Bulk High-temperature Superconductors Research Articles

Magnetic fields end-face effect investigation of HTS bulk over PMG with 3D-modeling numerical method

In this paper, the magnetic fields end-face effect of high temperature superconducting (HTS) bulk over a permanent magnetic guideway (PMG) is researched with 3D-modeling numerical method. The electromagnetic behavior of the bulk is simulated using finite element method (FEM). The framework is formulated by the magnetic field vector method (H-method). A superconducting levitation system composed of one rectangular HTS bulk and one infinite long PMG is successfully investigated using the proposed method. The simulation results show that for finite geometrical HTS bulk, even the applied magnetic field is only distributed in [Formula: see text]–[Formula: see text] plane, the magnetic field component Hz which is along the [Formula: see text]-axis can be observed interior the HTS bulk.

Influence of critical current density on magnetic force of HTSC bulk above PMR with 3D-modeling numerical solutions

Numerical simulations of thermo-electromagnetic properties of a high temperature superconducting (HTS) bulk levitating over a permanent magnetic guideway (PMG) are performed by resorting to the quasistatic approximation of the H-method coupling with the classical description of the heat conduction equation. The numerical resolving codes are practiced with the help of the finite element program generation system (FEPG) platform using finite element method (FEM). The [Formula: see text]–[Formula: see text] power law is used to describe the electric current nonlinear characteristics of HTS bulk. The simulation results show that the heat conduction and the critical current density are tightly relative to the thermal effects of the HTS bulk over the PMG. The heat intensity which responds to the heat loss of the HTS bulk is mainly distributed at the two bottom-corners of the bulk sample.

3-D Transient Modeling of Bulk High-Temperature Superconducting Material in Passive Magnetic Bearing Applications

Bulk high-temperature superconductors are being considered for use in several engineering applications, including passive magnetic bearings. These bearings, apart from being passive, i.e., inherently stable, also offer the promise of lower bearing losses; thus, they are being considered for use with flywheels for energy storage in applications related to frequency regulation and for correcting forecasting errors associated with renewable energy sources. The effort presented in this paper was undertaken to characterize the performance of these bearings such as longitudinal and transverse stiffness and loss characteristics. To this end, a finite-element method (FEM) using the T– $\Omega$ potentials was used for the formulation. The results of the FEM were verified with experiments. These experiments are described. This FEM tool was also used to guide the development of a reduced-order model, which could run faster and, therefore, could be used in larger system simulations. Some discussions about the run time on a desktop PC are also presented.

Magnetic characteristics of permanent magnet guideways at low temperature and its effect on the levitation force of bulk YBaCuO superconductors

It is important to understand the influence of environmental temperature variation on high temperature superconducting (HTS) maglev performance in practice. As the on-board bulk high temperature superconductors (HTSCs) are kept in liquid nitrogen cryostats at all times, the actual temperature effects relevant to maglev performance are on the permanent magnet guideway (PMG), and these have been examined by exploring the PMG magnetic characteristics and the related interaction force with the on-board bulk HTSCs above it in this paper. The experimental temperature range was set from room temperature to 77 K. The experimental results show that the typical room temperature (RT) vertical magnetic flux density (defined as BPMG), 565 mT, of the PMG segment first increases as the temperature decreases to 138 K, before it reaches a peak of 634 mT within the temperature range from 138 K to 120 K, with a rapid growth rate to 12.2% over the RT value. And then BPMG decreases quickly to a stable value of 576 mT at 77 K. A final 1.9% enhancement ratio was obtained compared with the initial BPMG at room temperature. Furthermore, the levitation force exhibits a similar trend to the BPMG whatever the field cooling process and the zero field cooling process was conducted when the PMG temperature is reduced. Growth of 21.8% and 13.4% for the levitation force occurs for the field-cooling process and the zero-field-cooling process, respectively. The results provide basic data for environmental temperature effects on HTS maglev systems, which implies that the low temperature environment can have benefits towards enhancement of their levitation performance.

