References of "Fagnard, Jean-François"
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See detailBehaviour of bulk superconducting trapped field magnets subjected to magnetic fields with a transverse component
Vanderbemden, Philippe ULg; Caps, Hervé ULg; Vanderheyden, Benoît ULg et al

Conference (2016, November 10)

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating ... [more ▼]

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating machines and levitation devices. Compared to conventional permanent magnets, however, the magnetic field generated by a superconducting trapped field magnet is much more sensitive to the presence of an external field that is not perfectly aligned with the superconductor magnetic moment, i.e. when the external field has a transverse component. A situation where the superconductor experiences a magnetic field with a transverse component is likely to happen several times during running operation of the device. As an example, in a “trapped flux” synchronous machine, any variation of the load of the shaft may lead to a slight misalignment of the magnetic moment of the superconductor (placed in the rotor) with respect to the rotating field (generated by the stator). In such a situation, the superconductor is subjected to a rotating field, and the situation is likely to be different from a situation where the superconductor is subjected to a pure transverse field. In the present work, we aim at examining different scenarios (i.e. transverse or rotating) where the trapped field magnet is subjected to a transverse component. These configurations are examined both experimentally and numerically. The numerical model is used to determine the distribution of currents in the cross-section of the superconductor. We examine the conditions where the rotating field leads to a partial remagnetization of the sample and is therefore less detrimental that the application of a pure transverse field. We also show that the magnetic moment of the sample may behave differently that the field at the surface of the superconductor. Finally, we investigate the behavior of structures where the superconductor is attached to a thin ferromagnetic disk, and show the beneficial effect of such hybrid structures compared to the superconductor alone. Acknowledgements : The research was funded through the University of Liège (ULg) and an ARC grant for Concerted Research Actions, financed by the French Community of Belgium (Wallonia-Brussels Federation), under reference ARC 11/16-03. We greatly acknowledge M. Morita, S. Nariki and H. Teshima from Nippon Steel & Sumitomo Metal Corporation, Futtsu, Chiba. [less ▲]

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See detailMagnetic shielding above 1 T at 20 K with bulk, large grain YBCO tubes made by Buffer-aided Top Seeded Melt Growth.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Kumar, Nambury Devendra et al

Poster (2016, September 07)

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the ... [more ▼]

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the present work is to study experimentally the shielding performance of YBCO tubes obtained by Buffer-aided Top Seeded Melt Growth fabrication process (BA-TSMG). This fabrication process enables the tube to be closed at one extremity by a cap containing the seed and there is no air gap between the cap and the tube. The shielding effectiveness is characterized by two parameters: (i) the shielding factor, defined as the ratio between the applied magnetic induction and the local magnetic induction measured inside the shield, and (ii) the threshold induction, i.e. the applied magnetic induction above which a given value of the shielding factor cannot be achieved. The magnetic shielding performances of tubes with different geometry are characterized at 77 K in liquid nitrogen. Further magnetic shielding measurements are carried out on one tube at various temperatures ranging from 20 K to 77 K using a Physical Property Measurement System (PPMS). The tubes are subjected to a quasi-static (“DC”) uniform magnetic field. A Hall probe measures the axial component of the local magnetic induction along the axis of the tube as a function of the applied magnetic induction. In order to investigate how the cap prevents the magnetic flux from penetrating inside the tube, we also characterize open tubes where the cap is removed and compare their properties to those of closed tubes. Magnetic shielding measurements show that the threshold induction increases by a factor of 9 as temperature decreases from 77 K to 20 K. Measurements also show that the presence of the cap improves the shielding performance at the closed extremity of the order of 1000 as it reduces the penetration through the open end. Near the closed extremity, a threshold induction of 1.5 T was reached at 20 K. To our knowledge this threshold induction is the best value reported so far at 20 K, and is comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. These results give evidence that efficient magnetic shields can be obtained with this fabrication technique. [less ▲]

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See detailNon-destructive measurements of the volume magnetic behavior of large bulk GdBCO single domains and ferromagnet/superconductor structures
Vanderbemden, Philippe ULg; Egan, Raphael; Morita, Mitsuru et al

Conference (2016, September 05)

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device ... [more ▼]

