WINDOW BREAKDOWN CAUSED BY HIGH-POWER MICROWAVES

Physical mechanisms leading to microwave breakdown on windows are inve stigated for power levels on the order of 100 MW at 2.85 GHz, The test stand uses a 3-MW magnetron coupled to an S-hand traveling wave reson ator. Various configurations of dielectric windows are investigated. I n a standard pillbox geometry with a pressure of less than 10(-6) Pa, surface discharges on an alumina window and multipactor-like discharge s starting at the waveguide edges occur simultaneously. To clarify phy sical mechanisms, window breakdown with purely tangential electrical m icrowave fields is investigated for special geometries, Diagnostics in clude the measurement of incident/reflected power, measurement of loca l microwave fields, discharge luminosity, and x-ray emission. All quan tities are recorded with 0.2-1-ns resolution. In addition, a framing c amera with gating times of 5 ns is used. The breakdown processes for t he case with a purely tangential electric field is similar to de flash over across insulators, and similar methods to increase the flashover field are expected to be applicable.