GdBa2Cu3O7-x (GdBCO) superconducting tape is a notable candidate for conductive materials with high current-carrying capacities owing to its high critical current density, irreversible magnetic field, and critical transition temperature. However, the effects of mechanical deformation at the microscopic level on the microstructural evolution, microcrack initiation, and ductility between the layers of GdBCO superconducting tapes still remain insufficiently understood. In this study, an in-situ transmission electron microscopy tensile (In-situ TEM) technique is used to characterize the microstructural evolution of GdBCO superconducting tape layers at different moments during tensile deformation. The results demonstrate that a small number of amorphous or semi-crystalline Y2O3 nanoparticles in the Y2O3 layer are transformed into Y2O3 nanocrystals. Simultaneously, new stacking and linear dislocations gradually appear in the GdBCO superconducting layer. Moreover, a small number of heterogeneous nanoparticles in the GdBCO superconducting layer undergo rotational and morphological changes. Microcracks first appear in the CeO2 layer, and the microcrack tip then propagates to the adjacent GdBCO superconducting and transition layers on both sides. Finally, fracture analysis confirms the tearing-mode fracture behavior of the CeO2 layer. This study provides insights into the sprout origin layer of microcracks and the microcrack propagation mode. The findings also enhance the understanding of synergistic evolution mechanism of composites with complex laminated structures.

A 16.5 T NbTi-Nb3Sn superconducting magnet with a 150 mm bore has been successfully developed at the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS). The magnet is designed as a general-purpose background field magnet and is composed of four concentric coaxial solenoid coils, including three Nb3Sn coils and one NbTi coil. To achieve optimized current density distribution, the Nb3Sn coils were wound using multiple Nb3Sn conductors with different performance characteristics, enabling effective classification and optimization of operating current density. Comprehensive stress and strain simulations were conducted to evaluate the mechanical behavior of the magnet. In addition, a passive quench protection system was specifically designed to ensure reliable protection of the magnet during quench events. Quench simulations confirmed that the passive protection circuit effectively limits coil voltage, mechanical stress, and temperature rise to safe levels. Experimental results demonstrate that the magnet successfully generated a central magnetic field of 16.5 T at 4.2 K under liquid helium immersion, validating the overall design and performance. This paper provides a detailed description of the electromagnetic design, stress and strain analysis, quench protection strategy, as well as the fabrication and experimental testing of the magnet.

In this work, five different RE123 samples were prepared via solid-state sintering in flowing air, including (Eu0.2Gd0.2Y0.2Er0.2Yb0.2)BCO, (Eu0.25Gd0.25Y0.25Er0.25)BCO, (Eu0.33Gd0.33Y0.33)BCO, (Nd0.25Eu0.25Gd0.25Y0.25)BCO and (La0.2Nd0.2Y0.2Er0.2Yb0.2)BCO. Their phase constituents, phase stabilities, superconducting properties and microstructures were systematically investigated. Results indicated that RE123 samples with almost pure superconducting phase could be prepared; the superconducting transition temperature (T-c) of each sample was > 90.0 K. The lattice parameters and peritectic decomposition temperature (T-P) increased with the average ionic radius of RE3 + at the RE site in RE123 samples. The critical current density (J(c)) was considerably affected by the constituent elements at the RE site. Specifically, of all samples, (Eu0.25Gd0.25Y0.25Er0.25)BCO exhibited the highest self-field J(c) of 373 kA/(c)m(2) at 4.2 K; this remarkable superconducting performance could be mainly attributed to its higher superconducting volume fraction, but meanwhile, the configuration entropy at the RE site also have contributed to its high J(c) value due to the formation of nanoscale SFs.

