A. L. Donaldson

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 >

Electrode erosion in high current, high energy transient arcs

Electrode erosion in high current, high energy spark gaps was examined both theoretically and experimentally. A brief description is given of the production of high velocity, high temperature electrode vapor jets which are shown experimentally to be the major source of electrode erosion under these conditions. Consideration of a simple one dimensional thermal model with the vapor jet considered as a transient heat flux onto the electrode surface led to a figure of merit for the electrode materials which was in close agreement with experiments. The rapid decrease in erosion with increasing gap separation was also measured and explained by the vapor jet mechanism. Erosion rates are given for a large number of electrode materials including many copper composites, such as copper niobium and copper tungsten. Possible implications of these results on rail gun design are also given.

Electrode Erosion Phenomena in a High-Energy Pulsed Discharge

The erosion rates for hemispherical electrodes, 2.5 cm in diameter, made of graphite, copper-graphite, brass, two types of copper-tungsten, and three types of stainless steel, have been examined in a spark gap filled with air or nitrogen at one atmosphere. The electrodes were subjected to 50 000 unipolar pulses (25¿s, 4-25 kA, 5-30 kV, 0.1-0.6 C/shot) at repetition rates ranging from 0.5 to 5 pulses per second (pps). Severe surface conditioning occurred, resulting in the formation of several spectacular surface patterns (craters up to 0.6 cm in diameter and nipples and dendrites up to 0.2 cm in height). Surface damage was limited to approximately 80 ¿m in depth and was considerably less in nitrogen gas than in air. Anode erosion rates varied from a slight gain (a negative erosion rate), for several materials in nitrogen, to 5 ¿cm3/C for graphite in air. Cathode erosion rates of 0.4 ¿cm3/C for copper-tungsten in nitrogen to 25 ¿cm3/C for graphite in air were also measured.

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. >

Utilization of a thermal model to predict electrode erosion parameters of engineering importance

A simple solution to the one-dimensional heat conduction equation has been shown to describe quite accurately the scaling laws obtained experimentally for electrode erosion. For erosion where vaporization or ablation is the dominant material removal mechanism the correct scaling parameter is either f/sub 1/ proportional to Q/sub e/I/sub p/ (t/sub p/)/sup 1/2/, or Q/sub e/, where Q/sub e/ is the effective charge transferred, I/sub p/ is the peak current and t/sub p/ is the pulse width (defined as the width of the first half cycle for oscillatory pulses). It is shown that the electrode erosion results for many materials, for several different experimenters, scale linearly with f/sub 1/ regardless of whether the material is vaporized or ablated. The relative magnitude of the erosion for different materials is characterized quite well by the energy required to vaporize or melt the material.<<ETX>>

Modeling of self‐breakdown voltage statistics in high‐energy spark gaps

A model which incorporates the influence of electrode surface conditions, gas pressure, and charging rate on the voltage stability of high energy spark gaps is discussed. Experimental results support several predictions of the model; namely, that increasing the pressure and the rate of voltage charging both produce a broadening of the self‐breakdown voltage distribution, whereas a narrow voltage distribution can be produced by supplying a copious source of electrons at the cathode surface. Experimental results also indicate that two different mechanisms can produce this broadening, both of which can be taken into account with the use of the model presented. Further implications of the model include changes in the width of the self‐breakdown voltage probability density function as the primary emission characteristics of the cathode are modified by, for example, oxide or nitride coatings and/or deposits from the insulator. Overall, the model provides a useful and physically sound framework from which the pr...

The Erosion Performance Of Graphite Electrodes In High Current, High Coulomb, Spark Gaps

The erosion performance of graphite electrodes has been characterized in high current (100-500 kA), high coulomb (1-900 C), switches. Six different graphite materials were tested including state-of-the-art pyrolytic graphite which exhibited superior erosion performance at high-Q. At least two different regimes of operation were identified, low Q (< 5 C), and high Q (> 25 C). For high Q, the erosion is primarily a function of the energy required to sublime the material and its thermal conductivity. In addition, at high Q the thermal model for erosion predicted, within 10%, the scaling law (proportional to Q) and the erosion rate of a recently developed high Q switch at Physics International Corp.

Surface studies of dielectric materials used in spark gaps

Dielectric materials commonly used as insulators in spark gaps (lexan, nylon, lucite,macor, boron nitride, delrin, and G‐10) have been exposed to the byproducts of arcs in three different spark gap experiments. The first was a 60‐kV, 0.05‐C/shot spark gap using copper‐tungsten or graphite electrodes at various pressures of N2 and SF6 gas. The second was a 5–30‐kV, 4–25‐kA, 0.1–0.6‐C/shot, unipolar, pulsed spark gap using graphite, copper‐graphite, copper‐tungsten, brass, and stainless steel electrodes in N2 gas or air. The third was a 45‐kV, 0.009‐C/shot surface discharge switch. Surface analysis of these insulators indicates that most become coated with a thick layer of electrode material depending upon the type of gas, electrode, and insulator material used, and the conditions of the arc. However, lucite insulators inserted in the second spark gap using graphite electrodes and air showed no indications of deposited electrode material on the surface but did show small particles of graphite imbedded in th...

The Effect Of Repetition Rate On Electrode Material Performance In High Current Switches

Many applications of high current switches require operation in the burst mode or at high repetition rates. Under these conditions, it has been observed that electrode erosion rates increase signifi-cantly. An analytical solution to the heat conduction equation is discussed which determines the onset of the bulk surface erosion mode as a function of peak current, pulse width, rep-rate and material properties. The solution indicates that materials with lower thermal diffusivities can operate at higher rep-rates before gross melting of the electrode surface occurs. The overall effect of increasing rep-rate is to shift the point of transition on the standard electrode erosion graph to lower values of peak current or charge transfer.