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Effect of wire diameter and annealing on the properties of internal magnesium diffusion (IMD)-processed MgB 2 composite wires is studied by establishing a correlation with the characteristics of the interfacial reaction zones developed at Mg − B and barrier (Nb) – sheath (Al + Al 2 O 3) interfaces. The MgB 2 superconductor phase grows at the Mg − B interface along with the B-rich MgB 4 phase. Formation of MgB 2 prior to MgB 4 is anticipated owing to a relatively lower Gibbs free energy at annealing temperatures studied. An intermetallic NbAl 3 phase has developed at the barrier – sheath interface whose thickness decreases with an increase in the wire diameter. Development of NbAl 3 further provides mechanical reinforcement and aids at yielding a dense superconductor phase. Resistivity – voltage characteristics of wire indicates the formation of MgB 2 through a huge drop in resistivity values across the core of the wire. An annealing temperature of 645 °C near the melting point of pure Mg (650 °C) and annealing time of 120 min results in highest critical current of the MgB 2 wire. Annealing time beyond 120 min has shown an impediment to the flow current due to crack propagation possibly because of accumulation of stresses within the developed superconductor phase due to grain growth. A significantly greater resistance for the wire with the smallest diameter is attributed to the formation of a thicker NbAl 3 phase. However, highest engineering current density has also been attained by the wire with the smallest diameter annealed at 645 °C for 60 min. [ABSTRACT FROM AUTHOR]

To meet the application requirements of high magnetic field and high current in future fusion magnet systems, a compact twisted stacked tape cable with high current density is proposed. Three stepped grooves are evenly distributed around a circular former, and a 20+10 superconducting tapes configuration is arranged in each single groove. The performance of the cable under self-field is investigated through simulation modeling and experiment on a 1-meter sample. Firstly, the electromagnetic and mechanical properties of cables are studied through simulation. According to the simulation results, the prospective critical current reaches 5090 A under self-field at 77 K while the maximum von Mises stress and volumetric strain obtained from the simulation during the current-carrying process are within acceptable ranges. A sample with a twist pitch of 300 mm is fabricated, consisting of 18 superconducting tapes and 72 copper tapes. The measured and simulated results of the critical current of this sample are 1900 A and 1970 A respectively, with a deviation of 3.5 percent. The feasibility of the proposed cable has been preliminarily demonstrated through these experimental and simulation results. Subsequent research will be conducted on the optimization of cable parameters, including geometric dimensions and tape capacity, to achieve better performance. • A stepped, twisted stacked-tape cable layout with high current density is proposed. • Electromagnetic and mechanical simulations are conducted to demonstrate its feasibility. • A cable sample is fabricated and the experimental result validates simulations. [ABSTRACT FROM AUTHOR]

This paper presents a design for five conventional superconducting magnet structures to meet the high magnetic load demand of superconducting motors while reducing the superconducting tapes consumption. A 45° highest efficiency line is proposed based on the graphical method, and the magnet structure is optimized by combining finite element and PSO algorithms. Additionally, a correlation function is established using the vertical magnetic field to estimate the critical current, describing the nonlinear relationship between the two. The optimization method can maintain the airgap flux density waveform before and after optimization, reduce tapes consumption and perpendicular field on the tapes, refine the field distribution, and improve the safety margin of superconducting coils. The magnet design scheme can be selected with low tape consumption or high safety margin according to actual demand. This research can be used to optimize the electromagnetic design of superconducting electric motors, as well as other superconducting magnet applications, such as superconducting magnetic bearings, nuclear magnetic resonance, and large-scale scientific installations for high-energy physics. The aim is to achieve the exhaustive use of superconducting tapes. [ABSTRACT FROM AUTHOR]

Low activation isotopic boron (11B) based magnesium diboride (Mg11B 2) superconductors doped with biomass-derived activated carbon were synthesized using 11B and magnesium powder via solid-state reaction. The effect of carbon doping on the lattice structure and superconducting properties of Mg11B 2 bulks were evaluated using X-ray powder diffraction, high resolution transmission electron microscopy, scanning electron microscopy and magnetization measurements. Precise refinement of structural parameters indicates successful substitution of carbon in Mg11B 2 bulks. The critical current density (J c) of carbon doped Mg11B 2 synthesized at 650 °C was enhanced more than two times compared with the pure Mg11B 2 bulk. Similar improvement was observed for the Mg11B 2 bulks heat-treated at 800 °C. This enhancement is due to successful substitution of biomass-derived carbon with high surface area into Mg11B 2 lattice. The flux pinning mechanism of pure and doped Mg11B 2 bulks were investigated using the Dew-Hughes model. This study provides information regarding enhancement of the J c of low activation Mg11B 2 superconductors suitable for next-generation fusion magnets. [ABSTRACT FROM AUTHOR]

• We investigated the microscopic morphology of the prepared multi-filamentary tapes. • We compare the short sample current-carrying performance of non-striated tapes with that of 2, 3, 6, 10-filament tapes. • The current-carrying characteristics of multi-filamentary tapes wound on metal cores of different sizes were compared for the first time. The second generation of high temperature superconductivity (HTS) is one of the candidate materials for future superconducting cables (such as CORC), CICC conductors and high field magnets due to its high current carrying performance and excellent mechanical strength. Currently, it has been demonstrated that the utilization of REBCO multi-filamentary tapes in the fabrication of CORC cables can further reduce AC losses. This study involved the preparation of various types of REBCO multi-filamentary tapes through reel-to-reel ultraviolet picosecond laser cutting technology, as well as the manual winding of multiple single-layer CORC cable samples. This work focused on the impact of core diameter on the current-carrying performance of REBCO multi-filamentary tapes under self-field conditions at 77 K. The results indicate that the critical current (I C) of REBCO multi-filamentary tapes decreases as the core diameter decreases. Furthermore, the decrease in I C becomes more significant with an increasing number of cores. When the cable core diameter is 4.6 mm, the critical current of the 3-filament tape is 180.49 A with an n -value of 24.84, which is only a 3.1 % recession compared to the critical current of the commercialized tape. The conclusion of this paper will provide a certain data reference for the subsequent selection of CORC cable core size prepared by REBCO multi-filamentary tapes. [ABSTRACT FROM AUTHOR]

• Essential characteristics of low temperature water ice are reviewed. • I c (B,T) characteristics of MgB 2 coil were measured in the water ice at 10–36 K and 0–6 T. • The heating of MgB 2 coil in water ice was observed only about its critical current. • Using of sub-cooled water ice is interesting for practical MgB 2 windings. Highly sub-cooled water ice, with temperatures as low as 10–20 K is not commonly utilized and, as a result, its fundamental properties remain generally unknown. Therefore, the thermal characteristics of water ice have been thoroughly reviewed in the scientific literature and compared with the performance of other solid materials, which can be potentially used as a cooling media in superconducting applications. The effectiveness of water ice as a cooling agent was demonstrated through experimental measurements of the temperature and the magnetic background field effects on the critical current of small MgB 2 solenoid immersed in water ice with temperatures ranging from 10 K to 36 K and external magnetic fields from 0 to 6 T. Increase of the solenoid's temperature was observed when the transport current exceeded the critical threshold, which is determined by the conventional criterion of 1 µV/cm. The obtained results confirm that sub-cooled water ice is a promising, cost-effective, and safe coolant suitable for superconducting systems. [ABSTRACT FROM AUTHOR]

