REBCO-coated Conductors Research Articles

Error analysis for determining transverse tensile delamination strength of REBCO coated conductors by anvil test: numerical simulation demonstrations

RE–Ba–Cu–O (REBCO, where RE = Y, Gd, Sm, and other rare earth elements) coated conductor (CC) tapes exhibit considerable potential for application within the domains of high-energy physics and high-field science. Nevertheless, weak interfacial properties pose a significant obstacle, impeding the progress and practical implementation in high-field scenarios. The anvil tension method has been extensively employed for the assessment of transverse delamination strength of REBCO CC tapes. However, the outcomes derived from anvil tension exhibit severe dispersion, thereby impeding its efficacy in evaluating material performance. The underlying cause of this phenomenon remains unidentified. In this study, error analysis of anvil measurement method in determining the transverse tensile delamination strength (TTDS) of REBCO CC tapes was conducted based on finite element (FE) numerical simulations. A two-dimensional multilayer elastic-plastic delamination FE model with main layers of REBCO CC tapes, solder connecting layers and anvil materials were developed based on the bilinear cohesive zone model. The effects of anvil test conditions and the structural configuration of the conductor itself on the test results were discussed. Simulation results show that localized premature cracking of the interface due to stress concentration and plastic yielding of the CC tape around the loading boundary is the root reason for the discrepancy between the anvil test results and the true interfacial strength. Therefore, anvil test conditions (including top anvil dimensions, soldering conditions, loading eccentricity angle, and anvil material properties) as well as the structural configuration of the conductor itself (including edge initial crack length, edge encapsulation width, and stabilizer thickness) have a significant impact on anvil test-based TTDS results.

Enhancing in-field performance of GdBCO coated conductors by cooperative irradiation with Ti ions and protons

Irradiation can accurately manipulate defects and adjust pinning landscapes within REBa2Cu3O7-δ (REBCO, RE: rare earths) coated conductors (CCs). This study reports a productive method to dramatically boost the in-field critical current density (Jc) for GdBCO CCs using cooperative irradiation with Ti ions and protons. Remarkably, the in-field Jc of commercial CCs can be almost doubled at a wide range of temperatures and magnetic fields. Defects of various sizes induced by cooperative irradiation are more uniform distribution through the entire GdBCO film to improve the vortex pinning characteristics, thereby enhancing the in-field performance of the GdBCO CC. This method highlights how combining different particle irradiation types can tailor defect size and distribution, optimizing pinning landscapes for commercial REBCO CCs.

Numerical Study on Mechanical Behavior and Electromechanical Properties of Solder-Jointed REBCO-Coated Conductors.

As the second-generation high-temperature superconducting conductors, rare earth-barium-copper-oxide (REBCO) coated conductor (CC) tapes have good potential as high-field and high-energy superconductors. In superconducting applications, several joints are required for conjugating comparatively short REBCO CC tapes. Soldering lap joints are the simplest and most commonly applied REBCO CC joints. In addition to joint resistance, the mechanical behavior and electromechanical properties are also crucial for superconducting applications. In this paper, the electromechanical properties and mechanical behaviors of soldering lap joints at 77 K under a self-field were studied. The mechanical behavior was addressed by using a full three-dimensional multilayer elastic-plastic finite element model (FEM) with REBCO CC tape main layers and solder connecting layers. Then, the electromechanical properties were analyzed by using Gao's strain-Ic degradation general model on the basis of the FEM results. Both the mechanical behavior and electromechanical properties were verified by experimental results. The effects of soldering lap conditions including lap length, soldering thickness and lap style on the electromechanical properties and mechanical behaviors were discussed. The results indicate that shorter overlap lengths and a thinner solder can reduce the premature degradation of Ic due to stress concentrations nearby the joint edges; moreover, the irreversible critical strain is significantly higher in the back-to-back joint approach compared to the widely used face-to-face joint approach.

Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa2Cu3O7-x Superconducting Film

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility χ1″ related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density Jc and the pinning energy U of the sample have been extracted at 77 K by using the χ1″ measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the χ1″ at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the χ1″ as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the χ1″ with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications.

