Trap-dominated breakdown processes in an insulator bridged vacuum gap

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Abstract
Measurements of coordinated, time-resolved breakdown current and luminosity phenomena associated with a pulsed surface flashover event in an insulator bridged vacuum gap are presented. It was observed that the luminosity and current waveforms differ vastly in their temporal character. The luminosity profile has a sharp pulse with no counterpart in the current waveform. A significant afterglow activity is also observed after the cessation of the breakdown current. Further, the profiles of the luminosity and current waveforms changed with successive breakdowns. The rise times and decay times of the luminosity waveform, the time delay between the onset of the luminosity and current waveforms, and the rise time of the current waveform changed with successive breakdowns showing regular trends. The above modifications in the temporal profiles of luminosity with successive breakdowns are analyzed on the basis of carrier trapping and recombination processes within the localized levels of the forbidden gap associated with the insulator-vacuum interface. The experimental results reported here qualitatively support the surface flashover model based on carrier trapping for low mobility, large band-gap insulators. The analysis of the results is suggestive of a breakdown model in which hot-electron generation culminates in impact ionization-induced breakdown in the subsurface layers of the insulator.