High-temperature Superconducting Tapes Research Articles

Review of commercial nuclear fusion projects

Nuclear fusion technologies have re-gained momentum in the last decade thanks to their disruptive potential in different fields, such as energy production and space propulsion, and to new technological developments, especially high temperature superconductor tapes, which allow overcoming previous performance or design limits. To date, reviews of recent nuclear fusion designs are lacking. Therefore, this paper aims at giving a comprehensive overview of nuclear fusion concepts for industrial applications with a focus on the private sector. The designs are classified according to the three leading concepts for plasma confinement, namely, magnetic confinement, inertial confinement and magneto-inertial confinement. The working principles of the main devices are described in detail to highlight strengths and weaknesses of the different designs. The importance of the public sector on private projects is discussed. The technological maturity is estimated, and the main criticalities for each project are identified. Finally, the geographical distribution of the companies (or public institutions) pursuing the design of fusion devices for commercial applications is reported.

Development of metal-organic deposition-derived second-generation high-temperature superconductor tapes and artificial flux pinning

Development of metal-organic deposition-derived second-generation high-temperature superconductor tapes and artificial flux pinning

Miniaturized HTS linear flux pump with a charging capability of 120 A

Owing to the presence of joint resistance and flux creep, high-temperature superconducting (HTS) coils without a power supply inevitably suffer from current decay. A flux pump is a voltage supply that requires connections with smaller footprints and a lower heat load than traditional current leads. In this study, we explain the principle of the upper limit for the output current of the traveling wave flux pumps. Based on this principle, a miniaturized linear flux pump device was developed. With narrow and misaligned iron teeth, elaborate 3D geometry of the iron pieces, and optimized driving current waveform, the miniaturized flux pump can support more than 120 A output current with only a 10 mm wide HTS tape and a compact size of 4.6 × 4.6 × 3.4 cm. Our experimental results show that the critical current of the HTS tape has a significant effect on the flux pump output. An HTS tape with a larger critical current supports a higher maximum transport current, whereas an HTS tape with a smaller critical current requires less applied current for positive output. Finally, excitation tests on HTS coils were performed. Charge/active discharge and field supplement experiments were done on a maglev HTS racetrack coil of 0.4 H, where charging/field supplement capability of the miniaturized flux pump were demonstrated up to 46.8 A (close to the critical current of the coil). It has also been proved that the flux pump can work together with an external power supply with persistent current switch. The miniaturized flux pump can also independently charge an HTS coil of 60 μH to 91.6 A, which is the critical current of the coil at a low voltage criterion.

Experimental study of a high-temperature superconducting induction/synchronous motor with REBCO bulk bars for transportation applications

Lightweight and highly efficient high-temperature superconducting (HTS) rotating electric machines show great potential for transportation applications. An advantage of RE-Ba-Cu-O (REBCO, where RE=rare earth) bulk superconductors over stacked HTS tapes is their ability to carry larger and more homogenous currents. This study is the first in which an HTS induction/synchronous motor (HTS-ISM) with Gd-Ba-Cu-O (GdBCO) bulk superconductors has been successfully designed, fabricated, and tested. Our fabricated prototype featured a squirrel-cage rotor wound with 44 GdBCO bulk bars that were embedded in the rotor iron and connected at both ends using Y-Ba-Cu-O (YBCO) tapes to form a short circuit. First, the critical current of the bulk sample was investigated under a magnetic field perpendicular to its top surface at 77 K. Subsequently, no-load and load tests were performed in liquid nitrogen at different frequencies to verify the performance of the prototype. Finally, we thoroughly analyzed the differences between the proposed HTS-ISM that utilizes GdBCO bulk bars and developed HTS-ISMs that utilize a few turns of YBCO or Bi-Sr-Ca-Cu-O (BSCCO) tapes per bar.

