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A number of models for the critical surface of Nb3 Sn, and in general A15 superconductors, have been developed in the past years. This paper compares the most common parameterizations using consistent notation. Although the parameterizations appear dissimilar at first sight, they are in reality all based on a fit of the normalized pinning force vs. the reduced field, and have similar scalings for the critical field and critical temperature based on a Unified Scaling Law. In this paper we take the various parameterizations as a basis for a generic scaling proposed for the characterization and production follow-up of the ITER Nb3 Sn strands. The accuracy of the scaling is estimated using the fitting residuals on various sets of Ic (B, T, ϵ) data available in literature. We discuss the results, and give our view of the work towards a unified, practical parameterization. [ABSTRACT FROM AUTHOR]

The role of inter-bundle resistive barriers (metal sheet wraps), introduced to reduce the inter-bundle coupling loss in multistage cabled Cable-In-Conduit Conductors (CICCI for the International Thermonuclear Experimental Reactor (ITER) is evaluated, based on results gained recently on short sample experiments in the Twente Cable Press and SULTAN. The obvious benefit of limiting the inter bundle coupling loss unavoidably goes together with impeding the redistribution of nonuniform currents in the coil winding introduced at the terminations, as well as reduction of the heat exchange between the bundles. Six-element numerical electromagnetic code simulations are presented that qualitatively explain the effect of wraps on the DC performance, strongly depending on the testing geometry. The computations illustrate that wraps can reduce the DC performance in short sample tests. At the same time simulations of the Poloidal Field Coil Insert (PFCI), with a winding length of 50 m, have shown that omitting sub-stage wraps, can even degrade the DC performance of coils due to the short current transfer length in combination with current nonuniformity causing peak voltages in the most overloaded petals. [ABSTRACT FROM AUTHOR]

The ITER model coil experiments have revealed that degradation of the critical current (Ic) and n index occurred in large Nb3Sn CICC's and the larger the electromagnetic force, the larger was the degradation. However, such degradation was not observed in the Nb3Al CICC. The authors developed a new model, in which the degradation of the Ic and n index of an individual strand was taken into account, to interpret these results. Analytical model corrected from experimental results of a single strand is used in this model to estimate the degradation of each strand. The calculation results are in good agreement with the CICCs' test results. In addition, it is indicated that in case of the Nb3Sn CICC, the normal transition occurs from the strands at low field side in CICC cross section, where transverse load due to the electromagnetic force is the largest, resulting in the significant degradation in the Ic and the n index. In contrast, in the case of the Nb3Al CICC, the normal transition takes place from the strands at high field in the CICC cross section, where the transverse load is small, resulting in no degradation in the Ic and the n index. [ABSTRACT FROM AUTHOR]