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

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

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor 150 150 Mathew

In this paper we a first qualitative 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 a wide operational regime of detachment of the MAST-U super-X divertor, which can be roughly separated in four phases: 1) The ionisation front detaches off the target during the detachment onset. 2) The front of molecular-activated recombination detaches off the target with a sub-eV temperature region below it. 3) The onset of strong emission and particle losses associated with electron-ion recombination (EIR). 4) The electron density front detaches from the target. The analysis in this paper suggests that plasma-molecule interactions play a critical role in interpreting the hydrogen atomic line emission characteristics on MAST-U. These interactions result in lower Balmer line ratios, which are (within experimental uncertainty) identical to those expected from an ionising plasma. As a result, there is a strong spatial mismatch (of almost the entire divertor chamber) between the ionising region and the region where Balmer line ratios indicate that electron-ion recombination emission starts becoming dominant over electron-impact excitation. The plasma parameters in the MAST-U have been experimentally estimated. The electron density has been estiamted using Stark broadneing, which has indicated that the electron densities in the MAST-U Super-X divertor are fairly modest ($0.5 – 3 times 10^{19} m^{-3}$. We provide various evidence for the presence of low electron temperatures ($<0.5$ eV), based on quantitative spectroscopy analysis developed previously, a Boltzmann relation of the high-n Balmer line transitions as well as an analysis of the brightness of high-n Balmer lines.

Collection:
Journals
Journal:
Nuclear Fusion
Publisher:
IOP (Institute of Physics)