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

Dr. John Jerome Mankowski, PE

Associate Professor

Contact Information

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

Phone: (806)742-0526
Fax: (806)742-1281
john.mankowski@ttu.edu

Education

Research Interests

Awards and Professional Societies

Licensing

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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+ 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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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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+ 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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+ 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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+ 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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Publication Year:  2014
+ 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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+ 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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Publication Year:  2010
+ Development of a 40-stage distributed energy railgun
  Karhi, R.; Giesselmann, M.; Wetz, D.; Diehl, J.; "Development of a 40-stage distributed energy railgun "; Pulsed Power Conference, 2009. PPC '09. IEEE Digital Object Identifier: 10.1109/PPC.2009.5386257 Publication Year: 2009 , Page(s): 747 - 752
Abstract:  The development process pertaining to the design, fabrication, and testing of a 40-stage free-running arc synchronous distributed energy railgun is presented. Investigation of this type of system will determine the effectiveness of a distributed energy scheme to suppress the plasma restrike phenomenon and increase plasma armature railgun performance. Determined by a computer simulation, the proposed system will have a 1.0 cm ? 1.0 cm square bore cross section and a stage length of 15.24 cm producing a total rail length of 6 meters for 40 stages. A free-arc is utilized to relieve the financial burden of a large stored energy facility. A velocity of 8 km/s is desired to emulate conditions during a high altitude microsatellite launch. To achieve this velocity, pulsed power in conjunction with a low pressure (~ 10 Torr) air environment is required. The pulsed power supplies 15 kJ of energy to provide an armature current (~ 50 kA) for 1 millisecond. A real-time feedback control system will accurately release the stage energy upon arc arrival. Experimental data collected from a 7-stage prototype distributed system is discussed which will mimic the design and operation of the first 7 stages associated with the 40-stage railgun. The copper rail length is 1.2 m long with a 1 cm ? 1 cm square bore cross section and a 15.24 cm stage length. Each distributed energy stage contains a 750 ?F capacitor bank, a thyristor with an anti-parallel diode, and a driver board for triggering. The armature is formed using a plasma injector that is powered by a 40 kV Marx generator. Diagnostics for this examination include rail B-dot probes as well as independent Rogowski coils for each stage. Data collected from the rail B-dot probes will be used to measure the armature position and velocity as a function of time. There is no target velocity for this prototype; repeatable energy module operation, accurate stage triggering, and arc propagation toward the muzzle are the main areas focus. Outcom- es of these initial experimental results will aid the development of the 40-stage system.

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Publication Year:  2008
+ 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.

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+ Imaging of the Explosive Emission Cathode Plasma in a Vircator High-Power Microwave Source
  Walter, J.; Mankowski, J.; Dickens, J.; Imaging of the Explosive Emission Cathode Plasma in a Vircator High-Power Microwave Source; Plasma Science, IEEE Transactions on Volume 36, Issue 4, Part 1, Aug. 2008 Page(s):1388 - 1389 Digital Object Identifier 10.1109/TPS.2008.924489
Abstract:  Most pulsed high-power microwave sources use explosive electron emission cathodes to generate high current electron beams. In the explosive emission process, the current emitted through small field emission points becomes high enough to cause the cathode material to vaporize and form a plasma. Plasma characteristics, such as uniformity and expansion rate, will affect the performance of the microwave source. High-speed optical imaging can be used to resolve some characteristics of the plasma in time. The images of the cathode plasma during the operation of a triode-geometry virtual cathode oscillator high-power microwave source are presented for three different cathode materials.

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+ Low-Jitter Triggered Spark Gap With High-Pressure Gas Mixtures
  Yeong-Jer Chen; Mankowski, J.J.; Dickens, J.C.; Walter, J.; Kristiansen, M.; Low-Jitter Triggered Spark Gap With High-Pressure Gas Mixtures; Plasma Science, IEEE Transactions on Volume 36, Issue 5, Part 3, Oct. 2008 Page(s):2546 - 2553 ; Digital Object Identifier 10.1109/TPS.2008.2004366
Abstract:  Recent attention to impulse antenna phased array has necessitated the need to develop a reliable high-voltage high-repetition-rate switch that will operate with ultralow jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact that gases and gas mixtures have on switch performance which includes recovery rate and, in particular, jitter. A 50-Omega 1-nF pulse-forming line is charged to 30 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid-state opening switch voltage source that supplies >100-kV 10-ns rise-time pulses at a rep rate of up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high-pressure gas. The system includes a gas-mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6. Switch operations in 30 kV and 10 Hz have shown reliable subnanosecond jitter times with pure gases, including dry air, H2, N2, and with H2 - N2 and N2 - SF6 gas mixtures. The system was then modified for 50-kV 100-Hz operations with data collected for each of the pure gases. Recovery was monitored with no major problems at the 100-Hz operation, and subnanosecond jitter results for H2 , N2, and SF6 are also recorded.

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+ Secondary Arc Formation Within a Distributed Energy Railgun
  Karhi, R.W.; Mankowski, J.J.; Dickens, J.C.; Kristiansen, M.; Wetz, D.A.; Secondary Arc Formation Within a Distributed Energy Railgun; Plasma Science, IEEE Transactions on Volume 36, Issue 5, Part 3, Oct. 2008 Page(s):2738 - 2746 Digital Object Identifier 10.1109/TPS.2008.2004228
Abstract:  Experimental results comparing a breech-fed scheme and two distributed energy schemes for a free-running arc are presented. Analysis and observations of the issues associated with distributed energy switching of a plasma arc in the railgun are explored. The use of a free-running arc allows experiments to emulate the ablation and restrike phenomenon of a plasma armature railgun at high speeds (> 5 km/s) without the requirement of a large amount of stored energy. Numerous experimental tests were conducted to investigate the dynamics of plasma arcs within a distributed energy source railgun. Variations of switch timing, bore pressure, bore material, current amplitude, and current pulse length within each stage have been tested. These data reveal important design parameters for distributed energy railguns. The arc length, stage length, and stage trigger timing play a crucial role in distributed energy railgun performance. Failure to take these parameters into consideration will result in velocity reduction through plasma arc restrike and/or splitting.

