Experimental checking of an AC losses model for superconducting twisted cable-in-conduit

In superconducting machines, such as tokamaks for fusion experiments, the coils are subjected to fast variations in magnetic field. Superconductors have then to be designed in order to reduce a.c. losses to an acceptable level. It is therefore necessary to understand the influence of the design parameters on a.c. losses. For that purpose, an analytical model for calculating a.c. losses in twisted cable-in-conduit conductors has been developed. This model takes into account the strand characteristics and the detailed cabling pattern. To perform an experimental checking, a systematic analysis on subsize-cables, representative of the first three stages of the conductor that is foreseen for the toroidal field coils of ITER, has been performed. To see the influence of the strand on the cable losses, these cables have been made with various Nb3Sn composites, the other parameters being kept as constant as possible. This study includes a.c. losses measurements on composites and on sub-size cables, as well as inter-strand resistance measurements. Thanks to the model, the loss measurements are cross-checked to the resistance measurements. On the other hand, we correlate these resistance measurements to the strand inner characteristics thanks to a finite element code. The effect of the resistive barrier location is discussed.