The flashover process based on the finding of surface charge distribution on alumina insulator surface in vacuum

The measurement of insulator surface charge distribution on the whole area of alumina YSA 998 and alumina UHA 99 surface area after impulse voltage application revealed that the surface charging can be initiated either from the anode or from the cathode triple junction. The charging initiated from the anode triple junction (for positive polarity) produced positive charge at the anode region and the density is dependent on applied voltage, while the charging initiated from the cathode triple junction region (for negative polarity) produced negative charge around the cathode region. For positive polarity, the critical value of charge density to the flashover for alumina YSA 998 and alumina UHA 98 are 5.70 μC/m² and 17.2 μC/m², respectively