High-temperature Superconducting Tapes Research Articles

Expected radiation environment and damage for YBCO tapes in compact fusion reactors

We investigate the neutron damage expected in high-temperature superconducting tapes that will be employed in compact fusion reactors. Monte Carlo simulations yield the expected neutron spectrum and fluence at the magnet position, from which the primary knock-on atom energy distributions can be computed for each atomic species comprising the superconductor. This information is then employed to characterize the displacement cascades, in terms of size and morphology, through molecular dynamics simulations. The expected radiation environment is then compared with the neutron spectrum and fluences achievable at the facilities currently available for experimental investigation in order to highlight similarities and differences that could be relevant to the understanding of the radiation hardness of these materials in real fusion conditions. We find that the different neutron spectra result in different damage regimes, the irradiation temperature influences the number of generated defects, and the interaction of the neutrons with the superconductor results in a local increase in temperature. These observations suggest that further experimental investigations are needed in different regimes and that some neutron shielding will be necessary in compact fusion reactors.

Flux penetration of an HTS coated-conductor tape by an approaching permanent magnet

Interactions between inhomogeneous magnetic fields and a superconductor lie at the heart of several high-temperature superconducting (HTS) devices. An example is the situation where the magnetic field from a small permanent magnet (PM) is applied to a coated-conductor HTS tape, such that flux penetrates the superconductor. The PM field is highly spatially inhomogeneous, such that there is significant variation in the applied field magnitude and direction across the tape. This leads to a varying local critical current density, Jc(B , θ ), across the tape, and consequently different ‘shielding’ (magnetic flux penetration) behaviour than is the case for a uniform applied field. Here, we report results from measurements and numerical simulations of the penetrating magnetic field within an HTS coated-conductor tape which occur as a PM dipole approaches from a distance. Electromagnetic simulations were performed using a finite-element model based on the H -formulation, and which incorporated experimentally-measured anisotropic Jc(B , θ ) properties from a coated-conductor tape. This showed good agreement with experimental measurements. The effects of varying key modelling assumptions and parameters were then studied, including changing the widths of the PM and HTS tape and the magnitude of Jc(B , θ ). The inhomogeneity of the PM field leads to a characteristic gull-wing distribution of magnetic flux across the superconductor at elevated applied fields. Close approach of the PM to the tape suppresses Jc in the centre of the tape, and this results in the observation of a characteristic maxima for the total shielding currents circulating in the tape. A figure of merit is introduced which uses this effect to provide a threshold definition for ‘full flux penetration’ of an HTS tape by an inhomogeneous dipole field.

Numerical Analysis on Performances of Critical Current for Rutherford Cable Made From HTS Quasi-Isotropic Strands

2G high temperature superconducting (HTS) tapes are frequently cabled into strands that can be assembled as conductors in cables with various layouts and thus potentially employed in large-scale superconducting magnets with high magnetic fields and large transport currents. One option meeting the requirements for manufacturing high-current cables is the Rutherford cable configuration. This article proposes a flat Rutherford cable with a copper core made up of 12 quasi-isotropic strands. The critical current characteristics of the Rutherford cable are numerically analyzed at 77, 20, and 4.2 K by using the finite element method and a self-consistent model in self-field and applied magnetic fields with different magnitudes and orientations, respectively. Additionally, the simulated results of the Rutherford cable are compared with those of a directly stacked conductor. The results show that the proposed Rutherford cable can attain critical currents in the hundreds of kA range at 20 and 4.2 K in high fields. This Rutherford cable design is a potential and prospective solution for high-current superconducting cables in power transmissions and large-scale magnets with a high magnetic field.

A Noninductive Bifilar Coil to Design Compact Flux Pumps for HTS Magnets

High-temperature superconducting (HTS) flux pump is a promising way of energizing superconducting magnets without direct electrical contacts. It can remove the resistive heating and heat leakage from the current leads at room temperature. It is well known that applying the current over the critical current of the superconductor will force the superconducting layer to enter into the flux flow regime. This flux flow resistivity generates a dc voltage across the length of the HTS tape. This phenomenon is well understood; however, it is difficult to generate a high dc charging voltage without a high current. In this article, we demonstrate a novel flux pumping technique by using a noninductive bifilar bridge wound in parallel to the HTS coil. This configuration can generate large dc voltages using relatively small currents. It results in effectively pumping dc currents equal to the critical current for large HTS magnets.

