Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron

Results are reported demonstrating that radio-frequency (rf) plasma cleanin g is an effective technique for mitigating microwave-pulse shortening (i.e. , lengthening the pulse) in a multimegawatt, large-orbit, coaxial gyrotron. Cleaning plasmas were generated by 50 W of rf power at 13.56 MHz in nitrog en fill gas in the pressure range 15-25 mTorr. Improvements in the averaged microwave energy output of this high-power-microwave device ranged from 15 % to 245% for different initial conditions and cleaning protocols. The mech anism for this improvement is believed to be rf plasma sputtering of excess water vapor from the cavity/waveguide and subsequent removal of the contam inant by cryogenic vacuum pumps. (C) 2000 American Institute of Physics.[S0 003-6951(00)01849-0].