Bulk High-temperature Superconductors Research Articles

Three-Dimensional Numerical Characterization of High-Temperature Superconductor Bulks Subjected to Rotating Magnetic Fields

High-temperature superconductor (HTS) bulks have shown very promising potential for industrial applications due to the ability to trap much higher magnetic fields compared to traditional permanent magnets. In rotating electrical machines, the magnetic field is a combination of alternating and rotating fields. On the contrary, all studies on electromagnetic characterization of HTS presented in the literature so far have only focused on alternating AC magnetic fields and alternating AC loss due to the unavailability of robust experimental techniques and analytical models. This paper presents a numerical investigation on the characterization of HTS bulks subjected to rotating magnetic fields showing AC loss, current density distribution in three-dimensional axes, and trapped field analysis. A three-dimensional numerical model has been developed using H-formulation based on finite element analysis. An HTS cubic sample is magnetized and demagnetized with two-dimensional magnetic flux density vectors rotating in circular orientation around the XOY, XOZ, and YOZ planes.

Inverse analysis of critical current density in a bulk high-temperature superconducting undulator

In order to optimize the design of undulators using high-temperature superconductor (HTS) bulks we have developed a method to estimate the critical current density (${J}_{c}$) of each bulk from the overall measured magnetic field of an undulator. The vertical magnetic field was measured along the electron-beam axis in a HTS bulk-based undulator consisting of twenty Gd-Ba-Cu-O (GdBCO) bulks inserted in a 12-T solenoid. The ${J}_{c}$ values of the bulks were estimated by an inverse analysis approach in which the magnetic field was calculated by the forward simulation of the shielding currents in each HTS bulk with a given ${J}_{c}$. Subsequently the ${J}_{c}$ values were iteratively updated using the precalculated response matrix of the undulator magnetic field to ${J}_{c}$. We demonstrate that it is possible to determine the ${J}_{c}$ of each HTS bulk with sufficient accuracy for practical application within around 10 iterations. The precalculated response matrix, created in advance, enables the inverse analysis to be performed within a practically short time, on the order of several hours. The measurement error, which destroys the uniqueness of the solution, was investigated and the points to be noted for future magnetic field measurements were clarified. The results show that this inverse-analysis method allows the estimation of the ${J}_{c}$ of each bulk comprising an HTS bulk undulator.

Theoretical optimization and experimental verification of a non-contact damper for high temperature superconducting Maglev systems

High-temperature superconducting (HTS) maglev has excellent potential in high-speed transportation due to its capability of passive stabilization levitation. However, in applications, the vibration of maglev vehicle with a large amplitude will be triggered for some external incentives or interference owing to its weak damping characteristics. Hence, it is preferable and necessary to find a way to reduce the vibration without mechanical contact. In terms of mechanical structure, the HTS maglev system mainly consists of HTS bulks and permanent magnets guideway (PMG). Utilizing the magnetic field provided by PMG, we designed and assessed an electromagnetic shunt damper (EMSD) for the HTS maglev system, by which the vibrations can be attenuated without contact. At first, the parameters of EMSD were theoretically optimized, respectively. Second, according to the structure of the Halbach PMG, the experimental EMSD was presented. In this damper, two groups of coils in reverse series were set above the two magnetic poles of the PMG to get a large induced electromotive force. Eventually, the experimental results show that the vibrations of the HTS maglev model vehicle can be attenuated more than 80% in the resonance region when the designed EMSD was employed in the system. Namely, this non-contacting damper will highly enhance the dynamic characteristics of HTS maglev systems.

