The magnetic field, temperature, strain and angular dependence of the critical current density for Nb-Ti

Abstract

A scaling law for J c in commercial Nb-Ti wire is proposed that describes its magnetic field, temperature and strain dependence. The scaling law is used to fit extensive measurements of the total strand critical current density, J c,TS(B, T, ε), with the applied field orthogonal to the axis of the wire. We present critical current density, heat capacity and resistivity measurements to obtain B c 2 * ( θ ) , which shows clear angular anisotropy. At 4.2 K, the resistivity data show B c 2 * ( B ∥ J ) − B c 2 * ( B ⊥ J ) ≈ 1 T . We also discuss whether the fusion community should consider re-optimising standard commercial Nb-Ti wires that were developed for MRI applications at ~ 5 T, to produce higher J c at say 10T, and higher upper critical fields, perhaps using quaternary Nb-Ti alloys with artificial pinning centres.

Computational Simulations Using Time-Dependent Ginzburg–Landau Theory for Nb–Ti-Like Microstructures

“…Comparing our simulations at optimum composition, with the experimental J c data at 4.2 K from optimised technological conductors, real materials reach a maximum volume pinning force at roughly 25 vol.% ppt., whereas the optimum found in our model is 32 vol.% ppt. The pinning force in state-ofthe-art conductors Nb-Ti conductors scales with temperature and strain [29], with the maximum occurring at B = 0.5B c2 . At 4.2 K, B c2 = 11 T and the maximum pinning force density is approximately F max p = 17 GN•m −3 [24], which gives an equivalent dimensionless pinning force density in the best materials of F max p /J D B c2 = 4.8 × 10 −3 (given the zero-field depairing current density, J D , at 4.2 K is 3.22 × 10 11 A•m −2 [30]).…”