The Effect of Thermo-Mechanical Treatments on $J_{c}(T,B)$ and $T_{\cs}$ of Nb-Ti Strands

An extended database of critical current densities J _c(T,B) and current sharing temperatures T_cs for Nb-Ti strands was obtained in this work. This data set allows one to make a more realistic forecast of the ITER poloidal field (PF) coil critical current at temperatures above 6 K and in a magnetic field of 6 T. It is found that Jc (4.2 K, 5 T) lies in the range from 700 to 2950 A/mm², depending on the heat treatment regimes, and that J_c(6.5 K, 6 T) is 28 to 54 A/mm² almost regardless of the applied heat treatment, i.e. the microstructure does not significantly affect J_c(6.5 K, 6 T) under such ldquoextremerdquo conditions. It is shown, that T _cs (6 T) at a current of 31.2 A, which is close to the assumed operating current for the strand in the ITER PF1&6 coils, is 6.1-6.25 K. It is shown furthermore, that the application of multiparametric functions for the description of J _c(T, B) or V(I, T, B) leads to a reliable prediction of J_c(T, B) only in a temperature and field range, where experimental data points are available (extrapolations result in large deviations).