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This paper proposes a novel approach to the modelling of HTS transformer-rectifier flux pumps that can charge HTS magnet-ic loads without the need for bulky current leads or expensive cryogenic requirements. The model was built in MATLAB/Simulink for a flux pump with 2 HTS switches, an HTS transformer with a primary inductance of 15 mH, second-ary inductance of 50 H and an HTS magnetic load with an in-ductance of 2 mH. With a 5 A pk-pk triangular current wave-form at 2 Hz the system can load the HTS load to 34.63 A in 5 s. This incremental addition of flux to the load demonstrates the advantage of flux pumps as they can charge high-field magnets without the need for large power sources. The speed of flux pumping is improved by increasing the system frequency, the magnitude of the dynamic voltage developed in the HTS switches and by increasing the primary inductance of the trans-former. This has widespread industrial applicability in fields such as the charging of high-field magnets for MRI systems.

In recent years, along with the unceasing development of High-temperature Superconductor (HTS) technology, the performance of HTS tape has been improved significantly, including the higher critical current, lower joint resistance, and longer tape length etc. To use HTS instead of Low-temperature Superconductor (LTS) in Magnetic Resonance Imaging (MRI) applications, the flux pumping method with travelling wave has been investigated. In this paper, our new experimental results were regarding to the frequency, amplitude, and wavelength characteristics of charging an HTS coil with an HTS square bridge, using an electromagnetic flux pump designed at the University of Cambridge.

Magnetic Resonance Imaging (MRI) is an indispensable tool in monitoring lipid-rich tissues and the musculoskeletal system. It however requires maintaining a huge homogeneous (1 to 7 T) field. That is where superconductors come into play since they are the only feasible way to create such fields. The state-of-the-art technology utilizes low critical temperature superconductors (LTS) which require cooling down to liquid helium temperatures (4.2K). The new generation superconductors, aka the high critical temperature superconductors (HTS), can operate up to relatively elevated temperatures (technically above 100K's). In addition to that, they can withstand larger magnetic field strengths which can theoretically go up to the order of hundred Teslas in magnitude. It is evident that LTS wires of current MRI magnet technology is at their limits and new pursuits such as reaching higher fields and reducing operational/maintenance costs are in the favor of HTS tapes. There is, in fact, another advantage beyond minimization of the cryogenic costs, namely the compactness of the overall system. Utilizing certain auxiliary techniques such as flux pumping and conductive cooling of the tapes, MRI magnets can be reduced in size substantially. That, in turn, enables constructing much smaller thus mobile MRI scanners which can be brought to patients’ doorsteps. Here we present the current status of our project aiming to build an HTS MRI machine dedicated to use as a mobile head scanner.

We describe a novel use for superconducting flux pumping, namely a method for magnetizing a planar magnetic field source. The method is useful for many applications, but we illustrate it with the planar Penning trap with applications in quantum technology. A flux pumping mechanism and associated mathematical model are described. The method allows for complete control of the induced persistent current in a double-loop superconducting structure, whereby the final current is a known function of the applied flux and the loop geometry. We have tested our flux pumping technique and found excellent agreement with the theory.

Superconducting flux pump is a wireless charging technique utilizing electromagnetic induction law for exciting the superconducting magnet by pumping the magnetic flux into the superconducting closed-loop circuit without any electrical contact in order to compensate current attenuation and ensure the stability of the magnetic field. The contactless flux pump not only charges the magnet, but also quantitatively compensates the current decay of the magnet, which makes it possible for superconducting coils to work in the persistent current mode. This method of magnetizing can effectively reduce contact losses and heat losses, making the whole magnet system more compact and effective. This paper introduces the working principle and research progress of the recent flux pumping technology, reviews different types of high temperature superconducting (HTS) flux pumps proposed during the last decade, summarizes the underlying physics as well as merits and drawbacks of each type, and finally gives a potential developing trend for the future flux pumps where power engineering is largely applied.

