T.G. Engel

The pulsed discharge arc resistance and its functional behavior

A generalized comparison of several theoretical and empirical arc-resistance equations was conducted by normalizing the theoretical and experimental arc-resistance values at t=0.5 mu s (approximate time of maximum arc current). It was found that the arc-resistance equations considered could be grouped according to their functional form and were either of the inverse integral or inverse exponential form. The accuracies of both are discussed. At pressures approaching one atmosphere, it has been shown that the equations developed by I.V. Demenik et al. (1968), Kushner et al. (1985), Rompe and W. Weizel (1944), and A.E. Vlastos (1972) were equally accurate predictions of arc resistance for 0.1 mu s >

State-of-the-art insulator and electrode materials for use in high current high energy switching

An investigation into the failure of ceramic insulators that are used in a surface discharge switch (SDS) was conducted. The materials analyzed are Al/sub 2/O/sub 3/-25% SiC, MTF (modified alumina titanate), and CZA 500 (zirconia-alumina composite) ceramics. These insulators were subjected to high-current ( approximately 300 kA) surface discharges in atmospheric air and nitrogen. Energy-dispersive X-ray surface analysis was performed on the insulator surfaces in order to determine the contaminants that are present and the possible failure modes associated with the plasma arc environments mentioned above. Electrode erosion rates have been measured as a function of total charge transfer (up to 50 C/shot) for several in-situ materials including Cu-Nb, Cu-Nb+LaB/sub 6/, and Cu-Ta. Results from comparisons with standard Cu and CuW materials indicate that the in-situ materials represent an efficient method of retaining the copper in the bulk until it vaporizes and thus yield significantly lower erosion rates at high Coulomb transfer rates. >

Review of the mechanisms of electrode and insulator erosion and degradation in high current arc environments

The mechanisms responsible for the erosion and degradation of electrode and insulator materials in high current arc discharge environments such as electromagnetic launchers are reviewed. The review represents the experimental results obtained from several studies and are from investigations into materials performance in high current closing switches such as spark gaps and surface discharge switches. Parameters of interest that affect electrode and insulator material erosion and degradation include peak current, charge transfer, mass erosion, the surface voltage holdoff recovery, and arc velocity. Other parameters that have been shown to affect materials performance include the synergistics produced by certain electrodes, insulators, and gas combinations. Models that describe the erosion and degradation processes are presented and compared to the experimental results. >

Estimating the erosion and degradation performance of ceramic and polymeric insulator materials in high current arc environments

Modeling the erosion and holdoff degradation performance of various commercially available polymeric and ceramic insulators is addressed. The insulators are tested on a surface discharge switch at approximately 300 kA in atmospheric air. Test diagnostics include the surface voltage holdoff recovery and the eroded mass loss of the insulator and electrode materials used. The ceramic materials tested include several types of aluminum and magnesium silicates, several alumina and zirconia composites, and aluminum and silicon nitride. The polymeric insulators include polyvinyl chloride, low- and high-molecular-weight polyethylene, polytetrafluoroethylene, polyamide, acetyl, polyamide-imide, and several types of glass-reinforced epoxies, melamines, and phenolics. The test results indicate that the holdoff degradation resistance and erosion rates can be qualitatively predicated by the use of merit figures which are based on the thermochemical properties of the insulator. The results also show that the holdoff degradation and erosion rates can be improved for some thermoset polymers by a suitable choice of electrode material and/or by the ultraviolet stabilization of the insulator. >

A single gap transformer coupled L-C generator with resonant frequency compensation

A three stage L-C generator has been constructed to provide an output approaching its design ideal. Three dissimilar ferrite core transformers and capacitor pairs were chosen to couple each of the three stages with one switching gap. The resonant frequencies governed by the appropriate total leakage inductance and capacitance of each stage were equalized in this compensated design. The output voltage was switched to a resistive load at various levels below the maximum available. Measurements have been made of the magnitude of the load voltage for decreasing resistance values. Computer simulation studies were conducted to confirm these experimental findings.

Expansion of hydrogen arcs driven by oscillating currents

Calculations of the arc channel radius using the arc current as the independent variable are presented and discussed. Previously reported experimental results for the expansion of approximately 300-A to approximately 3000-A discharges in 460-torr hydrogen are compared with the theoretical radius calculations presented. Errors introduced in the calculated channel radius are investigated by first assuming the channel conductivity to vary according to the Spitzer conductivity, and then assuming the channel conductivity to be constant and comparing the numerical results of these two methods with the experimentally measured values of channel radius. It is shown that according to Hugoniot adiabatics, the apparent relationship between the arc channel pressure and the directed energy of the shock front is given by the pressure-velocity relationship of the gas leaving the shock front instead of the more commonly used pressure-velocity relationship for the gas entering the shock front. >

