Window and cavity breakdown caused by high power microwaves

Physical mechanisms leading to microwave breakdown on windows and in cavities are investigated for power levels of the order of 100 MW at 2.85 GHz. The test stand uses a 3 MW magnetron coupled to an S-band traveling wave resonator. Various configurations of dielectric windows are investigated. In a standard pillbox geometry with a pressure of less than 10/sup -8/ torr, surface discharges on an alumina window and multipactor-like discharges starting at the waveguide edges occur simultaneously. To clarify physical mechanisms, window breakdown with purely tangential electrical microwave fields is investigated for special geometries. Other configurations, such as air filled two window setups, relevant for vacuum-air interfaces, can be investigated as well. Diagnostics include the measurement of incident/reflected power, measurement of local microwave fields, discharge luminosity, and X-ray emission. All quantities are recorded with 0.2 to 1 ns resolution. In addition, a framing camera with gating times of 5 ns is used. Based on the experimental results, methods to increase the power density which can be transmitted through windows, such as surface coatings and window profiles, are investigated as well.