References of "Vanderheyden, Benoît"
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See detailClassical analogy for the deflection of flux avalanches by a metallic layer
Brisbois, Jérémy ULg; Vanderheyden, Benoît ULg; Colauto, Fabiano et al

in New Journal of Physics (2014), 16(10), 103003

Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably ... [more ▼]

Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably, in some cases the flux is totally excluded from the area covered by the conductive layer. We present a simple classical model that accounts for this behaviour and considers a magnetic monopole approaching a semi-infinite conductive plane. This model suggests that magnetic braking is an important mechanism responsible for avalanche deflection. [less ▲]

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

Poster (2014, August 12)

Bulk superconducting materials are well suited for magnetic shielding applications. At low frequencies, the performances of superconductors are higher than those of conventional ferromagnetic materials ... [more ▼]

Bulk superconducting materials are well suited for magnetic shielding applications. At low frequencies, the performances of superconductors are higher than those of conventional ferromagnetic materials. In shielding applications, two situations may be encountered. The first one corresponds to the case where the screen has to shield a volume from the magnetic field in its environment; this corresponds to an “immunity” problem. In the second situation, the screen has to prevent the magnetic field generated by an electronic device from perturbing its environment; this is an “emission” problem. So far, superconducting screens have been extensively studied in “immunity” and were subjected to uniform magnetic fields. In “emission”, the magnetic field is no longer uniform because a local magnetic source has to be placed inside the screen. In this work, we have studied experimentally at 77K the magnetic flux penetration in a Bi-2223 superconducting hollow cylinder subjected to a non-uniform quasi-static magnetic field generated by a small coil placed inside the sample. Two configurations were investigated: axial and transverse; corresponding respectively to the situation where the axis of the coil is coaxial or perpendicular to the axis of tube. We also investigate the influence of the sweep rate of the magnetic field on the magnetic shielding performances. Planar and circular (i.e. at constant distance of the tube) mappings of the magnetic field at proximity of the external surface of the tube were obtained thanks to a bespoke experimental setup using a three axes miniature Hall probe. It was observed that the three components of the magnetic field measured outside are affected differently by the superconducting screen. A simple one-dimensional model based on the conservation of magnetic flux and the Bean model was developed for the axial configuration. It was found to be in accordance with experimental data. It allows one to predict the maximal magnetic flux that can be generated inside the coil before the tube is fully penetrated and a magnetic field is measured outside the tube. Finally, it was observed that the inner surface of the tube is subjected to a magnetic field higher than the one at the same place without the tube. This concentration phenomenon arises because of the diamagnetic behaviour of the superconductor. [less ▲]

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See detailExperimental study of the magnetic shielding performances of a Bi2223 tube closed by a Bi2223 cap.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2014, August 12)

A superconducting tube can be used as an efficient magnetic shield. However, the shielding factor (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside ... [more ▼]

A superconducting tube can be used as an efficient magnetic shield. However, the shielding factor (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside the tube) decreases towards tube extremities because of the field penetration through the open ends of the tube. To improve the performances at tube extremities, the tube should be closed. This can be achieved by using a superconducting vessel or by closing both extremities with a cap. The purpose of the present work is to study experimentally the shielding performances of a Bi2223 tube closed by a superconducting Bi2223 cap. The cap is a circular plate with a diameter equal to the outer diameter of the tube and there is no superconducting joint between the cap and the tube. Our interest is to characterize the effect of the cap on the shielding factor distribution along the tube axis when only one extremity of the tube is closed. We also study the effect of the gap size between the cap and the tube in axial configuration. Finally, a tube closed at its both ends is also characterized. All experiments are carried out at 77K. The tube is subjected to a quasi-static (“DC”) uniform magnetic field. A Hall probe measures the three components of the local magnetic induction along the axis of the tube as a function of the applied magnetic induction. Results show that the shielding performances in the axial configuration are highly improved at the closed extremity as the cap reduces the penetration through the open end. The shielding factor distribution along the tube axis is affected by the presence of the cap. For an open tube, the shielding factor is maximum near the center and decreases towards both extremities. For the tube closed at one extremity, the shielding factor is maximum at the closed extremity and decreases towards the center. Moreover, the shielding factor distribution between the tube center and the open extremity is not affected by the presence of the cap. The experimental results show that, a small gap between the tube and the cap, which can be necessary for applications, does not strongly affect the magnetic shielding performances of the assembly. [less ▲]

