Potential application of magnesium diboride for accelerator magnet applications

This paper reviews the potential of the newly discovered and unexpected 39 K superconductor MgB₂ as a material for accelerator magnet strands. Unlike the high temperature cuprate superconductors, intergranular current flow in MgB₂ is not obstructed by weak links, removing one serious obstacle to the fabrication of wires. The critical current density J_c exceeds 10⁵ A/cm² in 1 T field at 4.2 K and in self-field at 20 K. However, the compound is anisotropic and the perpendicular upper critical field H_c2⊥ in present bulk samples is comparable to or less than that in Nb47wt.%Ti at 4.2 K. Thin films have shown significantly higher J_c and H_c2. Magnetization J_c values at 4.2 K exceed 10⁶ A/cm ² in many reports, and approach 10⁷ A/cm² for films with T_c close to 39 K. In alloyed films with small grain size and high resistivity, the irreversibility field H* (~0.85 H_c2) is 16 T at 4.2 K in perpendicular field (and double that in parallel field), and J_c exceeds 10⁵ A/cm² up to 10 T at 4.2 K and 5 T at 20 K. Simple wires, which achieve the properties of sintered bulk samples, are being made by the powder-in-tube route, indicating potential application capability, particularly if alloyed MgB₂ wires replicating thin film properties can be made