Transverse load optimization inNb3Sn CICC design; influence of cabling, void fraction and strand stiffness

Abstract

We have developed a model that describes the transverse load degradation inNb3Sn CICCs, based on strand and cable properties, and that is capable of predicting how suchdegradation can be prevented.The Nb3Sn cable in conduit conductors (CICCs) for the International Thermonuclear ExperimentalReactor (ITER) show a significant degradation in their performance with increasingelectromagnetic load. Not only do the differences in the thermal contraction of thecomposite materials affect the critical current and temperature margin, but mostlyelectromagnetic forces cause significant transverse strand contact and bending strain in theNb3Sn layers.Here, we present the model for transverse electro-magnetic load optimization (TEMLOP)and report the first results of computations for the ITER type of conductors, based on themeasured properties of the internal tin strand used for the toroidal field model coil(TFMC). As input, the model uses data describing the behaviour of single strandsunder periodic bending and contact loads, measured with the TARSIS set-up,enabling a discrimination in performance reduction per specific load and strand type.The most important conclusion of the model computations is that the problem of thesevere degradation of large CICCs can be drastically and straightforwardly improvedby increasing the pitch length of subsequent cabling stages. It is the first timethat an increase of the pitches has been proposed and no experimental data areavailable yet to confirm this beneficial outcome of the TEMLOP model. Largerpitch lengths will result in a more homogeneous distribution of the stresses andstrains in the cable by significantly moderating the local peak stresses associatedwith the intermediate-length twist pitches. The twist pitch scheme of the presentconductor layout turns out to be unfortunately close to a worst-case scenario.The model also makes clear that strand bending is the dominant mechanism causingdegradation. The transverse load on strand crossings and line contacts, abbreviated ascontact load, can locally reach 90 MPa but this occurs in the low field area of the conductorand does not play a significant role in the observed critical current degradation. The modelgives an accurate description for the mechanical response of the strands to a transverseload, from layer to layer in the cable, in agreement with mechanical experiments performedon cables.It is possible to improve the ITER conductor design or the operation margin, mainly by achange in the cabling scheme. We also find that a lower cable void fraction andlarger strand stiffness add to a further improvement of the conductor performance.