Diffusion in a two-dimensional anisotropic web map by extrinsic noise and divertor intersection of ion orbits induced by large edge transport in a tokamak

Abstract

A Diffusion by an extrinsic noise in a two-dimensional anisotropic web mapping is studied in the case where an extrinsic noise is applied to the intrinsically perturbed (intrinsically active) physical quantity when the intrinsic web diffusion is negligible. Contrary to the case where the extrinsic noise is applied to the other (intrinsically passive) physical quantity to yield a highly anisotropic diffusion scaling [Gunyoung Park and C. S. Chang, Phys. Rev. E 64, 026211 (2001)], the diffusion scaling in this case is found to be isotropic. An edge localized mode (ELM) event is known to transport a significant portion of pedestal plasma across the separatrix, and increases the divertor heat load to a possibly intolerable level in a tokamak fusion reactor. In the present work, a large random-walk transport is introduced on the low field side of the plasma edge in the guiding-center orbit following code XGC to understand the effect of the edge radial electric field on the wall strike location of the lost ion orbits. It is found that without an edge radial electric field $E_r$, the majority of ion orbital loss is to the lower magnetic field divertor (outer divertor) near the separatrix surface. However, with a large negative $E_r$ of H-mode type in the plasma edge with the ion ∇B drift into the single null divertor, large random walk of ions with their kinetic energy less than the potential energy shifts their wall-strike locations to the inner divertor while those with their kinetic energy higher than the potential energy retain their strike points at the outer divertor. On the other hand, smaller random walk size at the level of H-mode diffusion does not show this shift, keeping the ion loss to the outer divertor. If the ion ∇B direction is away from the single-null divertor, the ion orbital loss is always to the outer wall.