Time-resolved prebreakdown x-ray emission from polycrystalline alumina-bridged vacuum gaps for impulse stresses

Abstract

Time-resolved and time-coordinated observations of voltage and prebreakdown and breakdown x-ray emission associated with the surface flashover of ceramic-insulator-bridged vacuum gaps for pulsed excitations 0.15/20 μs are described. 99.9% pure polycrystalline alumina right circular cylinders (25.4 mm diam, 10 mm tall) were investigated. The alumina samples were of two types, i.e., machined on the cylindrical surface to (i) 0.8 μm surface finish and (ii) 0.25 μm surface finish. An inverse correlation between the frequency of occurrence of prebreakdown x-ray emission and the surface flashover strength is observed for all samples. The relationship between the onset time of the prebreakdown x-ray peak from voltage zero and the surface flashover strength, however, is seen to be in direct proportion, i.e., the longer the onset time of the x-ray peak from voltage zero, the higher is the flashover voltage. These novel experimental observations are analyzed to infer the origin of x radiation in the bridged vacuum gap and their dependence on the nature of the insulating surface. The results could not be explained satisfactorily by the conventional secondary emission avalanche theory. A thermally activated, field-assisted exoelectron emission model is believed to be in better qualitative agreement with the prebreakdown x-ray emission results reported here.