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device enables the measurement of magnetic moments as large as 1 Am^2 (1000 emu) on large bulk samples up to 20 mm diameter at 77 K. This extends significantly the accessible measurement range of “off-the shelf” magnetometers (e.g. SQUID, VSM). Unlike Hall probe mapping, the measured signal is representative of the superconducting currents flowing across the entire height of the sample. Since the device is based on a flux extraction technique and does not include any Hall probe, another advantage is that no gap between a Hall probe and the sample needs to be taken into account to interpret the results. In the present work we first show how the dimensions of the sensing coils and integration bounds are chosen in such a way the magnetometer is sensitive to dipolar magnetic moment of the sample, while being insensitive to moments of higher order. Next we study the relaxation of the trapped magnetic moment under various magnetization processes. The sensitivity of the device allows small variations of m (e.g. ~ 2% for 1 hour) to be recorded. We show experimentally that the relaxation of the magnetic moment of the whole sample is reduced strongly if the superconductor is not fully magnetized, either in field cooled (FC) or zero-field cooled (ZFC) conditions. The practical consequence is that an abnormally slow magnetic relaxation (large E-J power law exponent n) under a conventional magnetization procedure indicates that the bulk superconductor could be magnetized further. In a second set of experiments, we study the increase of magnetic moment of a bulk superconductor when it is attached to a soft ferromagnetic disk. We also examine the irreversible demagnetization of these structures when subjected to crossed magnetic fields and compare to the magnetic moment of a classical (Nd-Fe-B) permanent magnets in the same conditions. Acknowledgements: This work is part of an "Action de Recherches Concertees" grant (ARC 11/16 -03) from the "Communaute Francaise de Belgique". [less ▲]

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See detailSuperconductors used as trapped field magnets : investigation of hybrid structures combining superconducting and ferromagnetic materials
Vanderbemden, Philippe ULg; Fagnard, Jean-François ULg; Caps, Hervé ULg et al

Conference (2016, August 30)

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible ... [more ▼]

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible superconductors, these properties are due to the pinning of individual flux lines (vortices) by defects at the nanoscale level. In this talk the influence of pinning on the magnetic properties of bulk superconductors will be explained and illustrated, with the emphasis placed on how pinning can be exploited to produce “trapped field magnets” for practical applications. We will then show how the properties of these trapped field magnets are enhanced when superconductors (SC) are combined to soft ferromagnetic (FM) alloys [1,2]. In particular we investigate the properties of macroscopic FM/SC/FM structures where the superconductor is sandwiched between two ferromagnetic sections, one of each circular face. In particular we show how the properties of the whole structure is influenced by the saturation magnetization and the geometry of the ferromagnetic material. References: 1. M P Philippe, J-F Fagnard, S Kirsch, Z Xu, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Magnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures. Physica C: 502 (2014) 20-30. 2. M P Philippe, M D Ainslie, L Wera, J-F Fagnard, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Influence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y–Ba–Cu–O superconductor. Superconductor Science and Technology: 28 (2015) 095008. [less ▲]

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See detailMagnetic shielding with bulk high-temperature superconducting hollow cylinders: improvement of the shielded volume and investigation of sequential orthogonal applied fields
Vanderbemden, Philippe ULg; Hogan, Kevin ULg; Wera, Laurent ULg et al

Conference (2016, April 26)

Due to their ability to trap magnetic fields that exceed the saturation magnetization of conventional ferromagnets, bulk high-temperature superconductors have great potential to act as efficient passive ... [more ▼]

Due to their ability to trap magnetic fields that exceed the saturation magnetization of conventional ferromagnets, bulk high-temperature superconductors have great potential to act as efficient passive magnetic shields. Shielding efficiency is usually demonstrated in bulk hollow cylinders. One of the main drawbacks of this geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed using e.g. a superconducting cap. Gaps or holes, however, are needed for allowing connections between the inner part of the tube and the external world. In this communication we examine the shielding efficiency and volume when a superconducting tube is closed either at one extremity or at both extremities. The effect of the gap size between the cap and the tube is also studied. Next we examine more complex magnetic shielding configurations where an external magnetic field is applied first one direction and a second field is subsequently applied along another (perpendicular) direction. We discuss the influence of pre-existing shielding supercurrents on the magnetic shielding properties. [less ▲]

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See detailMagnetic moment and local magnetic induction of superconducting/ferromagnetic structures subjected to crossed fields: experiments on GdBCO and modelling
Fagnard, Jean-François ULg; Morita, Mitsuru; Nariki, Shinya et al

in Superconductor Science and Technology (2016), 29(12), 125004

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been ... [more ▼]

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed. [less ▲]

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See detailMagnetic Shielding with Bulk High Temperature Superconductors: Improvement of the Shielded Volume in Hollow Cylinders
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

in Muralidhar, Miryala (Ed.) Superconductivity: Applications Today and Tomorrow (2016)

