Localizing CIII emission using multi-delay coherence imaging in the W7-X divertor
The movement of spectral emission fronts can be used to diagnose the transition into detachment.Measurements of the 2D spectral emission distribution are available in tokamaks via tomographic inversions, but this is more challenging in stellarators due to the lack of toroidal symmetry. The effect of Zeeman splitting can be exploited on W7-X to estimate the location of spectral lines, thanks to the high magnetic field strength and its precise mapping[1]. This method has been proven valid in a previous study based on dispersive spectroscopy on a 1D fan of lines of sight[2]. As a proof of principle, multi-delay coherence imaging[3] has been used here to distinguish the effect of Doppler broadening and Zeeman splitting on the CIII spectrum across a 2D image, estimating the position of the emission for each pixel in 3D space. Typical inferred uncertainties are ∼ 10 cm along the line of sight of each pixel, leading to usable results only on the vertical target, in qualitative agreement with the poloidal movement of the radiation observed by the spectrometers and the radial movement observed by toroidal cameras. Compared to dispersive spectroscopy, this allows increased spatial resolution in both poloidal and toroidal directions. The emission is inferred to be at the target in attached conditions and move away from it during detachment, highlighting a different stability of the front in different magnetic configurations, in agreement with previous measurements and modelling[4]. A toroidal variation in the time of detachment is inferred. The large uncertainties are partially due to a system optimized exclusively for ion temperature inference[5]. To improve the accuracy for future measurements, a new set of crystals has been optimized for simultaneous emission location and ion temperature measurements. [1] Pedersen, T.et al.2016 Nature Communications [2] Gradic D. et al. 2022 PPCF [3] Allcock J S et al. 2021 RSI [4] V.R. Winters et al 2024 NF [5] Kriete D M, et al. 2024 RSI