High-temperature superconducting guidance force enhancement by a novel permanent magnet guideway for maglev curve negotiation

In order to enhance the guidance force, adding the high-temperature superconducting (HTS) bulks and permanent magnets in lateral direction is the simplest way especially in curve negotiation situations. However, as the core component of maglev system, permanent magnet guideway (PMG) consists of a huge amount of NdFeB magnets along the operation line. Hence, it means a huge construction cost to add the magnets in lateral direction. To help solving this issue, this paper discusses a PMG improvement methodology based on the widely-used two-pole Halbach PMG. This proposed novel PMG can enhance the guidance force by only adding air intervals between neighbor magnets and the flux-concentration irons and raising the onboard HTS bulks’ width accordingly. Firstly, a 2D electromagnetic theoretical model of the HTS bulk-PMG maglev system was built in the COMSOL Multiphysics. Then, under the different external magnetic fields of the PMGs with different air intervals, levitation and guidance performances of the typical onboard HTS bulk array at typical working conditions were discussed. The results showed that, with the suitable HTS bulks’ width, the guidance force can increase with the intervals increasing in a range. And the HTS maglev system adopting this idea can achieve a guidance force growth of about 41%. This PMG idea is appropriate for curve negotiation situations because of its higher ratio of the guidance force to the levitation force, without changing the basic magnetic circuit and the magnet block number of the previous linear PMG.

Physical principles of development of magneto-levitation systems based on the second generation high temperature superconducting composites (Review)

An overview of experimental and theoretical studies of the characteristics of maglev systems using high-temperature superconductors (HTSC) is presented. Materials used in maglev technologies, namely bulk superconductors and HTSC tape composites, are considered. The main experimental data obtained on both bulk and tape superconductors assembled in stacks of various configurations are demonstrated. The factors influencing the magneto-force characteristics are analyzed: geometric parameters, the influence of external alternating magnetic fields, temperatures, relaxation phenomena. A significant part of the review is devoted to the description of various methods for calculating maglev systems, including those based on stacks of HTSC composites. The features of thermal processes in maglev systems with cryocooler and nitrogen cooling are considered. General recommendations for the creation of optimal maglev systems based on tape HTSC composites are given. Keywords: magnetic levitation, high-temperature superconductors, strip composites, levitation force, lateral force, methods for calculating magnetic force characteristics.

Coupling behavior of Bi2Sr2−xInxCaCu2O8+d

The interplay between intrinsic grain and inter-grain effects at the superconducting state defines the behavior of bulk high-temperature superconductors. In this work, we use DC resistivity and AC magnetic susceptibility measurements to show how the intrinsic and inter-grain properties influence the intergrain phase coherence and flux dynamics of Bi2Sr2−xInxCaCu2O8+d. High In-doped samples, x ≥ 0.4, have broad resistive and diamagnetic superconducting transitions due to weak intergrain coupling caused by segregated impurities. Low doping levels, x ≤ 0.3, have sharp superconducting resistive and diamagnetic transitions, with higher transition temperature TC attributed to an increase superconducting pair density. This results to enhanced intergrain coupling and strong diamagnetic screening of intergrain void networks. An applied AC field of 0.77 mT amplitude deteriorates the intergrain diamagnetic screening at x = 0.2 through AC flux slip between grains.

Optimization of the Guiding Stability of a Horizontal Axis HTS ZFC Radial Levitation Bearing

This document presents a study on the optimization of the 3D geometry of a horizontal axis radial levitation bearing with zero-field cooled (ZFC) high-temperature superconductor (HTS) bulks in the stator, and radially magnetized permanent magnet (PM) rings in the rotor. The optimization of component dimensions and spacing to minimize the volume or cost concerning only the maximization of the levitation force was previously studied. The guidance force and guiding stability depend on the spacing between PM rings in the rotor and between the rings of HTS bulks in the stator. This new optimization study aims to find the optimum spacing that maximize the guidance force with given HTS bulk and PM ring dimensions while maintaining the minimum required levitation force. Decisions are taken using the non-dominated sorting genetic algorithm (NSGA-II) over 3D finite element analysis (FEA). A simplified electromagnetic model of equivalent relative permeability is used on 3D FEA to reduce numerical processing and optimization time. Experimental prototypes were built to measure magnetic forces and validate appropriate values of equivalent magnetic permeability. An analysis of stable and unstable geometry domains depending on the spacing between rings of HTS bulks and PM rings is also done for two HTS bulk sizes.

