Vacuum Surface Flashover of High Gradient Insulators Under Nanosecond Pulse

Vacuum surface flashover of the high gradient insulator (HGI), which consists of many alternative layers of conductors and insulators, was found to generally start from one or several dielectric layers adjacent to the anode under nanosecond pulse by using the fast photography. A simple model is proposed to explain the experimental observations. The secondary electron emission avalanche (SEEA) along the dielectric surface adjacent to the anode will be greatly enhanced due to the upstream SEEA starting from the traditional and the intermediate cathode triple junctions. The 15-mm-thick HGI samples with different numbers of dielectric layers were tested and the samples with six and eight dielectric layers showed the best performance. The experimental result proves that as the number of dielectric layers reaches a certain level, the performance of the HGI will decrease as the number of dielectric layers increases.