Sub-Terahertz Gyrotron Developments for Collective Thomson Scattering in LHD

Abstract

Collective Thomson scattering (CTS) is expected to provide the spatially resolved velocity distribution functions of not only thermal and tail ions also alpha particles resulting from fusion reactions. To date,CTS has not been fully utilized because of the difficulties in achieving sufficient signal-to-noise ratios and appropriate scattering angles. The use of gyrotrons with frequency higher than the conventional ones may overcome these difficulties and avoid cut-off and refraction effects even in the high-density plasmas. Therefore,a high-power pulse gyrotron operating at approximately 400 GHz is being developed for CTS in large helical device (LHD). A single mode oscillation with a frequency greater than 400 GHz,applying the second harmonic resonance,was successfully demonstrated in the first stage. Achieving the oscillation at higher power levels by improving the electron gun and cavity is under study. At the same time,concrete feasibility study based on ray tracing,scattering spectra and electron cyclotron emission calculations has been conducted