A novel hydrogenic spectroscopic technique for inferring the role of plasma–molecule interaction on power and particle balance during detached conditions

Abstract

Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma–atom and plasma–molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. Both plasma–atom as well as plasma–molecule interactions can result in excited hydrogen atoms which emit atomic line emission. In this work, we investigate a new Balmer Spectroscopy technique for Plasma–Molecule Interaction—BaSPMI. This first disentangles the Balmer line emission from the various plasma–atom and plasma–molecule interactions and secondly quantifies their contributions to particle (ionisation and recombination) and power balance (radiative power losses). Its performance is verified using synthetic diagnostic techniques of both attached and detached TCV and MAST-U SOLPS-ITER simulations. We find that H 2 plasma chemistry involving H 2 + and/or H − can substantially elevate the Hα emission during detachment, which we show is an important precursor for Molecular Activated Recombination. An example illustration analysis of the full BaSPMI technique shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines, can be significantly influenced by plasma–molecule interactions by tens ofpercent. That has important implications for using atomic hydrogen spectroscopy for diagnosing divertor plasmas.

A study of the influence of plasma–molecule interactions on particle balance during detachment

“…The spectroscopic analysis in this work is performed using the BaSPMI technique (described in detail elsewhere [30,31,6]), which uses electron density estimates derived from a Stark broadening analysis. Dγ and Dδ measurements are analysed initially assuming only 'atomic' (e.g.…”

“…This indicates EIR does not (or barely for # 62972) contribute to the medium-n Balmer line emission. In contrast, during the density ramp, the atomic extrapolated Dδ/Dα ratio strongly rises from ∼ 0.0025 to ∼ 0.05 (figure 4 k) during detachment as electronion recombination strongly contributes to the medium-n Balmer line emission (> 70% for n=5,6 [30]). The measured Dδ/Dα ratio is smaller (∼ 0.008), however, due to plasma-molecule interactions contributions to Dα (figure 4 k).…”

“…In this paper we build on those previous experimental results [31] and use the BaSPMI [30] technique to investigate plasma detachment in TCV further to provide a more complete view of previous findings.…”

“…Recent developments in our spectroscopic analysis technique (BaSPMI [30]) have enabled the inference of the impact of plasma-molecule interactions through their effect on the hydrogenic line series intensities. Initial application of this technique to the TCV tokamak (n e ∼ 10 20 m −3 ) divertor plasma indicated that plasma-molecule interactions can: 1) have a strong impact on the observed Balmer line series; and 2) result in a large ion sink through MAR, that is significantly larger than electron-ion recombination [31].…”

“…To keep the analysis consistent between all discharges, only the atomic part of BaSPMI was utilised. Combining this with Dα measurements facilitated estimating EIR and MAR ion sinks [30], which are shown in figure 4 a,e.…”