Optimization of ITER Nb3Sn CICCs for coupling loss, transverse electromagnetic load and axial thermal contraction

Abstract

The ITER cable-in-conduit conductors (CICCs) are built up from sub-cable bundles, wound indifferent stages, which are twisted to counter coupling loss caused by time-changingexternal magnet fields. The selection of the twist pitch lengths has major implications forthe performance of the cable in the case of strain-sensitive superconductors, i.e. Nb3Sn, as the electromagnetic and thermal contraction loads are large but also for theheat load from the AC coupling loss. At present, this is a great challenge for theITER central solenoid (CS) CICCs and the solution presented here could be abreakthrough for not only the ITER CS but also for CICC applications in general.After proposing longer twist pitches in 2006 and successful confirmation by shortsample tests later on, the ITER toroidal field (TF) conductor cable pattern wasimproved accordingly. As the restrictions for coupling loss are more demandingfor the CS conductors than for the TF conductors, it was believed that longerpitches would not be applicable for the conductors in the CS coils. In this paper weexplain how, with the use of the TEMLOP model and the newly developed modelsJackPot-ACDC and CORD, the design of a CICC can be improved appreciably,particularly for the CS conductor layout. For the first time a large improvement ispredicted not only providing very low sensitivity to electromagnetic load and thermalaxial cable stress variations but at the same time much lower AC coupling loss.Reduction of the transverse load and warm-up–cool-down degradation can be reached byapplying longer twist pitches in a particular sequence for the sub-stages, offering a largecable transverse stiffness, adequate axial flexibility and maximum allowed lateral strandsupport. Analysis of short sample (TF conductor) data reveals that increasing the twistpitch can lead to a gain of the effective axial compressive strain of more than 0.3% withpractically no degradation from bending. This is probably explained by the distinctdifference in mechanical response of the cable during axial contraction for short and longpitches. For short pitches periodic bending in different directions with relatively shortwavelength is imposed because of a lack of sufficient lateral restraint of radial pressure.This can lead to high bending strain and eventually buckling. Whereas for cables withlong twist pitches, the strands are only able to react as coherent bundles, beingtightly supported by the surrounding strands, providing sufficient lateral restraintof radial pressure in combination with enough slippage to avoid single strandbending along detrimental short wavelengths. Experimental evidence of goodperformance was already provided with the test of the long pitch TFPRO2-OST2,which is still until today, the best ITER-type cable to strand performance everwithout any cyclic load (electromagnetic and thermal contraction) degradation.For reduction of the coupling loss, specific choices of the cabling twist sequence are neededto minimize the area of linked strands and bundles that are coupled and form loops withthe applied changing magnetic field, instead of simply avoiding longer pitches. In additionwe recommend increasing the wrap coverage of the CS conductor from 50% to at least 70%.A larger wrap coverage fraction enhances the overall strand bundle lateral restraint.The long pitch design seems the best solution to optimize the ITER CSconductor within the given restrictions of the present coil design envelope,only allowing marginal changes. The models predict significant improvementagainst strain sensitivity and substantial decrease of the AC coupling loss inNb3Sn CICCs, but also for NbTi CICCs minimization of the coupling loss can obviously beachieved. Although the success of long pitches to transverse load degradation was alreadydemonstrated, the prediction of the elegant innovative combination with low coupling lossneeds to be validated by a short sample test.