Performance Evaluation of NbTi Superconducting Joints Fabricated by Mechanical Pressing for Persistent Current Mode Operation

Abstract

A mechanical pressing technique to join multifilament niobium-titanium (NbTi) superconducting wires is proposed and experimentally verified. The fabricated joints enable superconducting magnets to operate in persistent current mode without conventional solder joints. Finite element analysis (FEA) was used to design a copper mold capable of withstanding compressive forces of up to 14 tons without mechanical failure. Two NbTi wires with diameters of 0.7 mm and 0.9 mm were prepared by chemically etching the copper matrix at the wire ends, after which the ends were mechanically compressed by applying different loads (10, 12, and 14 tons) to join the wires. Electrical characterization at 4.2 K showed that compression under high pressure significantly improved the superconducting performance. The critical current of the joints increased from ~ 10 A at 10 tons to ~ 140 A at 14 tons, and the n-value (current-voltage index) rose from ~ 3 to ~ 32. Cross-sectional scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) revealed that, at 14 tons, the NbTi filaments of the two wires were highly consolidated without intervening copper, whereas at lower pressures, the filament boundaries were clearly visible. These results demonstrate that NbTi joints compressed under appropriate conditions can achieve superconducting properties suitable for sustainable magnet operation. The joint fabrication technique based on mechanical compression provides an alternative to solderless NbTi superconductor joints and, subject to further optimization, is potentially applicable to magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) magnets.