Observation of Stem Branching in Long Laboratory Sparks under Positive Switching Impulses

The phenomenon of leader branching in natural lightning and laboratory sparks has raised broad and long-standing concerns, while the physical mechanism has not been fully understood. Recently, a stem-driven mechanism has been proposed to explain the leader branching behavior. To this end, this paper mainly studies the stem branching phenomenon to explain the branching behavior of positive leaders. Streamer stems were visualized with both the schlieren photography and the direct photography. The corresponding current pulses were recorded synchronously. Based on the experimental results, the relationship between the second streamer stem structure and its corresponding current waveform is classified into three types. Besides, the elongation of the main and secondary branches and the charge required per unit for them are calculated. The charge per unit required to elongate for the main and secondary branches is 7.09 µC/m and 28.32 µC/m, respectively. These findings provide new insights into the branching behavior of the stem and the direction variation during the leader propagation process.