Critical Current Degradation Research Articles

Thermal gradient-induced critical current degradation in mesoscopic superconducting thin film

Abstract Superconducting materials inevitably suffer from the sudden change of temperature in localized areas in practical applications, and the concomitant thermal gradient may be detrimental to their performance. Critical current density is a key factor affecting the performance of superconductors. However, the effect of thermal gradient on the critical current density has not been identified. Here, by combining the time-dependent Ginzburg–Landau equations and the heat transfer equation, the thermal gradient and magnetic field dependence of the critical current density are systematically investigated and rationalized by exploring the behavior of vortex and magnetization. For lower magnetic fields, it is found that the thermal gradients strongly reduce the local surface barriers, which inhibits vortex entry and movement, leading to a rapid deterioration of the current-carrying capability. Under moderate magnetic fields, the critical current density corresponding to higher thermal gradients decreases more slowly with increasing magnetic field, which results from the thermal gradient-induced entry and moving of vortices along the current direction. As the magnetic field continues to increase, the variation of the critical current density transitions into a platform period and even slightly rises. The enhanced critical current is primarily attributed to the excess entry of vortices, which increases the surface barrier of the sample. With the further increase in the magnetic field, the critical current density continues to decrease due to increased magnetic field penetration. These results unveil the fundamental interplay between thermal gradients, external magnetic field, vortex, magnetization and critical current density, and provide a theoretical basis for understanding the heat-induced quenching of mesoscopic superconducting thin films in practical applications.

Critical current degradation in an epoxy-impregnated rare-earth Ba2Cu3O7−x coated conductor caused by damage during a quench

Critical current degradation in an epoxy-impregnated rare-earth Ba2Cu3O7−x coated conductor caused by damage during a quench

Screening-current-induced mechanical damage and critical current degradation in epoxy-impregnated REBCO insert coils

Screening-current-induced mechanical damage and critical current degradation in epoxy-impregnated REBCO insert coils

A novel numerical approach for analyzing experimental data on critical current degradation in Nb3Sn wires caused by transverse deformations preceding heat treatment

In the framework of studies on high-field magnets for future accelerators, a specific project called ASTRACT focuses on the effect of transverse strain on the critical current of Nb3Sn wires. The first phase of the project addresses the effects of strain imposed on Nb3Sn wires before heat treatment and the development of a procedure to directly compare critical current measurements with values extracted from magnetization cycles. A Nb3Sn RRP-Ti wire was deformed to different values of transverse strain (10%, 15%, 20%, and 25%), and long samples were collected for transport measurements at 4.2 K, in a background field ranging from 10 to 14 T. Short samples were used for magnetization measurements (VSM technique). Additionally, SEM images of the cross-section were collected at different longitudinal positions along the wire for each strain value. This paper proposes a method based on SEM image analysis and magnetization measurements analysis to study the effect of bundle deformation on transport properties. The procedure requires morphological information provided by SEM images after appropriate numerical processing. Verification through statistical comparison between the Ic transport and VSM data is also conducted. Direct critical current measurements showed no degradation due to deformation up to 25%. The method introduced, independent of transport data, can reach the same conclusions by considering the real shape of the bundles.

Quench protection for high-temperature superconductor cables using active control of current distribution

Superconducting magnets of future fusion reactors are expected to rely on composite high-temperature superconductor (HTS) cable conductors. In presently used HTS cables, current sharing between components is limited due to poorly defined contact resistances between superconducting tapes or by design. The interplay between contact and termination resistances is the defining factor for power dissipation in these cables and ultimately defines their safe operational margins. However, the current distribution between components along the composite conductor and inside its terminations is a priori unknown, and presently, no means are available to actively tune current flow distribution in real-time to improve margins of quench protection. Also, the lack of ability to electrically probe individual components makes it impossible to identify conductor damage locations within the cable. In this work, we address both problems by introducing active current control of current distribution between components using cryogenically operated metal-oxide-semiconductor-field-effect transistors (MOSFETs). We demonstrate through simulation and experiments how real-time current controls can help to drastically reduce heat dissipation in a developing hot spot in a two-conductor model system and help identify critical current degradation of individual cable components. Prospects of other potential uses of MOSFET devices for improved voltage detection, AC loss-driven active quench protection, and remnant magnetization reduction in HTS magnets are also discussed.

