Improved kinetic test particle model for impurity transport in tokamaks

The expression for the Coulomb collision term in Fokker-Planck form is revised and generalized to the case of interaction of trace impurity ions with a fluid-like background plasma. The velocity distribution of the background ions is assumed as a Maxwellian in the 13 moment expansion, with parameters determined from a fluid consideration. The impurities are described in the framework of a linear drift kinetic test particle model. The new Coulomb term is suitable for linear Monte Carlo procedures and provides both a correct treatment of thermalization of the test particles and an accurate kinetic description of velocity distribution effects on the thermal and frictional forces. These features are missed out in existing codes for kinetic modelling of impurity transport, based on a reduced Coulomb term with these forces treated in the fluid approximation. Numerical results presented for TEXTOR specific edge plasma conditions emphasize the importance of the proposed more accurate treatment, in particular by consideration of the influence of thermal forces on low ionized impurities.