Mechanism for negative corona current pulse in CO2-SF6 mixtures

Current waveforms of first negative corona pulses have been measured in CO2–SF6 mixtures over a pressure range extending from 6.65 to 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. For a given set of experimental conditions it is concluded that in the mixtures containing up to 30% of SF6 the negative corona pulse is associated with the formation of a cathode-directed streamer-like ionizing wave in the immediate vicinity of the cathode. This is in contrast to the discharge behaviour in air–SF6 and N2–SF6 mixtures, where in similar conditions the discharge develops according to a multi-avalanche Townsend mechanism.