Photoionization mechanism of branching of streamer discharge in air*

Branching of a streamer channel is the most important process that controls the development of a streamer flash, the degree of filling of plasma-chemical reactors with plasma, and the uniformity of distribution of active particles in a streamer flash. The point-to-plate geometry was used to study streamer branching. The length of the discharge gap was 30 mm.  The voltage amplitude was varied from 20 to 50 kV, the pulse rise time was 5 ns and the duration at half height was 20 ns. The experiments were performed in the pressure range from 0.5 to 1.5 bar. Integral images of the cathode-directed streamer show that the number of branches increases with increasing pressure at fixed voltage and with increasing voltage at fixed pressure. Within the pressure range of 680 to 1300 Torr, a branched streamer flash was observed for all applied voltages. The flash had from 2 to 5 separate channels. As the pressure decreases, the number of branches decreases. The mean distance between branches can be approximated by the formula l = exp[90(E/P)/P], where P is in [Torr], E/P is the mean reduced electric field in the gap, [V/(cm×Torr)], l is in [mm]. The dependence shows an important role of electron impact ionization for the branching.