Mitigation of transient beam loading in a compact high duty cycle injector via hardware upgrade and multi-objective optimization

Compact normal-conducting photoinjectors, such as the Independently Tunable Cells (ITC) injector, are critical for high-power terahertz free electron laser (FEL) sources. However, under high duty cycle operation, the transient beam loading effect significantly degrades the uniformity of electron beam energy and phase within the macropulse, thereby compromising the FEL radiation performance. Here, we propose a strategy to mitigate these transient effects and enhance the macropulse beam quality of the ITC injector for the HUST THz-FEL facility. The proposed hardware upgrade decouples the RF power feeds for the standing wave (SW) cavity and the traveling wave (TW) linac to enable independent timing control, and introduces an adjustable coupling coefficient for the SW cavity. Furthermore, the Non-dominated Sorting Genetic Algorithm III (NSGA-III) is employed to perform multi-objective optimization on four key variables: beam injection time, RF phase, coupling coefficient, and the inter-structure RF delay. Simulation results indicate that single-variable optimization is insufficient to balance energy and phase stability. In contrast, global multi-objective optimization reveals that increasing the coupling coefficient effectively reduces the cavity filling time, while independent RF timing synchronizes the accelerating fields with the beam transient. The optimized configuration achieves a stable macropulse energy of approximately 14 MeV with minimized energy spread and phase variation. Consequently, the combination of independent RF feeding, adjustable coupling, and global optimization effectively compensates for transient beam loading effects. This approach markedly improves the macropulse uniformity of the ITC injector in simulation, offering a potential pathway for similar compact injectors.