oda, "Plasma Structures Observed in Gas Breakdown Using a 1.5-MW, 110-GHz Pulsed Gyrotron", Phys. Plasmas 16, 055702 (2009).
Abstract:  Not Available
+ Recent Advances in Explosive Pulsed Power
  L.L. Altgilbers, A.H. Stults, M. Kristiansen, A. Neuber, J. Dickens, A. Young, T. Holt, M. Elsayed, R. Curry, K. O’Connor, J. Baird, S. Shkuratov, B. Freeman, D. Hemmert, F. Rose, Z. Shotts, Z. Roberts, W. Hackenberger, E. Alberta, M. Rader, A. Dougherty, "Recent Advances in Explosive Pulsed Power,", Journal of Directed Energy, 3, 1-43 (2009)
Abstract:  Not Available
+ Short Pulse High Power Microwave Surface Flashover at 3 GHz
  J. Krile, L McQuage, G. Edmiston, J. Walter, A. Neuber, "Short Pulse High Power Microwave Surface Flashover at 3 GHz", IEEE Trans. On Plasma Sci. 37, pp. 2139-2145 (2009)
Abstract:  High power microwave (HPM) induced surface flashover is investigated in order to gain a better understanding of this phenomenon and reduce the limitations it imposes on transmitted power levels. This work builds on previous testing using a magnetron producing 5 MW for 4 ¿s at 2.85 GHz. Both the previous and current experimental setups are designed to produce a flashover on the high pressure side of a transmission window without the influence of a triple point. Limitations of the previous experiment included a maximum power of 5 MW and a pulse rise time of 50 ns. The current HPM source is an experimental virtual cathode oscillator (vircator), the output of which has been extensively characterized. The vircator is capable of producing 50 MW peak for 100 ns with an adjustable frequency from 3 to 5 GHz and a rise time of < 4 ns. The dominant modes of the vircator and magnetron are the circular TE11 and rectangular TE10 modes respectively, with the major electric field component in both setups normal to the direction of propagation, yielding comparable field geometries at the transmission window. The experimental setup permits the study of factors including gas pressure, composition, temperature, and air speed. Diagnostic equipment allows the analysis of power levels and flashover luminosity with sub-nanosecond resolution.

[PDF]

+ Variation of the statistical and formative time lags of high power microwave surface flashover utilizing a superimposed DC electric field
  J. Foster, M. Thomas, A. A. Neuber, "Variation of the statistical and formative time lags of high power microwave surface flashover utilizing a superimposed DC electric field," J. Appl. Phys. 106, pp. 063310-063310-4 (2009).
Abstract:  In an effort to investigate the physics involved in the initiation of high power microwave surface flashover, a strong direct current (dc) electric field was introduced to the flashover region. The primary objective of this study is to demonstrate that an external electric field can have a significant impact on the delay time for surface flashover. It has been observed experimentally that the statistical and formative delay times for surface flashover can be varied depending on the polarity of the applied dc field. This external electric field may sweep possible breakdown initiating electrons away from the flashover region prior to the application of the high power microwave pulse. A distinct increase in the average statistical delay was observed primarily with a superimposed dc field in a positive polarity geometry as well as a decrease in the formative delay for either polarity in 167 mbar of nitrogen.

[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]

+ Compact Electro-Explosive Fuses for Explosive Driven Pulsed Power
  D. R. McCauley, D. W. Belt, J. J. Mankowski, J. C. Dickens, A. A. Neuber, and M. Kristiansen, "Compact Electro-Explosive Fuses for Explosive Driven Pulsed Power", to appear in IEEE Transactions on Plasma Science (2008)
Abstract:  Compact electroexplosive fuses (EEFs) as part of an explosively driven system are of interest for the one-time single-shot generation of high-power pulses. For instance, the transition from a very large driving current produced by an explosively driven flux compression generator (FCG), i.e., low impedance, to a large voltage spike delivered to the load, i.e., high impedance, can be done using an inductive storage system and an EEF. Typically, the EEF can be as large as, if not larger than, the current driver attached to it, thus making it one of the largest components in the system. Reduction in the size of the fuse will allow for size reductions of the entire high-power microwave (HPM) system. The goal of optimizing an EEF as an opening switch is to produce the greatest voltage multiplication possible to drive a load under physical size constraints. To optimize the fuse, several parameters are taken into account, including, but not limited to, fuse material, fuse length, fuse shape, and quenching medium. Individual optimization of these parameters will lead to complete optimization of an EEF, therefore resulting in a compact fuse capable of consistently producing maximum voltage multiplication for HPM systems.

[PDF]

+ 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]

+ Fast-charging compact seed source for magnetic flux compression generators
  M. Elsayed, M. Kristiansen, A. Neuber, Fast-charging compact seed source for magnetic flux compression generators, 2008 Review of Scientific Instruments. Vol. 79, 124702.
Abstract:  Flux compression generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kiloampere range for midsized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark gaps and overvoltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS) using solid-state components for the switching scheme. The CSS developed is a system (0.005 m3 volume and weighing 3.9 kg) capable of delivering over 360 J (∼12 kA) into a 5.20 μH load with a trigger energy of microjoules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. A detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid-state components used in the system.

[PDF]

+ High Current Surface Flashover in a High Pressure SF6 Environment
  J. Krile, A. Neuber, R. Vela, "High Current Surface Flashover in a High Pressure SF6 Environment", to appear in IEEE Transactions on Plasma Science (2008)
Abstract:  A recent upgrade of Sandia National Laboratories' Z-machine has exposed a possible failure mode in the 5 MV laser-triggered gas switches (LTGS). During the closure of the cascade section of the switch, the surface flashover (SF) inside the dielectric switch housing occurred sporadically, affecting subsequent closing timing and damaging the switch housing. A small-scale experiment has been constructed to mimic conditions within the LTGS and to examine the survivability of various materials exposed to high-current SFs.

[PDF]

+ High Power Microwave Switching Utilizing A Waveguide Spark Gap
  J. Foster, G. Edmiston, M. Thomas, A. Neuber, High Power Microwave Switching Utilizing A Waveguide Spark Gap, Rev. Sci. Instrum. 79, 114701 (2008).
Abstract:  A reduction in the rise time of a 2.85 GHz high power microwave (HPM) pulse is achieved by implementing an overvoltaged spark gap inside a waveguide structure. The spark gap is oriented such that when triggered, the major electric field component of the dominant TE10 mode is shorted. The transition from a transmissive to a highly reflective microwave structure in a relatively short period of time (tens of nanoseconds) creates a means to switch multimegawatt power levels on a much faster timescale than mechanical switches. An experimental arrangement composed of the waveguide spark gap and a high power circulator is used to reduce the effective rise time of a HPM pulse from a U.S. Air Force AW/PFS-6 radar set from 600 ns down to 50 ns. The resulting HPM pulse exhibits a much more desirable excitation profile when investigating microwave induced dielectric window flashover. Since most theoretical discussions on microwave breakdown assume an ideal step excitation, achieving a “squarelike” pulse is needed if substantial comparison between experiment and theory is sought. An overview of the experimental setup is given along with relevant performance data and comparison with computer modeling of the structure.

[PDF]

+ High Power Microwave System
  T. Holt, A. Young, M. Elsayed, J. Walter, A. Neuber, and M. Kristiansen, "High Power Microwave System", to appear in IEEE Transactions on Plasma Science (2008)
Abstract:  Not Available
+ Images From the Development of a High-Power Microwave System
  T. Holt, A. Young, M. Elsayed, J. Walter, A. Neuber, and M. Kristiansen, Images From the Development of a High-Power Microwave System, IEEE Transactions on Plasma Science 36, 1414-1415 (2008).
Abstract:  A recently developed self-contained compact single-shot high-power microwave (HPM) system was tested and characterized. The explosive-driven system utilizes a reflex triode virtual cathode oscillator (vircator) as the HPM source. An open-shutter image acquired with a digital single-lens reflex camera during operation was used to show plasma development extending beyond the anode-cathode gap of the vircator. The plasma's self-emission is due to ionized material eroded and desorbed from both the cathode and the anode.

[PDF]

+ Imaging of High Power Microwave Induced Surface Flashover on a Corrugated Dielectric Window
  G. Edmiston, J. Krile, A. Neuber, "Imaging of High Power Microwave Induced Surface Flashover on a Corrugated Dielectric Window", to appear in IEEE Transactions on Plasma Science (2008)
Abstract:  Dielectric window flashover is a severe pulse-shortening phenomenon limiting the power levels radiated in high power microwave (HPM) systems. This type of flashover develops in regions under high field stress coinciding with the dielectric interfaces separating the vacuum and atmospheric pressure sections of a microwave system. The formation of plasma at the exit aperture of a transmitting system can have several detrimental effects, including premature termination of the radiated pulse and/or the reflection of potentially damaging levels of radiation back toward the microwave source. Experimental studies of HPM surface flashover have been conducted under a variety of conditions in the S-band at power levels up to 5 MW with the aim of quantifying the relative impact of parameters such as gas pressure, type, and window geometry. One particular geometry variant designed with grooves perpendicular to the major electric field component at the window surface exhibited superior flashover suppression characteristics when compared with smooth window geometries. Images of HPM surface flashover evolution on this corrugated dielectric window geometry are presented.

[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]

+ Utilization of a Nonexplosive Test Bed for Flux-Compression-Generator Electroexplosive Opening Switches
  D. W. Belt, J. J. Mankowski, A. A. Neuber, J.C. Dickens, M. Kristiansen, Utilization of a Nonexplosive Test Bed for Flux-Compression-Generator Electroexplosive Opening Switches, IEEE Transactions on Plasma Science 36, 2684 - 2690, (2008).
Abstract:  Helical flux compression generators (HFCGs) of a 50-mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3-muH inductor. One way to drive a high-power microwave source with an HFCG is by power conditioning, such as an inductive energy storage system (IESS). The output performance of the IESS is contingent upon the opening switch scheme, usually an electroexplosive fuse. Our previous work involving fuse parameter characterization has established a baseline for potential fuse performance. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor-based magnetic switching scheme to generate the near-exponential rise of the HFCG. The use of the nonexplosive HFCG test bed will allow the verification of scalability of the fuse parameter model and also allow testing of exotic fuse materials. The nonexplosive test bed has provided a more efficient method for electroexplosive switch development and has allowed us to expand the study of opening switches. We will also discuss the a priori calculated baseline fuse design and compare the experimental results of the gold-wire-material with the silver-wire-material baseline design. With the results presented, an accurate PSpice model applicable to our 45-kA HFCG systems will be available.

[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

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+ 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

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+ 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.

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+ 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

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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.

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+ Design and implementation of a flux compression generator nonexplosive test bed for electroexplosive fuses
  D Belt, J Mankowski, A Neuber, J Dickens, M Kristiansen, "Design and implementation of a flux compression generator nonexplosive test bed for electroexplosive fuses" Review of scientific instruments 77, 094702
Abstract:  Not Available
+ Design and implementation of a flux compression generator nonexplosive test bed for electroexplosive fuses
  D. Belt, J. Mankowski, A. Neuber, J. Dickens, and M. Kristiansen Center for Pulsed Power and Power Electronics, Texas Tech University, Lubbock, Texas 79409-3102; Departments of Electrical, Texas Tech University, Lubbock, Texas 79409-3102; and Computer Engineering and Physics, Texas Tech University, Lubbock, Texas 79409-3102 Review of Scientific Instruments, Vol 77, Article 094702 (2006) (7 pages)
Abstract:  Helical flux compression generators (HFCGs) of a 50 mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3 µH inductor. By utilizing an electroexplosive fuse, a large dI/dt into a coupled load is possible. Our previous work with a nonoptimized fuse has produced ~100 kV into a 15 load, which leads into a regime relevant for high power microwave systems. It is expected that ~300 kV can be achieved with the present two-stage HFCG driving an inductive storage system with electroexploding fuse. In order to optimize the electroexplosive wire fuse, we have constructed a nonexplosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor based, magnetic switching scheme to generate the near exponential rise of the HFCG. The varying inductance approach utilizes four stages of inductance change and is based upon a piecewise linear regression model of the HFCG wave form. The nonexplosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results. By utilizing the nonexplosive test bed, we hope to reduce the physical size of the inductive energy storage system and fuse substantially. ©2006 American Institute of Physics

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+ 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.

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+ 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
  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

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+ 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
+ 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
+ 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.

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+ Pulsed power technology development at Sandia.
  Dillon Heirman McDaniel, David Emery Bliss, Keith R LeChien, Larry Kevin Warne, James Albert Van Den Avyle, Kenneth William Struve, Andreas A Neuber, Kenneth Randel Prestwich, Joseph Ray Woodworth, Jane Marie Lehr, Keith Conquest Hodge, John Phillip Corley, Peter Eric Wakeland, Scott J MacGregor, Richard Miller, Randy McKee, John Eric Maenchen, Scott Kovaleski, Mark Edward Savage, David L Johnson, "Pulsed power technology development at Sandia." Sandia National Laboratories.
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
+ 5.1 Shocked Gases within the Flux Compression Generator Volume
  BL Freeman, AA Neuber, "5.1 Shocked Gases within the Flux Compression Generator Volume" Explosively Driven Pulsed Power: Helical Magnetic Flux Compression Generators 127
Abstract:  Not Available
+ 8.1 Seed Sources Basic
  JC Dickens, AA Neuber, "8.1 Seed Sources Basic" Explosively driven pulsed power: helical magnetic flux compression generators
Abstract:  Not Available
+ 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.

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+ Design and optimization of a compact, repetitive, high-power microwave system
  Y. J. Chen, A. A. Neuber, J. Mankowski, J. C. Dickens, and M. Kristiansen Texas Tech University, Center for Pulsed Power and Power Electronics, Lubbock, Texas 79409-3102 R. Gale Texas Tech University, Nano Tech Center, Lubbock, Texas 79409-3102 Review of Scientific Instruments, Vol 76, Article 104703 (2005) (8 pages)
Abstract:  The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate Marx generator for driving an high-power microwave (HPM) source are discussed. Benefiting from the large energy density of mica capacitors, four mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to ~18.5 characteristic Marx impedance. Using solely inductors, ~1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise time. Repetitive HPM generation with the Marx directly driving a small virtual cathode oscilator (Vircator) has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode, primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results. Design and testing of a low cost, all-metal Vircator cathode will also be discussed. ©2005 American Institute of Physics

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+ Design and optimization of a compact, repetitive, high-power microwave system
  YJ Chen, AA Neuber, J Mankowski, JC Dickens, M Kristiansen, R Gale, "Design and optimization of a compact, repetitive, high-power microwave system" Review of scientific instruments 76, 104703
Abstract:  Not Available
+ Generator Modeling
  M Giesselmann, I Smith, B Novac, A Neuber, "Generator Modeling", Explosively Driven Pulsed Power, 159-199
Abstract:  Not Available
+ Guest Editorial Special Issue on Power Modulators and Repetitive Pulsed Power
  Kirkici, H.; Neuber, A.; Umstattd, R.J.; IEEE Transactions on Plasma Science, Volume 33, Issue 4, Part 1, Aug. 2005 Page(s):1134 - 1135
Abstract:  Not Available

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+ Guest Editorial Special Issue on Power Modulators and Repetitve Pulsed Power
  H Kirkici, A Neuber, RJ Rmstattd, "Guest Editorial Special Issue on Power Modulators and Repetitve Pulsed Power" Plasma Science, IEEE Transactions on 33 (4), 1134-1154
Abstract:  Not Available
+ 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.

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+ 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.

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+ 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
+ Power Conditioning
  T Heeren, M Giesselmann, A Neuber, "Power Conditioning" Explosively Dirven Pulsed Power, 201-233
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.

