Impact of indentation on the performance of high Jc Nb3Sn strand

Self Cite

The future fusion reactor devices built by Institute of Plasma Physics, Chinese Academy of Science (ASIPP) will be a compact burning plasma facility to fill the gap between ITER and the China Fusion Engineering Test Reactor (CFETR), with the mission of studying the Deuterium-Tritium (D-T) plasma in steady-state operation. The winding-package (WP) of its toroidal field coil (TFC) is graded into high-field WP and low-field WP, and the high critical current density (Jc) Nb3Sn strand will be applied to the high-field WP based on the current design. The thermo-magnetic instability is the main issue in the application of high-Jc Nb3Sn strand, which may induce premature quenching in the low field regions. This issue can be improved by reducing the effective filament diameter to suppress flux jumps and increasing the residual resistivity ratio (RRR) to enhance the release of heat generated by flux jumps. However, in the conductor manufacturing process, local plastic deformations of strands can impact the sub-element layout and degrade the RRR of the strand, which would increase again the risk on instability. In this study, magnetization measurements and V-I tests were performed on samples with different indentation depths. The hysteresis loss and deff of the indented sample was obtained from the test results. The impact of local plastic deformation on the thermo-magnetic instability of two types of Nb3Sn strand was confirmed by cross-sectional metallographic observation and quantitative microstructure analysis. It was shown that flux jumps were suppressed at indentation depths below 0.3 mm. Further indentation increase leads to severe flux jumps.

Section: Sample Preparationmentioning

confidence: 99%

Section: Eff and Rrr Of The Indented Strandmentioning

confidence: 99%

Impact of local plastic deformation on thermo-magnetic instability of high-J _c Nb₃Sn strand

Liu,

Wu,

Nijhuis

et al. 2024

Self Cite

“…As shown in figure 5, cables sections were extracted from the compacted conductor and disassembled to inspect potential damage to superconducting strands caused by indentations formed during the cabling and compaction process, ensuring that the electromagnetic performance and thermo-magnetic stability of the Nb 3 Sn strands falls within the acceptable range [46].…”

Section: The Ciccs With New Cabling Designmentioning

confidence: 99%

High performance of an innovative cable-in-conduit conductor with CWS cable pattern

Guo,

Liu,

Dai

et al. 2024

Cable-in-conduit conductors, known as CICCs, were developed for constructing superconducting coils in tokamak fusion reactors. To achieve large currents in high magnetic field, CICCs were utilized with a short-twist-pitch (STP) cable pattern to prevent irreversible performance degradation, but also inducing higher AC losses. Institute Of Plasma Physics Chinese Academy Of Sciences (ASIPP) designed and manufactured three innovative CICCs, all featuring CWS (copper wire with a short-twist-pitch wound around superconducting strands with a long-twist-pitch) structure to increase both the current density and structure stiffness of CICC cable. These CICCs had the same new CWS cable pattern but the Nb3Sn superconducting strands were from different suppliers. All samples were subsequently tested under electromagnetic cycling tests in SULTAN. For similar electromagnetic performance degradation, the Lorentz load threshold of the CWS cable pattern exhibited to be higher than that of STP cable pattern. Moreover, the AC losses of CWS were 15% lower than that of STP cable pattern for low frequencies of the applied alternating magnetic field. Both results indicated that the CWS cable pattern has a higher margin of engineering safety and lower AC losses than STP cable pattern under the target operating conditions. This provides new insights in finding solutions for optimizing the CICCs’ cable pattern and preventing its electromagnetic performance degradation.

Local plastic deformation effects on the critical current of high-J_c Nb₃Sn strands under uniaxial strain

Liu,

Sun,

Gao

et al. 2024

In order to meet the target operating parameters of the toroidal field coils (TFCs) for the next-generation Chinese compact burning plasma tokamak, high critical current density (J c) Nb3Sn strand will be applied to the high-field winding-package of the TFC. To improve the transverse stiffness of the cable in withstanding the huge Lorentz force and avoiding conductor performance degradation, the short-twist-pitch and copper-wound-strand cable patterns were taken into consideration. In the processes of cabling and compaction of the conductor, the tight cable configurations lead to severe local plastic deformation (LPD) within the strands. The strands in the conductor are subjected to strain caused by thermal contraction and Lorentz force during conductor cooling down and operation. So far it is unknown, whether the LPD could impact the critical current (I c) versus uniaxial applied strain behavior of high-J c Nb3Sn strand. Aiming to investigate the effect of LPD on the I c of strands under uniaxial strain, three types of high J c Nb3Sn strand with different indentation depths were tested on a U-shaped bending spring. The axial strain ranges from −0.9% to +0.4% at 14 T and 4.2 K. The three types of strands showed more strain sensitivity and lower tensile irreversible strain limit with increasing LPD, while even irreversible degradation of the I c could be observed in the compressive strain region. The sample preparation, test process, test results and analysis are reported.