Abstract Heating of the REBa2Cu3O7-δ (REBCO) layer is a proximate trigger for degradation in high temperature superconducting tapes. Yet, a loophole-free characterization for probing the degradation mechanism remains unavailable. Here, we introduce in-plane grazing incidence X-ray diffractometry to directly observe the orthorhombic-to-tetragonal phase transition behaviors in the superconducting layer of heat-treated tapes. Based on the obtained diffraction patterns, oxygen out-diffusion is confirmed to be the origin of thermal degradation in REBCO-based tapes. During thermal degradation, the degradation rate of the tapes varies linearly with the lattice parameter difference between the a-axis and b-axis, whereas the proportions of in-plane a- and b-axis components in the biaxially textured REBCO layer remain unchanged. These pattern-derived values can serve as inherent indicators when commercial tapes undergo thermal quench events. In addition, the precisely determined electronic and chemical structures of heat-treated REBCO offer indirect evidence for the phase transition. These critical findings provide valuable insights into the thermal degradation mechanism of superconducting tapes, paving the way for the reliable fabrication and stable operation of superconducting devices based on REBCO coated conductors.
Abstract Nb3Sn-coated superconducting radio frequency (SRF) cavities are being developed for operation at 4 K to significantly improve the energy efficiency of SRF-based accelerators. However, trapped magnetic flux generated during cooldown by thermoelectric currents in the bimetallic system can enhance their losses considerably. We present direct measurements of magnetic flux density generated during cooldown in Nb 3 Sn-coated samples prepared by DC magnetron sputtering and by the bronze-route technique. During cooldown, magnetic fields several times larger than the Earth's magnetic field are generated and subsequently trapped as the samples become superconducting. The generated field increases with increasing temperature gradient across the sample, and the effect varies by more than two orders of magnitude depending on the sample preparation. We also present a simulation of thermoelectric currents that provides insight into the underlying mechanism and suggests strategies to reduce the effect. These results are relevant for the production and operation of Nb 3 Sn cavities.
Abstract Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-field applications. In epoxy-impregnated REBCO superconducting windings, during the cooling process, the delamination induced by thermal mismatch stress significantly threatens stable operation due to the weak c-axis strength of REBCO CC tapes. Therefore, reducing the coefficient of thermal expansion (CTE) of the impregnation material will serve as an effective measure to enhance the structural safety of impregnated REBCO superconducting coils. In this research, a series of ZrW2O8/epoxy composites were fabricated by doping different proportions of negative thermal expansion powders into epoxy. The CTE within the temperature range of 4.2–285 K, along with the tensile and shear properties at room temperature and 77 K, were examined. Moreover, high temperature superconducting coils (HTS) were impregnated with optimized improved epoxy composite material, and the Voltage-Ampere characteristics at 77 K in self-field were explored. The results indicate that ZrW2O8/epoxy resin as an impregnation material effectively mitigates the thermal mismatch stress of superconducting coils and enhances their structural stability at extremely low temperatures. Finite element simulation provides supporting evidence of the experimental results. This study will offer an important foundation for optimizing the impregnation materials of high-field superconducting magnets and is highly significant for enhancing the performance and reliability of superconducting magnets.
Abstract The performance of REBCO multi-stack cables is crucial for next-generation high-field fusion magnets, it is often limited by uneven current distribution. This study employs a 3D finite-element model based on the weak form of the H-formulation to analyze current non-uniformity within a novel BRAided Stacks of Tapes (BRAST) assembly. The model rigorously accounts for the nonlinear E–J power law of the REBCO material, as well as the effects of inter-tape and terminal resistances. The model’s accuracy is verified experimentally. Simulation results establish a clear hierarchy of stability drivers. First, non-uniform terminal resistance determines the inter- tape current imbalance. Low inter-tape contact resistance has a dual role: it amplifies short-range current imbalances at the cable entry point, leading to increased local current loading, while simultaneously promoting a more uniform longitudinal current distribution within the cable, thereby mitigating current inhomogeneity. Furthermore, the analysis reveals that tape regions with reduced critical current can be the onset of overall stack transition, and that the overall stability of the cable is determined by the minimum current-carrying capacity within the tape stack. This study provides quantitative guidance for controlling terminal resistance uniformity and optimizing contact resistance, thereby improving the stability and robustness of BRAST and similar multi- tape superconducting cable systems.
Abstract Since its discovery in 2001, magnesium diboride (MgB 2 ) has attracted significant interest for large-scale superconducting applications due to its high transition temperature (39 K), low material cost, and weak crystallographic anisotropy. This review critically assesses progress and remaining challenges in the industrialization of MgB 2 superconducting wires, covering material optimization, fabrication techniques, and emerging applications. Key material strategies include precursor powder control, carbon doping, and high-pressure sintering to enhance wire performance. Various fabrication routes are evaluated for their industrial maturity and capability to produce kilometer-length wires with improved current-carrying capacity. Recent application progress in superconducting joints, magnetic resonance imaging systems, superconducting cables, and superconducting magnetic energy storage is also reviewed. Although full-scale commercialization has not yet been realized, recent demonstrations indicate that MgB 2 technology is transitioning from proof of concept to early-stage industrial adoption. Current technical bottlenecks and future directions are discussed, offering perspectives on the path toward cost-effective, large-scale superconducting applications.