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding ... [more ▼]

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding performances can be characterized by two parameters: (i) the shielding factor SF, defined as the ratio between the applied magnetic induction Bapp and the magnetic induction inside the shield Bin and (ii) the threshold induction Blim for which the shielding factor becomes lower than a given level. The main drawback of the hollow cylinder geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed. This can be achieved by using a superconducting vessel or by closing one or both extremities with a cap. This chapter describes the experimental and the numerical analysis of the DC shielding performances of a Bi-2223 tube closed by a superconducting Bi-2223 cap, for an axial configuration. The first part is dedicated to the experimental study of two tubes of different lengths when they are closed at one extremity or closed at both extremities. We also study the effect of the gap size between the cap and the tube. In the second part, we use numerical simulations to highlight the possible improvements and to optimize the design. In particular we study the influence of the critical current density and of the thickness of the cap. [less ▲]

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See detailBulk high-temperature superconducting hollow cylinders used for magnetic shielding: effect of an initial trapped field on the shielding performances.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2016)

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a ... [more ▼]

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a superconducting shield is a bulk hollow cylinder. In this work, we investigate the magnetic shielding properties when the tube is not in the virgin state. Such a situation happens in practice when the tube has been initially subjected to a first magnetic field. The induced superconducting currents that are flowing in the tube generate a trapped magnetic moment which may deteriorate the shielding of a second field. Here we study experimentally how an initial trapped moment affects the shielding of a magnetic field, when they are both perpendicular to each other. In particular, we focus on the effect of an axial trapped moment on the transverse shielding performances and on the effect of a transverse trapped field on the axial or the transverse shielding performances. Finally, we show how the pristine state of the tube can be restored when applying an external field of adequate amplitude. The consequence for practical applications is that it is possible to recover shielding properties similar to those obtained in the virgin state without heating up the shield above its critical temperature TC. [less ▲]

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See detailInfluence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

in Journal of Physics: Conference Series (2016), 695

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When ... [more ▼]

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) “trapped field magnets” are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a “crossed-field” configuration. Experimental results show that the usual “crossed-field demagnetization” caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet. [less ▲]

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See detailOrigin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity
Geng, J.; Matsuda, K.; Fu, L. et al

in Journal of Physics : D Applied Physics (2016), 49(11), 1101

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the ... [more ▼]

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the superconducting load by ac fields. Giaever (1966 IEEE Spectr . 3 [http://dx.doi.org/10.1109/MSPEC.1966.5217702] 117 ) pointed out flux motion in superconductors will induce a dc voltage, and demonstrated a rectifier model which depended on breaking superconductivity. van de Klundert et al (1981 Cryogenics 21 [http://dx.doi.org/10.1016/0011-2275(81)90195-8] 195, 267 ) in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a dc voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the dc voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps. [less ▲]

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See detailCritical analysis of the paramagnetic to ferromagnetic phase transition in Pr0.55K0.05Sr0.4MnO3
Thaljaoui, R.; Pękała, M.; Fagnard, Jean-François ULg et al

in Physica B: Condensed Matter (2016), 482

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots ... [more ▼]

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots (MAP), the Kouvel-Fisher method and the critical isotherm analysis. Data obtained near Tc were examined in the framework of the mean field theory, the 3D–Heisenberg model, the 3D–Ising model, and tricritical mean field. The deduced critical exponents values obtained using MAP method were found to be β = 0.44(4) with TC ~ 303 K and γ = 1.04(1) with TC ~ 302 K. Kouvel-Fisher method supplies the critical values to be β = 0.41(2) with TC ~ 302 K and γ = 1.09(1) with TC ~ 302 K. The obtained critical parameters show a tendency towards the mean-field behavior, suggesting the existence of long-range ferromagnetic order in the compound studied. The exponent δ deduced separately from isotherm analysis at T= 303 K was found to obey to the Widom scaling relation δ = 1+ γ/ β. The reliability of obtained exponents was confirmed by using the universal scaling hypothesis. The itinerant character of ferromagnetism in the present system was also tested by using Rhodes-Wohlfarth’s criterion. [less ▲]

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See detailThermal properties of Ti-doped Cu–Zn soft ferrites used as thermally actuated material for magnetizing superconductors
Stachowiak, Piotr; Mucha, Jan; Szewczyk, Daria et al

in Journal of Physics : D Applied Physics (2016), 49

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process ... [more ▼]