Magnesium diboride (MgB2) bulks, as a superconducting material with a relatively high superconducting transition temperature, low raw material cost, absence of weak-link effects, and flexible fabrication routes, have shown great potential for applications in various fields. Among the available fabrication methods, the gas-solid method has attracted considerable attention due to its ability to significantly reduce the MgO impurity content in the samples. However, cracking is prone to occur during the gas-solid reaction process, which adversely affects the structural integrity of the bulks. The introduction of molds can effectively suppress crack formation and improve the shaping quality of MgB2 bulks. Nevertheless, the use of molds also reduces the diffusion rate of Mg vapor, resulting in a slower reaction process and consequently lower production efficiency. In this work, the optimal sintering time for MgB2 bulk fabrication via the mold-assisted gas-solid method is optimized, thereby reducing experimental cost. A series of MgB2 bulk samples with sintering times ranging from 12 h to 72 h were fabricated using the mold-assisted gas-solid method. The results show that all samples are dominated by the MgB2 phase with a small amount of MgO impurity. With increasing sintering time, the grain size gradually increases, mechanical properties such as microhardness are significantly enhanced, and electrical resistivity decreases. Meanwhile, the Af, Jc, Fp, and Hirr are all improved. Nano-CT results reveal that Mg vapor diffuses from the exterior to the interior of the boron compact and reacts progressively. The internal defects evolve from dispersed pores at the early stage of sintering to planar cracks at later stages. Among all samples, the one sintered for 60 h exhibits the optimal comprehensive performance, indicating that 60 h is the optimal sintering time for the mold-assisted gas-solid fabrication of MgB2 bulks. This work provides important experimental evidence and theoretical guidance for further optimization of the fabrication process, microstructural control, and performance enhancement of MgB2 bulk materials.

Ship magnetic field modeling is a core element in the development of naval magnetic detection and stealth technologies. Currently, equivalent source modeling methods predominantly employ magnetic sources. However, their inversion processes often suffer from abstract physical interpretations and the ill-posed nature of equations, typically requiring multi-objective optimization or regularization to mitigate these issues. To fundamentally circumvent ill-posed problems, this paper proposes an electric equivalent source modeling method based on iterative modeling and a hierarchical optimization strategy. The proposed model utilizes current-carrying circular loops with distinct physical significance as fundamental units, ensuring the model can directly serve engineering construction. The iterative modeling follows a "simple-to-complex" principle, evolving from a low-dimensional coarse model with few loops to a high-dimensional refined model. Results from the previous generation serve as prior information to guide the construction of the subsequent model, ensuring the entire inversion process remains well-posed and controllable. Once the quantity and structure of the loops are determined, a position-current hierarchical optimization strategy is employed for parameter solving. Specifically, the outer layer utilizes linear optimization to search for loop positions, while the inner layer leverages the explosive search capability of the Fireworks Algorithm to determine optimal current magnitudes. This strategy successfully transforms high-dimensional ill-posed inversion into a series of low-dimensional, well-posed optimization problems. Numerical experiments based on finite element models under two geomagnetic environments demonstrate the method's superior accuracy and stability. The relative error on the modeling plane is consistently controlled within 5%. Even with superimposed measurement noise, the error remains at a comparable level, exhibiting strong robustness. Meanwhile, inward and outward magnetic field extrapolation errors are maintained within the 6%-9% range. This study confirms that the proposed method effectively circumvents the ill-posed pitfalls of traditional inversion, providing a novel pathway with significant engineering application value for high-precision and robust ship magnetic field modeling.

A co-located integration quantum electrical standard is essential to reduce reliance on distributed traceability in high-accuracy metrology, especially for portable and on-site use. Metrologically, realizing any two of voltage, resistance, and current is sufficient, as the third follows from Ohm's law. The combination of Josephson voltage and quantum Hall resistance offers better uncertainty, but conflicts with the tesla-level field for quantum Hall and near-zero field for Josephson operation. Here we report a compact unified platform enabling co-realization of quantum voltage and resistance in a single cryostat near 4 K, with quantum current derived via Ohm's law. A hierarchical magnetic shielding with staged attenuation and spatial confinement allows 6 T and below 50 nT to coexist within 270 mm axial separation with negligible cross-coupling. In integrated operation, the Josephson and quantum Hall subsystems agree with expected quantized values within relative standard uncertainties of 2.6E-9 and 1.4E-8, respectively. Linking them via an improved cryogenic current comparator realizes a 50 μA quantum current with relative uncertainty of 6.6E-8. These results demonstrate that three basic electrical units can be synchronously realized with superior metrological consistency on a single integrated platform, offering a viable transition from distributed calibration chains toward compact-integrated quantum-based realization.

