Design and nonlinear optimization of a one-fold lattice for the Super Tau-Charm Facility collider rings

The Super Tau-Charm Facility (STCF) is a proposed high-luminosity electron-positron collider operating at beam energies of 1–3.5 GeV, with a nominal peak-luminosity target exceeding 0.5 × 10 35 cm − 2 s − 1 at 2 GeV. At such low energies, the combined effects of beam-beam interactions in the crab-waist collision scheme, collective effects, and Touschek scattering impose stringent requirements on the dynamic aperture, momentum acceptance, and beam lifetime. This paper presents the design and nonlinear optimization of a one-fold lattice for the STCF collider rings. The design procedure consists of three main steps: lattice-independent optimization of the global machine parameters under luminosity, beam-beam, collective effects, and lifetime constraints; design of a compact interaction region (IR) with local chromatic correction and dedicated crab-waist sextupoles; and full-ring nonlinear optimization. The last step employs a two-stage strategy combining analysis-driven preoptimization with tracking-based refinement. Particular emphasis is placed on controlling momentum- and amplitude-dependent nonlinearities in the IR and preserving the nonlinear transparency of the crab-waist sextupole pair. At 2 GeV, the optimized lattice achieves a theoretical peak luminosity of 1.1 × 10 35 cm − 2 s − 1 while maintaining sufficient dynamic aperture, momentum acceptance, and a Touschek lifetime of approximately 600 s. These results demonstrate the effectiveness of the proposed staged optimization strategy and provide a systematic framework for the design of high-luminosity, low-energy electron-positron colliders.