DIELECTRIC SURFACE PREFLASHOVER PROCESSES IN VACUUM

Surface preflashover phenomena across alumina ceramics in vacuum are p resented. The localized plasmas are identified by the spatially resolv ed images that appear in a pulse surface preflashover phase using an i ntensified charge-coupled-device camera, along with coordinated, time- resolved preflashover current and luminosity. It was observed that sig nificant preflashover activity associated with the localized plasmas o n the surface appears after a flashover event and disappears after a f ew pulse voltage applications. Large bursts of current pulses (several hundred milliamperes) with no counterpart in the luminosity signal we re also observed during preflashover. These phenomena cannot be explai ned satisfactorily by electron stimulated gas desorption. The effect o f adsorbed gases and surface polishing on preflashover are presented a nd discussed. The experimental results indicate that preflashover esse ntially depends on the surface state (physical and chemical) of the so lid insulator. The preflashover conduction associated with the localiz ed plasmas is attributed to the detrapping and impact-ionization proce sses associated with defect centers. The results reported support the surface flashover model based on trapping/detrapping and impact ioniza tion processes.