THE CORRELATION BETWEEN SURFACE-CHARGE DISTRIBUTION AND FLASHOVER PROPERTIES OF ALUMINA SURFACE IN VACUUM

The surface charge distribution of disc-type (diameter, 23 mm; thickne ss, 5 mm) alumina YSA 998 and UHA 99 insulator were measured after imp ulse voltage application. The measurement of the surface charge distri bution was carried out using an in situ electrostatic probe connected to an electrometer. The electrode configuration included needle and ba ck-side electrodes that made it possible to measure trapped charge ove r the whole area of the insulator surface. For non-flashover condition s (positive polarity) the surface charge density is dependent on the a pplied voltage, and that the highest surface charge density occurs at an anode position decaying toward the cathode. Up to the critical volt age which caused a flashover, no charge was detected at the cathode re gion. On the other hand, for a flashover, the entire sample surfaces w ere charged positively, and generally, the highest surface charge dens ity occurred at the cathode region. For negative polarity (non-flashov er) the cathode region is charged positively, and a small positive cha rge is detected on the other part of the anode region.