High-temperature Superconducting Devices Research Articles

Performance evaluation of a racetrack high temperature superconductor winding in an HTS levitation platform

Abstract High temperature superconducting (HTS) is a key technology of next generation high-speed railway systems, which guarantees high current and highly efficient traction power supply. HTS devices can reduce system weight and thus improve loads of vehicles. On high-speed maglev trains, HTS windings wound by second-generation (2G) HTS tapes play an important role in suspension and traction. However, in complex ferromagnetic environments, the performances of windings are notably influenced by the varying magnetic fields. In this paper, a new maglev platform is designed and comprised of a winding of six racetrack HTS coils, an E shape iron core and a magnetic guide rail. The air core structure of the winding is modeled，tested and the electromagnetic characteristics are analyzed for three dimensional (3D) finite element analysis of the platform. An A formulation is used to calculate magnetic vectors A and infer magnetic flux densities and current densities for the structures. After the winding is installed on the iron core and the magnetic guide rail is suspended above the iron core, the air gap between the iron core and the rail is varied from 0 mm to 20 mm in models and experiments. The magnetic fields, critical currents and losses of the winding in cases of the air core and the iron core conditions are used for analysis. The comparison between the air core and the iron core conditions confirms the reliability of the proposed models. This study provides means for analysis of complex superconducting maglev systems.

Performance Analysis of Data Acquisition Instruments for High-Temperature Superconducting Devices

Performance Analysis of Data Acquisition Instruments for High-Temperature Superconducting Devices

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.

Levitation force measurement and characteristics analysis of double sided high temperature superconducting thin film device

In this paper, the levitation characteristics of double sided high temperature superconducting (HTS) thin film device is proposed and measured. Based on the unique advantages such as, passive self-stable levitation, higher payload, lower energy consumption, less maintenance cost, etc., HTS-magnetic levitation (maglev) technology has been applied in the traditional fields (rail transit and energy storage fields) and novel fields (astrophysics, space station microgravity, vibration isolation fields). As the key core components, HTS device should meet the requirements of volume, weight and cost, to improve the system operation efficiency. Thus, considering the low profile, miniaturization and lightweight characteristics of the double sided YBa2Cu3O7-x (YBCO) thin film, we measured its levitation and relaxation characteristics. Meanwhile, we also designed the YBCO thin film with meander-lines, to explore the levitation characteristic evolution mechanism. It can be found from the experimental results that, the single piece of YBCO thin film with/without meander-lines can offer the levitation force more than 400 mN and 800 mN, respectively. The signature of this work is that the double sided YBCO thin film has the potential to be employed as the core device in the special application fields (such as astrophysics and space station), to supply the sufficient micro levitation force in limited volume and weight.

Optimization of La2−xSrxCuO4 Single Crystal Film Growth via Molecular Beam Epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) combined with ozone is one of the best methods to fabricate single-crystal thin films of complex oxides. Cuprate such as La2−xSrxCuO4 (LSCO) is a representative complex-oxide high-temperature superconductor (HTS) material. Our group utilizes this method to produce high-quality single-crystal HTS films with atomically smooth surfaces and interfaces. In addition, ALL-MBE enables us to engineer multilayer heterostructures with atomic precision. This allows the fabrication of tunnel junctions, various nanostructures, and other HTS devices of interest for superconducting electronics. We have synthesized over three thousand LSCO thin films in the past two decades. These films’ structural and electronic properties have been studied and characterized by various methods. Here, we distill the extensive experience we accumulated into a step-by-step protocol to fabricate atomically perfect LSCO films. The recipe includes substrate preparation, ozone generation and distillation, source calibration, the in situ monitoring of the film synthesis, post-growth annealing, and ex situ characterization. It discloses a reproducible way to fabricate single-crystal LSCO films for basic research and HTS electronic 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.

Synthesis of La2−xSrxCuO4 films via atomic layer-by-layer molecular beam epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) is a sophisticated technique to synthesize high-temperature superconductor (HTS) materials. ALL-MBE produces single-crystal HTS films with atomically smooth surfaces and interfaces, as well as precise multilayer heterostructures engineered down to a single atomic layer level. This enables the fabrication of tunnel junctions, nanowires, nanorings, and other HTS devices of interest. Our group has focused on ALL-MBE synthesis and materials science of La2−xSrxCuO4 (LSCO), a representative HTS cuprate. In the past two decades, we have synthesized over three thousand LSCO thin films and characterized them by a range of analytical techniques. Here, we present in full detail a systematic process for the synthesis and engineering of atomically perfect LSCO films. The procedure includes the preparation of substrates, calibration of the elemental sources, the recipe for ALL growth of LSCO films without any secondary-phase precipitates, post-growth annealing of the films, and ex situ film characterization. This report should aid replication and dissemination of this technique of synthesizing single-crystal LSCO films for basic research as well as for HTS electronic applications.

