Low field flux jumping in high performance multifilamentary Nb/sub 3/Al and Nb/sub 3/Sn 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/sub c/ may be increasing at a rate faster than effective filament diameter, d/sub eff/, can be reduced. This is particularly true for Nb-clad melt-processed Nb/sub 3/Al strands which at fields below H/sub c2,Nb/ have subelements which are superconductively coupled via the Nb matrix. Control of d/sub eff/, while still requiring attention, is less of a problem for the new very high J/sub c/ Nb/sub 3/Sn 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.