Rare-earth Barium Copper Oxide Tapes Research Articles

Investigation on characteristic of vanadium trioxide insulation mixed with metal powder for rare-earth barium copper oxide coils

This study examined the turn-to-turn contact resistance (R ct) between rare-earth barium copper oxide (REBCO) tapes and layers of vanadium trioxide (V2O3) and V2O3 mixed with metal powder mixture. V2O3 in single crystal structure was electrically characterised to exhibit resistivity with negative temperature dependence, allowing the turn-to-turn insulation to self-regulate the current bypass between REBCO tapes. To facilitate effective quench protection of V2O3-insulated REBCO magnets above the metal-insulator transition temperature (T rt), R ct must be further reduced to a level similar to those of non- and metal as insulated (NI and MI) REBCO magnets. Thus, we explored the mixing of conductive metal powders such as molybdenum (Mo) with V2O3 paste and investigated the transition properties of R ct. The resistance versus temperature characteristics, microscopic morphologies of the V2O3 layers, and thermal conductivity (k v) were appropriately assessed to determine the effects of mixing the metal powder with V2O3. The R ct of virgin V2O3 exhibited variations of 107–105 μΩ cm2 under 77–293 K. As the mixing concentration of the metal powder was increased, the reduction magnitude on R ct increased for > T rt (approximately 150 K). Furthermore, the transition slope became gentler for a wider temperature range of < T rt. For metal powder concentrations exceeding 50 wt%, R ct decreased by approximately 2 orders of magnitude (∼103 μΩ cm2) for > 150 K compared with that for virgin V2O3 paste. Moreover, compared to that of pure V2O3, k v demonstrated a remarkable increase of approximately 352% at 91 K for Mo powder mixed at a concentration of 60 wt%. The improved electrical and thermal properties of the V2O3 insulation layer owing to the mixing of metal powders can help REBCO magnets operate in an insulated state under normal conditions and effectively convert to a non-insulated state under quenching.

A newly developed 10 kA-level HTS conductor: innovative tenon-mortise-based modularized conductor (TMMC) based on China ancient architecture

We proposed a new type of high-temperature superconducting (HTS) conductor concept: modularized conductors (MCs) connected by Chinese traditional tenon-mortise (TM) connection structure, referred as TMMC (tenon-mortise modularized conductor). The conductor consisted of multiple concentric round sub-conductors with slots for stacking rare-earth-barium-copper-oxide (REBCO) tapes. Innovatively, the REBCO stacks in the adjacent sub-conductors were arranged with the fully-misaligned configuration to enhance the critical current’s isotropy with respect to magnetic field and reduce ac loss. For example, the angle between the adjacent stacks in the two adjacent sub-conductors was 45° if each sub-conductor contains 4 REBCO stacks. In order to construct the fully-misaligned configuration, the sub-conductors were designed with two open half-circular formers and connected by TM structure which makes the conductor modularized and simply to assembly and disassembly. Based on the design concept, a prototype conductor containing 160 REBCO tapes distributed in the four concentric sub-conductors was fabricated. The conductor’s measured critical current was 13.69 kA at 77 K and self-field, which was consistent to the simulation result. In order to further improve the TMMC’s engineering critical current density (J ce) and bending performance, we proposed two enhancement approaches: reducing the former’s thickness and re-arrange stacks in the outer sub-conductors. With the enhancements, both TMMC’s radius and J ce were comparable to the existing slotted-core conductor. The study shows the TMMC’s advantages of non-twisted structures, easy assembly, high-current carrying and low ac losses, which made it promising for constructing large-scale scientific devices.

Experimental study on the an-isotropic critical current of REBCO tape.

Superconducting magnets are widely used in nuclear fusion reactors, high-energy particle accelerators, steady-state high magnetic fields, etc. Higher magnetic fields and higher operating temperatures are two application trends. High temperature superconducting (HTS) materials are the only choice for high temperature and high field magnets in the future. The first- and second-generation HTS materials have a typical tape structure; their critical performance is magnetic field angle and temperature dependent. A new test facility is developed for an experimental study on the an-isotropic critical current. The field angle can be changed from 0° to 360° with a resolution of 1°. The rotation deviation angle is measured to be 0.2° when the upper part rotates 90°. The temperature can be changed from 4.2 to 80K. The temperature errors are ±50, ±80, and ±135 mK for 4.2-20, 20-40, and 40-80K, respectively. The angle dependence of critical current (Ic) of the tested rare-earth barium copper oxide tape within 0°-30° is strong. From 30° to 90°, the sample Ic almost does not change with the magnetic field angle. The implementation of the project will not only promote the structural optimization of HTS tapes but also promote the miniaturization and economical application of HTS magnets.

