References of "Vanderheyden, Benoît"
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See detailStudy of buoyancy driven heat transport in silicone oils and in liquid nitrogen in view of cooling applications
Satpathy, Kamalakanta ULiege; Duchesne, Alexis; Dubois, Charles et al

in International Journal of Heat & Mass Transfer (2018), 118

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural ... [more ▼]

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural convection. In one recent experimental investigation (Dubois et al., 2016) carried on silicone oils and liquid nitrogen, we have demonstrated that the velocity field satisfies specific scaling laws with respect to the temperature increase in the liquid pool. In this work, we pursue the analysis by modeling the heat transfer in a parallelepiped enclosure for a steady laminar flow regime. The Navier-Stokes equations are solved using a finite volume approach to obtain the detailed three-dimensional flow and heat transfer characteristics. A quantitative analysis of the velocity field over the temperature field shows that the experimental power laws are reproduced in simulations. Following Dubois and Berge (1978), a theoretical law originally introduced in the context of the classical Rayleigh-Bénard experiment is shown to be satisfied in the simulations over a wide range of Rayleigh numbers (Ra), assuming the definition of the characteristic convection length is adapted to the investigated geometry. Moreover, the simulations are shown to correctly reproduce the main features of the flow, including the characteristic convection length, for different heater lengths. [less ▲]

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See detailMagnetic shielding of open and semi-closed tubes made of bulk superconductors : the role of a cap
Vanderbemden, Philippe ULiege; Wera, Laurent ULiege; Hogan, Kevin ULiege et al

Conference (2017, December 11)

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic ... [more ▼]

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic materials which exhibit a relatively low saturation magnetization (e.g. µ0Msat ~ 0.75 T for mumetal), superconductors do not suffer from this drawback. They are therefore well suited to shield magnetic flux densities above 1 tesla. This can be achieved by hollow bulk superconductors of cylindrical geometry. The shielding efficiency and shielded volume are both strongly improved when the hollowed tube is closed at one extremity. As an example, we showed recently [1] that YBa2Cu3O7 (YBCO) tubes of ~ 10 mm inner diameter closed at one extremity and engineered by a buffer-aided top seeded melt growth fabrication process were able to attenuate axial flux densities of 1.5 T by a factor of 100 at 20 K. Such record performances are partly due to the presence of a cap at one extremity. The aim of the present work is to examine further and understand the influence of a cap on the shielding properties for various configurations. In this communication we investigate the shielding properties of hollow and semi-closed bulk superconducting tubes at 77 K. For the semi-closed configuration, we compare situations the cap is either placed against the extremity (with a small controllable gap between them) or welded naturally. The field to be shielded is either parallel or perpendicular to the main axis of the tube. Both YBCO tubes produced by buffer-aided top seeded melt growth and commercial Bi-2223 tubes are studied. The experimental results are compared to finite element numerical modelling carried out either in 2D (for the axial configuration) or 3D (for the transverse configuration). In the axial configuration, the results show that a small gap between the tube and the cap is not detrimental to the shielding properties. We also show that the shielded volume can be easily enhanced by increasing the thickness of the cap or its critical current density. In the transverse configuration, the results point out the crucial role of the superconducting joint between the tube and the cap. The need for a superconducting joint depends therefore on the main direction of the magnetic field to be shielded. References: [1] L. Wéra. et al., IEEE. Trans. Appl. Supercond. 27 (2017) 6800305 [less ▲]

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See detailBulk superconducting tube subjected to the stray magnetic field of a solenoid
Hogan, Kevin ULiege; Fagnard, Jean-François ULiege; Wera, Laurent ULiege et al

in Superconductor Science and Technology (2017), 31(1),

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have ... [more ▼]

