Abstract
Dedicated experimental and modeling research studies on the performance of superconducting cable-in-conduit conductor (CICC) have been massively performed and are still ongoing in order to determine the operational limits of the conductors and to optimize their design. Strand strain distribution and crack formation in the filaments after cabling and compaction, and under cooling down and electromagnetic load have been considered as the main cause for the degradation of the CICC’s transport properties. In combination with the strain maps generated by the mechanical model MULTIFIL and the electromagnetic code JackPot with the basic electrical and strain properties of the superconducting strand, the current sharing temperature (Tcs) of the CICC of the ITER Central Solenoid has been simulated and analyzed. A quantitative analysis of the Tcs degradation due to strain variation and filament fracture, respectively, is still missing. Here, the approach of analyzing the performance of CICC (e.g., the short samples tested in the SULTAN facility, or the full-size CICC used in real magnets) has been presented. Consequently, the effect of filament fracture on the cable Tcs has been investigated and turns out to be limited. Instead, the dominant mechanism behind the degradation of the transport properties of ITER type Nb3Sn CICCs is shown to be the broadening and shift in the strain distribution of the superconducting filaments.
Highlights
In order to quantitatively describe the behavior of the large and complex cable-in-conduit conductor (CICCs), experiments and models have been designed and massively performed
The results indicate that the degradation of ITER Nb3Sn CICCs can only be marginally attributed to filament fracture, but is mainly caused by the broadening and shift in the strain distribution of the superconducting filaments and corresponding inter-filamentary current redistribution
Given the amount of filament fracture observed in post-mortem TARSIS and full-size ITER samples, the 3D strand model effectively quantifies the impact of cracks on the strand and the final CICC performance
Summary
In order to quantitatively describe the behavior of the large and complex cable-in-conduit conductor (CICCs) (e.g., the ITER magnets1), experiments and models have been designed and massively performed. As the two main causes of the degradation of CICC’s transport performance, strand strain distribution and filament fracture have been investigated and analyzed. Many of the tested short cable samples show a systematic degradation of the current sharing temperature Tcs over several thousand current cycles under a background magnetic field of 11 T, which has been attributed mostly to filament fracture due to the electromagnetic loads on the strands.. The 3D strand model is applied to analyze the quantitative effect of filament cracks on the performance degradation of full-size ITER CICCs. The results indicate that the degradation of ITER Nb3Sn CICCs can only be marginally attributed to filament fracture, but is mainly caused by the broadening and shift in the strain distribution of the superconducting filaments and corresponding inter-filamentary current redistribution
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