Characterization of residual-resistance-ratio of Cu stabilizer in commercial REBCO tapes

Residual-resistance-ratio (RRR) of Cu stabilizer in REBCO coated conductor is an important design parameter for REBCO magnets. Cu stabilizer with high RRR is especially beneficial for quench protections of REBCO magnets. In this work, we study RRR of electroplated Cu stabilizer in commercial REBCO tapes. We present RRR of over 180 samples measured for the quality assurance programs of REBCO magnet projects at the National High Magnetic Field Laboratory, USA (NHMFL). To investigate the factors that influence RRR, several samples were analyzed extensively by using scanning electron microscopy, secondary ion mass spectroscopy, and inductively coupled plasma mass spectroscopy. We found that RRR is strongly correlated with the grain size of Cu, which suggests that resistivity at low temperatures is dominated by grain boundary resistivity. In addition, low RRR corresponds to high concentration of chlorine impurity. This is explained by that higher chlorine impurity hindered the grain growth in the self-annealing process at room temperature which resulted in smaller grain size and low RRR. Annealing at 300C significantly enlarged the grain size and enhanced RRR. Due to the concern of critical current degradation, however, annealing is not recommended as a practical method to improve RRR of Cu in REBCO tapes.

Winding angle optimization and testing of small-scale, non-planar, high-temperature superconducting stellarator coils

We designed and constructed two non-planar coils with high-temperature superconductors (HTS) based on shapes from the Wendelstein 7-X stellarator. Tape track orientation of the HTS was optimized to reduce the coil size as much as possible while staying within the strain limits of the gadolinium barium copper oxide (GdBCO) superconductor. This resulted in average coil radii of 0.23 m and 0.48 m at strain limits of up 0.45% to for the coil shapes that were chosen. The coils were produced by winding the GdBCO tapes onto 3D-printed plastic frames. We confirmed the integrity of the superconducting layer after winding by spatially resolved measurement of the critical current and by energizing the coils in liquid nitrogen. Coil 1 showed a resistance of 1.75μΩ and did not have any critical current degradation, while coil 5 had a resistance of 195μΩ and showed only one dropout, attributable to a handling error. We measured the magnetic field of the coil with a three-axis Hall probe system and found good agreement with predictions. This work demonstrates the manufacturing of small-scale, non-planar magnetic coils from commercially available HTS.

Accelerator magnet development based on COMB technology with STAR® wires

This paper reports progress in the development of COMB magnet technology with STAR® wires. A two-layer dipole magnet with 60 mm clear bore has been recently fabricated and tested in liquid nitrogen. The purpose of the test was to determine what kind of critical current degradation occurs in the process of winding the STAR® wire into the COMB structure.

Micromechanical modelling on the elastoplastic damage and irreversible critical current degradation of the twisted multi-filamentary Nb3Sn superconducting strand

Micromechanical modelling on the elastoplastic damage and irreversible critical current degradation of the twisted multi-filamentary Nb3Sn superconducting strand

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.

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.

Bending performance of the CORC cable with flexible interlocked stainless steel former

The high temperature superconducting cable on round core (CORC) is a kind of cable that could be used in fusion projects. Nevertheless, conventional copper former CORC cables require a large external force to allow the cable to endure plastic deformation and be tightly wound into solenoids. In this case, the superconducting tape will be affected by concentrated stress, resulting in a risk of critical current degradation. Therefore, this paper proposes a new CORC cable with flexible interlocked stainless steel former, which can be wound into a solenoid by applying a small external force. To verify the bending performance of this interlocked former CORC cable, a double-layer and a ten-layer interlocked stainless steel former CORC cable, as well as a double-layer traditional copper former CORC cable, are fabricated. And these three CORC cables are used to wind solenoids of various radius sizes respectively. The experimental results show that the critical bending radius of the double-layer interlocked stainless steel former CORC cable is less than 20 mm, the critical bending radius of the ten-layer interlocked stainless steel former CORC cable is less than 50 mm, and the critical bending radius of the double-layer traditional copper former CORC cable is larger than 55 mm. A self-consistent finite element model for the critical current of the CORC cable solenoid is also established. And the critical current experimental results are in good agreement with the simulation results. The results of this paper verify the excellent bending performance of the interlocked former CORC cable, which provides a good option for the preparation of insert magnets for future fusion projects.

