Current drive by lower hybrid heating of thermonuclear alpha particles in Tokamak reactors

Modification of the fusion born alpha particle current by off-axis radiofrequency (RF) heating of energetic alphas with intense localized waves is investigated by Monte Carlo simulations and by an analytical method for tokamak fusion reactors. The alpha current is found to be increased significantly by a prolongation of the slowing down time and the concomitant increase of the hot alpha particle concentration, as well as by the increase of trapped alpha fraction through the elevation of the perpendicular energy content of the magnetized alpha population with the Landau interaction between the alpha particles and lower hybrid waves. For a sufficiently localized wave with a power absorption density comparable to the local alpha power production density, the wave generated bootstrap current turns out to dominate the wave driven alpha current based on momentum transfer, leading to a bidirectional current density profile. The current enhancement is shown to increase with the wave diffusion coefficient, inverse aspect ratio and electron temperature, and is maximized for wave perpendicular phase velocities somewhat below the alpha birth velocity.