Optimization of high harmonic ECRH scenario to extend a heating plasma parameter range in LHD

Effectiveness of high harmonic Electron Cyclotron Resonance Heating (ECRH) was investigated by both experiments and ray-trace analyses. The condition of both the EC wave injection and the magnetic field configuration was optimized in the Large Helical Device (LHD). In the case of the second harmonic ordinary (O2) mode injection with a frequency of 77 GHz and with the optimized injection angle, about 30 - 40% absorption could be kept beyond the cut-off density of the second harmonic extraordinary (X2) mode, which is 3.7 x 10¹⁹ m^-3. In the third harmonic X (X3) mode heating experiment, the dependence of the absorption rate on the plasma density and the temperature of the target plasma was precisely investigated and compared with the ray-trace (TRAVIS code) calculation. The calculation results of the absorption rate show fairly good agreement with the experimentally obtained ones on the plasma-parameter dependences. The maximum absorption rate of more than 40% was achieved. Superposed stepwise injection from three gyrotrons with a total of 3 MW increased the central electron temperature to about 3.5 times of the initial target plasma temperature of 0.6 keV. This shows that the temperature increase improves the absorption rate of the subsequent injection. (author)