Study of the Maglev Performance of the Side- Mounted High-Temperature Superconductor Maglev Rotating System

A new type of maglev system, a side-mounted high-temperature superconductor (HTS) maglev system, designed to run at high speed in an evacuated circular tube is presented in this paper. The levitation performance of the system was investigated. Results show that the stability of the maglev system is strongly dependent on the arrangements of HTS bulks along the center of permanent-magnet guideway. For the linear arrangement, the guidance force tends to drive HTS bulks away from the magnetic hill with the decrease of suspending gap. This may cause the levitation force decrease at the same time. In the comparison with the quadrilateral, square and liner arrangement of the HTS bulks, the triangular arrangement possesses the best maglev performance.

Controllability of Flux-Pinned Docking Interface

The flux-pinned docking interface (FPDI) is composed of an YBCO high-temperature superconductor bulk and an electromagnet (EM). They are installed on the spherical target spacecraft module and another spherical tracking spacecraft module, respectively. This novel docking interface can enable the active control of the docking process to become an additional option rather than a necessary option. The vertical force between two flux-pinned spacecraft modules (FPSMs) can be analytically calculated based on the improved image dipole model. The controllability of the FPDI is achieved through varying current flowing through the EM. Two FPSMs can be linked or detached by changing the direction of current. The distinct equilibrium connection state of two FPSMs can be transformed by changing the current intensity. The orbital dynamic docking equation of two FPSMs is established based on Clohessy–Wiltshire's equation. The numerical calculation based on the Runge–Kutta algorithm can calculate the derived dynamic docking equation. The simulated consequence shows that proportional-derivative active feedback control applied to this docking interface can effectively suppress the vibration amplitude and significantly shorten the settling time, which suggests that this potential novel docking interface can be readily controlled.

Development of Magnetic Field Control System Using HTS Bulks and HTS Coil for MDDS

Magnetic drug delivery system (MDDS) is a key technology effects in the medical applications, and also the magnetic force control is a very important issue in MDDS. In general, the high magnetic field along the axial direction and high magnetic field gradient along the longitudinal direction are very useful for MDDS. So, we proposed the new magnetic force control system consists of superconducting magnet, high temperature superconductors (HTS) bulks or tapes and irons. In this system, the shielding currents in HTS bulks and tapes due to the diamagnetism of the superconductor generate the magnetic field with same direction of external magnetic field, and we can control the magnetic field gradient along the longitudinal direction by the arrangement of the superconductors and irons. In this study, the stacked GdBCO tapes were used in experiments and electromagnetic analysis based on finite-element method was carried out to optimize the arrangement of the superconductors and the irons. It was confirmed the control of magnetic field gradient along the longitudinal direction was possible by arranging the HTS bulks, tapes and irons on the superconducting magnet. In analysis, the magnetic field of 164.6 mT and field gradient of 1.89 T/m were obtained.

The Levitation Characteristics of the Noncontact Rotating System Using HTS Bulks Trapped by Permanent Magnets

We have studied a noncontact magnetic levitation system using the ring-shaped high-temperature superconducting (HTS) bulks and ring-shaped permanent magnets (PMs). Because HTS bulks were magnetized by PMs fixed between the HTS bulks, these PMs levitated with strong restoring force. We used the stator coil to rotate the iron shaft that was combined with PMs. In order to rotate the shaft stable, a restoring force of the radial direction was important. Thus, we placed ring-shaped PMs that magnetized in the radial direction with the inner part of the HTS bulks. In this rotating system, we focused on two parameters, the rotation and the control. In the rotation, we measured the maximum rotational speed and the axial vibration caused by increasing of rotational speed. In the control, we measured slippage from the convergence position when rotated by 60°. In addition, we compared the slippage by changing the conditions.

