Critical current anisotropy in conventional and artificial pinningcenter round wire Nb-Ti superconductors

摘要

Critical current density anisotropy was detected in conventional and artificial pinning center (APC) Nb-Ti round wires by means of a relevant increase of the magnetization M as the wire aspect ratio Aw (length L/filament diameter df) was increased. However, contrary to what is observed in APC wires, the critical current density J|| calculated from the SQUID measured saturated-magnetization in conventional wires using an anisotropic Bean model (BM), is significantly different from the transport critical current density Jct. We suggest that the mismatch between SQUID measured J|| and Jct is due to a combination of the discontinuity of the ribbons along the wire length in conventional Nb-Ti wires and also to the low electric fields generated in SQUID measurements. The low electric fields allows the magnetization-induced currents to redistribute inside the wire due to the local inhomogeneities associated with the presence of α-Ti ribbons. In this case anisotropic BM cannot be used to extract Jc from M because it will be function of the intrinsic lengths of these ribbons