Effect of drifts on the diffusion of runaway electrons in tokamak stochastic magnetic fields

The quasilinear diffusion of runaway electrons in tokamak stochastic magnetic fields is examined. Previous models are generalized with respect to the spatial location and coherency of the perturbing magnetic fields, treatment of the radial as well as poloidal drift motion of the electrons, and the role of sidebands that arise from the beating of the electron drift motion with the applied perturbing fields. It is found that drift effects act to reduce the level of quasilinear diffusion by an amount that depends on the poloidal distribution of the magnetic turbulence. The results are employed to estimate the internal magnetic fluctuation levels at the edge during recent experiments on the TEXT tokamak [Phys. Fluids B 3, 2038 (1991)], where the drift modification effects are shown to be small. It is inferred that intrinsic magnetic turbulence controls runaway diffusion, but not the thermal diffusivity of the background electrons.