The role of photoionization in positive streamer dynamics

The results of two-dimensional modelling of positive streamer dynamics in 2 cm point-to-plane gap are presented, including the process of photoionization in a gas volume. The model of photoionization in air, developed by Zhelezniak, Mnatsakanian and Sizykh is considered in detail and a comparison with the experimental data of Penney and Hummert is presented. The model of photoionization in nitrogen, based on the data of Penney and Hummert is described. The simulations of streamers were performed for air and nitrogen-like gas. it is shown, that fundamental space scale of streamer-the width of space charge layer l(rho) (ionization domain) around its head is defined by the length of absorption of photoionizing radiation l(alpha). This length defines all other parameters. electron density, radius of streamer channel etc, so one may speak about standard streamer in air, whose properties depend only on pressure. Based on simulation results one may assume that streamer branching occurs in high field, when l(rho) exceeds l(alpha). The process of branching may be interpreted as an instability which transforms the non-standard streamer into a number of standard ones.