Numerical study of divertor detachment in the MAST-U tokamak

Divertor detachment is a promising method to reduce heat loading and erosion in tokamak devices or even in future magnetic fusion reactors. In this thesis, two detachment regimes (increasing upstream density and seeding impurity) leading to the decrease of the divertor ion flux is numerically studied through modelling the super-X divertor in MAST-U like conditions. This thesis builds on previous work using the original SD1D modules of BOUT++, which is established to simulate parallel transport process from upstream to the target. We implement an upgrade in SD1D module by adding molecule-plasma interactions and impurity seeding in order to making simulations more self-consistent. To understand the role of molecules in density ramp detachment process, comparisons are made between the cases with different recycling conditions. It is found that if the recycling in divertor is more likely to produce neutral molecule, the roll-over of ion flux at the target occurs at a higher upstream density and a lower target temperature. We also find that molecule–plasma interactions are as crucial as atom–plasma interactions during divertor detachment, both of which account for the main plasma momentum loss. Molecule–plasma interactions can even cause a strong rise of Halpha signal in the detachment process, which agrees with the measurement on other devices (e.g TCV tokamak). The divertor detachment induced by seeding impurity (e.g. neon) is simulated in order to understand the difference between the two detachment regimes. It is found that increasing the puffing rate of neon impurity cannot quickly reduce the target temperature, thus the density of molecule species is small during detachment due to the high molecule dissociation rate, while atom-plasma interactions become dominant and account for the most of plasma momentum loss. Different from the density ramp induced detachment, we cannot find the strong rise of Halpha signal in this case.