Progress in the Development of Conduction-Cooled REBCO Magnets for Ultrahigh-Field MRI Systems

We started developing REBa₂Cu₃O_7-δ (REBCO) magnets for ultrahigh-field magnetic resonance imaging (MRI) systems in 2013. Our final targets are 9.4 T MRI systems for whole-body and brain imaging. In this paper, a conduction-cooled 1.5 T REBCO MRI magnet having a room-temperature bore of 396 mm was fabricated and tested in order to evaluate the magnetic field homogeneity and stability. The magnet was composed of 60 single pancakes whose inner diameter was 500 mm. The total conductor length was 10.3 km, and the total inductance was 12.4 H. The size of the homogeneous magnetic field region was 200 mm diameter spherical volume. The central magnetic field was as high as 1.5 T at 192.7 A, and the current density of single pancakes was 301 A/mm ². The magnet was cooled from room temperature to 4.7 K in 55 hours, and the temperature difference among the coils was 0.1 K or less during both initial cooling and excitation. The magnetic field inhomogeneity was 249.7 parts per million (ppm), and the Z² coefficient was largest at 579.6 ppm. The main reason for the error magnetic field was dimensional errors in the positions on the z-axis. The magnetic field inhomogeneity was improved to 4.1 ppm by passive shimming using iron pieces. The magnetic field stability was about 2 ppm/h because of a reduction in screening-current induced in the REBCO-coated conductors. Current sweep reversal improved the magnetic field stability from 2 ppm/h to 0.8 ppm/h.