Influence of SF<inf>6</inf> admixtures to nitrogen on electrodeless streamer inception at an epoxy resin surface

Abstract

The influence of SF 6 -admixtures to N 2 on electrodeless streamer inception at the surface of a dielectric solid (epoxy resin) is investigated for different SF 6 /N 2 ratios and gas pressures, with focus on the lower pressure region (below 1 bar) which is only sparsely discussed in literature. For this purpose, we use a special set-up with cylindrical electrodes embedded out of axis in a cylindrical epoxy rod. The influence of the dielectric surface on streamer development is analyzed by estimating the ionization rate. Ionization rates obtained from experimental measurements seem to be higher than those available for discharges in bulk gas under similar conditions. The main influence on the inception voltage, however, seems to come from the change in the field distribution due to the presence of the dielectric solid. Furthermore, it turns out that the streamer inception voltage increases with gas pressure for a constant SF 6 /N 2 ratio, increases rapidly as a function of the SF 6 /N 2 ratio at small amounts of SF 6 (∼ 5%), and increases at lower rates for higher amounts. The total duration of the discharge decreases almost by a factor of 10, i.e., from tens of nanoseconds to several nanoseconds for pure nitrogen and 5% SF 6 -admixture, respectively. Overall, it is shown, that the major inception parameters (critical voltage, rise and fall times, current shapes) of streamer discharges on a gas-solid dielectric interface change considerably starting already with a small (5%) admixture of SF 6 to nitrogen.