MITHRA 1.0: A full-wave simulation tool for free electron lasers

Free Electron Lasers (FELs) are a solution for providing intense, coherent and bright radiation in the hard X-ray regime. Due to the low wall-plug efficiency of FEL facilities, it is crucial and additionally very useful to develop complete and accurate simulation tools for better optimizing a FEL interaction. The highly sophisticated dynamics involved in a FEL process was the main obstacle hindering the development of general simulation tools for this problem. We present a numerical algorithm based on finite difference time domain/Particle in cell (FDTD/PIC) in a Lorentz boosted coordinate system which is able to fulfill a full-wave simulation of a FEL process. The developed software offers a suitable tool for the analysis of FEL interactions without considering any of the usual approximations. A coordinate transformation to bunch rest frame makes the very different length scales of bunch size, optical wavelengths and the undulator period transform to values with the same order. Consequently, FDTD/PIC simulations in conjunction with efficient parallelization techniques make the full-wave simulation feasible using the available computational resources. Several examples of free electron lasers are analyzed using the developed software, the results are benchmarked based on standard FEL codes and discussed in detail.

自由电子激光器（Free Electron Lasers, FELs）是实现硬X射线波段（hard X-ray regime）高强度、高相干性与高亮度辐射的核心方案。由于FEL装置的壁插效率（wall-plug efficiency）较低，开发完整且精准的仿真工具以优化FEL相互作用过程，不仅至关重要，更具备极高的实用价值。FEL过程中涉及的极为复杂的动力学行为，曾是阻碍通用仿真工具开发的主要障碍。本文提出一种基于洛伦兹boost坐标系（Lorentz boosted coordinate system）下的时域有限差分法/粒子网格法（finite difference time domain/Particle in cell, FDTD/PIC）数值算法，可实现FEL过程的全波仿真（full-wave simulation）。所研发的软件无需采用任何常规近似，即可为FEL相互作用分析提供可靠的分析工具。通过将坐标系转换至束团静止参考系（bunch rest frame），可将束团尺寸、光波长以及波荡器周期（undulator period）这三个尺度差异极大的物理量，统一至同一数量级。结合高效并行计算技术（parallelization techniques），依托现有计算资源即可完成全波仿真。本文利用所开发的软件对多例自由电子激光器实例开展了分析，结果以标准FEL代码（standard FEL codes）为基准进行了对标验证，并展开了详细讨论。