A New Coherence Imaging Spectroscopy Diagnostic Optimized for Ion Temperature Measurements in the W7-X Scrape-Off Layer

To investigate scrape-off layer (SOL) physics in the island divertor of the W7-X stellarator a new coherence imaging spectroscopy (CIS) diagnostic is being optimized for ion temperature measurements. The CIS technique provides high-spatial-resolution measurements of ion velocity in the SOL of W7-X but ion temperature measurements are challenging due to the contribution of Zeeman splitting at high magnetic fields (B ~ 2.5 T). A technique is described for estimating the uncertainty in the ion temperature due to the spatial variation of emission along the CIS lines of sight. The impact of bremsstrahlung radiation on CIS measurements may be substantial for high-density ( > 1 × 10²⁰ m^-3) W7-X plasmas. A design is presented for the new W7-X CIS instrument that utilizes multiple birefringent crystals and a micropolarized camera in a multi-delay configuration to provide more accurate ion temperature measurements. The birefringent crystals have been optimized to minimize sensitivity to Zeeman splitting while maintaining sufficient sensitivity to ion temperature. The new multi-delay CIS instrument has been tested on the Magnetized Dusty Plasma Experiment (MDPX) to quantify the contribution of Zeeman splitting over a range of magnetic fields relevant for W7-X measurements.