Boltzmann equation analysis of the electron swarm development in SF₆

Abstract

The electron swarm development in SF6 gas is studied for E/p20 values from 15 approximately 200 V cm-1 Torr-1 (E/N=45.6-607 Td) by a Boltzmann equation method in which the effect of ionisation and electron attachment is considered properly. The momentum transfer, vibration, ionisation and attachment cross-sections are taken from experiments, but the electronic excitation cross-section is determined by fitting the calculated ionisation and attachment coefficients to previous measurements. The calculation is performed for the steady-state Townsend, pulsed Townsend and time-of-flight experiments. The results show that the deduced electron drift velocities and diffusion coefficients agree reasonably well with experiment and the value of a swarm parameter should depend on the type of experiment not only at high E/p20 values, at which ionisation is appreciable, but also at low E/p20 values, at which electron attachment is appreciable. The electron energy distribution, which is found to have depressions at low E/p20 values, the electron mean energy and the excitation coefficients are also calculated and discussed.