Filament-size dependent critical current of multifilamentary Nb3Sn conductors

The critical current density
j\min{c}\max{ov}
of multifilamentary Nb
3
Sn conductors depends on the diameter of the filaments. At low fields (less than about 12 T)
j\min{c}\max{ov}
increases with decreasing filament diameter because of the finer grain size. At high fields
j\min{c}\max{ov}
increases with increasing filament diameter. Measurements of the critical current under externally applied tensile stress revealed that changes of the internal compression of the Nb
3
Sn within the composite account for this effect. The larger the filament diameter, the smaller the compressive strain, the higher the upper critical field and therefore
j\min{c}\max{ov}
at high fields. The decrease of compressive strain with increasing filament size results from a decrease in the yield stress of the bronze which is caused by an increase in grain size of the bronze between the filaments.