CRITICAL-CURRENT ANISOTROPY IN CONVENTIONAL AND ARTIFICIAL PINNING CENTER ROUND WIRE NB-TI SUPERCONDUCTORS

Critical current density anisotropy was detected in conventional and a rtificial pinning center (APC) Nb-Ti round wires by means of a relevan t increase of the magnetization M as the wire aspect ratio A(w) (lengt h L/filament diameter d(f)) was increased. However, contrary to what i s observed in APC wires, the critical current density J(parallel to) c alculated from the SQUID measured saturated-magnetization in conventio nal wires using an anisotropic Bean Model (BM), is significantly diffe rent from the transport critical current density J(ct). We suggest tha t the mismatch between SQUID measured J(parallel to) and J(ct) is due a combination of the discontinuity of the ribbons along the wire lengt h in conventional Nb-Ti wires and also to the low electric fields gene rated in SQUID measurements. The low electric fields allows the magnet ization-induced currents to redistribute inside the wire due to the lo cal inhomogeneities associated with the presence of alpha-Ti ribbons. In this case anisotropic BM cannot be used to extract J(c) from M beca use it will be function of the intrinsic lengths of these ribbons.