Trapped Magnetic Field Research Articles

The Use of Very Thin TFMs in Electromechanical Devices

The contribution offered by this paper is to point out the advantage gained by the use of ultra thin trapped field magnets (TFMs) in electromechanical devices. They closely match the torque and force generation capability of their thicker counterparts due to the small reluctance paths they present to the stator side of the magnetic circuit. These conclusions are based on recent experimental progress in the production of these thin TFMs which show they can support critical current densities in the range of 47 kA/cm 2 at 77 K. The results prove that the default electromechanical design objective of maximizing the TFM air gap magnetic field density should be reconsidered. Because of the reduction in the reluctance of the stator field path, substantial torques result even when the volume of superconducting material is vanishingly small. A second advantage is that the stator windings can more easily activate the rotor TFMs.

Effects of Fullerene Addition on Pinning Properties and Microstructures in Melt-Textured Y-Ba-Cu-O Superconductors

Bulk Y-Ba-Cu-O superconductors have significant potential for engineering applications due to high critical current density ( J c) and high trapped magnetic fields, which is attributed to the presence of pinning centers such as micro-sized Y2BaCuO5 (Y211) inclusions. The introduction of nanosized particles is known to function as more effective pinning centers than micro-sized ones. It has been reported that J c values were enhanced with the addition of nanosized particles such as ZrO2 and Y2Ba4CuMOy (M = Nb, Zr, etc.) in the YBa2Cu3O x (Y123) matrix. We have focused on nanocarbon as another candidate of the pinning center and reported that the addition of carbon nanotube (CNT) and nanodiamond led to the enhancement of J c in Y-Ba-Cu-O superconductors. Fullerene (C60) is also expected to improve the flux pinning performance. We then fabricated melt-processed Y-Ba-Cu-O bulk samples with the addition of fullerene and investigated the pinning properties and microstructures. SEM observation showed that nanosized particles were distributed in the Y123 matrix. J c exhibited the highest values for the sample with 1.0 wt% fullerene addition. These results suggest that fullerenes act as effective pinning centers.

Analysis of the microstructure of bulk MgB2 using TEM, EBSD and t-EBSD.

EBSD analysis can provide information about grain orientation, texture and grain boundary misorientation of bulk superconducting MgB2 samples intended for supermagnet applications. However, as the grain size of the MgB2 bulks is preferably in the 100-200 nm range, the common EBSD technique operating in reflection mode works only properly on highly dense samples. In order to achieve reasonably good Kikuchi pattern quality on all types of MgB2 samples, we apply here the newly developed transmission EBSD (t-EBSD) technique to spark-plasma sintered MgB2 samples. This method requires the preparation of TEM slices by means of focused ion-beam milling, which are then analysed within the SEM, operating with a custom-built sample holder. To obtain multiphase scans, we identified the Kikuchi pattern of the MgB4 phase which appears at higher reaction temperatures and may act as additional flux pinning sites. We present here for the first time EBSD mappings of multiple phases, which include MgB2 , MgB4 and MgO. LAY DESCRIPTION: The electron backscatter diffraction (EBSD) technique operating in the scanning electron microscope provides information on the crystallographic orientation the material by recording Kikuchi patterns. In polycrystalline samples, it becomes possible to analyse the orientations of the grains to each other. The metallic superconductor with the currently highest superconducting transition temperature, MgB2 with a Tc of 38.5 K, can be used in applications in polycrystalline form. One such application of interest are trapped field magnets or supermagnets, where the superconductor cooled in an applied magnetic field can trap the magnetic field as vortices at numerous flux pinning sites in the sample. When the external magnetic field is removed, the sample will stay magnetised as long as it is kept cool, and importantly, the trapped magnetic fields can be much higher as for any permanent magnet. However, the small size of the MgB2 grains in the 100-200 nanometre range requires a different approach when using the EBSD technique on such samples. The recently developed EBSD technique working in transmission mode (t-EBSD) helps considerably to image such materials. In this approach, a tiny TEM slice has to be milled out from the original sample by using focused ion beam milling. To understand the properties of the flux pinning in the spark-plasma sintered MgB2 sample, we had to identify the Kikuchi pattern of MgB4 , which is another, non-superconducting phase appearing at higher reaction temperatures required to compact the material. Using this information, we could perform EBSD scans using three different phases, MgB2 , MgB4 and MgO. The EBSD mappings enable to see where the secondary phase particles are located in the sample, and to judge if the particles could work as flux pinning sites.

Superconducting YBCO Foams as Trapped Field Magnets.

