Rare-earth Barium Copper Oxide Research Articles

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.

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.

Performance degradation due to thermal cycling in soldered Faraday and SuperPower HTS tape stacks

Soldering stacks of high temperature superconducting (HTS) tapes results in a current carrying matrix with high electrical, thermal, and mechanical stability. In some applications, these tape stacks may be exposed to elevated temperatures during thermal heat cycles. Previous research on thermal degradation of high temperature superconductors has been performed with isolated crystals or individual HTS tapes, showing that oxygen-out diffusion from the rare-earth barium copper oxide (REBCO) results in a reduction of the superconducting performance. In this work, the performance of Pb37Sn63 soldered HTS tape stacks with thermal cycling to 170∘ C is quantified through determination of the critical current Ic , the n-value, the linear resistance R in the pre-transition region of the tape stack current–voltage (I–V) traces, and the diffusion coefficient D associated with oxygen-out diffusion. The effect of tape manufacturer and nickel electroplating were considered; three soldered stacks each of unplated Faraday, nickel electroplated Faraday, unplated SuperPower, and nickel electroplated SuperPower tapes were fabricated. Across 20–30 one hour thermal cycles, there are no significant differences in degradation profiles observed between the unplated and nickel electroplated tape stacks for a given manufacturer. There is however a marked difference in degradation profiles observed between the SuperPower and Faraday tape stacks. For the unplated and plated SuperPower tape stacks, the degradation in Ic generally follows the model to describe oxygen-out diffusion from the superconducting REBCO layer. Significantly less degradation is observed in the unplated and plated Faraday tape stacks, and in some sections Ic even increases slightly across cycles. Further, cases of increasing Ic accompanied with decreasing n are observed, a decoupling between the two parameters. Across all samples, the linear resistance was highest in current-redistribution regions near the leads. Consistent with the model of oxygen-out diffusion and the formation of an increasingly wide oxygen deficient layer in the REBCO, the linear resistance increased with cycle number.

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.

Effect of doping on evolution of He<sup>+ </sup>ion irradiation defects and superconductivity in EuBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–δ</sub> superconducting strips

Rare-earth barium copper oxide (REBCO) as a representative of the second-generation high-temperature superconducting materials possesses superior physical advantages such as high critical magnetic field, elevated critical temperature, and superior current density, which has been applied to many domains. Although the introduction of non-superconducting nanoscale particle dopants, as a critical method, can enhance the magnetic flux pinning capability of REBCO strips, the effect of the doping on the performance change and microstructure evolution of the strips under irradiation is ignored. In this work, undoped and 3.5 mol% BaHfO<sub>3</sub> (BHO) doped EuBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–δ</sub> strips are investigated in the room-temperature irradiation experiments (1.4 MeV He<sup>+</sup> ions) with three distinct doses of 5×10<sup>14</sup>, 5×10<sup>15</sup>, and 5×10<sup>16</sup> ions/cm<sup>2</sup>, respectively. Electrical performance tests reveal that the undoped strips exhibit a slight increase in <i>J</i><sub>c</sub> after the low-dose irradiation. However, with dose increasing, <i>J</i><sub>c</sub> decreases by over 60%. In contrast, doped strips experience a significantly smaller decline in <i>J</i><sub>c</sub>, ranging only between 30% and 40% at high-dose irradiation. Raman spectroscopy and transmission electron microscopy characterizations confirm that the defects induced by He<sup>+</sup> ion irradiation lead to amorphization and structural disorder within the superconducting layers, which is the primary reason for the decline in the superconducting properties of the strips. The results show that the introduction of localized strain through BHO nanophase in the superconducting layer changes the migration and aggregation behavior of irradiation-induced defects, repairing the damaged superconductor structure. Furthermore, the field dependence and temperature dependence of <i>J</i><sub>c</sub> of doped strips are irradiation-resistant due to BHO nanocrystals as strong pinning centers. Additionally, unlike the superconducting properties of the REBCO strips that can be repaired through oxygen annealing after neutron or heavy ion irradiation, the electrical properties of the two types of strips irradiated with high doses of He<sup>+</sup> ions in this work are further deteriorated after being annealed. It is worth noting that compared with the undoped strip, the localized strain generated by BHO in the doped strip inhibits the size growth of helium defects in the three-dimensional direction at high temperatures, which changes the magnetic flux pinning characteristics and delays the disorder and amorphization of the superconducting layer structure caused by the severe growth of helium bubbles. This study provides a reference for the application of REBCO superconducting strips in the irradiation environment.

