Upper critical field in multifilamentary Nb3Sn conductors

The critical current of bronze processed multifilamentary Nb3Sn conductors was measured at 4.2 K up to 23 T. The flux pinning theory of Kramer does not fit the results at flux densities higher than about 18 T. The critical field B*c2extrapolated from data below 18 T is smaller than the field Bc2at which superconductivity is completely suppressed. From independent measurements of the temperature dependence of Bc2on the same samples (using very small current) Bc2at 4.2 K can be calculated by applying the GLAG theory. A nonvanishing resistivity at fields close to Bc2is evidence of inhomogeneities which considerably broaden the transition.