Measurement of AC losses in MgB2 wire and bulk conductors at different temperatures

AC loss measurements were carried out on MgB₂ wire and bulk conductors at temperatures between 25 K and 40 K in applied field up to 0.2 T transverse to conductor length. For bulk conductors in the shape of a flat plate (3 mm × 0.5 mm), loss behavior of a thin strip Q∝H₀⁴ was observed. A reduction of width to 1 mm leads to a change in loss behavior similar to a round wire with Q∝H₀³. For the ex-situ MgB₂ wire of Ni sheath, losses were totally dominated by the hysteresis of ferromagnetic Ni. While the losses in the superconducting core is greatly reduced by the strong magnetic shielding by the Ni sheath, a small increase in the Ni loss was observed upon superconducting transition in the core. This is attributed to the compression of magnetic flux due to the diamagnetic superconducting core. Such an interpretation is supported by measurements of PbBi2223 tape in a cavity of an iron block.