A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi2Sr2Ca2Cu3Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.

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A project on the development of REBa2Cu3O7−δ (REBCO) magnets for ultra-high-field magnetic resonance imaging (MRI) was started in 2013. Since REBCO-coated conductors feature high mechanical strength under tensile stress and high critical current density, use of REBCO coils would allow superconducting magnets to be made smaller and lighter than conventional ones. In addition, a conduction-cooled superconducting magnet is simpler to use than one cooled by a liquid helium bath because the operation and maintenance of the cryogenic system become simpler, without the need to handle cryogenic fluid. Superconducting magnets for MRI require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coil shape and position. Moreover, in REBCO magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field, a 1 T model magnet and some test coils were fabricated. From an evaluation of the 1 T model magnet, it was found that the main reason for magnetic field inhomogeneity was the tolerances in the z-axis positions of the coils, and therefore, it is important to control the gap between the single pancakes. In addition, we have already demonstrated the generation of an 8.27 T central magnetic field at 10 K with a small test coil. The screening-current-induced magnetic field was 0.43 T and was predictable by using an electromagnetic field simulation program. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems. The magnet was composed of six main coils and two active shield coils. The total conductor length was 581 km, and the stored energy was 293 kJ. The field inhomogeneity was 24 ppm peak to peak and 3 ppm volume-root-mean-square (VRMS) for a 500 mm diameter spherical volume (DSV). The axial and radial 5 gauss line locations were less than 5 m and 4.2 m respectively.

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