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+ Utilization of a Nonexplosive Test Bed for Flux-Compression-Generator Electroexplosive Opening Switches
  D. W. Belt, J. J. Mankowski, A. A. Neuber, J.C. Dickens, M. Kristiansen, 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.

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Publication Year:  2007
+ A Bench Top Railgun With Distributed Energy Sources
  Mankowski, J.; Dickens, J.; Giesselmann, M.; McDaniel, B.; McHale, B.; Kristiansen, M.; IEEE Transactions on Magnetics, Volume 43, Issue 1, Part 2, Jan. 2007 Page(s):167 - 169
Abstract:  Experimental results of a distributed energy source railgun are presented. Distributed energy source railguns were first proposed by Marshal in an asynchronous scheme and later by Parker synchronously. Both schemes employ a "traveling excitation wave" to push the projectile along the rail. The primary advantages of such a scheme over the common breech-fed is higher efficiency due to less energy remaining in the rail and lower rail resistive loses. Another advantage is the reduction in the probability of re-strike. However, these advantages are achieved at a cost of higher switching complexity. As a proof of principle experiment, we have constructed a bench-top solid armature railgun with distributed energy sources. Instead of a single, capacitive, breech-fed, energy source, the current is supplied by two storage capacitor banks, placed at different positions along the rail. The switching configuration, which requires a dedicated switch at each capacitor, is realized with sold state switches. The railgun is diagnosed in order to evaluate performance and to appropriately trigger the switches. In addition, experimental results are compared to simulation

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+ Cathode and Anode Optimization in a Virtual Cathode Oscillator
  Chen, Y.; Mankowski, J.; Walter, J.; Kristiansen, M.; Gale, R.; IEEE Transactions on Dielectrics and Electrical Insulation, [see also IEEE Transactions on Electrical Insulation] Volume 14, Issue 4, Aug. 2007 Page(s):1037 - 1044
Abstract:  Not Available

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+ Jitter and Recovery Rate of a Triggered Spark Gap with High Pressure Gas Mixtures
  Yeong-Jer Chen; Mankowski, J.J.; Walter, J.W.; Dickens, J.C.; Jitter and Recovery Rate of a Triggered Spark Gap with High Pressure Gas Mixtures; Pulsed Power Plasma Science, 2007. PPPS 2007. Conference Record - Abstracts. IEEE 17-22 June 2007 Page(s):255 - 25
Abstract:  Summary form only given. Recent attention in impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact, gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Omega, 2.4 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. The system will also introduce a high pressure gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, and SF6.

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Publication Year:  2006
+ 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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+ 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
+ The Impact of Field Enhancements and Charge Injection on the Pulsed Breakdown Strength of Water
  Wetz, D.A.; Mankowski, J.J.; Dickens, J.C.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 34, Issue 5, Part 1, Oct. 2006 Page(s):1670 - 1679
Abstract:  A unique theoretical model of the breakdown mechanism in water has been developed and further tested in both simulation software and experimentation. The conducted experiments test the degree to which electrode material, surface roughness, and surface area impact the dielectric strength of water. Voltage pulses with respective rise times of roughly 200 and 20 ns were applied to a water test gap producing electric fields in excess of 1.5 MV/cm. In experiments testing various electrode materials, thin film coatings of various metallic alloys and oxides were applied to Bruce-profiled stainless steel electrodes, with an effective area of 5$hboxcm^2$, through ion beam deposition. Similar Bruceprofiled stainless steel electrodes with surface roughness ranging from 0.26 to 1.96$muhboxm$and effective areas ranging from 0.5 to 75$hboxcm^2$were used in the study of surface roughness and area. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur.

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Publication Year:  2005
+ 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
+ The Impact of Surface Conditioning and Area on the Pulsed Breakdown Strength of Water
  Wetz, D.A., Jr.; Truman, K.P.; Mankowski, J.J.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 33, Issue 4, Part 1, Aug. 2005 Page(s):1161 - 1169
Abstract:  Experimental results are presented on the degree to which electrode surface conditioning and surface area impact the dielectric strength of water. The applied pulse to the test gap reached electric field levels greater than 1.5 MV/cm with risetimes around 200 and 50 ns, respectively. The test gap is composed of 304 stainless steel electrodes machined with a Bruce profile. Electrode surface roughness ranged from 0.34 to 1.41$muhbox m$and effective areas ranged from 0.5 to 75$hbox cm^2$. Additional results are presented on the pulsed breakdown strength of Rexolite with various surface finishes ranging from .025 to 5.715$muhbox m$. Conclusions are made as to the effect electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. Conclusions are also made as to the impact of the surface condition of Rexolite has on its bulk breakdown strength.

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Publication Year:  2004
+ Guest Editorial Fifth Special Issue on Pulsed Power Science and Technology
  Dickens, J.C.; Lehr, J.M.; Mankowski, J.; IEEE Transactions on Plasma Science, Volume 32, Issue 5, Part 1, Oct. 2004 Page(s):1763 - 1764
Abstract:  Not Available

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+ Microwave frequency determination mechanisms in a coaxial vircator
  Xupeng Chen; Dickens, J.; Mankowski, J.; Hatfield, L.L.; Eun Ha Choi; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 32, Issue 5, Part 1, Oct. 2004 Page(s):1799 - 1804
Abstract:  Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. In this paper, some experiments showing different results are reported. In particular, the E-beam is observed to play an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator with different geometries has been investigated in detail. Investigation of the E-beam performance will improve understanding of the interaction between the E-beam and microwaves, which is a key for determining the microwave frequency. These results are helpful in optimizing the design of a cylindrical diode to avoid microwave frequency shifting and mode competition.

