Impurity screening in hybrid scenario plasmas in JET-ILW*

Future fusion experiments such as ITER are designed with tungsten (W) as a first wall material to manage the large heat loads produced during high performance scenarios. W contamination can severely limit the operational performance of burning plasmas and potentially lead to the radiative collapse of the plasma due to impurity accumulation. Therefore, understanding impurity transport and demonstrating the capability to screen impurities from the confined plasma is essential for the successful operation of fusion reactors.

 

Recent experiments on JET have shown further evidence of impurity screening in high power, hybrid pulse scenarios with D plasmas, as previously demonstrated [A R Field NF 2022, J Hobirk NF 2023]. A strong ion temperature gradient at the pedestal, relative to the ion density gradient, creates an outward neoclassical convection of W impurities, preventing core impurity accumulation during the inter-ELM period. This work presents results from recent JET experiments that showcase impurity screening and attempts to gain further understanding into the driving forces behind this observed behaviour. Neoclassical transport of W impurities is calculated with NEO, using experimental kinetic profiles. In some cases, our analysis indicates that peripheral temperature gradient screening alone isn’t enough to recreate the screening observed in experiment. This could suggest that there are additional transport effects that influence impurity screening which are yet to be fully understood.