Whispering-Gallery-Mode Gyrotron Operation with a Quasi-Optical Antenna

Abstract

High frequency gyrotron operation in the TE 6 1 1 whispering gallery mode is reported.Powers as high as 112 kW and an efficiency as high as 25.4% have been obtained. In addition to the TE 6 1 1 mode, ten other fundamental modes with frequencies between 133.9GHz and 216.4 GHz were observed. A quasioptical antenna for whispering gallery modes has been tested for the TE 1 1 mode. The combination of this antenna and a reflector produces a well collimated, linearly polarized rf beam suitable for electron cyclotron resonance heating or plasma diagnostics. The experimental conversion efficiency was determined to be 89%, and the cross polarization was down by 25dB. A new quasioptical transmission line employing this antenna has also been tested with the gyrotron. The absence of mode competition for the TE 6 1 1 mode, as well as the efficient conversion of the output radiation into a linearly polarized Gaussian-like beam, substantiates the arguments for operation in surface modes in high power gyrotrons. The implications of quasioptical antennas for megawatt gyrotron window design are discussed.2