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 (D2 + and/or D2 - -> 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.

The role of plasma–atom and molecule interactions on power & particle balance during detachment on the MAST Upgrade Super-X divertor

“…The results from the first MAST Upgrade campaign indicate both a further reduction of the target heat fluxes as well as a reduced detachment onset, in terms of the core density in the SXD compared to the Conventional Divertor (CD), required for detachment. These observations are roughly consistent with analytic predictions and simulations [7,9,10,31]. Divertor spectroscopy analysis of density ramp discharges in the SXD configuration was performed to separate the Dα emission in terms of its different atomic and molecular processes.…”

“…the divertor ion source dominates any upstream flows (I i ≫ I u ). 10 In these conditions, the divertor is in 'high recycling' conditions and the 'closed box' approximation (I t ≈ I i − I r ) is valid.…”

“…However, this purely atomic analysis [11,16,21] could not explain all spectroscopic observations, particularly the brightness of the Dα emission. Further research on JET [22], TCV [3,23,24] and MAST-U [10] revealed that this emission arises from excited atoms born from plasma-molecular interactions involving molecular ions (D + 2 and/or D − 2 → D − + D). When those ions react with the plasma, they result in additional hydrogenic emission and power losses (hydrogenic radiation, potential energy losses), ion sources (Molecular Activated Ionisation-MAI), ion sinks (Molecular Activated Recombination-MAR-contributing to I r ), and neutral atom sources (Molecular Activated Dissociation-MAD).…”

“…Detachment in the SXD configuration can be distinguished by four phases [10,32], which are listed below and illustrated schematically in figure 1, adopted from [10].…”

“…One example of an ADC is the tightly baffled Super-X divertor (SXD), used by the novel MAST Upgrade tokamak. With the Super-X, the strike point is shifted to a larger major radius, leading to a larger gradient in the magnetic field along the flux tubes to the target, which reduces the heat flux and plasma temperature and makes plasma detachment more accessible [7][8][9][10].…”