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 photoionizat ion in a gas volume. The model of photoionization in air, developed by Zhel ezniak, Mnatsakanian and Sizykh is considered in detail and a comparison wi th the experimental data of Penney and Hummert is presented. The model of p hotoionization in nitrogen, based on the data of Penney and Hummert is desc ribed. The simulations of streamers were performed for air and nitrogen-lik e gas. it is shown, that fundamental space scale of streamer-the width of s pace charge layer l(rho) (ionization domain) around its head is defined by the length of absorption of photoionizing radiation l(alpha). This length d efines all other parameters. electron density, radius of streamer channel e tc, so one may speak about standard streamer in air, whose properties depen d only on pressure. Based on simulation results one may assume that streame r 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.