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Characterisation of detachment in the MAST-U Super-X divertor using multi-wavelength imaging of 2D atomic and molecular emission processes

Basil Duval, Kevin Henricus Annemarie Verhaegh

In this work, we provide the first 2D spatially resolved description of radiative detachment in MAST-U Super-X L-mode divertor plasmas. The Super-X magnetic configuration was designed to achieve reduced heat- and particle loads at the divertor target compared to conventional exhaust solutions. We use filtered camera imaging to reconstruct 2D emissivity profiles in the poloidal plane for multiple atomic and molecular emission lines and bands. A set of deuterium fuelling scans is discussed that, together, span attached to deeply detached divertor states observed in MAST-U. Emissivity profiles facilitate separate analysis of locked-mode induced split branches of the scrape-off layer. Molecular deuterium Fulcher band emission front tracking reveals that the deuterium electron-impact ionisation front, for which it serves a proxy, detaches at different upstream electron densities in the split branches. Upon detachment of this ionisation front, Balmer emission attributed to molecular activated recombination appears near-target. We report a simultaneous radial broadening of the emission leg, consistent with previous SOLPS-ITER modelling. With increased fuelling this emission region detaches, implying electron temperatures below & SIM;1

IOP Publishing Ltd2023

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Basil Duval, Holger Reimerdes, Christian Gabriel Theiler, Kevin Henricus Annemarie Verhaegh

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms has been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium, indicating a molecular ion density (D-2(+)) that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate (D-2 + D+ -> D-2(+) + D) for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the D-2(+) content by >x100. By disabling ion isotope mass rescaling: (1) the total ion sinks are more than doubled due to the inclusion of MAR; (2) the additional MAR causes the ion target flux to roll-over during detachment; (3) the total Da emission in the divertor increases during deep detachment by roughly a factor of four; (4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50% increase in neutral pressure; (5) total hydrogenic power loss is increased by up to 60% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over. Extrapolating simplified scalings of divertor molecular densities (TCV & MAST-U) to reactor-relevant densities suggests the underestimation of molecular charge exchange could strongly impact divertor physics (neutral atom density, ions sinks) and hydrogen emission (which has implications for detachment control) in deeply detached conditions, warranting further study.

IOP Publishing Ltd2023

Model-based impurity emission front control using deuterium fueling and nitrogen seeding in TCV

Basil Duval, Cristian Galperti, Christian Gabriel Theiler

This paper presents the first result using nitrogen-seeded exhaust feedback control of the NII impurity emission front in TCV. The NII emission front position is consistently located below its commonly used CIII counterpart, indicating the NII emission front is representative of a colder plasma region. We demonstrate control of the NII impurity emission front position for two cases: (a) using nitrogen seeding as the sole actuator, and (b) using deuterium fueling as an actuator while injecting a small amount of nitrogen that remains a trace impurity. For sole nitrogen actuation, peak target current density is significantly reduced when the NII emission front approaches the x-point (approximate to 50% for the NII front at the halfway point). When actuating with deuterium, peak target current density is less affected, which is explained by changes in fueling engendering a different scrape-off-layer plasma density. Perturbative (system identification) experiments show that nitrogen actuation induces a stronger, but slower, response of the NII emission front than deuterium actuation. Moving the NII emission front back to the target after pushing it towards the x-point has proven difficult, where both the NII front position and total radiated power do not reach pre-seeding conditions within the discharge time following termination of nitrogen injection. This result highlights the need to account for impurity retention for such seeded discharges in exhaust control strategies.

IOP Publishing Ltd2023