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See detailMagnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Kirsch, Sébastien ULg et al

in Physica C: Superconductivity (2014), 502

Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In ... [more ▼]

Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In the present work, we investigate how the trapped field of such magnets can be improved by combining bulk YBCO with a soft FeNi, ferromagnetic alloy. This involves machining the alloy into components of various shapes, such as cylinders and rings, which are attached subsequently to the top surface of a solid, bulk HTS cylinder. The effect of these modifications on the magnetic hysteresis curve and trapped field of the bulk superconductor at 77 K are then studied using pick-up coil and Hall probe measurements. The experimental data are compared to finite element modelling of the magnetic flux distribution using Campbell’s algorithm. Initially we establish the validity of the technique involving pick-up coils wrapped around the bulk superconductor to obtain its magnetic hysteresis curve in a non-destructive way and highlight the difference between the measured signal and the true magnetization of the sample. We then consider the properties of hybrid ferromagnet/superconductor (F/S) structures. Hall probe measurements, together with the results of the model, establish that flux lines curve outwards through the ferromagnet, which acts, effectively, like a magnetic short circuit. Magnetic hysteresis curves show that the effects of the superconductor and the ferromagnet simply add when the ferromagnet is saturated fully by the applied field. The trapped field of the hybrid structure is always larger than that of the superconductor alone below this saturation level, and especially when the applied field is removed. The results of the study show further that the beneficial effects on the trapped field are enhanced when the ferromagnet covers the entire surface of the superconductor for different ferromagnetic components of various shapes and fixed volume. [less ▲]

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See detailDesign and fabrication of an electrode array sensor for probing the electric potential distribution at the mesoscopic scale in antistatic felts
Boutaayamou, Mohamed ULg; Lemaire, Philippe; Vanderheyden, Benoît ULg et al

in Measurement Science and Technology (2014), 25

We present an original voltage probe design for measuring the electric potential distribution at the mesoscopic scale (i.e., 1 mm–1 cm) in antistatic felts. The felts are composed of a mixture of non ... [more ▼]

We present an original voltage probe design for measuring the electric potential distribution at the mesoscopic scale (i.e., 1 mm–1 cm) in antistatic felts. The felts are composed of a mixture of non-conductive and metallic fibers and exhibit complex nonlinear electric behavior—including possibly nonlinearity and hysteresis effects—which may be due to localized electrical or electromechanical phenomena. The sensor consists of an array of 8 × 9 needle electrodes (phgr 160 µm at the shaft and less than phgr 50 µm toward the apex), which are mechanically maintained at fixed relative positions while their tips are inserted inside the fabric of the sample. The interelectrode distance is 1.5 mm and the overall active area is 12 × 12 mm². The electrical insulation resistance for nearest neighbor pairs of electrodes was found to be larger than 860 GΩ, thus making the sensor suitable for measuring antistatic felts with an electric resistance that typically does not exceed a few GΩ. The sensor was successfully used for measuring the distribution of the electric potential in a polyester fabric subjected to voltages of up to 6.2 kV, and in a sample containing 2% in weight of metallic fibers, demonstrating the presence of irreversible changes in that felt sample (i.e., with conductive fibers) at high voltages. It is concluded that the developed probe voltage is a promising technique that could be used for the assessment of the conduction mechanisms in the antistatic materials at the mesoscopic scale. [less ▲]

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See detailMeasurement of the magnetic hysteresis cycle of bulk superconductor / ferromagnet hybrids
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Conference (2013, November 07)

Large grain, bulk YBa2Cu3O7-δ (YBCO) high temperature superconductors (HTS) can be potentially used as powerful permanent magnets for magnetic levitation. The magnetic flux distribution above a bulk HTS ... [more ▼]

