On the structure of positive streamers in air between point-to-plane-electrodes

Pulsed corona discharges can form very complex patterns. The properties of individual streamers are studied by determining their diameter and velocity in a pressure range of 0.015 to 1 bar. It appears that streamers have a minimum diameter, d_min, at a voltage just above inception. In air, the product of pressure and minimal diameter p-d_min is 0.20plusmn0.02 bar-mm for all pressures of the experiments. This value also matches the minimum diameter of sprite discharges above thunderclouds at 75 km altitude. Streamers produced by high and fastly rising voltages, on the other hand, can be as thick as 2.5 bar-mm and 20 times faster than minimal streamers. If the electrode gap is sufficiently large, the corona streamers branch during propagation. Branching is inherently a three dimensional effect. Stereo-photographic images are taken to resolve the three-dimensional structure . Such photos show an approximately Gaussian distribution of branching angles of 43degplusmn12deg; these angles do not significantly depend on the distance from the needle or on the gas pressure. Furthermore, the branching parameter D/d is determined, where D is the length of the streamer between two branching points and d is the diameter. Its value is 12plusmn4, so there is a considerable statistical fluctuation but this value also does not depend on pressure or voltage.