Particle stochasticity due to magnetic perturbations of axisymmetric geometries

Abstract

The quasi-linear theory of collisionless test particle diffusion in stochastic magnetic fields is extended to include the effects of finite gyroradius ρ and particle drifts (including magnetic trapping). A canonical framework is used, in which both the criterion for onset of stochasticity and the diffusion tensor scale with field-particle coupling coefficients gl. The gl contain all the information about the unperturbed orbit of a given particle and the perturbation fields with which the particle interacts. The modification of transport due to finite ρ and drifts is thus found by comparison of the gl including these effects to their driftless, ρ→0 limit. It is found that runaway electron confinement is substantially improved over earlier, driftless estimates, and that trapped particles in microturbulence ought not be stochastic. The perturbations from proposed ripple injection schemes are large enough to induce stochasticity for certain classes of particles.