Impurity transport and density in TdeV

Abstract

The authors study the modelling of light impurity transport in TdeV (Tokamak de Varennes) Ohmic discharges with graphite poloidal limiters. The impurity fraction, the diffusivity and the convective velocity were determined by comparison of measured C IV, O VII and Ne IX line intensities (VUV spectroscopy) with MIST simulated profiles. It is found that the diffusion of the intrinsic impurities C and O, and of the artificial impurity Ne, is strongly anomalous (an order of magnitude above neoclassical values). To obtain good fits to the experimental data the diffusivity must be a varying function of minor radius. No convective velocity is needed to produce good fits. The oxygen content in the plasma is found to be strongly dependent on the electron density: I=I0.ne1.5.Ip-0.8. Carbon and oxygen densities are roughly equal, as found typically on 'all carbon' machines.