Large grain, bulk YBa2Cu3O7-δ (YBCO) high temperature superconductors (HTS) can be potentially used as powerful permanent magnets for magnetic levitation. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform compared to the rather flat distribution above a ferromagnet. In the present work, we study experimentally how cylindrical pieces of FeNi soft ferromagnetic alloys can be combined with a cylindrical, bulk, large grain YBCO superconductor to improve the distribution of the trapped field at the surface or its average value through the volume of the sample. The magnetic properties of each hybrid structure are measured at 77 K under axial magnetic field up to 3 T. The remanent induction distribution near the top and bottom surfaces is determined by miniature Hall probe mapping. Pick-up coils wound around the superconductor are used to measure the average flux density inside the superconductor. This technique relies on low level voltage measurements (typically a few microvolts) and allows non-destructive measurement of the average magnetic hysteresis curve of the entire bulk superconductor. [less ▲]

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See detailSemi-analytical study of AC losses in an infinitely long superconducting cylinder surrounded by a metallic sheath: magnetic field dependent critical current density and generation of harmonics
Kirsch, Sébastien ULg; Fagnard, Jean-François ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2013)

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by ... [more ▼]

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by assuming the Bean–Kim model for the superconductor and Ohmic dissipation for the metal. The time varying magnetic flux crossing the superconductor induces eddy currents in the metal sheath and, due to the nonlinear response of the superconducting material, generates harmonics in the metal current density. In turn, these currents generate distorted magnetic fields acting back on the superconductor. This coupling mechanism is sensitive to the magnetic constitutive law of the superconductor and affects both the waveform of the fields and the total losses. In this paper, we study the importance of the harmonics in the metal on the total losses, as well as their sensitivity to a field dependent critical current density following Kim's law. [less ▲]

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See detailCharacterization of structures with a complex disposition of YBCO coated conductors for magnetic shielding applications
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, G. A. et al

Poster (2013, September 16)

An efficient superconducting magnetic shield can be built as an assembly of YBCO 2G coated conductor sections. Each section is milled and placed around a cylindrical support in order to form a joint free ... [more ▼]

An efficient superconducting magnetic shield can be built as an assembly of YBCO 2G coated conductor sections. Each section is milled and placed around a cylindrical support in order to form a joint free superconducting loop where persistent currents can flow and provide a strong attenuation of a magnetic field. Our previous works have shown that this assembly is able to shield an axial quasi static (“DC”) magnetic field and that the shielding performances depend on the aspect ratio and the number of layers. The purpose of the present work is to study experimentally the shielding efficiency of several structures with a more complex orientation and position of YBCO coated conductors. Our aim is to design a magnetic shield that would be able to shield a magnetic field directed at any angle with respect to the superconducting loops. Such a structure can be obtained by placing pairs of coated conductors sections along three orthogonal axes. All experiments are carried out at 77K. The structure is subjected to a quasi-static (“DC”) magnetic field. A Hall probe measures the three components of the local magnetic induction inside the assembly as a function of the applied magnetic induction. The shielding efficiency of the structure is characterized as a function of (i) the magnetic field amplitude, (ii) the position of the Hall probe along the three axes, and (iii) the angles between the applied magnetic field and each axis. The experimental results allow us to determine the shielding efficiency in the central part of the new 3-axes structure. Although the shielding efficiency is lowered with respect to that of the traditional 1-axis-coil geometry, measurements at different field orientations allow us to identify the role played by each of the pairs of coils in screening the external magnetic field. [less ▲]

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See detailMagnetic hysteresis cycle and remnant field distribution of bulk high temperature superconductor / ferromagnet hybrids
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Poster (2013, September)

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean ... [more ▼]

