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The comprehension of the current re-distribution phenomenon inside multi-filamentary conductors is a crucial point for the design of ITER-relevant coils, as it is by now assessed that current non-uniformity among cable sub-stages may strongly deteriorate Cable-in-Conduit Conductors (CICC) performances. The only feasible way to get information about the current flowing inside CICC sub-stages is an indirect evaluation by self-field measurements in regions very close to conductor surface. A 7m full-size NbTi conductor (Bus-Bar III) has been used as short-circuit during the test of an ITER Toroidal Field Coil HTS current lead at FzK. Its relatively simple shape and the absence of any other magnetic field source (background coils, etc.), made BBIII one of the most desirable candidate for a reliable measurement of the current distribution under controlled conditions. This is why it has been ad hoc instrumented with different arrangements of Hall-probes (rings and arrays), as well as with transverse and longitudinal voltage taps. This paper gives a qualitative interpretation of the current (re-)distribution events inside the conductor as derived from the analysis of the Hall sensors and the voltage taps signals, during Tcs measurements and as a function of different dI/dt. It has been shown that Hall probes represent a very reliable tool to investigate this issue. In fact, re-distribution phenomena have been clearly observed during transition, and even far before reaching Tcs, when voltage transverse signals had not yet showed any appreciable onset.

The tests analysis of the International Thermonuclear Experimental Reactor (ITER) model coils showed lower performances than expected. For that reason, a specific program has been launched in Europe for investigating the influence of bending strain on the transport properties of high-performances Nb3Sn strands inside cable-in-conduit conductors when experiencing significant Lorentz forces. To this purpose, an experimental campaign has been initiated on single Nb3Sn strands jacketed inside a 0.2 mm thick stainless-steel (SS) tube in order to be tested in mechanical ITER Toroidal Field (TF) Coils relevant conditions. In this article we describe in a first part the procedure investigated at CEA to impose a controlled pure bending strain upon jacketed stands and we present the results of the qualification tests carried out in the ENEA facility to assess the reliability of the chosen method. In a second part we propose a new model to simulate the strands behaviour experiencing bending strain together with an initial compressive strain. To understand our results, a key parameter considered is a specific weighting function relying on strand interfilament current redistribution capacity. At the end, this model enables us to explain more reliably the experimental results but also shows discrepancies which requires further improvements and extra refinements

A facility for the characterization of superconducting strands at variable temperatures has been recently upgraded at ENEA. Measurement of transport properties of superconducting strands can be now carried out in the temperature range from 3.5 K to 15 K, with stability less than 10 mK, and in magnetic fields up to 12.5 T, covering a range of critical currents between a few Amperes, up to about 300 A. In this study we present the results of a measurement campaign carried out on NbTi wires, aimed also at comparing the performances of some available candidate strands, to be used in the design of the Toroidal Field Coils of the JT60-SA Tokamak, as well as in the design of the ITER high field Poloidal Field Coils, PF1 & 6. We measured strands with different diameters, and with Cu-nonCu ratios ranging between about 1.5, up to about 7. The measurements showed the critical behavior of the NbTi in the range of relatively high temperatures (T > 5.5 K) and high magnetic fields (B > 6 T), typical for large coils in fusion reactors, where Cable-in-Conduit Conductors are used and where temperature margins of at least 1K are required for a sound design. The experimental results highlighted the need for an optimization of the numerical fits normally used to predict NbTi critical current densities, starting from measurements performed at 4.2 K. Based on our results, we conclude that fit parameters for each NbTi strand should be inferred from experimental characterizations in the temperature and magnetic field range of interest for the design.