Study on Beam Dynamics and Beam Instabilities for a Next Generation Light Source Based on an Energy-Recovery Linac

能量回收直线加速器是下一代x射线光源的候选光源，它能产生相干x射线和飞秒x射线。本项目对决定ERL光源性能的关键问题进行了研究。问题是：(1)由于超导腔中激发的高阶模式导致的光束不稳定，(2)由于弯曲路径的集体效应导致的发射度增长。针对HOM不稳定性，我们开发了一个粒子跟踪模拟程序来估计光束破裂的阈值电流。仿真代码的结果与其他仿真代码的结果进行了比较。我们还在JAEA ERL上进行了HOM不稳定性的实验研究，其中注入光束在主腔处受到与HOM相同频率的光束偏转器的扰动。我们看到偏转的波束增强了HOM信号。对于弯曲路径上的发射度增长，我们提出了一种线性分析方法来计算电子束中由相干同步辐射和纵向空间电荷力这两种集体效应引起的发射度增长。线性分析可以用于寻找沿注入合并和返回回路等曲线路径的最佳光束包络线，以最小化发射度增长。

Energy-recovery linac is a candidate of next-generation X-ray light source, which produces coherent X-rays and femto-second X-rays. Critical issues which determine the performance of ERL light sources have been studied in this project. The issues are (1)beam instabilities due to higher-order modes excited in superconducting cavities, (2)emittance growth due to collective effects through a curved path.For the HOM instability, we developed a particle tracking simulation code to estimate the threshold current of beam break-up. The result of simulation code has been compared with results from other simulation codes. We have also made an experimental study of HOM instability at JAEA ERL, where the injection beam is perturbed by beam deflector at the same frequency as HOM at the main cavity. We have seen enhancement of HOM signals by the deflected beam.For the emittance growth in a curved path, we have proposed linear analysis to calculate the emittance growth due to the collective effects in an electron bunch, which are coherent synchrotron radiation and longitudinal space charge force. The linear analysis can be used to find an optimum beam envelope along a curved path such as injection merger and return loop to minimize the emittance growth.