Description
High energy gain fusion devices will exhibit for the first time both high opaqueness to neutrals and a very low collisionality Scrape-Off Layer (SOL). It is presently envisioned that in a Spherical Tokamak (ST) pilot plant, main chamber SOL plasma temperatures will be high enough to invalidate the fluid plasma approach, with kinetic effects becoming increasingly important to parallel heat transport, including flux limitation and nonlocal ‘preheat’. Accurate predictions of neutral dynamics and fueling efficiency rely on modelling of the plasma solution that accurately resolves these kinetic effects and their impact on upstream profiles.
‘Reduced-kinetic’ models offer an attractive method for calculating the SOL parallel heat flux within plasma fluid codes, with greater accuracy towards the “true” kinetic result at reduced computational cost compared to fully-kinetic codes. The SNB model [1] is applied to calculate the parallel thermal transport in the Hermes-3 code for a MAST-U connected double-null discharge, in a conventional divertor configuration. This model treats the parallel electron thermal conduction as a multigroup diffusion problem, dividing the electron velocity distribution into separate velocity groups and calculating corrections to the classical fluid heat flux for each group. Models are initially validated against the MAST-U Thomson scattering and Langmuir probe data for upstream and divertor plasma profiles. Numerical scans of plasma parameters (e.g. input power, density) are then performed with both the reduced-kinetic and standard fluid Hermes-3 thermal transport models employed, scaling SOL conditions beyond MAST-U attainable parameters towards those expected of a future ST pilot plant (e.g. STEP [2]). The results are analysed in terms of a transition from high to low SOL collisionality through which predictions of upstream plasma profiles diverge between models, and the consequences on neutral opaqueness in the main chamber for future ST reactors is assessed.
[1] G. Schurtz, P.D. Nicolai, and M. Busquet 2000 Phys. of Plasmas 7 4238.
[2] S.S. Henderson et al 2025 Nucl. Fusion 65 016033.
Acknowledgments: Work supported by US DoE Awards DE-SC0023289 and DE-SC0023372.