Time-resolved study on negative ion motion in SF6 at high pressures

We have used a pulsed Townsend technique to measure the drift velocity of two negative ion species in SF₆ at low values of the density-normalized electric field intensity E/N, from 2.3 to 100 Td (1 Townsend = 10^-17 V cm²), 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 cm² V^-1 s^-1 and 0.42 cm² V^-1 s^-1. This finding is consistent with the mobility ascribed to the clusters SF₆^-SF₆ and SF₆^-(SF₆)₂, respectively. These ions are assumed to be formed by ion-molecule reactions between SF₆^- and the gas neutrals. Our findings may be of relevance for the modelling of high pressure SF₆ discharges, like those taking place in circuit breakers.