Studies of simultaneous operation both round beam and flat beam in a diffraction-limited storage ring

Abstract

The diffraction-limited storage rings (DLSRs) can provide an electron beam emittance below 100 $\text{pm}\cdot \text{rad}$, approaching the hard X-rays diffraction limit with better transverse coherence and higher brightness compared to the current light sources, to the synchrotron light users. A round beam with equal horizontal and vertical emittances can effectively mitigate intra-beam scattering effects and increase the Touschek lifetime, however a horizontally elongated flat beam is preferable for some beamline applications with brighter photon beam. Depending on the specific requirement on the experiment, some users prefer high brightness with flat beam while others prefer high horizontal coherence with round beam. To address the limitation of being able to provide only one type of operation mode as either round beam or flat beam, this paper proposes two approaches for the simultaneous operation of both modes with several bunches in flat beam and the other in round beam, namely the flat beam generation in round-beam mode and the round beam generation in flat-beam mode. As an example to demonstrate its feasibility, the lattice design, dynamic aperture, and numerical simulations of these two approaches on the latest lattice of the High Energy Photon Source (HEPS) storage ring, are presented.