Effect of radio-frequency power injection on impurity profile in JET plasmas

To maximize the reactivity of a fusion plasma the impurity content should be as low as possible in the plasma centre. RF power has been seen to flatten the impurity density profiles in various experiments, especially when applied to electrons and deposited in the centre. To evaluate the potentiality of such phenomenon as a means towards an active control of the impurity profiles, JET H-Mode and Hybrid discharges at ITER relevant collisionality ($\nu $eff$<$0.2), with and without RF power applied to electrons, have been transiently doped with traces of Ne, Ar, Ni and Mo. With RF the pinch parameter (ratio of convection velocity to diffusion) of all the injected impurities reduces remarkably or even reverses its sign and the impurity profiles are flattened or become hollow. Such beneficial effect is to be weighted by the increase of the metal content in the plasma that can result from the RF application. Quasi linear gyro-kinetic simulations (GS2) do not explain the observed outward pinch and refer the analysis to a full non linear approach.