The MRI industry is reliant on persistent-mode superconducting electromagnets for high magnetic fields and increased stability. Achieving persistent coils is entirely dependent on the formation of high quality superconducting joints, currently made using Pb-Bi solder. Whilst this is an effective solution, there is legislative pressure to remove the lead even from medical instruments. There is also a technical driver to explore new joints in that the critical field of Pb-Bi is much lower than that of the NbTi wires, so joints must be located in regions that experience fields of 1 T or lower, meaning they must be removed from the highest field regions of the magnets. Recent studies of alternative lead-free solders have found no drop-in replacement for Pb-Bi, with even the best of these new alloys having much lower Bc2 values . Other lead-free approaches such as spot welding and cold pressing lack the reproducibility required for MRI production . We are developing new approaches for joint fabrication, using NbTi as the current-carrying intermediary material between commercial NbTi wires. Several designs of joints have been analysed by SEM, SQUID magnetometry and high current measurements and compared against the performance of Pb-Bi soldered joints. This presentation will compare the properties of these different joint strategies, and suggest which are the most promising directions for the development of lead-free persistent mode joints.
TD is supported by an EPSRC CASE studentship with Siemens Magnet Technology.
1. Mousavi et al., Supercond. Sci. Technol., 29, 2016
2. Brittles et al., Supercond. Sci. Technol., 28, 2015
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