Stripe Modelling Achieves 13% Carbon Retention in Diii-D Tokamak PMI Simulations

Researchers are gaining important insights into carbon sourcing and transport from helicon antenna surfaces during high-power plasma discharges in the DIII-D tokamak. A. Kumar, D. Nath, and W. Tierens, together with collaborators including J. D. Lore, R. Wilcox, and G. Ronchi, investigated plasma–material interactions to understand how carbon erodes from antenna structures and subsequently affects the core plasma. Using the STRIPE modelling framework, their work shows that although the current graphite wall configuration limits severe impurity accumulation, helicon antennas can still become sources of net erosion and core-directed impurity transport under certain operating conditions. This finding is particularly relevant as fusion devices transition toward high-Z first-wall materials, highlighting the need for sheath-aware antenna designs and reliable predictive modelling to preserve plasma purity and optimise performance.

The team performed detailed simulations of carbon erosion, re-deposition, and impurity transport by integrating SOLPS-ITER, COMSOL, RustBCA, and GITR/GITRm within the STRIPE framework. Their study addresses emerging plasma–material interaction challenges associated with rectified RF sheath potentials forming near the helicon antenna and surrounding tiles. COMSOL simulations predict rectified sheath potentials in the range of 1–5 kV, localised near the antenna base where magnetic field lines intersect the surface at shallow angles, strongly influencing erosion patterns and impurity trajectories.

Results show that carbon self-sputtering dominates the erosion process, while RF-accelerated deuterium ions contribute up to about 1% of the total erosion flux. Simulations were carried out for two H-mode discharges with different antenna–plasma gaps and RF power levels, enabling a comparative analysis of impurity behaviour. In the small-gap configuration, GITRm simulations indicate that only around 13% of eroded carbon is re-deposited locally, with approximately 58% transported into the plasma core. In contrast, the large-gap configuration exhibits lower overall erosion, reduced core penetration of about 35%, and weaker local re-deposition of roughly 4%, consistent with lower collisionality and reduced plasma contact.

The study establishes a clear relationship between the antenna–plasma gap and impurity transport, demonstrating that larger gaps help mitigate core contamination. Nevertheless, it cautions that under specific plasma conditions and magnetic geometries, helicon antennas can still act as sources of net erosion and core-directed impurity transport, potentially affecting the overall impurity balance. The STRIPE framework successfully captures these complex interactions, providing valuable insight into carbon sourcing and transport mechanisms. Overall, the work underscores the importance of controlling RF sheath potentials to minimise erosion and core contamination, contributing to the development of more efficient and sustainable fusion energy systems.