Superconducting foams of YBa2Cu3Oy (YBCO) are proposed as trapped field magnets or supermagnets. The foams with an open-porous structure are light-weight, mechanically strong and can be prepared in large sample sizes. The trapped field distributions were measured using a scanning Hall probe on various sides of an YBCO foam sample after field-cooling in a magnetic field of 0.5 T produced by a square Nd-Fe-B permanent magnet. The maximum trapped field (TF) measured is about 400 G (77 K) at the bottom of the sample. Several details of the TF distribution, the current flow and possible applicatons of such superconducting foam samples in space applications, e.g., as active elements in flux-pinning docking interfaces (FPDI) or as portable strong magnets to collect debris in space, are outlined.

2-D Numerical Modeling of a Bulk HTS Magnetization Based on <bold>H</bold> Formulation Coupled With Electrical Circuit

Bulk high temperature superconductors (HTS) can be magnetized and act as permanent magnet much stronger than conventional ones as NdFeB. The design of the inductor is a key point to perform the desired magnetization of the HTS bulk. In this paper, we focus on modeling a pulsed field magnetization (PFM) process of an HTS bulk using a coil powered with a magnetizer. The built model is a 2-D axisymmetric problem, based on the H formulation and coupled with electrical equations though the magnetic flux seen by the magnetizing coil. The calculation of this magnetic flux in the H formulation is not trivial and was validated using magnetic vector potential formulation on a coil in the air. Assuming different operating conditions, the bulk HTS is then modeled using four different properties corresponding to air, perfect diamagnetic, copper, and HTS. It was shown that simulating a PFM process could lead to different value of peak current and applied magnetic field to the bulk HTS, depending on the critical current density of the bulk, for example. These variations are in the range of the air and diamagnetic cases. Therefore, the proposed method should be used in order to predict a realistic trapped magnetic field in the HTS bulk by taking into account its reaction seen by the coil during the PFM process.

Numerical modelling of mechanical stresses in bulk superconductor magnets with and without mechanical reinforcement

The magnetic field trapping capability of a bulk superconductor is essentially determined by the critical current density, Jc(B, T), of the material. With state-of-the-art bulk (RE)BCO (where RE = rare earth or Y) materials it is clear that trapped fields of over 20 T are potentially achievable. However, the large Lorentz forces, FL = J × B, that develop during magnetisation of the sample lead to large mechanical stresses that can result in mechanical failure. The radial forces are tensile and the resulting stresses are not resisted well because of the brittle ceramic nature of (RE)BCO materials. Where fields of more than 17 T have been achieved, the samples were reinforced mechanically using resin impregnation and carbon-fibre wrapping or shrink-fit stainless steel. In this paper, two-dimensional (2D) axisymmetric and three-dimensional (3D) finite-element models based on the H-formulation, implemented in the commercial finite element software package COMSOL Multiphysics, are used to provide a comprehensive picture of the mechanical stresses in bulk superconductor magnets with and without mechanical reinforcement during field-cooled magnetisation. The chosen modelling framework couples together electromagnetic, thermal and structural mechanics models, and is extremely flexible in allowing the inclusion of various magnetisation processes and conditions, as well as detailed and realistic properties of the materials involved. The 2D model—a faster route to parametric optimisation—is firstly used to investigate the influence of the ramp rate of the applied field and any heat generated in the bulk. Finally, the 3D model is used to investigate the influence of inhomogeneous Jc(B, T) properties around the ab-plane of the bulk superconductor on the developed mechanical stress.

Trapped Field and Levitation Performance of a YBCO Superconductor Magnetized in Different External Magnetic Fields

In order to find a relative preferable method to improve the trapped magnetic field (Btrap) of a YBCO superconductor, the influences of field-cooling height (FCH), magnetization intensity, and magnetization process on the trapped fields of the high temperature superconductor (HTS) magnetized in the magnetic fields of the permanent magnet guideway (PMG) and the superconducting magnet (SM) were both studied. In addition, the levitation forces of the bulk magnetized in the two different magnetic fields were also investigated in this paper. The experimental results showed that the maximum trapped field ratio (δtrap) of the bulk magnetized above the PMG attained the largest value, when the initial FCH was 30 mm. The remagnetization process was a preferable method to increase the Btrap, in the case that the FCH was higher than 30 mm. When the exciting current (Ie) of the SM was smaller than 15 A, the remagnetization process was an effective method to improve the Btrap. The remagnetization performance of the bulk attained the saturation, under the condition that the Ie was larger than 15 A. In the aspect of the levitation performance, it can be found that the decreasing amplitude of the force with the Btrap became larger, when the levitation gap was lower. The force acted on the bulk increased with increasing the magnetic fields of the SM. Moreover, the results also indicated that the initial Btrap was larger; the maximum levitation force (Flmax) became smaller. The magnetic hysteresis of the bulk was found to decrease with increasing the Btrap.

