Alternative Lattice Design for the STCF Collider Rings
The Super Tau-Charm Facility (STCF) is a proposed high-luminosity electron-positron collider operating in the beam energy range of 1-3.5 GeV, targeting a peak luminosity larger than $0.5\times10^{35}\ \mathrm{cm^{-2}s^{-1}}$ at 2 GeV. In this regime, the combination of beam-beam interaction in the crab-waist scheme and low beam energy imposes stringent constraints on dynamic aperture, momentum acceptance, and Touschek lifetime. In this paper, we present an alternative one-fold lattice design for the STCF collider rings, developed within a systematic optimization framework. The approach consists of three stages: (i) lattice-agnostic global parameter optimization using a parameter optimization model that consistently incorporates luminosity performance, beam-beam limits, and collective effects; (ii) optics design based on a compact interaction region with local chromatic correction and crab-waist sextupoles; and (iii) global nonlinear optimization combining analysis-driven methods and tracking-based refinement. The optimized lattice achieves the more ambitious luminosity of $1\times10^{35}\ \mathrm{cm^{-2}s^{-1}}$ while maintaining a Touschek lifetime of about 600 s at 2 GeV, with sufficient dynamic aperture and momentum acceptance for stable operation. The results highlight the critical role of local nonlinear control in the interaction region and demonstrate that the proposed optimization strategy provides an effective and general methodology for the design of high-luminosity low-energy colliders.