Browsing by Subject "Electric spark gaps"
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Item A fast preionization source for diffuse discharges containing attachers(Texas Tech University, 1983-12) Cooper, James RandallDiffuse discharge opening switches and most excimer lasers must operate with gas mixtures containing attachers. In self sustained discharges, preionization is required for arc free initiation of these discharges. The preionizer must have a short rise and falltime and must have an extremely accurate timing system because of the attachers in the discharge gas. Also, this device must have sufficient power to produce large volumetrically uniform electron densities in gases at pressures in the one atmosphere range. This paper describes the design and construction of such a preionizer and gives an overview of various preionization techniques previously employed. The device presented here consists of eight coaxial cables for energy storage, which are switched through a master spark gap to eight cables terminated by individual multi-spark arrays. The preionizer and the main discharge are triggered by a single laser (with a beam splitter and with a variable optical delay on one of the beams). Experiments showing the influence of the delay, preionization pulse length, jitter, and durability of the spark sources on the main discharge are described.Item A new design concept for field distortion triggered spark gaps(Texas Tech University, 1986-12) Pashaie-Awal, BijanCommon field-distortion triggered spark gaps utilizing geometric field enhancement at sharp edges usually operate in a cascade mode via the trigger electrode. A new trigger concept is proposed allowing strong field enhancement and direct breakdown between the two main electrodes. A test device was designed to prove the feasibility of this concept. Experimental results on hold-off voltage and on delay and jitter dependent on percent breakdown voltage are presented. Best results achieved are a delay of 9 nanoseconds (ns) and a jitter of 2 ns at a self-breakdown voltage of 15 KV. The fields code ELF (Tetra Corporation, Albuquerque, NM) was used to verify the experimental results. An optimum location of the trigger electrode was calculated, and it matched the experimental data within a 5% accuracy. A full-scale field distribution graph was generated in both the triggered and the non-triggered cases, which are believed to represent the triggering action.Item A study of cathode erosion in high power arcjets(Texas Tech University, 2002-05) Harris, William JacksonCathode erosion continues to be one of the predominant technology concerns for high power arcjets. This study will show that cathode erosion in these devices is significantly affected by several mitigating factors, including propellant composition, propellant flowrate, current level, cathode material, and power supply current ripple. In a series of 50-hour and 100-hour long duration experiments, using a water-cooled 30-kilowatt laboratory arcjet, variations in the steady-state cathode erosion rate were characterized for each of these factors using nitrogen propellant at a fixed arc current of 250 Amperes. A complementary series of measurements was made using hydrogen propellant at an arc current of 100 Amperes. The cold cathode erosion rate was also differentiated from the steady-state cathode erosion rate in a series of multi-start cathode erosion experiments. Results of these measurements are presented, along with an analysis of the significant effects of current ripple on arcjet cathode erosion. As part of this study, over a dozen refractory cathode materials were evaluated to measure their resistance to arcjet cathode erosion. Among the materials tested were W-ThO2 (1%, 2%, 4%), poly and mono-crystalline w, W-LaB6 , W-La2O3 , W-Ba02, W-BaCaAl204 , W-Y2O3 , and ZrB2. Based on these measurements, several critical material properties were identified, such work function, density, porosity, melting point, and evaporation rate. While the majority of the materials failed to outperform traditional W-ThO2, these experimental results are used to develop a parametric model of the arcjet cathode physics. The results of this model, and the results of a finite-element thermal analysis of the arcjet cathode, are presented to better explain the relative performance of the materials tested.Item An electron beam triggered spark gap(Texas Tech University, 1981-08) McDonald, Kenneth FoxThe triggering of a high-voltage gas-insulated spark gap by an electron (e) beam has been investigated. Rise times of approximately 2.5 ns with sub-nanosecond jitter (~0.2 ns) have been obtained for 3 cm gaps charged at voltages as low as 50 percent of the self-breakdown voltage (varied up to 0.5 MV). The switch delay (including the e-beam diode) was 52 ns. The triggering e-beam pulse has a duration of 15 ns and a 0-50 percent rise time of 1.5 ns. The e-beam current is 0.5 kA, and the electron energy can be varied in the range from 80 to 145 keV. The working media were N2, mixture of N2 and Ar, and N2 and SF6 at pressures of 1-3 atm. Voltage, current and jitter measurements have been made for a wide range of gap conditions and e-beam parameters. The effects of varying the e-beam cross-sectional