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean model) compared to the rather flat distribution above a ferromagnet. In the present work, we study how FeNi soft ferromagnetic alloys of different shapes can be combined with a bulk, large grain (RE)BCO superconductor (RE denotes a rare-earth element) to improve the distribution of trapped field at the surface or its average value through the volume of the sample. A FeNi ferromagnetic alloy was machined into pieces of various shapes (cylinders and rings) and attached to (i) the top surface of the bulk HTS cylinder to form bulk ferromagnet / superconductor (F/S) hybrids and (ii) to the top and bottom surfaces to form bulk F/S/F hybrids. The magnetic properties of each hybrid structure were measured under axial magnetic field at 77 K. Pick-up coils wound around the superconductor were used to measure the average magnetic induction inside the superconductor while the remnant induction distribution near the top and bottom surfaces was determined by miniature Hall probe mapping. The modifications of the hysteresis curves and flux distributions were analyzed by taking into account the ferromagnet intrinsic properties (intrinsic permeability, saturation) and geometrical properties (shape, size and volume). The results show that the effect of the ferromagnet increases with its volume. In presence of a ferromagnet, the superconductor hysteresis curve shows a combination of a diamagnetic and a ferromagnetic behaviour on which it is worth noting that (i) the bulk remnant magnetization increases and (ii) in the magnetic saturation regime of the ferromagnet, the magnetic effects of the superconductor and the ferromagnet are superimposed. The results also give evidence that flux lines curve through the ferromagnetic component, which produces a decrease of the self-demagnetizing field inside the superconductor. [less ▲]

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See detailLight absorption in conical silicon particles
Bogdanowicz, J; Gilbert, M; Innocenti, N et al

in Optics Express (2013), 21(3), 3891

The problem of the absorption of light by a nanoscale dielectric cone is discussed. A simplified solution based on the analytical Mie theory of scattering and absorption by cylindrical objects is proposed ... [more ▼]

The problem of the absorption of light by a nanoscale dielectric cone is discussed. A simplified solution based on the analytical Mie theory of scattering and absorption by cylindrical objects is proposed and supported by the experimental observation of sharply localized holes in conical silicon tips after high-fluence irradiation. This study reveals that light couples with tapered objects dominantly at specific locations, where the local radius corresponds to one of the resonant radii of a cylindrical object, as predicted by Mie theory. [less ▲]

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See detailMagnetic shielding with YBCO coated conductors: influence of the geometry on its performances
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, G. A. et al

in IEEE Transactions on Applied Superconductivity (2012), 23(3),

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong ... [more ▼]

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic shields made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in detail the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic shield is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly at a fixed sweep rate. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the appliedmagnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the appliedmagnetic induction and the magnetic induction measured inside the shield) is improved for increasing aspect ratios of the global coated conductor assembly and that the threshold magnetic induction (defined for SF = 10) increases with the number of layers. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized.(C) 2012 IEEE [less ▲]

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See detailNon-destructive measurement of volume magnetic properties of large, bulk superconductors
Vanderbemden, Philippe ULg; Dennis, T.; Shi, Y. H. et al

Conference (2012, December 07)

The development of large, single grain bulk superconductors by melt processing techniques has generated a need to characterize samples magnetically over large dimensions, exceeding typically 20 mm in ... [more ▼]

The development of large, single grain bulk superconductors by melt processing techniques has generated a need to characterize samples magnetically over large dimensions, exceeding typically 20 mm in diameter. The usual magnetic characterization gives relies on miniature Hall probe mapping and gives information about the field distribution above the sample surface. If volume properties are required (e.g. magnetization hysteresis loops), the sample needs to be cut in smaller pieces, since the DC magnetic characterization systems for measurements at cryogenic temperatures generally accommodate samples of relatively small size (typically < 5-10 mm diameter). In this work we describe how the hysteresis B(H) loops of large bulk superconducting samples exceeding 10 mm diameter can be determined using home-made sensing coils, either in liquid nitrogen or within the experimental chamber of a Quantum Design Physical Property Measurement System (PPMS). Magnetic properties are measured and compared to those given by several Hall probes attached to both faces of sample. The system is used successfully to measure the DC hysteresis loops of entire (RE)BCO bulk superconducting domains. A careful data acquisition and numerical integration of pick-up coil voltage enables the sweep rate of the magnetic field to be varied from 0.5 to 10 mT/s while keeping an excellent signal/noise ratio. A simple model based on demagnetizing field approach is used to emphasize how the hysteresis loops determined by this technique differ from “true” magnetization loops derived from classical magnetic moment measurements (e.g. SQUID or VSM). These differences are supported with numerical modelling of the average magnetization of the bulk sample using the Brandt method. [less ▲]

