Schlieren studies of impulse breakdown in air gaps

The initial expansion of the leader channel and its subsequent cooling have been investigated in 10–30cm rod-plane gaps in air using a high-speed schlieren technique. The channel was found to expand to a maximum value of 0.6 mm and a shock wave, associated with the initial rapid expansion, was observed. When breakdown occurred, the main spark channel always followed the track of the leader. Thermal time-constant calculations indicate that the neutral gas density along the track of the leader would be reduced for periods of ∼100μs. As reillumination intervals in the case of the stepped leader are shorter than this, it is suggested that enhanced ionisation in the hot column would have to be included in any explanation of the mechanism governing the propagation of the stepped-leader discharge.