area and the beam energy are discussed.Item An investigation of electron beam initiated spark gap breakdown(Texas Tech University, 1979-12) Newton, Mark AllenA coaxial system was designed and constructed to investigate electron beam initiated discharges in high pressure gases. An electron beam was fired axially into a gap formed by an interruption in the center conductor of the coaxial system to initiate a discharge. Photographic techniques were used as diagnostics to examine the physical processes occurring in the discharge. Open shutter photographs revealed broad discharge channels with spatial dimensions approximately equal to the spatial dimensions of the electron beam. Streak photographs of the discharge provided evidence that an avalanche process was occurring in the discharge.Item Breakdown characteristics of an isolated conducting object in a uniform electric field(Texas Tech University, 1986-12) Grothaus, Michael GlennNot availableItem Breakdown processes in laser triggered switching(Texas Tech University, 1983-05) Dougal, Roger AdelbertAn investigation of laser triggered switching using a 1.06 µ NdrYAG laser to trigger a uniform field gap filled with 800 Torr of N2 has been performed in the following triggering geometries: 1) Transverse not striking electrodes, 2) Coaxial not striking electrodes, 3) Coaxial striking target electrode, and 4) Coaxial striking recess in target electrode. Streak photography, time resolved spectroscopy, and current measurements reveal great differences in the breakdown processes for the various geometries. The major differences are attributable to laser field interaction with the incipient arc channel. All breakdown processes except types 2 and 3 look quite similar to an overvolted breakdown. Diagnostics of type 2 triggering have shown a laser assisted streamer propagating from the laser fireball to the opposite electrode. Streak photography shows the streamer precursor of the breakdown channel initially proceeds across the gap at a few times 10 cm/sec, but slows to about 2X10 cm/sec as it advances in the focal cone to regions of lower laser intensity. The laser interaction with the streamer produces a uniform, high conductivity channel which emits intense continuum light once ohmic heating raises the channel temperature. When delay is greater than the laser pulse length two distinct regions can be detected in the arc channel; one, laser assisted, showing the abrupt, uniform continuum luminosity, and the other not laser assisted, appearing much like a weakly overvolted breakdown process, exhibiting a glow discharge stage followed by thermalization proceeding from the ends of the glow region towards the middle at 10 cm/sec.Item Design and construction of a diffuse discharge experiment(Texas Tech University, 1983-05) Hutcheson, George ZohnA definition and characterization of diffuse discharees is presented, and the conditions necessary for the production of diffuse discharges are discussed. The design and construction of an experimental device that produces diffuse discharges for the study of optically controlled diffuse discharge opening switches is described. It is shown that these control processes require diffuse discharges as a switch medium. Diagnostic voltage and current probes were built into the system and were found to have rise-times of less than 5 ns. The device constructed produces discharges in Helium at gas pressures up to 200 torr.Item Development of intense light flashes and their application to discharge initiation(Texas Tech University, 1982-12) Alley, Michael PaulAn ultraviolet light flasher has been built which consists of a spark gap (Vs = 7 kV) mounted at the end of a coaxial transmission line so that current pulses of five nanosecond width are produced. Intensity studies of the flasher in the spectral range capable of producing photoelectric emission from the electrode in a pulsed discharge experiment were performed and statistical plots showing the variation in intensity at particular wavelengths are presented. The light from the flasher was focused on an electrode arrangement in a pressure-vacuum chamber. Current curves for single avalanches were recorded and the statistical distribution of the number of electrons produced by the flasher were found.Item Electrode erosion measurements in a high energy spark gap(Texas Tech University, 1982-08) Donaldson, Anthony LyleNot availableItem Erosion phenomena of arcing electrodes(Texas Tech University, 1980-05) Petr, Rodney AlanNot availableItem High voltage subnanosecond dielectric breakdown(Texas Tech University, 1997-12) Mankowski, John J.Current interests in ultrawideband radar sources are in the microwave regime, which corresponds to voltage pulse risetimes less than a nanosecond. Some new sources, including the Phillips Laboratory Hindenberg series of hydrogen gas switched pulsers, use hydrogen at hundreds of atmospheres of pressure in the switch. Unfortunately, the published data of electrical breakdown of gas and liquid media at times less than a nanosecond are relatively scarce. A study was conducted on the electrical breakdown properties of liquid and gas dielectrics at subnanosecond and nanoseconds. Two separate voltage sources with pulse risetimes less