AC permeability of the flux-line liquid in the anisotropic high-Tc superconducting crystals

AC properties of the flux-line liquid in the anisotropic high-temperature superconducting crystals in a parallel field are theoretically investigated. The ac responses in the simple flux-flow regime are analyzed from associated effective ac-magnetic permeability calculated on the hydrodynamic theory basis. The responses are studied as functions of anisotropic ratio and sample dimensions. The results illustrate the influence of the platelet crystal's size on permeability in the anisotropic superconductors while in the isotropic superconductors, the relationship of responses between a square rod and cylinder is found. It indicates that the permeability of a cylinder can be essentially replaced by that of a square rod and vice versa. The geometric effect on response is also elucidated in the isotropic superconductors     

Influence of flux creep on dynamic behavior of magnetic levitation systems with a high-T/sub c/ superconductor

Significant flux creep may be generated in some high-T/sub c/ superconductors with weak pinning, which could yield an influence on the dynamic behavior of a high-T/sub c/ superconductor-magnet levitation system. To investigate this influence, this article presents a numerical analysis of dynamic features of the levitation generated by an interaction between a high-T/sub c/ superconductor (HTSC) and a permanent magnet (PM) after the flux creep in the superconductor is taken into account in a macro-model of superconductivity. The influence is comprehensively displayed by comparing the predictions of dynamic responses of such systems in which the flux creep in the superconductor is and is not taken into account. The obtained results show that whether or not the flux creep results in a noticeable influence to the levitation of superconductor-magnet systems is mainly dependent upon properties of superconductivity and applied excitation, e.g., critical current density of superconductors, and amplitude and frequency of external excitations. When the critical current density is less than 4.5/spl times/10/sup 8/ A/m/sup 2/, and the system is subjected to a periodic excitation, the influence of flux creep should be taken into account in the theoretical analysis.     

Flywheels on a roll

Superconducting bearings promise to reduce friction by up to two orders of magnitude, auguring flywheels in the 10-kWh class. The author discusses the principles of stable magnetic levitation of permanent magnets using superconductors. The author then discusses the application of the principles to bearings using high temperature superconductors. Key design parameters and application to flywheels are also discussed     

Numerical evaluation of levitation force of HTSC flywheel

Levitation force between permanent magnets and high-T_c superconductors (HTSC's) is evaluated numerically based on the critical state model. The bulk HTSC is approximated to a thin plate multilayer model from its anisotropic structure. The thin plate eddy current code with the finite element method is applied to the analysis of the HTSC flywheel, where ring magnets and the HTSC are arranged in triple concentric circles. The size and the combination of the magnets are examined to obtain the large levitation force. It is shown that the levitation force of the HTSC flywheel depends strongly on the magnetic field configuration     

抗磁稳定悬浮结构的阻尼特性研究

研究了一种抗磁稳定悬浮结构的涡流阻尼力的阻尼特性。建立了求解涡流阻尼及阻尼系数的数学模型,借助MATLAB和COMSOL 5.6通过三维电磁场有限元仿真分析方法研究了涡流阻尼的特性以及外部激励频率、振幅、悬浮磁体厚度、导体板厚度等对涡流阻尼的影响,给出了其变化规律。结果表明：增大磁体半径与厚度、增大抗磁体边长和厚度以及减小悬浮间隙都可增大涡流阻尼。除此之外,增大外部激励的振幅与频率也可以增大涡流阻尼,但增大激励频率时涡流阻尼会出现相位滞后现象。该结果对于分析研究抗磁悬浮结构的振动特性具有一定的参考意义。     

Improvement of accuracy in measurements of the surface resistanceof superconductors using dielectric resonators

Two techniques to minimize the influence of parasitic losses on surface resistance measurements of superconductors employing dielectric resonators have been described. The first method optimizes the sapphire resonator aspect ratio to minimize the parasitic losses for given superconductor sample dimensions and measurement frequency. The second utilizes a reference resonator with a perfect conductor plane to cancel out the influence of parasitic losses by measurements of the resonant frequencies and Q-factors of two resonators. The second technique is recommended when materials having noticeable dielectric losses are used to construct dielectric resonators     

Development of a Ground-Based Simulated Experimental Platform for Gravitational Biology

The purpose of this study is to develop a ground-based experimental platform which can provide a long-term, persistent, and stable weightless environment to provide technical support for the fundamental study of space life sciences. The platform included a superconducting magnet which can generate a large gradient high magnetic field, temperature control system, object stage, observing system, and gas control system. The platform can accurately measure and control comprehensive parameters, such as temperature, humidity, sample location, gravity level, and magnetic induction intensity. The biological experiment by means of the platform indicated that diamagnetic levitation affected cell morphology but had nonlethal effects on cell growth, which is consistent with the results of spaceflight. It suggests that the platform can meet the need of the simulated experimental conditions for gravitational biology research from molecule, cell, tissue, and organ to model animal.     