A reconstructed three-dimensional HTS bulk electromagnetic model considering J c spatial inhomogeneity and its implementation in a bulks’ combination system

High-temperature superconducting (HTS) bulks in HTS Maglev systems are always arrayed in a combination to make full use of the applied magnetic field of the permanent magnet guideway (PMG). An excellent combination scheme improves the overall levitation and guidance performance significantly. In this paper, a three-dimensional (3D) electromagnetic model of the real HTS-PMG maglev system with an HTS bulk array was established. This model comprehensively expresses the influence of various factors on the E – J relationship and the 3D spatial distribution of J c, including internal factors such as the inhomogeneity and anisotropy of electromagnetic characteristics, as well as external factors such as applied magnetic field and working temperature. A ternary function was proposed to describe the uneven distribution of J c caused by the bulk’s growth process, which is an interesting phenomenological modeling attempt. In the simulations of the bulks’ combinations, perfect magnetic conductor boundary conditions were applied on the contact surface to simulate two bulks touching each other. Besides, the research target includes reproducing the shapes, the orientations, and the combination scheme of HTS bulks in the real PMG magnetic field. The calculation results of levitation force of the cylindrical bulk under different spatial orientations above the PMG were compared with the experimental results, through which the accuracy of the model was verified. On this basis, the influence of the magnetic field generated by the superconducting current on the nearby bulk was further explored. It was found that this magnetic field has a small contribution to the total levitation force and a relatively obvious influence on the guidance force. When the lateral displacement is large, such as 5 mm, the magnetic field generated by the superconducting current slightly increases the total guidance force stiffness. According to more simulated conditions, some optimization strategies on bulk combinations were proposed. This work provides not only a 3D descriptive model for fitting the real multi-bulk-combination maglev scenarios but also some optimization strategies for the HTS maglev transportation applications.

The magnetic and levitation characteristics of single-grain YBaCuO and GdBaCuO-Ag bulk superconductors in high magnetic fields

In principle, high-temperature superconducting (HTS) flux-pinning maglev has the advantage of self-stable levitation and is therefore a good development prospect as a new rail transit technology. The magnetic field intensity is one of the key factors affecting the levitation performance of single-grain bulk superconductors in the HTS maglev system. To date, however, most researchers have focused on the levitation performance of HTS bulk superconductors in magnetic fields within a permanent magnet guideway (PMG) configuration. The magnitude of the external working magnetic field provided by a PMG is limited to about 0.5 T at 77 K, given that the superconductor usually levitates at a typical working height of 10 mm above the PMG. Therefore, the weak magnetic fields generated by the PMG significantly limits the levitation performance of HTS maglev systems to that of the permanent magnet (PM). In this article, a superconducting magnet test bench was utilized to provide a 0–3 T external magnetic field (B) environment for the study of the levitation force in HTS single grains. The difference in levitation performance between YBaCuO (YBCO) and GdBaCuO-Ag (GdBCO-Ag) bulk superconductors under large, changing magnetic fields was studied systematically. The relationships between levitation force and magnetic field characteristics, including vertical magnetic field component (Bz), transverse magnetic field component (Br), and vertical magnetic field variation (ΔBz) were investigated. The result of variation curves about YBCO and GdBCO bulks' levitation forces under different ΔBz shows that the levitation force of the bulk, single-grain superconductors will reach saturation under a varying ΔBz environment (0.72 and 1.5 T). The values of ΔBz corresponding to the saturation positions are found to be independent of the sizes of the HTS samples. At the same time, GdBCO-Ag exhibits a levitation force that is 112.8% greater than YBCO in B = 3 T magnetic environment. As a result, it is demonstrated that the levitation potential of single-grain HTS bulks can be better exploited by optimizing the distribution of the magnetic field.