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Publication Year:  2003
+ Efficiency results from a coaxial vircator using a simple feedback technique
  Mankowski, J.; Cheng, X.; Dickens, J.; Kritiansen, M.; Choi, E.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 1, 15-18 June 2003 Page(s):455 - 458 Vol.1
Abstract:  Experimental findings on the coaxial virtual cathode oscillator (vircator) at Texas Tech University are reported. A major modification to the system extended the pulse forming line several meters. The pulsed power system now delivers up to 750 kV and 60 kA for at least 70 nsec. Additional modifications include diagnostics to measure the diode voltage, current, and radiated field pattern of the output microwaves. In this initial phase of experiments copper reflectors were installed within the diode to provide a simple means of microwave feedback to assist in the e-beam modulation. Thus far we have observed microwave peak power output with feedback reflectors as high as 3.5 GW corresponding to an efficiency of /spl sim/9.5%. Measured radiated field patterns show evidence of multimode operation.

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

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+ Subnanosecond corona inception in an ultrawideband environment
  Mankowski, J.; Dickens, J.; Kristiansen, M.; Lehr, J.; Prather, W.; Gaudet, J.; IEEE Transactions on Plasma Science, Volume 30, Issue 3, Part 1, June 2002 Page(s):1211 - 1214
Abstract:  Corona discharges in ultrawideband radiating systems can have adverse effects on performance such as reflection, phase dispersion, and significant power losses. A test-bed has been assembled to experimentally observe corona created by voltage pulses similar to ultrawideband systems. The current work involves the attenuation of an incident pulse after propagation through a self-initiated corona and relative measurements of visible light emission from the photoionization produced during streamer development. Several gas dielectrics, including ambient air, N/sub 2/, H/sub 2/, and SF/sub 6/, were tested.

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Publication Year:  2000
+ A review of short pulse generator technology
  Mankowski, J.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 28, Issue 1, Feb. 2000 Page(s):102 - 108
Abstract:  Today's ultrafast, pulse generators are capable of producing high-voltage pulses, (>1 kV), with fast, leading-edge rise times, (<1 ns). A review of generator implementation methods is presented that includes a detailed discussion of the various circuit designs and a list of commercially available high-voltage pulse generators. All of these generators are capable of rise times less than a few ns and voltages greater than several hundred volts. Finally, a brief description of the three primary switch types, reed, spark gap, and solid state is presented

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Publication Year:  1998
+ High voltage subnanosecond breakdown
  Mankowski, J.; Dickens, J.; Kristiansen, M.; IEEE Transactions on Plasma Science, Volume 26, Issue 3, June 1998 Page(s):874 - 881
Abstract:  Present-day ultra-wideband radiation sources produce Megavolt pulses at hundreds of picosecond (ps) risetimes. Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or nonexistent. In support of the design of these ultra-wideband sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and high-pressure gases. These include air, N2, H2, SF6, and transformer oil. Gap voltages attained were over 700 kV and gas pressures were over 150 atm (15 MPa). Breakdown times achieved were on the order of 600 ps. Electric field strengths observed for given breakdown times were higher than predicted by other investigators. An empirical fit is presented for the data obtained

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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:  2010
+ 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
+ Effects of Gas Temperature and Gas Mixtures on a Triggered, Sub-ns Jitter, 50 kV, 100 Hz Spark Gap
  Y. Chen, J. Dickens, J. Mankowski, M. Kristiansen, "Effects of Gas Temperature and Gas Mixtures on a Triggered, Sub-ns Jitter, 50 kV, 100 Hz Spark Gap", 2010 IEEE International Power Modulators and High Voltage Conference, Atlanta GA, May 2010.
Abstract:  Recent research efforts at Texas Tech University on impulse antenna phased array have shown that an ideal jitter of a small fraction of the rise-time is required to accurately synchronize the array to steer and preserve the rise-time of the radiated pulse. This has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with very low jitter. This manuscript presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter. A 50 Ω, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Gases tested include N2, dry air, H2, and SF6, as well as N2-H2, N2-SF6, N2-Ar, and gas mixtures containing Kr85. This manuscript will discuss in detail 50 kV, 100 Hz triggered switch operations of such gases. Switch jitter as a result of triggering conditions is also discussed, also including an evaluation of jitter as a function of formative delay in various gases. An evaluation of switch jitter as a function of operation time and gas temperature is also included. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns rise-time pulses at a rep-rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F -PCTFE (PolyCholoroTriFluoroEthylene) lining is used to house the switch and high pressure gas. It is shown that jitter is strongly dependent on the triggering technique, as well as the trigger magnitude, with ionization rates playing an important role. Sub-ns jitter is seen with a variety of gases and gas mixtures with H2 producing the best results. Varying the gas temperature and addition of radioactive sources are seen to improve the switch jitter.

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+ Synchronization of Phased Array Pulsed Ring-down Sources using a GPS based timing system
  Y. Chen, J. Dickens, S. Holt, D. Reale, J. Mankowski, and M. Kristiansen, "Synchronization of Phased Array Pulsed Ring-down Sources using a GPS based timing system", 2010 IEEE International Power Modulators and High Voltage Conference, Atlanta GA, May 2010.
Abstract:  A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be multiplied by the number of sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the individual antennas to operate at different geo-locations to function in a coordinated fashion to deliver the peak power of each element to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser, low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maxim power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to ~10 ns without any correction factors. With the timing and geospatial errors taken into account, the proposed concept will show usable gains of up to several hundred MHz.

