Streamer formation and Monte Carlo space-charge field calculation in SF6

A Monte Carlo simulation is carried out for SF₆ in uniform electric fields, with 7% and 14% overvoltages and at two gas densities at each overvoltage. The electron motion and avalanche growth are simulated by tracing individual paths, and the effect of space charge is included by solving the Poisson equation. The streamer propagation, the electron and positive and negative ion distributions and space charge fields are studied in detail as more time has lapsed after voltage application. The simulated streamer shape explains for the first time the dark space in SF₆ streamers observed experimentally. It is found that the mechanism of streamer propagation in an attaching gas is different from that in a nonattaching gas. The maximum field enhancement is just behind the streamer or between two successive streamers in SF₆. The anode-directed streamer propagates with a velocity of 10⁷ to 10⁸ cm/s, depending on the percentage of overvoltage and the gas number density