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+ 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
+ Magnetic flux compression Generators
  Neuber, A.A.; Dickens, J.C.; Proceedings of the IEEE Volume 92, Issue 7, July 2004 Page(s):1205 - 1215
Abstract:  Magnetic flux compression generators offer the largest pulsed power output per unit size or weight when compared with other more conventional systems. They have found widespread use as pulsed power sources for hydrodynamics programs and high magnetic field research at national laboratories or in commercial applications, including exploration for oil and minerals and mine detection. Also, due to their nature as a true one-time-use device with superior energy density, a large portion of applications is defense related. A variety of basic magnetic flux compression generator designs have been developed and tested during the past four decades. All of them rely on the explosive-driven deformation of a system of conductors having an initial, preferably large, inductance. The most successful basic design is the helical flux compression generator, which is capable of producing a high-energy output into large impedance loads, just as it is needed for a practical pulsed power source. This paper will review the advances and state of the art of primarily helical magnetic flux compression generators mainly developed as pulsed power sources and will offer new insights gained as a result of a recently completed five-year AFOSR/DoD Multidisciplinary University Research Initiative program that studied the basic physics and engineering aspects of helical flux compression generators

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+ Modern Pulsed Power: Charlie Martin and Beyond
  Schamiloglu, E.; Barker, R.J.; Gundersen, M.; Neuber, A.A.; Proceedings of the IEEE on Pulsed Power Technology, Volume 92, Issue 7, July 2004 Page(s):1014 - 1020
Abstract:  In this introduction to the Special Issue on pulsed power and its applications, background information is provided for the nonspecialist to better understand the many challenges in designing pulsed power systems, and the wide diversity of applications that are now emerging. The approach to providing a tutorial on pulsed power technology is to make available to the reader the paper written by J. C. Martin which appeared in a Special Section of the Proceedings of the IEEE on pulsed power technology in June 1992. That paper is supplemented in this introduction with additional information that complements many of the invited papers composing this Special Issue.

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+ Quantification of ohmic and intrinsic flux losses in helical flux compression Generators
  Hernandez, J.C.; Neuber, A.A.; Dickens, J.C.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 32, Issue 5, Part 1, Oct. 2004 Page(s):1902 - 1908
Abstract:  Helical magnetic flux compression generators (MFCGs) are the most promising energy sources with respect to their current amplification and compactness. They are able of producing high current pulses required in many pulsed power applications with at least one order of magnitude higher energy density than capacitive storage with similar discharge characteristics. However, the main concern with MFCGs is their intrinsic flux loss that limits severely their performance and which is not yet well understood. In general, all flux losses have a differing degree of impact, depending on the generator's volume, current and energy amplification, size of the driven load, and angular frequency of armature-helix contact point. Although several computer models have been developed in the open literature, none of them truly quantify, starting from basic physics principles, the ohmic and intrinsic flux losses in helical MFCGs. This paper describes a novel method that provides a separate calculation of intrinsic flux losses (flux that is left behind in the conductors and lost for compression) and ohmic losses, being especially easy to implement and fast to calculate. We also provide a second method that uses a simple flux quantification, making a mathematical connection between the intrinsic flux losses, quantified by the first method, and the intrinsic flux losses observed in the generators. This second method can also be used to a priori estimate the MFCG performance. Further, we will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.

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+ 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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+ Scanning the Technology
  E. Schamiloglou, R. J. Barker, M. Gunderson, and A. A. Neuber, “Scanning the Technology,” Proceedings of the IEEE, vol. 92, pp. 1014-1020, 2004.
Abstract:  Not Available
+ 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
+ Helical MFCG For Driving A High Inductance Load
  Andreas A. Neuber, Juan-Carlos Hernández, James C. Dickens, Magne Kristiansen, Electromagnetic Phenomena, vol. 3, pp. 397-404, (2003).
Abstract:  Even at small dimensions of less than 0.5 meter in length end-initiated helical magnetic flux compression generators (MFCG) have at least one order of magnitude higher energy density (by weight or volume) than capacitive energy storage with similar discharge time characteristics. However, simple MFCGs with a single helix produce high output energy only into low inductance loads, thus producing several 100 kA of current at a voltage level of less than 10 kV. Many pulsed power devices require less current but a considerably higher voltage level. For effectively driving a high inductance load of several μH, a multistage MFCG design has been suggested. We successfully tested a dual stage MFCG with a total length of 250 mm, a helix inner diameter of 51 mm, which is wound with Teflon insulated stranded wire of different sizes in the range from AWG 12 to AWG 22. We have presently achieved an energy gain of ~ 13 into a 3 μH load and will discuss the generator performance based on experimental current/voltage waveforms and specify the observed losses.

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+ 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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+ High-speed optical diagnostic of an exploding wire fuse
  Giesselmann, M.; Heeren, T.; Neuber, A.; Walter, J.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 30, Issue 1, Part 1, Feb. 2002 Page(s):100 - 101
Abstract:  Explosive flux compression generators generate hundreds of kiloamperes and voltages of a few kilovolts. A power conditioning stage is required since typically voltages in the hundreds of kilovolts range are needed. Inductive energy storage systems with an opening switch provide the necessary voltage gain. In our application, the opening switch has been implemented as an exploding wire fuse. The voltage gain, and hence the performance of the system, is greatly dependent on the opening switch. We utilized high-speed optical imaging (up to 107 pictures/s) to assess the performance of the exploding wire fuse

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+ High-speed optical diagnostic of an exploding wire fuse
  M Giesselmann, T Heeren, A Neuber, J Walter, M Kristiansen, "High-speed optical diagnostic of an exploding wire fuse" Plasma Science, IEEE Transactions on 30 (1), 100-101
Abstract:  Not Available
+ Thermodynamic state of the magnetic flux compression generator volume
  Neuber, A.; Holt, T.; Dickens, J.C.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 30, Issue 5, Part 1, Oct. 2002 Page(s):1659 - 1664
Abstract:  The thermodynamic state of the gas trapped in the volume of helical magnetic flux compression generators was measured using optical emission spectroscopy and fast pressure probes. Three main stages of operation are discussed: (1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, as low as 2000 K; (2) the intermediate stage during 14-4 /spl mu/s before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV; (3) the last few /spl mu/s that reveal a highly compressed gas with temperatures of about 5000 K and pressures of about 1500 bar. Most experiments were conducted in air, initially at STP, some results are given for argon and sulfur hexafluoride initially at one atmosphere. Additionally, the thermodynamic state is linked to the electrical volume breakdown threshold via simple resistance measurements that were conducted in current-free flux compression generators.

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Publication Year:  2001
+ Advanced modeling of an exploding flux compression generator using lumped element models of magnetic diffusion
  M Giesselmann, T Heeren, A Neuber, M Kristiansen, "Advanced modeling of an exploding flux compression generator using lumped element models of magnetic diffusion" Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of Technical Papers 1 162-165
Abstract:  Not Available
+ Calculation of air temperature and pressure history during the operation of a flux compression generator
  X Le, J Rasty, A Neuber, J Dickens, M Kristiansen, "Calculation of air temperature and pressure history during the operation of a flux compression generator" Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of Technical Papers 2 (2) 939-942
Abstract:  Not Available
+ Electrical behavior of a simple helical flux compression generator for code benchmarking
  Neuber, A.; Dickens, J.; Cornette, J.B.; Jamison, K.; Parkinson, E.R.; Giesselmann, M.; Worsey, P.; Baird, J.; Schmidt, M.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 29, Issue 4, Aug. 2001 Page(s):573 - 581
Abstract:  A variety of basic magnetic flux compression (MFC) generator geometries have been tested during the last three decades. Though size and operating regimes differ widely, it is apparent that the helical flux compression generator is the most promising concept with respect to current amplification and compactness. Though the geometry of the helical generator (dynamically expanding armature in the center of a current carrying helix) seems to be basic, it turns out that the understanding of all involved processes is rather difficult. This fact is apparent from the present lack of a computer model that is solely based on physical principles and manages without heuristic factors. A simple generator was designed to address flux and current losses of the helical generator. The generator's maximum current amplitude is given as a function of the seed current and the resulting “seed-current” spread is compared to the output of state-of-the-art computer models. Temporally resolved current and current time derivative signals are compared as well. The detailed generator geometry is introduced in order to facilitate future computer code bench marking or development. The impact of this research on the present understanding of magnetic flux losses in helical MFC generators is briefly discussed

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+ High speed optical diagnostics of an exploding wire fuse for power conditioning of explosive flux compression generators
  M Giesselmann, T Heeren, A Neuber, J Walter, M Kristiansen, "High speed optical diagnostics of an exploding wire fuse for power conditioning of explosive flux compression generators" Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of Technical Papers 1 (1) 102-105
Abstract:  Not Available
+ Studies on a Helical Magnetic Flux Compression Generator
  A. Neuber, J. Dickens, M. Giesselmann, M. Kristiansen, B. Freeman, D. Dorsey, P. Worsey, J. Baird, M. Schmidt, “Studies on a Helical Magnetic Flux Compression Generator”, Paper 2000-01-3617, Journal of Aerospace, SAE 2000 Transactions, Section 1, ISBN 0-7680-0840-9, © 2001, p. 865…869.
Abstract:  Not Available
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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+ Studies on a helical magnetic flux compression generator
  A Neuber, J Dickens, M Giesselmann, M Kristiansen, B Freeman, D Dorsey, P Worsey, J Baird, M Schmidt, "Studies on a helical magnetic flux compression generator" SAE Transactions 109 (1), 865-869
Abstract:  Not Available
+ 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
  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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+ 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
+ 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
+ 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
+ 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
+ Finite Rate Chemistry and NO Molefraction in Non-Premixed Turbulent Flames
  A. Neuber, G. Krieger, M. Tacke, E. Hassel, and J. Janicka: Combustion and Flame, vol. 113, 198-211 (1998)
Abstract:  Finite rate chemistry is investigated in turbulent N2-diluted H2 diffusion flames by means of laser spectroscopic methods and a numerical model of combustion. The major species occurring in these hydrogen flames or the temperature are measured with SRS (Spontaneous Raman Spectroscopy) or Rayleigh spectroscopy. Some minor species, the radical OH and the pollutant NO, are simultaneously measured with linear LIF (Laser Induced Fluorescence). Multidimensional pdfs (probability density function) can be deduced from these measurements. Use is made of a numerical model with two principal variables, mixture fraction ξ and reaction progress variable η, as a basis for discussion of the experimental results. A k-ε-turbulence model together with a two-dimensional presumed pdf for the coupling of turbulence and chemistry are applied. So, experimental two-dimensional pdfs as well as mean values of ξ and η as functions of the position in the flame are deduced from the simultaneous measurements. The experimental and theoretical spatial maxima of the mean OH molefraction agree well in magnitude, despite the correlation coefficient between ξ and η of the measured pdf can be as high as 0.5. The neglect of this covariance for the calculation of the presumed pdf is quantified. It results in clear deviations for the OH molefraction. The experimental NO and OH molefractions are better simulated by flame calculations carried out with the presented combustion model than by the also shown calculations based on a single variable for description of chemistry.

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+ 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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Conference Paper/Presentation

Publication Year:  2015
+ Analysis of a tunable electrically small antenna
  B. Esser, S. Beeson, J. Dickens, J. Mankowski and A. Neuber, "Analysis of a tunable electrically small antenna," 2015 IEEE Pulsed Power Conference (PPC), Austin, TX, 2015, pp. 1-3.
Abstract:  A tunable, metamaterial-inspired, electrically small antenna topology is evaluated for a possible future use as the principle radiating element in a mobile Ionospheric Heating (MIH) system. The RF source signal is fed via a 50 Ω coaxial cable into a small semi-loop antenna (SLA). This inductively couples to a capacitively loaded loop (CLL) providing a natural 50 Ω match to the source. The resonant frequency of the antenna can be adjusted by varying the capacitance of the CLL via inserting a large permittivity dielectric. A simplified circuit model is used to show that the resonant frequency can be tuned between 40-100 MHz. Also, the maximum power handling capabilities achievable with this antenna topology at frequencies relevant to ionospheric heating (~ 10 MHz and below) are estimated.

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+ Characterization of the Optical Properties of GaN:Fe for High Voltage Photoconductive Semiconductor Switch Applications.
  V. Meyers, D. Mauch, J. Mankowski, J. Dickens, A. Neuber, "Characterization of the Optical Properties of GaN:Fe for High Voltage Photoconductive Semiconductor Switch Applications,", 2015 IEEE Pulsed Power Conference, pp. 1-4
Abstract:  The optical properties of bulk semi-insulating GaN:Fe are obtained to assess its future suitability as a high voltage photoconductive semiconductor switch (PCSS). The material properties of GaN:Fe hold significant promise to improve devices for pulsed power and other applications. Growth techniques of bulk GaN:Fe, which have hitherto been largely insufficient for commercial applications, are nearing the point that anticipatory characterization research is warranted. In this paper, the optical constants of bulk GaN:Fe (refractive index, absorption coefficient, and off-state dielectric function) were determined by optical reflection/transmission analysis. The results of this analysis are compared with a similar treatment of bulk 4H-SiC as well as possible elements of PCSS housing: Sylgard 184 elastomer, and EFI 20003/50013 electrical potting epoxy. The data presented provide foundational material characterization to enable assessment of the feasibility of GaN:Fe as a practical high voltage PCSS material. Beyond basic materials research, these properties inform design optimization in PCSS construction and implementation.

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Publication Year:  2011
+ Advanced Imaging of Pulsed Atmospheric Surface Flashover
  A. Fierro, G. Laity, L. Hatfield, J. Dickens, A. Neuber, "Advanced Imaging of Pulsed Atmospheric Surface Flashover," 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL, (2011).
Abstract:  Vacuum Ultraviolet (VUV) radiation is commonly thought to enhance streamer formation, as it is energetic enough to cause photoionization in the gaseous volume. Light with wavelengths below 180 nm, i.e. VUV, is highly absorbed in the atmosphere which increases the difficulty of measuring any VUV emission from gaseous breakdown at atmospheric pressure. Nevertheless, VUV emission from pulsed surface flashover at atmospheric conditions was previously recorded at Texas Tech. A second generation system was designed to image VUV and visible emission directly while also preserving the spatial profile. The visible emission is imaged through an air-side focused ICCD, while VUV emission is imaged through a vacuum spectrograph. The variable length gap was excited with a pulser designed for a 100 ns rise time and 50 kV peak output. Captured images of visible light emission from streamers produced in oxygen are diffuse whereas nitrogen produces streamers that are segmented. VUV spatial images taken in oxygen reveal stronger emission closer to the cathode region, while nitrogen produces a more distributed intensity profile across the gap. While MgF2 enabled transmission and measurement of VUV, streamer characteristics recorded in the visible light spectrum of surface flashover on BK7 dielectric windows were also investigated. In this paper, the observed streamer images in both visible and VUV wavelength range will be discussed as it relates to surface flashover at atmospheric pressure.

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+ Compact Electro-Explosive Fuse Optimization for a Helical Flux Compression Generator
  J. C. Stephens, A. A. Neuber, J. C. Dickens, M. Kristiansen, "Compact Electro-Explosive Fuse Optimization for a Helical Flux Compression Generator," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  This paper presents the optimization of a compact electro-explosive fuse designed for the power conditioning system to be driven by a helical flux compression generator (HFCG). An electro-explosive fuse interrupts the current flow from the HFCG through a storage inductor on a 50 to 100 ns timescale inducing a voltage large enough to close a peaking gap, which commutates the energy in the storage inductor into a 20 Ohm load at voltage levels above 200 kV. Experimental data has revealed that electro-explosive fuses with wires in closer proximity to one another have consistently produced lower pulsed voltages than fuses with larger wire spacings. This paper addresses possible factors that might contribute to this drop in performance. An electro-magnetic field solver is used to model the current redistribution in the fuse wires. The experimentally observed performance of compact fuses with varying wire spacings is presented.

