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

A Monte Carlo simulation is carried out in SF6 in uniform electric fie lds with 7 and 14% overvoltages and at two gas densities at each overv oltage. The electron motion and avalanche growth is simulated by traci ng individual paths and the effect of space charge is included by solv ing 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 simulate d streamer shape explains for the first time the dark space in SF6 str eamers observed experimentally. It is found that the mechanism of stre amer propagation in an attaching gas is different from that in a non-a ttaching gas. The maximum field enhancement is just behind the streame r or between two successive streamers in SF6. The anode directed strea mer propagates with a velocity of 10(7) to 10(8) cm/s, dependent on th e percentage of overvoltage and the gas number density.