Impurity production from the ion cyclotron resonance heating antennas in JET

Additional heating systems can be a source of impurities in fusion plasmas. Studying the behaviour of such impurities is important to understand and minimize their effects on tokamak plasma performance. In this paper we present a detailed study of the influence of ion cyclotron resonance heating (ICRH) on the impurity content of JET plasmas. Using spectroscopic methods we monitor the Ni impurity release by the two ICRH antenna types in JET: the so-called ITER-like antenna and the A2 antennas. The release of Ni during ICRH is presented as a function of the power density, total ICRH power applied, the relative phasing of the A2 antenna straps, the D ₂ gas injection level and the plasma-strap distance. We also estimate the contribution of Ni impurity to Z _eff and to plasma dilution. L-mode discharges are compared for different relative phasings of the four antenna straps of the A2 antennas: (0π0π), (0ππ0), (00ππ), (00π/2π/2), (0π/2π 3π/2), (π0) and (0π). We observe that for antenna phasings that allow the best coupling, the Ni impurity concentration is reduced in the central part of the plasma. The impurity content is also monitored in H-mode experiments aiming at improving the ICRH coupling at large antenna strap-separatrix distances, up to 19cm, by injecting gas from different inlets. Interestingly, with increasing gas injection rate the coupling improves and the Ni concentration decreases, and the actual location of gas injection is found to influence the Ni concentration.