基于超声驻波的径向悬浮移动系统研究

常用的声悬浮系统是采用一维单轴式超声悬浮装置,利用驻波特性仅能使少量小颗粒悬浮在声压节点处。为改善实验效果,该文在传统悬浮装置的基础上作了改进,设计了一种径向悬浮移动系统,采用相控阵探头板作为发射端和反射端,结合高频时钟驱动的现场可编程门阵列(FPGA)作延时控制,改变发射波束与反射波束间的相位差,使小颗粒可呈阵列式悬浮移动。系统中所设计的驱动电路是采用金属氧化物半导体场效应晶体管(MOS)结合变压器的电压驱动方式,无需考虑输入端信号的电流大小及MOS管无需接高上拉电压。通过对系统的硬件测试及仿真,结果表明,该文方法比传统方法能稳定悬浮多个小颗粒,且能使悬浮小颗粒平滑移动,具有较好悬浮稳定性和可靠性。     

Permanent magnet pulse motor with high temperature superconductinglevitation

A simple but stable noncontact high T_c superconducting levitation system with a vertical shaft has been presented. The levitation system consists of a superconductor and permanent magnets. In the system, a high T_c superconductor supports a lower end of the shaft, and a pair of permanent magnets supports the other end. Although a levitation is stable when the levitation system is used, since the restoring force is small in the horizontal direction, the levitation stability is small in the horizontal direction. Hence, it is difficult to drive the shaft by electromagnetic forces in using such motors. This paper describes a driving system consisting of six coils and a permanent magnet with eight poles, in which balance forces act on two opposite sides of the disc type rotor in the axial direction. The system has no unbalance force, and stable rotation is obtained without control. To validate the proposed system, experimental tests have been carried out. The experimental motor achieved stable rotation at speeds up to 1250 rpm     

Computational study of variations in gap size for the electrostatic levitating force of MEMS device using dual BEM

For MEMS combdrive performance, the calculation of levitating force due to electrostatic field is very important, and an accurate electrostatic analysis is essential. Because the gap size between combdrive fingers and ground plane or movable finger and fixed finger, plays a very important role for levitation, a study of the effect of gap size variation is indispensable. For diverse gaps of MEMS comdrive design, the dual BEM (DBEM) has become a better method than the domain-type FEM because DBEM can provide a complete solution in terms of boundary values only, with substantial saving in modeling effort, hence the DBEM was used to simulate the fringing of field around the edges of the fixed finger and movable finger of MEMS combdrive for diverse gap size. Results show that the less the gaps between combdrive fingers and ground plane are, the larger the levitating force acting on the movable finger is. In addition, the levitating force becomes more predominant as the gaps between movable finger and fixed finger decrease. By way of DBEM presented in this article, an accurate electrostatic field can be obtained, and the follow-up control method of levitation force for MEMS combdrive can be implemented more precisely.     

Numerical analysis of high-temperature superconductors with the critical-state model

A numerical method is proposed to analyze the electromagnetic behavior of systems that include high-temperature superconductors. The algorithm uses the finite-element method and the critical-state model, and it solves two-dimensional and axially symmetric problems that include superconductors and other materials. The main advantages of the algorithm are its speed, its robustness, and its ease of coupling with circuit equations. This method is used to analyze two different systems: 1) Magnet-superconductor levitation system-the levitation forces are calculated. In order to validate the model, experimental results of such a system are obtained and compared with simulation results. 2) Nine-wire superconducting cable. Parallel and series-type configurations of the current driven in the wires are analyzed. Hysteresis ac losses are also calculated and compared for both scenarios.     

Large-scale applications of superconducting coils

Superconducting coils have been utilized in a broad range of applications which continues to expand. Initially, they were used for small physics experiments in the laboratory. More recent applications involve superconducting coils on a scale that would have been unrealistic only a few years ago. This paper is concerned with some of these applications. After a brief review of the main characteristics of superconductors and superconducting windings in general, specific applications are discussed. These include high-energy physics, controlled thermonuclear fusion, magnetohydrodynamic power generation, inductive energy storage, and levitation of high-speed ground vehicles. Photographs and illustrations are included to describe some of the existing systems as well as some systems which are presently conceptual in nature.     

Stabilization of one degree-of-freedom control type levitation table with permanent magnet repulsive forces

It is impossible to levitate an object by using only permanent magnets (PMs). Many researches have been reported to reduce the number of control degree-of-freedom (DOF) in magnetic levitation systems. This paper presents a one-DOF control type magnetic levitation table supported by repulsive forces of four sets of PMs. Each set of PMs is composed of two PMs on a fixed frame and one PM on the levitated table. The PMs are located at the four corners of the rectangular table and the corresponding positions of the frame. The magnetic poles of the PMs on the table are opposed to those of the fixed PMs, so that repulsive forces act on the table and the frame. This paper, first, discusses theoretically the relative position range of the sets of PMs to the center of gravity of the table from the viewpoint of the stability condition of the motion of equation. In the discussion, the repulsive force and stiffness produced by a set of the PMs are assumed to be known parameters because they can be numerically calculated. Then an experimental table is constructed, in which the unstable motion is actively stabilized by a linear motor. Dynamic responses are measured, and results prove that the proposed magnetic levitation table is successfully held in stable equilibrium by only one-DOF active control.     