Multi-Objective Optimization for Improving Weight and Fault Characteristics of a DC HTS Cable in Cryo-Electric Aircraft

The aim of the presented study is to optimize the different classes of high-temperature superconducting (HTS) DC cables for improving their performances in a cryo-electric aircraft considering their weight, peak temperature during faults, and the ratio of current passing through each tape to the critical current of HTS tapes. These terms were interpreted into three objective functions, and a multi-objective optimization algorithm known as non-dominated sorting genetic algorithm II was used to find the optimal solution clusters. The cable optimization was conducted for different former materials by changing the former thickness and radius. Results showed that the DC HTS cables with aluminum former have the lowest weight while cables with copper formers have the best thermal performance against faults.

Dependence of dynamic resistances in high-temperature semiconducting quasi-isotropic strands on magnitude and orientation of AC magnetic field

The dynamic resistance of a superconductor appears if it carries a direct current (DC) current and is simultaneously exposed to alternative current (AC) magnetic fields if its amplitude is higher than its full penetrated field. Due to its strong anisotropy, the dynamic resistance of high-temperature superconducting (HTS) tape is not only affected by the magnitude of AC magnetic fields but also their orientations. In particular, the dynamic resistance has considerable effect on the current distribution in HTS cables (except for Roebel cables) and even cable-in-conduit conductors made from those HTS cables, except for dissipative loss. This paper presents the behavior of dynamic resistance in an HTS quasi-isotropic strand (Q-IS), which consists of four sub-stacked 2G HTS tapes proposed by our group at North China Electric Power University, while it carries a DC current and simultaneously experiences an AC magnetic field with different magnitudes and orientations. An angular dependence of dynamic resistance in a single 2G HTS tape in an AC magnetic field with various orientations is first performed in simulations and experiments, respectively. The experimental results are in good agreement with the simulated ones, which shows that the dynamic resistance of a Q-IS also exhibits quasi-isotropic characteristics and has an important effect on the distribution of DC transport current. The related results are promising for the design and application of other DC HTS cables, feeders and magnets.

Analytical approximations for the self-field distribution of a superconducting tape between iron cores

Magnetic switches apply AC magnetic fields to DC current-carrying high temperature superconducting (HTS) tapes to generate DC voltages and are commonly used in the persistent current switches (PCSs) and flux pumps to charge HTS-coated conductor magnets. Normally, they are made of copper field coils and iron cores with narrow air gaps for the HTS tape to pass through. However, the perpendicular components of the self-field of the HTS tape in the air gap can be enhanced by the iron cores and cause a critical current reduction of up to 40% to the tape. If ignored, this reduction, rather than the magnets themselves, will limit the current carrying capability of the HTS magnets. To tackle this problem, we present analytical approximations to calculate the self-field distribution of a superconducting tape between iron cores. The approximate solutions are based on the method of images in electromagnetics to simplify the derivation and are then verified by the experiments and 3D finite element method models using the T–A formulation. The solutions are universal and can be applied to almost all the magnetic switches currently in use. A case study of typical magnetic switches shows that the solutions can be used to determine the critical current reduction quickly and accurately, analyse the influence of different parameters, and simplify the design process of magnetic switches. The results can significantly benefit the design and optimisation of PCSs and flux pumps for HTS magnet charging systems in the future.

Fabrication technology and performance tests for optical fiber-encapsulated, high-temperature superconducting tapes

Realizing fast and accurate quench detection is a great challenge for the application of long high-temperature superconducting (HTS) conductors. The combination of a distributed temperature sensing (DTS) system and optical fiber-encapsulated HTS (OFE-HTS) tape may become a promising approach to solve this problem. As a recently proposed composite HTS tape, its properties have not been studied systematically yet. Therefore, in this study, the electromagnetic and thermal behaviors of the OFE-HTS and traditional HTS tapes are compared by finite element simulation technology. The simulation results predict that the embedded optical fibers will hardly change the original electric, magnetic and thermal characteristics of the HTS tapes. In addition, a detailed fabrication method for OFE-HTS tape is introduced, and the composite tape performances including structural integrity, critical current uniformity, anti-bending/tensile force and quench response are tested seriatim. According to the microscope and x-ray detection results, the optical fibers are fully embedded in the OFE-HTS tape through the presented fabrication process. The critical current uniformity test results show the average critical current of the prepared 76 m long OFE-HTS tape is about 520 A, and the uniformity variation is about ±4%. The prepared OFE-HTS and traditional tapes have similar anti-bending/tensile properties. Finally, to check the effectiveness of the embedded optical fiber for quench detection, the fabricated OFE-HTS coil is tested. The quench detection results show that the temperatures in the same area measured by the optimized DTS system and a thermocouple are similar. Moreover, the temperature response ability of the optimized DTS system is better than that of the thermocouples, and the optimized DTS system is able to effectively avoid environmental electromagnetic field interference.

