High-power microwave generation by a plasma-loaded backward-wave oscillator

High-power microwaves were measured for a frequency range from 9.5 GHz to 13.4 GHz in a plasma-loaded backward-wave oscillator employing an annular relativistic electron beam with kinetic energy varying from 400 keV to 500 keV and current ranging from 1 kA to 3 kA. The relativistic electron beam, which was produced from a helium gas-loaded foil-less diode, was injected into the helium gas-loaded rippled-wall waveguide of the backward-wave oscillator, generating the plasma in the waveguide and then the high-power microwave. The gas-loaded diode operated stably without encountering the closure problem in the He gas pressure range (less than 100 mTorr) which was of interest to the operation of the backward-wave oscillator. An enhancement in the total microwave power emission over the vacuum case by a factor of up to 7 was found in the experiment, and the highest efficiency of electron beam to microwave radiation was estimated to be about 29%