Development of a Harmonic Rejection Mirror System at the NFPSS-BL17B Beamline

Harmonic contamination is a major issue in low-energy X-ray absorption fine structure (XAFS) experiments, leading to amplitude attenuation and spectral distortion, which compromise the reliability of structural analysis. The BL17B beamline, part of the National Facility for Protein Science in Shanghai (NFPSS) and operated within the Shanghai Synchrotron Radiation Facility (SSRF), also faces this challenge in its low-energy XAFS experiments. [Purpose] This study aims to develop a Harmonic Rejection Mirror (HRM) system tailored to the optical characteristics of the BL17B beamline to improve the quality and reliability of XAFS data. [Methods] First, XOP simulations were conducted to model the optical system of BL17B, confirming the feasibility of the HRM system based on a dual-plane mirror design. Key parameters, including the use of Ni and Rh coatings, a fixed grazing incidence angle of 5.8 mrad, and coating dimensions, were optimized accordingly. Second, a precise mechanical structure was designed, enabling multi-dimensional adjustments, including grazing incidence angle tuning, optical alignment, and coating switching to accommodate different experimental requirements. During installation and commissioning, the total reflection critical energy measurement method was employed to address the challenge of accurate grazing incidence angle control, ensuring the system met design specifications. Finally, the performance of HRM was evaluated using monochromator rocking curve measurements and XAFS experiments. [Results] The HRM effectively suppresses harmonic contamination to below 10−4 in 5–10 keV while maintaining a fundamental beam transmission above 70%. The total reflection critical energy measurement method achieved a grazing incidence angle calibration precision of 0.01 mrad, ensuring that the HRM operates as designed. At 5 keV, the HRM significantly enhanced beam monochromaticity and improved Ti foil XAFS data quality by eliminating spectral distortions. [Conclusions] The implementation of the HRM successfully extended the lower energy limit of XAFS experiments at BL17B from 10 keV to 5 keV, significantly enhancing the capability of low-energy XAFS experiments while providing a valuable technical reference for harmonic suppression in synchrotron radiation sources.