Negative corona current pulses in SF6: Air mixtures

Current waveforms of first negative corona pulses have been measured in dry air + SF 6 mixtures at a pressure of 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. It is concluded that in the mixtures containing less than 10 % of SF 6 the negative corona pulse is associated with the formation of a cathode-directed streamer in the immediate vicinity of the cathode. In the mixtures containing more than 20 % of SF 6 the streamer is quenched and, consequently, the discharge is governed by the Townsend ionisation mechanism fed by cathode photoemission processes.