The mechanism of positive streamer acceleration and expansion in air in a strong external field

Positive streamers in atmospheric pressure air in strong uniform external fields (40, 50 and 60 kV ) are simulated within the scope of the diffusion-drift model. Photo-ionization was taken into account. A simple model which describes streamer acceleration and expansion is proposed on the basis of the simulation results. It is shown that photo-ionization is essential only during the early stage of streamer formation. In strong external field further streamer evolution is governed by collisional multiplication of seed photoelectrons in a gap. The detailed structure of the streamer head is presented. It is shown that streamer advancement occurs due to avalanche ionization in a thin ionization domain (ID) with a width of the order of the space-charge layer's width. The ID can be considered as a region where some initial level of electron density is rapidly converted to the density in the streamer channel . This allows one to assume that the shape of the streamer head coincides with the instant position of the contour line on the seed electron density surface. This assumption leads to a simple relation for the streamer velocity which is consistent with the simulation results.