单谐振腔束团长度监测器耦合结构设计与仿真

单谐振腔束团长度监测器利用谐振腔内的两个本征模式测量ps量级的电子束团长度，它的关键是如何将两个不同频率的模式互不干扰地耦合提取出来。为解决这个问题，基于低通和带通滤波器的理论，提出了同轴滤波耦合结构和膜片加载波导滤波结构，借助CST微波工作室对滤波器进行建模并仿真得到其S参数。为测试耦合器的应用效果，设计了一套带有该耦合结构的单谐振腔束团长度监测器探头，根据国家同步辐射实验室基于可调谐红外激光的能源化学研究大型实验装置FELiChEM的束流特点，在CST内对所设计的探头进行束流模拟仿真。仿真结果表明，该耦合器可以实现对特定模式的耦合，并有效降低其它模式的干扰，采用同轴滤波耦合结构和膜片加载波导滤波结构的谐振腔监测器可以实现对FELiChEM装置束团长度的高精度测量，测量误差小于2 %。

Design and simulation of the coupler of single resonant cavity bunch length monitor

The single-cavity bunch length monitor uses two eigenmodes in the resonant cavity to measure the bunch length in the order of picoseconds. The key is how to couple and extract two modes of different frequencies without interfering with each other. To solve this problem, based on the theory of low-pass and band-pass filters, a coaxial filter coupling structure and a diaphragm-loaded waveguide filter structure are proposed. The filters are modeled and simulated in CST Microwave Studio to obtain S parameters. In order to test the effect of the coupler, a bunch length monitor with the coupling structure is designed. According to the beam characteristics of the National Synchrotron Radiation Laboratory based on the tunable infrared laser energy chemistry research large-scale experimental device (FELiChEM), a beam simulation is performed on the designed monitor in CST. The simulation results show that the coupler can realize the coupling of specific modes and effectively reduce the interference of other modes. The resonant cavity monitor adopting the coaxial filter and the diaphragm-loaded waveguide filter can achieve high-precision measurement of the bunch length of the FELiChEM, and the measurement error is less than 2%.