Yi Shi

Large transverse Lorentz forces in high-field superconducting magnets pose severe challenges to the mechanical integrity and operational reliability of REBa ₂ Cu ₃ O ₇ − δ (REBCO) conductors. However, cable-scale electromechanical studies above 10 ³ kN m ⁻¹ remain rare, and the mechanics-to-performance degradation pathway of mechanically heterogeneous, solder-encapsulated REBCO cables remains poorly understood, impeding technological advancement. In this study, cryogenic electromechanical experiments at 77 K were integrated with three-dimensional finite-element simulations to quantify the monotonic and cyclic transverse-load responses of twisted stacked slotted-core (TSSC) REBCO cables and provide mechanics-based interpretations of damage susceptibility. Monotonic tests reveal a pronounced loading-orientation effect: at 95% critical-current retention, the cable sustains 2080 kN m ⁻¹ under 0° loading, approximately 33% higher than the 1560 kN m ⁻¹ sustained under 45° loading. Simulations further link this difference to orientation-dependent load redistribution, resulting in distinct spatial patterns of local plasticity and solder damage initiation, together with corresponding variations in the mixed-mode interlaminar damage-driving index near the REBCO tape edges. Additionally, cyclic tests reveal load-level-dependent degradation behavior: at 1500 kN m ⁻¹ , the cable exhibits a three-stage degradation process with accelerated degradation after approximately 900 cycles, whereas at 1375 kN m ⁻¹ , the critical current remains nearly unchanged after 1000 cycles. Moreover, cross-sectional observations identify solder voids and insufficiently filled regions as processing-induced defects, while defect-containing finite-element models show that these defects amplify local stresses and disrupt load-transfer continuity. These findings establish a contact–plasticity–damage failure pathway and provide mechanics-based guidance for defect control and structural optimization of reliable TSSC conductors.

E-Ba2Cu3O7−δ (REBCO) coated conductors are promising candidates for next-generation high-field fusion magnets operating above 20 T. To meet the structural and electromagnetic demands of such environments, various cabling concepts based on REBCO coated conductors have been developed. However, inadequate mechanical protection can significantly degrade their transverse load tolerance under practical operating conditions. In particular, multiaxial stresses—such as axial pre-strain combined with transverse loading—may induce irreversible performance degradation, yet their coupled effects remain poorly understood, limiting reliable application of REBCO-based conductors in high-field fusion systems. In this study, the transverse compression behavior of a double-casing conductor (DCC) with a twisted stacked-tape cable (TSTC) was systematically investigated under monotonic, cyclic, and axial preload conditions. Real-time strain monitoring was enabled by fiber Bragg grating (FBG) sensors, rigorously calibrated to decouple mechanical, thermal, and clamping-induced strains. This approach allowed precise quantification of the actual strain state, demonstrating the strong potential of FBG diagnostics for in-situ integration into future conductor architectures. Experimental results show that the DCC strand sustained a critical transverse load of 1188 kN/m without axial pre-strain. However, this tolerance dropped by over 52% when 0.25% axial pre-strain was applied. Finite element analysis revealed a peak compressive strain of 0.36% at the critical load. Under six-around-one contact, monotonic loading of DCC-8 exhibited a degradation threshold of 1095 kN/m, while fully superconducting DCC-25 demonstrated superior fatigue resistance under cyclic loading, maintaining less than 5% degradation after 500 cycles at 1125 kN/m. These findings highlight the necessity of incorporating multiaxial stress criteria into conductor design and confirm DCC architecture with FBG diagnostics as a robust candidate for next-generation high-field fusion applications.

