Enhanced transport of impurity ions and electron heat in Tore Supra ohmic discharges following nickel laser blow-off injection

Abstract

Nickel has been injected into ohmic Tore Supra (TS) discharges with and without activation of the ergodic divertor (ED). The nickel ion transport is studied by means of an impurity transport code, and the radial profiles of the diffusion coefficient D and of the inward convection velocity V are obtained by simulating the soft X ray brightness and emissivity time evolutions. The simulations confirm two previously reported conclusions: the presence of a central core with reduced transport coefficients and, with the ED activated, the necessity of introducing a peripheral transport barrier (with increased inward convection and/or reduced diffusion). To make satisfactory simulations in both experimental conditions of the initial phase of the nickel ion inflow before the first inverted sawtooth, it is necessary to increase transiently the diffusion coefficient (by 1 order of magnitude or more) during the first few milliseconds after the injection. By comparison of the experimental and simulated bolometric signals, it is possible to verify quantitatively the available calculations of the global radiative losses for nickel. The observed discrepancy by a factor of 2 is discussed. The inward propagating cold wave, following the nickel injection, has been observed with a heterodyne radiometer (monitoring the electron temperature). Both with and without the ED, good simulations of the cold wave propagation are obtained using a predictive non-local transport model, previously successfully tested in JET during transient regimes. The model predicts large increases of the electron thermal diffusivity χe, by a factor of 5, immediately after the injection and describes well the electron temperature profile in the so-called heat transport dominated region outside the q = 1 surface. The experimental signals are smoothed with the `generalized singular value decomposition' technique, reducing the effect of the sawtooth modulation; the simulation can, therefore, be more clearly compared with the measurements. It is not possible, using present experimental data, to demonstrate the existence of an electron diffusivity transport barrier inside the q = 1 region, as done for the impurity transport, because the temperature profile is strongly affected by sawteeth in this region. Similarities have been found in the time evolutions of the electron temperature and of the Abel inverted soft X ray emissivities at fixed radii, implying similar time behaviour of D and χe outside the q = 1 surface.