目前世界上正在发展百皮米级甚至十皮米级发射度的衍射极限储存环光源，但要产生高度横向相干的同步辐射，还需要将发射度进一步降低到皮米级。为降低发射度，目前衍射极限储存环lattice都采用MBA结构。在更低的十皮米级发射度储存环设计中，目前被很多衍射极限储存环所采用的hybrid MBA类型结构呈现出了一定的局限性，难以获得较大的动量孔径。最近我们提出了两种MBA结构——局部对称MBA和具有交错式高色散区的MBA，它们在十皮米级发射度储存环设计的初步研究中获得了较大的动力学孔径和足够大的动量孔径。本项目将研究将这两种MBA拓展到更能降低发射度的结构并研究进一步增大其动力学孔径，使其可用于设计皮米级发射度的衍射极限储存环。此外，本项目还将研究用于设计皮米级发射度储存环的其它MBA结构。本项目研究成果将对未来设计皮米级发射度的衍射极限储存环和目前世界上正在设计的衍射极限储存环都具有重要的价值。

At present, diffraction-limited storage rings (DLSRs) are being developed around the world towards hundreds of picometers or even tens of picometers. To produce synchrotron light with highly transverse coherence, the emittance of a DLSR is required to be further reduced to picometer-order. In order to reduce the emittance, DLSRs adopt multi-bend achromat (MBA) lattices. The hybrid MBA concept that has been adopted in many DLSRs has shown some limitations in designing DLSRs with a much lower emittance of tens of pm-rad, because it is hard to achieve large dynamic momentum aperture (MA). Recently we have proposed two MBA concepts, locally symmetric MBA and MBA with interleaved dispersion bumps, which have achieved large dynamic aperture (DA) and large enough dynamic MA in designing a DLSR with an emittance of tens of pm-rad. In this research project, we will extend the two concepts to achieve a lower emittance and do a deep study to further increase the DA, so that they can be used to design DLSRs with picometer-order emittance. In addition, other lattice concepts for designing DLSRs with picometer-order emittance will also be studied in this project. The research results of our project will have an important value for designing DLSRs with picometer-order emittance in the future, as well as the DLSRs that are being designed around the world.