Time-resolved study on negative ion motion in SF/sub 6/ at high pressures

We have used a pulsed Townsend technique to measure the drift velocity of two negative ion species in SF/sub 6/ at low values of the density-normalized electric field intensity E/N, from 2.3 to 100 Td (1 Townsend = 10/sup -17/ V cm/sup 2/), and very high pressures in the range 50-720 torr. Within experimental uncertainties of between 2-3%, we could group the observed mass-unidentified ionic reduced mobilities near the value of 0.48 cm/sup 2/ V/sup -1/ s/sup -1/ and 0.42 cm/sup 2/ V/sup -1/ s/sup -1/. This finding is consistent with the mobility ascribed to the clusters SF/sub 6//sup -/SF/sub 6/ and SF/sub 6//sup -/(SF/sub 6/)/sub 2/, respectively. These ions are assumed to be formed by ion-molecule reactions between SF/sub 6//sup -/ and the gas neutrals. Our findings may be of relevance for the modelling of high pressure SF/sub 6/ discharges, like those taking place in circuit breakers.