In this article, the stripe shape stainless-steel coating on the commercial REBCO tape was attempted to adjust the turn-to-turn contact resistance. The original plan is to use a stripe SS coating to create the space between the two REBCO tapes and maintain the path on the SS coating part, which can significantly increase the contact resistance by decreasing the contact area. In real conditions, the SS coating did not reach its goal but created alternative pathways of SS-SS contact and Cu-Cu contact, which can also partially adjust the contact resistance. It is believed that the contact resistance can be controlled in a larger range by changing the coating materials.

Magnesium diboride (MgB2) bulks, have shown great potential for applications in various fields. Among the available fabrication methods, the gas-solid method has attracted considerable attention due to its ability to significantly reduce the MgO impurity content in the samples. However, cracking is prone to occur during the gas-solid reaction process, which adversely affects the structural integrity of the bulks. The introduction of molds can effectively suppress crack formation and improve the shaping quality of MgB2 bulks. Nevertheless, the use of molds also reduces the diffusion rate of Mg vapor, resulting in a slower reaction process and consequently lower production efficiency. In this work, the optimal sintering time for MgB2 bulk fabrication via the mold-assisted gas-solid method is optimized, thereby reducing experimental cost. A series of MgB2 bulk samples with sintering times ranging from 12 h to 72 h were fabricated using the mold-assisted gas-solid method. The results show that all samples are dominated by the MgB2 phase with a small amount of MgO impurity. With increasing sintering time, the grain size gradually increases, mechanical properties such as microhardness are significantly enhanced, and electrical resistivity decreases. Meanwhile, the Af, Jc, Fp, and Hirr are all improved. Nano-CT results reveal that Mg vapor diffuses from the exterior to the interior of the boron compact and reacts progressively. The internal defects evolve from dispersed pores at the early stage of sintering to planar cracks at later stages. Among all samples, the one sintered for 60 h exhibits the optimal comprehensive performance, indicating that 60 h is the optimal sintering time for the mold-assisted gas-solid fabrication of MgB2 bulks. This work provides important experimental evidence and theoretical guidance for further optimization of the fabrication process, microstructural control, and performance enhancement of MgB2 bulk materials

High-temperature superconducting (HTS) synchronous condensers improve grid stability and inertia through superconducting rotor magnets. This study investigates the dynamic AC loss behavior of racetrack-shaped HTS coils under strong excitation, with current ramping from the nominal value of 465 A up to 515 A (approximately 1.1 times the nominal current) within durations ranging from 0.5 to 2.5 s. A homogenized 2D cross-sectional Hformulation model is developed to compute cross-section-resolved loss density. Losses for straight and arc segments are obtained via a segment-wise equivalent approach, and then mapped as heat sources into a 3D transient thermal model that includes the detailed cryogenic structure. Simulation results reveal localized AC loss hotspots and temperature rise patterns, with all temperatures maintained within a safe operating range of 30 +/- 1.5 K. These insights offer guidance for rotor and cryogenic cooling system design. The modeling framework provides a basis for analyzing high-stress transient conditions in HTS rotors and supports the development of scalable, reliable synchronous condensers for future power grids.

In this article, we report an investigation of the REBCO-NbTi low-resistance joint for highly stable field superconducting magnets, such as NMR. The REBCO-NbTi joint used the most reliable engineering soldering method. The joint resistance at 4.2 K can be as low as 0.1 n ohm when the joint length reaches 1.5 m. When combined with the meter-length REBCO-REBCO joint that has a resistance of less than 0.1 n ohm, as reported before, it is confirmed that the current meter-length soldering of the REBCO-NbTi joint and the REBCO-REBCO joint can initially satisfy the magnetic field stability of 1 & times; 10(-8)/h using an LTS+HTS magnet design up to 1.2 GHz (28.2 T). It proved the feasibility of the soldering REBCO-NbTi joint for the high stable field magnet.

