The mechanism of leader breakdown in electronegative gases

A survey is given of the present state of understanding of leader breakdown of electronegative gases in nonuniform field gaps and under fast-rising voltage waveforms. The basic physical processes involved are the formation of a pulsed streamer corona, the transformation of one of the streamers into a leader step, the temporal development of the leader channel, and the stepped propagation of the leader through the gap. The growth and structural characteristics of the corona are modeled and the results obtained are used to derive simple approximations for the spatial corona extension and the corona charge. Two leader inception mechanisms are discussed, namely, the stem and the precursor mechanisms. Inception criteria and characteristic time scales are derived for both of them in terms of the experimental parameters. A simplified model, which also predicts the random aspects of leader propagation through the gap, is presented. Some typical gas-insulation design problems are treated as examples