High- $T_{c}$ superconducting (HTS) materials are extremely promising for high-field applications, due to their relatively high upper critical field. Flux pumps are a means ofenergizing closed superconducting magnets without direct electrical contact. Simulation is a good method to understand the physics of the flux pumping system. In this paper, we propose a simulation methodology for a transformer–rectifier HTS flux pump. A model is built, based on the electrical circuit. The simulation methodology opens up various opportunities for flux pump developers to design the whole superconducting magnet energizing system.

In this paper, we propose a novel kind of Josephson-Energy Suppression Pump (JESP) controlled by a fully magnetic flux drive. The device presented here is composed of two superconducting loops interrupted at one side by superconducting nanowires which are joined together by a superconducting island. The phase difference developed at the edges of the nanowires by means of the magnetic flux threading the loops can collapse their Cooper condensates, leading to complete suppression of the Josephson energies. This mechanism allows to greatly reduce the leakage current when performing Cooper pair pumping by a pure magnetic pumping cycle without involving any gate modulation. The pumping capability of the JESP is studied through a master equation approach in the non-adiabatic case.

Due to the improvement in the manufacturing of the superconductor, the superconducting magnets have been widely used in the areas such as magnetic resonance imaging (MRI), particle accelerators, and magnetic levitation systems. The transformer-rectifier flux pump is an effective method to energize the superconducting magnets. However, in the real system of the flux pump, some undesired dc current will be inevitably generated in the flux pumping system during the process of energizing the superconducting magnets. The undesired dc current will have many negative impacts such as slowing down the charging process, decreasing the final field of the superconducting magnets, and resulting in the failure of the whole system. This paper will focus on the origin of the undesired dc current in the flux pump, investigate the damage that the undesired dc current causes, and propose an effective method of removing the undesired dc current. The proposed method has greatly improved the performance of the flux pump. An experimental system is built and tests are carried out to verify the practicality and validity of the proposed method.

High-temperature superconducting (HTS) no-insulation (NI) coil is a promising design because of high current density, high thermal stability, and self-protection mechanism. Flux pumping is a contactless charging method for a superconducting magnet, which can reduce the cryogenic losses associated with current leads. In this study, we compared the charging performance of a rotating permanent magnets–based flux pump for two different types of HTS coils: an INS coil and an NI coil. Results showed that the rotating type flux pump works well with both coils. But the charging speed of NI coil is much slower, which results from the small parallel characteristic resistance of NI coil. Results also show that the output DC voltage and inherent dynamic resistance of the flux pump increase linearly with the increase of rotating speed.

The high-$T_c$ superconducting (HTS) dynamo exploits the nonlinear resistivity of an HTS tape to generate a DC voltage when subjected to a varying magnetic field. This leads to the so-called flux pumping phenomenon and enables the injection of DC current into a superconducting coil connected to the dynamo without current leads. In this work, the process of charging a coil by an HTS dynamo is examined in detail using two numerical models: the Minimum Electromagnetic Entropy Production and the segregated $\textbf{H}$-formulation finite element model. The numerical results are compared with an analytical method for various airgaps and frequencies. Firstly, the I-V curves of the modeled HTS dynamo are calculated to obtain the open-circuit voltage, short-circuit current and internal resistance. Afterward, the process of charging a coil by the dynamo including the charging current curve and its dynamic behavior are investigated. The results obtained by the two models show excellent quantitative and qualitative agreement with each other and with the analytical method. Although the general charging process of the coil can be obtained from the I-V curve of the flux pump, the current ripples within a cycle of dynamo rotation, which can cause ripple AC loss in the HTS dynamo, can only be captured via the presented models., 19 pages, 7 figures

In this research, we investigated the surface critical magnetic field (HC3) of magnetic superconductors by the Ginzburg-Landau (GL) Theory. We used the 1st GL equation to calculated HC3 by variation method analytically and modified to the temperature dependence properties. The analytical formula of surface critical magnetic field depended on magnetic properties of material: susceptibility and differential susceptibility. In addition, the temperature-dependent surface critical magnetic field was studied by four models; Chen's model, Zhu's model, Shanenko's model and Changjan & Udomsamuthirun ’s model. Focused on four models, our numerical calculation based on Changjan & Udomsamuthirun's model could find a good agreement with the experimental data of Pb82Bi18 superconductors vicinity the critical temperature scenery.