Study of Electromagnetic and Thermal Transients in a High-temperature Superconducting Transformer during a Short Circuit

Today, high-temperature superconducting (HTS) current limiters and transformers allow to limit the surge short circuit current during failure without negatively affecting on the power grid complex at the normal operation mode. However, the transition of a superconductor to a resistive state at the moment of current limitation can cause significant heat generation, which can destroy the transformer windings. The research goal is to provide optimal technical characteristics of the HTS transformer to achieve effective short circuit current limitation and prevent thermal breakdown of its windings. To achieve this goal, a mathematical model of a HTS transformer was developed. The presented method considers the material type and geometry of the superconducting tape, the critical parameters of the superconductor (current and temperature), the parameters of the cryogenic liquid, dependence of the resistance and heat capacity of the HTS tape layers on temperature. The simulation model was created in the Matlab/Simulink software. The most important result is the possibility of obtaining optimal electrical and thermal parameters of the HTS transformer windings during the short circuit current limitation, as well as ensuring the thermal stability of the superconducting tape at the quench moment. The obtained results are significant in the design and operation of HTS transformers. For efficient and safe operation in the current-limiting mode, it is necessary to take into account heat generation on the transformer windings. It is important for the superconductor returning to the superconducting state without causing significant overheating of the windings.

Fabrication of racetrack-type double-pancake coils using 2G tapes and online mechanical shock test

Racetrack-type double-pancake (DP) coil was wound successfully with YBCO high-temperature superconducting tapes. The characteristics and impact resistance of the coils were investigated. The characteristics of the dependent DP coil did not change after applying different acceleration values in the X and Y directions. With the increasing value of acceleration (i.e., up to 30 g in X direction and 15 g in Y direction), the voltage of the racetrack coil remained unaffected. Based on the tests, three racetrack-type coils were wound and assembled in a certain sequence to form a magnet. The optimum temperature range for impregnating the magnet was confirmed by investigating the viscosity and shear stress of paraffin at different temperatures. After the assembly of the coils, the magnet was vacuum impregnated with liquid paraffin. Subsequently, the magnet was fixed in a Dewar that was filled with liquid nitrogen to cool the coils. The magnet was energized after cooling down. Then, the mechanical shock resistance of the coils in Dewar was tested dynamically. Results showed that the coil characteristics did not change after paraffin impregnation. The voltage and magnetic field were almost unaffected by the dynamic increase in the acceleration value to up to 30 g every 5 g interval in the X direction. Thus, these methods of construction and paraffin impregnation and fixtures can be used in the racetrack-type magnet.

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

High-temperature superconducting (HTS) coils serving as DC magnets can be operated under non-negligible AC fields, like in synchronous machines of maglev trains and wind turbines. In these conditions, dynamic resistance is generated in HTS tapes, causing redistribution/bypassing of the transport current inside the no-insulation (NI) coil and its unique operational features. This issue was studied by experiments on an NI coil with DC current supply put into external AC fields. Due to the current redistribution induced by dynamic resistance, the central magnetic field and voltage of the NI magnet initially undergo various transient processes, and eventually exhibit a stable central magnetic field reduction and a DC voltage. These time evolutions have implications for the time-varying torque and loss of an HTS machine. These time evolutions are strongly affected by the contact resistivity distribution, and whether it is the first time that the NI magnet has been exposed to the AC field, showing several qualitatively different waveforms (e.g. some are even non-monotonic with time). The magnitudes of the stable central field reductions, and their observed linear correlation with the DC voltages are found to be decided by the local contact resistivity of the innermost and outermost several turns. It is also noted that the non-insulated turn-to-turn contact help lessening the loss induced by the dynamic resistance. A numerical model is established to analyze/explain these experimental results by observing the microscopic current distribution. Two risks of quench are noticed: (i) the azimuthal current of the middle part turns increases as the AC field is applied; (ii) a concentration of radial current is observed near the terminals of the NI coil.

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.

