EXCESS MAGNETIZATION DUE TO INTERFILAMENTARY PROXIMITY COUPLING IN NB-TI MULTIFILAMENTARY WIRES

It is shown theoretically that the excess magnetization of a multifilamentary superconducting wire caused by interfilamentary proximity coupling, DELTA-M(p), is proportional to the twist pitch of the wire, 1. It is also shown that this theoretical prediction can be confirmed experimentally for Nb - Ti multifilamentary wires. The critical current density of the super-conducting shielding current flowing through the matrix, which becomes a weak superconductor due to interfilamentary proximity coupling, is given by J(cp) = J(o)(B/B(c2p))-1.4 x [1 - (B/B(c2p)]1.4 with J(o) infinity [cosh(d(N)/2xi-N)]-2, where B is the magnetic flux density, and B(c2p)'xi-N and d(N) are the upper critical flux density, the effective coherence length and the width of the weakly superconducting matrix, respectively. It is concluded that DELTA-M(p) can be estimated quantitatively by measuring the 1 dependence of total magnetization, and also that the most effective method of decreasing DELTA-M(p) is to decrease xi-N to the dirty limit.