Magnetization loss of CORC cables under axial tensile loading

As a highly flexible and fully isotropic cable structure, CORC (Conductor on Round Core) cable wound with high-temperature superconducting (HTS) tapes has promise for giant magnets, including fusion magnets, high-energy accelerator magnets, and nuclear magnetic resonance. CORC cables are subjected to mechanical, thermal, and electromagnetic loadings during preparation and operation. Axial tension is one of the basic deformation modes of CORC cables. To investigate the impact of axial tension loads on the electromagnetic performance of CORC cables, a mechanical-electromagnetic model is created in this study. A comparison of numerical simulations and experiments was first performed for single straight tape and single-layer CORC cables to verify the model's reliability, with particular attention to the critical current reduction and the variation of magnetization loss during axial tension. The model developed in this paper may evaluate electromagnetic performance under applied AC magnetic fields with small amplitudes and mechanical response to axial tension loads. On this basis, the strain distribution of CORC cables under axial tension and their magnetization loss characteristics under various strain states are presented. The effects of different cable design parameters on the critical current and magnetization loss are discussed. The results show that the strain affects the magnetization loss of CORC cable significantly, especially for ones with larger winding angles. The magnetization loss can be reduced by choosing a narrower tape, a smaller winding angle, or a cable core material with a higher Poisson ratio. This study can provide a theoretical basis for the structure optimization design and practical application of CORC cables.

Nondestructive Testing of Multilayer HTS Tapes by Using Eddy Current Thermography

This article presents numerical and experimental investigations of the structural control of multilayer high temperature superconducting (HTS) tapes, using eddy current thermography at room temperature. A rotating magnet wheel is used as inductor to avoid the thermal disturbance of current supplied inductors. A 3-D magnetothermal modeling approach based on an integro-differential formulation in terms of the electric vector potential is developed for eddy current computation in the HTS tapes; associated to a thermal modeling using the finite difference method. The source magnetic field is calculated using the magnetic surface charge model. The simulation results are confirmed by the measurements, both highlighting the potential of such control to detect structural defects, such as delamination.

Conceptual Design and Sensitivity Analysis of MRI Magnets From REBCO HTS Tapes

An actively shielded wide-bore magnet system has been designed for magnetic resonance imaging. The gradient-based optimization solver of COMSOL Multiphysics has been used to obtain an optimum geometrical arrangement of symmetrical electromagnets in the magnetic resonance imaging (MRI) system. To generate a high and homogeneous magnetic field, we employed a second-generation rare-earth barium-copper-oxide (REBCO) high-temperature superconductor (HTS), which was wound into 10 double pancake coils (DPC). The dimensions of the DPC sets have been chosen considering the size of REBCO HTS tapes, and the optimized design solution provides magnetic field, stray field, and current passing through the HTS tapes on each DPC set. The design details of a 1.5 T actively shielded magnet as well as the sensitivity analysis of the inhomogeneity, stray field, and currents with respect to coil positions have been presented. Optimum parameters for various designs are listed. The inhomogeneity of 1.23 ppm in the 200 mm diameter of spherical volume (DSV) and a stray field of as low as 0.05 G outside of the 5 m distance were achieved. Additionally, a low peak field of 2.40 T in DPC sets has been accomplished.

Artificial intelligence-based models for reconstructing the critical current and index-value surfaces of HTS tapes

For modelling superconductors, interpolation and analytical formulas are commonly used to consider the relationship between the critical current density and other electromagnetic and physical quantities. However, look-up tables are not available in all modelling and coding environments, and interpolation methods must be manually implemented. Moreover, analytical formulas only approximate real physics of superconductors and, in many cases, lack a high level of accuracy. In this paper, we propose a new approach for addressing this problem involving artificial intelligence (AI) techniques for reconstructing the critical surface of high temperature superconducting (HTS) tapes and predicting their index value known as n-value. Different AI models were proposed and implemented, relying on a public experimental database for electromagnetic specifications of HTS tapes, including artificial neural networks (ANN), eXtreme Gradient Boosting (XGBoost), and kernel ridge regressor (KRR). The ANN model was the most accurate in predicting the critical current of HTS materials, performing goodness of fit very close to 1 and extremely low root mean squared error. The XGBoost model proved to be the fastest method, with training computational times under 1 s; whilst KRR could be used as an alternative solution with intermediate performance.