than 400 ps were developed. Diagnostic probes were designed and tested for their capability of detecting high voltage pulses at these fast risetimes. A thorough investigation into E-field strengths of liquid and gas dielectrics at breakdown times ranging from 0.4 to 5 ns was performed. The breakdown strength dependence on voltage polarity was observed. Streak camera images of streamer formation were taken. The effect of ultraviolet radiation, incident upon the gap, on statistical lag time was determined.Item Investigation of shock pressures in high current spark gap electrodes(Texas Tech University, 1995-08) Wofford, MicheleGenerally, the limiting component in a pulsed power system is the switch This is due to the fact at some point, the switch transfers all of the energy in the system High voltage and current create a rough environment for switch electrodes and insulators. In previous studies of switch reliability and lifetime, electrode erosion has received much attention. This investigation deals with the mechanisms behind electrode fracture, which presents an immediate problem, rather than erosion, which has long-term effects on switch performance. The switches under study are spark gaps with graphite electrodes. The spark gap closes when an electric arc forms between the electrodes, and current conduction begins Strong mechanical forces are generated as the arc forms and impact the electrodes. The purpose of this research is to determine the dominant pressure mechanism of electrode fracture, and the propagation of the pressure wavefront through the electrode. Energy is stored in a 5 kJ, 60 kV capacitor, and shorted through the spark gap. This is done to duplicate prior experimental work at Physics International Company. Operating parameters, such as gap spacing, air pressure, charging voltage, inductance, and magnetic pulse shaping, are varied, and mechanical pressure is measured using a strain gage mounted on a ceramic bar. Experimental data from the strain gage is compared to theoretical modeling done in Mathcad and PSPICE.Item Investigations of overvoltage breakdown(Texas Tech University, 1981-08) Levinson, Scott JosephAn experiment has been constructed to investigate the highly overvolted, electrical gas discharge for the following parameters: pressure (< 1 Torr - 1350 Torr), electrode spacing (1 and 2 cm), electric field (55-110 kV/cm) and electrode material (aluminum, brass, and graphite). In order to determine the effects of these parameters on the statistics associated with the observational time lag, an automated data acquisition system has been built to reduce large amounts of time lag data. An insulator has been placed over the anode in a number of experiments to examine the current waveforms resulting from electron space charge motion in the gap during the pre-breakdown stage. The results indicate that large^ fast rising (~ 1 ns) currents (~ 100 A) exist in the gap before a conducting plasma channel bridging the cathode and anode is formed. These initial currents appear to be the result of runaway electron space charge motion. The results of the statistical investigation indicate that: (1) the graphite electrode is a promising candidate in low jitter switching applications because of its large emissivity (~ le~/ns); (2) the statistics associated with the formative time lag are comparable to those of the statistical time lag (when graphite electrodes are used) at overvoltages as high as 400%. An apparent pressure dependency of the electron emission rate is a consequence of this result; (3) the formative time lag reaches an asymptotic value at the highest values of overvoltage investigated (- 2800%); and (4) the formative time lag is significantly larger than that predicted by the Streamer Theory (Raether's criteria), particularly at the highest values of overvoltage.Item Laser triggered spark gap using fiber optic transmission(Texas Tech University, 1981-05) Harjes, Henry CharlesThe use of an optical fiber in a laser triggering system is described. The fiber transmits a high power ruby laser pulse which triggers a high voltage spark gap. The spark gap is a gas switch on a water dielectric Blumlein generator which is pulse charged by a three stage Marx bank. Typical operating parameters for the spark gap are: 2 cm gap, 2700 Torr pressure, Ar-N^ gas mixture, and a charging voltage of 200 kV. The single strand, 1 mm, quartz, optical fiber is selected specifically for high power transmission capability. Laser pulses of 4 MW are coupled into and transmitted by the fiber with no observed damage. The overall optical system transmission efficiency is 62%. The triggering performance of the system is excellent in that switching delays of less than 10 ns and subnanosecond jitters are measured.Item Item Multichannel surface discharge switch(Texas Tech University, 1983-08) Johnson, Donald EA system designed to investigate the performance of dielectrics in a multichannel surface discharge switch was built and tested. The apparatus consisted of a constant current charging supply that produced 10 mA at 50 kV, a storage capacitor, a 6-ohm stripline, the surface switch, a matched 5-ohra copper sulfate water load, and diagnostics. Diagnostics to evaluate the dielectric performance