Thallium-based cuprates are known to exhibit high critical temperatures and currents. Usually, partial substitution of copper by transition elements leads to the suppression of superconducting properties. This contribution reports on the studies of Tl 2 Ba 2 Ca 2 Cu 3-x Co x O y. All specimens were found to be superconductors with critical temperatures of 102.4 K, 112.9 K, 112.2 K, 113.5 K, 112.8 K, and 94.8 K for x = 0.005, 0.010, 0.020, 0.030, 0.040, and 0.050, respectively. The samples were characterised by scanning electron microscopy, X-ray diffraction, AC susceptibility, resistance, and vibrating sample magnetometry. [ABSTRACT FROM AUTHOR]

Porosity affects the properties of high- T c superconductors and can improve their performance by enhancing oxygenation, cryocooling, etc. Among other factors, the presence of pores plays a significant role in the process of magnetic flux trapping. Relationships with the porosity manifest in the irreversibility field, the full penetration field, and the remnant magnetization of the samples. To account for the effect of porosity on the trapped magnetic flux into type-II superconductors, a simple toy model is suggested. Generally, as the porosity increases, the trapped flux and related parameters tend to diminish. However, in the case of microscopic samples, porosity can enhance magnetic flux trapping. [ABSTRACT FROM AUTHOR]

• A novel roll-to-roll mechanical incision device was developed for the preparation of REBCO multi-filament tapes. • We compared the current-carrying performance of short samples of non-striated and 3-filaments tapes as well as after making CORC cables. • We investigated the influence of the hysteresis loss of the non-striated and 3-filaments tapes at different temperatures. REBCO high-temperature superconductor (HTS) has widely used in high magnetic field applications, because of the excellent critical current properties and high critical temperature. However, REBCO tape has an extremely large width-to-thickness ratio (typically in the range of 1000–10000) to cause too high power dissipation in the applications. In order to reduce the shielding current effect and AC loss generated during the application of REBCO HTS tapes, a roll-to-roll mechanical incision device was developed for the preparation of REBCO multi-filaments tapes. The microscopic morphology, critical current I C , and hysteresis loss of the tapes before and after incision were comparatively characterized using optical microscope, standard four-probe method, and physical property measurement system (PPMS) and other equipment and methods. The results show that the developed device prepares multi-filaments tapes with incision grooves depths of ∼ 30 μm and grooves widths of ∼ 20 μm on the superconducting layer, and the hysteresis loss of the 3-filaments is reduced by up to 48.6 % compared to that of the non-striated tapes under different temperature conditions. In addition, the I C of the short samples of the 3-filaments tapes was reduced by ∼ 18 % compared with that of the non-striated tapes under 77 K and self-field test conditions, whereas the samples of the 3-filaments tapes and the non-striated tapes were hand-wound respectively onto a metal core with a diameter of 5.25 mm to make a single-layer CORC cable, and there was no significant reduction in the I C of either of them. [ABSTRACT FROM AUTHOR]

• A FE model is developed to simulate helical winding of 2G tape in HTS power cable. • The layered sandwich composite structure of 2G HTS tape is considered for FE model. • The effect of strain on Ic of tape at different loading condition is analysed. • Different loading condition includes tension, torsion, helical winding and bending. • The axial strains are obtained at different winding pitches and bending radii. In High Temperature Superconducting (HTS) power cable, HTS tapes are wound helically around a circular former and are subjected to different types of mechanical loads during manufacturing, transportation and installation of cable. The strain developed within the HTS tape during these processes may affect the superconducting properties of the tape. In the present work, 3D structural finite element (FE) models using ANSYS® are developed to obtain the strain behaviour of the tape under different loading conditions, considering the layered sandwich composite structure of 2G HTS tapes. The analysis is carried out with axial tensile and twisting loads on the tape, during helical winding of tape around the former and a combined effect of helical winding with external tensile and bending loads. The result obtained from FE simulation due to applied tensile load is compared with the established experimental result, which shows a good agreement. Further, the strain developed within the tape affects the critical current. Therefore, the strain is coupled with an empirical relation to obtain the critical current degradation of the tape. The FE analysis is carried out for five different winding pitches and four different bending radii with YBCO layer in the downward position and the corresponding critical currents are estimated. Further, a minimum winding pitch is obtained, below which the rapid degradation of critical current is initiated. The results show that the HTS tape with YBCO in downward position can withstand more amount of external load without affecting the critical axial strain of the tape. [ABSTRACT FROM AUTHOR]

Spinning electrodynamic tether systems are considered one of the most promising potential applications of tethered satellite systems, which require little fuel and avoid the equilibrium limit of conventional vertical electrodynamic tether systems. Therefore, spinning electrodynamic tether systems have good prospects in debris removal, orbit reboost, payload transportation, and so on. It has been found that, if not carefully controlled, tethers easily become slack or sagging during the transition process from equilibrium state into spin. In this regard, this paper mainly focuses on the transition process of spinning electrodynamic tether systems from the initial equilibrium state into the final spinning state with expected angular velocities. Conditions of dynamic equilibrium position and minimum current for acceleration (critical current) are firstly derived in this paper, which provide references for future space tether experiments. The motion of spinning electrodynamic tether systems is described by the Lagrangian model, which takes orbital motion into consideration. By considering power limits of electrodynamic tether systems, this paper proposes two open-loop control methods for the safe transition into spinning state as nominal control algorithms with different mission objectives. The first method (direct transition) provides a near time-minimum solution, and the second method (swinging transition) provides a minimum current-energy solution for acceleration into spin. Considering perturbations in space, including inhomogeneous distribution of magnetic induction, varying mass distribution of the system and so on, an adaptive sliding mode controller is proposed to regulate the system and to track nominal trajectories of acceleration. The effectiveness of the proposed control methods is validated by numerical results. • The work presents analysis of spinning electrodynamic tether systems in its transition into spin. • Critical current in the transition stage is derived and empirically predicted. • Two control methods are proposed to ensure safe transition into spin. • An adaptive sliding mode controller is proposed to ensure system stability under perturbations. [ABSTRACT FROM AUTHOR]