Following the energy upgrade of the BEPCII collider, a comparative analysis was conducted on the power deposition of synchrotron radiation and higher-order mode beam power on the injection kicker magnets before and after the upgrade. To enhance the operational reliability of the injection kicker magnets, improvements and design optimizations were made to the original scheme. These include replacing the ceramic plate coating for the mirror current path with a more reliable three-metal-strip structure, optimizing the water cooling system for better efficiency, and refining the design of the feedthrough connection structure. This paper also details the development process and measurement results of the three-metal-strip magnets. By changing the structural type of the mirror current path on the kicker magnet, the safety and reliability of the kicker magnets can be increased. The impedance and magnetic field measurement results shows that the three-metal-strip structure kicker magnets meet the engineering design requirements. The injection kicker magnets adopt a new three-metal-strip structure, which can meet the physical design requirements and have the advantages of safety and reliability.

A full-wave transformer-rectifier flux pump (TRFP) based on a single superconducting strip is proposed and experimentally demonstrated. The device operates under a combined AC magnetic field and DC gradient field applied to the superconducting strip, which periodically generates flux-flow and dynamic resistance in alternating regions of the strip and produces rectified voltages that charge a parallel HTS coil. The prototype successfully charged a 10-turn HTS coil to 35 A at 77 K, with an optimal operating frequency of approximately 10 Hz. For the first time, this study experimentally demonstrates that a TRFP can achieve the flux-pump effect on a single superconducting tape without requiring a travelling-wave magnetic field to traverse the superconductor, revealing a more general approach to flux-pump implementation, and provides new insight into the physical connection between transformer- rectifier and travelling-wave flux pumps.

Two TM01-TE11 mode converter with a center frequency of 4.3 GHz was designed in this paper. The mode converters were simulated and optimized using CST software. For the double-bend circular waveguide mode converter, the conversion efficiency is greater than $98 \%$ at 4.3 GHz and greater than $97 \%$ in $[4.1 \mathrm{GHz}, 4.6 \mathrm{GHz}]$. For the triple-bend circular waveguide mode converter, the conversion efficiency is greater than $99 \%$ at 4.3 GHz and greater than $98 \%$ in [$4.1 \mathrm{GHz}, 4.6 \mathrm{GHz}$]. A double-bend mode converter was fabricated and tested, and experimental results verified the effectiveness of the design.

In this paper, thermal damage of different degrees was artificially introduced on the REBCO tape. The Ic begins to show obvious degradation after 3 minutes at 350 °C. Different damage detection methods were used to characterize the thermal damage. For the superconducting properties, the four-probe method, 2D-Hall, and resistivity-temperature (R-T) measurements were chosen. For the microstructure, the optical observation, micro-CT, XRD, and SEM were used. After a systematic investigation of the measurement data, the susceptibility of the thermal damage detection method was analyzed. It is found that the large area Ic degradation in the 2D-Hall image, the widening phenomenon of the R-T curve, and most importantly, the crystal lattice parameter change of the REBCO were confirmed as the specific characteristics of thermal damage. Hence, if the REBCO tape is damaged and can meet the above three characteristics, then the source of this damage can be determined as temperature. This work provides a standard comparison sample for magnet workers to determine the source of damage.