Electromagnetic Performance Analysis of Novel HTS Doubly Fed Flux-Modulated Machines

The flux-modulated machine is designed based on a promising low-speed direct-drive concept for applications such as wind turbines, due to its inherent magnetic gearing effect. In this paper, a novel doubly fed flux-modulated machine using high-temperature superconducting (HTS) bulks is proposed. Compared with previous permanent-magnet (PM) flux-modulated machines, the beauty of the proposed machine is that electric excitation, instead of PMs, is exploited to improve its flux controllability with reduced cost. Unlike conventional electric-excited synchronous machines, neither slip rings nor brushes are needed in this novel machine because both the field windings and the armature windings are housed in the stator. In other words, the proposed machine combines the merits of a flux-modulated machine and an electric-excited machine. To alleviate the end effect in flux-modulated machines, HTS bulks are used for flux modulation. The performance of the machine is analyzed using finite-element method.

Increase of Levitation Force due to Soft Ferromagnetic Plate and Bar in Magnetic Levitation System Using <roman>GdBCO</roman> Bulk

We have studied a magnetic levitation system using a magnetic shielding effect of a high-temperature superconducting (HTS) bulk. The basic system is composed of a soft ferromagnetic bar, the HTS bulk, and a permanent magnet. To increase levitation force, we proposed two kinds of modified systems. One is that two basic systems are placed side by side. We name this the “2-set model.” The other is that two basic systems are commented with a common soft ferromagnetic bar. This system is called the “common bar model.” We experimentally obtained the levitation force of the modified systems. According to the experiments, the levitation force of the “2-set model” and the “common bar model” were 314 and 327%, respectively, of that of the basic model. We also numerically analyzed the magnetic flux distribution of the modified systems. From the analysis, the soft ferromagnetic bar and plate were effective to reduce magnetic resistance in a magnetic circuit of the systems, and hence, the levitation force became large.

The Study to Improve the Field Homogeneity of the NMR Relaxometry Magnets Using the Packaged HTS Bulks With Various Iron Rings

The compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) Gd-Ba-Cu-O bulk annuli was trapped by a held cooling method. However, it was difficult to trap the uniform magnetic held above 4.7 T (200 MHz-class NMR magnet) and held homogeneity under 0.01 ppm/cm3 at liquid nitrogen temperature (77.4 K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic held required for NMR relaxometry device are 1.5 T and 150 ppm/cm 3 respectively. It is easily to generate the trapped magnetic held over 1.5 T at 77.4 K using the stacked HTS bulks, but it is still hard to obtain 150 ppm/cm 3 held homogeneity with the conventional superconducting magnet (SCM) as the magnetizing magnets. Therefore, we have studied the compensation methods to improve the held homogeneity. In this paper, we proposed the packaged HTS bulk magnet consisted of HTS bulks annuli and various iron rings as the held compensation, and investigated it in analysis and experiment.

Study on the Dynamic Stability of 3-D Superconducting Actuator by Various Typed HTS Bulk Movers

High-temperature superconductor (HTS) material can trap high magnetic fields due to its strong flux pinning force. Herein, based on our belief that such a device would be useful as a transporter in clean rooms where silicon wafers are manufactured, we report on the development of a three-dimensional (3-D) superconducting actuator using HTS bulk material in a noncontact transportation device that will move freely in 3-D space. The proposed actuator consists of a trapped HTS, which provides the mover, along with two-dimensionally arranged copper-wound iron-core electromagnets that form the stator. The current and the polarity of each electromagnet can be individually controlled via the power supply. In our previous experiments, a single-component mover was observed to overshoot during operation, which led us to conclude that a more powerful levitation force was needed. In this study, single-component HTS movers (inner diameters of 46 and 60 mm) and multicomponent HTS movers were used to increase the levitation force and dynamic stability.

