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.