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have been extensively discussed in the literature. In the present work, we investigate the magnetic shielding and the penetration of magnetic flux in a bulk high temperature superconducting tube subjected to the inhomogeneous fringe field of a solenoidal coil. Thanks to a bespoke microdisplacement measurement system, we measure the magnetic field distribution around the tube. We develop a full 3D finite element model based on an H formulation to understand the flux penetration mechanisms and predict the shape of the current loops. Using constitutive law parameters obtained from previous independent experiments, our model is found to be in excellent agreement with the measurements. We discuss how to assess the degree of inhomogeneity of the magnetic field and show that, in our case study, the field can be treated as the magnetic field of an equivalent magnetic dipole. We also show that some features of the flux penetration in inhomogeneous field can be also observed when the tube is subjected to an oblique homogeneous magnetic field, which offers a better understanding of the shielding current density distribution inside the shield. Finally, we discuss the magnetic field concentration occurring around the shield for different magnetic field configurations. In particular, we show that the extremities of the tube on the side not facing the magnetic field source experience the highest flux concentration. [less ▲]

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See detailInfluence of the inhomogeneous properties of a superconducting film on the penetration of the magnetic flux
Burger, Loïc ULiege; Avila Osses, Jonathan ULiege; Nguyen, Ngoc Duy ULiege et al

Poster (2017, September 20)

We consider the penetration of magnetic flux perpendicular to a Nb superconducting film containing edge indentations and/or defects. The presence of such defects and indented edges forces the induced ... [more ▼]

We consider the penetration of magnetic flux perpendicular to a Nb superconducting film containing edge indentations and/or defects. The presence of such defects and indented edges forces the induced currents to change direction abruptly, which gives rise to discontinuity lines (d-lines). It has been recently shown, by magneto-optical means, that the shape of the d-lines contain information about the size and the shape of the indentations [1]. Moreover, in contrast with what is commonly assumed, thermomagnetic instabilities are not preferentially triggered near the manufactured indentations, but rather along smooth edges [1]. Such phenomena can lead to unexpected quenching of superconducting samples and considerably damage them, which must be avoided in applications. In this poster, we investigate whether inhomogeneous superconducting properties can be responsible for triggering magnetic flux avalanches. To this aim, we numerically model the flux penetration and determine the distribution of current lines and electric fields for a situation where the superconducting properties vary locally over a small region of the film. We consider the variation of the thickness of the film, the critical current density and the size of the discontinuity. The resulting shapes of the d-lines are deduced and compared to analytical critical state approximations. We also determine the location and the amplitude of the maximal electric field in the film and compare it to typical values which must be reached in order to trigger magnetic flux avalanches. We conclude that the investigated variations lead to levels of electric field which are of the same order of magnitude as those generated by artificial indentations. [less ▲]

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See detailStudies on convective cooling of cryogenic fluids towards superconducting applications
Satpathy, Kamalakanta ULiege; Dubois, Charles; Duchesne, Alexis et al

in WIT Transactions on Engineering Sciences (2017), 118

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form ... [more ▼]

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nuave). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nuave ~ Ra^0.374. Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results. [less ▲]

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See detailModel for the response of superconducting/ferromagnetic structures subjected to crossed fields
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2017, September 20)

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time ... [more ▼]

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time-varying parasitic fields perpendicular to the initial trapped flux density. In this work, we consider hybrid structures made of superconducting pellets and soft ferromagnetic rings or disks and investigate numerically how their magnetization varies under such crossed-field excitations. We consider a finite-element model in the A-V formulation, which is developed for both the full 3D configuration of the system and for a computationally lighter 2D approximation. The models provide the volumic distribution of current densities, the field lines, and the magnetization of the system, and the latter is compared to experimental measurements obtained with a bespoke magnetometer. A critical assessment is made outlining the advantages and limitations of the 2D approach. It is demonstrated that the ferromagnetic material helps in increasing the initial trapped field and in reducing the crossed-field effects. For the case of a single ferromagnetic disk put on top of the superconducting pellet, we study the dependence of the initial magnetization with respect to the disk thickness and the saturation field of the ferromagnetic material, and discuss the resulting optimal thickness. We also suggest different hybrid structures made of one superconducting pellet and one or several ferromagnetic disks or rings, and show numerically that a compromise is to be found between the maximum level of induction which can be reached and the improved protection against crossed-field effects. [less ▲]