High-temperature superconducting (HTS) synchronous condensers are regarded as promising reactive power compensation devices for renewable-energy-based power systems. Fast numerical analysis of the HTS excitation winding is of significant engineering importance. However, finite element methods are computationally expensive for large-scale HTS devices, which motivates the use of more efficient alternatives, such as the J-model. For HTS synchronous condensers without rotor iron cores, the operating characteristics of the HTS excitation winding can be evaluated using the J-model. Nevertheless, for configurations with rotor iron cores, the lack of an appropriate analytical magnetic-field model makes the direct application of the J-model difficult. In this article, an analytical magnetic-field model is derived based on the electromagnetic characteristics of a dual-iron-core structure using the mirror image method. By coupling this analytical model with the J-model, a J-A model is established for dual-iron-core HTS synchronous condensers. Furthermore, an iterative scheme for equivalent relative permeability is developed to incorporate iron-core saturation effects into the analytical model, enabling accurate calculation of nonlinear magnetic fields. The proposed J-A model is validated against the finite element method based on the H-A formulation. The results demonstrate that the proposed model achieves high accuracy in predicting losses, magnetic flux density, current density distribution, and critical current. Meanwhile, the computational efficiency of the J-A model is significantly higher than that of the finite element method, making it highly suitable for the design and optimization of HTS synchronous condensers.

ABSTRACT In this study, we employed commercial NdBCO single crystals as seeds and utilized the conventional top‐seeded melt growth (TSMG) method to fabricate GdBa 2 Cu 3 O 7‐δ (GdBCO) bulk samples, incorporating trace amounts (0.1 wt%) of micro‐ and nano‐sized BaTiO 3 (BTO) particles. The results demonstrated that single‐domain GdBCO bulk samples with highly oriented [00l] textures were prepared. Compared with the undoped samples, BTO doping could reduce the pore sizes within the bulks to some extent. Moreover, BTO‐doped samples exhibited better superconducting properties, including higher J c , and higher trapped field ( B tr ), and levitation force ( F L ). Particularly, the nano‐sized BTO‐doped sample exhibited the best superconducting performance and achieved a self‐field J c of 250 kA/cm 2 at 77 K, B tr of 0.403 T, and F L of 40.7 N. And this could probably be attributed to the formation of magnetic flux pinning centers from nano‐sized BTO and stacking fault defects.

In high-temperature superconducting (HTS) excitation motors, the excitation current of the HTS winding is limited by its critical current. The rated excitation current must remain below this limit, with an adequate safety margin to minimize the risk of quenches. Rapid determination of this critical current is essential for improving motor design efficiency. Conventional finite element methods (FEMs) obtain the critical current by calculating the E-I curve, but the computation becomes time-consuming when numerous HTS tapes are involved. To overcome this limitation, this paper combines the J-model with multiple current control strategies to quickly determine the critical current. Considering the nonlinear variation of superconducting current density, control methods with strong robustness and strong convergence are adopted, such as hysteresis control, bisection method and PI control. The FEM based on the H formulation is employed to identify key control parameter and verify accuracy of the method. The proposed method is further used to analyze how key electromagnetic parameters of HTS excitation motors affect the critical current of the HTS winding. Results demonstrate that the method achieves high accuracy and computational efficiency. Finally, a ring-shaped coil and a racetrack-shaped HTS coil are used to verify the universality and accuracy of the method in other superconducting devices. In summary, the proposed critical current calculation method significantly improves the efficiency of performance evaluation and optimization for superconducting coils across diverse applications.

Degradation of critical current (Ic) for ReBa2Cu3O7-x (ReBCO) superconductor tape under tensile stress is extremely crucial for the stability of magnets in general and fusion magnets in specific at elevated fields such as (20T). Extensive research is carried out on the effect of tensile stress on Ic values, considering the ReBCO tape with multiple nanocomposite layers as a homogeneous entity, while neglecting the interface between the layers. Herein this piece of work, for the very first time, the interface between ReBCO/CeO2 nanocomposite is investigated in detail under varying tensile stress. In-situ EBSD technology was followed to systematically examine the crystal orientation rotation within CeO2 nanofilms. Furthermore, a theoretical analysis for lattice-mismatch stress within ReBCO layer transmitted through ReBCO/CeO2 interface is also provided. Ic degradation due to ReBCO/CeO2 interface is calculated, and the relevant stress under realistic magnetic conditions was evaluated by utilizing a multi-physics simulation technique. This work provides a novel approach for in-depth investigation of the Ic degradation mechanism in ReBCO tape subjected to varying stress.