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 modified finite-element analysis (FEA) and hybrid numerical-analytical method for simulation of critical currents and magnetic fields in high-temperature bulk superconductors are discussed. It is shown that the hybrid method is more robust and efficient in terms of speed and accuracy as compared with the known integral method. The modified FEA method is more stable than a conventional transient solution especially at large n -value in J–E relation.

The effective pinning of magnetic flux in high-temperature superconductor bulk plays a basic role in a variety of engineering applications as superior magnetic field sources. We have studied the trapped magnetic flux behavior of large single-grain GdBa 2 Cu 3 O 7-δ (GdBCO) bulk cryomagnet. Here, an off-axis field cooled magnetization is proposed, in which magnetic field was applied with an angle θ with respect to the crystallographic c-axis of the bulk, i.e., off-axis geometry. To verify the trapped flux performance, the flux density parallel to the c-axis was scanned with Hall sensor on the surface of the bulk after off-axis field cooled magnetization. It was found that the trapped flux component along the c-axis still remains strong with up to θ = 30°, and the total flux over the bulk surface reaches over 97% compared with that when θ = 0°. These results demonstrate GdBCO bulk can nearly maintain their full trapped flux potential along the c-axis under off-axis field cooled magnetization with finite inclination angle. Thus, the present off-axis magnetization can provide an alternative choice for the magnetization geometry design in HTS applications using bulks, when the magnetizing field cannot be applied perfectly aligned with the crystallographic c-axis of the bulk.

The accessible performance range for most magnetic confinement plasma physics devices expands markedly with increasing magnetic flux density. The MIT Plasma Science and Fusion Center is investigating the use of high-temperature superconductors as a low cost means to significantly enhance the performance characteristics of small to moderate scale devices. Our initial investigation emphasized the no-insulation winding technique as a means to produce highly stable dc magnets for devices operating at moderate to high magnetic flux density. We present design, manufacture, and test results for a double pancake coil wound from a 500 m length of 12 mm wide REBCO tape. The coil provides on-axis magnetic flux density within an 8 cm clear bore in excess of 0.5 T when operated in liquid nitrogen and in excess of 6 T when operated in liquid helium. The ultimate aim of the program is to develop conduction-cooled HTS coil modules that can be used for both linear and toroidal plasma devices.

We investigated in detail the flux pinning properties around B || ab for GdBCO-coated conductors with crossed columnar defects produced by heavy-ion irradiation, where the crossing angle Θi was systematically controlled in the range from ±5° to ±15° relative to the ab plane. A single peak of critical current density Jc centered at B || ab occurs for the crossing angle of Θ i = ±5°, where an upward shift in Jc can be observed compared to an unirradiated sample. As the crossing angle is larger, the Jc peak becomes smaller and broader while maintaining the peak at B || ab. Increasing Θi up to ±15°, three Jc peaks emerges at B || ab and at the two irradiation angles, independently from the others. The crossover behavior from the single peak to the triple one around the ab plane is attributed to the strong line tension energy of flux lines around B || ab , which derives from the elliptical nature of the core of flux lines in anisotropic superconductors.

Low-temperature superconducting MRI magnets are usually cooled by a large amount of liquid helium (LHe); however, LHe is getting scarcer and its price has been rising. Therefore, the development of helium free high-temperature superconducting (HTS)-MRI magnet is expected. HTS-MRI magnet fabricated by REBCO tape superconductors has mainly two problems for producing a highly stabilized magnetic field for MRI imaging. One problem is current fluctuation of a power supply, which is necessary for excitation because the permanent current operation of HTS-MRI magnet is difficult. Another is a screening current in REBCO tapes, which affect the magnetic field stability. The MRI magnetic field feedback control test was carried out, that is, the overshooting and the current feedback control, using 32 H HTS-MRI test magnet and the highly stabilized power supply. The magnetic field was evaluated under the current feedback control at 1.5 T (66 A). By the overshooting and the current feedback control, the magnetic field stability was improved to 0.7 ppm/ h which is enough for MRI imaging.

