Real-time measurement of surface charge distributions on insulator before and after vacuum surface discharge events

It is said that surface charging on insulator has an influence on vacuum surface discharge. To clarify mechanism of that event, it is necessary to investigate the progress of surface charge distribution on insulator during voltage application. In this research, real-time measurement of surface charge distributions on insulators (Alumina plate and PET film) in vacuum under AC voltage application was carried out with Pockels effect. Moreover, we tried to calculate time dependence of surface charge density near needle electrode on insulator. Especially we focus on time dependence of the surface charge density just before and after surface discharge event. As a result, alumina and PET were not charged just before first surface discharge. After the discharge, they were negatively charged and the negative charge density increased as the negative applied voltage increased.