Very-high-frequency(VHF)gun photoinjectors,capable of producing high-brightness and high-repetition-rate electron bunches,are some of the best electron sources for driving MHz-class repetition-rate free-electron lasers.In this study,the beam dynamics optimization of a VHF gun photoinjector for Shanghai High Repetition Rate X-ray Free Electron Laser and Extreme Light Facility(SHINE)is systematically demonstrated using a genetic algorithm.Through the inclusion of the solenoid geometry as an optimization variable into the genetic algorithm,the opti-mum projected normalized emittance for 100 pC bunches with bunch length of 1 mm rms is reduced to 0.1 mm mrad for 100％of the particles and 0.075 mm mrad for 95％of the particles,proving that sub-100 nm emittance can be achieved in the SHINE injector using a single-cell Tsin-ghua University(THU)VHF gun.This emittance fulfills the requirements not only of SHINE and Linac Coherent Light Source(LCLS)-II but also of LCLS-II-High Energy(LCLS-II-HE).We demonstrate that the optimal emittance in the VHF gun injector is reduced via the optimization of the solenoid geometry,thereby reducing solenoid spherical aberration.Through the inclusion of high-order(H.O.)energy spread among the optimization objectives,the H.O.energy spread can be reduced by a factor of nearly six using a high-harmonic cavity despite a 38％emittance growth.Finally,the beam dynamics in the SHINE main accelerator show that reducing the H.O.energy spread in the injector is of great significance to improving com-pression efficiency and reducing bunch current spike.

Han Chen;Lian-Min Zheng;Bin Gao;Zi-Zheng Li;Ying-Chao Du;Ren-Kai Li;Wen-Hui Huang;Chuan-Xiang Tang;Duan Gu;Zheng Qi;Meng Zhang;Hai-Xiao Deng;Qiang Gu;Dong Wang

Department of Engineering Physics,Tsinghua University,Beijing 100084,China;Chinese Academy of Sciences,Shanghai Institute of Applied Physics,Shanghai 201800,China;Chinese Academy of Sciences,Shanghai Advanced Research Institute,Shanghai 201210,China

核技术（英文版）

[1]Han Chen;Lian-Min Zheng;Bin Gao;Zi-Zheng Li;Ying-Chao Du;Ren-Kai Li;Wen-Hui Huang;Chuan-Xiang Tang;Duan Gu;Zheng Qi;Meng Zhang;Hai-Xiao Deng;Qiang Gu;Dong Wang-.Beam dynamics optimization of very-high-frequency gun photoinjector)[J].核技术（英文版）,2022(09):80-92

photoinjector,photoinjectors,Tsin,ghua,LCLS

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