The Shanghai soft X-ray free-electron laser (SXFEL) user facility project started in 2016 and is expected to be open to users by 2022. It aims to deliver ultra-intense coherent femtosecond X-ray pulses to five endstations covering a range of 100–620 eV for ultrafast X-ray science. Two undulator lines are designed and constructed, based on different lasing modes: self-amplified spontaneous emission and echo-enabled harmonic generation. The coherent scattering and imaging (CSI) endstation is the first of five endstations to be commissioned online. It focuses on high-resolution single-shot imaging and the study of ultrafast dynamic processes using coherent forward scattering techniques. Both the single-shot holograms and coherent diffraction patterns were recorded and reconstructed for nanoscale imaging, indicating the excellent coherence and high peak power of the SXFEL and the possibility of “diffraction before destruction” experiments at the CSI endstation. In this study, we report the first commissioning results of the CSI endstation.
Shanghai Soft X-ray Free-Electron Laser (SXFEL) is the first X-ray free-electron laser facility in China. The initial commissioning of the beamline was carried out in May 2021. Herein, we present a status report and the first experimental results obtained during the early commissioning of Kirkpatrick-Baez (KB) mirrors for the Coherent Scattering and Imaging (CSI) endstation, including three types of diagnostics. A bright X-ray focal spot of less than 3 μm was achieved by using edge-scan and silicon ablation imprint measurements. In order to confirm the spot size, the attenuated beam and full beam are used respectively for the two measurement methods.
The characterization of X-ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X-ray free-electron lasers (XFELs) are the latest generation of X-ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X-ray Free Electron Laser Facility was characterized using the method. The full width at half-maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method.
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