Electric Aircraft Fueled by Liquid Hydrogen and Liquefied Natural Gas

The paper is a review of the opportunities and challenges of cryogenic power devices of electric aircraft, and the ongoing research and development efforts of the government agencies and the industry. Liquid Hydrogen (LH2) and Liquefied Natural Gas (LNG) are compared to support high temperature superconducting (HTS) and normal metal devices, respectively. The power devices were assumed to operate at the normal boiling point of the fuel used. The efficiencies of the electrical devices are estimated based on state-of-the-art technology. The mass flow rates and total fuel requirements for both the cryogenic fuels required to maintain the operating temperatures of the devices were simulated using thermal network models. A twin-aisle, 300 passenger aircraft with a 5.5 h flight duration was used for the models. The results show that the required masses of LH2 and LNG are 744 kg and 13,638 kg, respectively for the cooling requirement. The corresponding volumes of LH2 and LNG required are 9,760 and 30,300 L, respectively. In both cases, the estimated mass of the fuel needed for the aircraft is more than what is needed to maintain the cryogenic environment of the power devices. It was concluded that an electric aircraft with LNG cooled normal metal devices is feasible. However, an aircraft with HTS devices and cooled with LH2 is more attractive if the ongoing R&D efforts on HTS devices and LH2 infrastructure are successful. The emission reductions would be substantially higher with LH2, particularly when H2 is produced using renewable energy sources.

The calculation and measurement of electromagnetic energy flows for investigation of critical current estimation for high-temperature superconducting coil by means of pick-up coils

For practical operations of high-temperature superconducting (HTS) devices, we have proposed a monitoring and diagnosing system to detect the degradations of superconducting properties before normal transitions occur locally in the HTS winding. The proposed system estimates the distributions of critical current indirectly from measurement results of the active power components of the Poynting vector around the monitoring HTS devices conducting ac currents by means of pick-up coils pairs mounted surrounding the HTS devices. For this purpose, it is necessary to solve the inverse problem. In order to estimate the critical current distribution, therefore, it is first necessary to be able to calculate the measurement signal distribution from the critical current distribution of the monitoring coil. This paper first proposes the calculation method of signal distributions measured by our system, next reports the results of the experiments and analysis, which were carried out to verify the validity of the calculation method. A pancake coil wound with Bi-2223 multifilamentary tape exposed to a uniform AC magnetic field was assumed as a monitoring coil in the experiments and analyses. The good agreement between the measured signals obtained from the experiments and the numerical analysis demonstrates the effectiveness of the proposed calculation method of measured signals.

Effect of barrier thickness of Polytetrafluoroethylene on the Direct Current breakdown characteristics of liquid nitrogen

When high-temperature superconducting (HTS) tapes in high voltage DC HTS devices are quenched, amounts of thermal bubbles are generated in liquid nitrogen (LN2) that will reduce the insulation strength greatly. Especially when resistive-type superconducting fault current limiters meet a short-circuit fault, the insulation strength of the LN2 decreased significantly due to the thermal bubbles and electrothermal pressure. Solid insulating barriers can improve the insulation strength of the LN2. The objective of this study is to obtain the effect of barrier thickness on the DC dielectric breakdown characteristics of LN2. Insulating barrier Polytetrafluoroethylene with three different thicknesses were tested. Three types of electrodes such as rod-plane, needle-plane, and plane-plane were applied. The results showed that the barriers can increase the negative and positive breakdown voltage both with and without thermal bubbles in a slightly non-uniform field (SNF) and non-uniform field (NF). For the SNF, the DC breakdown voltage is higher when the barrier is thinner. The effect of the barrier thickness in the NF is lower than that in the SNF field. For the NF, the positive breakdown voltage is higher when the barrier is thicker.

