Studies of round beam at HEPS storage ring by driving linear difference coupling resonance

Abstract

Strong intra beam scattering (IBS) effect and short Touschek lifetime become common issues for ultra-low-emittance storage ring (USR). One efficient approach to reduce these effects is operating such a machine under round-beam mode, a beam with equal horizontal and vertical emittance. Operating on the coupling resonance to obtain round beam will be most likely the method of choice for many of the future diffraction limited storage ring light sources. In this paper, we carried out a systematic study on round beam generation by driving linear difference coupling resonance using skew quadrupoles with same phase difference between horizontal and vertical planes. Theoretical analysis and application of this method at High Energy Photon Source (HEPS) storage ring are presented. Based on the simulation results of HEPS, we find that round beam can effectively mitigate the horizontal emittance growth due to IBS and increase the Touschek lifetime. Furthermore, the dynamic aperture is still be accommodated using on-axis injection when introducing reasonably skew quadrupole fields along the ring and closing to the coupling resonance to achieve round beam.