The Single-peak and “V” Shaped Combined Permanent Magnet Guideway for High-Temperature Superconducting Magnetic Levitation

With the merits of the excellent self-stabilization, the high-temperature superconducting (HTS) magnetic levitation (maglev) system is relatively appropriate for high-speed railway transportation. The suspension force and guidance force are mainly produced by the interaction between the HTS bulks and the permanent magnet guideway (PMG). Before the HTS bulks turn into the superconducting state, it will capture the magnetic field generated by the PMG and interact with the magnetic field of PMG after turning into the superconducting state, which creates the suspension phenomena. Therefore, the research of the magnetic flux density of external magnetic fields excited by the PMG is very significant to an intensive discovery of HTS maglev properties. However, the previous single-peak guideway needs to be improved to supply HTS maglev with sufficient vertical magnetic fields gradients. Otherwise, it might cause the guidance force to be not great enough for practical engineering application. Hence, this paper proposed a better “V” shaped PMG, by combining the single-peak and “V” shaped PMG. Under the typical work condition of 30 mm field cooling height, 10 mm work height and 10 mm lateral displacement, the levitation force and guidance force of HTS bulks could attain 6779 N and 2778 N, respectively. Compared to the levitation force and guidance force produced by using single-peak PMG, these two forces increase by 12.6% and 137%. As a result, the new “V” shaped PMG will provide HTS maglev with more adequate guidance performance and load capacity.

Design and Optimization of a Novel HTS Flux-Modulated Linear Motor Using Halbach Permanent Magnet Arrays

For rope-less elevator applications, a novel high temperature superconducting (HTS) flux modulation linear motor (LM) using Halbach permanent magnet (PM) arrays is investigated in this paper. The flux modulation structure with iron segments can serve simultaneously and regulate the magnetic field. The HTS bulks are filled between the iron segments to further improve the flux modulation effect based on the Meissner effect. Compared with conventional counterparts, the force density of the proposed LM with HTS fillers is apparently higher. Furthermore, the mover with Halbach PM arrays is used to reduce the force ripple and increase the output force. The electromagnetic performances of the flux-modulated LM are researched by transient finite element analyses (FEA) to prove the effectiveness of the proposed structure. Finally, the parameters of the motor structural are optimized and analyzed by the combination of FEA and genetic algorithm (GA).

Design and Performance Analyses of Multi-pole HTS Maglev Guideways for the Electromagnetic Launch

A prototype of the high temperature superconducting (HTS) Maglev system for the electromagnetic launch was built in our laboratory. To meet the higher carrying capacity requirement of this practical application, multi-pole permanent-magnet guideways (PMGs) with different flux concentration methods and various arrangement schemes of HTS bulks have been proposed. The magnetic field distribution of these multi-pole PMG with the iron and permanent magnet (PM) concentration schemes has been estimated by employing both analytical and finite element methods. Meanwhile, a two-dimensional simulation model was established to evaluate the levitation and lateral performance of the Maglev system with several arrangement cases of HTS bulks. It has been validated that the total levitation force of the system can be significantly improved by increasing the pole number of the PMG and applying the optimal arrangement scheme.

Design and Testing of a New Cooling System using Solid Nitrogen for Pulsed Field Magnetization and Characterization of HTS Bulks

Solid Nitrogen (SN2) can provide a uniform and stable cryogenic environment for High Temperature Superconducting (HTS) systems such as bulk samples during their magnetization and/or characterization. In this paper, a SN2 cooling system consisting of a cryocooler Sumitomo CH-110 and an exchanger in a Liquid Nitrogen (LN2) bath is studied. In order to design this cooling system, an analytical model based on a nodal method coupled to a formulation of the thermal capacity was realized. The model considers the thermal parameters variation as well as the phase change of the Nitrogen. In order to compare our results, we performed a 3D simulation on COMSOL Multiphysics. The performance of the cooling system was evaluated by measurements and we estimate that 10 L of LN2 can be cooled down to 25 K in 45 hours.

