Examination of design parameters affecting Nb₃Sn CICC current sharing temperature using necessary condition analysis

Abstract

The performance of Nb3Sn cable-in-conduit conductor (CICC) especially its current sharing temperature (T cs) is known to depend on the various parameters of the CICC such as the Nb3Sn strand characteristics, its cabling, the final geometry of the CICC cross section, the void fraction, the current distribution and other parameters. However, the performance has not been very predictable. The change in CICC T cs performance with repeated operation or electromagnetic loading is of particular concern. Furthermore, evaluation of T cs performance with respect to parametric variation of some CICC parameters has resulted in conflicting conclusions for different CICC designs. Thus, a more comprehensive review of published CICC T cs performance with respect to the corresponding CICC cable characteristics has been performed and is reported here for CICC samples whose designs and test results have been published during the past decade and more. Necessary condition analysis as well as linear regression is applied in the interpretation of the results which has found that the first stage cable twist pitch which has been the focus of much research is not that significant in determining the T cs performance but does affect T cs degradation. The direct effect of strand critical current on T cs performance was also found to be weak. On the other hand, void fraction was found to be significant in both determining T cs performance and limiting T cs degradation. Other correlations between parameters have also been studied. Overall, it seems that there has been appropriate targeting as well as overemphasis on some CICC parameters for improving Nb3Sn CICC T cs performance. No single parameter studied has been found to be a determining factor for high stable CICC T cs performance. Research on additional parameters is warranted.

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“…However, since Nb 3 Sn is a brittle and strain sensitive material, serious degradation of the T cs of Nb 3 Sn CICCs were observed under cyclic electromagnetic load. After a series of investigations on performance degradation of CICCs under high Lorentz force, fracture of superconducting Nb 3 Sn filaments inside strands were found to be the main reason [24][25][26][27][28][29][30].…”