Investigation of dielectric window flashover under pulsed high power microwave excitation
Edmiston, Gregory Ford
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Surface fashover development at the output windows of high power microwave (HPM) systems present a major limitation to the power densities and pulse lengths transmitted through these interfaces. Given a high enough plasma density in the developed discharge, substantial power can either be absorbed thus terminating the radiated pulse or reflected back towards the microwave source potentially leading to damage or destruction of the device. Likewise, the physical damage to the dielectric window incurred by repetitive discharge production in close proximity can lead to cracking, fatigue, and the eventual structural failure of the interface, equally terminating device operation. For these reasons, developing a physical model accurate in predicting surface flashover initiation is of prime interest. The operational parameters of an aircraft-based HPM system are investigated and the impact of each variable on flashover initiation at or near the dielectric-gas interface is identified ed. These parameters include environmental variables such as gas pressure, type, and the presence of ultraviolet radiation incident upon the window surface in addition to physical variables associated with the window such as material, surface roughness, and geometry. A specially designed test environment has been constructed to study HPM surface flashover without the influence of electron injecting structures, such as metallic electrodes, which can dominate all other initiation mechanisms. Additionally, a Monte-Carlo type ion motion simulation was developed to investigate the origin of initiatory electrons which is of specific importance in environments largely devoid of residual free-electron populations, such as the electronegative gases SF6 and O2. The key parameters impacting high power microwave surface flashover are presented along with comparison to literature data. Simulation models designed to estimate the initiation and development of surface flashover will also be compared with empirical data.