AC loss properties of Cu-sheathed MgB₂ composites with Nb barriers

We designed and fabricated Cu-sheathed MgB₂ composites with Nb barriers for low AC loss property. The wires were produced with powder-in-tube method after mixing Mg and B powders. The wires have a diameter of 0.7 mm and six MgB₂ filaments. Each filament has a thin Nb barrier to suppress reaction between Mg and Cu. The volume ratio of the filaments is 0.13. The averaged critical current of an about 1-m coiled specimen with a twist pitch of 10 mm is 55 A at 1 T and liquid helium temperature. The AC losses of the coiled specimen exposed to the applied magnetic field parallel to the coil axis were measured by a standard pickup-coil method in liquid helium. The ranges of the amplitude and the frequency were extended up to 1 T and 5 Hz, respectively. The AC loss property including the frequency dependence was also estimated for the multifilamentary structure with a usual theoretical expression of the coupling loss among the filaments. It is found that the experimental results observed cannot be explained by the theoretical prediction. We discuss the discrepancy from viewpoint of a coupling among the filament with the Nb barrier. The theoretical estimation shows that the six filaments surrounding the Nb barrier are equivalent to a cylindrical hollow superconductor.