Current (re-)Distribution inside an ITER Full-Size Conductor: a Qualitative Analysis

Abstract

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.