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$\cdot$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.

衍射极限存储环（DLSR）光源正朝着达到衍射极限发射率的目标发展。为了减少发射，在DLSR设计中使用了具有许多弯曲单元的多弯曲消色差（MBA）晶格。结合了纵向梯度弯曲（LGB）和反向弯曲（RB）的弯曲单元电池可以实现更低的发射率，这产生了SLS-2 7BA晶格（Streun et al。，2018）。本文针对具有2.2 GeV能量和20个周期的DLSR设计了带有LGB / RB晶胞的6BA晶格。36 pm的超低自然发射率$\cdot$设计的DLSR可以达到rad，直线部分的beta函数也相对较低，以增强插入设备的辐射亮度。优化的动态光圈相对较大，在水平方向上大于4 mm，并且动量光圈也很合理。此外，对束内散射效应进行了估计和讨论。