Natural poloidal asymmetry and neoclassical transport of impurities in tokamak plasmas

The neoclassical transport of impurities is investigated for a plasma without toroidal rotation nor anisotropic ion temperature. It is shown that a natural poloidal asymmetry of the impurity density exists in this case, and that it can be described with a simple analytical model. The poloidal asymmetry tends naturally to cancel as the impurity profile evolves towards its steady state, so that the main effect of the poloidal asymmetry is to slow down the impurity flux compared to its predicted value without poloidal asymmetry. The contribution of the asymmetries of the electrostatic potential and of the main ion density can be included in the analytical derivation, thus forming a self-consistent description of the neoclassical impurity flux together with its poloidal distribution. Numerical simulations with a non linear fluid code confirm the analytical findings, showing that the neoclassical transport of impurities is strongly modified by its natural poloidal distribution.