Strain redistribution effects on current-sharing measurements on straight samples of large Nb3Sn cable-in-conduit conductors

Abstract

Recent results on short samples of ITER-type Nb3Sn cable-in-conduit conductors have shown degradation, with the current-sharing temperature dropping slowly over several thousand current cycles. However, although such behaviour has been linked to the magnetic loads on the strands, which cause filament fracture and plastic yielding of the surrounding copper, the detailed examination of the results shows a number of inconsistencies. These suggest that the degradation may be exaggerated by artefacts of the short sample and may provide an over-conservative assessment of the in-coil performance. The behaviour of a short sample and its frictional interaction with the jacket have been analysed to consider the impact of local material modulus changes. Both an analytical approach and a finite-element simulation have been used. This paper shows that, if transverse magnetic loading causes a local reduction in the longitudinal elastic modulus of the cable in the high-field region, then the thermal compression along the cable becomes non-uniform, with a higher compression in the high field. The process develops with load cycles, due to the jacket/cable friction. If the strain change is interpreted using Jc, T, B and the strain scaling law, the predicted current-sharing temperature drop is similar to those observed during testing.