Stable propagation of solitary positive streamer heads in air*

We studied positive streamers in STP air in a 4 cm gap and found a case wherein the streamer is not accelerating and expanding, but is propagating with a constant radius and velocity. We observed this for a simulated single streamer in a homogenous background field that is 17% of the breakdown field. This streamer got detached from the electrode from which it originated, so we dub it as a solitary streamer. The propagation behavior of the solitary streamer reminds us of the original definition of the streamer stability field, namely the homogenous electric field in which a streamer propagates in a stable manner. Our measurements agree well with empirical values for the stability field reported in earlier literature. The velocity and optical radius also agree well with earlier measurements of so-called minimal streamers. In electric fields above this limiting case of uniform translation, the streamers accelerated as they grew, and in electric fields below it, the streamers decelerated and some even stagnated.