Investigation on Thermal Strain in Full Size ITER Conductors

The TF magnetic system of the international thermonuclear experimental reactor (ITER) will be composed of 18 superconducting coils assembled in a torus. The design of these large coils relies on the use of a circular cable-in-conduit conductor (CICC) composed of a cable of about one thousand twisted Nb3Sn and copper strands embedded in a thin steel jacket. The different thermal contractions of the materials composing this conductor during the cool down from the Nb3Sn heat treatment reaction temperature (about 920 K) to the operating temperature (about 5 K) result in a compressive strain on the superconducting Nb3Sn filaments which leads to a degradation of the conductor critical current. The qualification of the ITER conductor prototypes will be performed by testing short straight full size samples in the unique European test facility SULTAN at PSI (Switzerland). A comparison of the performance of the toroidal field model coil (TFMC) conductor in a short straight sample and in the coil has shown indications that the sample performance is better, apparently because the thermal strain in Nb3Sn is lower. It looks possible that this could have been caused by a slippage between the cable and the jacket in the joints at the sample ends during the heat treatment. In this case, these kind of short straight samples could not be relevant for conductor qualification. An investigation on the thermal strain by destructive examination of different TFMC conductor samples, including one leg of the TFMC short sample tested in 1999 in the SULTAN facility, was performed at CEA Cadarache. The paper reports on the different strain measurements, proposes an analytical approach of the results in order to conclude on the relevance of straight samples.