Upper critical field in multifilamentary Nb3Sn conductors

The critical current of bronze processed multifilamentary Nb
3
Sn 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*
c2
extrapolated from data below 18 T is smaller than the field B
c2
at which superconductivity is completely suppressed. From independent measurements of the temperature dependence of B
c2
on the same samples (using very small current) B
c2
at 4.2 K can be calculated by applying the GLAG theory. A nonvanishing resistivity at fields close to B
c2
is evidence of inhomogeneities which considerably broaden the transition.