Effect of Desorbed Gas on Dielectric Surface Discharge in Tangential and Normal Microwave Electric Fields in Vacuum

The particle-in-cell–Monte Carlo collision (PIC–MCC) model is used to simulate the dielectric surface discharge caused by tangential and normal microwave electric fields in vacuum. We focus on the influence of gas desorbed from the dielectric surface on the discharge characteristics. To better understand the discharge characteristics, the analytical model is used to predict the electron transit times, which are consistent with the PIC–MCC simulation results. The simulation results also indicate the multipactor dominates in the early stages of discharge. However, the ionization of the desorbed gas leads to a further increase in the number of charged particles after the multipactor reaches saturation, which is different from the pure multipactor discharge. The pressure of desorbed gas in the tangential microwave electric field grows faster than in the normal microwave electric field. This is because electrons have shorter transit times in the tangential microwave electric field, and the collision rate between electrons and the dielectric surface is higher. The ionization of desorbed gas leads to a more significant increase in the number of electrons in the tangential microwave electric field compared with the normal microwave electric field. The peak density and thickness of plasma increase in time in the tangential microwave electric field, but the spatial profile of plasma does not change significantly in the normal microwave electric field.