High Gradient and rf Breakdown Measurements in a Millimeter-Wave Accelerating Cavity

We report the recent measurement of high gradient and rf breakdown rates (BDRs) in a millimeter-wave accelerating cavity powered by a megawatt gyrotron. The cavity is powered by 10 nanoseconds rf pulses, chopped from 3 microsecond gyrotron pulses using a laser-driven silicon switch. The highest achieved accelerating gradient is 230 MV/m corresponding to a peak surface electric field of 520 MV/m after processing the cavity with more than 10
⁵
pulses. The initial BDR is about 10
⁻³
[1/pulse] and it has not reached a steady state yet. We expect the BDR to further decrease after extended running time.