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.

Electrode Erosion Phenomena in a High-Energy Pulsed Discharge

An inexpensive, compact, three‐electrode (trigatron) switch utilizing transverse gas flow was developed for burst‐mode applications. The switch is cooled by adiabatic expansion from three built‐in miniature nozzles and the interacting flow streams serve to sweep hot discharge gases from the electrode region. Tests demonstrated control of an average power of 100 kW for 10 s bursts at repetition rates up to 100 pulses per second (pps) while maintaining a hold‐off voltage up to 50 kV. Under these conditions, the switch commuted currents up to 6 kA with a pulsewidth of 10 μs. The cost of operation is reduced by using oil‐free and dry air at a supply pressure of 500–1000 kPa and a mass flow rate of 0.5–8 g/s. Minimum switch jitter is obtained using a rounded‐tip trigger pin flush with the adjacent electrode and an optimum trigger voltage that causes breakdown first between the trigger pin and opposite electrode.

Transverse‐flow 50‐kV trigatron switch for 100‐pps burst‐mode operation

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...

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

In this paper, a study concerning self-breakdown voltage characteristics, electrode erosion, and degradation of the insulator, of a two-electrode spark-gap switch has been presented based on experiments. A 6 μF pulse capacitor and a DC high voltage source (set at −40 kV) were utilized to achieve ∼0.1 pulses per second repetitive arc discharge between two fixed Cu-W (Cu30/W70) alloy electrodes in the 1.2 L airtight chamber of the switch. With average transfer charge of 2.2 C per shot, this switch was tested for 5800 shots altogether. The results demonstrated the electrode erosion rate of the cathode was 6.3434×10−6 cm3/C whereas that of anode was 5.4691×10−6 cm3/C. Besides, by analyzing electrode surfaces' morphology and PTFE insulator surfaces' compositions after 5800 shots with the help of a scanning electron microscope, a dynamic 3D aspheric metrology instrument, an X-ray photoelectron spectrometer, a metallographic microscope, and a Mettler-Toledo analytical balance, some evidence of physical processes and chemical reactions between plasma channel, electrode materials, and insulation materials could be observed. Moreover, the chamber was opened and inspected for 10 times during the whole procedure, thus providing an overall description of the variation of the switch. It should also be pointed out that owing to complex mechanisms during the aging process, the self-breakdown voltage followed a mixed Normal distribution other than any other simple distributions.

Experimental study of electrode erosion and aging process of a specially designed gas switch under repetitive arc discharge

Pulsed power technology is one of the technological foundations in high and new technology research, which has extremely broad development and application prospect. Gas switch is one of the key elements in pulsed power devices, and electrode erosion is a key restrictive factor in high-power gas switch development and application. According to the 1-D equation of heat conduction and thermal equilibrium equation near the electrode surface, this paper researches the electrode heat conduction mechanism, establishes the electrode erosion calculation model, calculates electrode erosion heat fluxes and their peak powers caused under different discharge conditions, and proceeds to calculate the depth of electrode etch pit. Calculation results indicate that arc joule heat is the main reason for electrode erosion when discharge current is not too high. In the light of different discharge parameters, that is, peak current, electric quantity, and current waveform, as well as single discharge electrode erosion experiments, the calculation formula of etch pit depth is fitted. The results indicate that model and fitted calculation results are relatively approximate to the experiment results.

Research on Erosion Property of Field-Distortion Gas Switch Electrode in Nanosecond Pulse

The chemical interactions and physical processes occurring in a high energy spark gap with different combinations of gases, electrodes, and insulators were studied. The electrodes studied were graphite and a tungsten-copper composite; the insulators were Lexan and Blue Nylon; and the gases were N2 and SF6. The gas composition was monitored with a mass spectrometer. Spectroscopic techniques were used to observe the arc channel. The electrode surfaces were studied with several surface analysis techniques, including scanning electron microscopy, electron spectroscopy for chemical analysis, Auger electron spectroscopy, and X-ray fluorescence. The breakdown voltage distribution was examined for different material combinations. The plasma chemistry processes involving the gas, electrode, and insulator materials were found to affect the voltage self-breakdown distribution. The detailed surface analysis gave information about the nature of the chemical processes. The presence of Blue Nylon seemed to have a more adverse effect than Lexan and graphite seemed to have a narrower voltage distribution than the tungsten-copper composite.

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H. C. Kirbie

Papers

Material Studies in a High Energy Spark Gap