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Title: Metallographic autopsies of full-scale ITER prototype cable-in-conduit conductors after full testing in SULTAN: 1. The mechanical role of copper strands in a CICC

Cables made with Nb3Sn-based superconductor strands will provide the 13 T maximum peak magnetic field of the ITER Central Solenoid (CS) coils and they must survive up to 60,000 electromagnetic cycles. Accordingly, prototype designs of CS cable-in-conduit-conductors (CICC) were electromagnetically tested over multiple magnetic field cycles and warm-up-cool-down scenarios in the SULTAN facility at CRPP. We report here a post mortem metallographic analysis of two CS CICC prototypes which exhibited some rate of irreversible performance degradation during cycling. The standard ITER CS CICC cable design uses a combination of superconducting and Cu strands, and because the Lorentz force on the strand is proportional to the transport current in the strand, removing the copper strands (while increasing the Cu:SC ratio of the superconducting strands) was proposed as one way of reducing the strand load. In this study we compare the two alternative CICCs, with and without Cu strands, keeping in mind that the degradation after SULTAN test was lower for the CICC without Cu strands. The post mortem metallographic evaluation revealed that the overall strand transverse movement was 20% lower in the CICC without Cu strands and that the tensile filament fractures found were less, both indications of an overall reductionmore » in high tensile strain regions. Furthermore, it was interesting to see that the Cu strands in the mixed cable design (with higher degradation) helped reduce the contact stresses on the high pressure side of the CICC, but in either case, the strain reduction mechanisms were not enough to suppress cyclic degradation. Advantages and disadvantages of each conductor design are discussed here aimed to understand the sources of the degradation.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [1]
  1. Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center, NHMFL
  2. ITER International Fusion Energy Organization, Saint Paul lez Durance (France)
  3. ITER International Fusion Energy Organization, Saint Paul lez Durance (France); Univ. of Wisconsin, Eau Claire, WI (United States)
  4. ITER International Fusion Energy Organization, Saint Paul lez Durance (France); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  5. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
OSTI Identifier:
1295452
Grant/Contract Number:
FG02-06ER54881
Type:
Accepted Manuscript
Journal Name:
Superconductor Science and Technology
Additional Journal Information:
Journal Volume: 28; Journal Issue: 8; Journal ID: ISSN 0953-2048
Publisher:
IOP Publishing
Research Org:
Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center, NHMFL
Sponsoring Org:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY ITER; CICC; Lorentz force; Nb3Sn; image analysis; microscopy; metallography; large scale fusion magnets; niobium-tin; cable-in-conduit conductor