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

Abstract

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 a density front 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

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

“…Such interactions start becoming significant at detachment onset with a spatial preference towards the target side of the ionisation region where the molecular density builds up. The TCV simulations, consistent with TCV [5,6,7,10] and MAST-U [11] experimental results, indicate that plasma-molecular chemistry involving molecular charge exchange generating D + 2 and associated MAD neutral atom sources, MAR ion sinks and atomic hydrogen emission: 1) starts to occur from the detachment onset on-wards as the ionisation and electron-impact dissociation regions detach from the target; 2) increase in magnitude as higher molecular densities are obtained below the ionisation region when T e drops below 3 eV [34]. Neutral baffling may play a strong role in this point 2), which was brought forward as an explanation for why plasma-molecular effects plays a much more significant role in the MAST Upgrade Super-X divertor than the TCV open divertor (experiments from 2016 before baffles were present) [11].…”

“…MAR & MAD provide additional dissociation processes that are significant below 5 eV, by which time the electron-impact dissociation cross-sections are strongly reduced, but the molecular content is strongly ¶ It should be noted that the ion target flux roll-over is less clear in # 56567 than # 52065 as significantly lower core densities were obtained before the plasma disrupted. increased [34,11]. Therefore, MAR & MAD can be stronger neutral atom creation processes than electron-impact dissociation & EIR, particularly in detached regimes.…”

“…balancing the D + 2 creation/destruction rates) are compared. It is these conditions in where the molecular density greatly increases for decreasing temperatures [34,6,11].…”

“…The underlying vibrationally resolved cross-sections used in the Eirene rates are likely underestimated at low temperatures, as shown in section 4.3, which can result in a significant under-prediction of molecular charge exchange even if ion isotope mass rescaling is applied correctly. further, we have obtained scalings from previous SOLPS-ITER simulations of both MAST Upgrade [47] and TCV [15] (open divertor -without baffles) for the evolution of the spectroscopic line-of-sight integrated neutral atom and neutral molecular density relative to the electron density as function of T e [11] using all of the divertor spectroscopic lines of sight for both TCV and MAST-U. Using a simplified model for the rates [11], together with the D + 2 /D 2 ratios with the 'modified' rate setup (figure 2), the evolution of the various atomic and molecular rates can be calculated as function of T e for a fixed n e .…”

“…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].…”