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

High resolution Fulcher band spectroscopy was used in the MAST-U divertors during Super-X and elongated conventional divertor density ramps with $\text{D}_{2}$ fuelling from the mid-plane high-field side. In the Super-X case (density ramp from Greenwald fraction 0.12 to 0.24), the upper divertor showed ground state rotational temperatures of the $\text{D}_{2}$ molecules increasing from $\sim$6000 K, starting at the detachment onset, to $\sim$9000 K during deepening detachment. This was correlated with the movement of the Fulcher emission region, which is correlated with the ionisation source. The increase in rotational temperature did not occur near the divertor entrance, where the plasma was still ionising. Qualitative agreement was obtained between the lower and upper divertor. Similar rotational temperatures were obtained in the elongated divertor before the detachment onset, although the increase in rotational temperature during detachment was less clearly observed as less deep detachment was obtained. %In the elongated conventional divertor there was some qualitative agreement of this effect impeded by low signal. The measured vibrational distribution of the upper Fulcher state (first four bands) does not agree with a ground state Boltzmann distribution but shows a different characteristic with an elevated population especially in the $\nu = 2$ and $\nu = 3$ bands. The populations of the $\nu = 2$ and $\nu = 3$ band relative to the $\nu = 0$ band are roughly proportional to the $\textit{rotational}$ temperature

Transport and drift-driven plasma flow components in the Alcator C-Mod boundary plasma

Boundary layer flows in the Alcator C-Mod tokamak are systematically examined as magnetic topology (upper versus lower-null) and plasma density are changed. Utilizing a unique set of scanning Langmuir–Mach probes, including one on the high-field side (HFS) midplane, the poloidal variation of plasma flow components in the parallel, diamagnetic and radial directions are resolved in detail. It is found that the plasma flow pattern can be decomposed into two principal parts: (1) a drift-driven component, which lies within a magnetic flux surface and is divergence-free and (2) a transport-driven component, which gives rise to near-sonic parallel flows on the HFS scrape-off layer (SOL). Toroidal rotation, Pfirsch–Schlüter and transport-driven contributions are unambiguously identified. Transport-driven parallel flows are found to dominate the HFS particle fluxes; the total poloidal-directed flow accounts for ~1/3 to all of the ion flux arriving on the inner divertor. As a result, heat convection is found to be an important player in this region, consistent with the observation of divertor asymmetries that depend on the direction of B × ∇B relative to the active x-point. In contrast, the poloidal projection of parallel flow in the low-field SOL largely cancels with E[subscript r] × B flow; toroidal rotation is the dominant plasma motion there. The magnitude of the transport-driven poloidal flow is found to be quantitatively consistent with fluctuation-induced radial particle fluxes on the low-field side (LFS), identifying this as the primary drive mechanism. Fluctuation-induced fluxes on the HFS are found to be essentially zero, excluding turbulent inward transport as the mechanism that closes the circulation loop in this region.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99ER54512

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

First quantitative analysis of the detachment processes in the MAST Upgrade Super-X divertor show an unprecedented impact of plasma-molecular interactions involving molecular ions (likely $D_2^+$), resulting in strong ion sinks, leading to a reduction of ion target flux. This starts to occur as the ionisation source detaches from the target, leading to a build-up of molecules below the ionisation source who get excited, resulting in Molecular Activated Recombination (MAR) and Dissociation (MAD). The particle sinks in the divertor chamber exceed the ion sources in the middle of the detached operational regime before electron-ion recombination (EIR) starts to occur, demonstrating the strong capabilities for particle exhaust in the Super-X Configuration. MAD is the dominant volumetric neutral atom creation mechanism and results in significant power losses. This, combined with electron-impact excitation preceding ionisation, are the dominant power loss mechanisms in the divertor chamber. As the plasma becomes more deeply detached, EIR starts to occur and electron temperatures below 0.2 eV are achieved. Even at such low electron temperature conditions, MAR is observed to be an important ion sink mechanism, which suggests the presence of highly vibrationally excited molecules in the cold detached regime. The total radiative power loss is consistent with extrapolations of spectroscopic inferences to hydrogenic radiative power losses, which suggests that intrinsic impurity radiation, despite the carbon walls, is minor. These observations are observed in Ohmic L-mode, ELM-free H-mode and type I ELMy H-mode discharges

Impact of Divertor Shape on Divertor Performance in strongly Baffled Divertors on MAST Upgrade

Harnessing fusion energy efficiently requires optimising heat and particle exhaust in the edge from the fusion core plasma, which can be achieved through magnetic shaping of the divertor into Alternative Divertor Configurations (ADCs). In this study, we leverage MAST-U's unique shaping capabilities, which allow for a $\sim \times 2$ variation in the ratio of the magnetic field at the X-point and target ($B_{xpt}/B_t$), to investigate the power exhaust and core-edge compatibility of ADCs. Experiments show ADCs with large $B_{xpt}/B_t$ ratios drastically enhance divertor performance, with heat and particle loads reduced by factors up to $\sim 20$ and a 120 \% reduction in detachment onset. Notably, these benefits are achieved without compromising core plasma conditions. Our analysis attributes these improvements to the extra volume available below the ionisation front in longer leg-length divertors. This facilitates power dissipation and reduced particle loads through ion sinks from atomic (Electron-Ion Recombination) and molecular (Molecular-Activated Recombination) processes. The onset of divertor detachment and the evolution of the detachment front agrees with analytic models and divertor exhaust simulations. These insights emphasise the potential minor divertor geometry adjustments can have on power exhaust. This study illuminates pathways for devising optimised exhaust strategies in future fusion devices