Wall intersection of ion orbits induced by fast transport of pedestal plasma over an electrostatic potential hill in a tokamak plasma edge

Abstract

An edge localized mode event is known to transport a significant portion of pedestal plasma across the separatrix and increase 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 in a plasma edge in a guiding-center orbit following code XGC [C. S. Chang, S. H. Ku, and H. Weitzner, Phys. Plasmas 11, 2649 (2004)] to understand the effect of edge radial electric-field shear on the wall-intersection location of large-transport orbits. It is found that without an edge radial electric field, Er, the majority of large-diffusion-induced ion orbital loss is to the outer divertor near the separatrix surface. However, with a large negative Er in the plasma edge with the ion grad-B drift into the single-null divertor, ion orbits with their kinetic energy less than the potential energy shift their wall-intersection locations to the inner divertor, while those with their kinetic energy higher than the potential energy retain their wall intersections at the outer divertor. If the ion grad-B is away from the single-null divertor, the ion orbital loss is always to the outer wall.