The decreasing of critical current in the high-temperature tape superconductor (HTS) can be prevented by decreasing of the perpendicular component of self-magnetic field. The coating of this tape by the ferromagnetic material can deform the magnetic field around the HTS tape with flowing transport current. The present report contains the finite-element analysis of the effect of ferromagnetic coating near the HTS tape. The optimal configuration of the ferromagnetic coating for minimizing the perpendicular component of magnetic field was found. In present case, it reduces from 0.04 T to less than 0.01 T. The optimal shape of the coating contains the gap along the lateral side of the HTS tape. The influence of the twisting of the tape coated by ferromagnetic was also investigated. It was found that the difference between twisted and nontwisted cables was not large. The study was also performed for the stack of HTS tapes where neighbor tapes have the opposite direction of the transport current. It was found that the ferromagnetic coating and ribs make the magnetic field near edges smooth.

In the past years, different superconducting levitation systems were developed around the world. Most projects focused on optimizing the flux density in the working space of the superconductor in order to maximize the levitation force of the system. In general, these systems use tracks with permanent magnetic Halbach arrays or combine permanent magnets with steel yokes to maximize the usable flux density. Therefore, none of these tracks can easily be switched off to allow a safe crossing or to build a fast turnout switch without moving parts. A new track was designed containing permanent magnets and a YBCO-tape-coil to realize a fast switchable track. In this concept, the permanent magnets provide the flux used for levitation, whereas the coil controls the flux in the space above the track, where levitation usually takes place. The magnetic behavior of this new design was studied and improved with two-dimensional (2-D) and 3-D simulations. Afterwards, a demonstrator was built on which the induced magnetic field was measured in the different operation modes. The results of the measurements and the simulations indicate that the flux density above of the track might be controlled by the coil current. It was proven that the new track design generates magnetic fields comparable to the standard permanent magnetic track, whereas only residual fields are measured in inactive mode, which can be further reduced with negative coil currents.

The use of bulk high-temperature superconductor material has been proposed for various noncontact transportation systems. Commonly, such systems use permanent magnets to achieve levitation of a vehicle housing bulk superconductors, with motive force provided separately. Recent work has focused on using electromagnets to provide both the levitation and the motive force. Through energizing appropriately placed electromagnets in a correct pattern, straight line travel and rotation across a horizontal plane are possible. However, significant overshoot and instability can be incurred during motion events, leading to changes in the flux pinning state and misalignment of a vehicle. This paper proposes the design of a planar array of electromagnets and an energization pattern that achieves controlled rotation. Straight line travel is also achieved. Thus, using the proposed drive system, straight-line travel as well as rotation of a levitated vehicle across a horizontal plane is possible. Experimental verification is provided.

Applicability of the scanning permanent magnet method to the measurement of the critical current density in a high-temperature superconducting (HTS) film containing a crack with the consideration of a depth has been investigated numerically. To this end, a numerical code has been developed for analyzing the shielding current density in a multiple-layered HTS film with a crack. By using the code, the scanning permanent magnet method has been reproduced numerically. The results of the computations show that the disturbed area around a crack can be quantified by the introduction of a defect parameter. Moreover, it is found that the area increases with a strength of the magnetic flux density. On the other hand, the distinction between internal and surface cracks is not always obvious since the defect parameter for an internal crack is approximately equal to its parameter for a surface crack. However, the scanning permanent magnet method proved to detect the even internal crack.

© 2002-2011 IEEE. Stacked-coated conductor tapes have demonstrated superior properties over bulk superconductors, including the flexible stacking configuration, appealing mechanical properties, and the good cross field performance. This makes stacked high-temperature superconducting (HTS) tapes a promising alternative to HTS bulks in many applications despite of its cost, such as rotary motors and levitation. The conventional magnetization methods, such as pulse field magnetization and field cooling, require large magnetic field to be applied on an area to cover the whole HTS stacks, which seriously affects its applicability. In this paper, HTS flux pumping, a novel technique developed for HTS coil energization, is proposed to be used to magnetize stacked HTS tapes. HTS tapes used in this paper have slits in the middle between two tape ends, called partially slit tape (PST). The PSTs operate as flat HTS coils in persistent current mode once magnetized. The experimental results show flux pumping, with the traveling-wave based mechanical moving magnet design, is able to magnetize the stacked PSTs at a weak level compared to its potential.