Property Variations in Modern REBCO Coated Conductors from Multiple Manufacturers

The complex, multilayer structure of REBCO Coated Conductor (CC) poses significant challenges in the fabrication of high magnetic field devices where large stresses may initiate various forms of damage. Our goal is to peer below the cartoon representations of CC so that, amongst other things, we might better understand whether a CC from one manufacturer is interchangeable with that from another. This involves knowledge of a broad range of electromagnetic, geometric, microstructural, and Jc (θ,B,T) properties, and their variations that collectively pose challenges for the fault tolerance of REBCO CC devices. Accordingly, comparative measurements of Jc , visualization of flux penetration with Magneto-Optical Imaging (MOI), tape geometry from Scanning Electron Microscopy (SEM) of polished cross-sections, and extensive optical microscopy was performed on recently purchased samples from multiple manufacturers. Our analyses reveal many deviations from or characteristics absent from manufacturers’ specifications, while a comparison of different manufacturers’ mechanically and laser slit tapes shows a diverse array of slitting characteristics amongst the manufacturers and variation in properties those made to the same specification. Laser slit tapes from several manufacturers reveal ablated edges with damaged regions extending up to 50 μm, comparable to the damaged region found in the mechanically slit CC of this study. Overall, the aim of this study is to flesh out appropriate ways to understand the real conductor below the manufacturers’ cartoons to avoid surprises in our REBCO CC coil development program. The goal of this work was to perform a broad array of characterizations of the type needed for validation of purpose for making high field magnets: to our surprise we found a wide range of properties which greatly impact the mechanical strength and electromagnetic performance of solenoids composed of these conductors and reinforced the need for a broad characterization program for each conductor prior to its implementation.

Unlocking the performance evolution of GdBCO coated conductors irradiated by deuterium plasma

REBa2Cu3O7-δ (REBCO, RE: rare earth) coated conductors (CCs) can generate a powerful magnetic field and are optimal for sustaining the reaction in fusion devices. However, the REBCO CCs will inevitably be exposed to neutron radiation during device operation. Transmutation produces gas atoms such as hydrogen will affect their properties. In this paper, GdBCO CCs were irradiated by 20 eV deuterium plasma. It was found that the irradiation introduced smaller (2–4 nm) pinning sites that formed a mixed pinning landscape with rare earth oxides (tens of nanometres) in the pristine samples, leading to an improved vortex pinning. At an irradiation fluence of 5.0 × 1018 D cm−2, the J c values of the samples irradiated by deuterium can maintain the best enhancement of in-field performance at 4.2 K. Excess irradiation results in a decrease in J c, a broadening of the superconducting transition width, and an enhanced disorder in GdBCO. This study provides some valuable insights for potential changes in the properties of REBCO CCs irradiated in fusion devices.

An experimental study to investigate magnetic field and winding force-dependent contact resistance of NI REBCO coil

This paper reports the magnetic field and winding force-dependent contact resistance, also known as characteristic resistance, of no-insulation (NI) REBCO coil. Three NI coils were wound with different winding forces ( < 1 kgf, 3 kgf, and 5 kgf) using the same REBCO-coated conductor, and they were tested at 4.2 K in a background field of 0, 5, 10, and 14 T. The contact resistance of each coil is measured at each field. As a result of the measurements, we draw two conclusions. First, a quadratic function for magnetic field intensity reasonably reproduces the measurement results of each NI test coil’s magnetic field-dependent contact resistance, where the practical fit function’s coefficients are determined depending on the winding force. Second, the contact resistance of each coil is partially inconsistent with the magnetoresistance of the electroplated material, i.e. copper, of the REBCO-coated conductor. This paper will present the measurement results in detail, discuss them with Kohler’s rule, and formulate the contact resistance using winding force and magnetic field terms.