The improved model based on the H-A formulation in large-scale HTS magnet

The rapid progression of the rare earth barium copper oxide (REBCO) coated conductor have paved the way for the development of high-field superconducting magnets. However, the elevated aspect ratio of REBCO tapes results in the generation of substantial screening currents, presenting a formidable challenge in the electromagnetic analysis of high-field superconducting magnets. Currently, the finite element method (FEM) stands as the dominant technique employed in large-scale superconducting magnet system for screening current computations in REBCO tapes, encompassing primarily the H formulation and the T-A formulation. However, these models exhibit certain limitations. In this paper, the two-dimensional axisymmetric H-A FEM model is introduced into the computation of REBCO tapes in high-field magnet, and the corresponding homogeneous model, partial homogeneous model and multi-scale model are established. It can be demonstrated that the model not only significantly accelerates computational speed but also enhances precision in comparison to the T-A model. Remarkably, the computational time of the H-A homogeneous model is approximately a quarter of that required by the T-A homogeneous model. It is anticipated that the model harbors substantial potential for future applications in the computational analysis of large-scale superconducting magnets.

Intelligent estimation of critical current degradation in HTS tapes under repetitive overcurrent cycling for cryo-electric transportation applications

Overcurrent cycling refers to the procedure of imposing repetitive overcurrent to superconducting tapes/devices for characterizing their critical current reduction. Characterizing the overcurrent cycling behaviour of Rare Earth Barium Copper Oxide (ReBCO) tapes is a crucial step in the design process of High Temperature Superconducting (HTS) devices. Multiple overcurrent incidents during the operation of an HTS device can significantly decrease the total critical current, leading to potential quenches and failures. Data-driven models have been proposed in the literature to estimate the Critical Current Degradation Rate (CCDR) of ReBCO tapes under multiple overcurrent scenarios. However, these methods have exhibited notable errors in the range of 8%–11%, in the estimation of the critical current reduction. This paper proposed methods based on Artificial Intelligence (AI) techniques aimed at the challenges of conventional methods of CCDR estimation. Different AI-based techniques were proposed, tested, and compared to show the effectiveness of the proposed intelligent approach, including Support Vector Regression (SVR), Decision Tree (DT), Radial Basis Function (RBF), and Fuzzy Inference System (FIS). Experimental data on critical current values of ReBCO tapes subjected to multiple and repetitive overcurrent cycles were employed for this investigation. The results demonstrated that the Mean Relative Error (MRE) of the SVR method is 23%, for the DT model is approximately 0.61%, the MRE of the FIS model is well above 0.06%, and the MRE value for the RBF method is about 1.1 × 10−6%. Moreover, the proposed AI models offer fast test times, ranging from 1 to 11 ms. These findings highlighted the potential of using AI techniques to enhance the estimation accuracy of the risks associated with overcurrent events.

Electromechanical characteristic of stacked REBCO tapes under tension deformation

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 (Ic) 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 Ic and axial strain. By fitting experimental data, the key parameters are determined. The Ic 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.

Performance test of REBCO CICC sub-cables with 10 kA current under 20 T background field

While commercially manufactured rare earth barium copper oxide (REBCO) tapes show significant promise in facilitating the operation of fusion magnets with magnetic fields above 15 T, the design and development of highly stable cable in conduit conductor (CICC) technology is very important to achieve their practical application. To find a good solution for this demand, the Institute of Plasma Physics, Chinese Academy of Sciences, proposed two kinds of CICC design concepts, which are both manufactured from a sub-cable formed by winding REBCO tape around a stainless steel spiral tube. As part of the ongoing activities to develop an REBCO CICC, two sections of sub-cable specimens were manufactured and bent into a U-shape for testing under magnetic fields up to 20 T. A sub-cable specimen with 30 commercial 4 mm wide REBCO tapes displayed around 10 kA at 4.2 K and a background magnetic field of up to 20 T. It also showed stable operation under an electromagnetic (EM) load of around 200 kN m−1, which is above the 150 kN m−1 required by the designed CICC sub-cable. However, the calculated I c of the other specimen degraded from 8.8 kA to 8.5 kA when cycling with an EM load of around 160 kN m−1. The lower calculated n-value at 77 K and self-field as well as the observed imprints on the disassembled tape edges suggested that defects were generated in the cable during cabling, bending to the sample holder or operation with high EM and thermal loads. These results exhibit the potential and feasibility of using high flexible REBCO cable (HFRC) sub-cables for high-field fusion magnets. However, the winding parameters need to be optimized to ensure safe operation in more complex conditions, such as in tokamaks, especially if using tapes similar to those used in sample-B in this study. Moreover, it is imperative to establish much more rigorous requirements for coil manufacturing processes in order to avoid the occurrence of defects in the tapes.

