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

Abstract

A scaling law for Jc 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, Jc,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 ∗ c2(θ), which shows clear angular anisotropy. At 4.2 K, the resistivity data show B ∗ c2(B k J) − B ∗ c2(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 Jc at say 10 T, and higher upper critical fields, perhaps using quaternary Nb-Ti alloys with artificial pinning centres.