Evaluation of the impact of REBCO-coated conductors on the resistive wall impedance of the FCC-hh

The beam screen for the Future Circular hadron-hadron Collider (FCC-hh) has a baseline design based on a copper (Cu) coating. Calculations have indicated that the resistive wall impedance will be the major contributor to the beam impedance for the FCC-hh at both injection and collision and that Cu loss might be on the limit to ensure beam stability. To increase the safety margin, it is desirable to reduce the resistive wall impedance. In this contribution, we present an approach to reduce the beam impedance based on the reduction of the surface resistance of the beam screen coating by using High-Temperature Superconductors based on REBaCu3O7−x coated conductors (REBCO-CCs). These HTS-CCs have transition temperatures around 90 K, and critical current densities which are high enough even in the presence of a strong magnetic field, being therefore, good candidates to substitute Cu in the FCC-hh beam screen, which will be operating at around 50 K and under a magnetic field of 16 T. Using measured data of the surface impedance of REBCO-CCs, the beam impedance has been estimated for an elliptical beam screen with the same vertical dimension as that of the baseline design. A REBCO-CCs contribution dependence study to determine the optimum beam screen will be shown. Resistive wall impedance studies using an ellipse are a step forward towards determining the performance of the REBCO-CCs on the FCC-hh beam screen.

A protocol for mapping transport current density of REBCO-coated conductor by magneto-optical imaging

The measurement of transport current density is significant for investigations on improving the properties of REBa2Cu3O7−x (REBCO, where RE refers to rare-earth elements)-coated conductors (CCs). In this work, a protocol for mapping the transport current density of CC by magneto-optical imaging (MOI) is presented. A calibration method is developed based on the non-linear physical governing function for the MOI indicator, in which only two parameters are determined, i.e. the anisotropic magnetic field BA , and cMs , which is the multiplication of the constant c related to the thickness of the MOI indicator and the spontaneous magnetization Ms . The experimental results also showed that they were independent of the optical measure condition but dependent on temperature, making this calibration method comparative among different utilizers. The numerical results clearly manifested that the selected window size of the magnetic field around a long superconducting tape is closely related to the error of the reconstructed current density. A large window size of the magnetic field is needed to precisely reconstruct the transport current density. For actual MOI testing, a practical approach to extend the magnetic field data outside the MOI window was realized by fitting with a power function according to Ampere’s law, through which the false current density outside the sample is automatically suppressed to a large extent. On this basis, the mapping of the transport current density in the CC sample was achieved. It is believed that this work will improve MOI for a more precise measurement of the transport current density for long superconducting strips.

Striating of REBCO-Coated Conductors for AC Loss Reduction

Striating of REBCO-Coated Conductors for AC Loss Reduction

Simulation and experimental studies of mechanical delamination characteristics of REBCO tapes and lap joints under transverse tension

In this study, we investigate the delamination properties of high-temperature superconducting REBa2Cu3O7−x (REBCO) coated conductor (CC) tapes and lap joints under transverse tension by a finite element model (FEM) based on the cohesive zone model. The interfacial cohesive strength and the initial penalty stiffness used in the FEM calculations are adopted based on the experimental data obtained from the anvil tensile tests. Compared to previous theoretical studies, our calculation results of the delamination behaviors for the REBCO CC tapes and the lap joints are more approaching to the experimental data, and the abrupt failure behavior of the transverse load–displacement curves, which appears in the anvil tests, is reproduced by the simulations. Moreover, the effects of welding strength between the upper and lower tapes and internal tape damage during the welding process on the delamination characteristics, including the delamination strength, initial separation, delamination locations, and damage distribution of the CC lap joints, are presented.

Superconducting joints for the 1.3 GHz persistent NMR magnet under JST-Mirai Program

Superconducting joint technologies connecting high-T c superconducting (HTS) tapes, such as REBCO-coated conductors and Bi2223 silver-sheathed tapes, have been developed for the fabrication of superconducting magnets generating 30.5 T with persistent current operation at 4.2 K under the JST-Mirai Program since 2017. These are indirect joints using a superconducting intermediate layer to join highly crystallized HTS layers of tapes. Thus far, high I c values of above 100 A at 77 K in the self-field have been achieved by both joints. These performances are sufficient for installation to the 30.5 T magnet, while efforts to further improve the reproducibility of high I c for joints connecting long-length HTS tapes have been continuously made. The current status of HTS joint technologies is summarized in this paper partly from the view point of material science.

