Design and Optimization of the Superconducting Quadrupole Magnet Q1a in CEPC Interaction Region

To study the properties of Higgs, scientists from the Institute of High Energy Physics have proposed to build the Circular Electron Positron Collider (CEPC) and published a conceptual design report in 2018. In order to improve the collision luminosity of CEPC, the upgrade of the final focus system in the interaction region is on-going. According to the latest physical requirements of superconducting magnets in the CEPC interaction region, a high gradient, double-aperture quadrupole magnet Q1a is required, which is 1.9 m from the interaction point. In this paper, we present the magnetic design of superconducting quadrupole magnet Q1a, and discuss the advantages and disadvantages of the three kinds of quadrupole coil structures, including cos2 theta coil, CCT coil, and racetrack coil. Low-temperature superconductor NbTi is used in our magnet design process. In addition, optimization is performed to greatly reduce the weight of the magnet.