First-alpha diffusion and thermalization in tokamak reactors

Energy relaxation and spatial diffusion of fast alpha particles are incorporated into a multi-energy group model which is coupled to a fluid transport code for the thermal-plasma species. The multi-energy group equations evolve the temporal- and spatial-dependent alpha-particle distribution function and thus determine alpha-particle heating and loss rates for arbitrary thermalization and diffusion models. The effects of deviations from classical, local thermalization on plasma performance are discussed. It is shown that spatial diffusion can lead to inversion of the fast - ion distribution function even if thermalization remains classical. This inversion may drive instabilities and lead to anomalous thermalization. Ripple-induced spatial diffusion of fast alphas is used to illustrate the importance of extending the analysis to include pitch-angle dependence