Properties of streamers and streamer channels in SF6.

Abstract

A theoretical examination is made of the mechanism of formation of cathode- and anode-directed streamers and the resultant ionized channel in ${\mathrm{SF}}_{6}$, at 100 kPa, with a uniform applied field. The evolution of positive and negative ions, and electrons, is described by one-dimensional continuity equations, with the space-charged electric field determined by the method of disks. The main features of streamer formation may be understood by noting that any change in the electric field in ${\mathrm{SF}}_{6}$ causes abrupt changes in the electron density ${N}_{e}$ and current density J due to either strong net attachment or strong net ionization which balance at the critical field ${E}^{\mathrm{*}}$. In the streamer head the net charge enhances the field ahead of the streamer and depresses the field behind, causing a rapid increase in ${N}_{e}$ and J, followed by a rapid fall. The cathode-directed streamer propagates because photoionization releases electrons ahead of the streamer, while the anode-directed streamer can propagate, without photoionization, via electrons from the streamer head. The streamer channel is left with a uniform electric field ${E}_{Q}$>${E}^{\mathrm{*}}$ determined by a dynamic balance between conduction and displacement currents to maintain the total current.