Lattice upgrade design for the SSRF storage ring towards the diffraction limit

摘要: Shanghai Synchrotron Radiation Facility (SSRF) aims to be upgraded (SSRF-U) to the soft X-ray diffraction-limited light source. Beam emittance from double-bend achromat to 9-bend achromat structures was evaluated and compared to determine that 7-Bend Achromat (7BA) lattice is the baseline configuration of the SSRF-U storage ring. A hybrid optimization framework combining the multi-objective genetic algorithm and the gradient descent method was established to enhance design efficiency. The Dynamic Aperture Calculation from Action Variations (DACAV) method, which quantifies the difference of the Courant-Snyder invariant within short-term tracking data from the ideal linear one, was developed to address the computational cost in design and optimization process. Because the critical value to determine the Dynamic Aperture (DA) in DACAV method lacks a theoretical calculation, long-term tracking data need to be utilized to calibrate calculated DA. This developed method was applied to design and optimize the beam dynamics of the SSRF-U storage ring, resulting in a candidate lattice with 72.6 pm·rad of natural emittance. The lattice performances, including beam parameters, nonlinear behaviors and photon spectra etc., are presented in this paper.