Particle in cell investigation on two multipacting diffusion modes on insulating dielectric surface in vacuum flashover process

Vacuum surface flashover is a weak link and bottleneck issue in high-power pulse devices, and secondary electron emission avalanche (SEEA) is one of several physical phenomena required for the development of cathode-initiated flashover. The traditional theory holds that SEEA starts near the cathode triple junction (CTJ), and the high space charge density region spreads along the dielectric surface toward the anode. Xu et al. [Phys. Plasmas 29(11), 113901 (2022)] found in his experiments that the high space charge density region may also spread from the cathode to the anode or from the middle of the gap to both ends, and the physical mechanism behind this phenomenon is well worth exploring. The paper adopts the particle-in-cell simulation method to study the dynamic behavior of electrons after emission from the triple junction. The simulation reveals that when explosive emission occurs at the CTJ, a high-density electron region appears at a position far from the CTJ and spreads toward the electrode along the dielectric surface. This phenomenon is investigated through a combination of numerical simulation and theoretical analysis in the study.