Influence of plasma parameters on the absorption coefficient of alpha particles to lower hybrid waves in tokamaks

Abstract

In tokamaks, fusion generated α particles may absorb lower hybrid (LH) wave energy, thus reducing the LH current drive efficiency. The absorption coefficient γα of LH waves due to α particles changing with some typical parameters is calculated in this paper. Results show that γα increases with the parallel refraction index n‖, while decreases with the frequency of LH waves ω over a wide range. Higher background plasma temperature and toroidal magnetic field will increase the absorption. The absorption coefficient γα increases with ne when ne ≤ 8 × 1019 m−3, while decreases with ne when ne becomes larger, and there is a peak value of γα when ne ≈ 8 × 1019 m−1 for the ITER-like scenario. The influence of spectral broadening in parametric decay instabilities on the absorption coefficient is evaluated. The value of γα with n‖ being 2.5 is almost two times larger than that with n‖ being 2.0 and is even lager in the case of 2.9, which will obviously increase the absorption of the LH power by alpha particles.