The physics design of HEPS Linac bunching system

Purpose The High Energy Photon Source is a fourth-generation synchrotron radiation source being built in China.It is comprised of a storage ring,a full energy booster,a 500-MeV Linac and three transport lines.The pulse charge at the exit of the Linac is required to be up to 7 nC.Both single bunch and three bunches in a beam pulse might be needed according to the studies on instabilities of the booster and the beam dynamic of the main Linac.Methods A bunching system consisting of both sub-harmonic and fundamental frequency structures is designed to meet the requirements.To obtain high transmission efficiency and control the emittance growth,multi-objective genetic algorithm is introduced to optimize the electromagnetic parameters of these structures.Results In this paper,detailed optimization process and the final results are presented.The total transmission efficiency of the bunching system is 97%,and the transmission efficiency of the main bunch is 95%.The normalized RMS emittance of 8.5 nC included beam charge is smaller than 60 mm mrad.Conclusions The design of the bunching system is presented,and both the pulse charge and the beam emittance well meet the design requirement.