Fully-staggered-array bulk Re-Ba-Cu-O short-period undulator: large-scale 3D electromagnetic modelling and design optimization using A-V and H-formulation methods

The development of a new hard x-ray beamline I-TOMCAT equipped with a 1 m long short-period bulk high-temperature superconductor undulator (BHTSU) has been scheduled for the upgrade of the Swiss Light Source at the Paul Scherrer Institute. The very hard x-ray source generated by the BHTSU will increase the brilliance at the beamline by over one order of magnitude in comparison to other state-of-the-art undulator technologies and allow experiments to be carried out with photon energies in excess of 60 keV. One of the key challenges for designing a 1 m long (100 periods) BHTSU is the large-scale simulation of the magnetization currents inside 200 staggered-array bulk superconductors. A feasible approach to simplify the electromagnetic model is to retain five periods from both ends of the 1 m long BHTSU, reducing the number of degrees of freedom to the scale of millions. In this paper, the theory of the recently-proposed 2D A -V formulation-based backward computation method is extended to calculate the critical state magnetization currents in the ten-period staggered-array BHTSU in 3D. The simulation results of the magnetization currents and the associated undulator field along the electron beam axis are compared with the well-known 3D H -formulation and the highly efficient 3D H -ϕ formulation method, all methods showing excellent agreement with each other as well as with experimental results. The mixed H -ϕ formulation avoids computing the eddy currents in the air subdomain and is significantly faster than the full H -formulation method, but is slower in comparison to the A -V formulation-based backward computation. Finally, the fastest and the most efficient A -V formulation, implemented in ANSYS 2020R1 Academic, is adopted to optimize the integrals of the undulator field along the electron beam axis by optimizing the sizes of the end bulks.

Dynamic Characteristics of a YBCO Bulk Above an Electromagnet Guideway

For high-temperature superconducting (HTS) maglev, an electromagnet guideway unit (EMGU) that can form an electromagnet guideway (EMG) with a small gap, or even no gap, between EMGUs has been designed. The longitudinal magnetic fields along a single EMGU and two EMGUs arranged in a line were first investigated through measurement and simulation. The experimentally measured data validated the simulation results from a three-dimensional (3D) EMGU model implemented in COMSOL Multiphysics, indicating that the model is reliable and can be used for further studies. The dynamic responses of a high-temperature superconducting (HTS) bulk above a single EMGU and two EMGUs arranged in a line, including the dynamic levitation force (LF) and traveling directional force (TDF), under different operating conditions were investigated through experiment and simulation using a segregated H-formulation model. The magnetic field and current density distributions inside the superconductor are affected by the external magnetic field generated by the EMGU and are responsible for the dynamic characteristics. Finally, the segmented instant excitation (SIE) mode was investigated through simulation, which shows it is feasible by coordinating the currents of EMGUs.

Three-dimensional peridynamic modeling of crack initiation and propagation in bulk superconductor during field cooling magnetization

Bulk high-temperature superconductors exhibit outstanding electromagnetic properties and are capable of trapping very large magnetic fields. However, bulk superconductors are subjected to a large Lorentz force during field cooling magnetization (FCM), which can cause crack initiation and propagation. Superconducting performance is then limited by the damage to the bulk. In this paper, we study the mechanical behavior and brittle damage of a three-dimensional (3D) bulk GdBCO superconductor during FCM. Firstly, the distribution of electromagnetic field in the bulk is obtained based on the H-formulation with a finite element model. Then, a 3D bond-based peridynamic (PD) model is used to simulate the mechanical behavior and possible brittle damage to the bulk under a given electromagnetic force. The initiation and propagation of cracks can be predicted using the 3D bond-based PD model. The effect of residual stress due to the presence of a reinforcement ring during the cooling process is also discussed. The results show that mechanical stability can be improved by reinforcing the sample.

