Time characteristics of fast pulsed flashover in vacuum

The results of measurements of surface flashover across epoxy resin filled with Al(OH)3 between stainless electrodes are described. The test gap was subjected to an impulse voltage of 20 ns/20 μs. The flashovers were observed for various insulator shapes and insulator lengths. It is found that for the cylindrical shape, the flashover voltage, delay time td, and fall time tf are proportional to the insulator length. In comparison with the results in the absence of insulator with the same experimental conditions and length, the flashover voltage and fall time are 43 and 67% of those without insulator, respectively. The times td and tf are approximately 1 μs and approximately 1 ns, respectively. It is postulated that the time delay of the flashover occurs mainly at the initiation stage, while collapse of voltage takes place at the final stage of the flashover process. The shape of the insulator has a dominating influence on surface flashover. The positive angle cone and concave cylinder have a higher voltage holdoff capability than the negative angle cone and convex cylinder. It might be deduced that the fewer secondary electrons return to the insulator, the higher the voltage holdoff capability of the insulator shape.