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance

The optimum design of high-sensitivity Superconducting Quantum Interference Devices (SQUIDs) and other devices based on thin high-temperature superconductor (HTS) films requires accurate inductance modeling. This needs the London penetration depth λ to be well defined, not only at 77 K, but also for any operating temperature, given the increasingly widespread use of miniature low-noise single-stage cryocoolers. Temperature significantly affects all inductances in any active superconducting device, and cooling below 77 K can greatly improve device performance; however, accurate data for the temperature dependence of inductance and λ(T) for HTS devices are largely missing in the literature. We report here inductance measurements on a set of 20 different thin-film YBa2Cu3O7−x SQUIDs at 77 K with thickness t = 220 or 113 nm. By combining experimental data and inductance modeling, we find an average penetration depth λ(77)=391 nm at 77 K, which was independent of t. Using the same methods, we derive an empirical expression for λ(T) for a further three SQUIDs measured on a cryocooler from 50 to 79 K. Our measured value of λ(77) and our inductance extraction procedures were then used to estimate the inductances and the effective areas of directly coupled SQUID magnetometers with large washer-style pickup loops. The latter agrees better than 7% with experimentally measured values, validating our measured value of λ(77) and our inductance extraction methods.

Cryogenic testing of microphones and preamplifiers for MEMS quench detection in superconducting magnets

High Temperature Superconducting (HTS) tapes are needed for high field magnets in high-energy physics and nuclear fusion. However, quench events, in which the superconductor loses its superconductivity, can cause significant and costly damage to the device if allowed to propagate. Consequently, rapid quench detection methods are crucial. Existing methods such as voltage taps can be less effective in HTS applications, as the normal zone propagation velocities of HTS devices are slow, drastically increasing quench detection time (Iwasa, Cryogenics, 2003). We propose an alternative method that utilizes a linear array of MEMS microphones embedded in the cryogenic coolant channel (Takayasu, IEEE Transactions on Applied Superconductivity, 2019). The system requires amplifiers and acoustic sensors capable of operation at cryogenic temperatures as low as 4.2 K, although operation at 20 K may be sufficient. We report on cryogenic testing of the Vesper microphone preamplifier ASIC, as well as opamp based preamplifiers using the OPA838, LT1464, AD8591, and AD8057 ICs. The amplifiers are being used with the Vesper VM4 MEMS microphone to achieve acoustic measurements in gaseous helium at temperatures as low as 8 K. A charge amp scheme using the AD8591 achieved the best results to date. [Support from DOE STTR DE-SC0019905.]

Calculation and comparison of HTS electromagnetic characteristics with different models

To ensure the effective design and operation of high-temperature-superconducting (HTS) devices, the electromagnetic properties of HTS materials attract widespread attention. To date, the H-formulation and T-A formulation are two principal formulations applied to the finite element method to analyze the electromagnetic characteristics of HTS materials and devices. It is necessary to verify and to conduct a comparison of different effects of element order, mesh number along the width, and thickness expansion, with regard to calculation efficiency and accuracy under the H model and T-A model respectively. Compared to the H model, the T-A model with linear or quadratic element order can complete the calculation process more efficient having high accuracy and less time. The model with optimal parameters has been verified in a common case of a superconducting pancake coil. The optimal model of this work benefits to design or simulate HTS applications in building practical large-scale devices in terms of calculation efficiency and accuracy.

Analysis of Overcurrent Performance of YBCO Tape Considering Different Heat Exchange Conductions

The performance degradation of High Temperature Superconducting (HTS) tape, which may be influenced by various heat exchange conditions due to different cooling methods, is of significant for HTS devices under repeated overcurrent. In this article, considering different heat exchange conditions, a series of repeated overcurrent experiments were carried on YBCO tapes under 50Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeated overcurrent was recorded. the resistance characteristics of YBCO tapes under different cooling conditions were analyzed. Through finite element simulation, the temperature and current distribution of YBCO tape under different heat exchange conditions were simulated. The experimental results show that the overcurrent capability of the conductor under conduction cooling is far inferior to the liquid nitrogen immersion method.

Transport properties of Tl2Ba2CaCu2O8 microbridges on a low-angle step substrate

Tl-based superconducting devices have been drawn much attention for their high transition temperature (T c), which allow the high temperature superconductors (HTS) devices to operate at temperature near 100 K. The realization of Tl-based devices will promote the research and application of HTS devices. In this work, we present transport properties of Tl2Ba2CaCu2O8 (Tl-2212) microbridges across a low-angle step on LaAlO3 (LAO) substrate. We experimentally demonstrate intrinsic Josephson effects (IJEs) in Tl-2212 films by tailoring the geometry, i.e., reducing the width of the microbridges. In the case of a 1 μm width microbridge, in addition to the observation of voltage branches and remarkable hysteresis on the current–voltage (I–V) characteristics, the temperature dependence of differential resistance shows a finite resistance above 60 K when the bias current is below the critical current. For comparison, the wider microbridges are also investigated, exhibiting a highly critical current but do not showing obvious IJEs.

