Numerical simulations of edge localised mode instabilities in the MAST-U Super-X tokamak

Edge localised modes (ELMs) are tokamak instabilities that occur in high confinement mode (H-mode) plasmas. Filamentary structures of high density erupt
from the plasma edge transporting heat and particles from the core plasma to material surfaces, in particular the divertor targets. However, the high heat fluxes to the divertor have to be reduced for a sustainable future tokamak reactor. A solution to reduce the heat fluxes could be the Super-X divertor; the Super-X will be tested on the spherical tokamak MAST-U. In advance of MAST-U operation predictions are made for the behaviour of ELMs in this new magnetic configuration. The thesis starts with an introduction to the concepts and physics of fusion, tokamaks, MHD instabilities, ELMs, detachment and ELM burn-through. The nonlinear MHD code JOREK, which has been used for the ELM simulations through-out, is then described. A benchmark of JOREK and BOUT++ has been established for a circular plasma before the MAST-U tokamak ELM simulations are presented. ELM simulations are first performed for MAST-U Super-X plasmas using the single temperature reduced MHD model in JOREK, where comparisons of various divertor configurations have been conducted. The peak heat fluxes were found to reduce by an order of magnitude in the Super-X configuration in comparison to a conventional divertor configuration. The two temperature neutrals model in JOREK is explored, where an attempt to obtain a detached divertor for the MAST-U Super-X configuration is made. The detached divertor is then used as a starting point for the ELM simulations presented where the first ELM burn-through simulations for the MAST-U Super-X tokamak are performed. The plasma from the ELM burns through the neutrals front and the divertor plasma re-attaches. A few milliseconds after the ELM crash the divertor heat fluxes and temperatures recover to pre-ELM conditions and the plasma detaches again. These recovery times are shorter than the type-I inter-ELM phase, typically 10’s milliseconds, for MAST - a promising prediction for the future operations of MAST-U.