Improving the MWI threshold of the modified hybrid-7BA lattice design for SAPS

Abstract

The Southern Advanced Photon Source (SAPS), is a mid-energy ultra-low emittance ring-based light source proposed to be built adjacent to the China Spallation Neutron Source. Due to the capability of the hybrid-MBA (H-MBA) lattice to simultaneously realize excellent nonlinear performance and ultra-low emittance, a preliminary design has been designed for the SAPS based on modified H-MBAs where ultra-low emittance unit cells are combined therein to reach an emittance of 31.8 pm rad. Recently, a higher current is pursued to reach higher flux and brightness. To avoid the energy spread widening caused by microwave instability (MWI) at a higher current, it is particularly important to improve the MWI threshold. In this paper, we study the correlation between the lattice parameters and the MWI threshold. It is found that the circumference and momentum compaction factor are critical parameters in lattice design to improve the MWI threshold. By reducing the circumference from 1080 m to 810 m and raising the momentum compaction factor by nearly 2 times, the MWI threshold is significantly increased, resulting in an increase in the beam current from 200 mA to 400 mA. Furthermore, the emittance is reduced to 26.3 pm rad and the corresponding DA and MA meet the requirements of on-axis swap-out injection.