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+ Theoretical Performance of a GPS Linked Pulsed Ring Down Array
  D. Reale, J. Mankowski, S. Holt, Y. Chen, J. Walter, and J. Dickens, "Theoretical Performance of a GPS Linked Pulsed Ring Down Array", 2010 IEEE International Power Modulators and High Voltage Conference, Atlanta GA, May 2010.
Abstract:  Current research at Texas Tech University is focused on the development of a High-Power Pulsed Ring-Down Source (PRDS) Antenna Array. Previously, a Monte Carlo based analysis was conducted in order to predict the array performance based upon the estimated switching jitter between elements. This analysis showed good performance for jitter times between 0 to 2 periods of the ringing frequency. Therefore, for ringing frequencies up to 500 MHz, jitter times up to 4 nanoseconds can be tolerated. Subsequently, we have shown practical switching solutions capable of the sub-nanosecond switching performance needed for the frequencies of interest. Taking the analysis a step further, we introduce the uncertainty of the absolute position of each antenna element. To implement a randomly distributed array, where the position of elements is not fixed, a method of accurately resolving element positions relative to each other and the target location is required. The use of a variety of GPS technologies and techniques is explored as a method for position and timing resolution. The relative accuracy between elements and the absolute accuracy of each element is discussed. A Monte Carlo based analysis is conducted to predict array performance based upon GPS positional error, GPS timing error, and switch jitter.

[PDF]

Publication Year:  2008
+ A 50 kV, 100 Hz Low Jitter Triggered Spark Gap with High Pressure Gas Mixtures
  Chen, Yeong-Jer; Mankowski, John J.; Dickens, James C.; Walter, John; Kristiansen, Magne; A 50 kV, 100 Hz Low Jitter Triggered Spark Gap with High Pressure Gas Mixtures; IEEE International Power Modulators and High Voltage Conference, Proceedings of the 2008; 27-31 May 2008 Page(s):197 - 200; Digital Object Identifier 10.1109/IPMC.2008.4743614
Abstract:  Recent research efforts at Texas Tech University on impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. In (Y. Chen et al., 2007), we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. This paper discusses in detail 50 k, 100 Hz switch operations with different gases. Gases tested include, dry Air, H2, N2, and SF6, as well as H2-N2, and N2-SF6 gas mixtures. Switch jitter as a result of triggering conditions is discussed, also including a comprehensive evaluation of jitter as a function of formative delay in the various gases.

[PDF]

+ Analysis of Mesoband Single Element Pulsed Ring-Down Antennas for Implementation in Phased Array Systems
  Belt, D.; Mankowski, J.; Walter, J.; Dickens, J.; Kristiansen, M.; IEEE International Power Modulators and High Voltage Conference, Proceedings of the 2008 27-31 May 2008 Page(s):152 - 155 Digital Object Identifier 10.1109/IPMC.2008.4743602
Abstract:  In recent years, the pulsed ring-down antenna has become of great interest due to its compact size and high power on target potential. Since these systems are fairly new in study, it is often difficult to predict the overall performance without experimental evaluation. A pulsed ring-down antenna operates by charging the single element antenna with a high potential source and then closing a switch to develop transient wave reflections on the antenna, typical CW case analysis does not apply. For this reason, we have constructed a simulation model that allows us to predict the transient behavior of the structure. By utilizing the Comsol RF module transient analysis functions, we are able to characterize various parameters of different antennas, beginning with a dipole pulsed ring-down antenna operating around the 100 MHz range. After examining the simulated results against the experimental results for accuracy, we then moved to more complicated mesoband antenna structures. The simulation model developed within the COMSOL RF module allows us to examine various influential factors such as material losses, transient switching effects, structure capacitance, switch capacitance, and initial charging losses. With this, we are able to examine methods to improve the results in the far field such as capacitive spark gap loading and other capacitive storage methods. Utilizing the pulsed ring-down antenna model, we are able to give a better characterization of mesoband pulsed ring-down structures for implementation into a specific or multi-purpose phased array system.

[PDF]

+ Jitter and recovery rate of a 50 kV, 100 Hz triggered spark gap with high pressure gas mixtures
  Yeong-Jer Chen; Mankowski, J.J.; Walter, J.W.; Dickens, J.C.; Jitter and recovery rate of a 50 kV, 100 Hz triggered spark gap with high pressure gas mixtures; Plasma Science, 2008. ICOPS 2008. IEEE 35th International Conference on 15-19 June 2008 Page(s):1 - 1 Digital Object Identifier 10.1109/PLASMA.2008.4590809
Abstract:  Summary form only given.Recent attention to impulse antenna phased array has necessitated the need to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the rise time is required to accurately synchronize the array to steer and preserve the rise time of the radiated pulse. This paper presents the impact gases and gas mixtures have on switch performance which includes recovery rate and in particular, jitter. A 50 Omega, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source. Triggering is provided by an SOS voltage source that supplies >100 kV, 10 ns rise-time pulses at a rep rate up to 1 kHz in burst mode. A hermetically sealed spark gap with a Kel-F lining is used to house the switch and high pressure gas. The system includes a gas mixing chamber that can mix various gases up to 2000 psi. Gases tested include dry air, H2, N2, and SF6. Initial testing with 30 kV, 10 Hz switch operations have shown reliable sub-ns jitter times with pure gases including dry air, H2, N2, and with H2-N2, and N2-SF6 gas mixtures. The system was then modified for 50 kV, 100 Hz operations with no recovery issues. Jitter data for pure gases, H2-N2, N2-SF6, and various Kr85 gas mixtures at the 50 kV, 100 Hz operation is also documented and compared in this paper.

[PDF]

Publication Year:  2007
+ 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.

