The diffusion coefficient for the fast alpha particles produced in a thermonuclear plasma is derived numerically for the case of a magnetic field perturbed by ripple and magnetohydrodynamic (MHD) helical modes. It is found that this diffusion coefficient varies monotonously with the amplitude of the magnetic perturbations and that the transition from the classic to the stochastic regime occurs smoothly. The ripple perturbations as well as the MHD perturbations essentially affect the trapped-particle orbits. It is shown that above an MHD perturbation amplitude of some 10−3 of the total magnetic field, severe fast alpha-particle losses must be expected. Parametric studies have shown the dependence of the MHD helical diffusion coefficient on the amplitude of the perturbation, the helical mode number, and the energy of the alpha particles. An analytic expression for the MHD diffusion coefficient, based on these scalings, is proposed.