Analysis of critical current fluctuations as a means of checking the quality of high-temperature superconductor tape slitting

The patterns of longitudinal Ic fluctuations in 12 mm wide manufactured high-temperature superconductor tapes are compared with those obtained after slitting the tapes to widths of 6 mm, 4 mm and 3 mm, respectively. In the statistical analysis of Ic (x) data, the overall critical current, Ic ,ovrl , was first calculated. In the case of fluctuating critical current, Ic ,ovrl is lower than the average critical current, Ic ,aver . Also, the dissipation concentrates in ‘weak spots’ with reduced critical current. A simple model allows us to estimate the thermal runaway current, I tr , at which the weakest location would convert to a ‘hot spot’ with rapid increase in temperature. In the prediction of I tr , the absolute minimum of the Ic value, I cmin, is essential. Then, by comparing the I cmin /Ic ,aver ratio in the slit tapes to that established previously for the manufactured tape, one can deduce if some new defects have been introduced during slitting. It is also interesting to check if the relation between I tr and Ic ,ovrl remains unchanged, because the lower of these two quantities represents the limit of transportable current. We have found a reduction of both I cmin /Ic ,aver and I tr /Ic ,ovrl ratios in those tapes containing the edges of the originally manufactured 12 mm tape. Contrary to our expectations, the most serious drops in slit tapes were not in the same location as in the original tape. However, the systematically better performance of the central strips (the tapes with both edges cut) is a strong indication that the most serious defects are created during manufacturing, close to the tape edges. We conclude that tape cutting by femtosecond laser has not introduced additional non-uniformity. The analysis also shows that the criticality of defects in applications is largely governed by the width of said defects, the cooling conditions and the thickness of the stabilizing layer.

Fast modeling approach of large-scale non-inductive HTS coils under different current supply

This paper presents a fast-integral modeling approach for the design and characterization of large-scale non-inductive high temperature superconducting (HTS) coils made of first-generation HTS tape. The integral modeling is combined with a strategy of far tape approximation allowing to model a reduced set of consecutive tapes instead of the entire coil. It makes it possible to estimate the losses and the current density distribution in the coil, under different current supply. This leads to a considerable reduction in the computation time. The calculated AC losses show a good agreement with the Norris ellipse formula and measurements, highlighting the importance of considering the elliptical section of the superconductor in the tape, in the modeling. The developed modeling approach is in particular useful in the AC + DC mode, where Norris’s formulas cannot be used to determine the losses. Besides, some interesting phenomena have been observed, related to the current distribution in the tapes at different AC and DC current values.

Multiscale modelling and numerical homogenization of the coupled multiphysical behaviors of high-field high temperature superconducting magnets

Ensuring the safe and stable operation of the high temperature superconducting (HTS) magnet has become a key problem to be solved currently. In this paper, we developed a numerical homogenization scheme to simulate the coupled electromagnetic-thermal-mechanical behaviors of the REBCO (REBa2Cu3Ox, where RE = rare earth) high field magnets. The homogenized T-A formulation is adopted to model the electromagnetics of HTS tapes. The representative volume element (RVE) based on the micromechanics is created to predict the homogenization properties of the magnets. The field dependences of the critical current density of HTS tapes on the magnetic field, magnetic field angle, the temperature and the strain are considered. The effectiveness and efficiency of the proposed numerical scheme have been well validated through comparisons with experimental results. Moreover, the effects of the anisotropic plasticity on the mechanical behaviors of the HTS magnets are also evaluated. Our results show that compared with the Hill’s anisotropic model the commonly used isotropic von Mises plasticity model underestimates the plastic strain in the magnet. Under ultrahigh magnetic field, quench of the magnet is mainly triggered by the strain-induced degradation in the critical current of HTS tapes, which needs special attention in the design of high field magnets.

