NEGATIVE CORONA CURRENT PULSES AND CATHODE SHEATH INSTABILITIES IN A SHORT POINT PLANE GAP IN CO2

Waveforms of the first negative corona current pulses have been measured in CO2 and CO2+SF6 mixtures as a function of applied voltage and gas pressure in the range 6.67-66.7 kPa. A complex form of the pulses with a step on the pulse leading edge and a current hump on the pulse trailing part has been found at gas pressures below 40 kPa. Changing the cathode surface material from copper to copper iodide, which has an exceptionally high photoelectric yield, resulted in at least a twofold increase in height of the step on the pulse leading edge, but had little effect on the main current rise to the pulse peak. The use of unconditioned cathodes resulted in the appearance of peculiar current spikes on the pulse tail, which are tentatively attributed to positive-streamer-like instabilities of the cathode sheath. The implications of these results for theories of Trichel pulse formation are discussed, and a physical picture of the phenomenon based on the presumed existence of a cathode-directed streamer associated with the steep pulse rise is outlined.