Results of Thermal Strain and Conductor Elongation Upon Heat Treatment for ${\rm Nb}_{3}{\rm Sn}$ Cable-in-Conduit Conductors

The change of length upon heat treatment of steel jacketed, Nb₃Sn cable-in-conduit conductors (CICC) is driven by the difference of coefficient of thermal expansion between Nb₃Sn and the other components. The cold work of the steel jacket also plays an important role. During the preparation of CICC samples for test in the SULTAN facility, the change in length has been systematically measured for 16 sections of CICC. The broad range of changes, from elongation to shrinkage, is a concern for the manufacture of the ITER toroidal field (TF) coils, where the shape/length of the conductor after heat treatment must be exactly predicted to fit into the radial plates. For a reliable assessment of the performance of Nb₃Sn CICC, the axial, thermal strain in the Nb₃Sn filaments is a key parameter. To gain information on the subject, the residual strain on the steel jacket is systematically measured by strain gauges on 28 ITER CICC sections tested in SULTAN. The range of results is very broad and no clear correlation is found with the conductor performance, i.e. with the thermal strain in the filaments. To gain more inside about the actual thermal strain in the Nb₃Sn filaments embedded in a CICC, neutron diffraction techniques have been applied (with limited success) to deduce the thermal strain from the direct, in situ measurements of the lattice parameter.