A homogenised anisotropic J-model for accelerating computations of screening current profile in large-scale HTS magnets

Owing to the induced excessive strain, plastic deformation, and distortion of the magnetic field in high-temperature superconducting (HTS) tapes, there is a rapidly growing research interest in the screening current profile in high-field superconducting magnets. The J-model was developed to calculate the current density distributions in HTS stacks and cables, which improves the calculation efficiency because no air domain is considered in the calculation regions. However, for a large-scale high-field magnet wound with hundreds and thousands of layers of HTS tapes, there is still a big challenge for J-model simulation involving prohibitive time consumption and memory requirements. This study proposes an improved J-model approach based on a homogenised anisotropic (HA) scheme for large-scale HTS magnet systems to decrease the number of elements and enhance calculation efficiency. The reliability of the improved HA J-model was verified by solving 2000-tape HTS stacks, and a significant improvement in computational performance was achieved. Furthermore, the HA J-model was applied to a large-scale scenario of a 32 T hybrid all-superconducting magnet at the National High Magnetic Field Laboratory to numerically capture the electromagnetic responses. The magnetic flux and current densities in the HTS insert coils were obtained completely, and the screen current induced field, which is the most concerning, showed a good agreement with the literature. Meanwhile, the time consumption by the HA J-model is approximately half that of the traditional J-model and even less than 1–2 orders of magnitude of the others, such as the traditional T-A model and H model, with acceptable accuracy. The improved HA J-model may provide an appropriate prior design with rapid and real-time calculations for large-scale magnet systems.

Conceptual design, AC loss calculation, and optimization of an airborne fully high temperature superconducting generator

With the trend of multi-electrification and hybrid-electrification of aircraft, the airborne high temperature superconducting (HTS) generator is considered as the key technology of the new generation of aviation power generation system and has received wide attention. The high critical current and low loss of superconductors can significantly improve the output power, power density, efficiency and other performance of generators. In this work, conceptual design of a fully HTS generator for aircraft is carried out to address key issues in terms of AC loss distribution, which serves as the ground of cryogenic design in the future. A 10 MW fully superconducting generator is designed to be used in a large passenger aircraft (with more than 150 seats and a maximum takeoff weight of about 100 tons). Based on the finite element method (FEM) with the so-called T-A formulation, the electromagnetic characteristics of the generator are analyzed, and AC losses of the stator and rotor are calculated. To calculate the rotor loss in realistic dynamic-field environment, a technique named “rotation-domain exchange” is developed and implemented. By using such technique, accurate calculation of AC loss of the rotor in realistic dynamic electromagnetic environment is achieved with no need of simplified boundary condition as previously reported work. Based on the model, the stator coil structure is optimized to reduce the AC loss and the rotor coil structure is optimized to reduce the HTS tape usage. Results show that the optimized stator AC loss is 15.645 kW, and the rotor AC loss is 6.268 kW, indicating that the rotor AC loss of a fully superconducting generator with narrow air gap and large number of coil turns is not neglectable as previous work assumed. Thus, precise calculation and dedicated optimization of the rotor loss is critical since cooling of the rotor is much more challenging than the stator as the former remains in motion.

Shielding Characteristics of Solenoidal Superconducting Screens Made From HTS Tape, for SFCL Applications

In the design and prototyping of an Inductive-Resistive Superconductor Fault Current Limiter (IR-SFCL), previously presented by the authors, superconducting screens made from BSCO bulk have been used for shielding the inductive stage of the device. This kind of screens has several problems as rigidity, brittleness, commercial dependence on size and high cost. In order to avoid such issues, replacing the bulk-type superconducting screens by screens made from high temperature superconducting (HTS) tape is proposed. Tapes are more flexible and allow to easily make screens with several sizes and configurations, and better behavior against temperature changes and dynamic effects. To select a suitable configuration for this type of screen, a preliminary study of the shielding capacity of screens with different configurations, made from tape, is presented. This study, as well as literature, led us to select the solenoidal configuration as the most suitable for our application. In this work, we study solenoidal screens built with independent concentric layers, each one made from non-insulated tape, and all in contact just by the Ag external cover of the tape. A test method has been developed, and screens with 1 to 10 solenoid layers have been tested to find out their shielding factor. The shielding mechanism (diamagnetism vs. induced currents) has been studied by measuring of the transport current generated in between the non-insulated solenoids. The low incidence of these currents in the magnetic shielding process of the superconducting tape screens is reported.