A practical stress-based test method for evaluating reversible stress limit for critical current degradation in rare-earth barium copper oxide (REBCO) tapes.

The superior electromechanical properties of second-generation high-temperature superconducting rare-earth barium copper oxide (REBCO) coated conductor tapes make them viable candidates for high magnetic field applications. To characterize their electromechanical properties (EMPs) under operating conditions, the critical current degradation behavior of the REBCO tapes should be evaluated. Conventional evaluation methods for EMPs usually rely on a strain-based test method that utilizes an extensometer to measure the deformation induced on the coated conductor tape. This study aims to establish a practical stress-based test method that determines the reversible stress limit for critical current (Ic) degradation in REBCO tapes without using extensometers under uniaxial tension. For an efficient test procedure, Ic measurements were initially performed with broad stress intervals and then changed to narrow stress intervals before the critical current degraded irreversibly. Four commercially available REBCO tape samples were used to validate the reliability of the proposed stress-based test method. It was then assessed by comparing them with those obtained using the conventional strain-based test method. Statistical estimations were used to determine the reproducibility of the results. These results provide a basis for an international round-robin test guideline to establish a test method for measuring the electromechanical properties of high-temperature superconducting tapes at cryogenic temperatures.

Development of the first non-planar REBCO stellarator coil using VIPER cable

The benefits of operating fusion devices, such as tokamaks and stellarators, at high fields make high-temperature superconducting magnets necessary to realize a compact fusion power system. Superconducting stellarators, such as W7-X, have used standard low-temperature superconductor technology niobium-titanium. ARPA-E has recently funded a two-year project led by the startup Type One Energy and involving the Fusion Technology Institute at the University of Wisconsin-Madison, the Plasma Science to design and fabricate the first non-planar high-temperature superconductor (HTS) rare-earth barium copper oxide (REBCO) coil for a high-field stellarator based on the SPARC tokamak’s VIPER cable concept. The design consists of a 1.5-turn non-planar REBCO coil supported by a pair of 3D printed stainless steel radial plates. The ultimate goals of the project are to determine if commercial REBCO tapes and additive manufacturing can be used to fabricate high field () non-planar coils with tight bending radii () and with a degradation of the critical current smaller than 20% with respect to the expected performance. In this work we present numerical analysis for non-planar coils (critical current, magnetic field map, Lorentz forces and quench aspects) at the operating conditions of 77 K and 20 K and the fabrication and testing in liquid nitrogen (77 K) of the first two non-planar demonstrators for stellarators based on a VIPER cable. The first demonstrator is a short NOn-planar VIPER cabLe (cable demonstrator at which we will refer to as NOVEL) equipped with 100 HTS REBCO tapes and with a critical current of 5700 A at 77 K (self-field); the second is a MultIple turns (1.5-turns) NOn-plANar coil (coil demonstrator at which we will refer to as MINOAN) equipped with 30 HTS tapes and with a critical current of 2100 A at 77 K (self-field). Both demonstrators were tested at 77 K (liquid nitrogen bath) and the results showed that—even after being bent into non-planar shapes with bend radii —the degradation of the critical current was within 15%, meeting the expected goals of the project.

Aluminium-Stabilized High-Temperature Superconducting Cable for Particle Detector Magnets

Within the context of EP R&D, CERN is developing a high-temperature superconducting (HTS) conductor for future superconducting detector magnet projects. The conductor features a Rare-Earth Barium Copper Oxide (REBCO) tapes soldered to a copper-coated high-purity aluminium stabilizer. Critical currents of the 200 mm long straight cable samples measured with various numbers of REBCO tapes at 77 K are consistent with empirical scaling formulas for critical current within 10%. A few percent deviations are expected to arise from the critical current variation along the length of the HTS tape. No degradation was observed after multiple soldering and de-soldering cycles at 165 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\mathrm{o}$</tex-math></inline-formula> C with Bi–Sn based solder and after thermal cycling between 77 K and room temperature. However, extreme bending (of 100 mm radius) of the already soldered HTS cable leads to failure of the cable. We repeated the critical current measurements with a 650 mm long cable loop sample with 85 mm bending radius, where the HTS tapes were soldered after the aluminium profile was bent. The critical current of the HTS cable loop was 7% lower than the prediction. Based on the first critical current measurements, the HTS cable preparation method presented in this work results in repeatable quality aluminium-stabilized HTS cable.

