Free electron lasers (FEL) have emerged as significant advanced light sources owing to their unique advantages, including high power, excellent coherence, and wavelength tunability. Given that the peak and average brightness of an FEL depend on the quality of the electron beam generated by its injector, the optimization of the beam injector constitutes a key technical challenge in FEL development. The hefei infrared free electron laser facility is a state-of-the-art, oscillator-type user facility that provides continuously tunable mid-to-far-infrared radiation.

The injector structure of hefei infrared free electron laser is optimized to obtain electron beams with lower emittance, shorter beam length, smaller energy spread and higher peak current intensity, so as to improve the performance of driven infrared free electron laser light source.

The optimization research is carried out by combining beam dynamics simulation with numerical simulation. Based on the previous optimization of the electron gun’s grid structure, the improved design is carried out. A new 12th sub-harmonic buncher is added to the front stage of the existing 6th sub-harmonic buncher, and then the beam is bunched and accelerated using the appropriate traveling-wave buncher. Key parameters including the beam injection phase and the phase velocity variation in the traveling-wave buncher’s tapered section are systematically scanned to achieve 100% bunch capture efficiency and accelerate the electron beam to near-light-speed energy during the bunching stage.

Finally, the beam energy was increased to 64 MeV, and the root mean square length of the whole bunch reached 8.5 ps. The high-energy scattered electrons were filtered out, and the electron beams scattered by ±1% bunch energy were counted. The optimized beam core achieved a root-mean-square longitudinal bunch length of 3.1 ps with an energy spread below 0.23 MeV, while the normalized transverse emittance was reduced to 9.8 mm·mrad. At the same time, the peak current intensity reaches 270 A, which was 2.7 times that of the original optimization results.

The simulation shows that the longitudinal length, energy dispersion and emittance of the core region of the bunch are significantly reduced after optimization, and the peak current intensity is greatly improved. Compared with the original structure, this scheme has significant advantages in the key performance of free electron laser, which has important engineering value for light source upgrading. The optimization method can be extended to the design of other light source injectors.