Stochastic transport of MeV ions by low-n magnetic perturbations*

Abstract

Recent progress toward understanding two transport phenomena of importance to fusion reactors is described, in the framework of a theory of the stochastic transport of energetic ions in low-frequency, low-n perturbations. These are the observed enhancement of fusion product loss by magnetohydrodynamic (MHD) perturbations, and a new possible mechanism for ash removal and burn control. Significant features of the theory bearing on both problems are the variety of transport scalings with energy possible from perturbations of differing structure, and the contribution to the transport from perturbing ‘‘grad-B’’ drifts, neglected in earlier studies, but that can be dominant for MeV ions. Numerical evidence is presented substantiating these analytic expectations. The transition from above to below the global stochastic threshold, and some remaining disparities between experimental observations and numerical/theoretical results, are discussed.