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

Abstract

• The AC loss and interstrand contact resistance measured on three prototype Nb 3 Sn Cable-In-Conduit Conductors with significantly different cabling twist pitch patterns. • Simulations, done by using the JackPot model are in fair agreement, confirming the consistency of data and allowing reliable predictions with the JackPot model. • For CICC cables, a design petal wrap coverage of 70 % should be maintained for CS cables to reduce the AC loss. A critical threshold value of void fraction exists, which makes the presence of petal wraps ineffective in reducing coupling loss. • The JackPot simulations with derived corrections for inter-petal contact resistance representing 70 % petal wrap coverage show that the Twente cable design has the lowest coupling loss. • So far, the Twente cable design seems a suitable candidate for CFETR or DEMO conductors, made of strain-sensitive strands such as Nb 3 Sn or other materials, for minimization of coupling loss and strand indentation. 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 operating conditions like electromagnetic forces can be higher than in previous devices like ITER. The prototype conductors for the Central Solenoid (CS) coils in the CFETR, for example, are designed to produce a peak field of 19.9 T and are expected to be made of high current density Nb 3 Sn strands. Investigations are also ongoing on the application of bismuth strontium calcium copper oxide (BSCCO) and MgB 2 strands for CICCs in fusion reactors. The latter material, MgB 2, could be applied for superconductors subjected to lower magnetic fields, such as Poloidal Field coils, Correction Coils, and Feeders. The performance of all these strands is sensitive to strain, and the mechanical strength of the brittle filaments is relatively weak. This requires a thorough analysis of the cable pattern in terms of the mechanical support of the strands along their length in combination with the minimization of the interstrand coupling currents and strand indentation. As an initial step to finding the most appropriate cable pattern for CICCs, three prototype CICCs made of ITER type Nb 3 Sn strands with significantly different cable twist patterns are tested experimentally for AC coupling loss, interstrand contact resistance, and strand indentation. The three cabling patterns referred to as the Twente, CWS (copper wound superconducting strand), and CFETR-CSMC (CFETR Central Solenoid Model Coil) design. The numerical code JackPot ACDC developed at the University of Twente is used to analyze the interstrand coupling loss and contact resistance. The new ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) triplet modified CWS design is aimed at reducing strand pinching during cabling, which causes degradation of transport properties during compaction and cyclic loading. The Twente design has the same objective but also aims at reducing the coupling loss while maximizing the mechanical lateral support for the strands by making the twist pitch ratio of the sequential cabling stages close to one. The CFETR-CSMC, taken as a reference for comparison, has cable a pattern mostly similar to the ITER CS cable design.