The turbulent impurity (nickel) transport dependence on the normalized electron temperature gradient has been analyzed in sawtooth-free electron cyclotron wave heated Tore Supra plasmas. In the core, our experimental analysis shows that the lower R/L((T)(e)), the lower the nickel diffusion coefficient. The latter decreases until the instability threshold is reached. The experimental threshold is in agreement with the one computed by a gyrokinetic model. Further out, R/L((T)(e)) plays no role in the impurity diffusion. This set of experimental results is consistent with a quasilinear gyrokinetic analysis.
Experimental results on impurity transport in tokamaks are based on various techniques. We study here how the choice of the injection technique and of the analysis method influences the results. We have used three different injection techniques available in Tore Supra: laser blow-off, gas puff and supersonic molecular beam injection. We show that the long time duration of the gas puff injection compared with particle confinement time provides very limited information. The laser blow-off technique and supersonic pulsed injections give satisfactory results for diffusion but low quality convection estimates, presumably because the fast source term quenches the role of convection in the continuity equation. The best method is shown to be the combined analysis of supersonic pulsed injections and continuous puffing of a gaseous species. We obtain convection velocity profiles to an uncertainty of about 0.5 m s−1. This method is applied to ohmic, weakly sawtoothing plasmas. The diffusion coefficient is independent of the impurity charge and the convection velocity is inward. Neoclassical calculations show that these plasmas are dominated by anomalous transport. Quasilinear gyrokinetic simulations are in qualitative agreement with the above experimental results. We deduce from the simulations that convection is dominated by the curvature term, which means that no charge dependence should be expected in this situation.
Electron and impurity transport has been studied in sawtoothing plasmas in the Tore-Supra tokamak. High time and space resolution measurements of the electron density reveal the existence of a flat profile region encompassing the q = 1 surface, on which is superimposed a density peak building up between sawtooth relaxations. For the first time in this regime, we have determined the underlying transport of both nickel and electrons independently of the effect of sawteeth in the central part of the plasma. Electron transport is consistent with the neoclassical expectations only in the close vicinity of the magnetic axis. Further out, it exceeds the neoclassical values as calculated with the NCLASS code, although the turbulence level is very low in the whole central region region. In contrast, nickel transport is in good agreement with the neoclassical calculations in the same region. The neoclassical effect on trapped particles of a persisting mode due to incomplete reconnection of the magnetic surfaces is consistent with these observations.
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