ALPHA-PARTICLE LOSSES FROM TOROIDICITY-INDUCED ALFVEN EIGENMODES .2. MONTE-CARLO SIMULATIONS AND ANOMALOUS ALPHA-LOSS PROCESSES

Fusion-born alpha particles moving parallel to the magnetic field can resonate with toroidal Alfven eigenmodes (TAE) leading to anomalous alpha-orbit diffusion across the alpha-loss boundaries in a tokamak. This is analyzed using the Hamiltonian guiding center code ORBIT in conjunction with the kinetic magnetohydrodynamics (MHD) eigenmode solving code NOVA-K. Resonant single alpha orbits are studied below and above the threshold for orbit stochasticity and Monte Carlo randomized ensembles of alphas subjected to a finite amplitude time-dependent TAE are followed with respect to their radial losses using realistic MHD equilibria and numerically computed toroidal Alfven eigenfunctions for one toroidal eigenmode n = 1 and the full Fourier spectrum of poloidal harmonics m involved in the "gap mode". The alpha-loss mechanisms are resonant drift motion across the loss boundaries of alphas born near these boundaries and stochastic diffusion to the boundaries in constants of the motion (phase) space. After a first transient of resonant drift losses scaling as B(r) approximately/B0, the number of alphas lost via diffusion scales as (B(r) approximately/B0)2. For TAE amplitudes B(r) approximately/B0 greater-than-or-equal-to 10(-3), alpha orbit stochasticity sets in and, depending on the radial width of the fast alpha density n(alpha (r), a substantial fraction of alphas can be lost in one slowing down time. For B(r) approximately/B0 < 10(-4), the losses become insignificant.