To address the challenges of slow quench propagation in high-current high-temperature superconducting (HTS) components for fusion reactors and the susceptibility of conventional voltage detection to electromagnetic interference, this paper proposes a hybrid fiber optic sensing scheme based on Fiber Bragg Gratings (FBG) and Distributed Fiber Optic Sensing (DFOS). Utilizing the CRAFT 16 kA HTS current lead platform, simulated Loss of Flow Accident (LOFA) experiments and 3D electro-thermal coupled numerical simulations were conducted. Results demonstrate a dual-stage protection capability. For LOFA early warning, the FBG system identified thermal anomalies at 168.8 s (equal signal-to-noise ratio (SNR) criterion), leading the conventional voltage trigger (1354.3 s) by nearly 20 minutes. For quench-onset detection via a 3σ deviation threshold, continuous DFOS and discrete FBG captured hotspots at 1337.6 s and 1346.2 s, achieving response leads of 16.7 s and 8.1 s, respectively. This detection scheme provides an expanded safety margin that satisfies the thermal stability requirements for fusion-grade HTS components. By leveraging the continuous spatial sampling of DFOS and the hybrid sensing scheme, this study quantified the azimuthal thermal inhomogeneity within the HTS stacks. Performance data show that FBG yields an SNR of 898 and supports rapid response, making it suitable for millisecond-level protection triggering. Conversely, the continuous DFOS detected the quench onset 8.6 s earlier than the discrete FBG array, quantifying the coupled thermal lag induced by axial undersampling and stochastic azimuthal hotspots. This research confirms that a hybrid sensing strategy, integrating the rapid response of FBGs with the full-field diagnostics of DFOS, provides a measurement strategy for monitoring the operation of future large-scale fusion reactor magnets.

The development of high-field superconducting magnets for next-generation fusion devices requires robust, high-current cable-in-conduit conductors (CICCs) that withstand combined electromagnetic, thermal, and mechanical loads. In this work, a 2.8 m-long full-size prototype of a twisted stacked-tape rare-earth barium copper oxide (REBCO) CICC with a lateral support structure was designed, fabricated, and tested at the SULTAN facility. The prototype comprises three twisted stacked slotted-core sub-cables (336 REBCO tapes total), vacuum-solder consolidated onto a slotted copper former, wrapped with copper and 316 l tape and compacted into a CHSN01 stainless-steel jacket. Baseline tests at 77 K and self-field obtained a critical current ( I _c ) of 16.6 ± 0.3 kA and an n -value of approximately 27. At a magnetic field of 10.85 T and operating currents of 60 kA and 70 kA, the measured current sharing temperature ( T _cs ) values were approximately 23.5 K and 19.6 K, respectively, deviating within 1.0 K of the predicted average for REBCO tapes. Under electromagnetic cycling conditions with currents reaching 75–80 kA and a background field of 10.85 T, the prototype exhibited only a 1.3% reduction in T _cs after the 53rd cycle, and remained stable throughout subsequent cycles, demonstrating excellent capability to withstand high-field electromagnetic loads. This represents the successful operation of a twisted stacked-tape conductor-type high-temperature superconducting (HTS) CICC under an electromagnetic load of ∼830 kN·m ⁻¹ . The results confirm that vacuum soldering combined with a lateral support structure can effectively enhance the capability of HTS CICC to prevent the degradation under high transverse electromagnetic loads, providing crucial engineering experience for the future development of high-field HTS conductors for fusion magnet applications.

The next generation tokamak aiming to carry out D–T experiments and to demonstrate power generation, is under development at the Institution of Plasma Physics, Chinese Academy of China (ASIPP). A hybrid magnet configuration is proposed for the central solenoid (CS) coil of this tokamak, a high-temperature superconducting coil is inserted into an outer Nb ₃ Sn coil for increasing the flux. Minimization of the coupling losses is investigated in the Nb ₃ Sn cable-in-conduit conductor (CICC) with rectangular cross-section for the outer CS coils using numerical modeling and experimental verification. The AC loss was measured on several candidate conductors with a superconducting AC Dipole at ASIPP and the numerical modeling was performed with the JackPot-AC/DC ^© model, which was developed by University of Twente (UT). The data from the experiments tested in UT were used for calibrating and validating the model computations. With the model, further optimization on cable parameters was then carried out. The results show that the twist pitch (TP) of the first stage and the TPs ratio have an important impact on the rectangular CICC coupling loss, and the influences of these two factors are related. In addition, the use of a rectangular cross-section CICC can have significant advantages when used in CS magnets.

