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

Abstract

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 A/sub w/ (length L/filament diameter d/sub f/) was increased. However, contrary to what is observed in APC wires, the critical current density J/sub /spl par// 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 J/sub ct/. We suggest that the mismatch between SQUID measured J/sub /spl par// and J/sub ct/ 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 /spl alpha/-Ti ribbons. In this case anisotropic BM cannot be used to extract J/sub c/ from M because it will be function of the intrinsic lengths of these ribbons.