基于偏转腔的ns间隔逐束团纵向分布测量方法研究

Operation mode of high repetition frequency with ns-scale bunch interval is an important development direction of XFEL. The measurement of longitudinal bunch profile based on transverse deflecting cavity with high resolution is one of the key technologies, and the problem is the overlap of bunch profile because of the ultra-short bunch interval. Limited by the persistence and photon yield of screen as well as the shutter time and photon detection efficiency of cameras, it is difficult to obtain each bunch profile from the time angle. This project proposes the phase difference method based on transverse deflecting cavity with non-integer harmonic of bunch repetition frequency, which makes each bunch be at different deflecting phase and separates bunch profile at the screen from the spatial angle. The phase difference method eases the requirement of ns-scale fast time response for the screen and camera. The project will be carried out by the combination of simulation and experimental measurement. A transverse deflecting cavity satisfying the requirement of phase difference method will be developed, and measurement of microwave characteristics will be compared with the simulation results. Beam test will be done on a superconducting accelerator with bunch repetition frequency of hundreds of megahertz, in order to investigate the time resolution of the cavity, verify the phase difference method in principle, realize the measurement of longitudinal profile for bunches with ns-scale interval, and evaluate the measuring accuracy of the method and the influence of microwave phase.

ns量级短束团间隔的高重频运行模式是XFEL重要的发展方向，而基于偏转腔的高分辨率逐束团纵向分布测量是其中的关键技术之一。其中的核心难点在于极短束团间隔引起的束团间束斑重叠问题。受目前探测屏余辉时间与光产额、相机快门时间与光子探测效率的限制，单纯从时间角度获取各束团的束斑具有一定的难度。本项目基于工作在束流重复频率非整数倍的偏转腔提出相位差法，使各束团处于不同的偏转相位，从而在探测屏上实现束斑的空间分离，放宽了探测屏和相机对ns量级快时间响应的要求。本项目将以模拟仿真和实验测量相结合的方式开展：研制适用于相位差法的偏转腔，并对模拟仿真设计进行微波特性实验研究；在百MHz束流重复频率的超导加速器装置上对偏转腔进行束流实验研究，考察偏转腔的时间分辨表现，从原理上验证基于偏转腔的相位差法，实现ns间隔逐束团纵向分布测量，评估通过单幅束斑分布图像获取所有束团长度和微波相位选择对测量精度的影响。

针对高重频电子束逐束团纵向分布测量的难题，本项目研制了一套基于2856MHz横向偏转腔的束流参数测量系统，并原创性地采用相位差法解决了探测屏上由于极短束团间隔引起的束斑重叠问题，巧妙地将ns时间窗口的高速测量转换为易实现的空间分辨，从而实现了ns间隔高重频电子束的逐束团纵向分布测量。对于间隔2.3ns的4个束团（重频433MHz），通过调节偏转腔的微波相位，获得了每个束团的纵向分布，束团长度（FWHM）分别为17.8ps、18.3ps、17.9ps、18.4ps，测量相位分别为188°、337°、121°、275°，偏转腔的时间分辨率为~100fs。在高重频XFEL的发展态势下，该方法可应用于高重频自由电子激光等加速器装置中电子束的参数诊断。