High-temperature superconducting cables are being developed for high-field magnets operating above 20 T. The high current densities and magnetic fields produce significant Lorentz forces that can cause irreversible conductor degradation. In this high-load environment, achieving complete solidification of the conductor is a promising approach. However, such high-stress-resistant conductors are more vulnerable to strain-induced degradation, as individual strands are more likely to experience additional bending and twisting strains when being assembled into a full cable. This study presents a double-casing conductor (DCC) with a twisted stacked-tape cable (TSTC) configuration, designed as a fundamental sub-cable unit for future high-field, high-current fusion magnets. The DCC incorporates two copper casings and utilizes the vacuum soldering process, providing improved mechanical stability and reduced voids. To validate its performance comprehensively, we systematically investigated the electro-mechanical properties of the DCC under axial tension, bending and torsion. The irreversible strain threshold of the DCC (0.37%) and its safe mechanical operating limits (bending radius > 300 mm, twist pitch > 500 mm) align well with the stringent requirements of the CFETR central solenoid (20 T, 50 kA), where large Lorentz forces necessitate exceptional strain tolerance. Moreover, the bent-then-solder process effectively reduces strain-induced degradation, further enhancing the conductor’s mechanical integrity. These results confirm the robustness of the DCC and its suitability as a sub-cable unit for next-generation high-field fusion magnets. The identified irreversible stress limits and strain thresholds meet the stringent requirements of fusion reactor applications, laying a solid foundation for scaling up DCC-based cables and supporting their feasibility in future fusion energy systems.

The Super-X facility for testing large size superconducting conductors is making significant progress. However, the magnetic force between two coils arises when the centers of their respective magnetic fields do not coincide. Coil misalignment is nearly unavoidable during the installation of magnets which will lead to this eccentric force. In particular, for suspended background magnet in Super-X facility, the eccentric force amplifies the stress on structural components, thereby potentially compromising the magnet's safe operation. Therefore, it is imperative to evaluate its impact on the structural components of the magnet. The pull rope structure of the background magnet is elaborated upon in this article, followed by the analysis of the electromagnetic force generated under different eccentricities of the magnet. Subsequently, the stresses on the structural components of the magnet with and without the pull rope structure are analyzed separately. The results demonstrate that the presence of a pull rope structure is crucial for withstanding magnet eccentricity. Thus far, it can be concluded that the mechanical performance of the magnet still meets the design requirements, taking into account coil misalignment.

The demand for stronger magnetic fields in fusion devices necessitates robust conductors capable of withstanding greater loads. High-temperature superconducting (HTS) cables using REBCO tapes have demonstrated practicality by carrying high currents even in magnetic fields up to 20 T. Several projects have been initiated to demonstrate the feasibility of using HTS twisted stacked-tape cable (TSTC) conductors. However, these challenges, particularly those associated with soldering defects leading to cavities, failures, and cable cracking, have been effectively addressed, significantly enhancing load resistance and structural reliability when making REBCO tapes into cables. In this study, we conducted experiments on a double casing cable (DCC), a round HTS strand designed following the TSTC approach. We improved soldering quality using vacuum pressure impregnation: an inner casing provides additional tape support, reducing mechanical issues from weak solder strength, while an outer casing facilitates solder flow channels, preventing inner casing cracking under direct stress. Direct current (DC) tests on transverse electromagnetic characteristics demonstrate the DCC strand's potential for high-current cables and fusion magnet applications. Notably, the strand sustained transverse load amplitudes up to 1250 kN/m without degradation of critical current (I$_{c}$). We present experimental results and discuss structural optimizations based on visual inspection and analysis.