Elastoplastic mechanical behavior of high-field magnet of REBCO high temperature superconducting composite tape under extreme environment

Rare Earth-Barium-Copper-Oxide (REBCO) high temperature superconducting (HTS) coated conductors (CCs) are one of the best candidate materials for high-field magnets. However, due to its laminated structure, each component materials of the CCs are inevitably subjected to residual thermal stress and electromagnetic force during heat treatment, processing, cooling, coil winding, and operation of the magnet structure. It causes changes in its superconducting properties, and even leads to irreversible degradation of superconducting critical characteristics, brittle layer fracture and peeling delamination within the coil. To reveal the synthesis of the tape and the stress state of the high-field coil under multi-physical fields. The residual thermal stress/strain accumulation of each component material of the tape is investigated under two thermal processing and manufacturing methods by combining analytical theory with numerical calculation. In which, the two thermal processing methods are metal-organic chemical vapor deposition (MOCVD) process and one-step cooling simplified process respectively. The results show that the residual thermal stresses accumulated by five materials, Hastelloy, Buffer, REBCO, Ag and Cu, under the step-by-step synthesis process are about 14.7%, 97.3%, 96.3%, 80% and 49.2% of the simplified model, respectively. And the residual thermal strains are about 15.1%, 96.8%, 95.2%, 85.2% and 27.5% of the simplified model. In addition, after the five materials are manufactured according to the step-by-step thermal processing process, the accumulated residual thermal stresses are about 1.88%, 86.3%, 61%, 114.3%, and 34% of their respective yield strengths, respectively. The elastoplastic mechanical behavior of the magnet under the background of high magnetic field and strong current is also studied. The results indicate that the maximum values of effective and hoop stresses in each component material of the coil are distributed in the innermost side of the magnet, and decrease gradually along the radial direction of the magnet. In the innermost coil of the magnet, the effective stress of each component material increases with the increase of the transmission current. When the transmission current Iope = 183 A, the material Cu reaches the yield strength and enters the plastic deformation stage. Whereas the material Hastelloy reaches the yield strength and enters into the plastic deformation stage when the transmission current Iope = 222 A.

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.

3D numerical investigation on delamination behavior of the epoxy impregnated REBCO pancake coil

Superconducting coils made of rare-earth-barium-copper-oxide (REBCO) coated conductor (CC) exhibit superior electromagnetic performance. Employing epoxy impregnation can improve the structural integrity and mechanical property of the superconducting coils. However, due to the extreme work environment and weak adhesion strength of REBCO CC, the delamination induced by radial thermal stress and electromagnetic force significantly affects the electromagnetic property and the reliability of the superconducting coil. This study proposes a three-dimensional thermal-electromagnetic mechanical delamination model that incorporates the cohesive zone model to investigate the delamination mechanisms in epoxy impregnated REBCO pancake coils during the cooling and coil operation processes. The simulation employs a three-parameter Weibull distribution to account for the inhomogeneity of transverse tensile strength in the CCs. The delamination behavior and mechanisms of the coils under different conditions are analyzed. The simulation results show that the model considering random adhesion strength proves to be more effective in representing the delamination behavior of the coil. And large tensile radial stresses caused by thermal stresses and electromagnetic forces lead to the delamination behavior of the coil during cooling and operation. The main reason for the tensile radial stress is the mismatch in the thermal contraction among components of the coils during cooling process. Furthermore, we investigate the influence of the thermal expansion coefficient (CTE) and thickness of the mandrel, the CTE and prestress of the overband and the initial localized damage. The results indicate that these factors significantly affect the tensile radial stress and the extent of delamination in the windings. And the extent and distribution of delamination is related to the stress release caused by delamination to a certain degree.

Performance characteristics of REBCO coated conductor joints fabricated by flux-free hybrid welding

Recently, the joining of rare-earth barium copper oxide coated conductor (CC) tapes using ultrasonic welding (UW) has demonstrated outstanding potential in the in-line fabrication of longer tapes required for superconducting device applications. The UW method can produce CC joints by applying ultrasonic vibration in less than one second, and hybrid welding (HW) has been adopted to improve further the joint resistance (R j) and electromechanical properties of the UW CC joints. However, conventional methods for preparing the HW and soldered CC joints involve applying solder flux to remove the oxide film, which can cause corrosion to the surface of the CC tapes and affect the joint’s lifespan during device operation. Therefore, this study aims to fabricate a robust HW CC joint of pre-solder insertion without solder flux and compare its joint strength and electromechanical properties with the traditional cases with the solder flux. While similar R j can be obtained from both cases of HW CC joints, the flux-free HW CC joint has slightly higher joint strength and superior adhesive characteristics than those with flux. The difference in fracture mechanisms after lap-shear and T-peel tests between flux-free HW and with flux was extensively discussed. Additionally, the study investigates the correlation between a decrease in R j with longer joint length in differently stabilized and processed CC tapes for flux-free HW. Overall, this study demonstrated that the flux-free HW method could efficiently produce robust CC joints with a lesser risk of corrosion and enhanced joint characteristics.