Double armature HTS bulk synchronous machine

In many high-temperature superconducting (HTS) rotating machines of radial flux type, the magnetic field from one side surface of the field pole is employed facing the armature winding. However, both side surfaces of the field pole can be effectively utilised to face the armature windings. Thus, we have applied the double armature structure in which both inner and outer armature rotors sandwiched the field pole stator in the HTS synchronous generator. We introduce the first step design and the electromagnetic study of the double armature synchronous machine for tidal current power generation. The double armatures with copper winding were adopted with a field pole composed of the HTS bulks. The power and the torque density were calculated with a low speed that depends on a 500 kW contra-rotating turbine design. In this paper, we comparatively studied three electromagnetic designs in which (1) a permanent magnet was used for the field pole, (2) the HTS bulk was employed as the field pole and (3) the field pole HTS bulk was arranged at an angle in the x-y plane, reducing torque ripple due to torque consistency and tilting of the HTS bulk plate. A generator model consistent with the input mechanical torque of the turbine was obtained from the analysis of compatibility with the 500 kW class tidal current power generation.

Review on the Use of Superconducting Bulks for Magnetic Screening in Electrical Machines for Aircraft Applications.

High-Temperature Superconductors (HTS) considerably accelerate the development of superconducting machines for electrical engineering applications such as fully electrical aircraft. This present contribution is an overview of different superconducting materials that can be used as magnetic screens for the inductor of high specific power electrical machines. The impact of the material properties, such as the critical temperature () and the critical current density (), on the machine performances is evaluated. In addition, the relevance to flux modulation machines of different HTS bulk synthesis methods are addressed.

The mechanical characteristics and control of high temperature superconducting magnetic docking mechanism

The space docking mechanism is one of the important ways to set up a space station, which is an important platform to explore the space. The traditional docking mechanism has achieved a series of docking missions, but it has the problems of large impact and plume pollution, which has the risk to affect the normal work of high precise instrument and equipment installed on spacecraft. In order to solve the drawback of the traditional docking mechanism, the high temperature superconducting magnetic docking mechanism (a novel docking mechanism) consisting of a high temperature superconductor (HTS) bulk installed on a target spacecraft module and an electromagnet installed on a tracking spacecraft module is proposed to be designed based on the flux pinning effect of a HTS. The diamagnetic and trapped magnetic field characteristics of a HTS can be considered to propose an equivalent method of processing HTS (equivalent processing method). Moreover, the equivalent processing method is verified by way of experiment. The mechanical characteristics of the high temperature superconducting magnetic docking mechanism in the case of different parameters are studied using the equivalent processing method. Based on the mechanical characteristics, the dynamic model of the novel docking mechanism is established in order to analyze the dynamic response in the case of different control laws. Furthermore, the docking performance of the high temperature superconducting magnetic docking mechanism is evaluated based on three indicators, that is, maximum overshoot, peak time, and adjustment time.

Force analysis of HTS bulk in a reversed magnetic field

In the study of high temperature superconducting maglev, the analysis of the forces acted on a high temperature superconductor (HTS) bulk is a basic issue, which can directly reflect the levitation and guidance performances of an HTS maglev system. In previous studies, the electromagnetic guideway was verified to be an alternative guideway type for HTS maglev. Recently, we carried out experimental and simulation work to investigate how a field cooled HTS bulk to interact with a reversed magnetic field. An E-type electromagnet is used to generate the required magnetic field. During the field reversing, forces acted on the HTS bulk were measured with a 3D force measuring system. In this paper, the forces on an HTS bulk in a reversed magnetic field are analyzed according to the experimental data and the results of finite element simulation. The magnetized status of a field cooled HTS bulk in a reversed magnetic field is discussed with a two region supposition. The influences of a reversed magnetic field on HTS maglev are summarized. These results may be of great significance not only for further study of HTS maglev, but also for some other applications, such as superconducting magnetic bearing and superconducting magnetic shielding.