The effect of magnetic flux creep on the formation of resistive states of the composite superconductors has been studied taking into account their self-heating. The obtained results have been compared with the calculations carried out using the existing thermal stabilization theory, which is based on the model of a stepwise transition from the superconducting to normal state. It has been shown that, over a wide range of the superconductor temperature, this model leads to overrated effective electric resistances of the composite. As a result of its stable self-heating, the notions on the critical current, which determine the maximum transport current and on the resistive transition temperature, the higher of which in the transport current begins dividing between the superconductor and matrix, a loss a physical sense at magnetic flux creep, are used in the existing thermal stabilization theory. As a result, the limits of the theory of thermal stabilization of the composite superconductors can be extended if the theory has been used to describe stable sates at currents, which are higher than the conditionally defined critical current of the composite.

We devised a linear induction motor (LIM) that uses the close-ended coated superconductor coils as the secondary to explore the potential solutions of enhancing the thrust performance at large clearance, which is generally required for the linear propulsion of transit systems. In this proposal, the coated superconductor coils are embedded into a ferromagnetic yoke whose function is to guide and concentrate the travelling fields generated by the three-phase conventional stator. To make a preliminary study before the laboratory demonstration, we built a numerical model of this proposal by resorting to the finite-element software of MagNet, a recognized professional tool to simulate the electrical machines in the scientific community. Through a treatment of the power law, the developed numerical model has included the nonlinear constitutive law of superconductor, which is currently thought to be difficult in the MagNet environment. This general treatment takes a step forward toward the use of the professional software to perform complex simulations of the superconducting machines. Based on the numerical model, we carried out a set of transient studies on the proposed LIM to observe the distributions of travelling flux, the starting characteristics, and the time evolutions of thrust and speed. The achieved results clearly display that the proposed LIM has advantages over the conventional type and could be thereby regarded as a candidate for developing high thrust linear LIM with large clearance.

A high-frequency magnetometer with a sensitivity of 2.5·10–7 T/Hz1/2 based on two Hall transducers has been developed. It is used to measure magnetic field dependences of the density of captured magnetic flux in single-crystal samples of Bi2Sr2CaCu2O8+x and YBa2Cu3O7–x.

In order to study the commonly neglected magnetic field information in the course of levitation force measurement process in a superconducting maglev system, a multipoint magnetic field measurement platform was employed to acquire magnetic signals of a bulk high- $$T_{c}$$ superconductor on both the top and the bottom surface. Working conditions including field cooling (FC) and zero field cooling were investigated for these vertical down and up motions above a permanent magnet guideway performed on a HTS maglev measurement system. We have discussed the magnetic flux variation process based on the Bean model. A magnetic hysteresis effect similar to the levitation force hysteresis loop of the bulk superconductor was displayed and analyzed in this paper. What is more valuable, there exists some available magnetic flux on the top surface of the bulk superconductor, and the proportion is as high as 62.42% in the FC condition, which provides an experimental hint to design the superconductor bulk and the applied field for practical use in a more efficient way. In particular, this work reveals real-time magnetic flux variation of the bulk superconductor in the levitation application, which is the other important information in contrast to the macroscopic levitation and guidance force investigations in previous studies, and it enriches the existing research methods. The results are significant for understanding the magnetic characteristic of superconductors, and they can contribute to optimize the present HTS maglev system design.

The stability against magnetic flux jumps has experimentally been studied in the external magnetic field for three samples from NbTi composite superconductors, one monofilamentary and two multifilamentary. A comparison between the experiment and theory of thermomagnetic stability of composite superconductors has been carried out. We have determined threshold values of the rates of the external magnetic fields, starting from which heat capacity and conductivity of the normal composite matrix become determining stabilizing factors. For the first time, the increasing dependence of field of first magnetic flux jump on the rate of the rise in the external magnetic field has been experimentally registered in the superconducting wire for MRI. The reason for this effect is the shunting effect of a high pure copper matrix and the low volume fraction of a superconductor in the composite (~10%).

