Internal magnetic relaxation in levitating superconductors

The effect of stopping the levitation force relaxation was observed upon reversal of the superconductor magnetization. A model of the internal magnetic relaxation in levitating superconductors is proposed, according to which both the magnetic moment and the force acting upon the sample are constant.     

Relaxation Properties of the Trapped Flux of Bulk High-Temperature Superconductors at Different Magnetization Levels

The magnetic relaxation phenomenon is a crucial subject for the engineering applications of bulk high-temperature superconductors (HTS) in which the trapped field or levitation force behaves with a time-dependent decay due to the intrinsic flux creep inside the HTS materials. To fully exploit the high trapped field of bulk HTS, we have experimentally investigated the trapped flux relaxation properties, especially at different magnetization levels. With different excitation fields, the dependence between trapped field and relaxation rate was analyzed and compared in both field-cooling magnetization (FCM) and zero-field-cooling magnetization (ZFCM) conditions. In parallel, the relaxation rates for different trapped flux can be measured all at once during a single magnetization process. The relaxation rate is closely correlated to the trapped field and useful to reflect the effect of the following remagnetization processes. To suppress the relaxation, we further checked possible methods from the three aspects of material improvement, working temperature and flux annealing effect.     

Levitation Force Relaxation of HTS YBCO Bulk under Load

The loading capacity of the high temperature superconducting (HTS) maglev vehicle is closely relative to the maglev characteristics of the HTS bulk under the load. The investigation of the characteristics is necessary for the application of the HTS maglev vehicle. In our previous work, it was found that the levitation force relaxation increased with the increase of the reloading. In this paper, in terms of a HTS maglev system composed of the permanent magnetic guideway (PMG) and a HTS YBCO bulk, the levitation force relaxation under different loads is investigated by the numerical calculation and the experiment. It is found that the normalized relaxation rate of the levitation force increased with the increase of the load. The influence of the load on the current density distribution is also discussed in this paper. This work might be helpful for the research of the loading capacity of the HTS maglev vehicle.      

Magnetic Forces Relaxation Step Change Characteristics of YBCO Bulk with Mechanical Impact Load over a NdFeB Guideway

Magnetic force relaxation of YBCO bulk above the NdFeB permanent magnet guideway (PMG) with impact load has been investigated. An experimental setup has been built using a single YBCO bulk and a symmetrical center NdFeB PMG. There are two experimental methods: the case of magnetic levitation force relaxation measuring perturbed with impact load in zero-field-cooling (ZFC); and the case of magnetic levitation and guidance force relaxation measuring synchronously perturbed with impact load in field-cooling (FC). The results show that there is magnetic levitation and guidance forces relaxation step change at the time of the impact load. Two times impact loads are applied for each method. The first step change range is much larger than the second one. The Bean critical model and Anderson–Kim theory are used to analyze it.     

Levitation Force Relaxation of HTS Bulk above NdFeB Guideways at Different Approaching Speeds

The zero-field cooling levitation force relaxation of the YBCO bulk above two types of NdFeB guideways, which were used in the high-temperature superconducting maglev vehicle system, was studied experimentally when the bulk moved towards the guideways at different speeds. In this experiment, a sample with the diameter of 50 mm and thickness of 15 mm was used; the time range in the relaxation measurements was from 0 s to 200 s. As a result, a nearly linear logarithmic decay of the levitation forces with time was observed. It was found that the influence of the approaching speeds on the levitation force relaxation is different, and that the levitation force decay is closely related to the guideway structures. Based on the monopole peak PMG, the approaching speeds will influence more the levitation force decay than the multipole Halbach. So the maglev vehicle above mutipole Halbach structure PMG will work more stably than that above the monopole peak PMG.      

Influence of Maglev Force Relaxation on Hysteresis of Bulk HTSC Above the NdFeB Guideway at Different Lateral Displacements

After the maglev force (both levitation and guidance forces) relaxation property of a high-temperature superconductor (HTS) above a permanent-magnet guideway (PMG) was studied experimentally, the HTS bulk was horizontally moved in the lateral direction with different displacements above the permanent-magnet guideway. Both levitation and guidance force curves with lateral displacement were obtained by a SCML-2 measurement system synchronously. It was found that the levitation force hysteresis after relaxation was reduced compared to the case without relaxation, while the guidance force hysteresis curve almost did not change. This study provided a scientific analysis for the practical application of the bulk HTS.     

