Design and implementation of timing system for single-shot imaging at Shanghai soft X-ray free-electron laser

Abstract

X-ray free-electron laser (XFEL), as a novel advanced X-ray light source, has excellent properties such as ultra-high brightness, ultra-shot pulse duration, and full coherence. The coherent X-ray diffraction imaging (CDI) has a lot of advantages at high resolution and quantitative imaging compared with the traditional lens based X-ray imaging methods. By combining the excellent properties of XFEL and advantages of CDI, the single-shot imaging has been realized, based on the concept of “diffraction before destruction”. Shanghai soft X-ray free-electron laser facility (SXFEL) is the first XFEL facility operated at the X-ray wavelength in China. The coherent scattering and imaging (CSI) endstation is the first commissioned endstation at SXFEL, focusing on the high spatiotemporal imaging for nano materials and micro materials by using a single-shot imaging method. To realize the single-shot experiment at XFEL, especially for single-shot imaging, the timing system plays a crucial role in ensuring the operation of the equipment in sequence. This paper introduces the design and implementation process of SXFEL single-shot imaging timing. The timing system is implemented with White Rabbit (WR) and digital delay and pulse generator (BNC505). Single-shot imaging is realized by synchronously moving the sample scanning stages and X-ray shutter to select a single pulse to illuminate the sample. At the same time, the X-ray detector is triggered with the timing system to record the single-shot diffraction pattern. During debugging, a gold nanodisks each with a side length of approximately 300 nm and a thickness of about 30 nm, as test samples, are imaged at the CSI endstation. The nanodisks are uniformly dispersed on Si₃N₄ membranes for single-shot imaging. Because of the ultra-high peak intensity at the focus spot, the samples and membrane are ionized for each XFEL pulse shot. A raster scan is performed on the membranes at intervals of 50 μm to update the sample. With the timing system and X-ray shutter, single-shot diffraction patterns can be recorded by using an X-ray detector. From the image of the Si₃N₄ membrane after raster scanning, the ionized holes with an interval of 50 μm can be recognized. Finally, phase retrieval is applied to the single-shot diffraction pattern to obtain a real-space image of the sample. The resolution of the reconstructed image is estimated by calculating the phase-retrieval transfer function (PRTF). With a citation of the PRTF curve dropping below <inline-formula><tex-math id="M3">\begin{document}$ 1/{\mathrm{e}} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240383_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240383_M3.png"/></alternatives></inline-formula>, the spatial frequency cutoff is determined to be 22.6 μm^–1, corresponding to a half period resolution of 22.1 nm. The results show that the designed timing system can accurately control the time sequence of the imaging process, meeting the requirement for single-shot imaging within 50 Hz at SXFEL.