Practical Nb3Sn superconductive strands of composite material were utilized for a cable-in-conduit conductor (CICC) for ITER central solenoid (CS) which consist of 576 Nb3Sn strands, 288 Cu strands and a stainless-steel jacket. During the manufacture of CS, heat treatment up to 923 K is applied to the CICC for reaction of Sn and Nb in Nb3Sn strands. the CICC is cooled down to approximately 4 K to operate ITER magnets. Thermal strain on Nb3Sn filaments of the strand is induced by large temperature difference and different coefficients of thermal expansion among components of the strands and the jacket. Strain dependence of critical current, which is used for prediction of the conductor performance, is influenced by thermal strain. Therefore, it is important to understand mechanism of the inducing thermal strain on the Nb3Sn filament. To distinguish contributions of the Nb3Sn strand components and the jacket to thermal strain on Nb3Sn filaments, internal strain measurement of the Nb3Sn strand was carried out by using neutron diffraction during cool down from 300 K to 12 K. As the results of the measurement, it was found that compressive thermal strain on Nb3Sn filaments was -0.1% at 300 K and -0.2% at 12 K in axial direction. In this paper, stress-strain state of components of the Nb3Sn strand during cooldown is discussed.
Acknowledgement: We acknowledge the support of the Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, in providing the neutron research facilities used in this work.