The role of plasma–atom and molecule interactions on power & particle balance during detachment on the MAST Upgrade Super-X divertor

Abstract

This paper shows first quantitative analysis of the detachment processes in the MAST Upgrade Super-X divertor (SXD). We identify an unprecedented impact of plasma-molecular interactions involving molecular ions (likely $D_2^+$), resulting in strong ion sinks (Molecular Activated Recombination - MAR), leading to a reduction of ion target flux. The MAR ion sinks exceed the divertor ion sources before electron-ion recombination (EIR) starts to occur, suggesting that significant ionisation occurs outside of the divertor chamber. In the EIR region, $T_e \ll 0.2$ eV is observed and MAR remains significant in these deep detached phases. The total ion sink strength demonstrates the capability for particle ion exhaust in the Super-X Configuration. 

Molecular Activated Dissociation (MAD) is the dominant volumetric neutral atom creation process can lead to an electron cooling of 20\% of $P_{SOL}$. The measured total radiative power losses \emph{in the divertor chamber} are consistent with inferred hydrogenic radiative power losses. This suggests that intrinsic divertor impurity radiation, despite the carbon walls, is minor in the divertor chamber. This contrasts previous TCV results, which may be associated with enhanced plasma-neutral interactions and reduced chemical erosion in the detached, tightly baffled SXD. 

The above observations have also been observed in higher heat flux (narrower SOL width) type I ELMy H-mode discharges. This provides evidence that the characterisation in this paper may be general.

Benefits of the Super-X divertor configuration for scenario integration on MAST Upgrade

“…where R OSP and R OMP are the major radii of the outer strike point and the separatrix at the outer mid-plane respectively. These predictions are in qualitative agreement with previous Ohmic heated L-mode experiments on MAST-U [36,40], observing that the Super-X configurations facilitates access to the detached divertor regime.…”

“…This roll-over behaviour in the outer divertor D α emission is not commonly observed, because and is thought to be due to low electron temperature and density conditions that prevail when the Super-X divertor configuration is detached in MAST-U, where plasmamolecule interactions produce a significant proportion of the Dα emission. At separatrix densities significantly exceeding the detachment threshold in the Super-X configuration, the electron temperature at the divertor target drops below 1 eV, reducing the source of molecular ions and their associated D α emission [36,40], causing the total emission from the divertor to fall with increasing separatrix density.…”

“…However, strong reductions in the divertor ion saturation current density is suggestive of substantial dissipation of momentum and particles, via volumetric recombination and plasma-neutral interactions that occur at low temperatures (i.e. T e < 5 eV) [36,40,67].…”

“…During the design and initial operation of MAST Upgrade, extensive modelling studies were performed that predicted the Super-X configuration would offer significant benefits, including reduced divertor heat flux in the attached divertor regime [33,34] and improved access to the detached regime [33,35] with negligible impact on the radial plasma profiles at the midplane, upstream of the divertors [34]. Initial experiments in low confinement experiments [36] at low heating power confirmed these benefits of the Super-X configuration compared with a conventional divertor. The results presented here build on previous studies by performing initial experiments to study the impact of the Super-X divertor configuration in high performance H-mode pulses with significant auxiliary heating power.…”

Initial Fulcher band observations from high resolution spectroscopy in the MAST-U divertor

“…Detailed analysis of the physics mechanisms during detachment in the Super-X divertor have shown that plasmamolecular chemistry, including MAR and MAD, plays an unprecedentedly strong role in the tightly baffled divertor chambers when the ionisation source is significantly detached from the target. This results in a build-up of MAR downstream of the ionisation region towards the target [12,22].…”