Experimental and theoretical study of nickel transport dependence on gradients in Tore Supra

This paper reports on a Tore Supra experiment consisting in modifying locally the normalized electron temperature gradient R del T-e/T-e by playing with the electron cyclotron heating wave power deposition radius. Trace nickel was injected by the laser blow-off technique and its behaviour in the plasma has been analysed using a radial impurity transport code. The diffusion coefficient has been found to decrease steeply when vertical bar R del T-e/T-e vertical bar is decreased in conditions where linear gyrokinetic calculations find that turbulence is dominated by modes propagating in the electron drift direction. An experimental turbulence threshold for nickel transport has been deduced.