Monofilament MgB₂ Wire for a Whole-Body MRI Magnet: Superconducting Joints and Test Coils

This paper presents recent results from our continued development of a 0.5 T whole-body MRI magnet at the Francis Bitter Magnet Laboratory. HyperTech Research Corp. (Columbus, OH) manufactures the MgB₂ conductor for this project. During the past year, we have found that our technique, originally developed successfully to splice unreacted multifilament MgB₂ wires, works much better, i.e., of higher reliability, with unreacted monofilament MgB₂ wires. This has led us to wind the entire coil components in our persistent-mode MRI magnet with unreacted monofilament MgB₂ wire, having a MgB₂ core of 0.4 mm in diameter, an overall diameter of 0.8 mm bare, 1 mm S-glass insulated. To verify that these coils would not suffer from flux jumping, as they would if wound with monofilament NbTi wire, magnetization studies were performed on monofilament wires of MgB₂ and NbTi (as a reference) at 4.2 K. For the monofilament MgB₂ wire, the results were affirmative. To further ensure the absence of flux jumping that may quench these current-carrying coils, two test coils were wound with unreacted monofilament MgB₂ wire. One MgB₂ coil was operated in driven mode, while the other MgB₂ coil, equipped with a persistent current switch and terminated with a superconducting joint, was operated in persistent mode. The operating temperature range was 4.2-15 K for these MgB₂ coils. The driven mode coil was operated in self-field. The persistent mode coil achieved a persistent current of 100 A, corresponding to a self-field of ~1 T in the winding, for 1 hour with no measurable decay. Both test coils were operated quench free.