Comparison of surface charge distributions under vacuum and atmospheric condition due to surface discharge events

Surface charging on insulators in vacuum is recognized as one of the important phenomenon related to the occurrence of vacuum surface discharge events. In order to understand the vacuum discharge phenomenon deeply, it is necessary to investigate the influence of the surface charges on surface discharge events. Therefore the research on surface charges on an insulator is requested to measure the charge distribution or progress of charging before and after surface discharge event on the insulator surface. In this research, the measurement of 2-dimensional surface charge distributions in real time has been carried out in vacuum by Pockels effect. With this measuring method, the charge distributions on a dielectric sample can be measured from the high voltage application to the surface flashover event. This paper describes the comparison of the surface charge distributions in vacuum and those in atmospheric condition on dielectrics under non-uniform ac voltage application. Moreover, using this method, the charge distributions just before and after one flashover event on PET films have been measured. In these experimental results, we found that surface charge distribution in vacuum under negative cycle ac voltage application spread out over the sample surface after a single surface discharge event. On the other hand, surface charges due to negaive streamer-like-discharge are measured under atmospheric condition.