Stress and Deformation Analysis of REBCO Pancake Coils With Individual Turn Movement

In recent years, rare-earth barium copper oxide (REBCO) pancake coils have shown excellent performances in high magnetic field applications. The no-insulation (NI) winding technique enables to enhance the thermal stability and current density of REBCO pancake coils. Still, further developments of REBCO coils are expected; e.g., the mechanical deteriorations of REBCO tapes caused by electromagnetic forces have been reported. The screening currents are considered as one of the causes to damage the REBCO tapes because of a nonuniform current distribution. Meanwhile, the electromagnetic forces on non-impregnant NI REBCO coils, whose windings can be deformed separately with turns and moved easily, are complicated, and the detailed mechanism has not yet been clarified. Commonly, the circumferential movement/deformation of the winding is considered as a rigid body. In this study, we investigate the stresses and displacements due to the electromagnetic forces on NI REBCO pancake coils in high magnetic field using a current simulation and a 2D elastic finite element analysis in the radial and circumferential directions; i.e., each turn can individually move in the radial and circumferential directions considering the spiral winding structure. The simulation results show the circumferential movement/deformation of windings and the nonuniform hoop stress due to the electromagnetic force.

Mechanism of Recovery Performance Improvement With Porous-Stabilized REBCO Tape for Resistive-Type Superconducting Fault Current Limiters

Resistive-type superconducting fault current limiters (R-SFCLs) utilizing rare-earth barium copper oxide (REBCO) tapes have been developed to limit fault currents. To balance recovery and current-limiting performances, a porous-stabilized REBCO tape was proposed. The experimental results showed that using a porous stabilizer with the REBCO tape and insulating epoxy reduced recovery time to 25%. However, the underlying mechanisms of this improvement in recovery performance are not yet clear. The aim of the present study is to understand these mechanisms. To do so, a vapor-film-collapse model was proposed to clarify the impact of the insulating epoxy on vapor film collapse. This model suggests that when liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) comes into contact with the insulating epoxy, it causes a rapid decrease in the temperature of the insulating epoxy surface, leading to the collapse of the vapor film. Additionally, a liquid-retention model was proposed to investigate the effect of the porous stabilizer. This model indicates that if the porous stabilizer can retain LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the equivalent heat flux observed in experiments can be achieved through evaporation of the retained LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> .

Compressive Stress-Strain Behavior of REBCO Coated Conductors and Cables

Rare earth barium copper oxide (REBCO) coated conductor tapes and cables are promising material candidates for constructing high field magnets. In both the magnet constructing stage and operation stage, transverse pressure is exerted on the tapes/cables. Therefore, understanding the mechanical behavior of tapes/cable under transverse pressure and figuring out certain properties, such as modulus and stress limits, will be helpful for magnet design and magnet simulation study. We accomplished these goals by performing compression tests on several commonly used tapes/cables in high field magnets. We found that the mechanical behavior of tape stacks can be characterized in three distinct regimes. In the first regime (<20 MPa), tape stacks underwent flattening and self-relocation. Then in the second regime (20 - 60 MPa), the REBCO tape stack showed a modulus of 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 7% GPa, and the Ni-plated REBCO tape stack showed a modulus of 1.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 6% GPa. In the third regime (>60 MPa), the REBCO tape stack and its Ni-plated derivative had a modulus of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 11% GPa and 4.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 9% GPa respectively. Roebel cable had similar mechanical behavior to the tape stack with three regimes, and the second and third regimes showed a modulus of 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 5% GPa and 9.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 11% GPa respectively. The conductor on round core (CORC) cable was more vulnerable to stress, and its fracture stress was around 27 MPa.

Electromagnetic Behavior Simulation of REBCO Pancake Coils Considering REBCO Tape Rotation Under High Magnetic Field

Since the screening current induced in rare earth-barium-copper-oxide (REBCO) magnet generates an irregular magnetic field, a few screening current simulation methods have been proposed. For an insert REBCO magnet generating ultra-high magnetic field, a new screening current method has been proposed with consideration of the coil-deformation effect. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we have developed a new screening current method based on a partial element equivalent circuit (PEEC) model coupling with a two-dimensional stress finite element analysis to accurately the time-transient distribution of accurate screening current. In the presented method, the changes of self/mutual inductances are considered due to the coil deformation. As the simulation results, it was found that the coil deformation affected the screening current distribution. When considering the coil deformations, the screening current-induced fields to operating current are different for charging cycles. It is necessary to simulate the coil deformation to accurately estimate the screening current and the screening current-induced field.