Stabilization of magnetic mirror machine using rotating magnetic field

An increase in symmetry is observed for a low density non-collisional plasma, in a simple magnetic mirror machine, due to the application of external oscillating magnetic fields of 1.5 MHz frequency. The increase in symmetry is attributed to an increase in stability of the flute mode and is dependent on the field’s polarization and trap magnetic field strength.

Advantages of multi-seeded (RE)–Ba–Cu–O superconductors for magnetic levitation applications

Large, single grain (RE)Ba2Cu3O7 (where RE is a rare-earth element or yttrium) high temperature superconductors are technologically important materials due to their ability to trap large magnetic fields and to provide stable magnetic levitation for a number of potential high field applications. The fabrication of samples in the large singe grain form is a challenge, however, due to the characteristic slow growth rate of these materials and the need to produce samples that are electrically well-connected in order to generate trapped magnetic fields that are significantly greater than those produced by conventional permanent magnets (PM). In this work, we investigate whether large, single grain samples are optimum for the generation of high levitation forces for engineering applications. Three large bar-shaped Y–Ba–Cu–O samples of dimensions 60 × 20 × 12 mm3 were prepared for this investigation, including one single-seeded, one multi-seeded and one consisting of three square samples packed together closely in an array. The processing of these samples is described and their trapped field and levitation performance at 77 K measured using different PM arrays. We find that the multi-seeded samples and those assembled from smaller, individual bulk superconductors are able to achieve a higher levitation force than an equivalent single seed sample arrangement, at least in some geometries. This result is significant in that it suggests clearly that it is not always necessary to fabricate bulk (RE)BCO superconductors in the form of very large single grains for levitation applications, although the specific configuration of the system does need to be considered on an application-by-application basis.

Toward Optimization of Multi-Pulse, Pulsed Field Magnetization of Bulk High-Temperature Superconductors

Pulsed field magnetization (PFM) is the most practical method for magnetizing bulk superconducting materials as trapped field magnets, but the record trapped field achieved by PFM to date is still significantly less than the true trapped field capability of these materials. In this paper, a flexible numerical modeling technique based on the finite element method is used to provide a comprehensive and realistic picture of multi-pulse PFM, which has been shown to be effective in increasing the trapped field/flux over a single pulse. First, the maximum trapped field capability of a representative sample is determined using two types of numerical model simulating field-cooling and zero-field-cooling magnetization. Next, various sets of magnetic field pulses are applied to the bulk to analyze multi-pulse PFM. An increase in the trapped field can be achieved after a second pulse and to do so an increased amplitude of applied field is required to maximize the trapped field fully. The numerical analysis shows that this occurs in subsequent pulses because it is more difficult for the magnetic flux to penetrate the sample and there is a lower temperature rise.

Modifying the Pulsed-Field-Magnetization Technique for HTS Bulks in Electrical Machines Without Magnetic Field Sensors

This paper studies the high-temperature superconductor (HTS) YBCO and GdBCO bulks pulse-field magnetization (PFM) in loco , that is, when inserted in an electric machine's magnetic circuit. The PFM typically uses cryogenic hall sensors to monitor the magnetization process. However, to install cryogenic hall sensors in the magnetic excitation circuit of an electrical machine may increase the air gap and increase the machine cost. To surpass that, this paper presents an indirect technique capable of monitoring the superconductor magnetization process through the applied pulse voltage and associated electric current transient waveform. Last, different levels and the numbers of pulses were applied to magnetize the HTS bulks inside the magnetic circuit to analyze the resultant trapped magnetic field. Differences between the YBCO and GdBCO are highlighted and discussed.

Measurements of crossed-field demagnetisation rate of trapped field magnets at high frequencies and below 77 K

Design requirements of the next generation of electric aircraft place stringent requirements on the power density required from electric motors. A future prototype planned in the scope of the European project ‘Advanced Superconducting Motor Experimental Demonstrator’ (ASuMED) considers a permanent magnet synchronous motor, where the conventional ferromagnets are replaced with superconducting trapped field magnets, which promise higher flux densities and thus higher output power without adding weight. Previous work has indicated that stacks of tape show lower cross-field demagnetisation rates to bulk (RE)BCO whilst retaining similar performance for their size, however the crossed-field demagnetisation rate has not been studied in the temperature, the magnetic field and frequency range that are relevant for the operational prototype motor. This work investigates crossed-field demagnetisation in 2G high temperature superconducting stacks at temperatures below 77 K and a frequency range above 10 Hz. This information is crucial in developing designs and determining operational time before re-magnetisation could be required.

