Correlation of alumina surface finish with the pulsed surface flashover performance

摘要

A systematic investigation of insulator surface finish effect on the pulsed surface flashover performance, in vacuum, for 99.9% pure alumina, and the probable causes for the observed effects are described. Two different surface finishes i.e. 0.8 micron and 0.25 micron, are studied for voltage excitation with a 0.5/15 /spl mu/s profile. Time correlated, temporally resolved, breakdown current and luminosity measurements, with superior temporal and spectral response, provide several clues to the physics of the flashover event. It is found that not only do the luminosity and current waveforms differ vastly in their temporal character, but also that the profiles change dynamically with successive breakdowns. The above modifications are found to be substantial for the 0.25 micron finish indicative of the dominant role of carrier trapping in the localized states, associated with the forbidden gap of the insulator surface. It is proposed that the ceramic machining operations and the associated surface pretreatment affect the defect densities corresponding to the two surface finishes. The results suggest that the observed flashover strengths are influenced by the surface damage layer during processing rather than to the surface finish itself. The data are supportive of an energy bandgap model of surface flashover in which electron trapping is succeeded by impact ionization induced breakdown.