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, J_c, 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 J_c values were at 1times10⁵ A cm^-2 at 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 J_c. Optimization of transport properties through variation of the Mg/B stoichiometry was also studied. Pinning force curves at various temperatures were analysed.