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+ COMSED 2 - Recent Advances to an Explosively Driven High Power Microwave Pulsed Power System
  M. A. Elsayed, A. A. Neuber, A. J. Young, J. W. Walter, C. S. Anderson, S. L. Holt, J. R. Korn, M. Kristiansen, "COMSED 2 - Recent Advances to an Explosively Driven High Power Microwave Pulsed Power System," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL
Abstract:  Continued efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have led to improvements to the design, testing, and performance of a high power microwave (HPM) system, which is sourced by Lithium-ion Polymer batteries, a polypropylene capacitor, and high energetics. An indirectly seeded two-stage helical flux compression generator (HFCG) produces electrical energies in the kilo-Joule regime into a low impedance inductive load, varying from 2 μH to 3 μH. This high current output of the explosively driven generator is conditioned with a pure silver-wire-based electro-explosive opening switch, which reaches voltage levels in excess of 300 kV into a 18 Ohm load. Upon reaching levels high enough to close an integrated peaking switch, this high voltage is sufficient to drive a reflex triode virtual cathode oscillator, also known as a Vircator, into radiation. The Vircator employed in the system has reached microwave radiation levels well over 100 Megawatts from a cavity volume of less than 5 Liters at a microwave frequency of a few GHz. The complete system is governed through a microcontroller that regulates seed and detonator charging levels as well as discharge times using built-in feedback diagnostics. The complete system aforementioned is constrained to 15 centimeter diameter and measures 183 centimeter in length with an overall volume of less than 34 Liters. No external power or vacuum pumping for the HPM tube is required. This report will discuss recent design advancements and improvements on the HPM system and its sub-components that include the compact seed source, HFCG, and the power conditioning system. Improved safety features implemented as well as novel diagnostic integration will be discussed as well.

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+ 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.

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+ Diagnostic Measurements on Explosive Emission Cathodes Operating at High Current Densities and UHV Pressures
  C. F. Lynn, J. Walter, A. Neuber, M. Kristiansen, "Diagnostic Measurements on Explosive Emission Cathodes Operating at High Current Densities and UHV Pressures," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  Not Available
+ Guest Editorial, IEEE Transactions on Dielectrics and Electrical Insulation
  A. Neuber, B. M Novac, Guest Editorial, IEEE Transactions on Dielectrics and Electrical Insulation 18, pp. 937-937 (2011).
Abstract:  Not Available

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+ Improvements to a Small Scale Linear Transformer Driver
  D. W. Bolyard, A. Neuber, J. Krile, M. Kristiansen, "Improvements to a Small Scale Linear Transformer Driver," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  A linear transformer driver (LTD) is being constructed at Texas Tech University's Center for Pulsed Power and Power Electronics with the goal to achieve roughly 100 ns pulse width into an 18 Ohm load and energy densities similar to or greater than a previously designed and built 500 J compact Marx generator. A single experimental LTD stage, previously constructed, tested, and reported on by TTU, has undergone several improvements as well as the inclusion of additional diagnostics. Testing at a charging voltage of 14 kV into a 1.7 Ω resistive load has experimentally achieved a 10% to 90% risetime of ~60 ns, peak voltage of 9.2 kV, peak current of 5.5 kA, and instantaneous power of 50.6 MW. Simulations of the LTD stage correlate closely with the experimental results. Extrapolating the simulations to the final desired charging voltage indicate peak voltages and peak currents exceeding 15 kV and 10 kA respectively, with instantaneous power greater than 150 MW from a single LTD stage. The design for a multi-stage LTD using magnetic cores with a flux swing of 6 kV-μs will be presented. Improvements in diagnostics and modeling of the LTD will also be discussed as well.

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+ Initial Anode Optimization for a Compact Sealed Tube Vircator
  J. Walter, J. Vara, C. Lynn, J. Dickens, A. Neuber, M. Kristiansen, "Anode Optimization for a Compact Sealed Tube Vircator," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  During the development and optimization of a compact sealed tube virtual cathode oscillator (vircator) at Texas Tech University, it has become apparent that processes at the anode have a significant impact on tube performance. The impact of the high energy, high current density (100-200 A/cm2 or higher) beam on the anode will cause outgassing, plasma production, and anode melting and material ejection. The emitted material expands, eventually impacting the anode transparency and (combined with the plasma formed at the cathode) shorting out the anode-cathode gap. This expansion limits the maximum radiated pulse width, and can also limit the peak output power. The residual evolved gas also negatively impacts the maximum repetition rate of the tube. An effort is underway to study the thermal behavior, gases evolved, and transparency versus time for different vircator anode materials and material treatments. Several different anode materials are under investigation, including stainless steel, copper tungsten, tantalum, nickel, and molybdenum. The effect of different treatments on the anodes before tube assembly is also being studied. The gases that are evolved during operation have been characterized utilizing pressure and residual gas analyzer measurements. The pre-shot background pressure in the tube is in the ultra-high vacuum range (10-8 to 10-9 Torr), and the vircator is not pumped on during firing. The data collected for the different materials is presented.

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+ Investigation of the Transmission Properties of High Power Microwave Induced Surface Flashover Plasma
  S. Beeson, J. Foster, J. Dickens, A. Neuber, "Investigation of the Transmission Properties of High Power Microwave Induced Surface Flashover Plasma," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  When dealing with the propagation of High Power Microwaves (HPM), special precautions must be used to prevent the onset of plasma generation. In this paper, we investigate the plasma located on the high pressure side of the dielectric boundary separating the vacuum environment of the microwave source from the high pressure environment of the transmitting medium, e.g., atmosphere. Because the collisional ionization rates are a monotonously increasing function of Eeff/p in the range of interest, the effective electric field normalized with pressure, implementation of HPM in high altitude (low pressure) environments are subject to dielectric breakdown due to this generated plasma, more than at sea-level altitudes. Dielectric breakdown causes the interruption in transmission of electromagnetic radiation due to the reflection and absorption properties of the plasma generated on the dielectric surface. In this paper, transmission, reflection, and absorption data is presented for plasma generated under various pressures ranging from 5 to 155 torr in N2 and air environments. In addition, seed electrons from UV illumination of the dielectric surface and physical vapor deposited metallic points are implemented and their implications to the overall transmission properties are discussed.

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+ Nanosecond-Scale Spectroscopy of Vacuum Ultraviolet Emission from Pulsed Atmospheric Discharges
  G. Laity, A. Neuber, A. Fierro, J. Dickens, L. Hatfield, "Nanosecond-Scale Spectroscopy of Vacuum Ultraviolet Emission from Pulsed Atmospheric Discharges," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  This paper describes a 2nd-generation system for directly studying the emission of vacuum ultraviolet (VUV) light from pulsed dielectric surface flashover at atmospheric pressure. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is virtually unexplored, and yet could be a significant factor in the physics of fast breakdown of directed energy systems (such as MW-class high power microwave devices) in the aerospace community. First generation experiments at Texas Tech University have shown that VUV emission corresponding to nitrogen and oxygen excitation in the energy range 8 - 10 eV is easily produced, but the use of MgF2 optics inhibited future work with existing hardware due to the transmission cutoff of this dielectric material and chromatic aberration if used as a lensing medium. In an effort to enhance the detection capabilities of our hardware in the wavelength range from 115 - 135 nm, the current system utilizes a custom designed set of off-axis parabolic MgF2-Aluminium coated mirrors as the primary focusing element. High resolution spectroscopy with the upgraded system resulted in the observation of the nitrogen doublet at 149.5 nm, leading to a better fit for the appropriate line broadening parameters for an approximate 10 eV Boltzmann electronic temperature. Evidence of self-absorption for HI (121.5 nm) provides new insight into the generation of space charge in these plasma structures, which has been investigated quantitatively in both SF6-H2 and N2-H2 mixtures.

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+ Re-distribution of MRSA (Methicillin-resistant Staphylococcus aureus) during the laundering (normal machine washing) process on towel
  A. Laury, K. Fermin, D. Stull, A. Neuber, C. Brooks, T. Brashears, C. Alvarado, M. Brashears," Re-distribution of MRSA (Methicillin-resistant Staphylococcus aureus) during the laundering (normal machine washing) process on towels," ASM 2011, May 20th - 24th, New Orleans, LA.
Abstract:  Not Available
+ Reduction of MRSA (Methicillin-resistant Staphylococcus aureus) on towels utilizing Targeted Directional Microwave technology
  A. Laury, K. Fermin, D. Stull, A. Neuber, C. Brooks, T. Brashears, C. Alvarado, M. Brashears," Reduction of MRSA (Methicillin-resistant Staphylococcus aureus) on towels utilizing Targeted Directional Microwave technology," ASM 2011, May 20th - 24th, New Orleans, LA.
Abstract:  Not Available
+ Reduction of Salmonella on jalapeño peppers, peanuts, and dry dog food utilizing targeted directional microwave technology
  A. Laury, K. Fermin, D. Stull, A. Neuber, C. Brooks, C. Alvarado, T. Brashears, M. Brasheers, "Reduction of Salmonella on jalapeño peppers, peanuts, and dry dog food utilizing targeted directional microwave technology," Annual Meeting of the International Association for Food Protection, July 31 - August 3, Milwaukee, WI, 2011
Abstract:  Not Available
+ Temperature Dependence of Ferrimagnetic Based Nonlinear Transmission Line
  J.-W. B. Bragg, J. Dickens, A. Neuber, "Temperature Dependence of Ferrimagnetic Based Nonlinear Transmission Line," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  Ferrite loaded nonlinear transmission lines (NLTLs) are able to act as high power microwave sources, utilizing the nonlinearities present in ferrimagnetic materials and the excitation of damped gyromagnetic precession at high incident power levels. Ferrimagnetic properties depend greatly on operating temperatures; therefore, there exists a need to know the ideal temperature at which to operate ferrite loaded NLTLs. Ferrites are chilled or heated to a certain temperature for a time suitable to allow internal ferrite temperature uniformity. Experimental temperatures ranged from approximately -20 °C up to 150 °C, which is slightly above the Curie temperature of the loaded ferrites. This temperature range allows observation of precession dependence on temperature while maintaining ferrimagnetic properties and a single look at the behavior outside the ferrimagnetic regime. Above the Curie temperature the loaded ferrites become paramagnetic and lose ferrimagnetic properties. The design, testing, and results are detailed for an NLTL measuring 0.3 m length and ferrite inner and outer diameters of 3 mm and 6 mm respectively. Figures comparing output waveforms at different temperatures, output power versus temperature, and output frequency versus temperature are shown.

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+ The Effects of Stator Insulation Material and Methods of Fabrication on the Performance of Compact Helical Flux Compression Generators
  C. S. Anderson, A. A. Neuber, M. A. Elsayed, A. J. Young, "The Effects of Stator Insulation Material and Methods of Fabrication on the Performance of Compact Helical Flux Compression Generators," to be published in Proceedings of the 18th IEEE International Pulsed Power Conference, June 19th - 23rd, Chicago, IL (2011)
Abstract:  Helical Flux Compression Generators, HFCGs, are powerful high current sources for pulsed power applications. Due to the single shot nature of HFCGs, electrical output reproducibility is of great importance. One factor known to contribute to unpredictable performance is mechanical inconsistencies introduced during manufacturing of the stator. In an attempt to minimize these deviations during productions, two different winding forms for stator coils, designed to ensure repeatable generator dimensions, turn and coil pitch, were investigated. The differences between the methods were quantified by comparison of measurements made of the physical parameters of the coil (i.e. radius, inductance, etc.), as well as analysis of experiments conducted with the HFCGs fired into a 3 μH load inductor. With any particular fabrication method, the stator insulation material has a distinct impact on generator operation. Quad-built Polyimide coated magnet wire as stator insulation material and Teflon Fluorinated Ethylene Propylene (FEP) as field coil insulation material were investigate to improve HFCG performance. Insulation testing was carried out by firing HFCGs into the inductive load mentioned above. Experimental data and analysis, as well as conclusions on insulation material, will be presented along with a brief discussion of the optimum fabrication method.

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Publication Year:  2010
+ A Low Impedance 500 kV 2.75 kJ Marx Generator as Testbed for Vacuum Diodes
  C. Lynn, A. Neuber, E. Matthews, J. Walter, M. Kristiansen, "A Low Impedance 500 kV 2.75 kJ Marx Generator as Testbed for Vacuum Diodes," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  A low impedance Marx generator was developed as part of a test bed for vacuum diodes of various electrode materials and geometries. The generator supplies sufficient energy to initiate and sustain the typically unwanted plasma formation within the diode; which facilitates the observation of the plasma, current uniformity, and electron current densities of various diode structures. The generator consists of ten stages; each stage utilizes a 220 nF 50 kV capacitor, with a series inductance of ~20 nH. When charged to the rated voltage of the capacitors the energy density of the complete generator with case, spark gaps, insulation, etc., is 19.2 mJ/cm3; this is roughly the energy density of a typical ceramic doorknob capacitor without any supporting structure or isolation. The energy density of the capacitors utilized in the Marx generator by themselves is 104 mJ/cm3. Fired into a low inductance short, the ringing frequency was measured to be 1.4 MHz resulting in an output impedance of 5.2 Ω. Erection of the Marx required adding forward feeding capacitors as the stray capacitance to ground is smaller than the capacitance (~60 pF) of the low inductance, low profile spark gap switches. The design and construction of this generator are discussed as well as selected experimental results obtained with the generator.

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+ 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]

+ Atmospheric Flashover in a Symmetric Electric Field Geometry
  G. Rogers, A. Neuber, L. Hatfield, G. Laity, K. Frank, J. Dickens, "Atmospheric Flashover in a Symmetric Electric Field Geometry," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, surface flashover across a magnesium fluoride (MgF2) window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. That is, neither electrode was favored with respect to flashover/breakdown initiation. A semiconductor-switched 32 kV pulse with 140 ns rise time was applied to the 8 mm wide flashover gap in air, nitrogen, and oxygen at atmospheric pressure. Fast voltage and current measurements along with nanosecond imaging revealed four stages of flashover development: (1) Onset of a cathode directed streamer with a charge on the order of 100 pC and traveling with a speed of ~1 mm/ns at a macroscopic field level of ~10 kV/cm associated with a slow current rise (on the order of 10-3 A/ns) temporarily augmented by (2) a 5 ns wide current spike at the moment when the streamer reaches the cathode followed by (3) a cathode directed streamer focused toward the center of the flashover gap with a slow rising current leading to (4) a sharp current rise (on the order of 10 A/ns) reaching roughly a circuit limited 45 A about 20 ns after the return strike meets an anode directed streamer. Although present in all tested gases, the current spike at the end of stage (2) is most different for all three gases and having the greatest impact in air.

[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
+ COMSED 1 - A Compact, Gigawatt Class Microwave Source Utilizing Helical Flux Compression Generator Based Pulsed Power
  A. Young, A .Neuber, M. Elsayed, J. Korn, J. Walter, S. Holt, J. Dickens, M. Kristiansen, L. Altgilbers, "COMSED 1 - A Compact, Gigawatt Class Microwave Source Utilizing Helical Flux Compression Generator Based Pulsed Power," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA. Invited.
Abstract:  Recent progress in the development of a compact, portable, explosively-driven high power microwave source is presented. The envelope to which the system must fit has a 15 cm diameter, which means each sub-system fits within this dimension, with an optimized overall length. The system includes an autonomous prime energy source, which provides the initial energy for a two-stage, flux-trapping helical flux compression generator (FCG). Typical output from the FCG is several kilojoules into a 3 μH inductor. The amplified energy from the generator, after pulse conditioning, is used to drive a virtual cathode oscillator (vircator). Recorded voltages at the vircator with this arrangement were greater than 200 kV in experiments, where radiated output powers of greater than 100 MW have been measured. Voltages of at least 300 kV, with an electrical output power of 4 GW or greater, were generated by the FCG driven pulsed power source into a water resistor load with an impedance similar to the operating impedance of the vircator. A description of each component of the compact microwave source will be given, along with waveforms from tests performed with the components independent of the rest of the system. Data from experiments with the fully integrated microwave system will be shown, and analysis will be offered to detail the performance of the system in its present state.

