Losses in laminar composite Nb3Sn at 60 Hz

A detailed study has been made of the ac loss in cylindrical samples of Nb3Sn carrying a transport current at 60 Hz. Samples were prepared by a vacuum codeposition process. At low currents, up to 550 A/cm at 4.2 °K in some samples, the loss is dominated by a surface fluxoid pinning mechanism, varies no faster than the square of the current, and in many cases less rapidly, and is temperature independent. At higher currents, the loss is described by the critical-state model. Formation of the superconductor in cylindrical layers separated by another material provides enhanced bulk fluxoid pinning, thereby increasing the critical current density and decreasing the loss at high currents. The magnitude of the ac loss in this material up to 10–12 °K is comparable to that reported in niobium at 4.2 °K.