Drift velocity and ion temperature measurements in the W7-X scrape-off layer using coherence imaging spectroscopy

To investigate island divertor physics, 2D ion velocity and temperature measurements are achieved in the scrape-off layer (SOL) of W7-X using coherence imaging spectroscopy (CIS). The CIS technique encodes information about line-integrated ion velocity and temperature into a spatial interference pattern that is overlaid on an image of the plasma emissivity. Velocity measurements in plasmas with matched upstream parameters but opposite magnetic field directions show that in low edge-iota magnetic island configurations (ι = 5/6) drifts contribute substantially to SOL flows. The density scaling of the drift velocity varies throughout the SOL: in some regions it increases with density, while in others it decreases. Initial 2D ion temperature measurements derived from the contrast of CIS interference patterns are presented. Due to the high magnetic field of W7-X (B₀ = 2.5 T), Zeeman splitting substantially affects CIS contrast and must be accounted for to obtain accurate ion temperatures. A technique is described for estimating the Zeeman contrast, which varies across the CIS field of view. A design is also presented for a new CIS instrument simultaneously optimized for maximum sensitivity to ion temperature and minimum sensitivity to Zeeman splitting.