Beam emission spectroscopy diagnostic design and capabilities for two-dimensional turbulence measurement on Wendelstein 7-X

Turbulence transport is found to play a significant role in heat and particle confinements on Wendelstein 7-X (W7-X). A beam emission spectroscopy (BES) diagnostic is designed for studying 2D plasma turbulence dynamics at r/a∼0.5-1 by measuring the localized Doppler-shifted Balmer-alpha emission (n=3→2) from neutral heating beams on W7-X. We report the recent status of the diagnostic design, including optical assembly, detector platform, and the optical throughput. The field-aligned sightlines of BES is arranged to satisfy the constraints of the 3D magnetic topology and nearly radial heating beams while maintaining the sufficient Doppler shift to isolate the beam emission manifold on W7-X. The sightline grid with 1.4 cm spacing is evaluated to be sensitive to ion-scale turbulence with k_⊥ρ_i≦0.4 at r/a=0.75 in the post-pellet regime of W7-X. We report on point spread function (PSF) calculations for a BES sightline grid in a variety of magnetic configurations and operational regimes of W7-X. Finally, the BES capabilities for 2D ion-scale turbulence measurements are summarized, the plan for in-situ calibration and experimental measurement on W7-X is foreseen.