In this work, a dual-sided buffer template, which allows the deposition of YB 2 Cu 3 O 7-δ (YBCO) coated conductors on both sides, has been proposed and investigated. Theoretically, in contrast to traditional YBCO structure on one side, the performance of critical current in YBCO films has been improved by two times. In this buffer structure, the dual-sided Y 2 O 3 films were grown on a flexible metal template by a home-made reel-to-reel dynamic direct current sputtering system and the film growth parameters were systematically investigated. The microstructure were characterized by in situ reflection high-energy electron diffraction monitoring in real time during the deposition stage. X-ray diffraction, and atomic force microscopy were used to detect the biaxial texture and surface morphology, respectively. What's more, by the application of stopping and range of ions in matter software, it helped us to understand the influence of deposition parameter to the growth rate of Y 2 O 3 films. Finally, the biaxial texture of dual-sided Y 2 O 3 film yielded out-of-plane texture of Δ ω = 4.2° and in-plane texture of Δ Φ = 5.2° for one side, as well as Δ ω = 5° and Δ Φ = 5.3° for the second side. Subsequently, the dual-sided YBCO superconducting films were coated on the above Y 2 O 3 buffer layer by low-cost metal-organic chemical vapor deposition method with critical current of 30 A/cm and 60 A/cm for each side. [ABSTRACT FROM AUTHOR]

Chemical solution deposition is a promising technology to fabricate YBCO superconducting thin films due to its low-cost advantage. However, the decreasing of critical current density ( J c ) usually occurred in the fabrication of multi-layered YBCO films. In this study, Ag and Au heterogeneous interlayers were fabricated by using sputtering deposition in order to improve the bi-layered YBCO films, and the influence of crystallization temperature from 740 ∘C to 800 ∘C was investigated. It was found that J c could be enhanced by both interlayers relative to the counterparts without interlayers, and the effect of Ag interlayer was superior to that of Au interlayer. When the crystallization temperature was 800 ∘C, the optimal sample with Ag interlayer had a J c 3.65 MA/cm2, which was about twice of the J c of its counterpart. The surface morphologies and cross-sections were observed by using scanning electron microscope, and the content variations of c -axis oriented grains were measured by using Raman spectroscopy, which could help to investigate the influences of interlayers on the microstructure of YBCO films. The possible reasons for the improvement effects of Ag and Au interlayers were discussed based on the experimental results. [ABSTRACT FROM AUTHOR]

We studied the morphology of the tracks (irradiation damage) of 44, 66, and 84 MeV Au ions in YBa 2 Cu 3 O 7-x (YBCO) thin films using cross-sectional transmission electron microscopy (TEM). The electronic energy loss (S e = 11.6, 16.7, and 20.1 keV/nm) ranged near the onset of ion-track formation previously determined by plan-view TEM and critical-temperature measurements (about 13 keV/nm). The morphology changed from 1D arrays of large spheres of about 5 to 10 nm diameter in 44-MeV irradiation to their elongated variant (spheroid) that were statistically pierced by thin-lines of about 1 to 2 nm and 3 to 5 nm diameter in 66 and 84 MeV, respectively. In the same series of Au-irradiated YBCO films, we observed a sudden increase in critical current density at self-field and 77 K between 44- and 55-MeV irradiation, in agreement with the appearance of the thin-line damage in TEM (between 44- and 66-MeV). [ABSTRACT FROM AUTHOR]

Stacking REBCO tapes is an effective way to increase the current-carrying capacity of the cable. Various loading conditions, including combined tension-bend, are applied to these tapes during fabrication and operation of the magnets. This paper investigates the degradation behaviors of critical current (I c) in stacked rare earth–barium–copper–oxide (REBCO) coated conductor (CC) tapes under tension loading through experimental. Uniaxial tension experiments are conducted on single tapes and 4-tape cables with different stacking at 77 K. The critical current degrades rapidly when strain exceeds an irreversible threshold around 0.4-0.5%. A 3D finite element model analyzes the non-uniform stress distribution in the cable cross-section under tension, causing different degradation behaviors. An empirical model combining the Ekin law and Weibull distribution is proposed to relate I c and axial strain. By fitting experimental data, the key parameters are determined. The I c of a single tape reduces 20% at 0.4% strain. Face-to-face stacking decays at 0.45% irreversible strain, while back-to-back decays at 0.38%. The model provides an effective way to optimize the electromechanical performance of stacked REBCO cables. • This study focuses on the electromechanical behavior of REBCO cables under axial loads. • This study used 3D simulations to understand stress distributions in the cables. • This study conducted fitting of experimental results using a combination of Ekin law and Weibull distribution. [ABSTRACT FROM AUTHOR]

Development of reaction zones at the core Mg(Cu) – boron (B) and barrier (Ti) – sheath (Cu) interfaces of an internal magnesium diffusion (IMD) manufactured MgB 2 composite wire has been studied at varying temperatures of 560–665 °C for 30 min and varying time periods of 30–180 min at 560 °C. MgB 2 has predominantly formed at the Mg(Cu) – B interface with little solubility of Cu of 1–3 at%. A second phase enriched with Cu, Mg 2 Cu, has also formed at random locations within the MgB 2 phase matrix, indicating the advantage of doping Cu in the Mg rod. However, a layer of another B-rich phase, MgB 4 , has also formed at a higher temperature of 665 °C with negligible solubility of Cu at the MgB 2 – B interface. At the Ti – Cu interface, the formation of CuTi 2 , CuTi, Cu 4 Ti and Cu 4 Ti 3 intermetallic phases took place. However, the Cu 4 Ti and Cu 4 Ti 3 phases develop after annealing the wire at a relatively higher temperature of 620 °C and at 560 °C with progress in annealing time. Critical current density (J c) decreases with annealing time; however, the sensitivity of J c towards annealing time is low. Critical current density is found to be strongly dependent on the annealing temperature of the composite wires for the formation of the MgB 2 superconductor phase. A maximum critical current density of ∼ 2.5 × 104 A/cm2 at 5 T and 4.2 K is attained for the Cu-doped MgB 2 composite wire when annealed at 665 °C for 30 min. A maximum shift of only 0.5 K is measured for MgB 2 when formed at the highest annealed temperature compared to the lowest one in this study. [Display omitted] • Higher annealing time decreases critical current density, while higher temperature increases it. • Copper doping in the Mg rod enhances critical current density and catalyses MgB 2 layer growth. • Mg 2 Cu phase formation inside MgB2 enhances the overall functioning of composite wires. • Dense MgB 2 formation could result from intermetallic phases at the Cu-Ti interface. [ABSTRACT FROM AUTHOR]

Highlights • We constructed a transverse loading device that can apply a uniform compression perpendicular to superconducting tape. • The critical current and transport AC losses under transverse compression of YBCO CC were investigated. • The behaviors of Ic and /or transport AC losses in Bi2223-Ag during transverse compression were revealed. Abstract We designed a transverse loading device that can apply a more uniform compressive force perpendicular to superconducting tape than previously designed equipment. The effective compressive length and width reached up to 100 and 20 mm, respectively. The critical current (I C) and/or transport AC losses of the superconducting tape during transverse compression could be measured using this equipment. We used YBa 2 Cu 3 O 7−x -coated conductor (YBCO CC) specimens with copper stabilizing layers of different thickness to investigate the characteristics of the critical current and transport AC losses under transverse compression loading. For comparison, several Bi-2223/Ag samples were also tested. A critical stress was observed for the compression process (the same as that for axial tension), and beyond this stress, I C degraded irreversibly. For the transport AC losses behavior of the YBCO CC and Bi-2223/Ag specimens, below the critical stress value, the AC losses remained almost unchanged with the increase of the compressive stress at the same current; while above the critical stress value, at increasing compressive stress, the AC losses increased rapidly and the normalized AC losses for normalized current decreased. [ABSTRACT FROM AUTHOR]

