Design of a diffraction-limited storage ring lattice using longitudinal gradient bends and reverse bends

Diffraction-limited storage ring (DLSR) light sources are being developed towards the goal of reaching the diffraction-limited emittances. To reduce the emittances, multi-bend achromat (MBA) lattices with many bend unit cells are used in DLSR designs. The bend unit cell with the combination of longitudinal gradient bend (LGB) and reverse bend (RB) can achieve much lower emittance, which has produced the SLS-2 7BA lattice (Streun et al., 2018). In this paper, a 6BA lattice with LGB/RB unit cells is designed for a DLSR with an energy of 2.2 GeV and 20 periods. An ultra-low natural emittance of 36 pm center dot rad is achieved for the designed DLSR, and the beta functions at straight sections are also relatively low so as to enhance the radiation brightness of insertion devices. The optimized dynamic aperture is relatively large, larger than 4 mm in the horizontal direction, and the momentum aperture is also reasonable. Besides, the intra-beam scattering effect is estimated and discussed.