AC loss calculation in REBCO coils or stacks by solving the equation of motion for current using an integration approach

The flux motion and its collective behaviour known as AC loss is the basic characteristic of high temperature superconducting (HTS) materials and controls the economic efficiency, stability and magnetic field uniformity of superconducting equipment. In so far, we have no method to calculate the AC loss of HTS devices in an efficient way. Normally, it will take days to simulate AC loss of HTS coils even with moderate number of turns. In this paper, we surprisingly found that an integral method is remarkably efficient in calculating AC loss of the coils in large scale. By comparing with the fastest method at present, the integral method is proved to be 4.4 times faster than the present one. We give the calculation details and discuss why this method has an advantage in calculating AC loss of the coil with a large number of turns.

Cryogenic testing of MEMS microphones towards their utilization as a quench detection method in superconducting magnets

High Temperature Superconducting (HTS) tapes such as Rare Earth Barium Copper Oxide (REBCO) are an attractive option for high field magnets operated in cryogenic fluids. However, quench events can occur in which the conductor locally loses its superconducting properties. It is critical to rapidly detect and respond to such an event. Conventional quench detection methods using voltage taps are difficult in HTS devices, since the normal zone propagation velocities are 2–3 orders of magnitude lower in HTS compared to low temperature superconductors (Iwasa, Cryogenics, 2003). We propose an alternative in which a linear array of MEMS microphones is distributed down the central cooling channel of a cable in-conduit conductor. The array can detect the acoustic signature caused by a quench event which propagates in the cooling fluid. The proposed method differs from Acoustic Emission (AE) detection, which uses sensors mounted on the magnet surface to detect structural vibration (Tsukamoto, Appl. Phys. Lett., 1981). In order to implement the system, MEMS microphones and preamplifiers must operate in cryogenic fluids. We report on characterization of commercial MEMS microphones in cryogenic gaseous helium between 0.5 and 1.5 bar down to 20 K, and in liquid nitrogen at 1 bar and 77 K.

AC loss measurement and simulation in a REBCO coil assembly utilising low-loss magnetic flux diverters

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

Modelling analysis of periodically arranged high-temperature superconducting tapes

The practical operation of a high-temperature superconducting (HTS) tape to form a HTS device is strongly related to the HTS tape electromagnetic environment in the device. With regard to the HTS tapes commonly stacked for practical applications, it is necessary to study the electromagnetic interaction of the proximity HTS tapes to design and optimize the HTS tapes and devices. This paper presents a numerical model of periodically arranged HTS tapes using the finite element method (FEM). The numerical method has been developed to build the equivalent periodical arrangement of infinite HTS tapes and applied to simulate two basic arrangements, horizontal and vertical periodical arrangements. The magnetic field and current distributions of HTS tapes by periodical arrangements have been investigated. The effects on critical current with different arrangements have been illustrated. It is found that the Y-stack can reduce the current carrying capacity while the X-array can increase that in comparison to a single tape. The differences between the FEM results and analytical models have been analysed and also compared with the Norris model. The two mathematical models derived for AC loss prediction have shown higher accuracy than the existing analytical models. This study presents an effective method to optimize the electromagnetic design by considering favorable structure or environment for the stacked HTS tapes and estimate the AC loss, shows the phenomenon of central tapes in an HTS array, which could be further used in performance analysis of superconducting devices, e.g. HTS transformers.

Critical Current Degradation Behavior of Coated Conductor Subjected to Repeat Overcurrent

The performance degradation of superconducting equipment under repeat fault is a key issue for the operation of superconducting equipment in grid operation. The mechanism of the High Temperature Superconducting (HTS) tape performance degradation behavior is an important issue that need to be figured out. Therefore, it's necessary to carry out the performance degradation study of HTS tape under repeat overcurrent. In this paper, considering overcurrent duration, a series of repeated overcurrent experiments were carried on YBCO tapes under 50 Hz sinusoidal alternating current. The degradation of critical current of YBCO tapes after repeat overcurrent was recorded. The critical current density of the YBCO tape was scanned to track sample's structural change during experiment. The mechanism of the degradation development of YBCO tapes after their first degradation was described. The research could provide a reference for the design of fault protection and life assessment of HTS devices.