Influence of Time-Varying External Magnetic Fields on Trapped Fields in Bulk Superconductors

Large single-grain bulk high-temperature superconductors (HTS) can trap magnetic fields over 17 T below 30 K and up to 3 T at 77 K and have significant potential to replace permanent magnets, the fields from which are limited to significantly less than 2 T. Therefore, bulk HTS samples are ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems, and electric machines. In electric machines, in particular, the higher flux density improves the power density of the machine, resulting in smaller lighter devices. However, in a real electric machine environment, bulk HTS samples can be exposed to ac magnetic field fluctuations, which can affect the distribution of the supercurrent in the material and attenuate the trapped field, leading to a reduction in the magnetic loading of the machine, and in some cases, full demagnetization. In this paper, the variation of trapped field with the frequency and magnitude of an external time-varying magnetic field is analyzed numerically, and the mechanisms of the attenuation of the trapped field in HTS bulks are investigated using a two-dimensional axisymmetric finite-element model based on the H-formulation, considering both the electromagnetic and thermal behavior of the bulk sample.

Dynamic Force Characteristics in a Superconducting Interface Module

Bulk high-temperature superconductors (HTSs) with good flux trapping provide a feasible application to passive docking and assembling of space module systems. To learn dynamic process and stability of a superconducting interface module consisted of the flux-trapped HTS and an interfacing magnet, an experimental setup is designed to simulate the horizontal approaching process in the interface module and measure the dynamic force properties. Momentum difference in the module is given by changing the swinging angle of the interfacing magnet. Displacement of the interfacing magnet relative to the magnetized superconductor is recorded by a charge-coupled device to state dynamic behavior. Effects of different field-cooling (FC) magnetization conditions on the dynamic properties are demonstrated by analyzing the displacement dissipation. The FC magnetization can provide a stable interfacing process, although a long damping time is required. Influence of additional aluminum metal structure on the damping properties is also investigated, and the damping ability is increased by adding the metal thickness. Therefore, the aluminum structure producing eddy current loss is proposed to be applied in the superconducting interface module to improve the interface stability.

Analysis of Levitation Characteristics of a Radial-Type High-Temperature Superconducting Bearing Based on Numerical Simulation

A two-dimensional (2D) axisymmetric finite-element model was proposed to simulate the levitation characteristics of a radial-type high-temperature superconducting (HTS) bearing. The E-J power law together with an H-formulation was used to consider the superconducting state. The numerical model was successfully implemented using a FORTRAN program. A comparison of the experimental results with the simulation was given to validate the model. It shows that they have good agreement. The electromagnetic behavior inside the HTS bulks was also analyzed. The model presented in this paper can be used to optimize the design of a radial-type HTS bearing.

Preferential Magnetic Flux Invasion and Heat Generation Owing to Macrostructure in HTS Bulk Magnets in Pulse Field Magnetization Processes

In order to clarify the behavior of the magnetic flux that invade into the melt-processed Dy123-based high-temperature superconducting bulk magnet, the motions of magnetic fluxes and the phenomena of heat generation were precisely measured during the pulsed field magnetization processes. In the experiments, we estimated the time evolutional profiles that were measured at two portions in the growth sector region and the growth sector boundary. They are characterized as the areas of lower and higher critical current densities than each other, respectively. The serial pulsed magnetic fields of 5 T were applied to the sample at 30 K, and the data were obtained in turn by the Hall sensor and thermometer that were put at several positions along the radial rows on the growth sector boundary and the growth sector region. The profiles of magnetic flux during the pulsed field application actually proved that the magnetic flux invades the sample through the growth sector regions in shorter time and with stronger magnitude than through the growth sector boundaries. Since the low magnetic field application leads to low heat generation, it is expected to achieve the higher trapped field performances than ever.

Modeling of electrophysical properties of bulk high-temperature superconductors in calculations of magnetic systems

Advanced macroscopic models of electrical properties of materials that are represented by a set of transport and associated currents and identified, respectively, by the resistive current model and nonlinear hysteretic magnetization model are proposed in order to calculate magnetic systems with the bulk high-temperature superconductor (HTS) elements. Such models allow a more accurately reproduced behavior of superconductors in the electromagnetic field of simply and multiply connected domains in various HTS transition regimes to the superconducting state. The paper presents the results of the compared combined and resistive models of the HTS properties. The proposed models enable us to reproduce the nonlinear, anisotropic, and hysteretic properties of superconductors in a wide range of parameter variation on a macroscopic scale and can be recommended for their application in numerical analysis of electromagnetic fields. The results of the calculations allow the parameter determination methods for the combined model from the experimental data to be substantiated.

