Fabrication and Test of Nb3Sn-NbTi Superconducting Joint for a 14-T MRI Magnet

The 14-T magnetic resonance imaging (MRI) superconducting magnet utilizes reinforced multifilament Nb₃Sn wires and multifilament NbTi wires to generate the required magnetic field. The resistances of the Nb₃Sn-NbTi superconducting joints are crucial to the temporal field stability of the magnet operation in persistent mode. The resistance of each joint is required to be lower than 10⁻¹¹ Ω under a 0.5 T background magnetic field to keep the temporal field drift lower than 0.1 ppm/hour. This paper presents the fabrication of multifilament Nb₃Sn-NbTi superconducting joints and the measurement of their resistances. The joints were fabricated by the solder matrix replacement method using PbBi as the solder in an open-air environment. The samples were tested by an apparatus that was developed based on the current-decay method and can provide a background field up to 1 T. The sample loop with a Nb₃Sn wire and a NbTi wire contains two Nb₃Sn-NbTi joints. The Nb₃Sn wire undergoes the heat treatment before the fabrication of superconducting joints with the NbTi wire. The measured results indicate that the joints can carry a current up to 809.8 A with a resistance lower than 6.20 × 10⁻¹³ Ω without a background field. Under a 1 T background field, the joints can carry a current up to 481.5 A with a resistance lower than 1.24 × 10⁻¹² Ω, which meet the requirement of the 14-T MRI superconducting magnet.