Effect of trapped magnetic flux on ac losses of Nb3Sn

The effect of trapped magnetic flux on 60 Hz losses of Nb3Sn was investigated for a number of samples prepared by solid state diffusion and by reacting niobium with liquid tin. In the presence of trapped flux all samples showed a significant increase of loss at low current densities (e.g. from ∼1 to 10μW/cm2at 500 rms A/cm) and a modest decrease of loss at higher current densities (e.g. 30% at 1000 rms A/cm). These features are shown to be consistent with the critical state model provided one takes into account the field dependence of surface currents and assumes a considerable decrease of their magnitude with trapped flux. Even with large amounts of trapped flux the losses of most samples remain below 10μW/cm2at 500 rms A/cm, an acceptable loss level for ac power transmission applications.