2009 Volume 4 Pages 008
The energy dependence of the diffusion coefficients of α particles in rippled fields of tokamaks is numerically investigated with an orbit-following Monte Carlo code. The diffusion coefficients are enhanced around the ripple resonance energy while they are not so much enhanced in the neighborhood of it. Consequently, they have a local minimum near the resonance energy, and hence they have an M-shaped energy dependence. Ripple resonance is caused by a radial change of the toroidal precession of banana particles, and creates islands in (Nφ, ψ) phase space. Since particles outside the separatrix are the main contributors to diffusion enhancement, the M-shaped energy dependence is explained by the co-existence of open and closed orbits in the phase space. Ripple resonance diffusion is dominant for fusion-produced α particles since the resonance energy width occupies a large portion of the energy range in their slowing-down processes.