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 y ears 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 tha n 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 f or 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 fiel d magnetization experiments are presented and interpreted in terms of a sim ple model.