Abstract Chemical solution deposition is a promising technology to fabricate YBCO superconducting thin films due to its low-cost advantage. As found in our previous study, the addition of lead acetate in precursor solution could enhance the critical current density of YBCO films, since the deteriorating a / b -axis orientated grains could be suppressed. In this study, the precursor solutions with lead acetate weight percentages of 0%, 1%, 2% and 3% were prepared. The molar concentration of all metal ions was 2.0 mol/L for all solutions. Crystallization temperatures of 720°C, 740°C, 760°C, 780°C, 800°C and 820°C were used in the heat treatment. The thickness of the finally obtained YBCO films was about 400 nm. It was found that 1% of lead acetate addition was favorable to achieve high critical current at 77 K in self-field, and the critical current generally increased along with the crystallization temperature. The cross-sectional microstructure of YBCO thin films fabricated by using lead acetate addition was denser than that of the counterpart samples. In situ measurement of the film's resistance during the heat treatment was carried out to analyze the growth and nucleation processes of YBCO thin film deposition. The mechanism of lead acetate addition for the a / b -axis oriented grain suppression reported in our previous study was investigated. Highlights • Lead acetate modified precursor solution for YBCO superconducting film fabrication • 1 wt.% of lead acetate addition was favorable to achieve high critical current. • In situ measurement of the film resistance was carried out during heat treatment. • Influence of lead acetate addition on growth and nucleation processes was analyzed. [ABSTRACT FROM AUTHOR]

Highlights • Series measurements have been done on artificially indented Nb3Sn strand samples. • The indentation effects on strain sensitivity for Nb3Sn strand is a new research result. • The results in this manuscript showed that the indentation depth before heat-treatment should be controlled. Abstract The cable-in-conduit conductor (CICC) technology has been widely used in large scale superconducting magnet. The Short-Twist-Pitch (STP) design was aimed to avoid the conductor degradation during electromagnetic and thermal cycling. But manufacture of STP CICC deforms the cabled strands significantly. Indentations appear on superconducting strands after cabling and compaction. The conductor used for China Fusion Engineering Test Reactor (CFETR) magnet will be subjected to much higher Lorentz force than ITER. The STP CICC will be the first choice for the conductor design, since it showed no observed current sharing temperature T cs degradation after electromagnetic cycling. In order to estimate the effects of indentation on the superconducting and mechanical properties of Nb 3 Sn strands, series measurements have been done on an internal-tin (IT) Nb 3 Sn strand, fabricated by Western Superconducting Technologies Co., Ltd (WST). The strand was artificially indented and cut to several sections for different kinds of measurements, which mainly included the critical current I c versus axial strain measurements with Pacman device and stress-strain tests. The results show that the indentation has less impact on I c , but more impact on mechanical performance. The sample preparation, test results and analysis have been described in details. [ABSTRACT FROM AUTHOR]

• High current superconductors by texturing. • Scale-up of thin film superconductor tapes to manufacturing. • High magnetic field applications of HTS. This article is a lookback at the road traveled in the last 36 years to realize the abundant potential of high temperature superconductors (HTS) that had evoked worldwide excitement on their discovery. Numerous advances have been made in the materials science and manufacturing of HTS to overcome the many obstacles in implementing them in applications. [ABSTRACT FROM AUTHOR]

Highlights • The (Ba+Zr)/Cu composition of REBCO superconductors with 25% Zr addition affects its c-axis lattice parameter. • Increasing (Ba+Zr)/Cu content decreases lattice parameter mismatch relative to BZO nanocolumns. • The c-axis lattice parameter value significantly affects J c at (77 K, 0 T), (30 K, 3 and 9 T (B ||c)). • The reduction in BZO/REBCO mismatch increases BZO nanocolumn density leading to an improvement of in-field J c at 30 K. • The optimum c-axis value is temperature and field dependent, 11.74, 11.76 and 11.78 Å at (77 K, 0 T), (30 K, 3 T and 9 T (B ||c)). Abstract The relation between the c-axis lattice parameter of REBa 2 Cu 3 O 7−δ (REBCO and RE = Gd, Y) superconductors with 25 mol.% Zr addition and the critical current density (J c) was determined at (77 K, 0 T), (30 K, 3 T (B ||c)), and (30 K, 9 T (B ||c)). In this work, heavily doped Zr-added REBCO films with the thickness of 0.9 µm were fabricated by metal organic chemical vapor deposition (MOCVD) using a roll-to roll process. An increase in REBCO c-axis lattice parameter and a decrease in elastic mismatch strain relative to self-assembled BZO nanocolumns were observed with increasing (Ba + Zr)/Cu content. The optimum c-axis lattice parameter for achieving the highest J c in magnetic field was found to be temperature and field dependent, with optimum values of 11.74, 11.76 and 11.78 Å at (77 K, 0 T), (30 K, 3 T (B ||c)), and (30 K, 9 T (B ||c)), respectively. [ABSTRACT FROM AUTHOR]

In practical applications, such as superconducting magnetic energy storage (SMES) magnets or alternate current applications of superconducting power cables, high temperature superconducting (HTS) tapes are subject to cyclical loads caused by repeated thermal cycles and periodic electromagnetic forces. These repeated loads will affect electromechanical properties of the tapes. Previous electromechanical properties studies have provided useful information on the performance of multifilament Bi-2223 tapes under different forms of fatigue loading. The impact of axial tensile fatigue loading on the critical current ( I c ) under external magnetic field, however, has not been reported. In this work, we studied the variation of I c of the Bi-2223 tapes with the background magnetic field after different cycles of fatigue at 77 K. It was found that the effect of stress amplitude on the degradation of I c for Bi-2223 tapes was more significant than the maximum stress. When the maximum fatigue stress approached the irreversible stress limit, the copper alloy reinforced Bi-2223 tapes still had good current-carrying properties after 10 5 fatigue loadings. I c became less sensitive to the external magnetic field with the increase of the number of fatigue cycle. [ABSTRACT FROM AUTHOR]

