615 research outputs found
The operational phase space of the edge plasma and its sensitivity to magnetic topology in Alcator C-Mod
Comparison of neoclassical predictions with measured flows and evaluation of a poloidal impurity density asymmetry
Transport-Driven Scrape-off Layer Flows and the Boundary Conditions Imposed at the Magnetic Separatrix in a Tokamak Plasma
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
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 molecules increasing
from 6000 K, starting at the detachment onset, to 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 and bands. The populations of the and band
relative to the band are roughly proportional to the
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
Cross-Field Transport in the SOL: Its Relationship to Main Chamber and Divertor Neutral Control in Alcator C-Mod
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 ), 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 variation in the ratio of the magnetic field at the
X-point and target (), to investigate the power exhaust and
core-edge compatibility of ADCs.
Experiments show ADCs with large ratios drastically enhance
divertor performance, with heat and particle loads reduced by factors up to
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
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