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See detailModel for the penetration of magnetic flux in Nb superconducting films with lithographically defined indentations and defects due to inhomogeneities
Vanderheyden, Benoît ULiege; Avila Osses, Jonathan ULiege; Brisbois, Jérémy ULiege et al

Conference (2017, August 31)

We consider the penetration of magnetic flux perpendicularly to a Nb superconducting film containing edge indentations and/or defects. The induced currents undergo an abrupt change of direction around the ... [more ▼]

We consider the penetration of magnetic flux perpendicularly to a Nb superconducting film containing edge indentations and/or defects. The induced currents undergo an abrupt change of direction around the indentations and the defects, giving rise to discontinuity lines (d-lines). It has recently been shown, by means of magneto-optical imaging and numerical models, that the detailed structure of the d-lines generated with lithographically-defined micro-indentations carry information about their size and shape, and vary with temperature [1]. In this talk, we describe the models which can be used to describe the d-lines and discuss the effects of temperature, demagnetization, and flux creep, as well as the influence of a field-dependent critical current density on the distribution of the magnetic field. We extend the model to discontinuities produced by defects arising from inhomogeneous properties of the film and discuss the information contained in the resulting d-line structure. Last, in contrast to what has been repeatedly predicted in the literature, artificial indentations were not observed to act as preferred nucleation spots for flux avalanches [1]. To investigate this result, we estimate the levels of electric field strengthening arising near indentations and defects, and compare with the levels necessary for triggering thermomagnetic instabilities. [less ▲]

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See detailNon-destructive investigation of hybrid of ferromagnet /(RE)BCO large grain bulks by flux extraction magnetometry and levitation force
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wera, Laurent ULiege et al

Conference (2017, July 10)

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large ... [more ▼]

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large grain superconductors can be improved easily by attaching a short section of a soft ferromagnetic material (F) to one of the faces of the bulk superconductor (S), thereby producing a hybrid F/S structure [1]. Here we investigate the contactless determination of the magnetic behavior of such structures using a recently constructed bespoke magnetometer based on the flux extraction technique [2]. This device allows magnetic moments as large as 1 Am² to be measured at 77 K and accommodates large bulk samples up to 20 mm diameter. This extends significantly the accessible measurement range of “off-the shelf” magnetometers. Unlike techniques based on recording the distribution of flux at the surface of the sample, the measured signal is representative of the superconducting currents flowing across the entire volume of the sample. In the present work we examine the properties of permanently magnetized superconductors and hybrid structures, and measure the irreversible demagnetization of these structures when they are subjected to magnetic field cycles that are not parallel to their magnetization. We also investigate the levitation behavior of hybrid structures subjected to the non-uniform field of a permanent magnet or a combination of permanent magnets used as guideway for levitation applications, and compare the results to those obtained with a bulk superconductor alone. This abstract is for the special session on bulk superconductivity. References: [1] Egan R. et al., Rev. Sci. Instrum. 86 (2015) 025107 [2] M. P. Philippe et al., Supercond. Sci. Technol. 28 (2015) 095008 Acknowledgments: We greatly acknowledge Nippon Steel & Sumitomo Metal Corporation for providing bulk large grain GdBa2Cu3O7 (GdBCO) samples. [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 ULiege; Fagnard, Jean-François ULiege; Kumar, Numbury Devendra et al

in IEEE Transactions on Applied Superconductivity (2017), 27(4),

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and ... [more ▼]

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding “dc” stray fields generated by HTS magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K) or liquid neon (27 K). [less ▲]

