Design and Optimization of Combined-Function Quadrupole-Sextupole Magnets

A lightweight superconducting (SC) gantry with large momentum acceptance is under development at Huazhong University of Science and Technology (HUST). Three types of combined-function quadrupole-sextupole (QS) magnets are used to suppress the chromatic dispersion for the large momentum acceptance. This paper introduces the design and optimization of the QS magnets with adjustable sextupole to quadrupole (S/Q) field ratio. The pole shaping method and asymmetric excitation method are discussed in detail for the design. Considering the magnetic field quality deterioration caused by the asymmetry of the pole face, the contour of the pole face and the pole end chamfer are optimized to minimize the harmonics field. After several iterations, the maximum harmonics of the QS2 magnet can be limited within 1E-03. In addition, we investigate the magnetic center shift, when the S/Q ratio changes.