Positive Streamer Spark Breakdown at Low Pressures in Air

Abstract

No data existed as to the lower pressure limits of streamer breakdown in positive point‐to‐plane geometry. Using a combination of Lichtenberg figure techniques in the coplanar mode and a two‐photomultiplier analysis of the movement of luminous pulses between the electrodes, the streamer mechanism has been observed to 4 Torr in room air; below this pressure, it is too faint to observe. A primary streamer starts from the anode. Its speed of the order of 10⁷ cm/sec varies across the gap, depending on overvoltage and pressure. It is influenced by encounter with photoelectrically initiated avalanches from the cathode and by streamer‐tip‐cathode fields. On arrival at the cathode, fast space waves of potential gradient travel up the branch tips. Where such waves converge at the junction of the channel branches near the anode, a bright secondary streamer pulse appears. The tips of the midgap event move towards cathode and anode as secondary positive and negative streamers, respectively, at speeds around 10⁶ cm/sec. The luminosity engendered when the streamers are launched initiates a new negative photoelectrically triggered group of electron avalanches that move up the channel at about 10⁶ cm/sec. When this group meets the positive secondary streamer tip, the arc channel is completed. Observations leading to this picture constitute the content of this paper.