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See detailBehaviour of bulk superconducting trapped field magnets subjected to magnetic fields with a transverse component
Vanderbemden, Philippe ULiege; Caps, Hervé ULiege; Vanderheyden, Benoît ULiege 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 detailBulk high-temperature superconducting hollow cylinders used for magnetic shielding: effect of an initial trapped field on the shielding performances.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2016, November 09)

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 detailMagnetic shielding above 1 T at 20 K with bulk, large grain YBCO tubes made by Buffer-aided Top Seeded Melt Growth.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; 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 ULiege; 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 ULiege; Fagnard, Jean-François ULiege; Caps, Hervé ULiege 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 detailModel for the decay of the magnetization of a bulk superconductor for different sources of magnetic fields with a transverse component
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2016, August)

Bulk (RE)BCO samples can be used as efficient trapped-field magnets with surface flux densities of a few teslas. However, their magnetization may decay over time when subjected to time-varying parasitic ... [more ▼]

Bulk (RE)BCO samples can be used as efficient trapped-field magnets with surface flux densities of a few teslas. However, their magnetization may decay over time when subjected to time-varying parasitic crossed fields (applied perpendicular to the initially trapped flux density). In this work, we consider and compare the magnetization decays that occur under different types of crossed-field perturbations, mimicking realistic perturbations that are expected in synchronous motors. The model is based on a finite element approach carried in the GetDP environment. The superconductor is modelled with an isotropic E(J) power law and a field-independent current density. Under a magnetic field perturbation with a non-vanishing transverse component, the superconductor magnetization is modified due to a rearrangement of currents. We first investigate perturbations with different amplitudes that are applied perpendicular to the magnetization axis and have either a bipolar waveform with a zero DC component, or a unipolar waveform with a non-zero DC component. Then, these perturbations are compared to those obtained with an external field applied along the magnetization axis and being rotated away at different angles. The model predicts the evolution over time of the distributions of currents and the net magnetic moment in each case. It is demonstrated that the patterns of reorganized currents strongly depend on the waveform of the applied perturbation and an analysis of these patterns is provided. [less ▲]

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See detailComparison of the magnetization decay due to crossed or rotated magnetic field measured on a Ferromagnet disk/GdBCO pellet structure
Fagnard, Jean-François ULiege; Morita, Mitsuru; Teshima, Hidekazu et al

Poster (2016, August)

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can ... [more ▼]

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can experience a certain misalignment with the magnetic field generated by the stator. This situation is likely to happen during transients or non-ideal behavior of the machine: the angle between the stator field and the SC magnetization can change because of a sudden change of mechanical load or vibrations of the device. The consequence is that the SC pellet can be subjected to a transverse component of magnetic field which is likely to cause a significant demagnetization and therefore a decrease of the machine performances. In previous works, we already have studied the problem of a “pure” transverse magnetic field (also called “crossed field”) as well as the beneficial influence of a ferromagnetic (FM) disk placed on the top of the SC pellet. To this aim, a bespoke insertion tool for a PPMS was designed and constructed. The tool can be used to characterize a SC sample whose orientation is changed from a parallel to a perpendicular direction with respect to the magnetic field direction. Here, this insertion tool was used to rotate the sample at various angles regarding to the field direction. A GdBCO pellet of 9 mm in diameter and 5 mm in height was characterized at 77 K in various experimental conditions by measuring the magnetic induction below the centre of the sample surface. We compared the results obtained with the GdBCO pellet alone to those with a FM disk of various thicknesses attached on the top of the sample. A first set of experiments consisted in applying pure transverse magnetic field cycles of 50 mT and 100 mT at low frequency, the polarity of the field being either always the same (unipolar) or reversed after half of the period (bipolar). A second set of experiments was carried out by rotating the applied magnetic field (applied first in the pellet magnetization direction) in several steps to a given angle (up to 90°) and back to 0°. All these experiments were carried out after having fully magnetized the sample and recorded the magnetization decay due to the flux creep during 30 min. The results show that (i) the FM disk reduces the magnetization decay in both pure transverse and rotated magnetic field experiments, (ii) rotated magnetic fields have a less detrimental effect than the “pure transverse” magnetic field. This phenomenon is likely to be caused by a partial remagnetization of the sample when the angle of the magnetic field is reduced back to 0°. Finite element modelling was carried out to sustain the measurements of the local magnetic induction and gives additional information on the supercurrent distribution inside the SC pellet during “pure transverse” and “rotated” applied fields. [less ▲]