[PDF]

+ Modeling of a Single Element Pulsed Ring-Down Antenna for Implementation in a Phased Array System
  Belt, D.; Walter, J.; Mankowski, J.; Dickens, J.; Modeling of a Single Element Pulsed Ring-Down Antenna for Implementation in a Phased Array System; Pulsed Power Plasma Science, 2007. PPPS 2007. Conference Record - Abstracts. IEEE 17-22 June 2007 Page(s):816 - 816
Abstract:  Summary form only given. A pulsed ring-down phased array antenna provides substantial energy deposition in the far field region in addition to a broad range main beam with scanning capabilities. This allows remote neutralization of Improvised explosive devices (IEDs) at far field distances and in virtually any direction. The pulsed ring-down antenna operates by charging the single element antenna with a high potential source and closing a switch to develop transient wave reflections on the antenna which then propagate in air. The performance of a pulsed ring-down phased arrav is highly contingent upon the design and performance of the individual antenna elements within the array. Such factors as operating voltage, antenna capacitance, material losses, antenna geometry and closing switch conductance characteristics must be examined for optimal performance to be achieved. By utilizing the Comsol RF module transient analysis functions, we are able to characterize the various parameters beginning with a monopole and a dipole pulsed ring-down antenna operating in the hundreds of MHz range. We have examined and compared the results achieved from the experimental setup to the simulation model in order to better characterize the individual components of the antenna. We have also examined the discrepancies between an ideal closing switch and the experimental setup closing switch, which dramatically affects the far field range of the antenna. We have examined the material properties of the antenna to improve losses and increase system capacitance allowing an increase in the number of RF cycles per antenna discharge. With the results presented, an accurate model of pulsed ring-down antennas is available and will allow future development of more complex geometries that will improve the operation of pulsed ring- down phased array.

[PDF]

+ 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
+ Theoretical Pulsed Ring Down Antenna Array Performance
  Walter, J.; Dickens, J.; Mankowski, J.; Kristiansen, M.; Theoretical Pulsed Ring Down Antenna Array Performance Pulsed Power Plasma Science, 2007. PPPS 2007. Conference Record - Abstracts. IEEE 17-22 June 2007 Page(s):482 - 482
Abstract:  Summary form only given. High-power transient RF transmitters are currently being developed for multiple applications. One potential high- power pulsed source being considered is the pulsed ring- down source (PRDS). In a typical PRDS, electrical energy that is stored in the device structure is discharged through a switch, generating a damped ringing radiated waveform. The magnitude of the transmitted field is limited by parameters such as the overall device geometry and switch characteristics. The potential exists to increase the radiated far field power density by utilizing an array of PRDS transmitters. The performance of such an array depends on the triggering jitter of the device switches and the design of the individual radiating elements. The application of the array concept to pulsed ring-down sources using gas spark gap switches is examined. Theoretical array performance is discussed, taking into account practical switch jitter distributions, practical switch triggering schemes, and individual element characteristics.

[PDF]

Publication Year:  2006
+ A Flux Compression Generator Non-Explosive Test Bed for Explosive Opening Switches
  Belt, D.; Mankowski, J.; Neuber, A.; Dickens, J.; Kristiansen, M.; 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

[PDF]

+ 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
+ The Impact of Water Conductivity, Electrode Material, and Electrode Surface Roughness on the Pulsed Breakdown Strength of Water
  Wetz, D.; Mankowski, J.; McCauley, D.; Dickens, J.; Kristiansen, M.; Conference Record of the 2006 Twenty-Seventh International Power Modulator Symposium, 2006. 14-18 May 2006 Page(s):104 - 107
Abstract:  Experimental results are presented on the impact water conductivity, electrode material, and electrode surface roughness have on the dielectric strength of water. A 2 mm water gap was tested under pulsed conditions with maximum electric fields in excess of 1.5 MV/cm. Bruce profiled electrodes manufactured from various materials including molybdenum, aluminum, copper, tungsten, nickel, stainless steel, and zinc oxide, which have work functions ranging from 4.2 eV to 5.3 eV, have been tested. The roughness average of the various surfaces was varied from 0.1 mum to 1.96 mum and the conductivity of the water within the gap was varied from 1muS/cm to 39 muS/cm to determine the impact each has when tested in combination with the various electrode materials and surface roughnesses. Conclusions are made as to the effect each of these factors has on the holdoff strength of a water dielectric system

[PDF]

+ Virtual Cathode Oscillator Component Optimization
  Chen, Y.-J.; Mankowski, J.; Walter, J.; Kristiansen, M.; Conference Record of the 2006 Twenty-Seventh International Power Modulator Symposium, 2006. 14-18 May 2006 Page(s):157 - 160
Abstract:  Recent research efforts at TTU include a simple aluminum cathode for E-beam generation in a virtual cathode oscillator (vircator). As a continuation of these results, we are fabricating and testing several different types of cathodes for the vircator driven by a Marx generator and a pulse forming line (400 kV, 60 ns, 30 Omega). The cathodes, each with an emitting area of ~32 cm2, are made from aluminum and oxygen free copper fashioned to similar geometries with either a chemical etch or a CNC machine process. The vircator is tested with all of these cathodes and evaluated for beam voltage, current, and HPM generation. In addition to the cathode testing, several stainless steel and tungsten anode meshes with varying transparencies (50%-80%) are evaluated

[PDF]

Publication Year:  2005
+ A Low-Cost Metallic Cathode for a Vircator HPM Source
  Mankowski, J.; Chen, Y.; Dickens, J.; Neuber, A.; Gale, R.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):66 - 69
Abstract:  Recent research efforts at TTU include the testing of a rapidly charged, rep-rated Marx generator driving a reflex triode vircator [1]. As expected, the burning of the cathode material (ordinary cloth velvet), was the primary failure mechanism during repetitive operation. In an effort to achieve a repetitive vircator (>10 Hz), we are exploring a low-cost, all-metal cathode. The cathode is made from aluminum with a patterned surface. A typical pattern is composed of peaks and troughs with dimensions on the order of tens of microns. The pattern is achieved with a simple, low-cost chemical etching process. Results include current, voltage, and microwave waveforms from two solid metal cathodes and a cloth velvet cathode.