Retardation of the Magnetic Relaxation in High-Temperature Superconductors Near a Ferromagnet

The creep of a magnetic flux trapped in a bulk high-temperature superconductor has been studied. It has been found that the magnetic relaxation is retarded when the superconductor is placed near a ferromagnet. The value of the retardation effect depends on the sequence of magnetization and the approach of the superconductor to a ferromagnet. The magnetic relaxation is fully suppressed when a superconducting sample first is magnetized and then is brought close to a ferromagnet. An interpretation of this effect has been discussed. Being magnetized, a ferromagnet produces its own magnetic field. While penetrating into a disk sample through its planes, the ferromagnet field induces screening currents, which circulate oppositely to the current that arises upon trapping of the magnetic flux. As a result, the stability of the magnetic structure is sharply improved since opposite driving forces can act on different sections of the vortices.     

Observation of the Field,Current and Force Distributions in an Optimized Superconducting Levitation with Translational Symmetry

The superconducting levitation realized by immersing the high-temperature superconductors (HTSs) into nonuniform magnetic field is deemed promising in a wide range of industrial applications such as maglev transportation and kinetic energy storage. Using a well-established electromagnetic model to mathematically describe the HTS, we have developed an efficient scheme that is capable of intelligently and globally optimizing the permanent magnet guideway (PMG) with single or multiple HTSs levitated above for the maglev transportation applications. With maximizing the levitation force as the principal objective, we optimized the dimensions of a Halbach-derived PMG to observe how the field, current and force distribute inside the HTSs when the optimized situation is achieved. Using a pristine PMG as a reference, we have analyzed the critical issues for enhancing the levitation force through comparing the field, current and force distributions between the optimized and pristine PMGs. It was also found that the optimized dimensions of the PMG are highly dependent upon the levitated HTS. Moreover, the guidance force is not always contradictory to the levitation force and may also be enhanced when the levitation force is prescribed to be the principle objective, depending on the configuration of levitation system and lateral displacement.     

静磁场在材料生产过程中的应用研究评述

在材料电磁过程研究中，静磁场尤其是强磁场材料科学是当今世界的研究热点。本文从静磁场作用下生成的洛仑兹力和磁化力两个角度系统地归纳总结了静磁场技术在材料生产领域的应用原理和实践。对静磁场下的洛仑兹力，主要介绍了流体流动、波动和对流控制、电磁振动及电磁超声波等方面的研究现状；对强磁场下的磁化力，主要介绍了其在相变、结晶配向、磁悬浮、磁对流等方面的研究进展。最后对强磁场材料科学的研究趋势和发展前景做了展望。     

Mathematical Model of Magnetic Aftereffect in Hydrogenated Ferromagnets

We consider a mathematical model of magnetic aftereffect in ferromagnets saturated with hydrogen. The influence of the concentration of hydrogen on the magnetic aftereffect is taken into account by the dependence of the coefficient of relaxation of magnetization on the level of strains, which, in turn, depends on the concentration of hydrogen. The time dependence of the electromotive force in a contour enclosing a long cylinder is established for various values of the rate of magnetic relaxation. The obtained dependences can be used for the interpretation of the results of measurements aimed at the evaluation of the concentration of hydrogen.     

Pinning and dissipative effects in Bi-Pb-Sr-Ca-CuO superconductor

Pinning and dissipative effects have been investigated in Bi-Pb-Sr-Ca-CuO sintered compounds by means of DC magnetization and magnetic relaxation measurements. Hysteresis cycles have been performed up to 5 T from 18 to 50 K, and the time decay of the zero field cooled magnetization (M _zfc) has been recorded both as a function of field and temperature. The observed temperature dependence of the reduced pinning force and the crossover from a logarithmic to a nonlogarithmic time decay ofM _zfc (forT>50 K), are discussed in relation to the collective pinning and the related extensions to flux creep.     

