Analysis and optimizations of macro-pulse performances for a constructed high-power beam injector with a compact layout

The performance of terahertz (THz) radiation from free electron lasers (FEL) depends on the consistency of electron bunches within the macro-pulse, not just the properties of individual bunches. In accelerator-based injectors, bunch-to-bunch deviations often arise due to transient beam loading effects. This study investigates the impact of such transient responses on macro-pulse inconsistencies by combining theoretical analysis and particle simulations, using the FEL injector at Huazhong University of Science and Technology (HUST) as a case study. The goal is to optimize macro-pulse performance without requiring additional equipment or facility upgrades. To achieve this, an improved multi-objective optimization algorithm, NSGA-III, is introduced to optimize RF injection timing and phase settings, targeting energy and phase consistency within the macro-pulse and stable energy output. The approach is validated through extensive virtual simulations, which demonstrate its effectiveness in providing optimal initial settings for the HUST-FEL injector's macro-pulse. Subsequent online experiments conducted on this injector have preliminarily validated the reliability of the proposed method as well. The results are applicable to other multi-bunch/macro-pulse injectors, offering a cost-effective solution for efficient beam commissioning and engineering implementation.