[PDF]

+ 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
+ An Ion Beam Sputtering System for Electrode Surface Conditioning
  Wetz, D.; Mankowski, J.; Kristiansen, M.; IEEE Conference Record - Abstracts. IEEE International Conference on Plasma Science, 2005. ICOPS '05. 20-23 June 2005 Page(s):123 - 123
Abstract:  Summary form only given. In the past there has been considerable research performed using electromagnetic plasma thrusters as the primary ion source in sputtering applications. In the experiments conducted here, thin film coatings of various hard metallic alloys and oxides were applied to stainless steel electrodes using ion beam deposition. An SPT type Hall effect thruster capable of producing an ion beam with energies between 150 eV and 350 eV was used as the deposition ion source. The surface coatings were applied to Bruce profiled stainless steel electrodes which have an effective surface area of 5 cm2 and a roughness average of roughly 70 nm. The electrodes were then examined using an optical profiler to accurately measure the roughness average of the thin film coating as well as the effective layer thickness. A comparison of the results obtained from each surface material is performed

[PDF]

+ Multi-Frequency and Multi-Mode Microwave Identification in a Vircator Research
  Xupeng Chen; Dickens, J.C.; Hatfield, L.L.; Mankowski, J.; Kristiansen, M.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):190 - 193
Abstract:  In virtual cathode oscillator (VIRCATOR) research, multi-frequency and multi-mode microwaves are coupled out and propagate along a waveguide. Identifying the microwave propagating modes has become a priority work for further research on microwave radiation physical mechanisms. Generally, an antenna matrix at the end of an open waveguide, which can shape the radiated microwave power distribution, is used to identify the microwave propagating modes [1-2]. Actual microwave radiation mechanisms in a VIRCATOR are complicated. Multi propagating modes and multifrequency microwaves coexist simultaneously, which challenges the antenna matrix method. In this paper, based on microwave propagating theory in a waveguide, a new method is proposed to identify multi-frequency and multi-mode microwaves, which is partially justified by the experimental data at Texas Tech University.

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+ 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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+ The Impact of Electrode Area and Surface Roughness on the Pulsed Breakdown Strength Water
  Wetz, D.; Mankowski, J.; Kristiansen, M.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):1163 - 1166
Abstract:  Experimental results are presented on the degree to which electrode surface area and surface roughness impacts the dielectric strength of water. A 2 mm water gap was tested under pulsed conditions with maximum electric fields in excess of 1 MV/cm and maximum currents of more than 5 kA. Six different pairs of stainless steel electrodes, each having a unique Bruce profile and thus a different effective surface area, were used to achieve a uniform electric field across the gap. The differing electrode pair profiles, with effective areas ranging from 0.5 cm2 to 76 cm2, were designed to minimize the change in gap capacitance. Prior to each test, a different roughness average, ranging from 0.26 ¿m to 1.96 ¿m, was applied to the electrodes. Conclusions are made as to the effect both electrode surface area and surface roughness has on the holdoff voltage of water dielectric systems. In addition, shadowgraph images of pre-breakdown events are presented.

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+ The Impact of Electrode Material on the Pulsed Breakdown Strength of Water
  Wetz, D.; Mankowski, J.; Dickens, J.; Kristiansen, M.; 2005 IEEE Pulsed Power Conference, June 2005 Page(s):935 - 938
Abstract:  In the experiments presented here, various electrode materials were tested in an effort to determine the impact each has on increasing the dielectric strength of water. Prior investigations have tested materials such as stainless steel, copper, nickel, gold, silver, and cuprous oxide [1-4]. In our experiments, thin film coatings of various metallic alloys and oxides were applied to Bruce profiled stainless steel electrodes with an effective area of 5 cm2. An ion beam sputtering process was used to apply the coatings with thicknesses of several hundred nm. The electrodes were then tested across a water gap, with pulse lengths in both the microsecond and nanosecond time regimes. Electric fields in excess of 8 MV/cm were applied. Conclusions are made as to the impact electrode material has on the pulsed breakdown strength of water.

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Publication Year:  2004
+ Compact Pulsed Power
  M. Kristiansen, J. Dickens, H. Krompholz, M. Giesselmann, A. Neuber, J. Mankowski, L. Hatfield, “Compact Pulsed Power,“ Proceedings of the 5th International Symposium on Pulsed Power and Plasma Applications, Oct. 18-20, Chan-Won, Korea, p. 10-15, 2004. (Invited paper)
Abstract:  Not Available
+ Impact of surface conditioning of large area electrodes on the dielectric strength of water
  Wetz, D.; Mankowski, J.; Truman, K.; Kristiansen, M.; Conference Record of the Twenty-Sixth International Power Modulator Symposium, 2004 and 2004 High-Voltage Workshop. 23-26 May 2004 Page(s):512 - 515
Abstract:  Experimental results are presented on the impact electrode material and surface treatment has on the dielectric strength of water. A 4 mm water gap was tested under pulsed conditions with pulse widths greater than 1 /spl mu/s, peak electric fields over 2 MV/cm and peak currents over 15 kA. 304 stainless steel electrodes machined with a Bruce profile and an effective area of 5 cm/sup 2/ were tested with surface roughnesses ranging from .34 /spl mu/m to 1.41 /spl mu/m. Results comparing electrodes that have additionally been electropolished are compared to those without an additional treatment. These various surface treatments remove microprotrusions from the electrode's surface which reduce field enhancements across the electrode surface. It is believed that this technique will minimize the number of breakdown initiation points thereby increasing the dielectric strength. Results are given on the degree to which each finish improves the dielectric strength of water.

