Advances in understanding the impact of isotope mass on pedestal structure and transport

Abstract Understanding the physics of the H-mode pedestal structure and dynamics and predicting its performance in future fusion reactors is key to reducing the uncertainties associated with the realization of burning plasma conditions. ITER and future fusion reactors will operate with mixed deuterium–tritium (D–T) plasmas. Perhaps the greatest challenge to date related to understanding the impact of hydrogen isotope mass on plasma transport lies in the processes governing the plasma edge region and H-mode pedestal. This paper, based on a review talk at the 19th H-mode workshop (Mito, Japan, September 2024), reviews recent progress in understanding the impact of isotope mass on pedestal structure and inter-ELM transport. Recent T and D–T experiments on JET have confirmed a strong mass dependence on the pedestal density and provided new and unique results. State of the art pedestal diagnostics were employed to measure the pedestal structure in D, D–T and T and the most advanced theoretical models were used to identify the physics mechanisms linked to the dependence of pedestal stability and inter-ELM transport on isotope mass. Different modelling and analysis workflows are consistent in finding that changes in pedestal stability and in inter-ELM transport with isotope mass are required to explain the isotope mass dependencies of pedestal density and temperature. The competing levels of turbulent and neoclassical transport with varying isotope mass has been quantified using the gyrokinetic code GENE for selected JET pedestals. However, progress in this area is hindered by the difficulty to obtain accurate measurements of the edge particle source. The isotope dependence of neutral fuelling in the H-mode pedestal region was examined quantitatively in DIII-D experiments using direct spectroscopic measurements of neutral hydrogen penetration in H vs D plasmas. Projecting to D–T H-modes in future reactors, which will be largely opaque to neutrals, the impact of isotope mass dependence of edge neutral fuelling is expected to be negligible, while the pedestal density structure will depend primarily on its transport properties, in turn affecting pedestal stability.