Streamer branching in a short gap: the influence of the power supply

The formation of streamers in a 25-mm gap in air is studied with an intensified charge coupled device camera with high resolution in space and time. Strong branching is observed and streamers reach the cathode in an area that is much wider than the gap length. Switching the high voltage with either a spark gap or a semiconductor stack has a large influence on the branching: much more discharge channels appear when using the semiconductor switch. Pictures taken with 0.8-ns resolution show that the streamers propagate with 3 ms/mm near the electrodes and with 0.5 mm/ns in the middle of the gap.