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+ The effect of area on pulsed breakdown in water
  Truman, K.; Mankowski, J.; Wetz, D.; Kristiansen, M.; Conference Record of the Twenty-Sixth International Power Modulator Symposium, 2004 and 2004 High-Voltage Workshop. 23-26 May 2004 Page(s):126 - 128
Abstract:  Experimental results are presented on the degree to which electrode surface area impacts the dielectric strength of water. A water gap of 4 mm was tested under pulsed conditions with a maximum electric field in excess of 1 MV/cm and a maximum current of more than 20 kA. Stainless steel electrodes with a Bruce profile were used to generate a uniform electric field across the water gap. The profile of the electrodes enabled effective areas ranging from 0.5 cm/sup 2/ to 75 cm/sup 2/ while minimizing the change in capacitance of the water gap. Conclusions are made as to the effect electrode surface area has on the holdoff voltage and holdoff time of water dielectric systems.

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Publication Year:  2003
+ Cavity resonance effect on a coaxial vircator
  Xupeng Chen; Dickens, J.; Eun Ha Choi; Mankowski, J.; Hatfield, L.L.; Kristiansen, M.; Digest of Technical Papers. PPC-2003. 14th IEEE International Pulsed Power Conference, 2003. Volume 2, 15-18 June 2003 Page(s):1165 - 1168 Vol.2
Abstract:  Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. However, some experiments at TTU show that the interaction between the virtual cathode oscillation and the cavity is a key in determining the microwave frequency and propagating mode. Particularly, we observe that the E-beam plays an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator has been investigated in detail. The understanding of the E-beam characteristics is critical in optimizing the design of the cylindrical diode to avoid the microwave frequency shifting and mode competition.

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+ 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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+ Experimental results of a 2 GW vircator
  Mankowski, J.J.; Choi, E.H.; Dickens, J.C.; Xupeng Chen; Kristiansen, M.; IEEE Conference Record - Abstracts. The 30th International Conference on Plasma Science, 2003. ICOPS 2003. 2-5 June 2003 Page(s):187
Abstract:  Summary form only given. Experimental findings on the coaxial virtual cathode oscillator (vircator) at Texas Tech University are reported. Recent modifications to the system include extending the pulse forming line that now provides an applied diode voltage of 600 kV for 70 nanoseconds at 60 kA. Other modifications include diode voltage, current, and radiated field diagnostics. In this initial phase of experiments, copper reflectors were installed within the diode to provide a simple means of microwave feedback to assist in the e-beam modulation. Thus far we have observed microwave peak power output with feedback reflectors as high as 2 GW corresponding to an efficiency of -6%. The mode observed at these high efficiencies is almost entirely TE/sub 11/. At lower efficiencies, both TM/sub 01/, and TE/sub 11/ modes are observed. These results suggest mode competition at lower efficiencies.

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+ Reflex-triode geometry of the virtual-cathode oscillator
  Lara, M.B.; Mankowski, J.; 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):1161 - 1164 Vol.2
Abstract:  An eight-stage four-hundred kilovolt Marx bank, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a planar version of the virtual cathode oscillator (vircator). Eight .1/spl mu/F capacitors, charged to 50 kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-300 kV pulse to the diode. The design of the planar or reflex-triode geometry vircator is based upon claims of high efficiency by Didenko et al. A previously constructed TTU vircator includes a unique E-beam source, the brush cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a virtual cathode. This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.

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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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Publication Year:  2002
+ 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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Publication Year:  2001
+ Gas breakdown in the sub-nanosecond regime with voltages below 15 kV
  Krompholz, H.; Hatfield, L.L.; Kristiansen, M.; Hemmert, D.; Short, B.; Mankowski, J.; Brown, M.; Altgilbers, L.; Digest of Technical Papers, Pulsed Power Plasma Science, 2001. PPPS-2001. Volume 1, 17-22 June 2001 Page(s):487 - 490 vol.1
Abstract:  Gaseous breakdown in the sub-nanosecond regime is of interest for fast pulsed power switching, short pulse electromagnetics and for plasma limiters to protect devices from high power microwave radiation. Previous investigations of sub-nanosecond breakdown were mainly limited to high-pressure gases or liquids, with applied voltages in excess of 100 kV. In this paper, the authors investigate possibilities to achieve sub-nanosecond breakdown at applied voltages below 7.5 kV in point-plane geometries. The setup consists of a pulser (risetime between 400 ps to 1 ns), 50-/spl Omega/ transmission line, axial needle-plane gap with outer coaxial conductor, and a 50-/spl Omega/ load line. The needle consists of tungsten and has a radius of curvature below 0.5 /spl mu/m. The constant system impedance of 50 /spl Omega/ (except in the vicinity of the gap) and a special transmission-line-type current sensors enables current and voltage measurements with a dynamic range covering several orders of magnitude, with temporal resolution down to 80 ps. For pulse amplitudes of 1.7 kV (which are doubled at the open gap before breakdown) delay times between start of the pulse and start of a measurable current flow (amplitude > several milliamperes) have a minimum of about 8 ns, at a pressure of 50 torr in argon. Voltages of 7.5 kV produce breakdowns with a delay of about 1 ns. With negative pulses applied to the tip, at an amplitude of 7.5 kV, breakdown is always observed during the rising part of the pulse, with breakdown delay times below 800 ps, at pressures between 1 and 100 torr. At lower pressure, a longer delay time (8 ns at 50 mtorr) is observed. They authors expect the breakdown mechanism to be dominated by electron field emission, but still influenced by gaseous amplification.

