Activities on Realization of High‐Power and Steady‐State ECRH System and Achievement of High Performance Plasmas in LHD

Electron Cyclotron Resonance Heating (ECRH) has contributed to the achievement of high performance plasma production, high electron temperature plasmas and sustainment of steady‐state plasmas in the Large Helical Device (LHD). Our immediate targets of upgrading the ECRH system are 5 MW several seconds and 1 MW longer than one hour power injection into LHD. The improvement will greatly extend the plasma parameter regime. For that purpose, we have been promoting the development and installation of 77 GHz/1–1.5 MW/several seconds and 0.3 MW/CW gyrotrons in collaboration with University of Tsukuba. The transmission lines are re‐examined and improved for high and CW power transmission. In the recent experimental campaign, two 77 GHz gyrotrons were operated. One more gyrotron, which was designed for 1.5 MW/2 s output, was constructed and is tested. We have been promoting to improve total ECRH efficiency for efficient gyrotron‐power use and efficient plasma heating, e.g. a new waveguide alignment method and mode‐content analysis and the feedback control of the injection polarization. In the last experimental campaign, the 77 GHz gyrotrons were used in combination with the existing 84 GHz range and 168 GHz gyrotrons. Multi‐frequency ECRH system is more flexible in plasma heating experiments and diagnostics. A lot of experiments have been performed in relation to high electron temperature plasmas by realization of the core electron‐root confinement (CERC), electron cyclotron current drive (ECCD), Electron Bernstein Wave heating, and steady‐state plasma sustainment. Some of the experimental results are briefly described.