Development of high gradient quadrupole magnets in HEPS storage ring

The High Energy Photon Source is the first fourth-generation light source in China, featuring a hybrid multibend achromat lattice. This innovative design incorporates high gradient quadrupoles with an aperture of 26 mm, a gradient of about 80 T / m , and high-order field harmonics of less than 4 × 10 − 4 . This paper details the optimized magnetic design, precise fabrication techniques, batch field measurements, and a method for field harmonic compensation utilizing a magic finger. A comparative analysis was conducted on nine types of the 480 pure quadrupole magnets that share the same cross section but differ in iron lengths. These variations include laminated or solid cores and closed or open yokes. The cross section, characterized by a narrow concave and convex pole contour, a tapered pole root, and saddle coils, is designed to mitigate magnetic saturation while enhancing the iron filling ratio. The impact of iron saturation on the transfer function and the evolution of current-dependent multipole fields has been systematically investigated, along with the effects of mechanical errors and inhomogeneities in the magnetic properties of the iron core on harmonics. We demonstrate a comprehensive engineering solution for high gradient small-aperture quadrupole magnets, which has been validated through mass production.