Comparative analysis on twisting characteristics of HTS quasi-isotropic strand and stacked-tape strand

With the gradual maturity of high temperature superconducting (HTS) technology, several HTS conductors with large critical current like quasi-isotropic strand (Q-IS) and twisted-stacked-tape cable (TSTC) were proposed, which can be assembled into cable-in-conduit conductor (CICC) and applied in power system and fusion magnets. The purpose of this paper is to study the twisting characteristics of Q-IS and stacked-tape strand (STS) with different stacking patterns under self-field at 77 K. The critical currents of two Q-IS samples and two STS samples are measured under increasing twist angle until they break. Each sample is composed of copper tapes and second generation (2G) REBa2Cu3Ox (REBCO, RE = rare earth) HTS tapes. For each type of conductor, one sample has a structure where REBCO tapes are stacked near the outside, while the REBCO tapes are stacked close to the inside in another sample. Simulation based on finite element method (FEM) and H-formulation is carried out for comparison. Numerical calculation of the normalized critical current versus the twist pitch is also conducted. Results show that each kind of strand with REBCO tapes stacked close to the inside have smaller critical twist pitches. Besides, self-field effect is dominant while the twist angle is small, but the effect of longitudinal strain gradually becomes the main factor as the twist angle increases. In general, the critical current degrading rates of STS samples are smaller than that of Q-IS samples. However, when the twist angle is relatively small, Q-IS samples show better critical current characteristics than STS samples.

Enhancement of electro-mechanical behaviors in RE–Ba–Cu–O composite superconducting tapes with laser slit edges

The slit technique is an essential process for narrowing the second-generation high-temperature superconducting (2G-HTS) tapes. It is still very challenging to eliminate edge defects introduced during slitting. In this work, we developed a set of reel-to-reel laser slit (LS) equipment for narrowing 2G-HTS tapes using a femtosecond infrared laser. In comparison with the commonly used mechanical slit (MS) technique, the LS technique produced no cracks at the tape edges. Electro-mechanical behaviors of 2G-HTS tapes with MS and LS edges were compared in liquid nitrogen conditions. The results of the uniaxial tensile tests showed only a minor difference in irreversible tensile stress/strain of the two kinds of tapes. However, uniaxial tensile fatigue tests showed that the critical current (I c) retention of the LS tapes was about 14% higher than that of the MS tapes after 104 cycles (@ 77 K, σ max = 0.8σ y, and a stress ratio of 0.1). Moreover, U-Spring compressive tests revealed that no irreversible I c degradation occurred on the LS tapes under compressive strain up to −0.8%, with high reproducibility. On the contrary, the irreversible compressive strains of the MS tapes were rather scattered, ranging from −0.75% to −0.2%. Enhancement of electro-mechanical behaviors is strongly linked to fewer edge defects in the LS tapes. This research demonstrates significant advantages of the LS technique over the conventional MS technique, showing its promising prospects for demanding applications.

Coupling electromagnetic numerical models of HTS coils to electrical circuits: multi-scale and homogeneous methodologies using the T-A formulation

Numerical simulation is an effective tool for predicting the electromagnetic behavior of superconductors. Recently, a finite element method-based model coupling the T-A formulation with an electrical circuit has been proposed: the model presents the superconducting constituent as a global voltage parameter in the electrical circuit. This allows assessing the overall behavior of complex high-temperature superconductor (HTS) systems involving multiple power items, while keeping a high degree of precision on the presentation of local effects. In this work, the applicability of this model has been extended to large-scale HTS applications with hundreds or thousands of tapes by referring to two widely recognized methodologies, multi-scale and homogenization, to improve the computation efficiency. Based on the two approaches, three different models were developed and their effectiveness was assessed using the case study of a 1000 turn cylindrical HTS coil charged by a DC voltage source. The comparison of the calculated global circuit parameters, local field distributions, losses, and computation time proves that the computation efficiency can be improved with respect to a model simulating all HTS tapes, without compromising accuracy. The results indicate that the developed models can therefore be efficient tools to design and optimize large-scale HTS devices used in electrical machines and power grids. It is also found that the inductance of an HTS coil is varied according to the transport current and can be even higher than that of a normal conductor coil with the same geometry. We attribute this result to the superconductor’s non-uniform current distribution and relaxation effect during the dynamic process.

