Earticle

This review surveys the physical origin of flux pinning in high-temperature superconductors (HTS), with a focus on REBCO coated conductors. We begin with the microscopic origin of Cooper pairing, contrasting conventional phonon-mediated coupling with the strong electronic correlations and antiferromagnetic spin fluctuations that are widely regarded as playing a central role in cuprate pairing, and then examine the thermodynamic limits imposed by temperature, magnetic field, and current density. Vortex formation and motion in type-II superconductors are described within Ginzburg-Landau phenomenology, clarifying how the coherence length, the penetration depth, and the Ginzburg-Landau parameter determine the vortex core structure and the stability of the vortex lattice. On this basis, artificial pinning centers are classified by dimensionality - 0D point defects, 1D nanorods, 2D planar defects such as stacking faults and twin boundaries, and 3D nanoparticles - and their reported pinning force densities are compared over a broad range of temperature and magnetic field. We further discuss the intrinsic electromagnetic anisotropy arising from the layered perovskite structure of REBCO, which makes the critical current density particularly sensitive to magnetic fields applied along the c-axis, and review hybrid defect-engineering strategies that combine anisotropic and isotropic pinning centers to reduce that angular dependence. Taken together, the results indicate that a multidimensional combination of defects, matched to the intended operating temperature, field strength, and field orientation, provides a practical design route for REBCO conductors in high-field applications.

The influence of BaTiO3 (BTO) addition on phase formation, microstructure, and flux pinning properties of Bi1.6Pb0.4Sr2Ca2Cu3O10+δ((Bi,Pb)-2223)) bulk superconductors were systematically investigated. Samples containing 00.7 wt% BTO were synthesized using the conventional solid-state method. Phase composition, microstructure, and elemental distribution were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). Superconducting properties were evaluated through resistance-temperature and magnetization measurements, with the critical current density (Jc) determined from magnetic hysteresis loops using the Bean’s critical-state model. XRD analysis showed that increasing BTO content reduced the Bi-2223 phase fraction while promoting the formation of Bi-2212 and CaPbO4 secondary phases. SEM observations indicated that BTO addition modified the plate-like grain morphology and generally promoted grain refinement. EDS mapping showed distinct spatial distributions of Ba and Ti, suggesting partial decomposition of BTO and subsequent redistribution within the (Bi,Pb)-2223 matrix during sintering. Normalized pinning-force analysis demonstrated that at high magnetic fields, pure and 0.3 wt.% BTO samples exhibited close to normal point-pinning behavior, whereas other BTO-added samples showed behavior close to surface-pinning. Overall, these results confirm that BTO addition significantly modifies the phase evolution, microstructure, and flux pinning mechanisms of (Bi,Pb)-2223 bulk superconductors.

Using a SQUID-VSM magnetometer, we measured the magnetic moment as functions of applied magnetic field and temperature, denoted m(H) and m(T), respectively, for untreated MgB2 powder and MgB2 powders heat-treated in an Ar atmosphere. To quantify the symmetry and asymmetry of the magnetic hysteresis loops, we decomposed the m(H) data into symmetric and asymmetric components, denoted by |Δm(H)| and |Σ(H)|, respectively, and attributed these components to bulk pinning and surface barriers. For all samples investigated, the magnitude of the symmetric component was approximately two to three times that of the asymmetric component. The degree of asymmetry was further quantified using an asymmetry index, η(H) = |Σ(H)| / |Δm(H)|. In addition, the critical current density, Jc ∞ ΔM, can be estimated for each sample using the conventional Bean critical-state model. The variation in the irreversible magnetization ΔM with heat-treatment temperature, Tan, was examined in relation to changes in the asymmetry index. In particular, the increase in ΔM at Tan = 800 °C relative to that at 750 °C coincided with a pronounced decrease in the asymmetry index. This decrease was accompanied by an increase in the ΔM value, which may be related to the higher Jc value estimated by the Bean model.

We fabricated a two-stage LCLC low-pass filter using a 100-nm-thick NbTi thin film on a sapphire substrate and investigated its microwave transmission characteristics as functions of temperature and magnetic field applied parallel to the substrate plane. At T = 1.8 K, the fabricated filter exhibited a cutoff frequency of approximately 1 GHz and an attenuation of approximately 20 dB in the stopband. By analyzing the measured ∣S21∣ characteristics using an ABCD-matrix model, we extracted an effective inductance of Lmea = 46.1 nH and an interdigitated-capacitor capacitance of CIDC = 1.59 pF for each LC cell. Temperature-dependent measurements enabled the effective inductance to be separated into temperature-independent and kinetic-inductance contributions, yielding a sheet kinetic inductance of 1.61 pH/□. Under parallel magnetic fields, the cutoff frequency shifted toward lower frequencies because of an increase in the effective inductance, while the low-pass response was maintained up to B|| = 9 T. These results demonstrate that NbTi-based LCLC filters can serve as on-chip bias-line components for suppressing high-frequency noise and microwave leakage in cryogenic, high-magnetic-field environments.