Role of Insulation Materials and Cryogenic Coolants on Fault Performance of MW-Scale Fault-Tolerant Current-Limiting Superconducting Transformers

Fault-tolerant current-limiting (FTCL) high-temperature superconducting (HTS) transformers are promising components for playing a role in renewable energy integrated modern power systems. In this article, the impact of different insulation materials and cryogenic coolant fluids on the electro-thermal characteristic of an MW-scale FTCL-HTS transformer is investigated. For this purpose, an equivalent circuit model (ECM) is established to characterize the temperature, recovery time, and fault tolerability of FTCL-HTS transformer under different conditions. The proposed ECM is firstly validated using experimental results of a typical HTS transformer. After that the model is developed for a 50 MVA, 132 kV/13.8 kV HTS transformer. In order to add fault tolerance capability to the 50 MVA HTS transformer, three strategies were considered in this article. In the first strategy, the effect of three insulation materials (including Kapton tapes, Nomex papers, and Acrylated Urethane solid insulation) covering HTS tapes was investigated on fault performance. The second strategy was changing cryogenic fluid to liquid hydrogen instead of liquid nitrogen. As for the last strategy, the impacts of thickness and material properties of stabilizer in HTS tape were investigated on the maximum temperature and recovery time of HTS transformer. Results show that by using these strategies, the maximum temperature of HTS tape of transformer winding under a short circuit fault was reduced.

Numerical and Experimental Investigations of Flaws Detection in Multilayer HTS Tapes by Magnetothermal Technique

This paper presents a numerical and experimental studies on the non-destructive testing of high temperature superconducting (HTS) multilayer tapes at room temperature, by eddy current thermography (ECT). An experimental setup is developed in which a rotating magnet wheel inductor is used instead of electromagnetic inductors to avoid the inherent thermal disturbance. A 3-D magnetothermal modelling approach is developed for the verification of the experimental results and the simulation of situations that cannot be reproduced experimentally, in particular the verification of the detection limits as a function of the shape and location of the defects. Simulation and experimental results are in good agreement.

Numerical analysis of the mechanical and electrical properties of CORC cables under torsional loading

The type II high-temperature superconducting (HTS) ReBCO tapes have been widely used in high-field magnets. Conductors on round core (CORC) cables can carry very high currents in background magnetic fields exceeding 20 T, making it one of the most important types of HTS cables. However, the cables of the magnets in the fusion reactor are subjected to large mechanical and electromagnetic loads that may twist the CORC cables. The intrinsic strain of the superconducting cable is constrained by the brittleness of the HTS tape, which, if exceeded, will cause irreversible damage by producing cracks in the ReBCO layer. A finite element (FE) model was developed to predict how the CORC cable will perform in torsional cases. A comparison of numerical simulations with experiments for a single- and double-layer CORC cable is first performed to validate the model's reliability. For single-layer CORC cables, our model reduces the error between FE results and experiments from 46.7% to 6.7%. Variations in the winding angle, tape width, Poisson's ratio of the CORC cable core material, and core radius were made as part of the parametric analysis. The critical torsion angle is analyzed from the perspective of critical current density reduction. The results show that maintaining a small tape gap (smaller winding angle (26−30°), smaller core diameter, and larger tape width) and a small Poisson ratio can improve the strain limit of the ReBCO layer. The reason for the plateau phenomenon in the normalized critical current density curve of the double-layer CORC cable is reinterpreted. It is also confirmed that multi-layer CORC cables still need to maintain a small tape gap. The FE model can guide optimizing a cable design for specific application conditions.

