Superconducting Joint and Persistent Current Switch for a 7-T Animal MRI Magnet

The fabrication and test of the multifilament NbTi superconducting joints and NbTi/CuNi persistent current switch for a 7-T animal magnetic resonance imaging magnet are presented. The joints were fabricated by the solder matrix replacement method in the open air environment, and the samples were tested by a measuring facility that was developed based on the current-decay method and can supply a background magnetic field up to 0.6 T. The measured results indicate that the NbTi/Cu sample joints can carry a current up to 1000 A with a resistance lower than 3 ×10^-14 Ω, while the NbTi/CuNi sample joint can carry a current up to 630 A with a resistance lower than 4 ×10^-14 Ω under a magnetic field of 0.6 T. The thermally triggered switch was designed based on a two-dimensional axisymmetric finite-element analysis thermal model that is built with the commercial code ANSYS. It has an off-state resistance of 5 Ω and an equal switch-off to recovery time of about 2.5 s at a heating current of 120 mA.