Water vapor-enhanced electron-avalanche growth in SF6 for nonuniform fields

Abstract

When water vapor content is increased from 10 to 100 ppmv in SF6 at pressures from 200 to 300 kPa, a dramatic enhancement occurs in the mean size of electron avalanches formed near a positive-point electrode. Although this effect can be attributed to a change in gas composition, it is not due to a change in the ionization rate for the gas. It is proposed that the avalanche enhancement is due primarily to an increase in the probability for initiating electron release from minor negative ions associated with water vapor that collisionally detach more readily at a given field strength than the predominant negative ions associated with SF6. The profiles of the electron avalanche size distributions exhibit a monotonic decrease of probability with increasing number of electrons for avalanches with fewer than 107 electrons, but pronounced peaks appear in the distributions as the mean electron number exceeds 107. These peaks are not consistent with the behavior expected from a stochastic model of electron-avalanche growth in nonuniform electric fields which neglects the influence of space charge.