Experimental distinction of the molecularly induced Balmer emission contribution and its application for inferring molecular divertor density with 2D filtered camera measurements during detachment in JET L-mode plasmas

Abstract

A previously presented model for generating 2D estimates of the divertor plasma conditions at JET from deuterium Balmer line intensity ratios, obtained from tomographic reconstructions of divertor camera images, was amended to consider also the Balmer emission arising from molecular processes. Utilizing the AMJUEL and H2VIRB atomic and molecular databases of EIRENE enabled also inference of the molecular divertor density from the distinguished molecularly induced emission. Analysis of a JET L-mode density scan suggests the molecularly induced emission accounting for up to 60-70% and 10-20% of the Balmer Dα and Dγ intensities, respectively, at the onset of detachment, while electron-ion recombination becomes increasingly dominant with deepening detachment. Similar observations were made by post-processing EDGE2D-EIRENE simulations, which indicated significant roles of molecular D2 + ions and vibrational excitation of the D2 molecules as precursors for the molecularly induced emission. The inferred molecular density at the outer strike point was found to increase monotonously with decreasing strike point temperature, reaching approximately 30-50% of the local electron density at nmol,osp = 1-2×1020 m-3 at Te,osp ≈ 0.7 eV. A further steep increase by a factor of 3-5 was observed with decrease of Te,osp to 0.5 eV. The observations are in qualitative and reasonable quantitative agreement with EDGE2D-EIRENE predictions of nmol,osp within the uncertainties of the experimental data.

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

“…Quantitative evidence of the importance of plasmamolecular interactions in terms of spectroscopic interpretation as well as divertor physics is now available for TCV [3,21,22] as well as MAST-U (this work), with preliminary work also indicating the importance of these interactions in spectroscopic interpretations during deep detachment for JET [24][25][26]. As such plasma-molecular interactions are not fully included in plasma-edge modelling [22], further investigating such reactions is important for reducing the uncertainties in the extrapolation of current day devices to future reactors-particularly for reactor concepts with ADCs.…”

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

“…MAR & MAD not only have an impact on the divertor physics, but also result in a significant content of excited hydrogen atoms and thus hydrogen Balmer line emission as was shown in literature [48,14,13,12,5,7,6,11] and is indicated by the increase in Dα emission during detachment as shown in figure 3. Since that strong increase in Dα emission is not captured by the 'default' setup, this causes strong concerns on the synthetic deuterium (and tritium) atomic emission diagnostic signals predicted from plasma-edge modelling.…”

“…Answering that question requires further investigation, including further studies on the applicable molecular charge exchange as well as applying those to a range of reactor conditions, which is outside the scope of this work. Although there are large uncertainties regarding the molecular charge exchange rates, the molecular vibrational distribution (that determines these effective rates to a large extent) and their applicability to reactors, signatures of the impact of molecular ions on the hydrogen emission are being observed in JET with the ITER-like wall [12,13,14] during deep detachment.…”

“…Plasma-molecular effects result in a significantly enhanced population of the hydrogen atom n = 3 state, which may have implications for the treatment of photon opacity to the Lyman series in simulations [51,52] as well as the diagnosis of photon opacity [7]. Accounting for molecular ions in the analysis of hydrogen atomic line emission required the creation of novel analysis techniques [14,13,6], which need to be further expanded to include photon opacity effects.…”

“…Recent experimental investigations have used spectroscopy and filtered camera imaging on TCV [2,3,4,5,6,7,8,9,10], MAST-Upgrade [11] and JET [12,13,14] that register significant Balmer line emission after the break-up of molecular ions. For TCV [6,7], it was shown that MAR from molecular ions is a dominant contributor to the reduction of the ion target flux in contradiction with SOLPS-ITER simulations [6,15] that did not show a roll-over of the ion target flux [15,16,17,18].…”