Impact of magnetic turbulence on rf current drive

The kinetic theory of rf current drive in tokamak plasmas in the presence of magnetic turbulence is investigated. The main effect of the turbulence is to cause radial diffusion of the superthermal electrons. For typical parameters of large machines, the diffusion time can be of the same order of the slowing-down time of the current carriers. In these conditions, a significant regime is identified in which the total driven current is enhanced by the effect of the radial diffusion. This is demonstrated analytically in the weak turbulence limit, and numerically confirmed in the general case, by means of a fully three-dimensional Fokker--Planck code, for both parallel and perpendicular velocity-space diffusion