Design and Initial Tests of a Gas-Puff Imaging Diagnostic for W7-X.*

A Gas-Puff-Imaging (GPI) diagnostic is being designed for use on the W7-X stellarator. It will allow for detailed study of boundary and scrape-off-layer physics during the long-pulse W7-X operation period OP2. The main components of the diagnostic are 1) a system for puffing controlled amounts of H$_{\mathrm{2}}$ or He gas into the outboard edge of the W7-X plasma, and 2) a system that images, with high time-resolution(\textasciitilde 1 $\mu $s) and good 2d spatial resolution (\textasciitilde 5 mm), the emission resulting from the interaction of the puff with the local edge plasma. The nozzles used for the gas puff are novel in that they are ``converging-diverging'' cones inducing supersonic flow at and beyond the nozzle ``throat'', thereby providing a more collimated beam. Good collimation is critical because the nozzle is \textasciitilde 120 mm outside the LCFS. Gas flow rates and expansion angles for the puffed beam are measured and will be presented. The design of the imaging system includes a water-cooled re-entrant tube with a metal turning-mirror that is deployed as part of the tube shutter. The turning-mirror allows observation of the emission along sightlines that are within 15$^{\mathrm{o}}$ of the field-lines local to the emission region. The collected light is relayed up the tube and imaged directly onto an 8x16 array of APD detectors.