Modeling the spectral modification of lower hybrid wave in the presence of drift-wave type density fluctuation in the scrape-off-layer of the EAST tokamak

Abstract

The spectrum change of lower hybrid (LH) waves caused by low-frequency density fluctuation in the scrape-off layer (SOL) is studied by applying the wave scattering model developed by Bonoli and Ott [Bonoli and Ott, Physics of Fluids 25, 359 (1982)] via a Monte Carlo method. Due to the influence of density fluctuation, the perpendicular component of the LH wave-vector can be rotated in the 2D perpendicular space, which will further change the ray trajectory of the LH wave. A ray-tracing model specific to this purpose is developed to evaluate the probability distribution of both poloidal refractive index (N_theta) and the parallel refractive index (n||) of the LH wave at the last closed flux surface (LCFS), assuming wave propagation through the turbulent SOL plasma from the launcher at the far SOL to the LCFS. In the presence of the drift-wave-type density fluctuations, the Monte-Carlo approach is adopted to characterize the scattering probability and the scattering angle of the perpendicular LH wave-vector. The scattering probability and the rotation angle are determined by the combined effect from the geometric optics approximation term and the E×B drift term in the LH tensor elements. The probability distributions of N|| and N_theta at the LCFS are studied using the EAST parameters as a function of wave frequency, the initial n||, and the polar injection position, which may influence the LHCD efficiency.