Design 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 OP 2. GPI requires an in-vessel nozzle close to the plasma, through which non-perturbing amounts of H₂ or He gas are puffed; a re-entrant view of the puff, ideally along local field lines; and an optical system that relays the imaged emission to a fast 2D detector. This must be done in the harsh long-pulse environment, where re-entrant hardware must be actively cooled and collecting optics shuttered. We have prepared a design for viewing a roughly 50 x 70 mm region at the plasma boundary with spatial resolution of ~5 mm and time resolution of ~1 μsec. We propose to puff the gas through 4 converging-diverging nozzle apertures in order to collimate the gas cloud. We have performed finite-element analysis of fluid flow down the feed tube and through the nozzles. We have modeled the puff emission using DEGAS 2. We have designed a water-cooled re-entrant tube to hold the light collection optics and modeled the optics to attain sufficient resolution and light throughput. The details of the design and modeling will be shown.