Corrosion resistance is a crucial property to achieve successful superconducting joints of Y 0.5 Gd 0.5 Ba 2 Cu 3 O 7-z (YGdBCO) coated conductors (CCs). Cu and Ag metallic layers need to be fully removed from the area of conductor to be joint to allow for a superconducting path across the joint. Therefore, when using a wet etching process to remove the metallic layers, the joint performance can be significantly influenced by the etching conditions. The effects of chemical etching with ammonia water and hydrogen peroxide mixture on crystal structure, surface microstructure and critical current (I c ) of YGdBCO CCs were systematically investigated. We found the set of etching parameters that does not affect conductor performance, leaving the I c of the YGdBCO conductor unchanged upon etching. However, when the etching conditions are not optimal, decrease in I c was found and the underlying reasons driving the degradation were investigated. Raman spectroscopy and XRD analysis indicated that the reduced I c is mainly due to oxygen deficiency in the YGdBCO crystal lattice. [ABSTRACT FROM AUTHOR]

With the continuous reduction of the production costs and improvement of the transport performance, YBCO coated conductor is the most promising candidate for the high field magnet application due to its high irreversibility field and strong mechanical properties. Presently a stable production capacity of the YBCO conductors has been achieved by Shanghai Superconducting Technology Co., Ltd (SSTC) in China. Therefore, the demand in high field application with YBCO conductors is growing in China. This paper describes the design, fabrication and preliminary experiment of a solenoid coil with YBCO conductors supplied by SSTC to validate the possibility of high field application. Four same double pancakes were manufactured and assembled for the YBCO coil where the outer diameter and height was 54.3 and 48 mm respectively to match the dimensional limitation of the 14 T background magnets. The critical current (I c ) of YBCO conductors was obtained by measuring as a function of the applied field perpendicular to the YBCO conductor surface which provides the necessary input parameters for preliminary performance evaluation of the coil. Finally the preliminary test and discussion at 77 and 4.2 K were carried out. The consistency of four double pancakes I c was achieved. The measured results indicate that the fabrication technology of HTS coil is reliable which gives the conference for the in-field test in high field application. This YBCO coil is the first demonstration of the SSTC YBCO coated conductors. [ABSTRACT FROM AUTHOR]

MgB 2 superconductor samples co-doped with 0.02 wt. Al 2 O 3 and 0–0.05 wt. SiC were studied by magnetization – magnetic field (M-H) loop measurements at different temperatures. The critical current density has been calculated by the Bean model, and the irreversibility field, H irr , has been obtained by the Kramer method. The pinning mechanism of the co-doped sample with 2% Al and 5% SiC was investigated in particular due to its having the highest H irr . The normalized volume pinning force f  =  F / F max as a function of reduced magnetic field h  =  H / H irr has been obtained, and the pinning mechanism was studied by the Dew-Houghes model. It was found that the normal point pinning ( NPP ), the normal surface pinning ( NSP ), and the normal volume pinning ( NVP ) mechanisms play the main roles. The magnetic field and temperature dependence of contributions of the NPP, NSP , and NVP pinning mechanisms were obtained. The results show that the contributions of the pinning mechanisms depend on the temperature and magnetic field. From the temperature dependence of the critical current density within the collective pinning theory, it was found that both the δl pinning due to spatial fluctuations of the charge-carrier mean free path and the δ T c pinning due to randomly distributed spatial variations in the transition temperature coexist at zero magnetic field in co-doped samples. Yet, the charge-carrier mean-free-path fluctuation pinning ( δl ) is the only important pinning mechanism at non-zero magnetic fields. [ABSTRACT FROM AUTHOR]

A deformation-induced degradation model based on Weibull distribution function of statistical damage of superconducting filaments is developed to explore the influence of the torsional displacement on the critical current of Bi-2223 high temperature superconducting (HTS) composite tapes. It is extended in view of the critical current degradation mechanism of HTS tapes under uniaxial deformation. A longitudinal strain is introduced to unify the relation between uniaxial and torsional strain for the tapes. The extended model shows good predictions on the critical current degradation of the HTS tapes with torsional deformation in comparison with experiment data. The quantitative analysis indicates that the tensile longitudinal strain arised from torsional deformation plays a dominant role for superconducting filaments damage evolution compared to the compressive one, in which the irreversible degradation is always induced to remarkably reduce the critical current. Furthermore, a structural optimization design to enhance the carrying-current ability is achieved where the superconducting core should be less located at the tensile region of the HTS composite tapes. [ABSTRACT FROM AUTHOR]

The performance of a multi-layer composite superconducting tape depends on operating conditions, such as the applied field, temperature and stress. Therefore, accurate analysis of the electromagnetic-thermo-mechanical coupling behaviors of the high-temperature superconducting (HTS) cable is important to understanding the mechanism of interaction among different physical fields and the success of optimizing the design of the HTS cable structure. This paper develops a three-dimensional finite element model based on the twisted-stack slotted-core HTS cable structure for electro-thermo-mechanical analysis. Critical currents of a single multi-layer composite superconducting tape and the whole HTS cable structure are calculated. Profiles of the magnetic field, current density, temperature and stress are also obtained. It is found that the selection of tapes having higher critical current enhances the transport performance of the cable. Current densities are nonuniformly distributed in the stack. From the bottom to the top of the stack, the current density first increases and then decreases, in inverse relation to the magnetic field. Cooling with liquid helium improves the cable transport capacity. The temperature of the superconducting tapes is slightly higher than that of the aluminum core, but the stress in the aluminum core is greater than that in the superconducting tapes. The results imply that the transport capacity of the cable can be improved by selecting superconducting tapes with high critical current density and arranging them in stacks in a manner consistent with the electromagnetic field distribution. The model also explains the mechanism of interaction between different physical fields. It is a useful tool that clarifies the multiphysics coupling behaviors of an HTS cable with complex structure, and provides a theoretical reference for the design of novel HTS cable structures. [ABSTRACT FROM AUTHOR]

High-temperature superconductor (HTS) rare-earth-barium-copper-oxide (REBCO) tapes are very promising for use in high-current cables. The cable geometry and the layout of the superconducting tapes are directly related to the performance of the HTS cable. In this paper, we use numerical methods to perform a comparison study of multiple-stage twisted stacked-tape cable (TSTC) conductors to find better cable structures that can both improve the critical current and minimize the alternating current (AC) losses of the cable. The sub-cable geometry is designed to have a stair-step shape. Three superconducting tape layouts are chosen and their transport performance and AC losses are evaluated. The magnetic field and current density profiles of the cables are obtained. The results show that arrangement of the superconducting tapes from the interior towards the exterior of the cable based on their critical current values in descending order can enhance the cable’s transport capacity while significantly reducing the AC losses. These results imply that cable transport capacity improvements can be achieved by arranging the superconducting tapes in a manner consistent with the electromagnetic field distribution. Through comparison of the critical currents and AC losses of four types of HTS cables, we determine the best structural choice among these cables. [ABSTRACT FROM AUTHOR]

