Test-particle simulations of collisional impurity transport in rotating spherical tokamak plasmas

A full orbit test-particle approach is used to study the collisional transport of impurity (carbon) ions in spherical tokamak (ST) plasmas with transonic and subsonic toroidal flows. The efficacy of this approach is demonstrated by reproducing the results of classical transport theory in the large aspect ratio limit. The equilibrium parameters used in the ST modelling are similar to those of plasmas in the MAST experiment. The effects on impurity ion confinement of both counter-current and co-current rotation are determined. Various majority ion density and temperature profiles, approximating measured profiles in rotating and non-rotating MAST plasmas, are used in the modelling. It is shown that transonic rotation (both counter-current and co-current) has the effect of reducing substantially the confinement time of the impurity ions. This effect arises primarily because the impurity ions, displaced by the centrifugal force to the low field region of the tokamak, are subject to a collisional diffusivity that is greater than the flux surface-averaged value of this quantity (Helander 1998 Phys. Plasmas 5 1209). For a given set of plasma profiles, the ions are found to be significantly less well confined in co-rotating plasmas than in counter-rotating plasmas. The poloidal distribution of losses exhibits a pronounced up/down asymmetry that is consistent with the direction of the net vertical drift of the impurity ions