Design of a 7.5 T superconducting quadrupole magnet for magnetic circular dichroism experiments

This investigation presents the design of a superconducting quadrupole magnet capable of generating a 7.5 Tesla magnetic field in three directions for X-ray magnetic circular dichroism (XMCD) experiments. The quadrupole magnet has a bore diameter of 25 mm at its center and an outside diameter of 288 mm; and a 4-mm horizontal slit enables photon scattering. High permeability material-holmium pole and superconducting wire-Nb₃Sn were employed to achieve high magnetic field strength in limited space. The magnetic field and structure were optimized to prevent magnet quenching. The mechanical stress under cooling to 4.2 K and magnet magnetization was also considered. During the XMCD experiments, the magnet is installed and allowed to rotate in an ultra-high vacuum chamber. The magnet is thus cryogen-free and it is cooled by a 1.5 W Gifford-McMahon cryocooler. Cryogenic considerations and analysis are also offered.