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.

Induced delamination in REBCO coated-conductor tape by a scratch line and bending

The aim of this work is to study delamination promotion of the coated conductor (CC) tape under the influences of the scratches on the Ag surface when bending. For this purpose, the microstructural and superconducting properties of the CC tape bent in helical fashion with different former diameters were analyzed. Such CC tape that exhibits visible damage along its length in the form of “scratch line” caused on the final product was used. The CC tape was bent at a constant lay angle of 45° with a superconducting layer positioned inward to the former core experiencing compression. Transport current measurements in liquid nitrogen were performed to study superconducting properties of the CC tape.The results showed that delamination of the REBCO (Rare-Earth Barium Copper Oxide) layer started to form exclusively on the scratch location at tube diameter of 7 mm (corresponding to strain ε = −0.71 %) while the rest of the REBCO surface started to delaminate at tube diameter of 5 mm (ε = −0.99 %). As the superconducting properties of the CC tape are related with the REBCO delamination, the critical current degraded when first signs of REBCO delamination appeared as expected. Thus, it was observed that the scratches induce REBCO delamination during the bending process resulting in increase of critical bending radius.

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.

Electromagnetic force behavior of high temperature superconducting coils coupled with multi-physics fields under different models

Rare earth barium copper oxide (REBCO) superconducting tape is the second generation of high temperature superconducting material with strong current carrying capacity and high mechanical strength, which has been studied most recently. In order to better investigate the electromagnetic force distribution characteristics of REBCO superconducting materials after electromagnetic excitation, firstly, the electromagnetic characteristics of superconducting tapes are numerically calculated by H method and T-A method on the basis of finite elements (FE) in this paper, and the effectiveness of T-A method is obtained. Based on this method, the influence of real size model (RS model) and homogenization equivalent model (HE model) on superconducting tape is further studied. The results show that the radial magnetic field reaches a peak of 1.01 T at the middle position of the upper surface of the pancake, while the axial magnetic field reaches a peak of 16.4 T at the inner side of the pancake, which indicates that the axial magnetic field contributes more to the total magnetic field. At the same time, the rationality of the equivalent method is obtained by comparing the calculation results of the RS and HE models. In addition, the mechanical changes of superconducting tapes are studied by T-A method under the HE model. It is found that the hoop stress and radial stress reach the peak values of 64.7 MPa and 6.84 MPa, respectively, and both are tensile stress, indicating that the hoop stress is more likely to cause damage to the superconducting tapes. Finally, the effect of different external tensile strains on the superconducting tape is discussed.

Electromagnetic characteristics of NI-REBCO coils according to contact surface conditions controlled by the oxidation state and roughness of the REBCO conductor

Although no-insulation (NI) rare-earth barium copper oxide (REBCO) coils offer technical advantages, such as high-energy density, mechanical strength, and self-stabilizing performance, sometimes their applications are limited owing to current leaks through turn-to-turns inside the coils, which are caused by their very low turn-to-turn contact resistance. This results in temporal magnetic field delays and limits their applicability in fields requiring fast response performance in magnetic field control. These shortcomings can be addressed by increasing the turn-to-turn contact resistance of the NI coils. A NI winding method with REBCO conductor variations of surface-oxidation and roughness was proposed in this study; this method improved the magnetic field response by reducing the temporal delays of the magnetic field via the suppression of the turn-to-turn bypass current. This was achieved by increasing the turn-to-turn contact resistance within the NI coils through adjustments to the oxidation and roughness of the surface of a copper stabilizer layer, which was coated on the outermost part of the REBCO conductor by electroplating to ensure electrical and mechanical protection. In this study, the steady- and transient-state characteristics of NI coils were experimentally analyzed according to changes in the oxidation and roughness levels of the surface of the REBCO conductor. To verify the effectiveness of the proposed winding approach, basic tests were conducted in a liquid nitrogen cooling environment by constructing different types of sample coils using REBCO conductors, which were thermally oxidized at different temperatures in a convection oven and scratched using sandpaper. Further, the decay time constant and the time delay of the center magnetic field in the sample coils were measured in sudden current discharging and current charging/discharging tests, respectively. In addition, overcurrent tests using currents exceeding the critical current were conducted to analyze the thermal stability of the sample coils. In conclusion, it was experimentally verified that the time delays of the NI coils were reduced as the surface oxidation and roughness of the REBCO conductor increased. Furthermore, when the oxidizing and scratching approaches were employed, the contact resistance values of the NI coils (3.48 µΩ·cm2), which were not subjected to any surface treatment, increased to 155.01 and 4.43 µΩ·cm2, which were oxidized at 230 °C and sanded using P400-grit sandpaper, respectively. For oxidizing approach, heat treat temperatures at 200 °C – 230 °C will be suitable for increasing the contact resistance of NI coils without inserting any resistive material, thus resulting in a enhancing the magnetic response performance of NI coils.

