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Description
In this study, we develop a diagnostic technique to detect detachment onset. The technique utilizes a characteristic feature of the atomic line emission profile in the detached plasma, namely the emergence of atomic line emission peaks near the hydrogen recombination front. Our aim is to establish a simple and robust technique suitable for future fusion reactors. The recombination front is defined as the position where the hydrogen recombination flux reaches its maximum, and the electron density also peaks at this position. Consequently, the intensities of hydrogen and impurity atomic emission lines also exhibit local maxima. Our concept is to measure these peak positions with Zeeman spectroscopy, in which the line emission peak position along a given chord can be measured from the Zeeman splitting observed in the chord-integrated spectra. The use of near-infrared emission lines enhances the relative magnitude of the Zeeman splitting compared with the Doppler broadening [1, 2].
Numerical analyses were performed on the detached plasma data with Ar and Ne injection for JT-60SA calculated using the integrated divertor code SONIC [3]. Our previous work demonstrated that detachment onset can be detected with the deuterium Paschen-α line at 1875 nm. In the present study, we examine the feasibility of applying this technique to two relatively bright Ar I lines at 912 and 965 nm. Compared with Paschen-α, these Ar I lines have narrower line widths due to smaller Stark and Doppler broadenings, which enables clearer observation of the Zeeman splitting. A virtual viewing chord which is nearly parallel to the outer separatrix and passes through the hydrogen recombination front was assumed. Chord-integrated spectra were synthesized using excited argon atom densities calculated with a collisional–radiative model [4, 5], considering both Zeeman splitting and Doppler broadening. From the synthesized spectra, we evaluated the line emission peak positions and found that they are close to the recombination front.
[1] T. Chatani, et al., Sci. Rep. 12, 15567 (2022).
[2] M. Murakumo, et al., Rev. Sci. Instrum. 96, 043501 (2025).
[3] K. Shimizu, et al., Nucl. Fusion, 49, 065028 (2009).
[4] J. Vlcek, J. Phys. D 22, 623 (1989).
[5] H. Akatsuka, Adv. Phys. X 4, 257 (2019).