The processes of the formation of the macroscopic states of a superconducting tape induced by the transport current at magnetic flux creep have been studied. It has been shown that there are characteristic values of electric field intensity that are affected by a rate of current injection, properties of a superconductor, cooling conditions, and properties of a stabilizing matrix. These values are the basis of thermal electrodynamic mechanism, which determines the slope of a rise of IVC of technical superconductors. The conditions of formation of current instabilities have been studied taking into account a nonuniform temperature distribution over the cross section of a technical superconductor. The conditions of the existence of the IVC of technical superconductors have been formulated. These conditions allow for the stable heating of a superconductor as high as the critical temperature. The results of the carried out studies should be taken into account when measuring the IVC of superconducting materials and determining their critical parameters and the current of instability occurrence.

As diamagnetic materials, superconductors confine magnetic fields efficiently. Based on this property, we designed and fabricated a novel contactless power transfer (CPT) coupler consisting of a ferrite core wrapped with superconductor layers, which results in an obvious enhancement of the coupling coefficient. We analyzed the underlying causes of coupling enhancement with a magnetic circuit model. Additionally, we used finite-element simulations to investigate the performance of the structure as a function of applied frequency. The results predict that the coupler would be efficient up to 50 kHz. Thus, the modified coupler should be practical for CPT systems.

A phenomenon of magnetic relaxation stopping in a levitating superconductor was studied. It was experimentally shown that magnetic flux creep (diffusion of flux lines to regions with lower vortex density) is absent in magnetic suspension of the superconductor. Magnetic relaxation arises, when a rigid constraint that fixes a position of the superconductor relative to a magnet is imposed on a levitating object. It is assumed that oscillations of magnetic structure, which is due to free oscillations of the levitating superconductor, stop magnetic relaxation.

The nonlinear characteristics of high-temperature superconductors of the Bi–Sr–Ca–Cu–O system have been experimentally investigated in the temperature range of the superconducting transition under the influence of a harmonic alternating magnetic field. The effect of the generation of odd harmonics in the signal of response to a harmonic alternating magnetic field for multiphase high-temperature superconductors containing regions with different values of the critical temperature in their bulk has been observed for the first time. The mechanism of harmonic generation in a superconductor in the resistive state, which is associated with the switch effect, i.e., with the redistribution of eddy current density between the local regions of the superconductor, has been considered.

In this paper, we compare $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ samples fabricated by the infiltration growth process: the chosen samples have microstructures that are considered ideal for supporting high current densities; they have small spherical Y2BaCuO5 precipitates uniformly distributed in their superconductor matrices. However, they differ dramatically in their critical current density $J_{c}$ versus $H$ curves at high magnetic fields. It is expected from theory that flux pinning at high magnetic fields would be modified by defects of nanometric dimensions occurring in the superconductor. We have investigated the superconductor matrices of the samples using a nanoindenter, with the objective of probing differences in nanometric volumes within them. The measurements show highly enhanced nanohardness and elastic modulus in the superconductor matrices of samples that support high current densities at high magnetic fields; such an enhancement is absent in a sample whose current density, although starting at a high value at zero magnetic field, falls rapidly with increasing field. The enhanced nanohardness in samples showing enhanced current density at high magnetic fields has been attributed to nanometer-sized defects associated with twinning present in the superconductor matrix.

Superconducting flux pumps enable dc supercurrents to be injected into a superconducting coil without direct connection to an external current supply. Here, we report experimental data from a dynamo-type flux pump employing a high-T c superconducting stator. This device employs a mechanical rotor that rotates outside the cryogenic envelope, and excites the superconducting circuit through the cryostat wall via a magnetic circuit formed between the rotor and stator. We show that the width of the stator wire employed in this device has a significant effect on its output performance. At low frequencies, both the short circuit current, I sc , and the open-circuit voltage, V oc increase with stator width, and we obtain a maximum value of I sc > 340 A using a stator wire width of 46 mm. However at higher operating frequencies, we observe that I sc reaches a maximum value before dropping with any further increase in stator width. We attribute this to thermally dissipative effects due to circulating eddy currents in these wide superconducting stators. At frequencies above ~400 Hz, we observe a sharp rise in the internal resistance of the stator wire that we attribute to a local quench due to the eddy currents driven by the local electromotive force (EMF) beneath the rotor magnet. This paper emphasizes the importance of frequency dependence when defining optimum operating parameters of a high-T c superconductor (HTS) flux pump. Our results also offer the promise that very high current flux pumps may be developed through maximizing the ratio of the stator wire width to the peak width of the rotor field profile.

