Identification of Spurious Modes of High-Power 77-GHz Gyrotron for Collective Thomson Scattering in LHD

Abstract

Frequency measurements at 74-80 GHz were conducted for the identification and suppression of spurious modes from the 77-GHz gyrotron, which was originally introduced as a power source for electron cyclotron resonance heating and is now also used as a probe beam for the collective Thomson scattering (CTS) diagnostic. The spurious modes are excited as the gyrotron output powe ris turned on and off by controlling the anode voltage. These modes are harmful for the CTS diagnostic, even though their power is less than approximately 50 dB below than that of the main mode. The measured frequency of one of the spurious modes is approximately 74.7 GHz. The cavity structure, starting current calculation, and mode competition calculation suggest that the spurious mode is the TE_17,6 mode. The result is important for optimizing the gyrotron operation as a CTS probe beam and suppressing or minimizing the spurious mode excitation.