Effects of geometry and polarity on the prebreakdown and flashover behavior of conical high-voltage insulators in vacuum

Concurrent optical and electrical measurements have been used to characterize the physical phenomena associated with the prebreakdown and flashover behavior of high-voltage alumina ceramic insulator samples having both conventional and unconventional conical geometries. The critical parameters are the threshold voltage for initiating flashover events, the stable interelectrode current-voltage relationship, the luminescence pattern, and the breakdown rate as a function of time. These measurements have revealed that a well-conditioned sample of unconventional conical geometry can have very low, i.e., approaching zero, breakdown rate. Physical explanations of the findings are based on the current understanding of flashover mechanisms and the prediction from simulations using a field plotting technique.