This invention has applications in magnetic resonance imaging for medical diagnostics, nuclear magnetic resonance imaging for biomedical research, as well as other areas where superconducting magnets are used.
Most existing MRI machines have low-temperature superconducting magnets made from multifilament niobium-titanium (NbTi) wires operating at 4K. These magnets require cooling with liquid helium, which is quickly becoming a scarce and expensive commodity. Reacted magnesium diboride (MgB2) is an attractive alternative material which is capable of operation at higher temperatures, and which enables the use of larger-diameter monofilament wires. However, the lack of a reliable method of making superconductive joints between MgB2 wires in their reacted form is an obstacle to wider adoption of MgB2 as a magnet winding material. Existing manufacturing techniques require coils to be wound from unreacted wire, and entire magnets to undergo heat treatment to react the wires, driving up production costs and increasing scrap rates due to imperfect joints that cannot be repaired.
This invention demonstrates a novel method to form superconducting joints between reacted MgB2 wires.
A superconductive joint can be formed between two ends of reacted MgB2 wire by shearing the ends at acute angles and inserting them into the cavity of a stainless steel billet which is filled with a powdered mixture of magnesium and boron. A copper plug is then pressed into the cavity, partially sealing the billet. Finally, the billet is completely sealed using a ceramic paste and heat treated. In tests conducted on both short lengths of wires and test coils, magnets fabricated using this technique have been shown to be capable of quench-free operation at fields up to 90% of their critical currents, despite evidence of multiple flux jumping events. Additionally, they were shown to be capable of carrying persistent currents of up to 100 A while subjected to self-fields up to 0.94 T. This suggests that monofilament MgB2 wire is a viable material option for the fabrication of persistent-mode superconducting magnets.
Technique for joining reacted wires allow react-and-wind manufacturing workflow
Reduces reliance on liquid helium
Resistant to quenching despite occurrence of flux jumping