Numerical Analysis of Dynamic Resistance and Total Loss in REBCO-Coated Conductors at Low Temperature Under High Perpendicular AC Magnetic Fields

In several applications of high-temperature superconductors (HTS), a REBCO-coated conductor (CC) carries a DC current under varying magnetic fields. This generates AC loss which can be an important factor to consider in the thermal design of the application. In this work, the dynamic resistance (Rdyn), and total loss in a 4 mm wide Superpower REBCO CC are numerically investigated for AC perpendicular magnetic fields ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> m) up to 1 T and temperatures ranging from 30 K to 50 K. For <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> > 0.1 (where Idc is the DC current level and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> is the self-field critical current of the conductor), either of the previously developed equations, the non-linear or the simple linear expression for determining B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> (threshold magnetic field), can be used to predict R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dyn</sub> . Finite dynamic loss is observed even below the B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> points due to the emergence of resistance. The resistance arises from the superposition of subcritical shielding and transport currents which introduces a finite electric field in the CC. The total loss at higher fields decreases as the temperature increases from 30 K to 50 K for all current values, except for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> = 0.9. A lift-off in the total loss is evident for fields around 1 T for all temperatures investigated.

Characterization of Electromechanical Delamination Behaviors in Practical REBCO Coated Conductor Tapes Under Transverse Tension at 77 K

In high-temperature superconducting coils, the high transport current density and magnetic field generate a large electromagnetic force on REBCO-coated conductor (CC) tapes. A large Lorentz force can cause interlayer delamination damage to the CC tape under radial or transverse tensile loads. The structural delamination characteristics of multilayered REBCO CC tape are essential for the fabrication of mechanically reliable CC coils. In this study, the electromechanical delamination behaviors of practical REBCO CC tapes were characterized using an anvil test method. Our recently established continuous critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measurement system was used to minimize the test time and can directly evaluate the effect of the transverse tensile load on I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> behavior without time-consuming intermittent I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measurements during testing. Characterization of the delamination mechanism, including damage initiation, was performed through scanning electron microscopy observation of the fractured sample. Two-parameter Weibull distribution analysis was used to specify the data scattering of the electromechanical delamination strength.

High-field critical current density enhancement in GdBCO coated conductors by cooperative defects

Irradiation can precisely control defects in, and improve the superconducting properties of, REBa2Cu3O7−δ (REBCO, RE: rare earth) coated conductors (CCs). Here we report an effective approach for enhancing the in-field performance of GdBCO CCs. The critical current density (J c) of GdBCO films was significantly improved through cooperative defects created by co-irradiation with O ions and protons, especially at low temperatures and high magnetic fields. Surprisingly, the in-field J c of commercial CCs can be nearly doubled. The cooperative irradiation-induced defects are uniformly distributed throughout the GdBCO layer, which promotes the overall performance of the CC. Moreover, the dimensions of these irradiation-induced defects closely match the coherence length of REBCO. This simple and efficient method is a practical post-production solution to improve the in-field performance of commercial REBCO CCs.

Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions

Particle accelerators with superconducting magnets operating at cryogenic temperatures use a beam screen (BS) liner that extracts heat generated by the circulating bunched charge particle beam before it can reach the magnets. The BS surface, commonly made of high–conductivity copper, provides a low impedance for beam stability reasons, low secondary electron yield (SEY) to mitigate the electron–cloud (EC) effect, and low electron–stimulated desorption yield (ESD) to limit the dynamic pressure rise due to EC. Rare–earth barium copper oxide (REBCO) high–temperature superconductors (HTSs) recently reached technical maturity, are produced as coated conductor tapes (REBCO–CCs), and will be considered for application in future colliders to decrease the BS impedance and enable operation at around 50 K, consequently relaxing the cryogenic requirements. Aside from HTS properties, industry–grade REBCO–CCs also need qualification for EC and dynamic vacuum compatibility under accelerator–like conditions. Hence, we report the SEY and ESD measured at cryogenic temperatures of 12 K under low–energy electron irradiation of 0–1.4 keV. We also verify the sample compositions and morphologies using the XPS, SEM, and EDS methods. The energy and dose dependencies of ESD are comparable to those of technical–grade metals and one sample reached SEYMAX = 1.2 after electron conditioning.

