Study of the formation and propagation of a leader channel in air

Results are presented from calculations of the initial stage of leader channel formation in air. It is shown that the channel forms in two stages: one occurring at an essentially constant gas density on time scales much shorter than the characteristic gas-dynamic time and another in which gas-dynamic rarefaction of the channel becomes important and thermal-ionizational instability develops. The leader propagation velocity is largely determined by the time of channel contraction. The dependence of the leader velocity on the current and initial pressure is calculated. The results of calculations agree with experiment for relatively low leader currents (I L = 0.5–5 A) and for leader currents above tens of amperes. A comparative analysis of the leader propagation velocity in air at different initial pressures is given. In the current range I L = 0.5–30 A, the leader velocity depends weakly on the pressure; however, for currents as high as I L ≥ 50 A, it increases appreciably with pressure.