Effect of heat treatments on superconducting properties of Nb3Sn strands developed at GEC ALSTHOM

Two different Nb3Sn strands have been processed at GEC ALSTHOM using the internal tin process. Conductors were composed with seven sub-elements, made up with NbTa7,5wt% filaments, surrounded by a Nb/Ta diffusion barrier to isolate the core from the copper shell. Effects of 9 different heat treatments (HT) on superconducting properties such as critical current densities, critical temperatures, critical magnetic fields and hysteresis losses have been investigated. The use of heat treatments adapted to the strand design showed an increase in critical current up to 48 % with only a slight increase in the level of hysteresis losses. The low variation of losses is correlated to filament locations and bridging distributions across the conductor section. The critical current enhancement is achieved under a more complete transformation of the filaments into Nb,Sn and also by the refinement of Nb3Sn grains controlled by particular conditions of germination. As a result, critical current density Jc(non Cu) = 800 A/mm(2) at 12 T with hysteresis losses Q(non Cu+/-3T)=490 mJ/cm(3) have been measured.