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

X-ray free-electron laser (XFEL) as the novel advanced x-ray light source, has excellent properties, such as ultra-high brightness, ultra-shot pulse duration and full coherence. The development of XFEL provides unprecedented opportunities for ultra-fast and ultra-fine science. The XFEL based experimental methods have been widely applied to the frontier research in physics, chemistry, biology etc. High resolution imaging as one of the most promising methods, can provide a direct view of the microworld. The coherent X-ray diffraction imaging (CDI) is a lensless imaging method. It has a lot of advantages at high resolution and quantitative imaging compared with the traditional lens based X-ray imaging methods. As one of the driving forces to constructing XFEL, it has become one of the most important imaging methods at XFEL facilities. By combing the excellent properties of XFEL and advantages of CDI, the single-shot imaging has been realized, based on the concept of “diffraction before destruction”. With the femtosecond XFEL pulse, structural information of the sample can be captured in a single-shot without multiple measurements or data accumulation. The single-shot imaging can effectively avoid radiation damage and improve the spatial resolution of the images. Shanghai soft X-ray free-electron laser facility (SXFEL) is the first XFEL facility operated at the X-ray wavelength in China. The SXFEL can generate ultra-intense coherent femtosecond X-ray pulses with wavelengths spanning 2-15 nm (80-620 eV). There are two undulator lines and two beamlines. Five endstations were designed and constructed for ultrafast chemistry and physics science, atomic and molecular science and biological imaging. The coherent scattering and imaging (CSI) endstation is the first commissioned endstation at SXFEL and focuses on the high spatiotemporal imaging for nano and micro materials with 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 to ensure the operation of the equipment in sequence. The timing system is responsible for generating precise and adjustable trigger signals that are used to trigger different devices. These signals can be adjusted according to the specific requirements of the devices being triggered, ensuring that the devices are triggered at the desired moments. In a single-shot experiment, only a single pulse should be transmitted to interact with the sample, and all others must be blocked before the previous single-shot experiment is finished. To carry out the single-shot imaging at CSI endstation, a timing system was designed and commissioned at SXFEL. As the maximum repetition rate of SXFEL is 50Hz, a fast X-ray shutter was applied to select only one XFEL pulse. This paper introduces the design and implement procession of timing for SXFEL single-shot imaging. The timing system implemented with White Rabbit(WR) and digital delay and pulse generator (BNC505). Single-shot imaging is realized by synchronizing the sample scanning stages movement and X-ray shutter to select a single pulse to illuminate the sample. At the same time, the X-ray detector was triggered with the timing system to record the single-shot diffraction pattern. During commissioning, the gold nanodisks with a side length of approximately 300 nm and a thickness of about 30nm were imaged at the CSI endstation as test sample. The nanodisks were uniformly dispersed on Si₃N₄ membranes for single-shot imaging. Because of the ultra-high peak intensity at the focus spot, the samples and membrane were ionized for each XFEL pulse shot. A raster scan was performed on the membranes with an interval of 50 μm to update the sample. With the timing system and X-ray shutter, single-shot diffraction patterns could be recorded 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 was applied to the single-shot diffraction pattern to obtain a real-space image of the sample. The resolution of the reconstructed image was estimated by calculating the phase-retrieval transfer function (PRTF). With a citation of the PRTF curve dropping below 1⁄e, the spatial frequency cutoff was determined to be 22.6μm^-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 50Hz at SXFEL.