Variation of N || and its Effect on Fast Wave Electron Heating on LHD

It is predicted in a theory that the MHD stability of LHD plasma depends strongly on the profile of rotational transform, suggesting that the performance of the plasma is improved by controlling it. In order to control the profile, we have proposed a combline antenna for the ICRF fast wave current drive. The propagation and absorption of the wave excited by this antenna is studied in this paper by use of ray tracing analysis. It is concluded that the strong electron heating occurs when the frequency and wave number are adequately chosen. It is also found that up-shift of parallel wave number occurs as the ray travels and the wave absorption is enhanced. The same calculation is carried out in the tokamak with the similar size and plasma parameters, and it is found that the k||-shift in the helical configuration is much larger than that in the tokamak.