Partial discharge in vacuum on insulator surface under AC voltage application

In high voltage vacuum apparatus, e.g. vacuum circuit breaker, a partial discharge phenomenon, which is not breakdown but has a potential to lead breakdown, is observed under AC voltage application. We investigate the partial discharge along an insulator surface in vacuum under AC voltage application by the measurement of the charging and the light emission. Under sudden AC high voltage application, the partial discharge along the alumina surface occurred at the peak voltage of both polarities, but mainly at the negative polarity. The partial discharge was initiated near the rod electrode and spread out over the whole alumina surface. The partial discharge and subsequent charging behavior on the alumina surface under AC voltage application was verified to be dominated by the distortion of electric field due to not only the negative charges on the alumina surface but also the decrease phase of applied AC voltage.