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process of superconductors, however, a low value of Tc is required. Here we report and analyze in detail the thermal properties of bulk Ti-doped Cu–Zn ferrite ceramics Cu0.3Zn0.7Ti0.04Fe1.96O4 and Mg0.15Cu0.15Zn0.7Ti0.04Fe1.96O4. They are characterized by a Curie temperature in the range 120–170 K and a maximum DC magnetic susceptibility exceeding 20 for the Cu0.3Zn0.7Ti0.04Fe1.96O4 material. The temperature dependence of both the specific heat Cp and of the thermal conductivity κ, determined between 2 and 300 K, are found not to exhibit any peculiar feature at the magnetic transition temperature. The low-temperature dependence of both κ and the mean free path of phonons suggests a mesoscopic fractal structure of the grains. From the measured data, the characteristics of thermally actuated waves are estimated. The low magnetic phase transition temperature and suitable thermal parameters make the investigated ferrite ceramics applicable as magnetic wave producers in devices designed for magnetization of high-temperature superconductors. [less ▲]

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See detailDesign of the superconducting end caps of a tubular magnetic shield.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2015, September 09)

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device ... [more ▼]

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device, which is protected against external fields until the magnetic flux diffuses either across the tube wall or through the end openings. A better shielding vessel is obtained by using superconducting end caps. In the simplest design, the end caps, consisting of superconducting disks placed at the extremities of the tube, are making a 90 degree angle with the tube wall. Because of demagnetization effects, this arrangement results in a 'weak spot' for the penetrating magnetic flux. In this work, we investigate numerically different shapes for the end caps so as to avoid sharp angles and obtain a smoother magnetic flux penetration. The external magnetic induction is applied parallel to the shield axis. The magnetic flux distribution and the shielding currents are calculated with a finite element approach. The shielded volume is defined as the region where the local magnetic flux density is attenuated below a given fraction of the external field. It is studied as a function of the shape of the cap, its superconducting properties, the aspect ratio of the assembly, and the presence of a cabling hole. Curved and elongated caps, which could in principle be obtained by melt cast processing or different coating techniques, are shown to substantially increase the shielded volume. [less ▲]

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See detailMagnetic shielding properties of a bulk Bi-2223 superconducting hollow cylinder subjected to the magnetic stray field of a nearby magnetic source
Hogan, Kevin ULg; Wera, Laurent ULg; Fagnard, Jean-François ULg et al

Poster (2015, September 09)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials. Therefore, they can be very useful for many applications which require low or very low magnetic field in intense quasi-static magnetic environments. The vast majority of studies on superconducting screens has been conducted in homogeneous magnetic field configurations. Few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the shielding properties of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of a coil placed in the vicinity. All experiments are carried out at 77K. [less ▲]

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See detailCharacterization of the magnetic properties of a continuously coated YBCO coated conductor cylinder in persistent current mode
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2015, September 09)

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials ... [more ▼]

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials. In particular, an efficient superconducting magnetic shield can be built as an assembly of milled YBCO 2G coated conductor tape sections, or, as in this case, a continuously coated cylinder. The purpose of the present work is to characterize experimentally the DC magnetic properties of a continuous band of YBCO film (about 1 cm width) deposited around a hastelloy cylinder of 6.4 cm in diameter. In particular, we characterize the distribution and the time dependence of the trapped field inside the cylinder due to persistent current loops. We also investigate the magnetic shielding properties at several sweep rates when the applied magnetic field is parallel to the main axis of the cylinder. All measurements are carried out at 77 K. The measurement of the trapped field of the cylinder gives evidence that a persistent current of about 80 A can be induced in the YBCO band. Magnetic shielding measurements show that, thanks to the persistent current flowing in the band, the cylinder is able to attenuate by a factor of 2 the applied magnetic field. These results open interesting perspectives for a further development of magnetic shields made of coated conductor cylinders. We investigate how the shielding performances could be further improved by increasing the number of layers and by increasing the height of the cylinder. The results are compared to a multilayer coated conductor cylinder with joints and to persistent current loops obtained with milled coated conductor tape sections. [less ▲]

