Persistent-mode operation and magnetization behavior of a solid-nitrogen-cooled MgB2 small-scale test coil towards a tabletop 1.5-T osteoporosis MRI. (29th October 2020)

• Record Type: Journal Article
• Authors: Choi, Yoonhyuck; Park, Dongkeun; Li, Yi; Tanaka, Hiromi; Lee, Wooseung; Bascuñán, Juan; Iwasa, Yukikazu
• Is Part Of: Superconductor science & technology. Volume 33:Number 12(2020:Dec.)
• Publication Date: 2020-10-29
• Subjects: current density redistribution -- H formulation -- magnetic resonance imaging -- magnetization -- persistent-mode operation -- screening-current; Superconductivity -- Periodicals; Superconductors -- Periodicals; 537.623
• DOI: 10.1088/1361-6668/abba07
• Languages: English
• ISSNs: 0953-2048

Abstract

Abstract: 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 <0.6 ppm h −1, which is still considered low enough to meet the temporal stability requirement of <0.1 ppm h −1, for most magnetic resonance imaging magnets. This decay rate translates to a calculated circuit resistance of <1.79 × 10 –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.