Branching characteristics of positive streamers in nitrogen-oxygen gas mixtures

One of the most important feature of streamers is that they frequently branch during the development process. In this paper, the branching characteristics of positive streamers are experimentally investigated in nitrogen-oxygen gas mixtures. Pulsed voltages with the amplitude of 10-20 kV are applied to a point-plane gap. The gas pressure is 100 mbar and the concentrations of the oxygen are 0.01 and 20%. The branch angle, the cross-sectional area of streamer channels before and after branching, as well as the ratio between streamer branching length and width are obtained. It is found that the streamer branching angle for 0.01% oxygen is less than that for 20% oxygen. The average ratio of the streamer cross-section area after branching to that before branching is 0.711 and changes slightly for different oxygen concentrations. The branching ratio decreases significantly from 12.6±2.6 to 6.7±1.5 when lowering the oxygen content from 20% to 0.01%, which indicates that the streamers branch more frequently at a certain distance. Since the nitrogen-oxygen ratio directly decides the electron production rate by photoionization ahead of streamers, the results imply that the possibility of streamer branching may correlate with the photoionization rate in the streamer tip.