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See detailMagnetic shielding properties of tubular bulk superconductors subjected to stray magnetic fields
Hogan, Kevin ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Conference (2015, September 04)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous magnetic field configurations. Only few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the magnetic response of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of various magnetic sources. Thanks to a bespoke 3D mapping system and a three-axis Hall probe, we measure the magnetic flux density distribution in the vicinity of the cylinder when the latter is subjected to those stray fields. All experiments are carried out in liquid nitrogen. Our experimental data allow us to apprehend the magnetic field penetration inside the superconductor in inhomogeneous field configurations and to compare those to results obtained with homogeneous fields. Then we compare our results to a finite element model of an infinite tube subjected to an inhomogeneous magnetic field in order to gather information on the magnetic field penetration and the current density distribution inside the superconductor. [less ▲]

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See detailInfluence of soft ferromagnetic materials on the magnetic flux density above large grain, bulk high temperature (RE)BCO superconductors: measurements and modelling
Philippe, Matthieu ULg; Ainslie, Mark D; Fagnard, Jean-François ULg et al

Poster (2015, September)

Due to their ability to trap high magnetic fields, bulk (RE)BCO superconductors can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic ... [more ▼]

Due to their ability to trap high magnetic fields, bulk (RE)BCO superconductors can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We combine soft ferromagnetic pieces with bulk, large grain (RE)BCO high temperature superconductors to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic pieces influence (i) the profile shape of the trapped magnetic flux density at the surface of each structure and (ii) the decrease of the trapped field under an applied field that is orthogonal to the main magnetization (crossed field configuration). Hall probe mappings of the trapped magnetic flux density profile above the hybrid structures at 77 K are compared to modelled profiles using a 2D finite element method. Modelling results are in excellent qualitative and quantitative agreement with the measurements. The model is then used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and ferromagnet geometries. For the modelled configurations, the magnetic flux density is enhanced on the face opposite to the ferromagnet. Both thickness and saturation magnetization of the ferromagnetic material are found to be important parameters. The saturation regime of the ferromagnet can be predicted using a simple analytical model. We show that thin ferromagnetic discs can be driven to full saturation even though the superconductor magnetic field is much smaller than their saturation magnetization. Remarkably, the beneficial influence of the ferromagnet on the trapped field can be observed even if the trapped flux density above the superconductor is much larger than the ferromagnet saturation magnetization. Finally, we show that the ferromagnetic material acts as a magnetic shield and lowers the relative demagnetization effect caused by the application of transverse (crossed) magnetic field cycles. [less ▲]

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See detailTrapped magnetic flux density and influence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
Philippe, Matthieu ULg; Ainslie, Mark D; Fagnard, Jean-François ULg et al

Conference (2015, September)

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We ... [more ▼]

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We combine soft ferromagnetic pieces with bulk, large grain (RE)BCO high temperature superconductors to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how a ferromagnetic disc influences the trapped flux density at the surface of a SC/FM structure at 77 K and the magnetic moment of the whole structure. The flux density generated by the superconductor is shielded above the ferromagnet. The saturation of the ferromagnet by the flux of the superconductor is investigated with the help of a finite element model and a simple analytical formula is suggested to determine the saturation threshold. Finally, we show that the ferromagnetic disc lowers the relative demagnetization effect caused by the application of magnetic field cycles orthogonal to the main magnetization (crossed field configuration). [less ▲]

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See detailInfluence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y-Ba-Cu-O superconductor
Philippe, Matthieu ULg; Ainslie, Mark D; Wera, Laurent ULg et al

in Superconductor Science and Technology (2015), 28

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this ... [more ▼]

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this paper, soft ferromagnetic sections are combined with a bulk, large grain Y-Ba-Cu-O (YBCO) high temperature superconductor to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic sections influence the shape of the profile of the trapped magnetic induction at the surface of each structure and report the surface magnetic flux density measured by Hall probe mapping. These configurations have been modelled using a 2D axisymmetric finite element method based on the H-formulation and the results show excellent qualitative and quantitative agreement with the experimental measurements. The model has also been used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and geometries. The results show that the ferromagnetic material acts as a magnetic shield, but the flux density and its gradient are enhanced on the face opposite to the ferromagnet. The thickness and saturation magnetization of the ferromagnetic material are important and a characteristic ferromagnet thickness d* is derived: below d*, saturation of the ferromagnet occurs, and above d*, a weak thickness-dependence is observed. The influence of the ferromagnet is observed even if its saturation magnetization is lower than the trapped flux density of the superconductor. Conversely, thin ferromagnetic discs can be driven to full saturation even though the outer magnetic field is much smaller than their saturation magnetization. [less ▲]

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See detailA comparative study of triaxial and uniaxial magnetic shields made out of YBCO coated conductors
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, George et al

in Superconductor Science and Technology (2015), 28

Detailed reference viewed: 39 (24 ULg)