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∇Te/Te 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 |R∇Te/Te| 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.