Critical current density and low field losses in Nb3Sn

Assuming a dependence of critical current density J
c
on magnetic field H of the form J
c
=βH
^-p
, where β and p are positive constants, ac losses have been calculated and compared with measured losses in multifilamentary Nb
3
Sn. For p=0.5, good agreement is found for fields parallel to the filament axes, provided a strongly field dependent surface current is taken into account. A small discrepancy between critical current densities calculated from losses and those measured directly may arise from the irregular filament shape. In transverse field, surface effects can be neglected but good agreement with theory is found only when filament cross-linkage is absent. Comparison of calculated and directly measured critical current densities may indicate that the theory requires a minor modification. Losses measured as a function of bias field show a well-defined minimum which, in parallel field, agrees with a surface screening model. In transverse field no agreement with theoretical models is found but the influence of the reversible magnetisation may be observable.