X-band deflecting cavity design for ultra-short bunch length measurement of SXFEL at SINAP

For developing the X-ray Free Electron Lasers test facility(SXFEL) at Shanghai Institute of Applied Physics,Chinese Academy of Sciences(SINAP), ultra-short bunch is the crucial requirement for excellent lasing performance. It is a big challenge for deflecting cavity to measure the length of ultra-short bunch, and higher deflecting gradient is required for higher measurement resolution. X-band travelling wave deflecting structure has features of higher deflecting voltage and compact structure, which has good performance at ultra-short bunch length measurement. In this paper, a new X-band deflecting structure is designed to operate in HEM11-2π/3 mode.For suppressing the polarization of deflecting plane of the HEM11 mode, two symmetrical caves are added on the cavity wall to separate two polarized modes.     

Design and study of a C-band pulse compressor for the SXFEL linac

A C-band RF pulse compressor is in development at SINAP It comprises of two resonant cavities,two mode convertors and a 3 dB power divider.TE_(0.1.15)mode is selected for obtaining higher quality factor Q_0 of the RF pulse compressor cavities,so that the power gain factor can be 3.2,which is supposed to multiply the RF power from 50 MW to 1 60 MW.In this paper,we report our work on C-band RF pulse compressor,namely the design simulation and cold test results.     

Simulation studies on two-frequency RF gun

Photocathode RF gun is widely used for particle accelerators as an electron source.When driving an RF electron gun at the fundamental frequency and a higher harmonic frequency simultaneously with proper field ratio and relative phase,it generates electron beams of ultralow emittance and a linear longitudinal phase space distribution.Such a gun provides high quality electron beam with low energy spread,small traverse emittance and high brightness.In this paper,the RF design of a 1.5 cell cavity is presented.Simulation results of beam dynamics for the two-frequency gun and a standard single-frequency RF gun are also shown in this paper.In addition,bunch compression with a two-frequency gun is explored.     

基于响应矩阵的RFQ新调谐算法的验证与实现

射频四极场加速器（Radio Frequency Quadrupole,RFQ）广泛应用于质子加速器装置中,高频率、紧凑型RFQ的发展可以推动质子加速器装置的小型化,同时也面临调谐困难的问题。紧凑型RFQ的性能对腔体尺寸的敏感度高,基于理论物理特性的传统调谐算法难以取得良好效果,而基于腔体实际物理特性的调谐算法则可以解决紧凑型RFQ的调谐难点。本文提出了一种基于响应矩阵和最小二乘法的调谐算法,该算法可以限定求解的范围,避免计算结果超出调谐器可调谐范围的问题;为二极场分量赋予权重,可以高效地降低二极场误差,解决二极场分量难以调谐的问题。根据新的调谐算法分别在模拟环境下的RFQ样机上完成了调谐实验,在搭建的测量平台获得的结果显示：初始状态下二、四极场误差分别为24.09%和1.57%,经过5次调谐后两者误差分别降为2.33%和1.39%,验证了调谐算法的有效性。该调谐算法具有普适性,也可以用于其他频率的RFQ,将来还可以促进质子加速器装置的小型化,推动医用质子装置的普及。     

用于实现能量倍增器平顶输出的双环PI控制技术研究及验证

上海软X射线自由电子激光（Shanghai Soft X-ray Free-electron Laser,SXFEL）升级改造工程中，升级项目之一是使装置在双束团模式下运行，即在一个微波脉冲内加速得到两个相同能量的电子束团。为此设计开发了基于双环比例积分（Proportional Integral,PI）控制的微波低电平（Low Level Radio Frequency,LLRF）控制系统，用于调制微波脉冲能量倍增器的输出波形，使其变为平顶脉冲，以满足双束团模式的要求。该方法使用了两个反馈闭环分别控制输出脉冲整体的幅相稳定性和保持波形的平坦化。通过在微波实验平台上的连续测试，证明了双环PI控制法可以长时间维持能量倍增器输出脉冲的平坦度，并保证整体的幅相稳定性在0.09%和0.1°以内（均方根值），满足了设计要求。当对系统施加较大的外部干扰时，无需人为干预，算法能自动恢复平顶输出。     

软X射线自由电子激光试验装置微波系统

软X射线自由电子激光装置(SXFEL)是中国第一台X射线相干光源,其最短波长可达到2 nm。这台基于1.5 GeV的C波段高梯度电子直线加速器的激光装置分试验装置(SXFEL-TF)和用户装置(SXFEL-UF)两个阶段进行研制,最终形成包含1条种子型自由电子激光束线、1条自放大自发辐射束线以及5个实验站的用户装置。试验装置SXFEL-TF已经通过了国家验收,其上游段由0.84 GeV直线加速器组成,主体由S波段光阴极注入器、基于X波段线性化微波单元的束团压缩系统和C波段高梯度直线加速器组成,同时还应用了X、C和S波段的能量倍增器、偏转腔和波导等微波技术,创造了多项高水平的技术成果。本文将详细介绍SXFEL试验装置的微波系统及相关成果。