Multispectral Schwarzschild imaging system for the extreme ultraviolet emission measurement of tokamak plasma

A multispectral extreme ultraviolet imaging system based on a separated multichannel Schwarzschild objective was developed for emission measurements of tokamak plasma, with a wide field of view, high spatial resolution, and multispectral resolution. Multiple energy responses were realized by depositing different multilayer coatings on each mirror, targeting the interesting spectral lines of common impurities, such as Ne, C, and W, observed in tokamak experiments. Direct-current magnetron sputtering in conjunction with mask reshaping was employed to fabricate highly uniform multilayers on large-curvature substrates, achieving a thickness control accuracy of ±0.1 nm. Furthermore, this paper proposes an assembly and adjustment scheme that transitions from single-channel centering to multichannel integration, ensuring high-precision alignment across all channels. The design, preparation, and experimental validation of the proposed system are reported. The system was experimentally validated in the Experimental and Advanced Superconducting Tokamak facility, successfully detecting all target spectral lines. The results of the study demonstrate the feasibility of electron temperature measurements using the proposed system.