High power microwave window breakdown under vacuum and atmospheric conditions

Microwave window breakdown is investigated in vacuum and atmospheric conditions. An S-band resonant ring with a frequency of 2.85 GHz and a power of 80 MW with a 4 MW magnetron as a source is used. Window breakdown on the vacuum side is simulated using a dielectric slab partially filling an evacuated waveguide. Various high-speed diagnostic methods yield a complete picture on the breakdown phenomenology, with far reaching similarities to dc surface flashover. During the initiation phase, free electrons are present, which can be influenced by magnetic fields, followed by a saturated secondary electron avalanche with electron-induced outgassing. Final breakdown occurs in the desorbed gas layer above the surface. In order to simulate window breakdown on the gas-side, a segment of the resonant ring separated by two windows was filled with gas at variable pressure, and breakdown was initiated by field-enhancement tips on one of the gas-side surfaces. Threshold power densities for breakdown are measured, and first results on the phenomenology of this gas breakdown are compared with the processes of flashover in vacuum.