Development of low-AC-loss Bi-2223 superconducting multifilamentary wires

We have been designing and developing advanced Ag-sheathed Bi-2223 multifilamentary wires for the future applications to superconducting transformers of electric rolling stocks. Trial wires were twisted without remarkable degradation in the transport property for suppressing AC losses in a perpendicular magnetic field configuration. The AC losses were measured by a saddle-shaped pickup coil in a wide range of frequency up to a commercial frequency. The experimental results show that the AC loss per cycle has linear frequency dependence in a region of the practical amplitude of AC magnetic field for the transformers, where the excess loss may come from an electromagnetic coupling between the filaments. The hysteresis loss evaluated as a low frequency limit of the AC loss was also reduced with the decrease in the twist pitch, which may be considered as a similar effect to the so-called "twist effect" observed in in- situ Nb₃Sn wires.