AC loss and contact resistance of different CICC cable patterns: Experiments and numerical modeling

For upcoming nuclear fusion energy reactors, like the China Fusion Engineering Test Reactor (CFETR) and EU-DEMO, the superconducting Cable-In-Conduit Conductors (CICC) are in the design phase, and the operatingconditions like electromagnetic forces can be higher than in previous devices like ITER. The prototype con-ductors for the Central Solenoid (CS) coils in the CFETR, for example, are designed to produce a peakfield of19.9 T and are expected to be made of high current density Nb3Sn strands. Investigations are also ongoing on theapplication of bismuth strontium calcium copper oxide (BSCCO) and MgB2strands for CICCs in fusion reactors.The latter material, MgB2,could be applied for superconductors subjected to lower magneticfields, such asPoloidal Field coils, Correction Coils, and Feeders. The performance of all these strands is sensitive to strain, andthe mechanical strength of the brittlefilaments is relatively weak. This requires a thorough analysis of the cablepattern in terms of the mechanical support of the strands along their length in combination with the mini-mization of the interstrand coupling currents and strand indentation. As an initial step tofinding the mostappropriate cable pattern for CICCs, three prototype CICCs made of ITER type Nb3Sn strands with significantlydifferent cable twist patterns are tested experimentally for AC coupling loss, interstrand contact resistance, andstrand indentation. The three cabling patterns referred to as the Twente, CWS (copper wound superconductingstrand), and CFETR-CSMC (CFETR Central Solenoid Model Coil) design. The numerical code JackPot ACDCdeveloped at the University of Twente is used to analyze the interstrand coupling loss and contact resistance. Thenew ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) triplet modified CWS design is aimed atreducing strand pinching during cabling, which causes degradation of transport properties during compactionand cyclic loading. The Twente design has the same objective but also aims at reducing the coupling loss whilemaximizing the mechanical lateral support for the strands by making the twist pitch ratio of the sequentialcabling stages close to one. The CFETR-CSMC, taken as a reference for comparison, has cable a pattern mostlysimilar to the ITER CS cable design.