Delamination analysis of high-temperature superconducting tapes based on a random defect model

Random microscopic defects have been found in the fabrication of rare-earth-barium-copper-oxide (REBCO) high-temperature superconducting (HTS) tapes. Interlaminar failure of the tape is more likely when the interfacial bonding strength is reduced as a result of defects, which seriously jeopardizes the dependability of large magnet systems. In this paper, a three-dimensional finite element model (FEM) of the tape with random defects is developed using the Weibull distribution function to evaluate the effect of defects on the delamination of the tape. The delamination behavior under two loading scenarios were conducted (Case-I: out-of-plane tensile and Case-II: axial compression). The result show that the interfacial bonding strength is significantly affected by the defect fraction. The random characteristics of defects and their impact levels can be traced through different parameters. For case-I, the theoretical results of the effective interfacial bonding strength are in good agreement with the numerical results. During the delamination process, the occurrence of defects leads to stress oscillations in the constituent layers. Under axial compression loading (Case-II), the defect dramatically reduces the critical capacity of the tape. With increasing defect fraction, the rate of delamination propagation grows noticeably.

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.

In-situ critical current measurements of REBCO coated conductors during gamma irradiation

Rare-earth-barium-copper-oxide (REBCO) coated conductor tapes within next-generation tokamak pilot and power plant magnets will be exposed to broad-spectrum gamma-ray and neutron irradiation concurrently. It has been known since the 1980s that cumulative neutron fluence affects the superconducting properties of REBCO, but the effects of gamma rays are less certain, as are the effects of radiation (of any kind) during current flow. However, the use of superconductors as photon detectors suggests that energetic photons interact directly with the superconducting state, locally destroying superconductivity. Hence, as well as the effect of the overall radiation dose (fluence), the effect of radiation dose rate (flux) on the superconductor’s properties must be quantified to understand how REBCO magnets will perform during fusion magnet operation. In-situ measurements of the self-field critical current at 77 K, of several REBCO coated conductor tapes were performed during Co-60 gamma ray exposure at a dose rate of 86 Gy min−1. Samples were fully submerged in liquid nitrogen throughout the measurements. No change in the critical current of any sample during or after irradiation was observed within standard error. These are the first reported in-situ measurements of critical current during fusion-relevant gamma irradiation. Two samples were irradiated to a further dose of 208 kGy at room temperature and a second round of in-situ measurements was performed. No change in the critical current of these samples was observed within standard error. This corroborates recent studies, but is in conflict with older literature.

Mechanical-thermal coupling model of solidnitrogen cryostat for electrodynamic suspension system

High-temperature superconducting magnets wound by REBCO (rare-earth-barium-copper-oxide) coated conductors are promising candidates for electrodynamic suspension system due to their high current-carrying capability, excellent mechanical tolerance and low cooling cost. Solid nitrogen as coolant to protect HTS magnets can significantly promote their dynamic performance and reduce the use of auxiliary equipment, which reduce the system dimension and weight. In this study, we propose a mechanical-thermal coupling model of solid nitrogen cryostat for electrodynamic suspension system. This model is capable of calculating the temperature distribution during cooling process and the transient behavior for the rewarming process of the solid nitrogen cryostat. The cooling capacity map for the two-stage RDK-415D cryocooler was extended for accuracy. The steady state temperature distribution of cooling process and the thermal evolution of three different rewarming processes, including cryocooler reserved, 5 W losses applied and cryocooler detached, were calculated and verified experimentally, followed by the investigation of the structural stresses and displacement of the solid nitrogen cryostat due to thermal displacement among different components. The developed model can serve as a reference and guide for the design and optimization of solid nitrogen cryostat for electrodynamic suspension system.