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See detailMagnetic shielding with YBCO coated conductors: influence of the geometry on its performances.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, George et al

Poster (2012, October 10)

A superconducting magnetic screen can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong ... [more ▼]

A superconducting magnetic screen can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic screens made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in details the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic screen is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly a fixed sweep rates. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the applied magnetic induction. Results show that the threshold magnetic induction (above which magnetic shielding ceases) increases with the number of layers and that the shielding factor (defined as to ratio between the applied magnetic induction and the magnetic induction measured inside the screen) is improved for increasing aspect ratios of the global coated conductor assembly. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. The experimental results are compared to theoretical predictions for a perfect (type-I) magnetic shield with the same aspect ratio. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized. [less ▲]

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See detailMagnetic behaviour of soft ferromagnetic alloys attached to bulk (RE)BCO superconductors
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Dennis, Anthony et al

Poster (2012, October)

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic field distribution around a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the ... [more ▼]

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic field distribution around a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean model) compared to the rather flatter distribution around a ferromagnet. In the present work, we study how FeNi soft ferromagnetic alloys of different shapes can be combined with (RE)BCO bulk superconductors to improve the distribution of trapped field or its average value at the surface of the bulk sample. The FeNi ferromagnetic alloys are machined into pieces of various shapes (e.g. cylinder, cone) attached to the bottom surface of the bulk HTS disks. The magnetic properties of each ferromagnetic/superconductor (F/S) assembly are measured under axial magnetic field at 77 K. Small coils and Hall probe mappings provide the average magnetization and field distribution respectively. The results are compared to the intrinsic magnetic hysteresis curves of the magnetic alloys and of the superconductor. Measurements of the average magnetization show that the effects of the ferromagnetic alloys depend mainly on the volume of the ferromagnet and add up to the effect of the superconductor. However, at constant ferromagnetic volume, the shape of the ferromagnetic piece is found to influence the magnetic field penetration and distribution against the top surface of the superconductor. [less ▲]

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See detailMagnetic shielding properties of a superconducting hollow cylinder containing slits: Modelling and experiment
Fagnard, Jean-François ULg; Elschner, S.; Hobl, A. et al

in Superconductor Science and Technology (2012), 25(10), 104006

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a ... [more ▼]

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a hollow cylinder with two axial slits which cut the cylinder in equal halves. Finite element method modelling has been used with a three-dimensional geometry to help us in understanding how the superconducting currents flow in such a cut cylinder and therefore how the magnetic shielding properties are affected, depending on the magnetic field orientation. Modelling results show that the slits block the shielding current flow and act as an entrance channel for the magnetic flux lines. The contribution of the slits to the total flux density that enters the cylinder is studied through the angle formed between the applied field and the internal field. The modelled data agree nicely with magnetic shielding properties measured on a bulk Bi-2212 hollow cylinder at 77K. The results demonstrate that the magnetic flux penetration in such a geometry can be modelled successfully using only two parameters of the superconductor (constant J c and n value), which were determined from magnetic measurements on the plain cylinder. © 2012 IOP Publishing Ltd. [less ▲]

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See detailMagnetic Shielding with Bulk High Temperature Superconductors: Factors Influencing the Magnetic Field Penetration in Hollow Cylinders
Fagnard, Jean-François ULg; Vanderheyden, Benoît ULg; Vanderbemden, Philippe ULg

in Muralidhar, Miryala (Ed.) Superconductivity: Recent Developments and New Production Technologies (2012)

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See detailStudy of Conduction Mechanisms in Antistatic Felts at the Mesoscopic Scale
Boutaayamou, Mohamed ULg; Léonard, Angélique ULg; Vanderheyden, Benoît ULg et al

in International Symposium on New Frontiers in Fiber Materials Science; Conference Proceedings (2011, October 11)

This work is part of a project that deals with the optimization of the quantity and the nature of conductive fibers in antistatic felts used for filtering and sieving powders. Our research concerns the ... [more ▼]