Determination of <inline-formula> <tex-math notation="LaTeX">$J_C$</tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$n$</tex-math></inline-formula>-Value of HTS Pellets by Measurement and Simulation of Magnetic Field Penetration

Complete penetration magnetic field BP is a feature of a superconducting sample submitted to an applied magnetic field. It is very important to know this for applications such as an electrical motor or levitation. The electric E-J characteristics of a high-temperature superconductor (HTS) bulk is generally described by a power law. The main purpose of this paper is to investigate the influence of the n-value and the applied magnetic field rise rate Vb on the BP of a cylindrical HTS pellet. The numerical results presented come from the resolution of a nonlinear diffusion problem with commercial software. In this paper, cylindrical HTS pellets are submitted to an axial applied magnetic field. With the help of these simulations, a linear relationship between BP , Vb, and the n-value has been found. A comparison between measurements and simulations is done for the magnetization of cylindrical bulk superconducting samples. This comparison allows to determine the critical current density J C and n-value of the power law E(J) = EC(J/J C )n. The experiment is based on the direct measurement of the local magnetic field in the gap between two bulk HTS pellets. The field penetration measurements have been carried out on HTS pellets at 77 K by applying increasing magnetic fields with a quasi-constant sweep rate for the axial direction of the applied magnetic field. Two values of complete penetration magnetic field BP have been measured at two different rise rates Vb. The n-value of the real HTS pellet has been deduced.

Buffer Pellets for High-Yield, Top-Seeded Melt Growth of Large Grain Y–Ba–Cu–O Superconductors

Single-grain, (RE)–Ba–Cu–O [(RE)BCO)] bulk high-temperature superconductors have significant potential for application as trapped field magnets in a range of engineering devices. However, it is not trivial to fabricate single grains of (RE)BCO due to the complexity of the growth process, especially when the sample diameter exceeds 25 mm. In particular, difficulties associated with the seed crystal can lead to poor grain growth or to complete growth failure. We have employed an optimized buffer technique, which was determined by optimizing targeted critical parameters of the buffer pellet, including the choice of the buffer pellet composition and its aspect ratio, for the reliable fabrication of large, single grains of (RE)BCO. Potential candidates for the buffer pellet composition have been identified to yield successful grain growth and good superconducting properties. The optimum aspect ratio of the buffer pellet was also determined as part of this study. The optimized buffer pellet capped with the seed crystal has been demonstrated to work effectively as an efficient seed crystal and to aid significantly the growth of the Y-123 phase. We show that this optimized buffer technique ameliorates problems associated with both interfacial stress (commonly occurring at the seed/sample interface) and problems of grain contamination. We have fabricated a 40.8 mm diameter single-grain bulk superconductor and more than 25 single-grain YBCO samples with diameters in the range 25–35 mm by a significantly improved top-seeded melt growth process.

Levitation Performance of Rectangular Bulk Superconductor Arrays Above Applied Permanent-Magnet Guideways

Bulk high-temperature superconductors with a rectangular shape are easy to realize a compact assembly, which is promising for maglev applications. In order to find a reasonable array pattern, we have studied the levitation force of four multiseeded rectangular YBaCuO blocks with three possible arrays above two different permanent-magnet guideways (PMGs). Experimental results show that the levitation force can be improved by optimizing the array pattern of the bulk superconductors. An effective method to improve the utilization rate of the applied magnetic field is to avoid the joint air gaps between bulks located in a strong magnetic field area of the PMG. The optimal array pattern is able to enhance the levitation performance of the present superconducting maglev vehicle.