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

Abstract

Cables made with Nb 3 Sn-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-cooldown 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 the SULTAN test was lower for the CICC without Cu strands. The postmortem 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 reduction in high tensile strain regions. 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.

On the mechanisms governing the critical current reduction in Nb3Sn Rutherford cables under transverse stress

“…At very high pressure values, due to their brittleness, fracture of Nb 3 Sn filaments may occur if the mechanical loading exceeds the Nb 3 Sn crack initiation It is worth mentioning here the extensive metallographic analysis 59 , 60 carried out for the ITER cable-in-conduit conductors, in order to explore the role of the Lorentz loads in producing fractures in the brittle Nb 3 Sn filaments. Detailed filament crack count and fracture analysis classification have unambiguously evidenced the presence of ratcheted crack opening under electromagnetic cycling.…”

Metallographic analysis of 11 T dipole coils for High Luminosity-Large Hadron Collider (HL-LHC)

“…Composite theory predicts iso-strain in the composite constituents under axial loading [16]. This has been confirmed experimentally for the Nb 3 Sn filaments and Cu matrix in different Nb 3 Sn wires by high energy synchrotron x-ray diffraction [16], and in ten-stack samples by neutron diffraction [8]. At 210 MPa (sample 28), the macroscopic axial ten-stack stresses in the Nb 3 Sn and Cu are about 450 MPa and 100 MPa, respectively [17].…”

“…A post-mortem (destructive post magnet testing) metallographic analysis technique has been developed in order to provide more in-depth insight, at the conductor level, into the degradation and complexity in stress states. This approach was previously demonstrated in Nb 3 Sn cable-in-conduit magnet sections tested for the ITER project [7][8][9].…”

Research progress on the mechanical behavior of the cable in conduit conductor for the international thermonuclear experimental reactor project

“…这与Mitchell [39] 以及Nijhuis [77] 的预测结果截然相反. [72] 的实验结 果冲突. Nabara等人 [85] 对两种具有相同节距的CSJA 导体进行电磁循环加载测试发现两个CSJA导体分流 温度均随着电磁循环次数的增加而下降, 而且下降 趋势大致相同.…”

“…Calzolaio和Bruzzone [107] 的磁化测试等效应 变实验同样显示, 短节距导体内部等效压缩应变随 循环次数的增加而减小, Harjo等人 [86] 用中子散射的 方法发现了相同的规律. 近年来Takahashi等人 [113] , Sanabria等人 [72,115,116] , Qin等人 [117] 和Tomone等人 [118] 研究发现横向电磁力作用下的股线弯曲、挤压变形、 芯丝断裂等并非是导致T cs 退化的主要原因. 并且认 为电缆与导体之间可能存在slip-stick(黏滑)行为是造 成T cs 变化的主要机制 [116] .…”