Simulation of surface charging phenomena on insulator prior to flashover in vacuum

Before flashover across an insulator in vacuum, there is charging phenomena on the insulator surface, which significantly affects the developing process of flashover. However, up to now it is still very difficult to measure the phenomena with applying voltage. Based on the secondary electron emission avalanche (SEEA) model, and by using the Monte Carlo method, a two-dimensional analysis of surface charge density on cylindrical and conical insulators prior to flashover has been performed in vacuum. Different materials are employed, i.e., alumina ceramic, Polytetrafluoroethylene (PTFE), Polyimide (PI) and Polymethyl Methacrylate (PMMA). The influences of different materials, applied voltages and conical angles on surface charge distribution are investigated. The results reveal that, negative charges exist in a small surface region near the cathode and then turn to positive charges in a large surface region away from cathode. And the surface positive charge density is lower for the insulator with the small secondary electron emission rate. While increasing the voltage applied on the insulator, the density and area of negative charges decreases, but the density and area of positive charges increases, and the peak of positive charge region moves towards the cathode. For the conical insulator with negative conical angle, its surface positive charge density is greater than that with positive conical angle, and while the angle is between -22.5° and -30°, the surface positive charge density reaches the crest, corresponding to a lowest flashover voltage. The simulation results are consistent with experiments.