Impurity transport, turbulence transitions and intrinsic rotation in Alcator C-Mod plasmas

Abstract

Linear and nonlinear gyrokinetic simulations are used to probe turbulent impurity transport in intrinsically rotating tokamak plasmas. For this simulation-based study, experimental input parameters are taken from a pair of ICRF heated Alcator C-Mod discharges exhibiting a change in the sign of the normalized toroidal rotation gradient at mid-radius (i.e. a change from hollow to peaked intrinsic rotation profiles). The simulations show that there is no change in the peaking of the calcium impurity between the plasmas with peaked and hollow rotation profiles, suggesting that the impurity transport and the shape of the rotation do not always change together. Furthermore, near mid-radius, r/a = 0.5 (normalized midplane minor radius), the linear and nonlinear gyrokinetic simulations exhibit no evidence of a transition from ion temperature gradient (ITG) to trapped electron mode dominance when the intrinsic rotation profile changes from peaked to hollow. Extensive nonlinear sensitivity analysis is performed, and there is no change in the ITG critical gradient or in the stiffness of ion heat transport with the change in the intrinsic toroidal rotation profile shape, which suggests that the shape of the rotation profile is not dominated by the ITG onset in these cases.