Physics and Engineering of the Gas-Puff Imaging Diagnostic for W7-X*

A Gas Puff Imaging diagnostic is being installed on the Wendelstein 7-X Stellarator. It will be commissioned at the start of the next run campaign. The diagnostic will measure the 2d structure and dynamics of plasma fluctuations in a 75x40 mm region near the outboard boundary. In the ``standard'' magnetic configuration, the field-of-view will span (radially) one of the 5 magnetic islands at the boundary, thereby interrogating some of the turbulence dynamics at the island. The field-of-view will also include the last closed flux surface in some magnetic configurations. We expect a spatial resolution of $\approx $5 mm within the field-of-view. The engineering of the system is challenging and features a number of innovative elements. The local gas puff (H$_{\mathrm{2}}$ or He) used to ``illuminate'' the plasma fluctuations is provided by two ``converging-diverging'' nozzles that collimate the gas cloud to a half-angle of $\approx $12$^{\mathrm{o}}$. The emission from the gas cloud's interaction with the plasma is gathered by a high-throughput re-entrant lens system inserted in a port adjacent to the gas-puff port. It features a pop-up mirror that turns the view through a 115$^{\mathrm{o}}$ angle and acts as a shutter for the mirror and optics when retracted.