Localization Masks for Phase Contrast Imaging at Wendelstein 7-X

Phase contrast imaging (PCI) is the main core density turbulence diagnostic at Wendelstein 7-X (W7-X), the largest neoclassically optimized stellarator currently in operation. PCI therefore plays a key role in advancing the understanding of stellarator turbulence, which is paramount for the development of future stellarator fusion reactors. The W7-X PCI system measures electron density fluctuations integrated along 32 central lines-of-sight. Since PCI measurements are line-integrated in their basic form, it is often difficult to radially localize the measured fluctuations. However, this problem can be overcome by installing localization masks in the PCI system which only allow fluctuations originating from a limited range of magnetic pitch angles (radial range) to be detected. This contribution describes such localization masks designed for W7-X, particularly focusing on the optimization of the mask design performed using synthetic PCI techniques. Further, measurements from the PCI system with localization masks will be presented to the extent that they are available.