Magnetic field dependence of ac losses in multi-filamentary superconducting wires

The magnetic field dependence of ac losses due to nonlinear flux penetration into superconducting filaments, ie, nonlinearity between an applied magnetic field and a penetrated flux, has been studied experimentally for multi-filamentary superconducting wires with Nb-Tl filaments and Cu matrix. In order to observe this effect, ac loss measurements were extensively carried out for the cases of applied transverse ac magnetic fields ranging from 0.06 to 50 Hz in frequency and from 10^?3 to 0.2 T in amplitude. Shifting of the frequency corresponding to the peak of the eddy current loss with the amplitude of applied magnetic fields was revealed experimentally. The results obtained were explained by taking into account the magnetic field dependence of the effective permeability of the wire originated from nonlinear flux penetration into superconducting filaments. The associated frequency dependence of the hysteresis loss is also discussed.