Localized Phase Contrast Imaging of Turbulence and Magnetohydrodynamic Modes at the Wendelstein 7-X Stellarator*

Phase contrast imaging (PCI) measures line-integrated electron density fluctuations along the line-of-sight (LoS) of a laser beam. At the Wendelstein 7-X stellarator, the PCI system has been used to investigate turbulence [1] and magnetohydrodynamic (MHD) modes individually [2], as well as the interplay between plasma turbulence and MHD modes [3]. In the above studies, PCI was limited by the lack of localization of the signal along the LoS. While the location of the main turbulence features could be inferred through modeling and synthetic PCI techniques [1,4], no localization of the MHD modes could be obtained from PCI alone. In the recently completed W7-X experimental campaign, masks allowing localization by blocking the signal from part of the LoS [5] were used to confirm the inferred location of the dominant turbulent fluctuations and to locate other features observed by PCI, including MHD modes. Specifically, these investigations showed that localized measurements of short-wavelength MHD modes on the outboard side of W7-X are possible, while the ability to locate long-wavelength modes is limited, which is supported by synthetic PCI results [4].

[1] J.-P. Bähner et al., J. Plasma Phys. 87, 905870314 (2021)

[2] C. Slaby et al., Nucl. Fusion 60, 112004 (2020)

[3] S. Vaz Mendes et al., submitted to: Plasma Phys. Controlled Fusion (2023)

[4] S. K. Hansen et al., Plasma Phys. Controlled Fusion 64, 095011 (2022)

[5] J. R. Dorris, J. C. Rost, and M. Porkolab, Rev. Sci. Instrum. 80, 023503 (2009)