Modelling tokamak power exhaust and scrape-off-layer thermal transport in high-power fusion devices

Managing the steady-state power loading onto the divertor target plates remains a major unresolved challenge facing tokamak fusion energy, that will be crucial for the success of the next generation of high-power reactor-level devices. This thesis will tackle two topics within this wide research area: assessing the performance of advanced divertor geometries in the context of the ARC reactor concept, and studying the impact of 'nonlocal' thermal transport on modelling predictions for the ITER tokamak SOL.

Numerical simulations are performed using UEDGE for the Super-X divertor (SXD) and X-point target divertor (XPTD) configurations proposed for the ARC reactor design. The SXD, combined with 0.5% fixed-fraction neon impurity concentration, produced passively stable, detached divertor regimes for power exhausts in the range of 80-108 MW. The XPTD configuration is found to reduce the strike-point temperature by a factor of ~10 compared to the SXD for small X-point radial separations (~1.4lambda_{q||}). Even greater potential reductions are identified for separations of ≤1lambda_{q||}. Raising the separatrix density by a factor 1.5, stable detached divertor solutions were obtained that fully accommodate the ARC exhaust power without impurity seeding.

In the presence of steep temperature gradients, classical local transport theory breaks down, and thermal transport becomes nonlocal, depending on conditions in distant regions of the plasma. An advanced nonlocal thermal transport model is implemented into the 'SD1D' complex SOL code to create 'SD1D-nonlocal', and applied to study typical ITER steady-state conditions. Results suggest that nonlocal transport effects will have importance for the ITER SOL, with discrepancies observed between nonlocal/local transport model predictions in low-density scenarios. Heat flux models employing global flux limiters are shown to be inadequate to capture the spatially/temporally changing SOL conditions. An analysis of SOL collisionality and nonlocality suggests nonlocal effects will be significant for future devices such as DEMO and ARC as well.