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The ITER cable-in-conduit conductors (CICCs) are built up from sub-cablebundles, wound in different stages, which are twisted to counter coupling losscaused by time-changing external magnet fields. The selection of the twistpitch lengths has major implications for the performance of the cable in thecase of strain sensitive superconductors, i.e. Nb3Sn, as the electromagneticand thermal contraction loads are large but also for the heat load from the ACcoupling loss. Reduction of the transverse load and warm-up cool-downdegradation can be reached by applying longer twist pitches in a particularsequence for the sub-stages, offering a large cable transverse stiffness,adequate axial flexibility and maximum allowed lateral strand support. Analysisof short sample (TF conductor) data reveals that increasing the twist pitch canlead to a gain of the effective axial compressive strain of more than 0.3 %with practically no degradation from bending. For reduction of the couplingloss, specific choices of the cabling twist sequence are needed with the aim tominimize the area of linked strands and bundles that are coupled and form loopswith the applied changing magnetic field, instead of simply avoiding longerpitches. In addition we recommend increasing the wrap coverage of the CSconductor from 50 % to at least 70 %. The models predict significantimprovement against strain sensitivity and substantial decrease of the ACcoupling loss in Nb3Sn CICCs, but also for NbTi CICCs minimization of thecoupling loss can be achieved. Although the success of long pitches totransverse load degradation was already demonstrated, the prediction of thecombination with low coupling loss needs to be validated by a short sampletest.

Optimisation of ITER Nb3Sn CICCs for coupling loss, transverse electromagnetic load and axial thermal contraction