A solution for transverse load degradation in ITER Nb3Sn CICCs: verification of cabling effect on Lorentz force response
We present the latest results of the novel model for transverse electromagnetic load optimization (TEMLOP) especially developed for the ITER type of cable-in-conduit conductors (CICCs). The Nb3Sn CICCs for the International Thermonuclear Experimental Reactor (ITER) showed a substantial degradation in their performance correlated with increasing electromagnetic load. Not only do the differences in the thermal contraction of the composite materials affect the critical current (Ic) and temperature margin, but electromagnetic forces cause a significant transverse strand contact and bending strain in the Nb3Sn layers, resulting in localized filament cracking and permanent degradation.The most essential feature of the a priori TEMLOP predictions presented in May 2006 is that the severe degradation in CICCs can be improved greatly and straightforwardly by increasing the pitch length in subsequent cabling stages and by reducing the void fraction. These corrective measures give more support to the strands, sufficiently reduce the strain, and therefore avoid filament damage at the strand crossover points in the cables. It was the first time that an increase of the cable twist pitches has been proposed and no experimental evidence was available at that time. A full-size European prototype TF conductor sample (TFPRO-2), manufactured in autumn 2006, was adapted according to this new insight and tested in April 2007 in SULTAN for experimental validation of the predictions. The results were outstanding: for the first time an Nb3Sn CICC conductor achieved the performance that can be expected based on the single-strand properties, with high n value and no sign of degradation. As input, besides the cable properties, the model directly uses the measured data from single strands under uni-axial stress and strain, periodic bending and contact loads. The recent test results of the ITER OST strands used for the manufacture of the TFPRO-2 obtained with the TARSIS set-up are presented. With these most recent strand results, the model substantiates that not only strand bending is causing degradation but, depending on the strand and cable layout, the strand contact stress can also play a critical role. TEMLOP demonstrates that the twist pitch scheme and void fraction, of the proposed ITER reference TF conductor layout with a first-stage triplet twist pitch of 45 mm, turns out to be practically a worst-case scenario. It is also shown that shorter pitches can lead to an improvement but this requires more Nb3Sn material per metre composite conductor. However, it has been experimentally proven now that the proposed changes recover the ITER TF conductor operational margin up to the expected strand performance. The ITER TF conductor specification is being adapted now and it becomes possible to gain significant savings on the strand design, as degradation no longer needs to be compensated for.