Critical Currents and Superconducting Boundary Effects in S-N-S Multifilamentary Composites

Abstract

The proximity effect critical current density (J_cp) has been measured for number of NbTi multifilamentary composites with Cu matrices. Representative strands with filamentary Nb barriers of 0, 2, and 4% of the filamentary radius have been tested. For most strand types, several strand diameters have been taken, and several twist pitches of each have been measured. In all cases, L ≫ L_p. All measurements are made using a vibrating sample magnetometer in stepping mode, with field wait times equal to 5 minutes or greater. J_cp values as a function of fields of up to 16 kOe are reported. Measurements for different strand designs and strand manufacturers are discussed and intercompared. Several results were found. First, the presence of Nb barriers with thicknesses which are similar to or greater than the coherence length in the Nb enhance J_cp. Additionally, all strands without Nb barriers have comparable J_c,s in zero field at corresponding values of interfilamentary separation (d_N). However, while two of the strand types had quite similar field dependencies, the field dependence of the third strand type was very different.