X-transport: A baseline nonambipolar transport in a diverted tokamak plasma edge

Abstract

A new nonambipolar neoclassical transport (X-transport) mechanism has been identified which can be an irreducible baseline source of a strong radial electric field and edge pedestal formation immediately inside the separatrix in a diverted tokamak. Due to the vanishingly small poloidal magnetic field in the vicinity of a divertor X-point, there exists an ion velocity space hole at the thermal energy level. This becomes a source of a nonambipolar, collisional, convective radial transport of plasma ions, as the ions scatter into and out of the loss hole in the vicinity of an X-point. The widths of the E-r and edge pedestal layers are somewhat smaller than the ion poloidal gyroradius, measured at the midplane. A simple estimate shows that the X-transport rate can be significant enough to influence a high mode transition and edge pedestal formation. (C) 2002 American Institute of Physics.