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+ First Results of Streamer Formation During Dielectric Surface Flashover at Atmospheric Conditions
  K. Frank, J. Dickens, L. Hatfield, M. Kristiansen, G. Laity, A. Neuber, G. Rogers, "First Results of Streamer Formation During Dielectric Surface Flashover at Atmospheric Conditions," DPG Spring Meeting of the Atoms, Molecules, Optics, and Plasmas Section, March 8th - 12th, Hannover, Germany, (2010).
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.

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+ 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
+ Magnetic Biasing of Ferrite Filled Nonlinear Transmission Line
  J. Bragg, J. Dickens, A. Neuber, "Magnetic Biasing of Ferrite Filled Nonlinear Transmission Line," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Not Available
+ Material Emission Investigation of Explosive Emission Cathodes in Vacuum Sealed Tubes
  J. Parson, J. Dickens, A. Neuber, J. Walter, "Material Emission Investigation of Explosive Emission Cathodes in Vacuum Sealed Tubes," 37th IEEE International Conference on Plasma Science, June 20th - 24th, Norfolk, VA, (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
+ Modeling Of Compact Explosively-Driven Ferroelectric Generators
  D. Bolyard, A. Neuber, J. Krile, M. Kristiansen, "Modeling Of Compact Explosively-Driven Ferroelectric Generators," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Hydrodynamic pressure simulations combined with an empirical algorithm are used to model the open-circuit voltage output of several explosively compressed ferroelectric materials. The empirical algorithm was initially developed using detonating cord containing PETN and a metal driver element to compress the ferroelectric materials while the open-circuit voltage is recorded. A hydrodynamic code suite, CTH from Sandia National Labs, enables calculating Shockwave propagation and localized pressures. The resulting pressure profile in the ferroelectric material is then used as input for an empirically derived algorithm to calculate the predicted open-circuit voltage of the ferroelectric material. This previously developed empirical algorithm exhibited reasonable correlation between experimental and calculated open-circuit output voltages, but began to deviate when more powerful explosives were used. Hence, the amount of explosive material and geometry of the metal drive was varied to produce a wide range of peak pressures, including pressures higher then the maximum of 3.1 GPa previously modeled by the empirical algorithm. This data serves as the base to further develop the empirical algorithm for various ferroelectric materials and to more accurately model the open-circuit output voltage (experimentally observed range, normalized for thickness, of 1.3 to 3.8 kV/mm) over the wide range of applied pressures.

[PDF]

+ Monte Carlo Simulation of High Power Microwave Surface Flashover under UV Illumination
  J. Krile, J. Foster, M. Thomas, A. Neuber, "Monte Carlo Simulation of High Power Microwave Surface Flashover under UV Illumination," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  High Power Microwave (HPM) induced surface flashover is currently being investigated in order to gain a better understanding of the underlying processes involved and reduce the limitations it places on transmittable pulse lengths. The present experimental setup is designed to produce a flashover on the high pressure side of a transmission window without the influence of a triple point. A 2.5 MW magnetron produces a 900 ns pulse at 2.85 GHz with a 50 ns rise time. The experimental setup allows for the control of several parameters including gas pressure, gas composition, and external UV illumination of the window. Diagnostic equipment enables the analysis of incident, reflected, and transmitted power levels with sub-nanosecond resolution. A previously developed Monte Carlo simulation is used to model the processes involved in the flashover discharge formation. This Monte Carlo code is upgraded to account for the occurrence of field induced electron detachment from negative ion clusters within the high field region, >; 10 kV/cm, near the window. The code has also been expanded to include the occurrence of photoelectrons, emitted from the window while under UV illumination. Such illumination of the transmission window was experimentally shown to reduce the time to flashover by over 100 ns in air at 155 torr, and thereby the total pulse energy that can be transmitted. In addition, UV illumination also reduces the variation in flashover delay times from shot to shot, up to 67% in air at 155 torr. The simulation will determine if the observed reductions in delay time and variation can be explained by the addition of initiatory electrons via UV illumination of the surface.

[PDF]

+ Optimization of a Fuse Opening Switch for a Compact Power Conditioning Unit
  J. Korn, A. Young, A. Neuber, C. Davis, M. Elsayed, M. Kristiansen, L. Altgilbers, "Optimization of a Fuse Opening Switch for a Compact Power Conditioning Unit," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  The results from an investigation into the performance of a fuse opening switch, to be used as a component of a compact power conditioning unit, PCU, in an explosively driven high power microwave system, is presented. A pulse forming network designed to mimic the current action of a flux compression generator is utilized for these experiments. The investigation focused on the effect of altering fuse parameters, such as the diameter of fuse conductors and conductor winding geometry, on voltage and power levels delivered to a resistive load. Also, experiments were conducted with a thin layer of Semicosil, a commercial silicon material used in slower fusing opening switches, applied to fuse conductors, to investigate the possible advantages of using such a coating. Experiments showed that an increased number of conductor wires (with an approximately constant total cross-sectional area) resulted in similar voltages delivered to the resistive load, and that coating fuse conductors with Semicosil had a negligible effect on power delivery. A detailed description of these experiments will be given, along with data and waveforms illustrating the effects of these parameter variations on power levels delivered to the load.

[PDF]

+ Optimizing Wire Parameters in Exploding Wire Arrays
  C. Davis, A. Neuber, J. Stephens, A. Young, M. Kristiansen, "Optimizing Wire Parameters in Exploding Wire Arrays," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Exploding wire arrays as fast switches are of interest for explosive pulsed power applications utilizing magnetic flux compression. This type of opening switch has proven effective in producing pulses of several hundred kilovolts into vacuum diode loads. The research presented here discusses an exploding wire array capable of producing single digit kilo-joules, 100 nanosecond pulses when driven by a 45 kilo-Amp current with a waveform closely resembling typical magnetic flux compression output. For this specific parameter range, the optimal fuse design was developed based on the experimental behavior of the fuse under variation of parameters such as wire spacing, shielding, and quenching medium. Each fuse is composed of several silver wires arranged in a straight wire cylindrical array and is typically pressurized in a chamber filled with about 0.6 MPa of SF6. The tradeoff between wire spacing and voltage output was addressed by designing four different fuse termination pairs each with a diameter that increased wire spacing from 5 to 20 mm in 5 mm increments. A wire shield test was also conducted as an extension to the wire spacing experiment to uncover any mutual radiative effects between wires on fuse opening behavior. The optimum fuse design, including the optimum fuse wire diameter, will be discussed with a 20 Ohm resistive load as well as a vacuum diode load with similar impedance.

[PDF]

+ Performance of a Dual-Stage Helical Flux Compression Generator Under Varying Background Gas And Pressure
  M. Elsayed, A. Neuber, A. Young, J. Korn, J. Dickens, M. Kristiansen, C. Lynn, L. Altgilbers, "Performance of a Dual-Stage Helical Flux Compression Generator Under Varying Background Gas And Pressure," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  Recent efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have been focused on the development of a compact and explosively driven High Power Microwave, HPM, system. The primary energy source (other than the seed energy source) driving the microwave load in this system is a mid-sized, dual-stage helical flux compression generator, HFCG. The HFCG has a constant stator inner diameter of 7.6 cm, a length of 26 cm, with a working volume of 890 cm3. Testing at the Center has revealed energy gains in the 30's and 40's with output energy levels in the kilo-joules regime into loads of several micro-Henries. Over the last few years, close to one hundred shots have been taken with these generators into various loads consisting of dummy inductive loads, power conditioning systems, and HPM sources. Throughout these tests, the working volume of the HFCG, i.e. the volume in between the wire stator and the explosive-filled aluminum armature, was filled with SF6 at atmospheric pressure. This was primarily done do avoid electrical breakdown in the generator volume during operation, resulting in flux loss. Recent design updates enable pressurizing the generator volume to pressures up to 0.5 MPa, which is needed, for instance, to replace the SF6 with other gases such as air or nitrogen. The performance of the dual-stage HFCG with pressurized working volume (SF6 and N2) is presented in this paper along with an analysis of the maximum electric field amplitude held off in the volume during operation. The design technique to seal the HFCG will also be briefly discussed.

[PDF]

+ 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
+ Rapid Charging Seed Source with Integrated Fire Set for Flux Compression Generator Applications
  S. L. Holt, M. A. Elsayed, B. Gaston, J. C. Dickens, A. A. Neuber, M. Kristiansen, "Rapid Charging Seed Source with Integrated Fire Set for Flux Compression Generator Applications," 2010 IEEE International Power Modulator and High Voltage Conference, May 23 - 27, 2010 in Atlanta, GA.
Abstract:  The design and testing of an integrated front-end power and control system for helical flux compression generators (HFCG) is presented. A current up to 12 kiloamps needs to be pushed into the 5.8 microhenry field coil of the HFCG to establish the necessary seed flux for generator operation. This current is created with the discharge of a 5 kilovolt, 50 microfarad metalized polypropylene film capacitor using a single-use semiconductor closing switch. Once peak current/flux is obtained in the seed coil an exploding bridge wire (EBW) detonator is initiated with a discharge from a 1 kilovolt, 500 millijoule capacitor array contained in the compact fire set. Both capacitances, seed and fire set, are charged using a rapid capacitor charger system. The rapid capacitor charger is a solid state step up converter supplied by lithium-ion polymer (LiPo) batteries. It provides the 5 kilovolts and 1 kilovolt dual output voltages required for the compact seed source and compact fire set, respectively. The rapid capacitor charger operates at an average output power of 3 kilowatts and charges both capacitances simultaneously in under 250 milliseconds. The rapid capacitor charger is reusable if protected from the explosive detonation.

[PDF]

+ Simulation of Cascaded Flux Compression Generators with Conventional Circuit Simulation Software
  A. Young, A. Neuber, M. Kristiansen, "Simulation of Cascaded Flux Compression Generators with Conventional Circuit Simulation Software," The 13th International Conference on Megagauss Magnetic Field Generation and Related Topics, July 06-10, 2010, Nanlin Hotel, Suzhou, China.
Abstract:  Not Available
+ Spectral Analysis of Vacuum Ultraviolet Emission from Pulsed Atmospheric Discharges
  G. Laity, A. Neuber, G. Rogers, K. Frank, L. Hatfield, J. Dickens, "Spectral Analysis of Vacuum Ultraviolet Emission from Pulsed Atmospheric Discharges," 37th IEEE International Conference on Plasma Science, June 20th - 24th, Norfolk, VA, (2010).
Abstract:  Summary form only given. It is commonly accepted that vacuum ultraviolet (VUV) radiation, corresponding to emission from 180 nm to 115 nm and below, is responsible for photoionization contributing to streamer propagation during the initial stages of atmospheric discharges. An experimental setup was constructed to observe the VUV emission of pulsed surface flashover along a dielectric surface between atmosphere and vacuum. However, VUV radiation is highly attenuated in the atmosphere, which makes observation of detailed spectra difficult. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. The high speed detection scheme consists of a VUV sensitive ICCD camera and a photomultiplier, both with nanosecond temporal resolution. Spectra were measured in various gas mixtures at atmospheric pressure with a flashover spark length of about 8 mm with a 35 kV pulsed excitation, and spectral calibration was done utilizing a VUV calibration lamp with a known emission spectrum. Virtually all lines from 115 to 180 nm can be identified as atomic oxygen and nitrogen transitions during flashover in dry air, with most VUV emission occurring during the initial breakdown stage (current rise). The extremely fast decay of VUV emission intensity following this initial stage is evidence of radiationless quenching of the excited energy levels associated with the observed spectral lines. Flashover studies were also performed in pure oxygen and nitrogen environments to reinforce the observed emission trends. Spectroscopy must be carefully detailed, for instance, the Oxygen-I line at 130.2 nm (which corresponds to a ground level transition) is shown to be strongly self absorbed in the atmospheric spark when compared to a similar oxygen emission line at 130.4 nm. Full spectra were simulated using SpectraPlot, a- - temperature dependent spectral software suite developed at Texas Tech. It has been concluded from the comparison of simulated and measured Nitrogen spectra between 140 and 150 nm that the electronic temperature is about 4.5 eV, assuming that the electronic nitrogen energy level population density is Boltzmann distributed. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

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+ Time-Resolved Spectral Investigations of Pulsed Atmospheric Dielectric Surface Flashover Discharges
  K. Frank, G. Laity, A. Neuber, G. Rogers, L. Hatfield, J. Dickens, M. Kristiansen, A. Fierro, "Time-Resolved Spectral Investigations of Pulsed Atmospheric Dielectric Surface Flashover Discharges," 63rd Gaseous Electronics Conference, October 4th - 8th, Paris, France, (2010).
Abstract:  In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, a surface flashover event occurring on a magnesium fluoride (MgF$_{2})$ window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. Sharpened stainless steel electrodes (estimated tip radius of 200$\mu $m) are attached to springs which press down onto the MgF$_{2}$ surface a distance of 8 mm apart. Diagnostics include time resolved emission spectroscopy in the VUV range and gated ICCD optical imaging of streamer progression during the first 30 nanoseconds of breakdown (with 3 nanosecond resolution) in the visible wavelength range. One important parameter on which the streamer formation and the subsequent breakdown strongly depends is the gas type and/or the gas composition. That is why the streamer formation was recorded for gated intervals from 3 to 50 ns in lab air, standard nitrogen, oxygen and SF$_{6}$. The results are compared to those ones in purified air, oxygen and nitrogen.

[PDF]

Publication Year:  2009
+ A 15 kA Linear Transformer Driver
  D. Matia, M. Giesselmann, A. Neuber, M. K. Kristiansen, "A 15 kA Linear Transformer Driver," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 926-929, Washington, DC, June 2009.
Abstract:  The design of a 15 kA linear transformer driver (LTD) is presented. The specific goal of this LTD was improved energy density over the 500 J compact Marx generator previously designed and built at Texas Tech's Pulsed Power lab. The design of an individual 50 joule, 30 kV stage is discussed. For successful operation of the LTD, multiple spark gaps have to be fired with low jitter. Possible approaches for the design of a compact, low jitter triggering circuit will be presented as well.

[PDF]

+ Electrical Conduction in Select Polymers under Shock Loading
  C. F. Lynn, A. A. Neuber, J. T. Krile, J. Dickens, M. Kristiansen, "Electrical Conduction in Select Polymers under Shock Loading," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 171-174, Washington, DC, June 2009.
Abstract:  It is known that polymers become conductive under shock loading, which can be critical to the operation of explosive driven high current/voltage devices. Hence, the propensity of several select polymers to conduct under shock loading was investigated. Four polymers, Nylon, Teflon, Polypropylene, and High Density Polyethylene, were tested under shock pressures up to ~22 GPa. Shock waves were generated with high explosives, and CTH, a hydrodynamic code developed at Sandia National Laboratories, was utilized to calculate pressure and temporal resolution of the shock waves. Time of arrival measurements of the shock waves were taken to correlate the hydrodynamic calculations with experimental results. A notable delay between shock front arrival and the onset of conduction is exhibited by each polymer. The delay tends to decrease with increasing pressure down to approximately 500 ns for HDPE at ~22 GPa under electric field strength of ~6.3 kV/cm. The data shows that some polymers exhibit more delay than others, thereby indicating better insulating properties under shock loading. Additionally, experiments revealed that the polymers conducted for a finite time on the microsecond time scale before recovering back to an insulating state. This recovery from a shock wave induced conducting state back to insulating state was investigated for a possible opening switch application.

