Heat treatment effect on the strain dependence of the critical current for an internal-tin processed Nb3Sn strand

A comparative study on the effects of heat treatment, especially, the duration of the A15 reaction temperature plateau on the strain dependence of the critical current for an internal-tin processed Nb3Sn strand has been carried out. The strain dependence of the critical current is measured by a variable temperature Walter spiral probe that we have developed. It was shown that prolonged heat treatment can be a very effective way to improve the strain dependency. For a quantitative analysis, the measured data were analyzed with various proposed scaling laws: the scaling law based on strong-coupling theory, the modified deviatoric strain scaling law, and the interpolative scaling law. We found that there is a slight increase in the critical temperature and a substantial improvement in the maximum pinning force. The origin of improved strain dependency is further discussed.