THE INFLUENCE OF THE DIFFUSION BARRIER ON THE AC LOSS OF NB3SN SUPERCONDUCTORS

Nb3Sn superconductors as they are applied in the ITER fusion programme , are equipped with diffusion barriers made of V, Ta, and V-Nb, all ha ving superconducting properties. If an external ac magnetic field is a pplied, superconducting shielding currents are induced in the barrier which enclose a bundle of Nb3Sn filaments. As a consequence they are s hielded and no ac loss will occur in the filaments. When the penetrati on field of the barrier material is exceeded, additionally the loss of the barrier and for higher fields also the loss of the Nb3Sn multifil amentary zone is generated. As long as the barrier is superconducting it will cause a substantial increase of ac loss. As a consequence the ac loss of the conductor in terms hysteresis loss per cycle and coupli ng time constants are strongly influenced. This aspect has to be consi dered carefully when properties of Nb3Sn conductors are determined at low magnetic fields.