Whispering gallery mode gyrotron operation with a quasioptical antenna

1 High frequency gyrotron operation in the TE6 11 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 TE6 11 mode, ten other fundamental modes with frequencies between 133.9 GHz 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 pro-duces 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 TE6 11 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 I