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 t he internal tin process. Conductors were composed with seven sub-eleme nts, made up with NbTa7,5wt% filaments, surrounded by a Nb/Ta diffusio n barrier to isolate the core from the copper shell. Effects of 9 diff erent heat treatments (HT) on superconducting properties such as criti cal current densities, critical temperatures, critical magnetic fields and hysteresis losses have been investigated. The use of heat treatme nts adapted to the strand design showed an increase in critical curren t up to 48 % with only a slight increase in the level of hysteresis lo sses. 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 o f the filaments into Nb,Sn and also by the refinement of Nb3Sn grains controlled by particular conditions of germination. As a result, criti cal current density Jc(non Cu) = 800 A/mm(2) at 12 T with hysteresis l osses Q(non Cu+/-3T)=490 mJ/cm(3) have been measured.