Effect of plasma density fluctuations on the lower hybrid power spectrum -

Abstract

Radio frequency waves at the Lower Hybrid (LH) frequency are widely used in tokamak plasmas for driving a large fraction of the toroidal current. Consequently, evolution of the LH wave inside the plasma has been an important research subject. LH wave transfers energy to electrons by kinetic resonance via Landau damping. Landau damping takes places most efficiently for electrons at parallel velocities around 3 times the electron thermal velocity. However, the particles number as a function of v-- is a Maxwellian distribution function in which electrons that satisfy Landau damping condition occupy the tail and hence are very few which creates the spectral gap problem. On the other hand, it has been heuristically validated that RF power spectrum emitted by LH launchers should couple to a wide range of electrons so that the spectral gap is filled. It has been noticed that if a perturbed power spectrum with fluctuation rate faster then fast electron slowing time is fed into the present models of plasma, is fed into the present models of plasma, it would come up with answers that explain all phenomena inside plasma and reproduce realistically what observations are up to. This study suggests a plausible mechanism for filling the LH spectral gap, that starting with standard theory, a deeper investigation would allow to redo the calculations with new perspective regarding density fluctuations and turbulence in the SOL. Using COMSOL Multiphysics, it is shown that parallel to magnetic field line density fluctuations in front the LH antenna, with parallel wavelength in the order of the LH wavelength modify the power spectrum at the separatrix by diffraction, such that experimental observations in wide spectral gap regimes at Tore Supra are successfully reproduced.