Basic Coil Structure for Rapid Charge in a Low-Frequency and High-Efficiency Wireless Power Transmission System Using High-Temperature Superconducting Coil for Railway Vehicle

We have been investigating the possibility of a low-frequency and high-efficiency wireless power transmission (WPT) system for the railway vehicle using a high-temperature superconducting (HTS) coil. We investigated a basic HTS coil structure having a low ac loss and a high coupling coefficient between the primary and secondary coils by adjusting a radial gap between turns. The devised HTS coil was configured to have a radial gap approximately equal to the HTS tape width and increase the number of turns while reducing the inner diameter of the coil. Also, we analyzed the power transmission characteristics in the 40 kW class WPT system using the devised HTS coils operated at a low frequency around 1 kHz for the railway vehicle. Furthermore, we fabricated the 1 kW class WPT model system at an operating frequency of 0.92 kHz using the devised HTS coil and demonstrated the effectiveness of the low-frequency and high-efficiency WPT system using the devised HTS coil structure. As a result, we clarified that the low-frequency WPT system using the devised HTS coil for the railway vehicle can perform more rapid charging and the high-efficiency transmission than that using the copper coils.

Intelligent Probability Estimation of Quenches Caused by Weak Points in High-Temperature Superconducting Tapes

Fluctuations in the critical current along the length of high-temperature superconducting (HTS) tapes manufactured in the form of coated conductors is a common manufacturing phenomenon. These fluctuations originate in the generation of weak points through the length of HTS tapes that may cause quenching later. By means of the propagation of quenches in HTS tapes, the reliability, stability, and the performance of the device and the system that contain HTS tapes could be seriously degraded. In this study, an artificial intelligence technique based on artificial neural networks (ANN) was proposed to estimate the probability of quenches in HTS tapes caused by weak points. For this purpose, six different HTS tapes were considered with different widths, total thicknesses, and thicknesses of sub-layers. Then, for each one of these tapes, different operating conditions were considered, where the operating temperature changed from 40 K to 80 K, in 1 K steps. Under each operating temperature, different operating currents were considered from 50% to 100% of tape critical current. All of these resulted in more than 5000 different data points. Then, for each of these data points, analytical modelling was performed to provide initial inputs and outputs for the ANN model. It should be noted that the performed simulations were conducted based on an analytical method that was experimentally validated in the literature. After that, a sensitivity analysis was conducted to select the hyperparameters and structure of the ANN-based model. The last step was to take advantage of the trained model, as a function in the MATLAB software package to estimate the probability of quenches in different case studies. The significant feature of the proposed model is the capability for estimating the probability of quenches under different operating temperatures and currents for different types of HTS tapes.

Quench detection and early warning based on thermoelastic strain rate for HTS tapes thermally triggered by heat spots

The rapid detection and comprehensive monitoring of quench onset and evolution in high-temperature superconducting (HTS) materials remains immensely challenging because the normal zone propagation velocity in HTS materials is two or three orders of magnitude less than that in low-temperature superconductors. In this study, we experimentally and numerically explore quench events triggered by heat spots in yttrium barium copper oxide HTS tapes to characterize the quench onset and propagation. A multiplexed fiber Bragg grating sensor with multiple gratings was used to perform highly accurate strain measurements. Conventional voltage and temperature measurements were performed synchronously on the HTS tapes. A systematic comparison of these multifield signals during quench onset and development illustrated that the evolution of thermoelastic strain and strain-rate in HTS tapes captures the quench onset and propagation. A distinct feature was exhibited during the quenching of a pre-tensioned HTS tape: the thermoelastic strain initially relaxed but subsequently increased until the strain rate exhibited a significant slope change, which corresponded to the quench onset time. The thermoelastic strain in a nearly unconstrained HTS tape gradually increased until quenching occurred. A prominent characteristic for detecting quench onset in HTS materials have been revealed based on the change in the slope of the thermoelastic strain-rate or the second derivative of the strain remaining nearly constant. For a pre-tensioned HTS tape, the minimization of the thermoelastic strain or the strain-rate becoming zero may be a predictor that preceded the quench by ∼1–2 s, which can be, to a certain extent, regarded as an early warning. Another important and novel result was the experimental demonstration of global strain responses distant from the quench location in the pre-tensioned HTS tape, while the temperature and voltage detection are commonly localized methods. The mechanism behind these thermoelastic strain characteristics was further discussed and simulated from the induced Joule heating throughout the quench event. The measurements and numerical predictions suggested a new paradigm of quench detection based on the thermoelastic strain-rate in HTS materials.