Increase in critical currents of hybrid superconducting samples in the presence of arrays of defects and applied magnetic field are studied numerically via the Time Dependent Ginzburg Landau equations, with the link variable technique. The results show that a gradient of density in the distribution of defects seems to be the most relevant factor for the highest increases in critical currents. Arching structure of the distribution is not a necessary feature for high increases, contrary to what was claimed before with conformal arrays. The results are obtained with the use of complex order parameter instead of a model of individual vortex dynamics, suggesting some potential limitations of this formulation. [ABSTRACT FROM AUTHOR]

Quasi-isotropic strands made of second generation (2G) high temperature superconducting (HTS) wires are attractive to applications of high-field magnets at low temperatures and power transmission cables at liquid nitrogen temperature in virtue of their high current carrying capability and well mechanical property. In this contribution, a 10 m length quasi-isotropic strand is manufactured and successfully tested in liquid nitrogen to verify the feasibility of an industrial scale production of the strand by the existing cabling technologies. The strand with copper sheath consists of 72 symmetrically assembled 2G wires. The uniformity of critical properties of long quasi-isotropic strands, including critical current and n -value, is very important for their using. Critical currents as well as n -values of the strand are measured every 1 m respectively and compared with the simulation results. Critical current and n -value of the strand are calculated basing on the self-consistent model solved by the finite element method (FEM). Effects of self-field on the critical current and n -value distributions in wires of the strand are analyzed in detail. The simulation results show good agreement with the experimental data and the 10 m quasi-isotropic strand has good critical properties uniformity. [ABSTRACT FROM AUTHOR]

The effect of local defects on the quench properties for YBCO tapes after applying bending strain was investigated at self-field in 77 K. The minimum quench energy (MQE) was related to the position of defects in the tape and the smallest MQE appears where the region of the defects existed in the position of the heater at the same transport current. The normal zone propagation velocity (NZPV) was related to the size and quantity of the regions of defects. The more defects were in the tape, the faster the normal-zone propagation velocity. [ABSTRACT FROM AUTHOR]

Electric-field-controlled piezoelectric strain has been used, recently, to modify the superconducting properties in a new class of piezoelectric/superconducting (PE/SC) hybrids. Here, we investigate the appearance of thermomagnetic instabilities (TMIs) and the respective modification of the critical current density (J C ) through the application of electric field (E ex ) in PE/SC hybrids. Specifically, the SC nanolayers are Nb (thickness, d SC   = 20 nm) deposited on both surfaces of PE macroscopic crystals of (1-x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-PT) with optimum composition x = 0.31 (thickness, d PE   = 0.5–0.8 mm). The appearance of TMIs and the modification of J C by E ex is studied for two PMN-PT crystals of drastically different surface roughness (Sa). In the case of the PMN-PT crystal with low Sa (on the order of a few tenths of nm) TMIs are absent so that J C does not change under the variation of E ex . On the contrary, in the case of the PMN-PT crystal with high Sa (on the order of a few hundreds of nm) E ex induces TMIs in the Nb nanolayers. Specifically, the number of TMIs exhibits a non-monotonic increase on E ex , thus causing a non-monotonic degradation of J C . These experimental data are interpreted in terms of the variation of both volume strain and surface roughness on E ex . This work highlights practical means to control the current-carrying capability of SC nanolayers through strain provided by PE substrates. [ABSTRACT FROM AUTHOR]

A rotating HTS flux pump is used to study the charging characteristics of superconducting synchronous motor. In order to design the rotating flux pump, the elliptical winding is essential. Numerous studies have been conducted on the degradation of the critical current with respect to the bending condition of the second generation (2G) high-temperature superconducting (HTS) tape. These studies focused on the same radius of curvature depending on the bending position. Therefore, in this paper, the critical current in a HTS coated conductor (CC) considering the curvature of elliptical shape is measured with regard to nine mandrels. These are a tall prolate ellipsoid, oblate ellipsoid and semicircle. In the tall prolate ellipsoid, the degradation of the critical current was the least. The diameters of each shape are 20, 30 and 40 mm, respectively. Finally, the experimental observations have been compared with the theoretical predictions. [ABSTRACT FROM AUTHOR]

• We found the transport performance of FeSe 0.5 Te 0.5 tapes protected only by the thin silver layer was poor at high background field, while it could be significantly improved by soldering a stabilizing layer. • We investigated the influence of different stabilizing layers on the performance of tapes. • We investigated the influence of soldering temperature on the performance of tapes. The analysis of the influence of different stabilizing layers and solders on the transport current is very necessary for the engineering application of FeSe 0.5 Te 0.5 coated conductors. The soldering temperature and thermal stress caused by cooling from room temperature to liquid helium can affect the critical current values of the superconducting tapes. For this, FeSe 0.5 Te 0.5 coated conductors with four types of stabilizing layers and three types of solders were prepared. The stabilizing layer includes the copper layer with a thickness of 0.02 mm, 0.05 mm, 0.1 mm, and the silver layer with a thickness of 0.1 mm. The solders are respectively Bi 50 Pb 25 Sn 12.5 Cd 12.5 , Bi 51.2 Pb 30.6 Cd 18.2, and Sn 63 Pb 37. The experimental results show that the critical current of the tape encapsulated by the 0.02 mm thick copper layer and Bi 50 Pb 25 Sn 12.5 Cd 12.5 is about 400 A at self-field and about 25 A at 10 T, which is superior to other samples. The details of the fabrication process and performance test results were presented in this paper. [ABSTRACT FROM AUTHOR]

• The feasibility of CICC cable based on Bi-2223 tape has been studied. The performances of cables with 3-, 4-, 5- and 6-slot are analyzed to investigate the relation between the quantity of grooves and tapes on the critical current. The comparison between the copper and aluminum frame on the quench behavior of cable is also discussed. • The critical current properties of the stacked-tape cable are studied by the self-consistent model and H-formula model. The hot-spot temperature during an emergency discharge is also estimated based on one-dimensional analyses. • The critical current of cable is mainly determined by the quantity of tape in cable where the self-field effect has little impact on the critical current in 4.2 K and 15 T. • The minimum quench energy (MQE) is proportional to the tape numbers where the MQE with copper frame is higher than that with aluminum frame. The hot spot temperature is decreased with tape numbers for cable with aluminum frame, however, the effect of tape numbers on the hot spot temperature of cable with copper frame is not obvious where the quench is triggered by MQE. The cable with lower quantity of grooves shows higher stability compared with other variants containing the same number of tapes. The cable-in-conduit-conductor (CICC) based on Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) tape has potential to be used in fusion magnets duo to its high current-carrying capacity under high field and low temperature. In this study, the optimized study on the stacked tape cable based on Bi-2223 tape was performed. The effect of the quantity of grooves and tapes on the critical current is studied by the finite element method, respectively. The comparison between the copper and aluminum frame on the quench behavior of cable is also discussed. The results shows that the critical current of cable is mainly determined by the quantity of tape in cable where the self-field effect has little impact on the critical current in 4.2 K and 15 T. The minimum quench energy (MQE) is proportional to the tape numbers where the MQE with copper frame is higher than that with aluminum frame. The hot spot temperature is decreased with tape numbers for cable with aluminum frame, however, the effect of tape numbers on the hot spot temperature of cable with copper frame is not obvious where the quench is triggered by MQE. Moreover, the cable with lower quantity of grooves shows higher stability compared with other variants containing the same number of tapes. [ABSTRACT FROM AUTHOR]