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Publication Year:  1999
+ Energy Efficiency Analysis of an Inductive Storage System
  J. Kim, J. Zhang, M. Giesselmann, J. Dickens, J. Mankowski, M. Kristiansen, “Energy Efficiency Analysis of an Inductive Storage System”, Proceedings of the 12th IEEE International Pulsed Power Conference, Monterey, California, June 27-30, 1999.
Abstract:  Not Available
+ Energy efficiency analysis of an inductive storage system
  Kim, J.; Zhang, J.; Giesselmann, M.; Dickens, J.; Mankowski, J.; Kristiansen, M.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 2, 27-30 June 1999 Page(s):1445 - 1448 vol.2
Abstract:  In this paper, the simulation and evaluation of energy efficiency and voltage amplification in an inductive energy storage system with resistive, capacitive and diode loads is presented. A numerical simulation with Mathcad shows that by proper reduction of the storage energy inductance and of the exploding fuse interruption time, the energy efficiency and voltage amplification can be greatly improved

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+ High voltage subnanosecond corona inception
  Mankowski, J.; Dickens, J.; Kristiansen, M.; Lehr, J.; Prather, W.; Gaudet, J.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 2, 27-30 June 1999 Page(s):1392 - 1395 vol.2
Abstract:  Corona discharges in ultra-wideband radiating systems can have adverse effects on performance such as reflection, phase dispersion, and significant power losses. A test-bed has been assembled to experimentally observe corona created by voltage pulses similar to ultra-wideband systems. The current work involves the voltage attenuation of an incident pulse after propagation through a self-initiated corona and relative measurements of visible light emission from the photoionization produced during streamer development. Several gas dielectrics, including ambient air, N2, H2 , and SF6, were tested

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+ The Design of a compact Pulse Transformer
  J. Zhang, J. Dickens, M. Giesselmann, J. Kim, E. Kristiansen, J. Mankowski, D. Garcia, M. Kristiansen, “The Design of a compact Pulse Transformer”, Proceedings of the 12th IEEE International Pulsed Power Conference, Monterey, California, June 27-30, 1999.
Abstract:  Not Available
+ The design of a compact pulse transformer
  Zhang, J.; Dickens, J.; Giesselmann, M.; Kim, J.; Kristiansen, E.; Mankowski, J.; Garcia, D.; Kristiansen, M.; Digest of Technical Papers. 12th IEEE International Pulsed Power Conference, 1999. Volume 2, 27-30 June 1999 Page(s):704 - 707 vol.2
Abstract:  To improve the pulse shape and to obtain μs order pulse duration on a diode load in an inductive energy storage system, an oil-submerged compact pulse transformer with diameter of 20 cm and length of 70 cm has been designed and tested. From theoretical calculations, the parameters with primary inductance Lp=3.4 μH, secondary inductance Ls=90 μH, and coupling coefficient K=0.772 are obtained. To prevent breakdown, a conical secondary design is adopted. Under the condition of 500 kV operation voltage, the maximum electric field in the transformer is 205 kV/cm (in oil). The axial voltage distribution on the secondary is linear. Experiments have been done to test the insulation, the mechanical force and the coupling coefficient. The test results are consistent with the design parameters

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Publication Year:  1998
+ High voltage subnanosecond dielectric breakdown
  Mankowski, J.; Dickens, J.; Kristiansen, M.; IEEE Conference Record - Abstracts. 1998 IEEE International on Plasma Science, 1998. 25th Anniversary. 1-4 June 1998 Page(s):270
Abstract:  Summary form only given, as follows. Present day ultra-wideband (UWB) radiation sources a produce Megavolt pulses at 100's of picosecond (ps) risetimes. Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or non-existent. In support of the design of these UWB sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and gases. These include air, N2, H2, He, SF6, and transformer oil. The two voltage sources used in the experiments are capable of delivering 400 and 700 kV with a 400 ps risetime into an open load. These pulses are applied to the test gap area, capable of housing various gases and liquids at pressures from less than 1 to 150 atm. An empirical relationship of E-field versus breakdown time for the observed dielectrics is presented. Several other breakdown phenomena at these fast risetimes are observed. Dielectric breakdown strength dependence on polarity is investigated. Streak camera images of arc formation are captured, providing information on gap closure velocity. Also observed is the effect of ultraviolet radiation on the statistical lag time of breakdown for gas dielectrics at various pressures

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Publication Year:  1997
+ A subnanosecond high voltage pulser for the investigation of dielectric breakdown
  Mankowski, J.; Dickens, J.; Kristiansen, M.; Pulsed Power Conference, 1997. 1997 11th IEEE International Digest of Technical Papers. Volume 1, 29 June-2 July 1997 Page(s):549 - 554 vol.1
Abstract:  A high voltage, sub-nanosecond pulser is designed and built for the purposes of investigating dielectric breakdown. The requirement for the pulser is a voltage pulse of several hundred kilovolts, pulsewidth less than a few nanoseconds, and a risetime <400 psec. This is achieved by using pulse sharpening techniques on the output of a 500 kV Marx bank. Originally designed to stimulate a lightning strike, the voltage waveform from the Marx generator has a slow decay time of tens of μsec. In order to obtain a more desirable pulse, the Marx bank is modified. By removing a lumped-element resistor a higher peak output voltage with a faster risetime can be obtained. Circuit simulations have shown the capability of achieving an 800 kV output in less than 40 nsec to a charging (pulse forming) line. The 50 Ω impedance, oil-filled, pulse forming line consists of a peaking gap and pulse slicing gap. The peaking gap decreases the risetime of the applied pulse down to about 300 psec. The pulse slicing gap is included to short the voltage applied to the delay line and test chamber

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+ High voltage subnanosecond dielectric breakdow - 1997
  Mankowski, J.; Hatfield, L.; Kristiansen, M.; Agee, F.J.; Lehr, J.M.; Wells, J.; IEEE Conference Record - Abstracts., 1997 IEEE International Conference on Plasma Science, 1997. 19-22 May 1997 Page(s):317 - 318
Abstract:  Summary form only given. Present day ultra wideband radiation sources produce megavolt pulses at 100's of picosecond pulsewidths. Empirical data on the breakdown characteristics for dielectric media at these short time lengths and high voltages are either extremely limited or non-existent. In support of the design of these ultra wideband sources, we are investigating the breakdown characteristics, at these voltages and time lengths, of several liquids and gases. These include air, N2, H2, He, SF6, transformer oil, and freon-12

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