Electrical breakdown of SF6 at small values of the product pd

The breakdown of SF₆ when static and impulse voltages are applied at small values (from 10^-4 bar-mm to 1 bar-mm) of the product pd was investigated. It was found that the prevailing breakdown mechanism up to 8×10^-3 bar-mm was the Townsend mechanism. Between 8×10^-3 and 5.5 bar-mm the Townsend and streamers mechanisms combined, whereas from 5.5 bar-mm on the streamers mechanism took over. The Paschen law was found to hold for the points lying to the right of the minimum when static voltage was applied, whereas edge-type breakdown occurred on the points lying to the left of the minimum. When impulse voltage was applied, the Paschen law was valid only for the points to the right of the 1 bar-mm point. The electrode material affected the static and impulse breakdowns through the values of its work function