Development of HTS Bulk NMR Relaxometry With Ring-Shaped Iron

The compact NMR magnets consisted of the stacked high-temperature superconducting (HTS) GdBCO bulk annuli trapped by field cooling (FC) method have been developed. The target values of strength and homogeneity of magnetic field for proposed NMR relaxometry are 1.5 T and 150 ppm/cm3, respectively, at liquid nitrogen temperature. Since last year, we have been developing a hardware and software to detect the NMR signals by ourselves. Therefore, we needed to determine the final structure of HTS bulk magnet to be used in the NMR relaxometry. We can obtain the trapped magnetic field over 1.5 T at 77.4 K experimentally. However, it is still difficult to obtain the magnetic field homogeneity under 150 ppm/cm3 experimentally, due to their different superconducting properties. Therefore, in this study, four types of HTS bulk magnets to determine the final structure of compact NMR relaxometry were designed and their field properties including the degraded HTS bulk were investigated using three-dimensional finite-element method analysis. The field properties of 10-stacked HTS bulk magnets with the ring-shaped irons for field compensation and the degraded HTS bulk were investigated experimentally.

A robust seeding technique for the growth of single grain (RE)BCO and (RE)BCO–Ag bulk superconductors

Bulk, single grains of RE–Ba–Cu–O [(RE)BCO] high temperature superconductors have significant potential for a wide range of applications, including trapped field magnets, energy storage flywheels, superconducting mixers and magnetic separators. One of the main challenges in the production of these materials by the so-called top seeded melt growth technique is the reliable seeding of large, single grains, which are required for high field applications. A chemically aggressive liquid phase comprising of BaCuO2 and CuO is generated during the single grain growth process, which comes into direct contact with the seed crystal either instantaneously or via infiltration through a buffer pellet, if employed in the process. This can cause either partial or complete melting of the seed, leading subsequently to growth failure. Here, the underlying mechanisms of seed crystal melting and the role of seed porosity in the single grain growth process are investigated. We identify seed porosity as a key limitation in the reliable and successful fabrication of large grain (RE)BCO bulk superconductors for the first time, and propose the use of Mg-doped NdBCO generic seeds fabricated via the infiltration growth technique to reduce the effects of seed porosity on the melt growth process. Finally, we demonstrate that the use of such seeds leads to better resistance to melting during the single grain growth process, and therefore to a more reliable fabrication technique.

Effect of cooldown and residual magnetic field on the performance of niobium–copper clad superconducting radio-frequency cavity

We present the results of rf measurements on a niobium–copper clad superconducting radio-frequency cavity with different cooldown conditions and residual magnetic field in a vertical test Dewar in order to explore the effect of thermal current induced magnetic field and its trapping on the performance of the cavity. The residual resistance, extracted from the Q0(T) curves in the temperature range 4.3–1.5 K, showed no dependence on a temperature gradient along the cavity during the cooldown across the critical temperature up to ∼50 K m−1. The rf losses due to the trapping of residual magnetic field during the cavity cooldown were found to be ∼4.3 nΩ μT−1, comparable to the values measured in bulk niobium cavities. An increase of residual resistance following multiple cavity quenches was observed along with evidence of trapping of magnetic flux generated by thermoelectric currents.

Microstructure and trapped field of YBCO bulk single-grain superconductors prepared by interior seeding

The microstructural analyses of YBCO bulk single-grain superconductors grown by interior seeding with taller and shorter upper pellets have shown that a suitable upper pellet height can lower the porosity in the upper part of the sample, produce a more appropriate distribution of pinning centres in the form of Y-211 particles and suppress subgrain formation with a higher crystal misalignment in the c-growth sector (c-GS), which can lead to a higher measured trapped magnetic field and a more uniform cone of the trapped-field profile. The observed bulging of the sample surface at the c-GS can be explained by the edge melt distribution model, which shows that macroscopic mass transport to the growth sector occurs with higher growth rates.

