Joints in MgB2 superconducting coils

Thesis

Superconducting materials have been utilized for over a century, with applications expanding in various fields, including Magnetic Resonance Imaging (MRI) systems, which require highly stable magnetic fields. These systems rely on superconducting coils that maintain a persistent current to generate stable magnetic fields at operating temperatures below the material’s critical temperature (T_c). Currently, most MRI systems use lowtemperature Nb-Ti superconductors, cooled to 4.2 K with liquid helium (LHe). However, the growing demand for such systems has increased helium consumption and costs, prompting the search for alternative materials. MgB2, with its higher critical temperature (39 K), offers a promising solution for helium-free superconducting magnets.

This thesis presents a method for fabricating persistent joints using monofilament reacted MgB2 wires. The process involves inserting the wires into pre-drilled holes filled with unreacted Mg+B powder, followed by pressing to ensure close mechanical contact. A twostep heat treatment is then applied to optimize density, connectivity, and reduce nonsuperconducting impurities. The resulting joints demonstrate excellent connectivity, achieving residual resistances below 10^-13 Ω. Persistent currents of 172 A at self-field and 160 A under a 1 T background field were measured, corresponding to a critical current ratio (CCR) of 78% at 20 K. These results indicate the feasibility of producing persistent-mode MgB2 magnets operating below 20 K, which could significantly reduce costs for MRI systems using cryogen-free technology.

Additionally, for the first time in the literature, a superconducting joint has been produced between Nb-Ti wires using an MgB₂ filler. A coil fabricated with this joint achieved an ultralow resistance of approximately 4.2 × 10^-15 Ω and a trapped current of 7 A under 4.7 K and self-field conditions. This study highlights the potential of MgB2 filler as a non-toxic alternative to the Pb-based filler currently used in Nb-Ti wire joints.

Authors: Guven, M

• DOI: 10.5287/ora-ppvv5vqya
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English