Resent Progress on MgB2 Wires

Densification of MgB$_{\text{2}}$ core in an in situ PIT MgB$_{\text{2}}$ wire is one of the key factors to increase J_c values. Hot pressing, cold pressing before a heat treatment and hot isostatic pressing (HIP) are effective in densifying the core and thus, increasing J_c values. Mechanical alloying or mechanical milling of Mg+B precursor powder by a high-energy ball milling is also effective in increasing the core density and J_c values. Upper critical field H_c2 is another important parameter. Carbon doping to MgB$_{\text{2}}$ not only increases H_c2 but also decreases the anisotropy in H_c2, both of which are effective in enhancing J_c of MgB$_{\text{2}}$ wires in magnetic fields. The doping of some kinds of hydrocarbon or carbohydrate to a Mg+B precursor powder and the use of carbon-coated B powder as a starting material is effective for in situ PIT MgB$_{\text{2}}$ wires. Internal Mg diffusion (IMD) method much increases the MgB$_{\text{2}}$ filling density and J_c higher than 10⁵ A/cm² is obtained at 20 K and 4 T by using carbon-doped B powder. Solenoid coils were fabricated with IMD wires and coil tests were carried out. Concerning to AC loss of MgB$_{\text{2}}$ wires, cabling of small diameter mono-core MgB$_{\text{2}}$ wires is one of the effective methods to reduce both hysteresis loss and coupling loss. Superconducting joints of unreacted C-doped multi-filamentary wire showed joint resistance of 10⁻¹⁴ Ω at 20 K. But joint I_c was only 1/4 of wire I_c. A joint of reacted mono-core wire was reported to have resistance of 10⁻¹⁴ Ω at 20 K, 1 T. Excellent joint I_c of 78% of wire I_c were obtained.