Design and integration of a crab-waist interaction region for the Super Tau-Charm Facility

摘要: The Super Tau-Charm Facility (STCF) is a next-generation electron-positron collider being developed in China, designed to achieve a peak luminosity exceeding 5×10^34 cm^(-2) s^(-1) at the optimal beam energy of 2 GeV. Achieving this goal relies on a crab-waist collision scheme with a large crossing angle-a configuration that introduces strong nonlinearities, severely constraining the dynamic and momentum apertures. This paper presents a comprehensive physics design for the STCF interaction region (IR) that systematically addresses these challenges. The design features a modular optics framework that incorporates local chromaticity correction up to the third order, exact -I transformations between chromatic sextupole pairs for nonlinear cancellation, and minimization of the dispersion invariant to improve local momentum acceptance. Optics at the crab sextupoles are optimized to reduce their strength and associated nonlinearities. When integrated into the collider ring, the design achieves a Touschek lifetime of over 300 s at 2 GeV, meeting the STCF requirement. Furthermore, fringe fields from superconducting quadrupoles are partially compensated using octupole correctors, and detector solenoid effects are fully suppressed via local anti-solenoids. The machine-detector interface layout is also optimized to avoid synchrotron radiation background at the interaction point. This IR design represents the current optimized solution for STCF and has been incorporated into the project's conceptual design report.