Symmetric double-double bend lattice for a potential EUV diffraction limited upgrade of the HLS

NSRL recently proposed a future plan to further upgrade the HLS to an EUV diffraction-limited storage ring, named HLS-III. In this paper, a symmetric double-double bend lattice with long and mid-straight sections is studied as a highly promising design for the HLS-III storage ring. The design achieves an ultra-low natural emittance of 2.82 nm·rad at 800 MeV, while maintaining the current eight straight sections but with significantly reduced beta functions in these straights. By minimizing the fluctuation of resonance driving terms, the nonlinear dynamics optimization yields a large horizontal dynamic aperture of about 40 mm. Additionally, error and intra-beam scattering effects are evaluated.