[PDF]

+ Energy Deposition Assessment and Electromagnetic Evaluation of Electroexplosive Devices in a Pulsed Power Environment
  J. Parson, J. Dickens, J. Walter, A. Neuber, "Energy Deposition Assessment and Electromagnetic Evaluation of Electroexplosive Devices in a Pulsed Power Environment," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 892-895, Washington, DC, June 2009.
Abstract:  This paper assesses critical activation limits of electroexplosive devices (EED), such as blasting caps, which have recently found more usage in pulsed power environments with high EMI background. These devices, EEDs, can be very sensitive to low levels of energy (7-8 mJ) which make them dangerous to unintended radiation produced by compact pulsed generators. Safe operation and use of these devices are paramount when in use near devices that produce pulsed electromagnetic interference. The scope of this paper is to provide an evaluation of activation characteristics for EEDs that include energy sensitivity tests, thermodynamic modeling, and electromagnetic compatibility from pulsed electromagnetic interference. Two methods of energy deposition into the bridgewire of the EED are used in the sensitivity tests. These methods include single and periodic pulses of current that represent the adiabatic and non-adiabatic heating of the bridgewire. The heating of the bridgewire is modeled by a solution to the heat equation using COMSOL¿ with physical geometries of the EED provided by the manufacturer.

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+ 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.

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+ Initial Results of Time-Resolved VUV Spectroscopy of Pulsed Dielectric Surface Flashover in Atmosphere
  G. Laity, K. Frank, G. Rogers, A. Neuber, J. Dickens, J.B. Moss, "Initial Results of Time-Resolved VUV Spectroscopy of Pulsed Dielectric Surface Flashover in Atmosphere," 51st Annual Meeting of the APS Division of Plasma Physics, November 2nd - 6th, Atlanta, GA, (2009).
Abstract:  Not Available
+ Integration of a Self-Contained Compact Seed Source and Trigger Set for Flux Compression Generators
  M. A. Elsayed, A. A. Neuber, M. Kristiansen, L. L. Altgilbers, "Integration of a Self-Contained Compact Seed Source and Trigger Set for Flux Compression Generators," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 297-300, Washington, DC, June 2009.
Abstract:  Two integral components that accompany an FCG in an explosively driven system is the prime power source and the trigger set. The objective of the prime power source or seed source is to provide the initial seed current/energy into the primary stage of an FCG. Another integral component in an FCG based pulsed power system is the trigger set. The trigger set is used to detonate an exploding bridge wire (EBW) which triggers the high explosives (HE's) in an FCG. This paper will discuss a recent design of a stand-alone apparatus that implements a self-contained (battery powered with full charge time less than 40 sec), single-use Compact Seed Source (CSS) using solid state components for the switching scheme along with a single-use Compact Trigger Set (CTS) that also implements a similar switching technique. The CSS and CTS stand-alone apparatus developed is a system (0.005-m3 volume and weighing 3.9 kg) capable of delivering over 360-J (~12 kA) into a 5.20-¿H FCG load and approximately 2-mJ (~600 A) into the EBW. Both the CSS and CTS have trigger energies of micro-Joules at the TTL triggering level.

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+ Power Conditioning Optimization for a Flux Compression Generator Using a Non-Explosive Testing System
  C. B. Davis, A. Young, A. A. Neuber, J. C. Dickens, M. Kristiansen, "Power Conditioning Optimization for a Flux Compression Generator Using a Non-Explosive Testing System," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 951-955, Washington, DC, June 2009
Abstract:  Not Available
+ Prediction of Compact Explosively-Driven Ferroelectric Generator Performance
  D. W. Bolyard, A. Neuber, J. Krile, J. Dickens, M. Kristiansen, "Prediction of Compact Explosively-Driven Ferroelectric Generator Performance," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp.167-170, Washington, DC, June 2009
Abstract:  Explosively-driven ferroelectric generators are attractive as potential prime energy sources for one-time use pulsed power systems. While the output voltages of small ferroelectric discs have been shown to be on the order of the theoretical maximum values, scaling the ferroelectric to larger thicknesses has proven less successful. The primary limiting factor is how much of the ferroelectric material is compressed simultaneously. This is difficult to control for thicker ferroelectric discs or stacks of discs due to pressure pulse attenuation in the material and rarefaction waves shortening the pressure pulse. A hydrodynamic code system is utilized to calculate the temporally and spatially resolved pressure. The calculated pressure values are converted into voltage produced by the ferroelectric through an algorithm based on an empirical polarization-pressure hysteresis curve. The validity of the algorithm has been verified for PZT EC-64 with experimental data from a flyer-plate experiment reported in literature and our own experiments with the shock wave from the explosives more directly applied to the ferroelectric. Both calculations and experiments produced normalized output voltages, ranging from 1.4 to 3.4 kV/mm for 2.54 cm diameter discs. We will discuss how this pressure to voltage algorithm along with pressure simulations aided in the scaling of the amount of ferroelectric material in a generator, as well as in the design of new driver elements with the goal to increase the peak output voltage of a generator while keeping the generator compact. The calculated voltage output results are compared with experimental data of explosively-driven ferroelectric generators.

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+ Stand-Alone, FCG-Driven High Power Microwave System
  A. Young, M. Elsayed, J. Walter, A. Neuber, J. Dickens, M. Kristiansen, L. Altgilbers, "Stand-Alone, FCG-Driven High Power Microwave System," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 292-296, Washington, DC, June 2009.
Abstract:  An explosively driven High Power Microwave (HPM) source has been developed that is based on the use of a Flux Compression Generator (FCG) as the primary driver. Four main components comprise the HPM system, and include a capacitor-based seed energy source, a dual-staged FCG, a power conditioning unit and an HPM diode (reflex-triode vircator). Volume constraints dictate that the entire system must fit within a tube having a 15 cm diameter, and a length no longer than 1.5 m. Additional design restrictions call for the entire system to be stand-alone (free from any external power sources). Presented here are the details of HPM system, with a description of each subcomponent and its role in the generation of HPM Waveforms will be shown which illustrate the development of power as it commutates through each stage of the system, as well as power radiated from the diode. Analysis and comparisons will be offered that will demonstrate the advantages of an explosively driven HPM system over more conventional pulsed power devices.

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+ 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.

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+ 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.

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+ Vacuum Ultraviolet Spectroscopy of Dielectric Surface Flashover at Atmospheric Pressure
  G. Laity, K. Frank, G. Rogers, M. Kristiansen, J. Dickens, A. Neuber, T. Schramm, "Vacuum Ultraviolet Spectroscopy of Dielectric Surface Flashover at Atmospheric Pressure," 36th International Conference on Plasma Science, May 31st - June 5th, San Diego, CA, (2009).
Abstract:  Summary form only given. Spectroscopic measurements in the vacuum ultraviolet (VUV) regime are difficult to make due to extremely large absorption of VUV radiation in most materials. This paper describes an experimental setup designed for studying the optical emission during pulsed surface flashover for the wavelength range between 115 nm to 300 nm at atmospheric pressures. A vacuum monochromator VM 505 from Acton Research Corporation was used as the spectrograph. For VUV transmission down to 115 nm the light emitted by surface flashover across an MgF2 window (front side of window in air, backside in vacuum) was focused by an MgF2 lens onto the entrance slit of the spectrograph. A quartz window with sodium salicylate coating exposed to the spectrograph's vacuum was placed in the exit focal plane of the collimating mirror of the spectrograph. This fluorescent coating down-converts the VUV light to longer wavelengths that were recorded with an Andor DH520 series ICCD camera in combination with a Nikon 105 mm lens. Spectra were measured at atmospheric pressure with a flashover spark length of about 9 mm and DC excitation with a capacitance of 4.1 nF. Emission spectra were measured from 300 nm down to 130 nm. In parallel, theoretical spectra were calculated primarily for the identification of radiating species and their temperature. Utilizing the NIST Atomic Spectra Database (ASD) data a library of temperature dependent optical emission spectra was generated with SpectraPlot, a spectral software suite developed at TTU. VUV spectral lines of nitrogen, carbon, magnesium and silicon were identified. In pure nitrogen, for instance, the nitrogen I double line at 174.3 nm and 174.5 nm is clearly visible in the spectrum along with a strong double line at 279.6 nm and 280.4 nm, which is emitted by Magnesium II, eroded from the surface exposed to flashover. Spectra were measured in ambient air, pure nitrogen, and argon. An experiment upgrade is currently u- nderway, increasing the VUV sensitivity of the setup. The measured spectra will be discussed in relation to the physics of surface flashover and volume breakdown at atmospheric pressure.

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+ VUV Emission from Dielectric Surface Flashover at Atmospheric Pressure
  T. G. Rogers, A. Neuber, G. Laity, K. Frank, J. Dickens, T. Schramm, "VUV Emission from Dielectric Surface Flashover at Atmospheric Pressure," Proceedings of the 2009 IEEE Pulsed Power Conference (PPC), pp. 855-859, Washington, DC, June 2009.
Abstract:  There is a growing interest in the physics of surface flashover between the interface of atmosphere and vacuum in some high-power systems. More specifically, the quantitative role of vacuum ultraviolet (VUV) radiation for the photoionization leading to a streamer development during the initial stages of a breakdown is unknown. This paper describes an experimental setup used to measure the VUV radiation emitted from atmospheric flashover as well as time-resolved imaging of the flashover event. A pulser providing the voltage to the gap was designed with special considerations in mind, including long lifetime, low noise, and high reproducibility. This enabled the study of the flashover in various background gases with an emphasis on spectroscopic measurements. The calculated spectra are compared with the measured spectra, and it is found that atomic oxygen and nitrogen are responsible for most of the VUV production in an air breakdown at atmospheric pressure in the wavelength range of 115–180 nm. Time-resolved spectroscopy reveals that the VUV radiation is emitted during the initial stages while the streamers are developing.

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Publication Year:  2008
+ A Compact, Self-Contained High Power Microwave Source Based on a Reflex-Triode Vircator and Explosively Driven Pulsed Power
  A. Young, T. Holt, M. Elsayed, J. Walter, J. Dickens, A. Neuber, M. Kristiansen, L.L. Altgilbers, and A.H. Stults, A Compact, Self-Contained High Power Microwave Source Based on a Reflex-Triode Vircator and Explosively Driven Pulsed Power, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  Single-shot high power microwave (HPM) systems are of particular interest in the defense industry for applications such as electronic warfare. Virtual cathode oscillators (vircators) are manufactured from relatively simple and inexpensive components, which make them ideal candidates in single-shot systems. The flux compression generator (FCG) is an attractive driver for these systems due to its potential for high energy amplification and inherent single-shot nature. A self-contained (battery operated prime power), compact (0.038 m3), FCG-based power delivery system has been developed that is capable of delivering gigawatts of power to a vircator. Experiments were conducted with the delivery system connected to a resistive dummy load and then to a reflex-triode vircator. In order to optimize the performance of the vircator when driven by the power delivery system, a second experimental setup was constructed using a Marx-generator based system operating at similar voltages and rise-times. Performance measures of the delivery system when discharged into a resistive load will be presented, as well as vircator output power levels and waveforms from both experimental setups.

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+ Capacitor Evaluation For Compact HV Pulse Generation
  E. J. Matthews, A. A. Neuber, M. Kristiansen, Capacitor Evaluation For Compact HV Pulse Generation, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  The size of compact pulsed power generators capable of producing pulses with ~100 ns duration at Gigawatt power levels is primarily determined by the specific energy density of the utilized energy storage medium. Capacitors capable of delivering large pulsed currents at several 10 kV voltage levels have been most frequently used as the energy storage medium for portable pulse generators. To increase the specific energy density of the pulsed power generator, the capacitors are often voltage overstressed at the cost of capacitor life. However, rapid charging (milliseconds) of the capacitor immediately followed by discharging alleviates somewhat of the lifetime problem. For repetitive operation of the pulsed power generator, the charging/discharging energy loss is the more important parameter. The energy, WC, needed to charge a capacitor to a set voltage is measured along with the energy released, WR, by the capacitor under conditions corresponding to a compact Marx generator operating with ~10 Hz rep-rate into a ~20 Ohm load. For the tested capacitors with Mica as dielectric, the capacitor efficiency, eta, i.e. the ratio between WR and WC, is roughly equal to 97% and largely independent of the charging time. Also tested ceramic capacitors revealed an efficiency of ~90% for fast charging and an efficiency of ~94% for slower charging (from ~2 to 35 mus time constant).

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+ Compact High Power Microwave Generation
  Neuber, A. Young, M. Elsayed, J. Dickens, M. Giesselmann, M. Kristiansen, L. Altgilbers, Compact High Power Microwave Generation, Proceedings of the 2008 Army Science Conference, Orlando Florida, December 1-4, 2008. Presenter, P3E team contribution, paper was invited by L. Altgilbers.
Abstract:  Not Available
+ Compact Silicon Carbide Switch For High Voltage Operation
  James, C.; Hettler, C.; Dickens, J.; Neuber, A.; Compact Silicon Carbide Switch For High Voltage Operation; IEEE International Power Modulators and High Voltage Conference, Proceedings of the 2008 27-31 May 2008 Page(s):17 - 20 Digital Object Identifier 10.1109/IPMC.2008.4743565
Abstract:  Vanadium compensated, 6H silicon carbide (SiC) is investigated as a compact, high-power, linear-mode photoconductive semiconductor switch (PCSS) material. SiC is an attractive material due to its high resistivity, high electrical breakdown strength, and long recombination times compared to other photoconductive materials. The PCSS is designed for fast-rise time, low-jitter (sub-nanosecond) operation in a matched 50 mu test bed. Ohmic contacts were applied by physical vapor deposition and initial test utilized an external Nd:YAG laser trigger source. Analysis of the optical properties of Va-compensated SiC and of switch conduction resistance are presented and performance of contact material is discussed.

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+ Comparison of HFCG and Compact Marx for Driving a High Power Microwave Load
  A. Neuber, A. Young, M. Elsayed, J. Walter, J. Dickens, M. Giesselmann, M. Kristiansen,and L.L. Altgilbers, Comparison of HFCG and Compact Marx for Driving a High Power Microwave Load, 12th IEE Conference on Megagauss Magnetic Field Generation, (Novosibirsk, Russia), July 2008, to be published.
Abstract:  Not Available
+ Energy Deposition and Electromagnetic Compatibility Assessment of Electroexplosive Devices
  Parson, J.; Dickens, J.; Walter, J.; Neuber, A.; IEEE International Power Modulators and High Voltage Conference, Proceedings of the 2008 27-31 May 2008 Page(s):439 - 442 Digital Object Identifier 10.1109/IPMC.2008.4743684
Abstract:  This paper assesses the critical activation energy required to set off electroexplosive devices (EED) at constant joule heating and rates of joule heating. Safe operation and use of these devices are of great concern in and around pulsed electromagnetic interference. Sensitivity characterization of EEDs include firing sensitivity plots, thermodynamic modeling and electromagnetic interference. Activation energy evaluation of single and periodic rectangular pulses are included to represent adiabatic and non-adiabatic bridge wire heating of the EED. The scope of this paper is to provide a short overview of sensitivity, thermodynamic, and electromagnetic compatibility of EEDs. The results provide crucial information in evaluating energy induced by pulsed electromagnetic fields of compact pulse generators.

