Monte Carlo simulation of temporal behavior of surface charging across insulator at flashover initial stage in vacuum

The temporal behaviors of charging on insulator surface at flashover initial stage is studied under dc voltage in vacuum. It suggests that the surface charging process develops synchronously with multipactor phenomena. A saturated region is firstly formed nearby the cathode triple junction (CTJ), and the insulator surface is charged gradually from cathode toward anode as the multipactor develops forward. The surface charge density increases linearly with applied electric field, but a stronger applied electric field will slow down the charging velocity as the results of violent collisions of secondaries with dielectric surface due to the stronger normal field built by surface charges. The charging velocity mainly depends on the secondary electron yield (SEY) for dielectric, i.e., the higher SEY, the greater the charging velocity is. The emitter height from CTJ has great influence on surface charging at areas adjacent to cathode, but little effect on charging behaviors at other areas. Once the multipactor is triggered by adequate initial electrons released from cathode, the surface will be charged spontaneously even if the field emission process at cathode is terminated, implying the generation of secondary electrons turns into a self-sustaining process. This study strongly supports the speculation in our previous publications regarding the mechanisms of stable luminescence preceding flashover, which is attributed to be initialized by Fowler-Nordheim emission of electrons at CTJ and finally generated by the radiative de-excitation of excited atoms/molecules in the self-stabilizing secondary electron emission (SSEE) process.