Development, Construction, and Commissioning of a Fully Automated Cabling Machine for Round Multi-Layer REBCO Cables

High-current Superconducting Links are being developed at CERN for the powering of the superconducting magnets of the LHC High Luminosity upgrade (HL-LHC). The Superconducting Links consist of an assembly of magnesium-diboride (MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) cables powering different magnet circuits. They have a total DC current capability of up to about |120| kA at 25 K. The MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cables are connected at one side, at 4.2 K, to the Nb-Ti bus bars of the superconducting magnets and at the other side, to round high temperature superconducting (HTS) rare-earth-barium-copper-oxide (REBCO) cables operated, in helium gas, up to 60 K. We have developed, constructed, and commissioned an automated cabling machine for producing long lengths of round, multi-layer, electrically insulated REBCO cables. The machine allows precise adjustment of the cable geometry and control of the cabling parameters. It controls in high speed, with field programmable gate array (FPGA) and Linux real-time (RT) based controllers, the tension applied to the REBCO tapes, to the polyimide insulating tapes, and to the copper core of the cable. It records all cabling and process parameters in addition to the mechanical tensions and it provides the reporting needed for the quality control procedures that are integral to the fabrication of components of the HL-LHC. In this manuscript we report on the development, design and construction of the machine from the concept to the mechanical realization, including control and data acquisition systems. We also report on the commissioning on the machine via the production of round REBCO cables.

Numerical analysis of the mechanical and electrical properties of (RE)BCO tapes with multiple edge cracks

One of the leading causes of critical current degradation in rare-earth barium–copper-oxide tapes is the micro-cracks produced by mechanical slitting. These cracks are scattered near the edge of the tape and vary in length and angle. In this work, a tape model with multiple edge cracks is established. Under tensile loading, the effects of the Poisson ratio, crack length, crack angle, crack spacing, and geometric mutation between cracks on the stress intensity factor are investigated using the extended finite element method (XFEM). Tensile experiments were conducted at room temperature to investigate the crack propagation behavior of tapes with multiple edge cracks. The results show that the stress intensity factor obtained using XFEM is more informative than the analytical solution, which ignores the Poisson effect. The stress intensity factor is sensitive to crack length and angle variations and exhibits an evident jump characteristic when a geometric mutation occurs. The jump level strongly depends on the geometric difference. The jump location is the initiation site for crack propagation, which is consistent with the experiment results. The strain analysis of the tape implies that high-strain regions exist at the crack tip before the tensile strain reaches the irreversible strain limit. The critical strain of crack propagation is closely related to the form of crack distribution. It dominates the irreversible strain limit of critical current degradation, which facilitates understanding the early degradation of critical current. Finally, some engineering suggestions are given.

Influence of Combined Tension and Torsion on the Performance of REBCO Superconducting Tapes

Rare-earth-barium-copper-oxide (REBCO) coated conductor tapes are promising materials for superconducting cables and wires. Simulation studies of the impact of the mechanical loading on the degradation of superconducting tapes give a better understanding on the role of different geometrical parameters like tape width and thicknesses of constituent layers. In this article, a detailed analysis of combined tensile and torsional loading on REBCO tape is carried out. The results show that the thickness of each constituting layer of the tape influences the ability of the tape to withstand tensile and torsional loads. Although increasing the width of the tape increases the maximum allowable tensile load, it decreases its ability to withstand the torsional load. The angle of twist is increased from 0.18 to 0.72 °/mm, and the maximum applied tensile strain that can be applied is decreased from 0.65% to 0.3%. The maximum allowable angle of twist is decreased from 4 to 0.93 °/mm with the increase in the tape width from 3 to 12 mm under pure torsion. The critical current retention capacity of the tape changes with different geometric parameters of the tape. The findings are useful in determining appropriate values for the thicknesses of each layer and width of the tape corresponding to the magnitude of pure tensile, pure torsional and combined tensile and torsional loads experienced.

