Influence of impulse waveshape on breakdown voltage of a nonuniform-field gap in compressed gases

Investication were made into the influence of the impulse waveshapes, gas pressure p and polarity on the breakdown voltage vs of a sphere-plane gap in compressed air and nitrogen. The pressure was varied from 01bf/in2 gauge to 150lbf/in2gauge, the gap spacings from 6.35mm to 102mm and the risetimes of the impulse voltages from 2.1¿s to 189¿s. For an impulse voltage of either polarity, certain gap lengths exhibited a certical-pressure effect, and the gap lengths and pressure at which this effect was observed depended on the risetimes of the impulse voltage. Observations of the dependence of vs on the risetime showed that vs decreased with increasing risetimes, for risetimes of 16¿s or 40¿s, beyond which it increased again to reach a maximum at risetimes of the orter of 100¿s. For risetimes longer than 100¿s, vs again decrased with increasing risetime. This behaviour was similar for both nitrogen and air. Possible mechanisms are suggested to explain this behaviour.