Dielectric surface preflashover processes in vacuum

Surface preflashover phenomena across alumina ceramics in vacuum are presented. The localized plasmas are identified by the spatially resolved images that appear in a pulse surface preflashover phase using an intensified charge-coupled-device camera, along with coordinated, time-resolved preflashover current and luminosity. It was observed that significant preflashover activity associated with the localized plasmas on the surface appears after a flashover event and disappears after a few pulse voltage applications. Large bursts of current pulses (several hundred milliamperes) with no counterpart in the luminosity signal were also observed during preflashover. These phenomena cannot be explained satisfactorily by electron stimulated gas desorption. The effect of adsorbed gases and surface polishing on preflashover are presented and discussed. The experimental results indicate that preflashover essentially depends on the surface state (physical and chemical) of the solid insulator. The preflashover conduction associated with the localized plasmas is attributed to the detrapping and impact-ionization processes associated with defect centers. The results reported support the surface flashover model based on trapping/detrapping and impact ionization processes.