Low field flux jumping in high performance multifilamentary Nb3Al and Nb3Sn composite strands

The problem of flux jumping in NbTi superconducting strands was solved 30 years ago by the introduction of suitably fine multifilamentary subdivision accompanied by twisting. But with recent advances in A15 superconductors we are entering a regime in which J(c) may be increasing at a rate faster than effective filament diameter, d(eff), can be reduced. This is particularly true for Nb-clad melt-processed Nb3Al strands which at fields below H-c2,H-Nb have subelements which are superconductively coupled via the Nb matrix. Control of d(eff), while still requiring attention, is less of a problem for the new very high J(c) Nb3Sn strands whose sub-elements, although fully bridged, are still able to be reduced in size. The results of some low field magnetization experiments are presented and interpreted in terms of a simple model.