Transverse load optimisation in Nb3Sn CICC design; influence of cabling, void fraction and strand stiffness

Abstract

We have developed a model that describes the transverse load degradation in Nb3Sn CICCs, based on strand and cable properties, and that predicts how such degradation can be prevented. We present the model for Transverse Electro-Magnetic Load Optimisation (TEMLOP) and report the first results of computations for ITER type of conductors, based on the measured properties of the internal tin strand used for the Toroidal Field Model Coil (TFMC). The most important conclusion of the model computations is that the problem of the severe degradation of large CICCs can be drastically and straightforwardly solved by increasing the pitch length of subsequent cabling stages. It is for the first time that an increase of the pitches is proposed and no experimental data are available to confirm this outcome of the TEMLOP model. Larger pitch lengths will result in a more homogeneous distribution of the stresses and strains in the cable by significantly moderating the local peak stresses associated with the intermediate-length twist pitches. The twist pitch scheme of the present conductor layout turns out to be unfortunately close to a worst case scenario. It is possible to improve the ITER conductor design or the operation margin, by mainly a change in the cabling scheme. We also find that a lower cable void fraction and larger strand stiffness add to a further improvement of the conductor performance.