As the only high-temperature superconductor (HTS) that can be made into round wire without anisotropy, Bi-2212 round wire holds significant potential for applications as CICC (cable in conduit conductor) for superconducting magnets in fusion reactors. However, Bi-2212 is brittle and sensitive to strain which leads to a low mechanical performance. The effort on studying the impact of strain on the transport properties is mainly focus on the axial strain. However, the conductor will be inevitably subjected to torsional strain in the CICC. The torsional strain of Bi-2212 under background magnetic field has been rarely reported. In this paper, the critical current ( I c ) of Bi-2212 tape and round wire under torsion and background magnetic field is investigated and compared with each other. The measurement is performed at 77 K and 0–20 mT. The result demonstrates that the I c of Bi-2212 tape is strongly dependent on the magnetic field angle, while magnetic field angle has little influence on round wires. Although the external magnetic field and torsional angle have impact on Bi-2212 tape and round wire, the I c degradation of tested samples is more dependent on external magnetic field intensity than that of torsional angle. [ABSTRACT FROM AUTHOR]

Bi2Sr2CaCu2Ox is a potential material for the superconducting magnets of the next generation of Fusion reactors. A R&D activity based on Bi2212 wire is running at ASIPP for the feasibility demonstration of CICC. One sub-size conductor cabled with 42 wires was designed and manufactured. A test facility was designed and set-up to measure the resistance of joints and the critical current of the Bi2212 CICC in liquid helium. A superconducting transformer, which consisted of two concentric layer-wound superconducting solenoids, was used to provide the current of the conductor sample. Both of the primary and secondary coils were immersed in liquid helium during the experiments. The highest current of the secondary loop was up to around 14 kA. The critical current of the conductor was 13.2 kA with criterion of 1 μV/cm and the joints resistances were lower than 50 nΩ. [ABSTRACT FROM AUTHOR]

Heavy-ion irradiation of high-temperature superconducting thin films has long been known to generate damage tracks of amorphized material that are of close-to-ideal dimension to effectively contribute to pinning of magnetic flux lines and thereby enhance the in-field critical current. At the same time, though, the presence of these tracks reduces the superconducting volume fraction available to transport current while the irradiation process itself generates oxygen depletion and disorder in the remaining superconducting material. We have irradiated commercially available superconducting coated conductors consisting of a thick film of (Y,Dy)Ba 2 Cu 3 O 7 deposited on a buffered metal tape substrate in a continuous reel-to-reel process. Irradiation was by 185 MeV 197 Au ions. A high fluence of 3 × 10 11 ions/cm 2 was chosen to emphasize the detrimental effects. The critical current was reduced following this irradiation, but annealing at relatively low temperatures of 200 °C and 400 °C substantially restore the critical current of the irradiated material. At high fields and high temperatures there is a net benefit of critical current compared to the untreated material. [ABSTRACT FROM AUTHOR]

Polycrystalline Y 1− x Nd x Ba 2 Cu 3 O 7−δ ( x =0.02, 0.11, and 0.25) superconductors are synthesized. Nd atoms are uniformly distributed over grains. The magnetization loops of the samples have a pronounced second peak in a wide temperature range. The magnetization data are analyzed using the extended critical state model. It is found that the order-disorder transition of the vortex lattice is affected by doping with neodymium and temperature; the second-peak field and width decrease monotonically with increasing x value. The undoped polycrystalline YBa 2 Cu 3 O 7−δ compound is assumed to exhibit the peak effect in higher magnetic fields. [ABSTRACT FROM AUTHOR]

REBCO coated conductors are now available from several industrial manufacturers and are expected to be promising conductors for high-field-magnet applications. Using these conductors, the development of solenoids capable of generating high magnetic fields of 20–30 T is ongoing in major high-field laboratories in the world. In addition, CERN recently launched a conceptual design study for the Future Circular Collider, in which a 20-T dipole magnet is listed as a candidate for the bending magnet of the main ring. However, there has been limited research published on the electrical transport properties of commercially available REBCO conductors in a high-field, low-temperature environment. For magnet designers, the transport properties are of the highest importance in choosing a suitable conductor, and the data form the bases for high-field magnet development. Therefore, in this work, a new sample holder, which allows the measurements of full-width conductors to be carried out relatively easily, was developed, and the transport properties of commercial REBCO conductors from seven manufacturers (AMSC, Fujikura, Shanghai Superconductor, SuNAM, SuperOx, SuperPower, and SWCC Showa) were investigated at 4.2 K in perpendicular fields of up to 18 T. The results show that the I c values at 4.2 K clearly vary to some extent among these commercial conductors and the higher-current 4-mm-wide conductors have I c values in the range of 230–305 A at 18 T and in the range of 320–424 A at 12 T. [ABSTRACT FROM AUTHOR]

Majorana bound states in topological Josephson junctions induce a 4 π period current-phase relation. Direct detection of the 4 π periodicity is complicated by the quasiparticle poisoning. We reveal that Majorana bound states are also signaled by the anomalous enhancement on the critical current of the junction. We show the landscape of the critical current for a nanowire Josephson junction under a varying Zeeman field, and reveal a sharp step feature at the topological quantum phase transition point, which comes from the anomalous enhancement of the critical current at the topological regime. In multi-band wires, the anomalous enhancement disappears for an even number of bands, where the Majorana bound states fuse into Andreev bound states. This anomalous critical current enhancement directly signals the existence of the Majorana bound states, and also provides a valid signature for the topological quantum phase transition. [ABSTRACT FROM AUTHOR]

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface or point) and on their character (normal cores or Δκ cores). Different samples have been produced by Pulsed Laser Deposition, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. They have been characterized by SQUID Magnetic Properties Measurement System and Physical Properties Measurement System, as well as by Transmission Electron Microscopy (TEM). Correlations between pinning architecture, TEM images, and critical currents at various fields and field orientations will be shown for a large number of YBa 2 Cu 3 O x films with various types and architectures of artificial pinning centres. [ABSTRACT FROM AUTHOR]

REBCO coated tapes have become one of the most practical superconducting materials, which have been considered in the design of high-field magnets for fusion in the future. At present, several manufacturers are able to produce commercial REBCO tapes. During the fabrication and operation of REBCO magnets, REBCO tapes are subjected to tension, compression, bending, and torsion loads, resulting in applied strains. However, the critical current (I c) of REBCO tape is strain dependence. Therefore, the clear I c performance of a single REBCO tape under applied uniaxial strain is required before designing REBCO conductors/magnets in order to help select an appropriate tape specification and manufacturer. In this paper, several measurements of I c and n-value versus applied uniaxial strain were performed on different kinds of REBCO tapes, which were produced by Shanghai Superconductor Technology and Fujikura at 77 K with self-field on the U-spring device. The width of the REBCO tape is from 3 mm to 5 mm, and the thickness of the substrate is from 25 μm to 50 μm. The results show that the I c of most of the tapes are reversible under applied uniaxial compressive strain. The width of the tape and the thickness of the substrate affects the irreversible tensile strain limit. The n-value of the REBCO tape hardly depends on the strain condition. [ABSTRACT FROM AUTHOR]