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+ High-Current Compact FCG Seed Source Implementing Solid State Switching
  M. Elsayed, T. Holt, A. Young, A. Neuber, J. Dickens, M. Kristiansen, L.L. Altgilbers, and A.H. Stults, High-Current Compact FCG Seed Source Implementing Solid State Switching, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  Flux Compression Generators (FCGs) are some of the most attractive sources of single-use compact pulsed power available today due to their high energy density output and mobility. Driving FCGs requires some seed energy, which is typically provided by applying a high seed current, usually in the kilo-Ampere range for mid-sized helical FCGs. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. High-current seed sources have typically been comprised of discharging large capacitors using spark-gaps and over-voltage triggering mechanisms to provide the prime power for FCGs. This paper will discuss a recent design of a self-contained (battery powered with full charge time less than 35 sec), single-use Compact Seed Source (CSS) using solid state components for the switching scheme developed at the Center for Pulsed Power and Power Electronics at Texas Tech University. The CSS developed is a system (0.007-m3 volume and weighing 13 lbs) capable of delivering over 250-J (~10 kA) into a 6-muH load with a trigger energy of micro-Joules at the TTL triggering level. The newly designed solid-state switching scheme of the CSS incorporates off-the-shelf high-voltage semiconductor components that minimize system cost and size as necessary for a single-use application. An in-depth and detailed evaluation of the CSS is presented primarily focusing on the switching mechanics and experimental characterization of the solid state components used in the system.

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+ Opening Switch Utilizing Stress Induced Conduction In PMMA
  C. Lynn, A. Neuber, J. Dickens, J. Krile, Opening Switch Utilizing Stress Induced Conduction In PMMA, presented at the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  It is known that polymethylmethacrylate, PMMA, becomes conductive under shock loading. To develop an opening switch utilizing shock induced conduction, the reversibility of this process must be studied. It is suggested in literature that changes in electrical properties begin at pressures as low a ~2 GPa. Applying the minimum pressure necessary for conduction is desirable in order to maximize the reversibility by limiting compression heating of the material. CTH, a hydrodynamic code written at Sandia National Laboratory, was used to design various drivers that deliver pressures in the range of ~2 GPa to ~6 GPa to the PMMA. By utilizing the switch to trigger an RC discharge, the resistance and on-time of the switch was characterized. Experiments have shown conduction durations on the order of ~4 mus. The switch was then placed into a capacitive driven inductive energy storage circuit, IES, to determine the polymer's ability to recover. This paper will present experimental data, CTH simulation results, and discuss the attained switching characteristics under varying shock pressure profiles.

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+ 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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+ Scaling and Improvement of Compact Explosively-Driven Ferroelectric Generators,
  D. Bolyard, A. Neuber, J. Krile, J. Walter, J. Dickens, and M. Kristiansen, Scaling and Improvement of Compact Explosively-Driven Ferroelectric Generators, to be published in the Proceedings of the 2008 IEEE International Power Modulator Conference Las Vegas, Nevada, USA, May 27-31, 2008.
Abstract:  Explosively-driven ferroelectric generators are capable of producing single-shot voltage pulses of more than 100 kV, while requiring no seed electrical source, being very compact, and shelf stable. Problems with ferroelectric generators are the low energy output, high dielectric constant of the ferroelectric material, low surface flashover voltage, inconsistent ferroelectric material quality, and uneven or excessive shockwave compression. Initial generator voltage waveforms show that breakdown occurred towards the end of the generator operation time. Several designs and methods have been tested and implemented to prevent surface flashover with varying results. The ferroelectric discs used for the generators were 0.4 inch thick, 1 inch diameter EC-64 PZT ceramic discs. Several six-disc generators were built and tested with resulting open-circuit voltage pulses of 80-140 kV with a FWHM of 2-4 mus. Further improvements to the generators have been designed to prevent surface flashover, improve the explosive driver element and propagating shockwave, as well as increasing the number of discs per generator. Measured output waveforms into varying loads including direct driven antennas will be shown and discussed.

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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.

[PDF]

+ 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
+ Dielectric Surface Flashover under Pulsed Unipolar and RF Excitation
  A. Neuber, “Dielectric Surface Flashover under Pulsed Unipolar and RF Excitation,” presented at the DPG Frühjahrstagung des AMOP, Düsseldorf, March 19-23, 2007, Germany (invited plenary presentation)
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
+ Electro-Explosive Fuse Optimization for Helical Flux Compression Generator Using a Non-Explosive Test Bed
  D. McCauley, D. Belt, J. Mankowski, J. Dickens, A. Neuber, M. Kristiansen, "Electro-Explosive Fuse Optimization for Helical Flux Compression Generator Using a Non-Explosive Test Bed," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
Abstract:  Helical Flux Compression Generators (HFCG) of 50 mm form factor have been shown to produce a maximum energy deposit of 3 kJ into a 3 muH inductor from a seed current. A large dl/dt into a coupled load is possible when an electro-explosive fuse is used. Previous work with a non-optimized fuse has produced ~100 kV into a 15Omega load which leads into a regime relevant for High Power Microwave (HPM) systems. It is expected that ~3()0kV can be achieved with the present 2 stage HFCG driving an inductive storage system with an electro-exploding fuse. In order to optimize the electro-explosive fuse design, a non-explosive test bed, which closely simulates the 45 kA HFCG output, is used. To optimize the tiise, effects of fuse material, fuse length, and fuse shape will be examined as well as the effects of various quenching materials. Our previous work has characterized fuse material but we are also looking into the effects of the processes used to create the fuse wire, such as tempered wire versus fully annealed wire. Additionally, to maximize the output voltage and minimize the fuse recovery time, we are optimizing the length of the fuse wire. For shorter fuse lengths, we are optimizing fuse shape as well as fuse length to find the best fuse recovery time. By optimizing the individual parameters of an electro-explosive fuse, the fuse as a whole will be optimized to produce maximum output voltage when used with an HFCG.

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+ 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
+ Fuse and Load Testing with Mid-Sized, High Energy Density Flux Compression Generators,
  A. J. Young, T. A. Holt, M. A. Elsayed, A. A. Neuber, M. Kristiansen, L. L. Altgilbers, A. H. Stults, "Fuse and Load Testing with Mid-Sized, High Energy Density Flux Compression Generators," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
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
+ Investigation of an FCG and Pulse Transformer Based Power Conditioning System
  T. A. Holt, A. J. Young, M. A. Elsayed, A. A. Neuber, M. Kristiansen, K. A. O'Connor, R. D. Curry, "Investigation of an FCG and Pulse Transformer Based Power Conditioning System," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
Abstract:  Not Available
+ Opening Switch Utilizing Shock Wave Induced Conduction in PMMA and PVC
  C. Lynn, A. Neuber, J. Dickens, "Opening Switch Utilizing Shock Wave Induced Conduction in PMMA and PVC," 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
+ Short Pulse High Power Microwave Surface Flashover
  L. M. McQuage, G. F. Edmiston, J. P. Mankowski, A. A. Neuber, "Short Pulse High Power Microwave Surface Flashover," presented at the 2007 IEEE Pulsed Power and Plasma Science Conference, Albuquerque, NM June 17-22, 2007.
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
+ A Fabrication Method for a Mid-Sized, High Energy-Density, Flux Compression Generator
  T.A. Holt, A.J. Young, A.A. Neuber, M. Kristiansen, “A Fabrication Method for a Mid-Sized, High Energy-Density, Flux Compression Generator” presented at the 2006 International Conference on Megagauss Magnetic Field Generation and Related Topics, November 5-10, 2006 Santa Fe, New Mexico, USA
Abstract:  Not Available
+ A Fabrication Method for Helical Flux Compression Generators
  T.A. Holt, A.J. Young, A.A. Neuber, M. Kristiansen, “A Fabrication Method for Helical Flux Compression Generators,” presented at the 2006 Megagauss XI Conference, Sept. 10-14, 2006, Imperial College, London, UK
Abstract:  Not Available
+ A Flux Compression Generator Non-Explosive Test Bed for Explosive Opening Switches
  Belt, D.; Mankowski, J.; Neuber, A.; Dickens, J.; Kristiansen, M.; Conference Record of the 2006 Twenty-Seventh International Power Modulator Symposium, 2006. 14-18 May 2006 Page(s):456 - 459
Abstract:  Helical flux compression generators (HFCG) of a 50 mm form factor have been shown to produce output energies on the order of ten times the seeded value and a typical deposited energy of 3 kJ into a 3 muH inductor. Our previous work with a non-optimized fuse has produced-100 kV into a 15 load, which leads into a regime relevant for high power microwave (HPM) systems. It is expected that-300 kV can be achieved with the present 2-stage HFCG driving an inductive storage system with electro-exploding fuse. In order to optimize the electro-explosive wire fuse, we have constructed a non-explosive test bed which simulates the HFCG output with high accuracy. We have designed and implemented a capacitor based, magnetic switching scheme to generate the near exponential rise of the HFCG. The varying inductance approach utilizes 4 stages of inductance change and is based upon a piecewise linear regression model of the HFCG waveform. The non-explosive test bed will provide a more efficient method of component testing and has demonstrated positive initial fuse results

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+ Advanced Compact Pulsed Power for Directed Energy Weapons
  A. Neuber, “Advanced Compact Pulsed Power for Directed Energy Weapons,” Directed Energy Weapons and Laser Development Conference, January 18-19, Arlington, VA, 2006
Abstract:  Not Available
+ 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 at Atmospheric Conditions under High Power Microwave Excitation
  A. Neuber, “Dielectric Surface Flashover at Atmospheric Conditions under High Power Microwave Excitation,” APS conference, 2006 48th Annual Meeting of the Division of Plasma Physics, October 30-November 3, 2006, Meeting Id: DPP06, Philadelphia, PA. (invited)
Abstract:  Not Available
+ 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
+ Electro-explosive fuse development for helical flux compression generators
  D. Belt, J. Dickens, J. Mankowski, A. Neuber, and M. Kristiansen, “Electro-explosive fuse development for helical flux compression generators,” presented at the 2006 International Conference on Megagauss Magnetic Field Generation and Related Topics, November 5-10, 2006 Santa Fe, New Mexico, USA
Abstract:  Not Available
+ Helical Flux Compression Generator Non- Explosive Test Bed for Electro-Explosive Fuses
  D. Belt, J. Dickens, J. Mankowski, A. Neuber, M. Kristiansen, “Helical Flux Compression Generator Non- Explosive Test Bed for Electro-Explosive Fuses”, presented at the 27th Power Modulator Symposium and 2006 High Voltage Workshop May 14-18, 2006, Washington, DC.
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
+ Pressure Induced Conductivity for High Power Switching
  Veselka, H.; Neuber, A.; Dickens, J.; Power Modulator Symposium, 2006. Conference Record of the 2006 Twenty-Seventh International 14-18 May 2006 Page(s):219 - 219
Abstract:  Summary form only given. Investigations of shock induced conductivity of non-crystalline insulators and crystalline semiconductors were performed. Although we measured the insulator-to-metallic state transition time and conductivity, the focus of this investigation was on the recovery phase of the induced conductivity (i.e. metallic-to-insulator state transition). The recovery time and shock conditions were measured with high speed electrical diagnostics. The goal of this research is to determine the feasibility of using shock induced conductivity as a means of producing a high power opening switch. To minimize switch losses, the insulator-to-metallic transition time and conductance is also important, but has been more widely studied. Initial impact studies have shown that certain insulator can be conductive for 100 microseconds and recover under modest voltage less than one microsecond using a ten gram explosive charge. Various shock intensities are used in the study. The shock is produced primarily with conventional commercial explosives. In addition, the impact of sample thickness and compression duration on the induced the conductivity were also studied. The correlation between modest voltage and high voltage recovery time and shut-off current were studied

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+ Pulsed Unipolar Surface Flashover at Atmospheric Conditions
  Morales, K.; Krile, J.; Neuber, A.; Krompholz, H.; Dickens, J.; 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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+ Rapid Capacitor Chargers for Rep-Rated Operation of Low-Inductance Compact Marx Generators
  Giesselmann, M.; McHale, B.; Neuber, A.; Conference Record of the 2006 Twenty-Seventh International Power Modulator Symposium, 2006. 14-18 May 2006 Page(s):588 - 591
Abstract:  We designed and tested several rapid capacitor chargers for rep-rated operation of low-inductance, compact Marx generators with rep-rates ranging from 10 Hz to 100 Hz. All chargers are designed to be packaged in cylindrical volumes with inside diameters in the range of 5 in-12 in. Our capacitor chargers are based on H-Bridge inverters using ultra fast 600 V class IGBTs. The high voltage is obtained by driving step-up transformers with nano-crystalline cores at 30 kHz. These chargers are capable of average DC output power levels of more than 5 kW for short time operation, during which the thermal inertia of the IGBT assembly provides effective cooling (up to seconds). To achieve reliable rep-rated operation of the chargers, we developed HV feedback sensors to monitor the charging process and solid state Marx-style trigger generators to command trigger the discharge of the main Marx

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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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+ Transition of Window Breakdown from the Vacuum Multipactor Discharge to the Collisional RF Plasm
  Kim, H.C.; Verboncoeur, J.P.; Edmiston, G.F.; Neuber, A.A.; Lau, Y.Y.; Gilgenbach, R.M.; IEEE International Vacuum Electronics Conference, 2006 held Jointly with 2006 IEEE International Vacuum Electron Sources, 25-27 April 2006 Page(s):31 - 32
Abstract:  In high-power microwave systems, we investigate the transition of breakdown from single surface vacuum multipactor discharge to collisional rf plasma in argon. As the gas pressure increases, electron-neutral collisions prevail against secondary electron emissions. In addition, the discharge formation time is obtained as a function of the gas pressure.

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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
+ A Compact, Repetitive, 500kV, 500 J, Marx Generator
  Neuber, A.A.; Chen, Y.J.; Dickens, J.C.; Kristiansen, M.; 2005 IEEE Pulsed Power Conference; June 2005 Page(s):1203 - 1206
Abstract:  The electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate, Marx generator are discussed. While benefiting from the large energy density of mica capacitors, 4 mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to ~ 18.5 ¿ characteristic Marx impedance. Using solely inductors, ~ 1 mH each, as charging elements instead of resistors enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise-time. Repetitive HPM generation with the Marx directly driving a small Vircator has been verified. The Marx is fitted into a tube with 30 cm diameter and a total length of 0.7 m. We discuss the Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode primarily into resistive loads. A lumped circuit description of the Marx is also given, closely matching the experimental results.

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+ A Low-Cost Metallic Cathode for a Vircator HPM Source
  Y.J. Chen, J.J. Mankowski, A. Neuber, J.C. Dickens, “A Low-Cost Metallic Cathode for a Vircator HPM Source,” Proceedings of the 15th Int. IEEE Pulsed Power Conference, pp. 66-69, Monterey, CA, June 13-17, 2005
Abstract:  Not Available
+ 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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+ Compact Pulsed Power at TTU
  A. Neuber, M. Kristiansen, J. Dickens, M. Giesselmann, “Compact Pulsed Power at TTU,” 3rd Annual Multi-Functional Warhead Workshop, 30 November – 1 December 2005, Redstone Arsenal, Huntsville, AL.
Abstract:  Not Available
+ 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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+ High Voltage Impulse Generator Using HV-IGBTs
  Giesselmann, M.; Palmer, B.; Neuber, A.; Donlon, J.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):763 - 766, Monterey, California.
Abstract:  We are reporting on a High-Voltage Impulse Generator, which consists of a step-up transformer, which is driven by new HV-IGBTs (High-Voltage Isolated Gate Bipolar Transistors). The new HV-IGBTs are individually packaged silicon-dies intended for Pulsed-Power Applications. The silicon dies are normally packaged in large modules for locomotive motor drives and similar traction applications. In our work we used the Powerex QIS4506001 discrete IGBT and the QRS4506001 discrete diode, both with a nominal rating of 4500V/60A, derived from continuous-duty applications. Our experiments have shown that the devices are capable of handling currents in excess of 1 kA during pulsed operation.

