Optics design of the Super Tau-Charm Facility collider rings

TL;DR

This paper presents a comprehensive optics design for the Super Tau-Charm Facility collider rings, addressing nonlinear effects and optimizing beam dynamics for high luminosity in a low-energy electron-positron collider.

Contribution

It introduces a novel quasi-two-fold symmetric lattice design with multi-objective genetic algorithm optimization to enhance dynamic aperture and momentum acceptance.

Findings

Achieved optimized lattice with minimized nonlinear effects.

Enhanced dynamic aperture and momentum bandwidth through advanced correction schemes.

Robust beam dynamics performance under error conditions.

Abstract

The Super Tau-Charm Facility (STCF), China's next-generation electron-positron collider, targets an unprecedented luminosity exceeding 5x10^34 cm^-2 s^-1 at a center-of-mass energy of 4 GeV. The implementation of a submillimeter vertical beta function at interaction point (< 1 mm) and crab-waist collision scheme in this low-energy regime introduces critical challenges through severe nonlinear effects that constrain dynamic aperture and degrade Touschek lifetime. To address these constraints, we propose a novel quasi-two-fold symmetric lattice design integrating several synergistic features: Linear optics optimization minimizing the H-invariant around the ring to maximize local momentum acceptance (LMA); Up to third-order of local chromaticity correction in the interaction region combined with second-order achromatic arc optics, enhancing off-momentum beam dynamics; Configured FODO arc structure with interleaved sextupole groups satisfying -I transformation, suppressing third-order geometric aberrations while optimizing Montague function distributions; Advanced final focus system integrating chromatic sextupoles, crab sextupoles, and strategically positioned octupoles to counteract final quadrupole fringe fields. Furthermore, we develop a multi-objective genetic algorithm using the in-house toolkit PAMKIT to simultaneously optimize 46 sextupole families, maximizing both dynamic aperture and momentum bandwidth. Optics performance is evaluated under error conditions with appropriate corrections, ensuring robust beam dynamics.