Levitation forces in single-sided linear induction motors for high-speed ground transport

The levitation force and thrust in a current-controlled single-sided linear induction motor which propels electrodynamically-levitated vehicles are discussed. It has been shown theoretically that the single-sided linear induction motor (SLIM) can operate satisfactorily over suitably large clearance gap compared with the effective height of electrodynamic suspension. The levitation force in SLIM can be used practically as an alternative levitation force in the low-speed region by supplying a primary current several times the rated current within the very short acceleration times. In high-speed Maglev vehicle system with electrodynamic suspension and SLIM propulsion, the aluminum plate for levitation can be saved for the distance to accelerate the vehicle to speeds sufficient for electrodynamic levitation by the cryogenic magnets.     

Influence of YBCO Bulk’s Movement Speed and Operation Time on Levitation Force at Different Temperatures

In order to enhance levitation performance of the high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle and have more stable and safer operation, levitation forces of YBCO bulk with different movement speeds and operation time at different temperatures are studied. The levitation force is measured using a cryogenic measurement system which has advanced instrumentation. Experimental results show that the levitation force is not only dependent on the temperature but also on the YBCO bulk’s movement speed and operation time. It is found that the effect of YBCO bulk’s movement speed and operation time on the levitation force is larger at higher temperatures, while such effect is smaller at lower temperatures.     

Oscillation Characteristics of Levitation Force of YBCO Bulk Exposed to AC Magnetic Field above NdFeB Guideway

In the present High Temperature Superconducting (HTS) maglev vehicle system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneity of the material property of the NdFeB magnet. In order to investigate the influence of the nonuniformity on the levitation performance of the HTS bulk, the experiment involved an electromagnet, which is supplying AC current to simulate the nonuniformity of the external magnetic field. The levitation force of the HTS bulk is measured when applying AC current on the electromagnet coils. The results indicate that the levitation force abruptly changes and oscillates after applying AC external magnetic field. The effect of the amplitude of the AC magnetic field on the levitation force is studied; the result shows that the oscillation amplitude of the levitation force increases with the amplitude of the AC external magnetic field and is independent of the Field Cooling Height (FCH) of the bulk.     

Correlations Between Magnetic Flux and Levitation Force of HTS Bulk Above a Permanent Magnet Guideway

In order to clarify the correlations between magnetic flux and levitation force of the high-temperature superconducting (HTS) bulk, we measured the magnetic flux density on bottom and top surfaces of a bulk superconductor while vertically moving above a permanent magnet guideway (PMG). The levitation force of the bulk superconductor was measured simultaneously. In this study, the HTS bulk was moved down and up for three times between field-cooling position and working position above the PMG, followed by a relaxation measurement of 300 s at the minimum height position. During the whole processes, the magnetic flux density and levitation force of the bulk superconductor were recorded and collected by a multipoint magnetic field measurement platform and a self-developed maglev measurement system, respectively. The magnetic flux density on the bottom surface reflected the induced field in the superconductor bulk, while on the top, it reveals the penetrated magnetic flux. The results show that the magnetic flux density and levitation force of the bulk superconductor are in direct correlation from the viewpoint of inner supercurrent. In general, this work is instructive for understanding the connection of the magnetic flux density, the inner current density and the levitation behavior of HTS bulk employed in a maglev system. Meanwhile, this magnetic flux density measurement method has enriched present experimental evaluation methods of maglev system.     

A new method of determining the reversible magnetization loop of bulk high-temperature superconductors

A new method of calculation of the magnetization loop of bulk high-temperature superconductors is proposed which employs the results of measurements of the magnetic levitation force.     

Influence of AC External Magnetic Field on the Levitation Force Relaxation of HTS Bulk Above NdFeB Guideway

In the present High-Temperature Superconducting (HTS) maglev vehicle system, the air gaps between the adjacent permanent magnets make the magnetic fields above the NdFeB guideway non-uniform. So one is required to study the characteristics of levitation force of the HTS bulk influenced by non-uniform fields. In this paper, we have studied the characteristics of the levitation force relaxation by an experiment in which the AC external magnetic field generated by an electromagnet is used to simulate the time-varying external magnetic field caused by the inhomogeneity of the NdFeB guideway. From the experiment results, it is found that the levitation force is attenuated with the application of the AC external magnetic field, and the decay is increased with the amplitude of the AC magnetic field, but the decay is almost independent of the frequency of the AC magnetic field.     

