Damage phenomena of dielectric window material under X-band high power microwave

The microwave discharge experiments of dielectric window were performed in vacuum under X-band HPM with the frequency of 9.4 GHz and the power of 1 GW. Several typical dielectric window materials, i.e., PTFE, PMMA, LDPE and HDPE, were employed, and the influence of different surface treatments as notching and polishing was investigated. After microwave breakdown experiments, the surface morphology of dielectric samples were analyzed with the macroscopic and microcosmic view. It was found that there were obvious single-surface tree-like breakage phenomena occurred along the direction of microwave electric field, and the surface destroy status was significantly affected by the different surface treatment. It is considered that the tree-like depredation channels are closely related to the applied microwave field. By observing the tree failure inside the transparent PMMA sample, it was found that, the tree developed along the surface and also to the inner of material. A physical model is proposed to describe the dielectric window breakdown under HPM. It is suggested that the electric field of microwave results in the tree development along the material surface, and the tree develops into the material under magnetic field. Moreover, the possible reasons are revealed to describe the tree initiation and development inside the dielectric window.