Potential application of magnesium diboride for accelerator magnet applications

This paper reviews the potential of the newly discovered and unexpected 39 K superconductor MgB/sub 2/ as a material for accelerator magnet strands. Unlike the high temperature cuprate superconductors, intergranular current flow in MgB/sub 2/ is not obstructed by weak links, removing one serious obstacle to the fabrication of wires. The critical current density J/sub c/ exceeds 10/sup 5/ A/cm/sup 2/ 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/sub c2/spl perp// 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/sub c/ and H/sub c2/. Magnetization J/sub c/ values at 4.2 K exceed 10/sup 6/ A/cm/sup 2/ in many reports, and approach 10/sup 7/ A/cm/sup 2/ for films with T/sub c/ close to 39 K. In alloyed films with small grain size and high resistivity, the irreversibility field H* (/spl sim/0.85 H/sub c2/) is 16 T at 4.2 K in perpendicular field (and double that in parallel field), and J/sub c/ exceeds 10/sup 5/ A/cm/sup 2/ 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 MgB/sub 2/ wires replicating thin film properties can be made.