AC loss is a critical issue in high-temperature superconducting (HTS) coils because it directly increases the cryogenic load of superconducting power devices. In this study, magnetization losses of HTS coils wound with conventional CORC and Straight-CORC hybrid conductors were analyzed using measured conductor magnetization loss data and finite element magnetic field calculations. The Straight-CORC hybrid conductor consists of a four-layer CORC cable with 24 REBCO tapes and a straight stack of 20 tapes positioned inside the round former. Magnetization loss per unit length measured under an external AC magnetic field was represented by piecewise interpolation functions and the magnetic flux density calculated at the center of each cable turn was mapped to the corresponding magnetization loss function. Solenoidal and pancake coils were compared at a peak operating current of 200 A and an inductance of 0.05 H. Compared with the CORC coils, the Straight-CORC coils showed approximately 13~36% lower estimated total magnetization loss for solenoidal geometries and 11~16% lower estimated total magnetization loss for pancake geometries. The estimated magnetization loss normalized by the number of HTS tapes was reduced by approximately 50~64%. These results indicate that the Straight-CORC hybrid conductor is a promising option for reducing magnetization loss in high-current HTS coil application.

High-temperature superconductor (HTS) coils are of critical importance in applications that require intense magnetic fields, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). However, these coils undergo significant mechanical deformation due to Lorentz forces. In this study, we investigated the mechanical stability and electrical performance of GdBCO superconducting tapes subjected to partial surface abrasion (30%, 60%, and 90%) with 80-grit sandpaper. The results show that increasing the abrasion-treated surface fraction progressively reduces mechanical deformation and enhances critical-current retention under applied nominal hoop stress, while also increasing the turn-to-turn characteristic resistance. These results demonstrate the coupled influence of partial surface abrasion on the mechanical and electrical characteristics of GdBCO NI coils.

A Straight-CORC hybrid high-temperature superconducting (HTS) cable combining a straight stacked conductor and an outer Conductor on Round Core (CORC) structure was fabricated and its magnetization loss characteristics were experimentally investigated. The hybrid cable was constructed using a straight stack of 20 REBCO coated conductors as an inner core, an enamel-braided-wire reinforcement layer, a Nomex flattening layer, and outer CORC winding layers. Hybrid samples with two, four, and six CORC layers were prepared, together with CORC-only samples having the same outer winding configurations. Magnetization losses were measured using a linked pick-up coil system under an externally applied AC magnetic field. In addition, the hybrid samples were rotated with respect to the applied magnetic field to investigate the angular dependence of the inner stacked conductor. The CORC-only samples showed a decrease in magnetization loss density with increasing winding layers due to magnetic screening effect. The Straight-CORC hybrid sample with two CORC layers exhibited relatively small angular dependence and lower magnetization loss density than the corresponding CORC-only samples. The magnetization loss density of the hybrid conductors was reduced by 75.09% compared with that of the CORC conductors under the same CORC-layer conditions. These results demonstrate the feasibility of the Straight-CORC hybrid configuration for increasing superconducting tape utilization while maintaining low magnetization loss characteristics.

The through-thickness breakdown of a single 12.5 μm-thick Kapton film and the pressure-dependent turn-to-turn alternating-current (AC) breakdown of Kapton-wrapped conductors were investigated for superconducting-coil insulation. A 20 mm-film stainless-steel dummy conductor was tested from 800 to 1×10−5 Torr; commercial rare-earth barium copper oxide (REBCO) conductors from SST (Shanghai Superconductor Technology Co., Ltd.) and 7 mm-film dummy conductors were compared at 800, 1×10−1, and 1×10−5 Torr; and 7 and 30 mm films from the same manufacturer and cutting process were compared at 1×10−1 Torr. All values are two-parameter Weibull characteristic voltages; the single film gave 1.84 kV at 800 Torr. The 20 mm dummy conductor showed a nonmonotonic pressure dependence: 7.01, 0.48, and 5.48 kV at 800, 1×10−1, and 1×10−5 Torr. The SST conductor showed 4.61, 0.47, and 5.07 kV, and the 7 mm dummy conductor 6.29, 0.56, and 5.09 kV. The 30 mm film yielded 0.89 kV, 55.6% higher than the 7 mm film (0.57 kV). The pressure dependence and absence of distinct puncture marks suggest gas-assisted surface or interfacial discharge paths. The dummy conductor reproduced the SST conductor's pressure dependence, and film width showed potential as a structural insulation-design variable. These room-temperature results are comparative data, not cryogenic design limits.

Pneumatically driven Gifford-McMahon(G-M) cryocoolers offer the advantage of system miniaturization by replacing the conventional Scotch-Yoke mechanism with a pressure-driven displacer. However, because the displacer's motion is kinematically unconstrained, designing the system requires a coupled analysis of the thermo-fluid dynamics and the dynamic motion of the displacer. In this study, a one-dimensional numerical model is developed to predict the performance of a pneumatically driven G-M cryocooler. The model comprehensively calculates the mass flow rate through the valves, heat transfer and pressure drop within the regenerator, and the dynamic behavior of the displacer to derive the PV work and the net cooling capacity at 80 K. A parametric study is conducted over 49 operating conditions by varying the drive stem diameter (10–40 mm) and the operating frequency (1–4 Hz). The numerical results demonstrate that these parameters critically govern the displacer's dynamic response and the resulting cooling performance. Under the given conditions, the maximum cooling power is achieved with a drive stem diameter of 30 mm and an operating frequency of 4 Hz. The results suggest that while higher frequencies theoretically yield greater cooling power, practical cryocooler design must strictly incorporate the compressor's performance curves, as increased mass flow rates inevitably degrade the supply pressure ratio.