A heterostructure consisting of a cuprate superconductor YBa2Cu3O7-δ and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve was studied using SQUID magnetometry, ferromagnetic resonance, and neutron reflectometry. It is shown that because of the magnetic proximity effect a magnetic moment is excited in the superconducting portion of the heterostructure, whereas the magnetic moment in the spin valve becomes suppressed. The experimentally obtained value of a typical penetration depth of a magnetic moment into the superconductor is significantly greater than the coherence length of the cuprate superconductor, which indicates that the induced magnetic moment mechanism of Cu atoms is dominant. The mesastructure prepared by adding niobium film as a second superconducting electrode to the existing heterostructure, exhibited a superconducting current (dc Josephson effect) at interlayer thicknesses that are much greater than the coherence length of the ferromagnetic materials. The maximum of the critical current density dependence on the thickness of the spin valve material corresponds to the interlayer coherence length, which agrees with the theoretical predictions associated with spin-triplet pairing. The superconducting current is observed at magnetic fields that are two orders of magnitude greater than the field corresponding to the occurrence of one magnetic flux quantum in the mesastructure. The ratio of the second harmonic of the current-phase dependence of the mesastructure superconducting current to the first, determined according to the dependence of the Shapiro steps on the amplitude of microwave exposure, did not exceed 50%.

A high-temperature superconductor (HTS) field coil with a novel, step-shaped cross section is proposed for large-capacity superconducting machine applications. The design of the structure is explained in detail, and comparisons are made with conventional coils. The electromagnetic characteristics are also analyzed in comparison with the conventional alternative. From the analysis, the overlapping topology can reduce the volume of the generator and achieve better electromagnetic characteristics by eliminating the magnetic flux sag produced by the adjacent field coils, while also decreasing the iron loss.

Three YBCO bulk superconductors with different critical current densities are used to investigate the magnetic force of a simple superconductor–magnet system under different relative speed values between YBCO and a magnet. The initial cooling processes for superconductors include zero field cooled (ZFC) and field cooled (FC). Crossing phenomena can be observed both in force–displacement hysteresis curves of superconductors with high and very low critical current densities. The maximum repulsive force under different relative speed values is analyzed. For the high-performance superconductor, the ZFC maximum repulsive force increases at first and reaches a peak value and then decreases with the increase of relative speed. When the magnet reaches 0.1 mm above the minimum clearance and its speed is changed to 0.1 mm/s for 1 s, the levitation force increases a lot at first and then decreases during this very slow approaching process to the minimum clearance. Based on the experimental results and analysis, the crossing phenomena reflect the effect of vortex dynamic and are directly related to the flux balance between trapped magnetic field and flux creep.

In thermodynamics studies, the problem of the work done by magnetized materials is often approached with a general theoretical approach. Sometimes, ambiguities can be found in the demonstrations, because some authors do not clearly specify the terms in the relations. Conversely, a definition of “heat of magnetization” is not given in a general complete way. Some engineers and physicists use practical relations to calculate power dissipated on superconducting materials during magnetization cycles, but it is not possible to find theoretical works that justify the validity of these relations in general situations, for example, as instantaneous losses. In this paper, we want to approach both these aspects with a more complete analysis.

The physical features of the formation of macroscopic states of superconducting composites consisting of a superconductor and a coating under flux creep are discussed. It is demonstrated that there exist characteristic electric field strengths depending on the properties of the superconductor, cooling conditions, and characteristics of the stabilizing coating, which affect the intensity of the E-I characteristics of the superconducting composites. Analysis shows that the measurements of the critical properties of superconductors can be accompanied by a nonuniform electric field distribution over the composite cross section and high stable superheating of the superconductor, which do not lead to superconductivity breaking.