We have investigated flux flow dissipation in typical two kinds of HTS tapes, i.e., a Bi-2223 multi-filamentary tape and a RE-123 coated conductor (CC) from the view point of heat load under over current conditions. Based on systematic measurements on current-voltage characteristics, the nonlinear flux flow dissipation has been described analytically by taking into account current sharing in metallic sheath or stabilization layer. Flux flow dissipation in the RE-123 CC shows much steeper temperature dependence than that of the Bi-2223 tape. As a result, attainable cooling power becomes smaller in the RE-123 CC in comparison with that of Bi-2223 tape even if the same cooling condition. In other word, acceptable temperature rise in the RE-123 CC is small at over current condition, whereas moderate temperature dependence in the Bi-2223 tape allows stable operation even if the bias current exceeds the critical current. Influence of spatial inhomogeneity in the both HTS tapes has also been investigated. Longitudinal variation of local critical current, I c , and its statistical behavior have been characterized by use of reel-to-reel scanning Hall probe microscopy. It has been found that the flux flow dissipation is possibly localized more than one order higher than that of the average value due to discrete local I c drops. [ABSTRACT FROM AUTHOR]

We present an approach for minimizing the critical current for the magnetization switching in magnetic tunnel junctions by optimizing the spatial distribution of the current density. We show that such a minimization is possible because critical current is determined by the condition of making one of the magnetization eigenstates grow in time. The excitation of the eigenstates is enhanced when the spatial distributions of the eigenstates and current density overlap. Critical current can be viewed as a functional of the current density spatial distribution and it can be minimized by optimizing this distribution. Such an optimization results in a major reduction of the critical current and increase of the switching efficiency, viz. the ratio between the energy barrier and critical current. The minimized critical current increases approximately linearly with the magnetic tunnel junction size, which is much slower than critical current for the case of a uniform current density. The optimized efficiency can be approximately a constant with respect to the magnetic tunnel junction size, which is much higher than the efficiency for the uniform current density. Additional optimization can be achieved by spatially modulating the material parameters, e.g., the saturation magnetization. The presented approach and obtained scaling of the critical current and efficiency offers opportunities for the magnetic tunnel junction optimization. [ABSTRACT FROM AUTHOR]

• The twisting characteristics of quasi-isotropic strand (Q-IS) and stacked-tape strand (STS) are investigated and compared with each other by calculation, simulation and experiment. • Influences of stacking patterns of REBCO tapes in Q-IS and STS on the twisting characteristics of strands are considered and analyzed. • The different strain distributions of Q-IS and STS are analyzed and calculated, which lead to the different twisting characteristics of Q-IS and STS. The conclusions are verified by twist experiments. • The strands are proved to have a good critical current carrying capacity under torsional loads, so they are suitable for fusion and high-field magnet applications. With the gradual maturity of high temperature superconducting (HTS) technology, several HTS conductors with large critical current like quasi-isotropic strand (Q-IS) and twisted-stacked-tape cable (TSTC) were proposed, which can be assembled into cable-in-conduit conductor (CICC) and applied in power system and fusion magnets. The purpose of this paper is to study the twisting characteristics of Q-IS and stacked-tape strand (STS) with different stacking patterns under self-field at 77 K. The critical currents of two Q-IS samples and two STS samples are measured under increasing twist angle until they break. Each sample is composed of copper tapes and second generation (2G) REBa 2 Cu 3 O x (REBCO, RE = rare earth) HTS tapes. For each type of conductor, one sample has a structure where REBCO tapes are stacked near the outside, while the REBCO tapes are stacked close to the inside in another sample. Simulation based on finite element method (FEM) and H -formulation is carried out for comparison. Numerical calculation of the normalized critical current versus the twist pitch is also conducted. Results show that each kind of strand with REBCO tapes stacked close to the inside have smaller critical twist pitches. Besides, self-field effect is dominant while the twist angle is small, but the effect of longitudinal strain gradually becomes the main factor as the twist angle increases. In general, the critical current degrading rates of STS samples are smaller than that of Q-IS samples. However, when the twist angle is relatively small, Q-IS samples show better critical current characteristics than STS samples. [ABSTRACT FROM AUTHOR]

• The I c of the high J c Nb 3 Sn strand with the indentation depth is investigated. • The variation of RRR of high J c Nb 3 Sn strands with indentation depth is investigated. • The variation law of effective wire diameter with indentation depth is determined. • The critical indentation depth for strand performance degradation is determined. To meet the peak magnetic field requirements of the central solenoid (CS) and toroidal field (TF) magnets in the China Fusion Engineering Test Reactor (CFETR), cable-in-conduit conductors (CICC) that are composed of Nb 3 Sn strands with high critical current density (J c) have been developed. The Short-Twist-Pitch (STP) design is adopted to avoid performance degradation caused by the electromagnetic cycles. But the STP design and tight cabling will cause severe indentation at the contact point between strands. Previous studies on Nb 3 Sn CICC show that the indentation has a significant effect on the critical current (I c) and residual resistivity ratio (RRR) of the superconducting strand. Therefore, the same study was carried out for the high J c Nb 3 Sn superconducting strand produced by Oxford Superconducting Technology (OST) and Kis wire Advanced Technology CO., Ltd. (KAT). In this paper, the I c and RRR measurements of sample at different indentation depths were made, and the results were compared with the previous Nb 3 Sn indentation strands and flat rolled samples. Also, the hysteresis loss of the samples with different indentation depth was measured, and the change of effective filament diameter with indentation depth was obtained. [ABSTRACT FROM AUTHOR]

In this paper, we have calculated the dependence of critical current density j c on the radius R and concentration of extended ferromagnetic defects in the system simulating a high-temperature superconductor (HTSC) layer. It was shown that at fixed volume concentration of ferromagnetic fraction the j c (R) dependence, as against nonmagnetic defects, has one maximum or two maxima at certain magnetization values of ferromagnetic fraction. We found that these maxima are due to the interplay of two parameters: concentration and effective depth of potential wells of defects. [ABSTRACT FROM AUTHOR]

A series of MgB 2 samples were irradiated with protons of 11.3 and 13.2 MeV. Magnetization data shows an insignificant reduction of the critical temperatures but a continuous decrease of the Meissner fraction with increasing fluence or energy. All samples show a consistent improvement of the critical current density compared to the virgin sample and an increase of the pinning energy at high fields as resulted from relaxation data. [ABSTRACT FROM AUTHOR]