Persistent-mode operation and magnetization behavior of a solid-nitrogen-cooled MgB2 small-scale test coil towards a tabletop 1.5-T osteoporosis MRI

We present results—cool-down, energization, and persistent-mode operation—of a solid-nitrogen (SN2)-cooled, magnesium diboride (MgB2) small-scale test coil. The test coil, immersed in a volume of solid nitrogen at 6 K, successfully operated in persistent-mode at 108 A for a period of 5 d. Although designated a ‘persistent-mode’ coil, its center field was measured to decay at a rate of −1, which is still considered low enough to meet the temporal stability requirement of −1, for most magnetic resonance imaging magnets. This decay rate translates to a calculated circuit resistance of –12 Ω, which is mainly from one MgB2-MgB2 joint in the circuit. However, when the coil temperature increased from 6 to 16 K, the field had dropped by 0.33%: we believe this was caused by the change of magnetization in the MgB2 superconductor, which in turn decreased a screening-current field (SCF) at the magnet center. We performed a finite element analysis with a simplified numerical model based on H formulation to verify whether magnetization-induced SCF is responsible for this 0.33% drop. Indeed, the model shows that the change of magnetization, i.e. screening-current reduction and current density redistribution, happens during temperature-cycle-induced Jc (T) variation, and thus affects the center magnetic field. However, the Jc (T) variation in the 2nd cycle had little effect on MgB2 magnetization and thus had negligible magnetic field change.