This work is part of a project that deals with the optimization of the quantity and the nature of conductive fibers in antistatic felts used for filtering and sieving powders. Our research concerns the electrical properties at the mesoscopic scale. It aims at determining the conduction mechanisms and the distribution of the electric potential at the scale of the distance between the conductive fibers. In this paper, current-voltage (I-V) measurement results are presented and discussed. X-ray microtomography is used to obtain the geometry of the conductive fibers inside the felts before and after these I-V tests. The studied textile material is based on polyester fibers and stainless steel conductive fibers. [less ▲]

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See detailRandom matrix models for phase diagrams
Vanderheyden, Benoît ULg; Jackson, A. D.

in Reports on Progress in Physics (2011), 74

We describe a random matrix approach that can provide generic and readily soluble mean-field descriptions of the phase diagram for a variety of systems ranging from QCD to high-T_c materials. Instead of ... [more ▼]

We describe a random matrix approach that can provide generic and readily soluble mean-field descriptions of the phase diagram for a variety of systems ranging from QCD to high-T_c materials. Instead of working from specific models, phase diagrams are constructed by averaging over the ensemble of theories that possesses the relevant symmetries of the problem. Although approximate in nature, this approach has a number of advantages. First, it can be useful in distinguishing generic features from model-dependent details. Second, it can help in understanding the `minimal' number of symmetry constraints required to reproduce specific phase structures. Third, the robustness of predictions can be checked with respect to variations in the detailed description of the interactions. Finally, near critical points, random matrix models bear strong similarities to Ginsburg-Landau theories with the advantage of additional constraints inherited from the symmetries of the underlying interaction. These constraints can be helpful in ruling out certain topologies in the phase diagram. In this Key Issue, we illustrate the basic structure of random matrix models, discuss their strengths and weaknesses, and consider the kinds of system to which they can be applied. [less ▲]

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See detailMagnetic properties of drilled bulk high-temperature superconductors filled with a ferromagnetic powder
Lousberg, Grégory; Fagnard, Jean-François ULg; Chaud, Xavier et al

in Superconductor Science and Technology (2011), 24

It is shown that filling the holes of a drilled bulk high-temperature superconductor (HTS) with a soft ferromagnetic powder enhances its trapping properties. The magnetic properties of the trapped field ... [more ▼]

It is shown that filling the holes of a drilled bulk high-temperature superconductor (HTS) with a soft ferromagnetic powder enhances its trapping properties. The magnetic properties of the trapped field magnet are characterized by Hall probe mapping and magnetization measurements. This analysis is completed by a numerical model based on a 3D finite-element method where the conductivity of the superconducting material is described by a power law while the permeability of the ferromagnetic material is fixed to a given value and is considered uniform. Numerical results support the experimental observations. In particular, they confirm the increase of trapped flux that is observed with Hall probe mapping after impregnation. [less ▲]

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See detailBulk melt-processed (RE)BCO superconductors subjected to AC fields in “parallel” and “crossed” direction, experiment and modelling
Vanderbemden, Philippe ULg; Fagnard, Jean-François ULg; Babu, N. H. et al

Conference (2010, November)

When bulk melt-processed (RE)BCO samples are used as permanent magnets in various engineering applications (e.g. rotating machines, magnetic bearings), they may experience transient or periodic variations ... [more ▼]

When bulk melt-processed (RE)BCO samples are used as permanent magnets in various engineering applications (e.g. rotating machines, magnetic bearings), they may experience transient or periodic variations of the applied magnetic field. In such a situation, the remnant magnetization of the sample is found to decrease in running operation, which is particularly inconvenient for the long-term reliability of the system. There are several possible causes for such a phenomenon. The first is the temperature increase resulting from the losses caused by the variable applied field, and may arise when the field is parallel to the main direction of the trapped flux. The second is caused by the component of the magnetic field perpendicular to the main direction of the trapped flux (“crossed field” configuration). In the present talk we review the relevant parameters which have a detrimental impact on the remnant magnetization in both cases. Modelling results are compared to experimental data obtained on bulk YBCO samples subjected to field oscillations of large amplitude. [less ▲]

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