The initiation mechanism of long sparks in point-plane gaps

Investigations have been made of the optical and electrical characteristics of the corona discharges and leader strokes preceding the impulse breakdown of positive-point/negative-plane gaps in air at atmospheric pressure. The results show that initially a corona discharge forms in the highly stressed region round the pointed electrode; this corona gives rise to a short-duration (~0.4 microsec.) pulse of light emission in the gap, accompanied by a similar pulse of current in the circuit. The size and shape of the light-emitting volume in this corona discharge have been studied. The next stage in the growth of the spark is the development of the leader stroke from the corona. The leader stroke grows across the gap in a predictable manner, and the current flowing in the circuit during the growth depends on the velocity of the leader stroke. The size and configuration of the light-emitting volume of the leader stroke are similar to those of the corona. The characteristics of the corona and leader stroke in air at reduced pressures and in oxygen, nitrogen and hydrogen are also briefly described.