Microstructure and associated properties of YBCO superconductors prepared by melt-processing techniques

From the standpoint of applications, melt-processed bulk YBa₂Cu₃O_x (YBCO) superconductors are of considerable interest. In this paper, we studied the microstructure and levitation force of melt-processed YBCO, YBCO plus Y₂BaCuO₅, and YBCO+Pt. Large single-crystal samples, grown by a seeding technique, were also studied. The levitation force was highest in melt-processed samples made by the seeding technique.     

Engineering and performance of a contactless linear slider based on superconducting magnetic levitation for precision positioning

In this paper, a contactless linear slider for precision positioning able to operate in cryogenic environments is presented. The device, based on superconducting magnetic levitation, does not present contact between the slider (composed of a permanent magnet) and the guideline (made of high-temperature superconducting disks) of the mechanism, thereby avoiding any tribological problems. Moreover, the slider is self-stable and the superconductors provide inherent guidance to the permanent magnet in the sliding DoF due to the high translational symmetry of the magnetic field that leads to low power consumption. A sub-micrometre resolution and a symmetric stroke over ±9 mm have been demonstrated at cryogenic temperatures. In addition, a set of design rules for this kind of mechanism has been proposed and experimentally validated. These rules demonstrate that the performance of the device can be tuned just by modifying some geometrical parameters of the mechanism. In this way, the sensitivity and stiffness, resolution, angular run outs and power consumption can be adjusted for different applications and requirements.     

Independent Control of Average Torque and Radial Force in Bearingless Switched-Reluctance Motors With Hybrid Excitations

Radial force and torque are the control objectives that determine the machine performance of levitation and rotation in a bearingless switched reluctance motor (BSRM). This paper proposes a control scheme for rotating and levitating a 12/8 BSRM. The motor average torque and radial force are independently controlled with hybrid excitations in main windings and levitation windings. First, the mathematical relationship between radial force and currents, which is utilized in this paper, is derived by using the Maxwell stress tensor method. Then, the proposed control scheme is analyzed. The average torque of each phase generated in the levitation region equals zero for its symmetry of the aligned position. Accordingly, the current calculating algorithm is deduced to minimize the magnitude of instantaneous torque in the levitation region. The principle and realization of the proposed scheme are demonstrated with finite-element (FE) analysis. Experimental results show that the proposed scheme is effective for a stable levitation.      

Superconductors come in from the cold

The author briefly discusses the discovery of superconductivity in 1911 and the two major breakthroughs in 1966 and 1986 in this field. In 1966 a stable composite conductor of Ni-Ti strands embedded in a Cu strip was developed. A superconducting magnet was made from this material. In 1986 superconductors that maintained superconductivity up to 90 K, cooled by liquid N₂, were discovered. The author goes on to briefly discuss the various research projects around the world, involved in high temperature superconductors     

Drift of Levitated/Suspended Body in High- Tc Superconducting Levitation Systems Under Vibration—Part II: Drift Velocity for Gap Varying With Time

To study levitation drift further, i.e., the gap between a superconductor and a permanent magnet varying with time in high-T_c superconducting levitation systems, drift velocity is introduced. Based on the numerical simulations of the dynamic response of a levitated body, and according to the essential reasons for drift, the drift velocity is first divided into two regimes: V_ff [related to flux flow (FF)] and V_tc [related to flux creep (FC)]. The drift velocity is shown to be mainly dependent upon properties of superconductivity (such as the critical current density of superconductors), initial disturbances, and applied excitations (such as the amplitude and the frequency of external excitations). Furthermore, the corresponding influences of the drift velocities V_ff and V_fc have been investigated quantitatively in this paper.     

Application of superconductors to motors, generators, andtransmission lines

Two specific applications of superconductors are considered: electric machinery (motors and generators) and transmission lines. Development work carried out with respect to both types of equipment indicates that the use of superconductors is feasible, even if only liquid-helium-cooled superconductors are considered. The high-T _c superconductors will have a beneficial impact on motors, generators, and transmission lines only if conductors with sufficient mechanical properties and current-carrying capabilities can be developed     

Superconducting magnets and their applications

Since the discovery of superconductivity almost a century ago, there has been a steady increase in the variety of superconducting magnet applications. Progress in superconducting magnet technology has resulted in applications in areas of basic science, medicine, separation, and levitation. Performance improvements in a variety of materials, from low-temperature to high-temperature superconductors, are the foundation of recent rapid development. In addition, large increases in affordable computing power, along with steady refinement of three-dimensional analytical tools and improved materials characterization, have allowed many more advanced magnet concepts to be realized directly in hardware without scale prototype testing than was previously possible. This in turn has broadened opportunities for new science and technology results in many fields including the basic sciences, medical imaging, fusion, environmental remediation, and transportation. In this paper, a few examples of these applications will be discussed, representing a range in magnetic field, current density, and overall size, from the practical to the developmental.