Electromagnetic and thermal characteristics of supplementary-conductor-based no-insulation REBCO pancake coils with comparison to conventional-based ones

A no-insulation (NI) winding technique was originally developed in 2011 to enhance the thermal stability of Rare-Earth Barium Copper Oxide (REBCO) pancake coils. Thereafter, several similar techniques of controlling turn-to-turn contact resistance have been proposed, and they can be categorized as the conventional NI winding technique, named a conventional-based NI (CNI) technique in this paper. Recently, a few novel winding techniques different from the CNI winding technique but similar have been proposed: an intra-layer no-insulation technique and a conductive-epoxy-resin-coating technique. These techniques utilize supplementary conductors attached or embedded on REBCO coils as bypassing current paths, which is, in this paper, categorized as supplementary-conductor-based no-insulation (SNI) techniques. These conventional-based and supplementary-conductor-based NI techniques have high a potential for high-field applications. As of today, the applications of these NI techniques are nuclear magnet resonance and magnetic field imaging, and compact nuclear reactors. Those applications require a high performance; however, it is still uncertain which NI technique is suitable for which application since the differences in the characteristics are not clarified well. Therefore, we have systematically investigated the thermal stability of SNI REBCO pancake coils with different circuit parameters in this paper. The thermal stability simulation results of SNI REBCO coils were compared with those of CNI REBCO coils. As a result, the SNI REBCO pancake coils are able to be operated with high thermal stability without deteriorating charging performance as long as the turn-to-turn contact resistances between REBCO tapes and supplementary conductors are low.

Thermodynamic optimization of 10–30 kA gas-cooled current leads with REBCO tapes for superconducting magnets at 20 K

Thermodynamic optimization is carried out to minimize the refrigeration work of gas-cooled current leads at a current level of 10–30 kA for superconducting magnets at 20 K. The binary HTS lead is a serial combination of REBCO (rare-earth barium copper oxide) tapes as cold part and copper conductor as warm part. In gas-cooled leads, liquid nitrogen is not used, but cold helium gas is supplied for forced-flow cooling through the channel between spiral fins of copper conductor. A special attention is paid to the conditions of gas-cooling, which can be integrated with a closed refrigeration cycle without any heat intercept or boil-off loss of liquid. The input power for refrigeration is rigorously calculated with the temperature-dependent properties of conductors. When a safety margin is selected on the critical current of REBCO, it is proven that there exists a unique optimum in the cooling-gas temperature and the dimensional size of copper conductor to minimize the required work for refrigeration. The results are compared with the optimized cases of conduction-cooled and vapor-cooled binary leads for 20 K magnets. The details of optimization procedure and design data are presented for practical application.

Typical electrical, mechanical, electromechanical characteristics of copper-encapsulated REBCO tapes after processing in temperature under 250 °C

Heat treatments are inevitable not only in the production of rare earth barium copper oxide (REBCO) tapes, but also in their post-processing for applications, typically, in soldering and epoxy/wax impregnation during the fabrication of REBCO coils. In general, the heat treatment of REBCO tapes should be carried out at lower temperature for a shorter time, but the specific safe boundary of heat-treatment temperature and time for REBCO tapes is still unknown. Therefore, a comprehensive study on the typical electrical, mechanical, and electromechanical characteristics of REBCO tapes after heat treatments under temperature of 250 °C is necessary. This work focus on the copper-encapsulated REBCO tapes, which are more robust (while with much lower engineering current density) to be processed in application systems than the tapes without encapsulation. The critical current degradation, stress–strain characteristic, and electromechanical properties of REBCO tapes were measured after heat treatments at different temperatures in argon and oxygen atmosphere. A 2D finite element (FE) analysis model was established for detailed stress/train analyzes under tension and bending based on the analysis of residual stress/strain. The results indicate that the critical current of the copper-encapsulated REBCO tapes decreases with increasing heat-treatment temperature and dwell time, and is of no evident relation to atmosphere. In addition, increased temperature of heat treatment leads to an obvious decrease in the yield strength and critical tensile stress. This effect is mainly attributed to the degradation of mechanical properties of the encapsulated copper layer, which is demonstrated by the combination of our FE simulation and the experiments results. Interestingly, the change in the critical bending radius due to heat treatments was slight, because the bending axial strain of the REBCO layer remained almost unchanged after heating. It is also worthy to note that all the properties tested in this study were irrelevant to the external oxygen partial pressure during the heating process. As a practical conclusion for the application systems, an upper and atmosphere-irrelevant limit of processing temperature of 130 °C or 150 °C (2 h dwell time) was proposed for copper-encapsulated REBCO tapes, under which the critical current, yield strength, critical tensile stress/strain and critical bending radius of the copper-encapsulated REBCO tapes decay by <1% or 3%, respectively.