Copper impurity in diffusion Nb3Sn tapes

Nb3Sn tapes are prepared by the method of diffusion under high vacuum. Tapes are also prepared under identical conditions with copper impurity (0.5-45 at.%) added into the tin. The critical current of the Nb3Sn tapes, measured at 4.2K and in a magnetic field of 30 kOe, is found to increase with copper impurity by a factor of as much as 2 for 1 at.% copper. Further additions of copper, on the other hand, result in an almost logarithmic decrease of the critical current. The latter observation is in disagreement with previous results.