Investigation on Dielectric Window Treelike Breakdown and Suppression Under HPM in Vacuum

Surface discharges of dielectric window seriously limit the generation and transmission of high-power microwaves (HPMs), which block the development of microwave technology. An S-band (2.856-GHz) HPM experimental system is established. Several kinds of dielectric window materials, i.e., polyethylene, polytetrafluoroethylene, and polymethyl methacrylate, are investigated under S-band HPMs in vacuum, and the characteristics of destroyed dielectric samples are analyzed with macroscopic and microcosmic observation. Treelike breakdown channels are found on the upstream face of dielectric window along the direction of microwave electric field, and a breakdown development model with an increase of cumulative breakdown times is proposed. Dielectric surface breakdown suppression technologies are also studied, i.e., surface-polishing and surface-notching treatments. The experimental results show that the polishing treatment on the dielectric surface can decrease breakdown degree and increase surface breakdown threshold. The surface grooves that are parallel to the direction of electric field will reduce the surface breakdown threshold, while the grooves that are perpendicular to the direction of electric field can suppress the surface breakdown obviously. The effect is influenced by the depth and width of grooves on the dielectric surface, and correlative theoretical analysis is discussed.