Key problems in design and commissioning of compact FEL-THz beam injector

Electron beam injectors are the core components that determine the performance of free electron laser terahertz sources (FEL-THz). RF accelerator-based injectors can generate high-quality electron beams required to drive high-power THz radiation. [Purpose]: This study aims to design and commission a compact Free Electron Laser for Terahertz (FEL-THz) injector using an Independently Tunable Cell (ITC) structure, aligning high quality, cost-effectiveness, and compactness. This configuration holds promising potential for advancing the miniaturization of high-power FEL-THz facilities. [Methods]: The research was conducted on a collaborative FEL-THz system developed by Huazhong University of Science and Technology and the University of Science and Technology of China, Initially, optimization concepts and design results based on the ITC structure were introduced. Following system stabilization, beam diagnostics and system optimization measures were detailed for beam commissioning and operation. Relevant schemes and research validation were completed through beam dynamics simulations and online experiments. [Results]: The experimental results show that the optimized ITC injector achieves an electron beam energy of approximately 13.7 MeV with an energy spread of 0.41% and a transverse emittance of 8.8 mm mrad, meeting high-power and miniaturization requirements. [Conclusion]: The simulation and experimental results both indicate that, through optimization design combined with indirect diagnostic techniques, the ITC injector can meet the demands for both high power and miniaturization of FEL-THz. The application of indirect diagnostic techniques can also compensate for the insufficient diagnostic methods during the commissioning process, laying the groundwork for further promotion of the ITC injector.

电子束注入器是决定自由电子激光太赫兹源（FEL-THz）性能的核心部件。基于射频加速器的注入器可产生驱动高功率太赫兹辐射所需的高品质电子束。

【目的】：本研究旨在采用独立可调单元（Independently Tunable Cell，ITC）结构，设计并调试一款紧凑的太赫兹自由电子激光（FEL-THz）注入器，兼顾高品质、经济性与紧凑性。该配置为推动高功率FEL-THz装置的小型化具有广阔前景。

【方法】：本研究基于华中科技大学与中国科学技术大学合作开发的FEL-THz系统开展。首先，介绍了基于ITC结构的优化理念与设计结果；系统稳定后，详细阐述了用于束流调试与运行的束流诊断及系统优化措施；通过束流动力学模拟与在线实验完成了相关方案与研究验证。

【结果】：实验结果表明，优化后的ITC注入器获得的电子束能量约为13.7 MeV，能量分散率为0.41%，横向发射度为8.8 mm·mrad，满足高功率与小型化需求。

【结论】：模拟与实验结果均表明，通过优化设计结合间接诊断技术，ITC注入器可满足FEL-THz对高功率与小型化的双重需求。间接诊断技术的应用还可弥补调试过程中诊断方法的不足，为ITC注入器的进一步推广奠定基础。