Nanosecond high power microwave window breakdown diagnostic and its mechanism

Summary form only given. Multipactor and plasma discharge at windows are the major limiting factors in high power microwave HPM transmission and radiation[1-6]. Breakdown at the vacuum/dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above the dielectric^1-10.The timeand space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric/air interface have been observed by nanosecond response four-framing ICCD cameras. The experimental observations indicate that plasma developed more intensely at the dielectric/air interface than at the free-space region with a higher electric-field amplitude. A thin layer of intense light emission above the dielectric was observed after the microwave pulse. The mechanisms of the new breakdown phenomena are analyzed by an electromagnetic field modeling and an electromagnetic particle-in-cell simulation, and the nonlinear positive feedbacks of ionization, higher electron mobility and ultraviolet-driven photoemission due to the elevated electron temperature are crucial for achieving the ultrafast discharge.