An electromagnetic-thermal-mechanical analysis model for high temperature superconducting magnets

High temperature superconducting magnets are the core component of superconducting power devices, and their stability is the key factor that restricts the safe operation of superconducting power devices. In order to accurately and effectively evaluate the stability of superconducting magnets during operation, an electromagnetic-thermal-mechanical numerical simulation method for high temperature superconducting magnets is studied in this paper. Based on the model, the 150 kJ SMES magnet as case is studied, the magnetic field and current density distribution are solved during its operation, and its temperature rise, AC loss and stress analysis of the magnet are achieved. In addition, this work further analyses the critical current degradation of superconducting tapes in the 150 kJ HTS magnet under multi-field coupling, the dangerous region in operation is obtained and suggestions are put forward to avoid quench. The electromagnetic-thermal-mechanical model may provide an appropriate stability assessment with rapid and real-time calculations for high temperature superconducting magnets.

HTS Tape Mechanical Behavior Sensitivity on Material Properties and Thickness of Material Layers

Multilayer 2G high-temperature superconductor (HTS) tapes undergo various mechanical loading steps as part of a superconducting magnet operation. The mechanical loading steps include cool-down to cryogenic temperatures and Lorentz forces when powering up the magnets. Studying the mechanical behavior of the constituent layers in a HTS tape can reveal the strain or stress level in each layer and help in predicting the probability of mechanical failure or critical current degradation. A detailed finite element method (FEM) -based simulation models enable us to estimate the mechanical behavior in each layer. However, defining the model parameters for material mechanical properties or thickness of each material layer can have consid-erable uncertainty due to variation in manufacturing processes and missing measurements of material properties within these tapes. The material properties in thin layers may not exactly comply with bulk materials of the same kind. In this paper we present a sensitivity analysis for mechanical behavior dependence on material properties variation in the constituent layers of a HTS tape. The results give important insight about the accuracy and reliability of mechanical simulation results and guide the priorities in future material characterization. In addition, we will investigate the effect of the thickness of Hastelloy layer on effective macroscopic mechanical behavior of the tape. A nonlinear elastoplastic FEM model is used for performing the simulations.

Transport AC Losses in Multiple-Layer Roebel Tapes

The Roebel tape is a novel high-temperature superconducting (HTS) wire with a multiple superconducting layer structure. Adjacent layers are patterned into meander-shaped multifilaments in opposite directions so that the magnetic fields generated by each layer can be mutually cancelled. The Roebel tape can be regarded as an HTS Roebel cable encapsulated into a tape, and hence has exclusive advantages such as high engineering current density, small critical current degradation, low AC losses, and a simple manufacturing process. This paper focuses on the transport AC losses of the Roebel tape with a multiple-layer structure. 3-D finite element method (FEM) models for the Roebel tapes are built based on the T-A formulation. Current distribution and AC losses of the Roebel tapes with up to 10 ReBCO layers are obtained. The results show that the tapes with even numbers of layers generate lower losses than those with odd numbers, which indicates the superconducting layers should always appear in pairs. Increasing layers can increase the engineering current density of the tape, but the average loss per layer also rises. Therefore, we suggest increasing the distance between the layer pairs or using double-layer Roebel tape stacks instead. The results indicate that the multilayer Roebel tapes can be a promising cable solution for industry and fusion applications because of the high current density and low AC losses.

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.

Structural analysis of REBCO coated conductors and quasi-isotropic strands under bending using continuum shell element

The electro-mechanical properties of Rare-Earth Barium Copper Oxide (REBCO) coated conductors (CCs) are crucial for practical structures such as power cables, magnet coils and so on. Many tests including tension and bending tests were conducted to estimate the critical current of REBCO CCs under different loadings. However, standard test method for the electro-mechanical properties of REBCO CCs has not been established, especially for bending tests. Therefore, finite element simulations of bending tests of REBCO CCs are still needed to further analyze the electro-mechanical behavior under loading. Structural responses of REBCO CCs and quasi-isotropic (Q-IS) strands under bending are analyzed using continuum shell element in this study. The numerical results indicate that bending strains obtained from finite element simulations match well with the analytical calculations in pure bending state. Furthermore, the influences of former shapes are also discussed to evaluate the bending tolerance of REBCO CCs in different cases. In addition, the mechanical properties and critical current degradation of Q-IS strands under bending are analyzed.

