Description
Managing power exhaust in next-generation fusion devices requires impurity seeding to induce divertor detachment and protect plasma-facing components; however, this must be balanced against the need to minimize core contamination to maintain plasma performance. While previous SOLPS-ITER modelling of the proposed STEP (Spherical Tokamak for Energy Production) power plant demonstrated that the D2 fuelling location has a strong impact on the core argon (Ar) levels [1], it is necessary to establish if these trends hold in the current, significantly larger iteration of the machine. In this new study, we use SOLPS-ITER to review this question for an updated STEP design which incorporates a novel divertor geometry with a dome optimized to improve helium (He) pumping from the inner divertor.
We use the Zhdanov closure [2] to improve the fidelity of the impurity transport physics over previous modelling, and we also include He in our simulations to simultaneously investigate how the D2 fuelling location impacts both seeded Ar and He levels in the core, which is critical for a self-consistent power plant assessment. In addition to studying fuelling from the private flux region and midplane, we evaluate fueling from the divertor entrances and the inner divertor target (close to the strike point), and further explore the impact of in-out asymmetric fuelling. The optimized dome provides neutrals with a more direct path to the pump situated in the outer divertor, allowing for Ar injection solely into the inner divertors - contrasting with previous work where seeding was required in all four divertors to achieve detachment. Furthermore, this study builds on previous work by utilizing profiles of heat and particle diffusivity that are ballooning in character (i.e. peak on the low field side) across all simulations.
To study the impact of the fueling location, the total throughput of D2 and Ar is first kept fixed to identify effects on the degree of detachment. The Ar injection rate is then varied to match the degree of detachment across all cases to compare Ar/He compression and enrichment. The analysis focusses on how the fuelling locations affect the main ion flow, and therefore the balance between the thermal force and ion friction for the impurities, which plays a crucial role in determining the core impurity levels.
Acknowledgements: This work has been funded by STEP Fusion, a major technology and infrastructure programme led by UK Industrial Fusion Solutions Ltd (UKIFS), which aims to deliver the UK’s prototype fusion powerplant and a path to the commercial viability of fusion.
[1] R.T. Osawa et al 2024 Nucl.Fusion 64 106007
[2] S.O. Makarov et al 2021 Phys. Plasmas 28 062308