included capacitive voltage probes in the stripline, a video camera to record multichannel events on videocassettes, and an optically isolated counter to record the number of events in a test run. Typical operating parameters were: 180 joules per pulse, a peak voltage of 40-50 kV, and a peak current of 7 kA. Dielectric samples were tested by placing them in the switch for 19,000 shots at a repetition rate of 1-1.2 pulses per second. The dielectrics were then compared by observing their erosion, changes in breakdown voltage, and the number of channels that occur during discharges. Further analysis to study possible chemical change in the surfaces of three of the dielectrics was performed using Electron Spectroscopy for Chemical Analysis (ESCA).Item Optically controlled diffuse discharges for switching applications(Texas Tech University, 1986-08) Hutcheson, George ZohnDiffuse discharges containing electronegative gases, at present, are the only means capable of fast, repetitive, long-life operation as opening switches. Optical control of diffuse discharge switches has been proposed as both a means of sustaining and of enhancing the performance of such switches. Processes considered in this dissertation are photo ionization. photo detachment. and several approaches to optically enhanced attachment. Ultraviolet (UV) ionization has been used for several years now as a means of preionizing self-sustained diffuse discharges and, recently, has been used as a sustainment mechanism for diffuse discharge opening switches. Time-resolved measurements of the optical emission of a spark array, similar to those used for preionization or sustainment of diffuse discharges, are presented. Results of experiments in atmospheric pressure diffuse discharges containing admixtures of attachers, sustained and initiated by such a spark array, are also presented. Photo detachment is considered as a mechanism which could decrease switch losses and decrease switch closure time by counteracting dissociative attachment. Experimental results are presented demonstrating optically increased current densities, of as much as 900%, in externally sustained and externally initiated, atmospheric pressure, diffuse discharges containing 0^. This process proceeds through photodetachment of 0 by 590 nm light generated by a dye laser (0 +hu -• 0+e). Optically enhanced attachment through the generation of vibrationally excited attachers appears particularly attractive as a means to decrease turn-off or opening times for diffuse discharge switches. For some molecules attachment cross-sections can be increased by orders of magnitude by vibrational excitation. The influence of this effect on the resistivity of a discharge through IR pumping of NH.;^ and CpH^aF by a low power CO^ laser in continuous low pressure discharges is presented. UV enhanced attachment is also shown in gases containing molecules such as C^HpF^. CpHpClp, and CpH-Cl where UV photo dissociation produced vibrationally excited, strongly attaching, molecules (e.g.. C2H2F2+hu-C2H2+HF(v>1)).Item The effect of space charge induced by an electon beam on spark gap operation(Texas Tech University, 1981-08) Tzeng, YonhuaAn investigation into the effect of electron beam induced space charge on the insulating property of a gas in a spark gap is presented. The characteristics of the gas transition from insulator to conductor show strong dependence on the amount and location of the space charge introduced. Investigations of the delay time and the characteristics of the conducting channel have been made. The delay time from the injection of the electron beam to the collapse of the gap voltage ranges from 10^-9 to 10^-3 second. From open shutter photography, we observe that the character of the conducting channel is quite varied. Dark, diffuse, filamentary, or diffuse followed by filamentary (single or multi) channels have been observed, depending on the space charge conditions. The fundamental processes leading to the collapse of insulating property of the gas for various experimental conditions are discussed.Item The effects of electrode divergence angle and mean gap velocity on the performance of gas-blown spark gap switches(Texas Tech University, 1984-08) Borger, Mark AlanThe effects of fluid velocity and electrode divergence angle on the performance of a gas-blown spark gap switch operating in a self-breakdown mode are presented. The switch was contained in a two-dimensional, axially blown, diverging wind tunnel with an aspect ratio of 40:1. The mean gap velocities, the mean flow velocities in the plane of the minimum separation point, ranged from 4 to 17 m/s. The divergence angles studied were 5.0, 7.5, and 10.0 degrees. The flushing gas used was instrument grade dry air ( < 10 ppm H20 ), and the electrode material was 304 stainless steel. High-speed interferograms were used to verify the data reduction program and to identify all events. The relationship between the minimum mean gap velocity, Ūth, and divergence angle, 0, required for a predetermined failure rate was determined in this study. This relationship is of the form, Ūth = A*e-(b0)+B, which illustrates the importance of the divergence angle on the flow rate required for successful switch operation.