Streamer propagation in long point-plane oil gaps

Streamer propagation in a 67-mm point-plane gap has been studied by means of current and light registrations and streak and framing mode fast photography of the light emission. A strong bush-shape of positive streamers decreases gradually with increasing overvoltage. Negative streamers tend to resemble branched trees. In both, single channels flash up more or less regularly with associated current pulses. Negative streamers, and positive streamers at low overvoltages, grow fairly steadily across the gap at high supersonic speeds. Positive streamers at higher overvoltages bridge the gap from a mid-gap position in a fast event. Positive streamers seem to have their electron multiplication in the tips