Theoretical investigation on possible operation of a 140 GHz 1 MW gyrotron at 175 GHz for CTS plasma diagnostics at W7-X

Collective Thomson scattering is a common diagnostic technique for ion temperature measurements in experimental fusion plasma reactors. Such a system was successfully installed and commissioned at the Wendelstein 7-X stellarator. For this purpose, a 140 GHz gyrotron of the Electron Cyclotron Resonance Heating system was used as a source of the required probing millimeter (mm)-wave beam. However, accurate measurements in the plasma core were not possible at this heating frequency due to the absorption of the mm-waves and the high electron cyclotron emission background. To suppress these effects and to enhance the accuracy of the measurements, it is required to increase the frequency of the probing beam. In this work, the possibility to operate the same gyrotron, which has its nominal operation at 140 GHz, at a higher frequency is comprehensively investigated.