Alpha-driven currents in tokamak reactors

The generation of a net toroidal plasma current in tokamak reactors by interaction of an input magnetosonic wave with the fusion product alpha particles is examined. The wave transfers momentum to the alpha particles by Landau damping and transit time damping and thereby modifies the alpha particle velocity distribution function in an anisotropic manner. The change in the alpha particle distribution function and the resulting toroidal current density and wave energy absorption is calculated for several assumed alpha particle velocity distribution functions. A Gaussian velocity distribution function to model a spread in the alpha 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 alpha 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. The efficiency of the process is found to be comparable to that of lower hybrid current drive and approaches values of 0.15 A/W at high electron temperatures. The magnitude of the induced current is found to be significantly less than that required for plasma confinement and equilibrium, although anomalously large values of the parallel wave magnetic field could produce the design current. Issues such as wave accessibility, absorption, and mode conversion are also examined and found to be favorable for the assumed wave frequency of about four times the cyclotron frequency of deuterium and parallel wave vectors constrained for resonance with the 3.52-MeV alpha particles.