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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
+ Multistage Helical Flux Compression Generator Non-Explosive Test Bed
  Belt, D.; Dickens, J.; Mankowski, J.; Neuber, A.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):525 - 528
Abstract:  Helical Flux Compression Generators of small dimensions have been shown to produce energy output around 3 kJ into an inductive load. Adding a fuse opening switch has allowed us to produce 300kV into a 15 Ohm load. We are investigating inductive energy storage with emphasis on an electro-explosive fuse opening switch in order to improve upon previous results. We have designed and constructed a non-explosive test bed composed of two pulse forming networks (PFN). Each PFN provides a linear approximation during two different time ranges of the exponential rise response of a typical HFCG. This approach will be more cost and time effective than to drive the fuse with an explosive generator. Our initial goal will be to simulate a 15 kA HFCG unit followed by the simulation of a 50 kA HFCG.

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+ 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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+ Shock Induced Conductivity for High Power Switching
  Veselka, H.; Neuber, A.; Dickens, J.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):734 - 737
Abstract:  Investigations of shock induced conductivity of insulators and semiconductors both crystalline and non-crystalline, were performed. Although the insulator-to-metallic state transition time was measured, the focus of this investigation was on the recovery phase of the induced conductivity (i.e. metallic-to-insulator state transition). The recovery time and shock conditions were measured with high speed electrical diagnostic equipment. The goal of this research is to determine the feasibility of using shock induced conductivity as a means of producing a high power opening switch. To minimize switch losses, the insulator-to-metallic transition time and conductance is also important, but has been more widely studied. Various shock profiles and intensities and used in the study. The shock is produced primarily with conventional commercial explosives. Shock reverberation is planed in some tests to minimize sample heating and to produce tailored shock time profiles. In addition, the impacts of sample thickness and compression duration on the induced conductivity were also studied.

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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 FCG Driven Inductive Energy Storage System
  J.-C. Hernandez, A. A. Neuber, M. Giesselmann, J. C. Dickens, and Magne Kristiansen, ”Compact FCG Driven Inductive Energy Storage System,” in Proceedings of MegaGauss X, Berlin, Germany, July 18 -23, p. 144-147, 2004,.
Abstract:  Not Available
+ 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
+ Compact Pulsed Power
  M. Kristiansen, A. Neuber, J. Dickens, M. Giesselmann, and S. Shkuratov, “Compact Pulsed Power,“ MegaGauss X, Berlin, Germany, July 18 -23, p. 169-175, 2004. (Invited)
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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+ Explosive Pulsed Power Sources for Directed Energy Weapons
  A. Neuber, M. Giesselmann, "Explosive Pulsed Power Sources for Directed Energy Weapons”, invited presentation at the 2004 Directed Energy Symposium, Rockville, Maryland
Abstract:  Not Available
+ 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.

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+ 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.

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+ 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
+ Conductivity measurements of explosively shocked aluminum and OFHC copper used for armature material in a magnetic flux compression generator
  Hemmert, D.; Mankowski, J.; Rasty, J.; Neuber, A.; Dickens, J.; Kristiansen, M.; Digest of Technical Papers, PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1073 - 1076 Vol.2
Abstract:  Modeling and characterization of a magnetic flux compression generator (MFCG) requires detailed knowledge of the changes in conductivity of the MFCG materials during the shock-loading phase. In the studies reported here, a thin metallic strip is shocked with an explosively generated shock wave produced from a charge of composition C-4. The shock wave is intended to simulate the shock wave and pressures produced in MFCG research currently being conducted at Texas Tech University. These pressures are estimated to be between 1 and 3 GPa. The experimental setup is arranged so that the shapes of the metallic strip and shock front are the same, as confirmed using optical fibers. This was to ensure that the test sample was shocked uniformly. The metallic test strip is pulsed with a 70 A current pulse during application of the shock wave. The current and voltage across the test sample are measured directly to determine the change in conductivity. Pressure measurements are conducted in separate tests under similar conditions using strain gauges. The results are then compared to results determined previously using a split Hopkinson pressure bar apparatus (SHPB).

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+ Design criteria for prevention of armature "turn-skipping" in helical magnetic flux compression generators
  Rasty, J.; Le, X.; Dickens, J.; Neuber, A.; Kristiansen, M.; Pulsed Power Conference, 2003. Digest of Technical Papers. PPC-2003. 14th IEEE International Volume 2, 15-18 June 2003 Page(s):1077 - 1080 Vol.2
Abstract:  Helical magnetic flux compression generators (MFCGs) are capable of producing ultra-high power electric pulses by trapping and compressing a seed magnetic field into a load coil via an explosive-driven armature. The efficiency of helical MFCGs is generally very low, about 10%, due to large magnetic flux losses. One of the main sources of magnetic flux loss is the "turn-skipping" phenomenon, in which the expanding armature fails to establish contact with every turn of the helical coil, resulting in magnetic flux loss in the skipped turns of the coil. The "turn-skipping" phenomenon is related to non-uniform or asymmetric expansion of the armature, as well as detonation end effects. Equations describing the "turn skipping" phenomenon are developed in terms of the eccentricity of the armature with respect to the helical coil, the armature's wall thickness variations and the length of the detonation end effect. Design criteria for prevention of "turn-skipping" are presented in order to achieve optimum MFCG performance.

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+ 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
+ Ferromagnetic and ferroelectric materials as seed sources for magnetic flux compressors
  Schoeneberg, N.; Walter, J.; Neuber, A.; Dickens, J.; Kristiansen, M.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1069 - 1072 Vol.2
Abstract:  permanent magnet based systems. Permanent magnets provide a larger energy-to-volume ratio given that the use of capacitors requires a power source as well as charging and firing circuitry. A recent design developed at Texas Tech University's Center for Pulsed Power and Power Electronics [S.I. Shkuratov, et al., 2002], focused on the demagnetization of a Neodymium-Iron-Boron magnet (Nd/sub 2/Fe/sub 14/B, B/sub r/= 1.23 T) by a shockwave generated from high explosives. The maximum specific energy achieved with this design was only 2.3 J/kg, which may not be sufficient for effectively seeding an FCG (fast capacitors have at least 20 J/kg). The same magnets were used with an alternative design, referred to as a strip FCG, which utilizes opposing magnets to generate initial magnetic field intensity within an air gap. The air gap exists between a central tube, filled with high explosives, and metal strips placed between the magnets a few millimeters off-axis. The detonation of the explosive causes the expansion of the central tube, subsequently compressing the flux into the strips and then into the load. The original strip FCG design [B.A. Bojko, et al., 1994] used oxide-barium magnets (B/sub r/= 0.2T), which produced an estimated specific energy of 5.27 J/kg into a low inductance load. These magnets were replaced with the Nd/sub 2/Fe/sub 14/B (B/sub r/= 1.23 T) magnets in order to achieve better performance, which will be analyzed with respect to the specific energy. In addition, a design utilizing a shocked ferroelectric material, PZT, as a seed current source is discussed. An explosively generated shock wave is passed through a ferroelectric material, generating a current that establishes the initial seed flux for an FCG. Preliminary tests of the ferroelectric sources indicate a possible theoretical specific energy of more than 11 J/kg or 25 J/dm/sup 3/. A comparison of the designs will be discussed with regard to the specific energy produced and the effectiveness of each to seed FCG's.

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+ 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
+ High Power Microwave Breakdown of Dielectric Interfaces
  A. Neuber, “High Power Microwave Breakdown of Dielectric Interfaces,” invited talk at the Workshop on High Gradient RF, Argonne, IL, Oct. 7-9, 2003.
Abstract:  Not Available
+ Marx generator using pseudospark switches
  Kuthi, A.; Alde, R.; Gundersen, M.; Neuber, A.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):241 - 244 Vol.1
Abstract:  The design and preliminary operation of the major subsystems of a Marx style pulse generator using advanced pseudospark devices are presented. The bank consists of three 150 nF / 40 kV capacitors connected with three floating FS2000 type pseudospark switches. These switches can hold off 35 kV and pass up to 10 kA at repetition rates approaching 1 kHz. The expected lifetime of >200 kC and the relatively low housekeeping power of <50 W make the pseudospark switch an excellent candidate in compact Marx generator applications. Preliminary operation of the floating housekeeping units essential to the Marx generator is presented in deta

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+ Nanosecond laser-triggered microwave switch
  McQuage, M.M.; Neuber, A.A.; Dickens, J.C.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):309 - 312 Vol.1
Abstract:  The design and experimental testing of a laser-triggered microwave switch with a nanosecond activation time is described. The objectives of the project include, confirming that a nanosecond to subnanosecond risetime is achievable in the X-band waveguide at 9 GHz with the laser-triggered switch and to determine the minimum laser energy necessary to obtain the fastest possible risetime. A 1 kW pulsed X-band source with a 500 ns output pulse provides the microwave power for the system. A variable power Nd:YAG laser with a maximum 450 mJ at 532 nm, 10 ns FWHM output pulse is used in conjunction with an applied high voltage pulse to trigger the microwave switch. The microwave signal is switched with the rapid formation of plasma caused by the breakdown of a gas contained by a quartz tube inserted through a section of waveguide. The centerpiece of the waveguide system is a magic tee, which controls the direction of power flow through the system. Compared to tests in air and N/sub 2/, the best results have been obtained in argon. Risetimes below 2 ns have been obtained using argon at a reduced pressure of 150 Torr and a high voltage pulse of 28 kV from a spark gap. The impact of gas pressure, applied voltage pulse and applied laser pulse on the risetime of the microwave switch are discussed.

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+ 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.

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+ 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.

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+ 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.

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+ 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.

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+ 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.

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+ Short pulse electric field sterilization of liquid media
  Wetz, D.; Truman, K.; Dickens, J.; Mankowski, J.; Neuber, A.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1124 - 1127 Vol.2
Abstract:  In recent years, researchers have investigated methods of liquid sterilization by applying pulsed high electric fields with some degree of success. The mechanism by which microorganisms are damaged has been shown to be a function of the local electric field and exposure time while independent from thermal and electrolytic effects from the applied pulse. Most published experiments have employed electrical pulses of 10's of kV and microsecond long pulse lengths. We are employing electrical pulses in the 100's of kV range with 100 nanosecond long pulse lengths. This type pulse should be more effective at killing microorganisms and minimize energy losses due to thermal processes in the liquid media.

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+ Small sized MFCG for driving a high impedance load
  Hernandez, J.-C.; Neuber, A.A.; Dickens, J.C.; Kristiansen, M.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1065 - 1068 Vol.2
Abstract:  End-initiated small volume magnetic flux compression generators (MFCG) have at least one order of magnitude higher energy density (by weight or volume) than capacitive energy storage with similar discharge time characteristics. Since the prime energy is built into the MFCG in form of HE, the capacitor looses even more ground if the necessary prime energy source and the charging supply are included in the weight/volume balance. However, simple MFCGs with a single helix produce high output energy only into low inductance loads, thus producing several 100 kA of current at a voltage level of only a few 10 kV. Many pulsed power devices require less current but a considerably higher voltage level. Two approaches for achieving a higher output voltage level, both utilizing two staged MFCGs, have been reported in the open literature. The first employs a more traditional transformer coupling; the second relies on a dynamic transformer or flux-trapping scheme. Although the traditional transformer coupling has theoretically the better efficiency, we chose the latter approach for our generator design, mostly since it requires a smaller number of components. Our generator has a total length of 250 mm, a helix inner diameter of 51 mm, and is wound with Teflon insulated stranded wire of different sizes in the range from AWG 12 to AWG 22. We have presently achieved an energy gain of /spl sim/ 8 and will discuss the generator performance based on experimental current/voltage waveforms.

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+ 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.

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Publication Year:  2002
+ All-Explosive Pulsed Power Generator System
  A. Neuber, S. Shkuratov, E. Talantsev, J. Walter, T. Holt, J. Dickens, and M. Kristiansen, “All-Explosive Pulsed Power Generator System,” at the Ninth International Conference on Megagauss Magnetic Field Generation and Related Topics, Moscow – St. Petersburg, July 7-14, 2002.
Abstract:  Not Available
+ Dependence of Flux Losses on MCG Helix Geometry
  A. Neuber, T. Holt, J. Hernandez, J. Dickens, and M. Kristiansen, “Dependence of Flux Losses on MCG Helix Geometry,” presented at the 14th International Conference on High-Power Particle Beams, Albuquerque, NM, June 23 - 28, 2002.
Abstract:  Not Available
+ 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.

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+ Explosive-Driven Generation of Transient Antennas
  A. Neuber, N. Schoeneberg, J. Dickens, and M. Kristiansen, “Explosive-Driven Generation of Transient Antennas,” presented at the Ninth International Conference on Megagauss Magnetic Field Generation and Related Topics, Moscow – St. Petersburg, July 7-14, 2002.
Abstract:  Not Available
+ 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.

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+ Feasibility study of an explosively formed transient antenna
  Neuber, A.; Schoeneberg, N.; Dickens, J.; Kristiansen, M.; Conference Record of the Twenty-Fifth International Power Modulator Symposium, 2002 and 2002 High-Voltage Workshop. 30 June-3 July 2002 Page(s):374 - 377
Abstract:  The feasibility of utilizing the chemical energy stored in high explosives to generate an antenna capable of radiating for several microseconds is studied. Crucial parameters such as conductivity as a function of time, maximum achievable antenna length for given initial device volume and weight, and material dependence are assessed by sub microsecond optical and electrical diagnostics. By utilizing a rotating framing mirror camera with up to 4 Megaframes per second, which produces a color image sequence consisting of 125 frames, possible premature breakup of the transient antenna rod is measured with adequate spatial and temporal resolution, thus revealing a rod growth velocity of a few millimeters/microsecond. Electrical diagnostic, primarily aimed at the conductivity between selected points along the rod propagation, enables to make a connection with the observed behavior based on the optical diagnostics. Maintaining electrical contact at the base of the device while the rod is fully formed is crucial and will be discussed in detail.

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+ 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.

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+ Geometry Impact on Flux Losses in MFCGs
  A. Neuber, T. Holt, J. Hernandez, J. Dickens, and M. Kristiansen, “Geometry Impact on Flux Losses in MFCGs,” presented at the Ninth International Conference on Megagauss Magnetic Field Generation and Related Topics, Moscow – St. Petersburg, July 7-14, 2002.
Abstract:  Not Available
+ 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

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+ 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
+ Microwave breakdown studies of He-N² mixtures in a pillbox cavity from 760 to 3040 torr
  Hemmert, D.; Neuber, A.; Dickens, J.; Conference Record of the Twenty-Fifth International Power Modulator Symposium, 2002 and 2002 High-Voltage Workshop. 30 June-3 July 2002 Page(s):232 - 235
Abstract:  Microwave breakdown of various He-N/sub 2/ mixture combinations is investigated utilizing an S-band double window pillbox cavity. The objective is to determine the best combination of the two gases to maximize the mixture's ability to cool a dielectric surface while maintaining high breakdown thresholds. Helium is known to be an excellent source to dissipate heat from a dielectric, but it also has a low microwave breakdown threshold. Nitrogen does not transport heat as well, but has a much higher breakdown threshold. The studies focused on generating a series of E-field vs pressure curves for breakdown to help identify optimum He-N/sub 2/ mixture ratios. The S-band double window pillbox has a 333 cm/sup 3/ cavity with two ports to flow the gas mixture through. The pillbox is placed in a traveling wave resonant ring (TWRR) coupled to a 2.85 GHz, 4 MW, magnetron. This combination of double window pillbox and TWRR allows for testing power levels up to 40 MW. High speed diagnostics are used to measure the incident/reflected power and discharge luminosity. Coupled mass flow controllers maintain the gas mixture ratio and continuous gas flow through the cavity. Investigations can be conducted with single or multiple pulsed operations. Results ranged from a minimum of 5 MW for pure helium at 760 torr, to greater than 40 MW for pure nitrogen at 3040 torr.

