Study of impurity behaviour in JET-ILW hybrid scenario with Deuterium, Tritium and Deuterium-Tritium plasmas
Experimental campaigns at the Joint European Torus with an ITER-like Be/W wall (JET-ILW) with pure deuterium (D), tritium (T) and Deuterium-Tritium (DT) were a unique opportunity to explore various aspects related to the ITER operation. It should also be emphasized that the JET campaign at the end of 2021 included the first tokamak experiments using a 50-50 D-T fuel mixture since 1997. Therefore, one of the most important challenges in recent years was the development of the hybrid scenario for D-T, based on reference deuterium and tritium plasmas. This kind of scenario, one of the foreseen for ITER, is characterised by a low current plasma and a high normalized beta βN factor compared to the parallel optimised baseline scenario. On JET-ILW, the hybrid scenario is usually obtained by modifying the q-profile to help avoid MHD instabilities, and has an increased normalised confinement time compared to the H98,y2-scaling [1].
As the experiments have shown, controlling the plasma edge in the different phases of the hybrid scenario becomes more difficult with higher isotope mass and therefore are also in risk of impurity accumulation [2]. For this reason, investigation of the impurity behaviour, as well as their control, constituted the crucial issue. The present contribution aims to compare mid-Z (e.g. nickel (Ni), copper (Cu), iron (Fe)) and high Z (tungsten (W)) impurities behaviour within H-mode hybrid discharges in D and T plasmas, as well as D and DT plasmas. The presented results rely mainly on the measurements collected by the VUV SPRED survey spectrometer and bolometry diagnostic. Detailed analysis shows that in the H-mode regime in the hybrid scenario, higher impurity radiation is observed for DT in comparison to D plasmas, as well as for T compared to D plasmas. Additionally, it was noticed that the most significant contribution to the plasma radiated power comes from W and to a lesser extent from Ni (~10%). Moreover, it was found that an earlier transition from small ELMs to ELM-free phase can result in the earlier increase of impurities and also that ICRH power in the H-mode entrance phase influence Ni impurity behaviour.