Numerical modelling of iron-pnictide bulk superconductor magnetization

Iron-based superconductors exhibit a number of properties attractive for applications, including low anisotropy, high upper critical magnetic fields (Hc2) in excess of 90 T and intrinsic critical current densities above 1 MA cm−2 (0 T, 4.2 K). It was shown recently that bulk iron-pnictide superconducting magnets capable of trapping over 1 T (5 K) and 0.5 T (20 K) can be fabricated with fine-grain polycrystalline Ba0.6K0.4Fe2As2 (Ba122). These Ba122 magnets were processed by a scalable, versatile and low-cost method using common industrial ceramic processing techniques. In this paper, a standard numerical modelling technique, based on a 2D axisymmetric finite-element model implementing the H-formulation, is used to investigate the magnetisation properties of such iron-pnictide bulk superconductors. Using the measured Jc(B, T) characteristics of a small specimen taken from a bulk Ba122 sample, experimentally measured trapped fields are reproduced well for a single bulk, as well as a stack of bulks. Additionally, the influence of the geometric dimensions (thickness and diameter) on the trapped field is analysed, with a view of fabricating larger samples to increase the magnetic field available from such trapped field magnets. It is shown that, with current state-of-the-art superconducting properties, surface trapped fields >2 T could readily be achieved at 5 K (and >1 T at 20 K) with a sample of diameter 50 mm. Finally, an aspect ratio of between 1 and 1.5 for R/H (radius/thickness) would be an appropriate compromise between the accessible, surface trapped field and volume of superconducting material for bulk Ba122 magnets.

Nonadiabatic losses from radio-frequency-dressed cold-atom traps: Beyond the Landau-Zener model

Non-adiabatic decay rates for a radio-frequency dressed magnetic trap are calculated using Fermi's Golden Rule: that is, we examine the probability for a single atom to make transitions out of the dressed trap and into a continuum in the adiabatic limit, where perturbation theory can be applied. This approach can be compared to the semi-classical Landau-Zener theory of a resonant dressed atom trap, and it is found that, when carefully implemented, the Landau-Zener theory overestimates the rate of non-adiabatic spin flip transitions in the adiabatic limit. This indicates that care is needed when determining requirements on trap Rabi frequency and magnetic field gradient in practical atom traps.

A new ring-shape high-temperature superconducting trapped-field magnet

This paper presents a new trapped-field magnet made of second-generation high-temperature superconducting (2G HTS) rings. This so-called ring-shape 2G HTS magnet has the potential to provide much stronger magnetic fields relative to existing permanent magnets. Compared to existing 2G HTS trapped- field magnets, e.g. 2G HTS bulks and stacks, this new ring-shape 2G HTS magnet is more flexible in size and can be made into magnets with large dimensions for industrial applications. Effective magnetization is the key to being able to use trapped-field magnets. Therefore, this paper focuses on the magnetization mechanism of this new magnet using both experimental and numerical methods. Unique features have been identified and quantified for this new type of HTS magnet in the field cooling and zero field cooling process. The magnetization mechanism can be understood by the interaction between shielding currents and the penetration of external magnetic fields. An accumulation in the trapped field was observed by using multiple pulse field cooling. Three types of demagnetization were studied to measure the trapped-field decay for practical applications. Our results show that this new ring-shape HTS magnet is very promising in the trapping of a high magnetic field. As a super-permanent magnet, it will have a significant impact on large-scale industrial applications, e.g. the development of HTS machines with a very high power density and HTS magnetic resonance imaging devices.

Establishment of a biophysical model to optimize endoscopic targeting of magnetic nanoparticles for cancer treatment.

Superparamagnetic iron oxide nanoparticles (SPION) may be used for local tumor treatment by coupling them to a drug and accumulating them locally with magnetic field traps, that is, a combination of permanent magnets and coils. Thereafter, an alternating magnetic field generates heat which may be used to release the thermosensitively bound drug and for hyperthermia. Until today, only superficial tumors can be treated with this method. Our aim was to transfer this method into an endoscopic setting to also reach the majority of tumors located inside the body. To find the ideal endoscopic magnetic field trap, which accumulates the most SPION, we first developed a biophysical model considering anatomical as well as physical conditions. Entities of choice were esophageal and prostate cancer. The magnetic susceptibilities of different porcine and rat tissues were measured with a superconducting quantum interference device. All tissues showed diamagnetic behavior. The evaluation of clinical data (computed tomography scan, endosonography, surgical reports, pathological evaluation) of patients gave insight into the topographical relationship between the tumor and its surroundings. Both were used to establish the biophysical model of the tumors and their surroundings, closely mirroring the clinical situation, in which we could virtually design, place and evaluate different electromagnetic coil configurations to find optimized magnetic field traps for each tumor entity. By simulation, we could show that the efficiency of the magnetic field traps can be enhanced by 38-fold for prostate and 8-fold for esophageal cancer. Therefore, our approach of endoscopic targeting is an improvement of the magnetic drug-targeting setups for SPION tumor therapy as it holds the possibility of reaching tumors inside the body in a minimal-invasive way. Future animal experiments must prove these findings in vivo.