  In recent years, a new generation of storage ring based light sources, known
as diffraction limited storage rings (DLSRs), whose emittance approaches the
diffraction limit for the range of X ray wavelengths of interest to the
scientific community, has garnered significant attention worldwide. Researchers
have begun to design and build DLSRs. Among various DLSR proposals, the hybrid
multibend achromat (HMBA) lattice enables sextupole strengths to be maintained
at a reasonable level when minimizing the emittance; hence, it has been adopted
in many DLSR designs. Based on the H7BA lattice, the design of the Advanced
Photon Source Upgrade Project (APSU) can effectively reduce emittance by
replacing six quadrupoles with antibends. Herein, we discuss the feasibility of
designing an APSU type HMBA lattice for the Southern Advanced Photon Source, a
mid energy DLSR light source with ultralow emittance that has been proposed to
be built adjacent to the China Spallation Neutron Source. Both linear and
nonlinear dynamics are optimized to obtain a detailed design of this type of
lattice. The emittance is minimized, while a sufficiently large dynamic
aperture (DA) and momentum acceptance (MA) are maintained. A design comprising
36 APSU type H7BAs, with an energy of 3 GeV and a circumference of 972 m, is
achieved. The horizontal natural emittance is 20 pm rad, with a horizontal DA
of 5.8 mm, a vertical DA of 4.5 mm, and an MA of 4%, as well as a long
longitudinal damping time of 120 ms. Subsequently, a few modifications are
performed based on the APSU type lattice to reduce the longitudinal damping
time from 120 to 44 ms while maintaining other performance parameters at the
same level.