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+ Surface Flashover Across Ceramic Disks in Vacuum at Cryogenic Temperatures
  Keene, H.; Dickens, J.; Neuber, A.; Conference Record of the Twenty-Fifth International Power Modulator Symposium, 2002 and 2002 High-Voltage Workshop. 30 June-3 July 2002 Page(s):293 - 296
Abstract:  Not Available
Publication Year:  2001
+ Advanced Modeling of an Exploding Flux Compression Generator using Lumped Element Models of Magnetic Diffusion
  M. Giesselmann, T. Heeren, A. Neuber and M. Kristiansen, “Advanced Modeling of an Exploding Flux Compression Generator using Lumped Element Models of Magnetic Diffusion”, Proceedings of the 2001 Conference on Pulsed Power and Plasma Science, Las Vegas, Nevada, June 17-22, 2001, p. 162...165.
Abstract:  Not Available
+ Calculating stator losses in a helical MFCG
  Benton, T.; Kuo-Ta Hsieh; Stefani, F.; Neuber, A.; Kristiansen, M.; Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of Technical Papers Volume 1, 17-22 June 2001 Page(s):85 - 89 vol.1
Abstract:  This paper analyzes the transient electromagnetic, thermal, and structural behavior of the stator turns in a simple helical magnetic flux compression generator (MFCG). The main objective is to quantify the losses due to Ohmic heating and flux trapping in the conductors, including the effect of armature motion and armature proximity on the current distribution. The electric current excitation used in the modeling is based on experimental data from tests on a research MFCG. The electromagnetic (EM) and thermal analyses were conducted using EMAP3D, a 3D finite element analysis (FEA) code developed at The University of Texas at Austin (UT), which has the capability to model relative motion and sliding between conductors. Structural analyses were conducted using a version of DYNA3D that allows state data from EMAP3D to be used as input. Details of the analyses include temperature dependence on electrical, thermal, and mechanical properties.

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+ Calculation of air temperature and pressure history during the operation of a flux compression generator
  Xiaobin Le; Rasty, J.; Neuber, A.; Dickens, J.; Kristiansen, M.; Pulsed Power Plasma Science, 2001. PPPS-2001. Digest of Technical Papers Volume 2, 17-22 June 2001 Page(s):939 - 942 vol.2
Abstract:  During the operation of magnetic flux compression generators (MFCG), the gas-plasma, shocked by the rapidly expanding armature, could lead to electrical arcing across the gas between the armature and the stator at locations where physical contact between the armature and stator has not yet occurred. This will result in a loss of magnetic flux and a decrease in the electrical efficiency of the MFCG. Therefore, knowledge of the ensuing gas temperature and pressure histories is necessary for identification of loss mechanisms in an effort to optimize the efficiency of MFCGs. This paper describes the procedure for estimating the air temperature and pressure histories via finite element (FE) simulation of the armature expansion and its ensuing contact with the stator in an MFCG. First, the validity of the FE model was verified by comparing deformation contours obtained from the simulations to those obtained experimentally via high-speed photography. Utilizing the pressure history data obtained from the FE results, the air temperature was theoretically calculated. The results indicate that the air pressure and temperature in an MFCG, having a compression ratio of 1.8, could be as high as 30 MPa and 4000/spl deg/ Kelvin, respectively.

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+ Compact explosive driven shock wave ferromagnetic generators
  Shkuratov, S.I.; Talantsev, E.F.; Kristiansen, M.; Dickens, J.; Hernandez, J.C.; Neuber, A.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 1, 17-22 June 2001 Page(s):158 - 161 vol.1
Abstract:  The results are presented of tests with compact, explosively driven shock wave ferromagnetic generators. The shock wave from a high explosive charge is passed along the axis of a cylindrical, hard magnet. Two types of permanent magnets were used in the experiments: rare-earth NdFeB cylinders (D = 2.5 cm, L = 1.9 cm) and hard ferrite BaFe/sub 2/O/sub 3/ cylinders (D = 2.2 cm, L = 2.5 cm). The shock wave demagnetizes the cylinder, reducing the flux from the remnant value to zero. This change in flux generates a voltage in the winding. The current generated in the loads of the generators yielded a peak of 0.75 kA. The operation of the shock wave ferromagnetic generators was analyzed by the Maxwell 3D code. An analysis is given on the specific features of pulse generation in a system like this.

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+ Conductivity measurements of MFCG armature material under shock and high strain rates utilizing a split-hopkinson pressure bar apparatus
  Hemmert, D.; Rasty, J.; Neuber, A.; Dickens, J.; Le, X.; Kristiansen, M.; Digest of Technical Papers, 2001 Pulsed Power Plasma Science, PPPS-2001. Volume 1, 17-22 June 2001 Page(s):265 - 268
Abstract:  Not Available
+ Experimental and numerical investigation of armature/stator contact in magnetic flux compression generators
  Rasty, J.; Xiaobin Le; Neuber, A.; Dickens, J.; Kristiansen, M.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 1, 17-22 June 2001 Page(s):106 - 109 vol.1
Abstract:  The efficiency of a magnetic flux compression generators (MFCG) is highly dependent on the expanding characteristics of the exploding armature and the nature of contact between the armature and the surrounding stator coil. A hydrodynamic finite element (FE) model was developed to simulate the expansion characteristics of the armature and its ensuing impact with the stator. The effectiveness of the FE model to simulate the explosive behavior of the armature was qualified by comparing the numerical results with experimentally measured parameters. Specifically, the radial displacement of the armature as well as the axial velocity of the armature/stator contact point were measured experimentally and compared with numerical results showing excellent agreement between the two. The results indicated that the radial and axial velocity with which the armature impacted the stator did not change through the length of the armature. However, the results showed that the velocity with which the contact point between the armature and the stator traveled along the length of the armature decreased as the explosion process went on. As expected, the axial propagation velocity of the contact point was found to be at its highest value (2.25 X detonation velocity) at the region close to the detonation end while approaching the detonation velocity at points away from the detonation end.

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+ Experimental study of compact explosive driven shock wave ferroelectric generators
  Shkuratov, S.I.; Kristiansen, M.; Dickens, J.; Neuber, A.; Altgilbers, L.L.; Tracy, P.T.; Tkach, Y.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 2, 17-22 June 2001 Page(s):959 - 962 vol.2
Abstract:  The design of explosive driven ferroelectric generators is presented and experimental data are discussed. The active elements are lead zirconium titanate (PZT) disks with diameter D=25 mm and thicknesses H=2.5 mm and H=6 mm and PZT cylinders with D=21 mm and H=25 mm. The high explosive charge was varied from 4.2 g to 30 g. Two different ways to initiate shock waves in the active elements were used: explosively driven flyer plates and direct action of high explosives. The data presented is for the maximum power into a resistive load.

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+ Experimental study of compact explosive-driven shock wave ferroelectric generators
  Shkuratov, S.; Kristiansen, M.; Dickens, J.; Neuber, A.; Altgilbers, L.L.; Tracy, P.T.; Tkach, Y.; IEEE Conference Record - Abstracts. Pulsed Power Plasma Science, 2001.17-22 June 2001 Page(s):227
Abstract:  Summary form only given. The design of shock wave ferroelectric generators driven by high explosives is presented and experimental data are discussed. The active elements are lead zirconium titanate (PZT) disks with diameter D=25 mm and thickness L=2.5 mm, and PZT cylinders with D=21 mm and L=25 mm. The high explosive charge was varied from 4.2 g to 30 g. Two different ways to initiate shock waves in the active elements were used: explosively driven flyer plates and direct action of high explosives. The data are presented on the maximum power released in the resistive load and on the effect of the load inductance and the load capacitance on the amplitude of the pulse produced. The experimental results are discussed in comparison to numerical simulation with the commercial circuit simulator PSPICE. An analysis is given of the specific features of pulse generation in a system like this

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+ High speed optical diagnostics of an exploding wire fuse for power conditioning of explosive flux compression generators
  Giesselmann, M.; Heeren, T.; Neuber, A.; Walter, J.; Kristiansen, M.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 1, 17-22 June 2001 Page(s):102 - 105 vol.1
Abstract:  This paper presents high-speed optical diagnostics of an exploding wire fuse, which is used in the power conditioning system for an explosive flux compression generator. The images were taken using an IMACON(R) 790 high-speed framing camera utilizing a gated image intensifier tube. For the authors' measurements, the camera was operated in the high-speed multiple frame mode, yielding 8-10 sequential, 2 dimensional pictures with 100 ns between exposures.

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+ Impact of helix geometry on MCG flux losses [magneto cumulative generators]
  Holt, T.; Neuber, A.; Dickens, J.; Kristiansen, M.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 2, 17-22 June 2001 Page(s):905 - 908 vol.2
Abstract:  Explosively driven magnetic flux compression has been the object of research for more than three decades. Recently heightened interest has been focused on the basic physical mechanisms that determine the performance of helical magneto cumulative generators (MCGs). Two single-pitch helical flux compression generators of different sizes have been tested using current-voltage probes and optical diagnostics. The main parameters used to characterize the experimental performance of the flux compression generators were the flux conservation and theoretical current gain of each type of generator. Helices with constant pitch and differing separation between wires as well as wires with different insulation thickness were tested and analyzed with respect to their flux conservation and theoretical current gain. Preliminary results show that the insulation thickness plays only a minor role for a change in flux conservation due to geometry in the range from 0.01 to 0.5 mm provided that no internal breakdown occurred. Additionally, the overall physical dimension of the generator was modified to allow for a substantial increase in initial inductance. The outer diameter of the generator armature was held constant at 1.5 inches and the coil diameter was varied from 2.6 to 3.5 inches (expansion ratio of 1.7 or 2.3, respectively). The results gained from the conservative expansion ratio of 1.7 were used as a base to compare to the generator performance at the more aggressive expansion ratio of 2.3. First results show that an expansion ratio of 2.3 produces viable results for a partially annealed Aluminum armature with a Gurney angle of approximately 15 degrees.

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+ Thermodynamic state of the magnetic flux compression generator volume
  Neuber, A.A.; Holt, T.A.; Dickens, J.C.; Kristiansen, M.; IEEE Conference Record - Abstracts, 2001 IEEE Conference on Pulsed Power Plasma Science, 17-22 June 2001 Page(s):151
Abstract:  Summary form only given, as follows.The knowledge of the thermodynamic state of the gas trapped in a helical flux compression generator is crucial for the assessment of flux loss due to internal electrical breakdown/arcing. Besides the helix deformation and armature deceleration at extremely high current amplitudes approaching 1 MA, the thermodynamic state of the shocked and compressed gas causes problems in the prediction of the generator output current vs. time towards the end of generator operation. Such a breakdown is experimentally detected as an abrupt change in the time derivative of the current waveform and it is easily distinguished from partial turn skipping by its sharper fall and non-periodic occurrence. The thermodynamic state of the generator was measured using primarily optical emission spectroscopy. Three main stages of operation are discussed: (1) The initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, possibly as low as 2,000 K. (2) The intermediate stage during 14 to 4 microseconds before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV. (3) The last few microseconds that reveal a highly compressed gas with temperatures of about 5,000 K and pressures of about 200 bar. Most experiments were conducted in air initially at STP, some results are given for SF6 initially at one atmosphere. In order to link the thermodynamic state to the breakdown sensitivity, additionally, simple conductivity measurements were conducted in current-free flux compression generator models

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+ Transient Analysis of Copper Stator Turns in MFCG
  T. Benton, K.-T. Hsieh, F. Stefani, A. Neuber, and M. Kristiansen, “Transient Analysis of Copper Stator Turns in MFCG,“ Paper No. H.06, International Conference on Pulsed Power Applications, Gelsenkirchen, March 29-30, 2001.
Abstract:  Not Available
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

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+ 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

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+ 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
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

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+ 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

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+ Development of improved triggered vacuum switches
  Warren, T.; Dickens, J.; Neuber, A.; Kristiansen, M.; Frazier, G.; McNab, I.R.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 2, 27-30 June 1999 Page(s):1264 - 1267 vol.2
Abstract:  Triggered vacuum switches (TVS) are being evaluated for use in energizing electrical loads that require rapid and efficient switching of currents on command as well as the ability to interrupt at current zero crossing. Relatively few switching alternatives are available at the high currents (>250 kA) and voltages (>10 kV) needed. The TVSs used in recent pulsed power systems have demonstrated an ability to switch significant currents and interrupt at the zero current crossing, but they are not yet as effective as desired. In particular, higher current operation and faster dielectric recovery would be beneficial. The intent of this research is to expand the understanding of the physics of the vacuum switch (especially the opening phase) and to extend the operating parameters

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+ 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

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+ Measurement of dynamic electrical conductivity of MFCG armature material under conditions of shock and high strain rate loading
  Rasty, J.; Xiaobin Le; Neuber, A.; Jiande Zhang; Dickens, J.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 2, 27-30 June 1999 Page(s):708 - 711 vol.2
Abstract:  Characterization of changes in the conductivity during the shock loading process is of paramount importance in estimating the efficiency of magnetic flux compression generators (MFCG). In this study, the main emphasis was the characterization of dynamic conductivity of the armature material during the shock-loading phase. A Split Hopkinson Pressure Bar apparatus was utilized to subject as-received and annealed specimens of OFHC copper to various shock pressures. Experiments conducted to measure the resistivity of Cu specimens indicated that resistivity initially decreases, followed by a sharp increase before decreasing to a steady state value. Depending on the magnitude of the shock pressure, resistivity changes in excess of 200% were recorded

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+ 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

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+ 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

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+ 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

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+ 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

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+ 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. 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. IEEE International Conference on Plasma Science, San Diego (1997)
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:  2005
+ Basic Physics, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  B. L. Freeman, and A. Neuber, “Basic Physics,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
Abstract:  Not Available
+ Chapter 6, Generator Modeling, Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generators
  Michael Giesselmann, Ivor R. Smith, Bucur M. Novac and Andreas Neuber, Chapter 6, Generator Modeling, "Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generators", Springer, Berlin, Heidelberg, New York, ISBN-13 978-3-540-26051-6.
Abstract:  Not Available
+ Chapter 7, Power Conditioning, "Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generators
  Tammo Heeren, Michael Giesselmann, and Andreas Neuber, Chapter 7, Power Conditioning, "Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generators", Springer, Berlin, Heidelberg, New York, ISBN-13 978-3-540-26051-6.
Abstract:  Not Available
+ FCG Overview, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  B. L. Freeman, and A. Neuber, “FCG Overview,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
Abstract:  Not Available
+ Generator Modeling, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  M. Giesselmann, I. R. Smith, B. M. Novac, and A. Neuber, “Generator Modeling,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
Abstract:  Not Available
+ Loss Mechanism Basics, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  B. L. Freeman, and A. Neuber, “Loss Mechanism Basics,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
Abstract:  Not Available
+ Power Conditioning, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  T. Heeren, M. Giesselmann, J. C. Dickens and A. Neuber, “Power Conditioning,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
Abstract:  Not Available
+ Practical FCG Pulsed Power System, In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator
  A. Neuber and Juan-Carlos Hernandez, “Practical FCG Pulsed Power System,” In: Explosively Driven Pulsed Power, Helical Magnetic Flux Compression Generator, A. Neuber Edtr., Springer Berlin Heidelberg New York, ISBN 3-540-26051-X, 2005.
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