[Paper Review] Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

K. Verhaegh, B. Lipschultz+18 more|arXiv (Cornell University)|Apr 5, 2022

Magnetic confinement fusion research59 references49 citations

TL;DR

This study presents the first spectroscopic analysis of detachment phases in the MAST Upgrade Super-X divertor, identifying four sequential stages driven by plasma-molecule interactions. It demonstrates that molecular activated recombination and dissociation (MAR/MAD) significantly influence hydrogen Balmer line emission and that Fulcher band emission serves as a proxy for the ionisation front and plasma temperature, with evidence of sub-eV electron temperatures during deep detachment.

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 $D_2^- ightarrow D + D^-$) 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 relation of the high-n Balmer line transitions together with an analysis of the brightness of high-n Balmer lines.

Motivation & Objective

• To understand the atomic and molecular processes during divertor detachment in the MAST-U Super-X divertor.
• To investigate the role of plasma-molecule interactions, particularly molecular activated recombination and dissociation (MAR/MAD), in shaping hydrogenic emission.
• To evaluate the diagnostic utility of Fulcher band emission for tracking the ionisation front and estimating plasma temperature.
• To quantify electron temperature and density evolution during detachment using high-n Balmer line fitting and spectroscopic analysis.
• To assess the implications of low-temperature plasma regimes (≪0.5 eV) for modeling and reactor extrapolation.

Proposed method

• Analysis of divertor spectroscopy data from MAST-U, focusing on hydrogen Balmer lines and D2 Fulcher band emission.
• Use of high-n Balmer line fitting to infer electron temperature and density, applying Boltzmann distribution analysis.
• Development of a simplified emission model to simulate expected Balmer line behavior across detachment phases.
• Application of quantitative spectroscopy to distinguish emission from electron-impact excitation versus plasma-molecule interactions.
• Use of nearest-neighbor extrapolation for ADAS atomic data below 0.2 eV to assess uncertainty in low-temperature regimes.
• Correlation of emission profiles with magnetic flux expansion and divertor geometry to infer spatial evolution of detachment.

Experimental results

Research questions

• RQ1 How do plasma-molecule interactions influence hydrogen Balmer line emission during detachment in the Super-X divertor?
• RQ2 Can Fulcher band emission serve as a proxy for the ionisation front and a diagnostic for electron temperature?
• RQ3 What is the evolution of electron temperature and density during the four phases of detachment?
• RQ4 To what extent do molecular processes like MAR and MAD contribute to emission and power dissipation in the divertor?
• RQ5 What are the implications of sub-eV electron temperatures for plasma-edge modeling and reactor extrapolation?

Key findings

• Detachment in the MAST-U Super-X divertor proceeds through four distinct phases: ionisation front detachment, MAR/MAD emission peak detachment, onset of electron-ion recombination (EIR) emission, and electron density bulk displacement upstream.
• Molecular activated recombination and dissociation (MAR/MAD) produce significant excited atoms, leading to strong Balmer line emission, particularly in phase II.
• Fulcher band emission brightness correlates with the ionisation front and can be used as a proxy for the ionisation region and a temperature diagnostic, with emission peaking at Te ≈ 0.7 eV.
• Evidence of sub-eV electron temperatures (≪0.5 eV, with preliminary signs of Te < 0.2 eV) is found in phases III and IV using high-n Balmer line fitting and Boltzmann analysis.
• Electron density near the target drops significantly in phase IV, reaching levels of ne ≪ 1 × 10^19 m⁻³, while remaining modest (ne < 2 × 10^19 m⁻³) in phases I–III.
• Plasma-molecule interactions, particularly MAR/MAD, have a larger impact on hydrogenic emission than previously reported and must be accounted for in diagnostic interpretation and modeling.