The correlation between surface charge distribution and flashover properties of alumina surface in vacuum

The surface charge distribution of disc-type (diameter, 23 mm; thickness, 5 mm) alumina YSA 998 and UHA 99 insulator were measured after impulse voltage application. The measurement of the surface charge distribution was carried out using an in situ electrostatic probe connected to an electrometer. The electrode configuration included needle and back-side electrodes that made it possible to measure trapped charge over the whole area of the insulator surface. For non-flashover conditions (positive polarity) the surface charge density is dependent on the applied voltage, and that the highest surface charge density occurs at an anode position decaying toward the cathode. Up to the critical voltage which caused a flashover, no charge was detected at the cathode region. On the other hand, for a flashover, the entire sample surfaces were charged positively, and generally, the highest surface charge density occurred at the cathode region. For negative polarity (non-flashover) the cathode region is charged positively, and a small positive charge is detected on the other part of the anode region.