High Critical Current Density Multifilamentary MgB₂ Strands

Magnesium diboride multifilamentary strands consisting of 7, 19, 37, and 61 filaments were fabricated using a Nb chemical barrier, 101 Cu as an internal stabilizer, and either monel or oxide-dispersion-strengthened Cu (ODS Cu) as an outer sheath. The various strands were given single-step heat treatments of 700degC for 20-40 minutes. Transport critical current density, Jc, measurements were performed at 4.2 K on 1-meter lengths of strand on ITER barrels in fields of up to 15 T. Higher temperature transport values at 0-12 T were subsequently measured on short samples. Typical Jcvalues were at 1times105A cm-2at 5 T and 4.2 K and similar values at 2 T and 20 K. The use of ODS Cu as the outer sheath material improved the ductility and stability of the strand without much sacrifice in transport Jc. Optimization of transport properties through variation of the Mg/B stoichiometry was also studied. Pinning force curves at various temperatures were analysed.