Analysis of mechanical behavior and electromechanical properties of REBCO-coated conductor tapes under combined bending-tension loads using numerical methods

Rare-earth barium copper oxide (REBCO) high-temperature superconducting-coated conductor (CC) tapes are potential conductors for high-field magnets. In an operating high-field magnet, REBCO CC tapes are bent into coils and simultaneously subjected to hoop electromagnetic forces. Under these combined bending-tension loads, the critical current () of the REBCO CC tapes is at risk of irreversible degradation. Therefore, investigating the mechanical behavior and electromechanical properties of REBCO CC tapes under combined bending-tension loads is necessary. In this study, the mechanical behavior of REBCO CC tapes was analyzed using a finite element model (FEM). First, in the fabrication-cooling process of REBCO CC tapes, the thermal residual stress/strain accumulated in the constituent layers was analyzed. Then, considering the thermal residual stress/strain as the initial stress–strain state, the mechanical behavior of REBCO CC tapes under various mechanical loads, such as axial tension, bending, and combined bending-tension at 77 K, was analyzed. Furthermore, a phenomenological model of the internal strain in the REBCO films dependence of was developed for analyzing the electromechanical properties of REBCO CC tapes under combined bending-tension loads at 77 K. Calculated results showed that the compressive thermal residual strain in REBCO films at 77 K was −0.25%, and the internal strain in the REBCO films corresponding to the irreversible degradation of was 0.45% under axial tension loads, which was verified by experimental results. Under bending and combined bending-tension loads, the distribution of the internal strain in the REBCO films along the width direction was non-uniform owing to the Poisson effect, and the onset of the irreversible degradation of occurred at the outer edge of the REBCO films. The phenomenological model was experimentally verified to be effective in degradation behavior prediction under combined bending-tension loads, and model predictions based on the FEM results indicated that the electromechanical properties were significantly influenced by the bending modes (tensile bending or compressive bending) of the REBCO CC tapes.

Review of progress and challenges of key mechanical issues in high-field superconducting magnets.

The development of modern science and technology requires high magnetic fields exceeding 25T. Second-generation high-temperature superconducting wires, i.e. REBCO (REBa2Cu3O7-x, RE refers to Y, Gd, Dy, Eu and other rare-earth elements) coated conductors (CCs), have become the first choice for high-field magnet construction because of their high irreversible magnetic field. The mechanical stresses caused by manufacturing, thermal mismatch and Lorenz forces closely influence electromagnetic performance during operation for REBCO CCs. In addition, the recently studied screen currents have effects on the mechanical characteristics of high-field REBCO magnets. In this review, the experimental and main theoretical works on critical current degradation, delamination and fatigue, and shear investigations on REBCO CCs, are reviewed at first. Then, research progress on the screening-current effect in the development of high-field superconducting magnets is introduced. Finally, the key mechanical problems facing the future development of high-field magnets based on REBCO CCs are prospected.

Potential impedance reduction by REBCO-coated conductors as beam screen coating for the Future Circular Hadron Collider

The Future Circular Collider study creates a conceptual design for a post-LHC particle accelerator using 16 T superconducting dipoles to achieve collision energies of up to 100 TeV in a 90 km circumference ring. A copper-coated beam screen, similar to that used in the LHC, is planned. However, the undertaken research indicates that copper at the high working temperature of 50 K has a strong influence on the accelerator's performance, particularly at injection energy. In this work, we relate the experimentally determined properties of REBCO-coated conductors with their potential performance in the FCC-hh beam screen. Specifically, we use a round pipe approximation to demonstrate that a beam screen coated with a combination of REBCO and copper can have a much lower resistive wall impedance than one using only copper. The reduction is substantial (several orders of magnitude), and is observed in both the longitudinal and transverse wall impedance. Such a reduction has important effects on beam stability, operating costs, potential reduction in beam screen size, and lowering the stringent specifications of the 16 T magnets required for the Future Circular Hadron Collider.

Quench Behaviors of REBCO-Coated Conductors Impregnated with Aluminum Nitride–Filled Epoxy Composites

Quench Behaviors of REBCO-Coated Conductors Impregnated with Aluminum Nitride–Filled Epoxy Composites