Abstract
Superconducting wires subjected to AC magnetic field dissipate losses. Recently developed AC wires with ultra-fine superconducting filaments and short twist-pitch have extremely low AC losses for the transverse field. However, AC losses of windings are mainly caused by longitudinal and azimuthal magnetic field components in AC superconducting apparatuses such as current limiters and transformers, the windings of which are subjected to relatively low transverse external field less than IT. Those non-transverse magnetic field components are produced by the wire transport currents and the external magnetic field. AC wires with low transverse field AC losses are not necessarily the wires with low AC losses caused by non-transverse magnetic field components. Technology for reducing the transverse field AC losses has been clarified but that for reducing the non-transverse field AC losses is not clear at the present state. To develop the technology for reducing the non-transverse field AC losses, it is important to analyze the relation between the loss characteristics and the configuration of the wire cross-section.
Energy dissipation of a superconducting wire subjected to external field with transport current is supplied from current source of the wire and the external field. We measured power supplied from the current source to various NbTi and Nb3Sn AC wires with different cross-section configurations and different twist pitches nder external AC magnetic field. We made an analytical model to evaluate the on-transverse field AC losses of the wires based on the experimental data. In the model, current density vs. electric field characteristics of the superconducting filaments are taken into account. The validity of the model is verified by comparing the calculated and measured results. In the paper, it is shown that the AC losses caused by the non-transverse field are evaluated by the analytical model and become much higher than those caused by the transverse magnetic field as the transport currents are close to the quench currents or critical currents. It is also shown that the losses are significantly dependent on the phase difference between the wire transport current and the back ground field and that our analytical model well explains the experimental results on the phase difference characteristics.