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See detailStudies of heat transport in cryogenic fluids toward superconducting applications
Satpathy, Kamalakanta ULiege; Dubois, Charles; Fagnard, Jean-François ULiege et al

Poster (2016, June 16)

To meet the increasing global energy demand with minimal green house effect, electrical energy is a viable option. But, it requires an efficient electrical power system with optimal loss. Towards this ... [more ▼]

To meet the increasing global energy demand with minimal green house effect, electrical energy is a viable option. But, it requires an efficient electrical power system with optimal loss. Towards this, superconducting materials represent a promising way of improving the existing systems. However, the efficiency may be affected by a small temperature increase due to electromagnetic losses induced by time-varying magnetic fields. To minimize this, two approaches can be considered (i) reduce the losses and (ii) improve the heat exchange with the cooling environment. Although experimental and theoretical studies of the superconducting losses have been extensively pursued [1], few attempts have been made to combine the 'losses' and the 'cooling' aspects in the current scenario. Such studies require a detailed characterization of magneto-thermal interaction of the superconductors, as well as fundamental aspects of heat draining by the cryogenic fluid. Towards this, numerical studies on natural convection in an enclosed cavity, interacting with a constant heat source (heater) are carried out to understand the cooling efficiency of the cryogenic fluids (viz., liquid nitrogen, silicon oil and water). Natural convection heat transfer in an enclosure continues to be an active research area, due to its significance for both fundamental interests and engineering applications. Majority of the published studies can be classified into two groups: enclosure heated from below and cooled from above (Rayleigh–Bénard problem) and differentially heated enclosures [2]. Considerable attention has been given to natural convection from vertical/horizontal enclosures specified either with constant temperature or heat flux, while limited studies has been done with constant volumetric heat source [3]. The resulting characteristics of fluid flow and heat transfer are quite interesting and deserve a detailed investigation, which motivates for the present work. As a priori, three dimensional steady forms of incompressible Navier-Stokes equations are solved through finite volume approach [4]. Detailed flow features are presented in terms of velocity, temperature and path line profiles for different heater lengths. Comparative analyses are further reinforced in terms of average Nusselt number (Nu) for different fluids. It shows that, effective heat transfer enhancement occurs for liquid nitrogen with the shortest heater length, resulting in an efficient cooling. The profile of temperature and velocity magnitude distributions agrees well with the in-house experiment. Benchmark validation is found to be satisfactorily against the reported result [5]. Complete descriptions on geometrical details, governing equations, boundary conditions and solution methodology adopted for the numerical solution will be provided in full length paper. This study will be a gateway for further magneto-thermal analysis in an electromagnetic environment. [less ▲]

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See detailImprinting superconducting vortex footsteps in a magnetic layer
Brisbois, Jérémy ULiege; Motta, Maycon; Avila Osses, Jonathan ULiege et al

in Scientific Reports (2016), 6

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit ... [more ▼]

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories. [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 ULiege; Hogan, Kevin ULiege; Wera, Laurent ULiege 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 flux penetration in Nb superconducting films with lithographically defined micro-indentations
Brisbois, Jérémy ULiege; Adami, Obaïd-Allah ULiege; Avila Osses, Jonathan ULiege et al

in Physical Review B (2016), 93(5), 054521

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ... [more ▼]

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines (d-lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent n. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities. [less ▲]

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