上海软X射线自由电子激光单脉冲成像定时的设计与实现

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

上海软X射线自由电子激光装置(SXFEL)是我国首台X射线自由电子激光用户装置,目前建有2条波荡器线、2条光束线以及5个实验站.装置可提供2—15 nm波长(80—620 eV)的X射线脉冲,用于高时空分辨的前沿科学研究.利用XFEL高亮度、短脉冲和全相干的特性实现单脉冲相干衍射成像,可以有效地减轻辐射损伤,提高图像的空间分辨率.SXFEL设计重复频率为50 Hz,实现单脉冲成像的关键在于通过定时系统能够精确地控制X射线脉冲到达样品点的时间,以确保只有一个脉冲被选中用于成像.同时,还需要与成像系统的触发进行同步,以确保成像系统在正确的时间采集X射线脉冲与样品作用后的图像.本文介绍了SXFEL单脉冲成像定时的设计与实现.通过单脉冲成像的结果表明该定时方案能满足在50 Hz的SXFEL开展单脉冲成像的需求.

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 Xray 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 Si_(3)N_(4) 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 50μm 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 Si_(3)N_(4) membrane after raster scanning,the ionized holes with an interval of 50μm 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 1/e,the spatial frequency cutoff is 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 50 Hz at SXFEL.