Ray tracing calculation for the fast wave electron heating on LHD

N. Takeuchia), R. Kumazawab), T. Watarib), T. Watanabeb), Y. Toriia), T. Sekib), K. Saitob), T. Mutohb), and LHD Experimental Groupb)

a)Nagoya University, Nagoya, Aichi 464-8603, Japan
b)National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

The electron heating using an Ion cyclotron range of frequencies (ICRF) fast wave has been studied on the Large Helical Device (LHD). There are two ways of fast wave electron heating, i.e. the heating via direct electron Landau damping and that via mode conversion to the slow wave. Though the direct electron Landau damping is a weak heating mechanism, it is predicted that the heating efficiency is increased with the electron temperature and the plasma electron density. As the LHD has a non-symmetric magnetic configuration, it is interesting how the parallel refractive index (N//) changes and how the electron heating property changes while the fast wave travels along the LHD torus.
The ICRF fast wave is analyzed by using ray tracing calculation. The ray tracing method provides the wave propagation and the power absorption of the wave energy into each plasma species. In the present analysis, the cold plasma dispersion relation is used in the ray tracing and the hot plasma dispersion relation is used in the power absorption. The plasma configuration is given using the measured plasma parameters and the magnetic field strength is given using the LHD magnetic date.
The temperature, the density, the minority ratio and the parallel refractive index in the initial condition are changed. The initial parallel refractive index is employed as 6.2 (f=38.47MHz, k//=5.0m-1) based on the conventional loop antenna and as 8.9 (f=75.0MHz, k//=13.9m-1) based on the new type combline antenna. In the standard experimental condition, i.e. Rax=3.6m, B=2.75T, the position of fundamental ion cyclotron resonance layer is located at the saddle point of the mod-B surface. On the other hand, the direct electron heating dominates over the ion cyclotron heating in the condition using the combline antenna. The difference of the direct electron heating property between the conventional loop antenna and the combline antenna is reported. In addition, the difference of the direct electron heating property between on the LHD and the tokamak is also discussed in this paper.