ELECTRODE PERFORMANCE OF A THREE ELECTRODE TRIGGERED HIGH ENERGY SPARK GAP SWITCH

Electrode erosion rates and voltage holdoff !ability are presented for various electrode materials sed in a triggered spark gap switch. The switch tilized an electrical trigger with the triggered electrode ositioned perpendicular to the main gap. Results show significant reduction in electrode erosion rates over Lose for non-triggered gaps. The lowered erosion rates re attributed in part to the lower voltage and current roduct during switch turn-on. The tests are conducted xoss a broad range of peak currents (i.e. 10 kA 400 A) and materials, includmg copper-tungsten, coppermgsten+LaBg, and tungsten. Introduction High energy switches are important components many pulse power systems. In particular, switches ith current capabilities greater than 1 O4 A and hold-off dtage capabilities greater than 1 O3 Y are of interest in stems used for nuclear effeds simulation, ermonuclear fhsion, high energy mono-pulse radar, id high energy lasers. Currently no single solid-state vitch can satisfy the above voltage and current quirements. Consequently, much attention has been ven to high energy spark gaps. Although spark gaps e capable of handling voltages and currents several ders of magnitude greater than solid-state devices, they we one main disadvantage. Because spark gaps are nechanicall” switches, they have a finite lifetime. lthough several factors limit the overall switch lifetime, e main reason for failure is related to electrode osion. Because of this, many studies have been nducted on electrode erosion in spark gaps. ork supported by SDIORNI through DNA/RAEV under ntract # DNA 001-9O-C-0110 The study conducted in this paper targets early arc formation erosion processes and their effects on electrode erosion performance. In an effort to reduce erosion caused by early arc formation, a method of triggering was employed to reduce the voltage-current product during tum-on. By reducing the power dissipated in the spark gap, the heat flux incident on the electrodes is correspondingly reduced. Another method to reduce the voltage current product during turn-on is the use of saturable inductors.[l,2]1 The principle behind both of these methods is the increased time for the conducting plasma to form before the high current conduction begins. Experimental Set Up Switch confimration The switch configuration used for this study is similar to that of a typical mid-plane electrically triggered spark gap. The main gap electrodes have a 1.25 cm spacing and are made from 1.25 cm rod stack with a 2 cm radius placed on the ends. The trigger electrode is made fiom 1.25 m Cu+W+LaB6 rod and has a 30 degree blunt point. A diagram of the switch layout is shown in Figure 1. As shown in the diagram, the trigger electrode is placed perpendicular to the main gap and is in m offset position (relative to the center of the main spark gap) such that the arc length between the trigger electrode and the cathode is “id during triggering. Although the trigger electrode is closer to the mode, the field is higher bmeen the trigger electrode and the cathode when the voltage across the main gap is at its rated value. The higher field is a result of the trigger-to-cathode pulse being of opposite polarity relative to the anodecathode polarity.

A compact high voltage vector inversion generator

The Pichugin pulser (named after its Russian inventor) is a compact, high reliability vector inversion generator. Although consisting of many stages, the Pichugin pulser has the distinct advantage that only one inversion switch is required. One-switch operation gives the generator its high degree of reliability and is accomplished by using transformer coupling between the generator stages. If efficient transformer coupling is maintained, the generator can be made very compact. This investigation reports on the design, performance, and characterization of a compact 500 kV, approximately 1 J Pichugin pulser. Without an output peaking switch (or spark gap) the output risetime of the pulser is approximately 0.5 /spl mu/s. Shorter, nanosecond risetimes have been measured with the use of an output peaking switch. These types of pulsers are used in our laboratory to trigger various spark gaps and multi-channel surface discharge switches with a good performance record and a high degree of reliability. The Pichugin pulser is an attractive alternative to conventional Marx-bank pulser design.

Insulator and electrode mass erosion and surface voltage holdoff recovery for transient, high current surface discharges

Several polymeric insulator materials commonly used as sidewall insulators in electromagnetic accelerators were subjected to repetitive (from approximately 0.1 to 1 discharge per second), high current (from approximately 100 to 300 kA peak or approximately 100 to 300 kA/cm), transient ( approximately 20 mu s pulse width) surface discharges. The insulator materials tested include the thermosetting polymers G-9, G-10, and G-11 (i.e., fiberglass reinforced melamine and epoxy) and the thermoplastic polymers Lexan (i.e., polycarbonate) and Delrin (i.e., polyacetyl). Empirical scaling relationships are given that relate the amount of insulator and electrode (i.e., molybdenum) mass erosion to the total amount of arc energy transferred. Scaling relationships are also given for the lifetime of the given polymer as a function of the initial discharge current. The lifetime of an insulator material is defined as the number of discharges required to reduce the initial surface holdoff voltage to its half-power level for three consecutive discharges and is a useful parameter when specifying insulator materials to be used in high-power switching devices. >

Surface-discharge switch design: the critical factor

Dielectric properties that are critical to designing a long-life surface-discharge switch (SDS) are investigated. Theory is correlated with experiment by evaluating the performance of a large group of polymeric and ceramic dielectrics. These dielectrics are tested in a single-channel, self-commutating SDS operating at approximately 35 kV and approximately 300 kA (oscillatory discharge) with a pulse length of approximately 20 mu s (1/4 period approximately 2 mu s). The performance of a dielectric is characterized by its shot-to-shot breakdown voltage and by its mass erosion. Theoretically, the voltage holdoff degradation resistance (HDR) and the arc melting/erosion resistance (AMR) of a dielectric can be qualitatively predicted from its formativity and its impulsivity, respectively. The formativity and impulsivity are figures of merit calculated from the known thermophysical properties of the dielectric. The effects produced in dielectric performance by choice of electrode material (e.g., molybdenum, graphite, and copper-tungsten) and discharge repetition rate are also discussed. >