  A system for studying the spatio-temporal dynamics of fluctuations in the
boundary of the W7-X plasma using the Gas-Puff Imaging (GPI) technique has been
designed, constructed, installed, and operated. This GPI system addresses a
number of challenges specific to long-pulse superconducting devices like W7-X,
including the long distance between the plasma and the vacuum vessel wall, the
long distance between the plasma and diagnostic ports, the range of last closed
flux surface locations for different magnetic configurations in W7-X, and
management of heat loads on the system's plasma-facing components. The system
features a pair of "converging-diverging" nozzles for partially collimating the
gas puffed locally $\approx$135 mm radially outboard of the plasma boundary, a
pop-up turning mirror for viewing the gas puff emission from the side (also
acting as a shutter for the re-entrant vacuum window), and a high-throughput
optical system that collects visible emission resulting from the interaction
between the puffed gas and the plasma and directs it along a water-cooled
re-entrant tube directly onto the 8 x 16 pixel detector array of the fast
camera. The DEGAS 2 neutrals code was used to simulate the H$_\alpha$ (656 nm)
and the HeI (587 nm) line emission expected from well-characterized gas-puffs
of H$_2$ and He and excited within typical edge plasma profiles in W7-X,
thereby predicting line brightnesses used to reduce the risks associated with
system sensitivity and placement of the field of view. Operation of GPI on W7-X
shows excellent signal to noise ratios (>100) over the field of view for
minimally perturbing gas puffs. The GPI system provides detailed measurements
of the 2-dimensional (radial and poloidal) dynamics of plasma fluctuations in
the W7-X edge, scrape-off layer, and in and around the magnetic islands that
make up the island divertor configuration employed on W7-X.
