Tree-like breakdown phenomena of dielectric window under X-band high power microwave in vacuum

Surface breakdown of dielectric window seriously limits the transmission of high power microwave (HPM), which blocks the development of microwave technology. Under X-band HPM with the power level of 1 GW at 9.4 GHz, dielectric breakdown experiments were conducted with several dielectric window materials including polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polymethyl methacrylate (PMMA) in vacuum. The influence of different surface treatments for PTFE with notching and polishing on its breakdown characteristics is also studied. After breakdown, the photographs of damaged dielectric samples are analyzed with macroscopic and microcosmic observations. There are obvious tree-like breakdown phenomena occurred on the upstream face of the dielectric window along the direction of microwave electric field. The damage is significantly affected by the different surface treatments. It is considered that the treelike breakdown channels are closely related to the applied microwave field. Based on in-depth observation on the tree failure inside the transparent PMMA sample, it was found that, the tree developed along the surface and also inside of the material. A physical model is proposed to describe the dielectric window breakdown under HPM. It is suggested that the electric field results in the tree developing along the material surface. Possible reason for the tree invading into the material is analyzed. Moreover, several kinds of possible manners are presented to describe the tree initiation and development inside the dielectric window.