Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

TY - JOUR

T1 - Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

AU - Verhaegh, K.

AU - Lipschultz, B.

AU - Harrison, J. R.

AU - Osborne, N.

AU - Williams, A. C.

AU - Ryan, P.

AU - Allcock, J.

AU - Clark, J. G.

AU - Federici, F.

AU - Kool, B.

AU - Wijkamp, T.

AU - Fil, A.

AU - Moulton, D.

AU - Myatra, O.

AU - Thornton, A.

AU - Bosman, T. O.S.J.

AU - Bowman, C.

AU - Cunningham, G.

AU - Duval, B. P.

AU - Henderson, S.

AU - Scannell, R.

N1 - © 2022 Crown copyright. Funding Information: This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200—EUROfusion), from the RCUK Energy Programme and EPSRC Grants EP/T012250/1 and EP/N023846/1. It has been supported in part by the Swiss National Science Foundation. To obtain further information on the data and models underlying this paper please contact [email protected] . Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

PY - 2022/12/16

Y1 - 2022/12/16

N2 - We present the first analysis of the atomic and molecular processes at play during detachment in the MAST-U Super-X divertor using divertor spectroscopy data. Our analysis indicates detachment in the MAST-U Super-X divertor can be separated into four sequential phases: first, the ionisation region detaches from the target at detachment onset leaving a region of increased molecular densities downstream. The plasma interacts with these molecules, resulting in molecular ions ( D 2 + and/or D 2 − → D + D − ) that further react with the plasma leading to molecular activated recombination and dissociation (MAR and MAD), which results in excited atoms and significant Balmer line emission. Second, the MAR region detaches from the target leaving a sub-eV temperature region downstream. Third, an onset of strong emission from electron-ion recombination (EIR) ensues. Finally, the electron density decays near the target, resulting in the bulk of the electron density moving upstream. The analysis in this paper indicates that plasma-molecule interactions have a larger impact than previously reported and play a critical role in the intensity and interpretation of hydrogen atomic line emission characteristics on MAST-U. Furthermore, we find that the Fulcher band emission profile in the divertor can be used as a proxy for the ionisation region and may also be employed as a plasma temperature diagnostic for improving the separation of hydrogenic emission arising from electron-impact excitation and that from plasma-molecular interactions. We provide evidences for the presence of low electron temperatures (≪0.5 eV) during detachment phases III-IV based on quantitative spectroscopy analysis, a Boltzmann relation of the high-n Balmer line transitions together with an analysis of the brightness of high-n Balmer lines.

AB - We present the first analysis of the atomic and molecular processes at play during detachment in the MAST-U Super-X divertor using divertor spectroscopy data. Our analysis indicates detachment in the MAST-U Super-X divertor can be separated into four sequential phases: first, the ionisation region detaches from the target at detachment onset leaving a region of increased molecular densities downstream. The plasma interacts with these molecules, resulting in molecular ions ( D 2 + and/or D 2 − → D + D − ) that further react with the plasma leading to molecular activated recombination and dissociation (MAR and MAD), which results in excited atoms and significant Balmer line emission. Second, the MAR region detaches from the target leaving a sub-eV temperature region downstream. Third, an onset of strong emission from electron-ion recombination (EIR) ensues. Finally, the electron density decays near the target, resulting in the bulk of the electron density moving upstream. The analysis in this paper indicates that plasma-molecule interactions have a larger impact than previously reported and play a critical role in the intensity and interpretation of hydrogen atomic line emission characteristics on MAST-U. Furthermore, we find that the Fulcher band emission profile in the divertor can be used as a proxy for the ionisation region and may also be employed as a plasma temperature diagnostic for improving the separation of hydrogenic emission arising from electron-impact excitation and that from plasma-molecular interactions. We provide evidences for the presence of low electron temperatures (≪0.5 eV) during detachment phases III-IV based on quantitative spectroscopy analysis, a Boltzmann relation of the high-n Balmer line transitions together with an analysis of the brightness of high-n Balmer lines.

KW - Balmer emission

KW - divertor detachment

KW - Fulcher emission

KW - MAST Upgrade

KW - plasma spectroscopy

KW - plasma-neutral interactions

KW - Super-X divertor

UR - http://www.scopus.com/inward/record.url?scp=85144522510&partnerID=8YFLogxK

U2 - 10.1088/1741-4326/aca10a

DO - 10.1088/1741-4326/aca10a

M3 - Article

AN - SCOPUS:85144522510

SN - 0029-5515

VL - 63

JO - Nuclear Fusion

JF - Nuclear Fusion

IS - 1

M1 - 016014

ER -