Transverse cable stiffness and mechanical losses associated with load cycles in ITER Nb3Sn and NbTi CICCs


Nijhuis, A. and Llyin, Y. (2009) Transverse cable stiffness and mechanical losses associated with load cycles in ITER Nb3Sn and NbTi CICCs. Superconductor science and technology, 22 (5). ISSN 0953-2048

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The flexible nature of the cable bundles in the sizeable cable-in-conduit-conductors for ITER containing more than a thousand strands, in combination with a void fraction of around 30%, gives scope for significant cable compression and strand deflection. In particular, the transverse stiffness of the Nb3Sn type of cabled superconductors, being subjected to large electromagnetic forces, is critical for their long-term performance considering the impact of the strain variation on the transport properties. What is more, the compression of the cable bundle under load and the permanent deformation and relaxation in time or that associated with quenches, have an effect on the cooling and pressure drop along the turns of the windings and are valuable to account for in large magnets such as for ITER. The electromagnetic AC losses of ITER Nb3Sn and NbTi CICCs, related to changing magnetic field and in this manner important for their stability, were broadly studied and reported but the associated mechanical losses have received less attention so far. The lifetime characteristics in terms of cable compression, changes in transverse stiffness and mechanical losses are experimentally determined on several prototype ITER NbTi and Nb3Sn conductors in the Twente press and a summary of the results is given. The nonlinear stress–strain characteristics of the cable bundle and its moderate time-dependent nature can be considered as a viscoelastic–plastic phenomenon. The evolution of the stiffness and the mechanical loss depends on the peak load, void fraction, strand type and strand coating and changes with the number of load cycles. The dissipated heat from mechanical energy is not a critical issue for ITER magnet operation but is not negligible, in particular in the case of NbTi conductors.
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