A New Generation of In Situ ${\rm MgB}_{2}$ Wires With Improved $J_{\rm c}$ and

By means of Cold High Pressure Densification (CHPD), the critical current density, J_c, of binary and alloyed MgB₂ wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB₂ mass density of binary MgB₂ wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C₄H₆O₅ (malic acid) alloyed wires, densification also caused the enhancement of B_irr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic J_c values were obtained for C₄H₆O₅ added wires of 1 × 0.6 mm² cross section, the values of J_c(4.2 K)=1 × 10⁴ A/cm² for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of B_irr^// at 20 K was 11 T. The positive effects of cold densification on J_c and B_irr on MgB₂ was also observed on 150 mm long wires alloyed with C₄H₆O₅ (malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding J_c(4.2 K) = 1 x 10⁴ A/cm² at 13.1 T. Further improvements are expected after optimization.