Decay Characteristics of Levitation Force of YBCO Bulk Exposed to AC Magnetic Field above NdFeB Guideway

The superconducting maglev vehicle is one of the most promising applications of HTS bulks. In such a system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneity of the material property of the NdFeB magnet. So it is required to study the characteristics of levitation force of the bulks affected by the non-uniform applied magnetic fields along the moving direction. In this paper, we will study the characteristics of the levitation force relaxation between the HTS bulk and the NdFeB guideway by an experiment in which AC external magnetic field generated by an electromagnet is used to simulate the time-varying external magnetic field caused by the inhomogeneity of the guideway. From the experimental results, it has found that the levitation force is decreasing with the application of the AC external magnetic field, and the decay increasing with the amplitude of the applied magnetic field and is almost independent of the frequency.     

Effect of AC Magnetic Field on the Levitation Force of YBCO Bulk above NdFeB Guideway

In the present High Temperature Superconducting (HTS) maglev vehicle system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneous of the material property of the NdFeB magnet. In order to investigate the influence of the nonuniformity on the levitation performance of the HTS bulk, an electromagnet supplied by AC current is used to simulate the nonuniformity of the external magnetic field. The levitation force of the HTS bulk is measured when applying AC currents to the electromagnet coils. Experimental results indicate that the levitation force changes abruptly and then oscillates after applying AC external magnetic field, and the levitation force is attenuated by the AC magnetic field after withdrawing the AC field. Moreover, the oscillation amplitude and the attenuation rate of the levitation force increase with the amplitude of the AC external magnetic field.     

Superconducting bulk magnet for maglev vehicle: Stable levitation performance above permanent magnet guideway

High-temperature superconducting (HTS) maglev vehicle is well known as one of the most potential applications of bulk high-temperature superconductors (HTSCs) in transported levitation system. Many efforts have promoted the practice of the HTS maglev vehicle in people's life by enhancing the load capability and stability. Besides improving the material performance of bulk HTSC and optimizing permanent magnet guideway (PMG), magnetization method of bulk HTSC is also very effective for more stable levitation. Up to now, applied onboard bulk HTSCs are directly magnetized by field cooling above the PMG for the present HTS maglev test vehicles or prototypes in China, Germany, Russia, Brazil, and Japan. By the direct-field-cooling-magnetization (DFCM) over PMG, maglev performances of the bulk HTSCs are mainly depended on the PMG's magnetic field. However, introducing HTS bulk magnet into the HTS maglev system breaks this dependence, which is magnetized by other non-PMG magnetic field. The feasibility of this HTS bulk magnet for maglev vehicle is investigated in the paper. The HTS bulk magnet is field-cooling magnetized by a Field Control Electromagnets Workbench (FCEW), which produces a constant magnetic field up to 1 T. The levitation and guidance forces of the HTS bulk magnet over PMG with different trapped flux at 15 mm working height (WH) were measured and compared with that by DFCM in the same applied PMG magnetic field at optimal field-cooling height (FCH) 30 mm, WH 15 mm. It is found that HTS bulk magnet can also realize a stable levitation above PMG. The trapped flux of HTS bulk magnet is easily controllable by the charging current of FCEW, which implies the maglev performances of HTS bulk magnet above PMG will be adjustable according to the practical requirement. The more trapped flux HTS bulk magnet will lead to bigger guidance force and smaller repulsion levitation force above PMG. In the case of saturated trapped flux for experimental HTS bulk magnet, it is not effective to improve its maglev performances by increasing of charging magnetic field, when the guidance force at WH 15 mm is 5.7 times larger than that by DFCM of FCH 30 mm. So introducing HTS bulk magnet into the present maglev system is feasible and more controllable to realize stable levitation above applied PMG, which is an important alternative for the present HTS maglev vehicle.