  Heat treatment is critically important to the performance of Nb3Sn
superconducting strands. For very large Nb3Sn magnet coils, such as the
International Thermonuclear Experimental Reactor (ITER) Central Solenoid (CS)
coils, heat treatment carries risk of temperature and time excursion, which may
result in performance degradation. Therefore, it is prudent to study the effect
of possible excursion on Nb3Sn performance. In this study, Nb3Sn strands used
for ITER CS coils are heat treated at different temperatures for different
times. Their critical current, residual resistance ratio and hysteresis losses
are measured. It is found that in the range we studied, critical current and
hysteresis losses do not change significantly. Residual resistance ratio,
however, decreases with increasing heat treatment temperature and time. This is
attributed to the diffusion of metallic elements from the plated Cr layer to
the copper stabilizer. Based on a model of metallic elements diffusion, a
numerical code is developed to predict residual resistance ratio as a function
of heat treatment temperature and time.
