Improvement in Plasma Heating at Higher-Density Region Using 106.4 GHz Electron Cyclotron Heating System for Compact Helical System

A 106.4 GHz electron cyclotron heating (ECH) system for compact helical system (CHS) has been newly constructed. Initial experiments to confirm the system's performance showed good controllability of beam direction and polarization for optimized heating. In the beam direction and polarization scan experiments, on-axis beam injection with second-harmonic X-mode polarization resulted in the highest stored energy and central electron temperature. Additional heating becomes possible for the plasmas sustained by neutral beam injection with a density higher than the plasma cutoff density of 3.5×10 19 m -3 for the 53.2 GHz waves of the existing ECH system. Applying the 106.4 GHz heating waves extends the CHS plasma parameter to a high electron temperature of the keV order at a high density up to the second-harmonic X-mode cutoff density of 7×10 19 m -3 .