The tokamak fusion reactor ITER, involving the largest and most integrated superconducting magnet system ever built, relies on superconducting strands used in the so-called Cable-In-Conduit conductors (CICC). The flexible nature of the cable bundles in the sizeable conductors gives scope for cable compression and thus substantial local strand deflection during the enormous electromagnetic loads. The various Nb3Sn CICCs tested at the start of the ITER construction, showed a substantial degradation in their performance due to the electromagnetic load on the strands and cycling as well. For predicting full-size conductor performance, Nb3Sn strands were characterized at various axial strain, temperature and magnetic field values to identify their critical limits and occurrence of micro cracks. The effects of spatial periodic loading by bending, crossing strands and a homogeneously distributed load on the transport properties were compared. The experimental results enabled discrimination in performance reduction per specific type of applying force and per type of Nb3Sn strands. A cryogenic cable press was built enabling investigation of the lifetime issues of full-size ITER conductors for 40,000 cycles at 4.2 K showing significant changes in interstrand contact resistance, coupling loss and mechanical properties with cycling. A new model called Transverse Electro-Magnetic Load Optimisation (TEMLOP) has been developed for analyzing the transverse load degradation in Nb3Sn CICCs, based on measured strand and cable properties. The model predicted that the observed severe degradation in CICCs, which was mainly caused by strand bending, can be avoided straightforwardly by either significantly decreasing or increasing the pitch length in subsequent cabling stages and by reducing the void fraction. A full-size prototype conductor sample, manufactured in autumn 2006, was adapted according the new insight and tested April 2007 in the Sultan conductor test facility in Switzerland for validation of the predictions. The results were outstanding. For the first time a Nb3Sn CICC conductor achieved the performance expected from the single strand properties, with a steep transition and no noteworthy degradation. Accordingly, the design of the ITER Toroid Field conductor was modified, allowing limited degradation for the benefit of AC loss reduction, such to meet the ITER requirement for all samples of a current sharing temperature higher than 5.8 K after 1000 electromagnetic cycles.
|Award date||7 Jul 2016|
|Place of Publication||Enschede|
|Publication status||Published - 7 Jul 2016|