Numerical modelling of detached plasmas in the MAST Upgrade super-X divertor

This thesis covers some of the recent predictive modelling work carried out as part of preparations for MAST-U physics operations. The SOLPS-ITER code package has been used to numerically study the behaviour of detached plasmas in the MAST-U Super-X divertor configuration. One of the two topics addressed in this work concerns the role of the divertor magnetic geometry in the control of detached Super-X plasmas. The second topic concerns the comparison of strongly detached conditions achieved through the density ramp and extrinsic impurity radiation approaches, with a focus on the impact of ion-molecule elastic collisions on the detached Super-X divertor conditions.

The subject of detachment control has been addressed by comparing the evolution of detached divertor plasma solutions obtained from SOLPS-ITER to the predictions of an analytical model for detachment control, called herein the `detachment location sensitivity' (DLS) model. Two sets of steady state SOLPS-ITER solutions ranging from detachment onset to strongly detached conditions are studied. One set of solutions is obtained by varying the main ion fuelling rate and the other by varying the nitrogen injection rate (or `seeding' rate) at a fixed fuelling rate for the same (2.5MW) input power. The movement of various features of the plasma solutions (which correspond to the extent of detachment) through the divertor volume is tracked as a function of control parameters and qualitative similarities are observed between the two scans - both scans indicate that strong gradients in the magnetic field strength along the field line in the MAST-U Super-X divertor may be contributing to a reduction in the sensitivity of the detachment extent to the fuelling or seeding rate, in qualitative agreement with the predictions of the DLS model. This indicates that strong gradients in the magnetic field strength along the field line could potentially help passively stabilise the detachment extent and enhance detachment control in Super-X divertors. However, plasma-neutral pressure balance considerations indicate that tight baffling of neutrals may also be playing a role.

While the evolution of the detachment extent is qualitatively similar between the two scans, qualitative differences are observed in the comparison of strongly detached divertor conditions. Significant plasma recombination near the divertor plate is observed in the main ion fuelling case whereas it is negligible in the nitrogen seeding case. The divertor plasma density profile in the fuelling case is strongly peaked near the target, qualitatively different from that in the seeding case in which the plasma density profile is mostly flat. Lower target temperatures are also achieved in the fuelling case. Qualitative features of the seeding case are recovered in the fuelling case by turning off ion-molecule elastic collisions. Analysis suggests that the elastic collisions are an important divertor plasma energy and momentum sink in strongly detached conditions which enhance access to recombining conditions. Further analysis also suggests that additional momentum boundary conditions for the divertor targets may need to be included in SOLPS-ITER to study strongly detached impurity seeded scenarios, and that the set of plasma neutral-interactions included in the simulations presented here may not be sufficient to accurately model MAST-U Super-X impurity seeding discharges.