Experimental impurity transport and theoretical interpretation in a Tore Supra lower-hybrid heated plasma

Abstract

Recent numerical works on turbulent impurity transport raise a new interest in experimental studies on impurity transport and more especially a comparison of its parametric dependences with theoretical predictions. In this context, we have studied the behaviour of four different impurity species in Tore Supra lower-hybrid heated, sawtooth-free plasmas. We observe a decrease in the central impurity confinement time τimp with increasing Z in the range explored (from 13 up to 32). This result is in contradiction to both neoclassical and turbulent theoretical predictions. An analysis with the 1D impurity transport code ITC shows the existence of a central zone with reduced diffusion and indication of an inward convection. These results are completed with theoretical simulations of neoclassical transport (NCLASS) and turbulent transport with the quasilinear gyrokinetic QuaLiKiz and nonlinear gyrofluid TRB models. These simulations do not provide an explanation of the confinement time dependence on the impurity charge. Transport outside r/a = 0.3 is well described by the nonlinear turbulent calculations but the central region is predicted to be below the turbulence threshold while the observed transport is one order of magnitude higher than neoclassical values. The gyrokinetic calculations evidence the dominant role of the curvature pinch in the total convective flux. This term is directed inwards and only weakly dependent on the impurity charge in the explored range of species.