In situ detection of delamination and critical current degradation caused by the thermal stress in epoxy impregnated REBa2Cu3O7−δ coils

Epoxy impregnated superconducting coils have better structural integrity and thermal stability. However, for REBa2Cu3O7−δ (REBCO, RE=Rare earth) coils, the mismatch of thermal expansion coefficients between epoxy and REBCO tapes is a serious problem. In this work, the temperature distribution, stress evolution during the cooling process, critical current distribution, and delamination sites inside REBCO coils impregnated using Stycast 2850FT have been studied. We measured the temperature distribution and the hoop strain in the penultimate turn of impregnated coils during the cooling process and analyzed the thermal stress evolution. No damage was observed for coils with the ratio between outer and inner diameter Ro/Ri<1.93. The delamination behavior occurred in coils with Ro/Ri>2.37, where the coils even exhibited a two-stage delamination. The delamination mechanism of REBCO coils was proposed from three aspects: the mechanical analysis, the critical current degradation, and the microscopic analysis. In this work, we found that the actual delamination behavior may appear earlier than the steady-state temperature, and temperature distribution will push the radial stress peak toward the inner radius of the coil, making the inner turn more susceptible to delamination. Multiple delamination locations were accurately predicted and confirmed. The measured compressive hoop strain first increased from −4212 με to −4684 με with the increase in Ro/Ri and then decreased to −3835 με obviously due to delamination. This work reveals in detail the delamination mechanism in impregnated REBCO coils, which is of great significance for the development of damage-free coils.

Research on Data-Driven Approaches for Life Prediction of YBCO Tapes Under Overcurrent

Superconducting power devices have many advantages. In recent years, prototypes of various superconducting power devices based on high-temperature superconducting wires have been successfully developed, and some prototypes have also been tested on the power grid, which has accelerated the development of superconducting power technology. However, the reliability of superconducting devices under overcurrent has not yet been supported by research, which is part of the important reason that restricts large-scale promotion of them. In this article, the critical current degradation and life distribution characteristics of yttrium barium copper oxide (YBCO) tapes under overcurrent were analyzed. The overcurrent test adopted a sinusoidal alternating current with an amplitude of 480 A (4–5 times the critical current) and a frequency of 50 Hz. The duration of each overcurrent was 0.44 s. The critical current of the sample was measured in liquid nitrogen using the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> V·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> criterion. The results show that there is an approximately exponential relationship between the critical current degradation rate of YBCO tapes and the number of overcurrent cycles. The life of YBCO tapes under overcurrent can be considered to obey the two-parameter Weibull distribution. Two different remaining useful life of YBCO tape prediction models based on data-driven methods were built. It is verified through cross-checking that the relative prediction errors of the two methods are 8.4% and 5.3%, respectively. And the validity of the models was verified through uncertainty analysis and the comparison with two traditional statistical methods.

Critical Current Degradation in HTS Tapes for Superconducting Fault Current Limiter under Repeated Overcurrent

Superconducting fault current limiters (SFCL) can be an alternative to conventional devices limiting short-circuit currents in power systems. SFCL use high-temperature superconducting tapes of the second generation (HTS 2G) in SFCL, which, after reaching the characteristic critical current of the tape, go into the resistive state (quenching), limiting the short-circuit current. The critical current determines the moment of activation of the SFCL. Therefore, its value should not change during the operation of the device due to repeated limitation of short-circuit currents. The constancy of the critical current is a prerequisite for proper cooperation with the power system protection devices. Multiple quenching can cause microdamage in the superconducting layers responsible for lowering of the value of the critical current of the HTS tapes. The article presents the research results on the degradation processes of 2G HTS tapes intended for the construction of SFCL due to the action of prospective short-circuit currents with values exceeding the critical current of the tested tapes. The decrease in the value of the critical current of the HTS tape as a result of multiple transitions to the resistive state was investigated. The amount of energy emitted during the test current pulse of 0.2 s duration was determined. The limitation values of the voltage drop on the tape, which does not cause accelerated degradation processes, were defined. The microstructural tests of cross-sections of new HTS tapes subjected to prospective short-circuit currents were performed.