Nonlinear neoclassical transport in toroidal edge plasmas

In conventional neoclassical theory, the density and temperature gradients are not allowed to be as steep as frequently observed in the tokamak edge. In this paper the theory of neoclassical transport in a collisional, impure plasma is extended to allow for steeper profiles than normally assumed. Th e dynamics of highly charged impurity ions then becomes nonlinear, which af fects the transport of all species. As earlier found in the banana regime, when the bulk plasma gradients are large the impurity ions undergo a poloid al redistribution, which reduces their parallel friction with the bulk ions and suppresses the neoclassical ion particle flux. The neoclassical confin ement is thus improved in regions with large radial gradients. When the pla sma is collisional and the gradients are large, the impurities accumulate o n the inboard side of the torus. (C) 2001 American Institute of Physics.