Relation between constituent material fraction in multifilamentary MgB2 wires and requirements for MRI magnets

Magnetic resonance imaging (MRI) occupies the largest segment of the commercial applications of superconductivity. The NbTi wire is typically applied to MRI magnets and fulfils their strict requirements. On the other hand, the dramatically large energy margin in the MgB2 wire is attractive for liquid helium-saving MRI magnets. However, there are many types of cross-sections in the MgB2 wires. This makes it difficult to analyse the applicability of the MgB2 wires to the MRI magnets systematically. This paper focuses on the in situ MgB2 wires with an iron matrix and a Monel reinforced member. Multiple evaluations are conducted for several types of MgB2 wires, and their applicability to the MRI magnets is discussed. Because the critical current density of the superconducting filaments does not largely depend on the cross-section of the wires, the engineering critical current density (J (e)) is roughly proportional to the superconducting fraction (lambda (sc)). The acceptable bending strain of the heat-treated wires increases with the Monel fraction and is in the range of 0.3%-0.65%, which is larger than the value required for coil winding of the MRI magnets. Two types of protection approaches of the magnet are considered. One is an active protection. This approach requires a large fraction of the copper stabilizer in the cross-section of the wire and relatively reduces lambda (sc) and J (e). The other is the avoidance of quenches over the product lifetime using quick ramp-down of the magnet for unfortunate events, such as cooling system failure and emergency rundown. This approach requires no copper stabilizer and increases lambda (sc) and J (e) thus widens the acceptable operational temperature range. The cross-section of the MgB2 wire can be designed with a certain level of freedom depending on its functional requirements.