Multi-delay coherence imaging spectroscopy optimized for ion temperature measurements in the divertor plasma of the Wendelstein 7-X stellarator

A new coherence imaging spectroscopy (CIS) diagnostic optimized to measure the C2+ impurity ion temperature Ti spatial distribution in the divertor plasma of the W7-X stellarator is designed, tested, and validated. Using CIS to obtain Ti in the edge of magnetically confined plasmas has historically been challenging because Doppler broadening and Zeeman splitting have comparable effects on the shape of spectral emission lines. To distinguish between these two mechanisms, a novel approach to birefringent crystal design is employed to minimize the diagnostic’s sensitivity to Zeeman splitting. The recently developed pixelated multi-delay CIS approach is also used to obtain four times as much spectral information as traditional CIS approaches. The Ti-optimized CIS diagnostic is validated in a long-pulse W7-X plasma by comparison with a high-resolution spectrometer whose sightlines overlap with the CIS field of view. The CIS and spectrometer Ti profiles have the same shape and agree to within 10% on average and 25% in the worst case. Images of the Ti distribution near the divertor show toroidally elongated bands aligned with the magnetic field, with Ti ranging between 10 and 40 eV.