AC losses in multifilamentary Nb3Sn conductors

We have studied losses at 4.2K in a number of multifilamentary Nb and Nb3Sn wires fabricated by the bronze route. Losses due to alternating current and to transverse alternating field showed a fourth power dependence on current or field which is incompatible with the critical state model, even when the observed field dependence of the bulk critical-current density is taken into account. In order to reveal the source of the low losses in our materials more clearly, we have studied losses with the field parallel to the filament surface. As expected, the results show that losses in Nb filaments are low because of the presence of the Meissner state. However, superficially similar results for Nb3Sn filaments cannot be interpreted as evidence for an anomalous lower critical field; a more likely explanation is the presence of a surface current. Although a simple surface current model cannot account in detail for the transverse field losses, one incorporating losses due to surface roughness, field orientation effects and the field dependence of the surface current can probably provide an adequate description.