Behaviour of intrinsic carbon and laser blow-off injected nickel in Tore Supra during ergodic divertor activation

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Abstract
The behaviour of intrinsic carbon (the dominating impurity) and laser blow-off injected nickel has been studied in Tore Supra during ergodic divertor (ED) activation. The carbon content is reduced in the plasma core as a result of the screening effect of the ergodic layer, which is due to increased transport in the layer, to increased recycling flux and to a modified impurity source term. Simulations of the C VI and C V intensity ratios require in the ergodic edge both an increased diffusion and a large (in the 1016-1017 m-3 range) neutral hydrogen isotope density. The nickel burst is not 'screened' and penetrates into the core plasma. The confinement time tau p of the injected elements is always increased when the ED is activated, but there is no modification of the core plasma transport (diffusion coefficients, convection velocities, central reduced transport region extension). To simulate the increased central tau p values, a peripheral transport barrier has to be introduced. Satisfactory nickel brightness simulations can be obtained by modifying the inward convection velocity and/or the diffusion coefficient. If the barrier is taken to be purely diffusive, satisfactory carbon line ratios can be recovered, but the simulated injected impurity confinement time tau p is too short. For a satisfactory nickel brightness simulation it is necessary to add to the barrier some inward convection, pushing also the intrinsic carbon ions inwards, but this hinders correct line ratio evaluations