Localized phase contrast imaging at the Wendelstein 7-X stellarator

Hansen, S. K.; Porkolab, M.; Bähner, J. -P.; Stechow, A. von; Grulke, O.; Edlund, E. M.

doi:10.1063/5.0197958

Localized phase contrast imaging at the Wendelstein 7-X stellarator

Journal Article · Wed Jun 12 00:00:00 EDT 2024 · Physics of Plasmas

DOI:https://doi.org/10.1063/5.0197958· OSTI ID:2567665

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In its basic form, phase contrast imaging (PCI) provides line-integrated measurements of electron density fluctuations in plasmas. As turbulent fluctuations in magnetically confined plasmas have wave vectors almost perpendicular to the background magnetic field, the signals scattered by fluctuations from different parts of the PCI line-of-sight (LoS) are spatially separated in focal planes of the plasma. This allows localized PCI measurements by placing a mask in such a plane, to only permit signals from specific parts of the LoS to reach the PCI detectors. The present paper describes modeling and design of localization masks for the PCI system at the Wendelstein 7-X (W7-X) stellarator as well as the first results obtained using the masks in the recent long-pulse W7-X experimental campaign. During this project, we have extended the theory describing the mask response within the Fraunhofer diffraction model. As a novel development, we show from first principles that the mask response is determined by the fraction of power of the scattered beam spots that passes the mask. These insights have been used to select the W7-X mask design, consisting of a circular cutout, allowing the unscattered beam spot to pass the mask, with wedges covering a fixed angular range outside the central cutout. In the recent W7-X experimental campaign, the masks have verified the location of the main turbulence features observed by the PCI system and provided new information about the location of short-wavelength magnetohydrodynamic modes.

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Research Organization:: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

Sponsoring Organization:: USDOE; USDOE Office of Science (SC)

Grant/Contract Number:: SC0014229

OSTI ID:: 2567665

Alternate ID(s):: OSTI ID: 2578504

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 6 Vol. 31; ISSN 1070-664X

Publisher:: American Institute of PhysicsCopyright Statement

Country of Publication:: United States

Language:: English

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