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Heading ##Molecular processes are known to influence divertor detachment, but conventional effective-rate models assume vibrationally excited molecules remain in local equilibrium and neglect their transport [1, 2]. In this work, vibrationally resolved D2 simulations using the X1EXT molecular database are applied to both simplified divertor-leg and full-device MAST Upgrade Super-X SOLPS-ITER simulations to investigate the role of vibrational transport,electron cooling, and plasma-surface interactions during detachment [3, 4].
In isolated divertor-leg simulations, vibrationally excited molecules were found to survive for tens of microseconds and travel distances of several tens of centimetres before reacting. More than 55% of molecular trajectories exceeded the local electron-temperature gradient length, demonstrating strongly non-local behaviour. This transport enables excited molecules generated in warmer regions to penetrate detached plasmas, enhancing molecular charge exchange (MCX), molecular-activated recombination (MAR), and molecular-activated dissociation (MAD) far from their point of origin. Electron-impact excitation of D2 was also identified as an important power-loss channel, accounting for approximately 10% of total divertor power losses. These excitation losses (“electron quenching”) reduce electron temperature, promote low-temperature plasma-molecular interactions, and accelerate the onset of detachment.
The impact of these mechanisms was assessed in full-device simulations of MAST Upgrade Super-X plasmas and compared with spectroscopic measurements. Relative to effective-rate approaches, the vibrationally resolved model predicts detachment rollover at lower upstream density through enhanced MCX-driven MAR and stronger divertor cooling. The simulations reproduces experimentally observed Dα emission profiles, Fulcher-band emission,ionisation-front migration, and target ion-flux rollover more accurately than either AMJUEL or vibrationally unresolved X1EXT simulations.
Sensitivity studies show that plasma-surface interaction models have a strong impact on the vibrational state distributions of recycled molecules. The Eley-Rideal recycling model produces an overpopulation of high vibrational states and leads to unrealistically early detachment rollover, whereas the partial-thermalisation approach gives the closest agreement with experimental observations [5, 6]. These results highlight that predictive modelling of detached divertor plasmas requires a self-consistent treatment of vibrational state distributions, non-local molecular transport, and electron quenching, alongside plasma-surface interactions.
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The role of plasma-atom and molecule interactions on power & particle balance during
detachment on the MAST Upgrade Super-X divertor. Nuclear Fusion, 63(126023), 2023.
[2] S. Kobussen, et.al.
Collisional radiative modelling with improved cross sections to investigate
plasma molecular interactions in divertor plasmas. Technical report, Masters Thesis, 2023.
[3] J Bryant, et.al.
Impact of yacora evaluated molecular effective rate coefficients
on detached solps-iter simulations. Nuclear Fusion, 2 2025.
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Yacora on the Web: Online collisional
radiative models for plasmas containing H, H2 or He. Journal of Quantitative Spectroscopy
and Radiative Transfer, 240(106695), 1 2020.
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Isotope effects in the
formation of molecular hydrogen on a graphite surface via an eley-rideal mechanism. Journal
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Divertor shaping with neutral baffling as a solution to the tokamak power
exhaust challenge. Communications Physics, 8, 5 2025