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Description
Achieving the design goal of a collision luminosity of 1035 cm-2s-1 for the Super Tau-Charm Facility (STCF) poses a critical challenge in optimizing the performance of the injected positron damping ring. This damping ring must damp a 1 GeV positron beam with a large injected transverse emittance (>1400 nm·rad) to an extracted transverse emittance below 11 nm·rad within the 166.7 ms storage time. This imposes stringent requirements on the magnetic field distribution accuracy, damping efficiency, and dynamic aperture of the magnet system. This paper proposes a damping ring design based on combined-function quadrupole-sextupole(QS) magnets. This design achieves an effective balance among key parameters such as emittance control, damping time, and dynamic aperture. Based on this concept, two types of QS magnets with adjustable sextupole to quadrupole field ratios are designed. The paper elaborates on their design and multi-objective optimization methods, with a particular focus on the application of the pole shaping method and asymmetric excitation method. By optimizing the pole face, the impact of higher-order harmonic fields is effectively suppressed. Multiple iterations demonstrate that the major higher-order harmonics in QS magnets are suppressed to the order of 2×10-4, meeting the damping ring's tolerance for magnetic field errors. This provides a reliable technical foundation for high-luminosity positron beam injection.
