Diagnostic design for beam emission spectroscopy at Wendelstein 7-X for ion-scale turbulence measurements*

We report on a feasibility study to perform fluctuation beam emission spectroscopy (BES) measurements at Wendelstein 7-X (W7-X) to observe ion-scale turbulence. W7-X is a neoclassically-optimized stellarator, and initial results indicate that turbulence transport plays a significant role in heat and particle confinement at r/a ≈ 0.7. BES measurements can provide 2D imaging of plasma turbulence on relevant spatial and time scales by observing the localized Doppler-shifted Balmer-alpha emission (n=3→2) from neutral heating beams. The constraints of field-aligned sightlines at the observation volume, sufficient Doppler shift to isolate the beam emission manifold and high optical throughput are challenging to achieve with the nearly radial heating beams on W7-X. We report on BES measurement configurations that satisfy the constraints and provide 2D coverage at ion-scales in the edge and mid-radius regions. In addition, we report on diagnostic performance factors such as spatial resolution, k-space coverage, emission spectra, and signal-to-noise estimates with high-throughput collection optics and high-speed, low-noise BES detectors. Finally, we layout plans to deploy a BES diagnostic system for ion-scale turbulence measurements at W7-X.