STREAMER FORMATION AND MONTE-CARLO SPACE-CHARGE FIELD CALCULATION IN SF(6)

A Monte Carlo simulation is carried out in SF6 in uniform electric fields with 7 and 14% overvoltages and at two gas densities at each overvoltage. The electron motion and avalanche growth is simulated by tracing individual paths and the effect of space charge is included by solving the Poisson equation. The streamer propagation, electron, positive and negative ion distribution 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 non-attaching 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 10(7) to 10(8) cm/s, dependent on the percentage of overvoltage and the gas number density.