Alpha-driven currents in tokamak reactors

The generation of net toroidal plasma current in tokamak reactors by interaction of an input magnetosonic wave with the fusion product α particles is examined. The wave transfers momentum to the α particles by Landau damping and transit time damping and thereby modifies the α particle velocity distribution function in an anisotropic manner. The change in the α particle distribution function and the resulting toroidal current density and wave energy absorption is calculated for several assumed α particle velocity distribution functions. A Gaussian velocity distribution function to model a spread in the α-particle birth energy due to kinematic effects as well as speed diffusion from collisions with the background plasma is evaluated and found to give identical results as a monoenergetic delta function velocity distribution in the proper limits. The physical processes producing the Gaussian distribution are shown to be small. A more realistic calculation with an α-particle velocity distribution function based on a Fokker-Planck model of slowing down against the background electrons is performed, and numerical estimates obtained for a reference tokamak reactor design. It is shown, for the assumed parallel wave vector spectrum, that the transit time damping is the dominant process and that interaction of the wave with background electrons can be neglected