Computation of AC loss of HTS coils wound by coated conductors with different widths

The second-generation high-temperature superconductor (HTS) tape is a promising material for the HTS application. However, the extremely large aspect ratio of the HTS coated tape leads to high power dissipation in the environment of a time-varying magnetic field. Reducing the width of HTS coated tape is a feasible way to decrease the alternating current (AC) loss of devices composed of HTS tapes. In this study, a numerical model of the HTS coil group composed of six sub-coils based on the T-A formulation is presented in which each HTS sub-coil is wound with original or narrow width Y-Ba-Cu-O (YBCO) tape. The 1/3 and 1/4 narrow width tapes are used to represent the tapes, which are divided into three and four equal parts, respectively. Then, we calculate the AC losses in 1/3 and 1/4 narrow width HTS coil groups in different cases. The estimated results of AC loss are compared with those of the HTS coil group wound by HTS tapes with original width. The numerical results show that AC transport losses of 1/3 and 1/4 narrow width HTS coil groups are smaller than that of the original HTS coil group for the applied high current. Furthermore, with the decrease in tape width, the reduction of AC loss becomes more significant. In contrast with the original width HTS coil group, the magnetization losses of the 1/3 and 1/4 narrow width HTS coil groups will decrease at the high magnetic field. In addition, the influences of harmonic components on AC loss are also considered.

Examination of HTSP Tape Coils by Computed Tomography

In manufacturing high-temperature superconductor (HTS) devices, outgoing control of their quality must be carried out. Conventionally, for coils made of HTS, electrical tests are carried out to determine the critical current. In case of an unsatisfactory result, the reason why the critical current is below its expected value should be determined. The HTS tape coil manufacturing quality should be checked on the basis of non-destructive test methods. The article proposes an HTS coil examination technology, according to which the actual locations of the tape and gaps between the coil turns and rows are determined with subsequently analyzing the obtained values. To this end, the samples were scanned using the computed tomography method on the Nordson Dage XD7600NT X-ray machine with the μCT module, and the data obtained were processed using the VolumeGraphics VGStudio 2.2 visualization software. The proposed technology can be used as part of a predictive analysis of the state of the HTS coils of electrical machine windings.

Edge cracking behavior of Y2O3 coatings by the solution deposition planarization method

As an efficient flattening technique for surface smoothing, solution deposition planarization (SDP) has recently drawn more attention as a fabrication method for second generation high temperature superconducting (2G-HTS) tapes. However, at present, it is still challenging to achieve edge crack-free surfaces while preparing a Y2O3 coating via SDP processing, especially during drying and heat treatment. Experimental results show that as the colloidal dispersion is coated and dried on a nonporous rigid substrate, the enormous stresses developed during the drying process can fracture the thin film. The finite element simulation results show that coating thickness, which is affected by the differences in pulling rate and solution concentration, is a critical factor determining the crack sies may be initiated. After optimizing the coating parameters, a 10-m long Hastelloy C-276 alloy base was successfully prepared. The average value of Ra was 1.28 nm, which is less than 5 nm (5 × 5 μm2), satisfying the industrial requirements for the preparation of superconducting materials using the IBAD (lon Beam Assisted Deposition) technology route.

Magnetization Loss of Multi-Layered CORC According to Various Winding Types

Conductor on round core (CORC) is one of high temperature superconducting (HTS) conductors with large current capacity. CORC stacks HTS tapes to increase the current capacity. CORC can have various structures according to how we stack HTS tapes. The direction of the spiral winding in each layer and the overlapped area of HTS tapes affect AC losses of CORC because the losses mainly depend on the perpendicular magnetic fields on the HTS tapes. We have measured the magnetization losses of CORC short samples with various structures. Each sample has a different number of layers, winding direction in each layer, and overlapped area. We confirmed the best structure of CORC with the least magnetization loss.

Thermal Conductivity of NI HTS Coil Fabricated by Diffusion Bonding Technique

The no-insulation (NI) winding technique shows superior thermal protection performance when quenching of a high- temperature superconducting (HTS) coil occurs compared to the insulation winding technique. However, the thermal conductivity of an NI HTS coil are not clearly known because the HTS tape has various surface conditions such as roughness and oxidation. In this study, the diffusion bonding technique was applied to predict the thermal conductivity of an NI HTS coil, and the core of this technique is to remove the interface between turns by applying Ag diffusion bonding. To investigate the thermal conductivity of an NI HTS coil, an NI HTS coil was fabricated using a diffusion bonding technique, and a stainless-steel 304 block was fabricated to verify the experimental results. The thermal conductivity of the NI coil was measured using a conduction cooling experimental apparatus by applying heat loads to the outside of the coil. The measured thermal conductivity was compared to the values calculated using the thermal resistance circuit at temperatures from 20 K to 77 K.