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On the magnetic properties of drilled bulk high-temperature superconductors containing an artifical array of holes Lousberg, Grégory Doctoral thesis (2010) Detailed reference viewed: 38 (16 ULg)Numerical Study of the Shielding properties of Macroscopic Hybrid Ferromagnetic/Superconductor Hollow Cylinders Lousberg, Grégory ; Fagnard, Jean-François ; Ausloos, Marcel et al in IEEE Transactions on Applied Superconductivity (2010), 20(1), 33 We study the magnetic shielding properties of hybrid ferromagnetic/superconductor (F/S) structures consisting of two coaxial cylinders, with one of each material.We use an axisymmetric finite-element ... [more ▼] We study the magnetic shielding properties of hybrid ferromagnetic/superconductor (F/S) structures consisting of two coaxial cylinders, with one of each material.We use an axisymmetric finite-element model in which the electrical properties of the superconducting tube are modeled by a nonlinear E-J power law with a magnetic-field-dependent critical current density whereas the magnetic properties of the ferromagnetic material take saturation into account. We study and compare the penetration of a uniform axial magnetic field in two cases: 1) a ferromagnetic tube placed inside a larger superconducting tube (Ferro-In configuration) and 2) a ferromagnetic tube placed outside the superconducting one (Ferro-Out configuration). In both cases, we assess how the ferromagnetic tube improves the shielding properties of the sole superconducting tube. The influence of the geometrical parameters of the ferromagnetic tube is also studied: It is shown that, upon an optimal choice of the geometrical parameters, the range of magnetic fields that are efficiently shielded by the hightemperature superconductor tube alone can be increased by a factor of up to 7 (2) in a Ferro-Out (Ferro-In) configuration. The optimal configuration uses a 1020 carbon steel with a thickness of 2 mm and a height that is half that of the superconducting cylinder (80 mm). [less ▲] Detailed reference viewed: 107 (58 ULg)Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface Lousberg, Grégory ; Fagnard, Jean-François ; et al in Superconductor Science and Technology (2009), (22), 125026 We present a method for characterizing the propagation of the magnetic flux in an artificially drilled bulk high-temperature superconductor (HTS) during pulsed-field magnetization. As the magnetic pulse ... [more ▼] We present a method for characterizing the propagation of the magnetic flux in an artificially drilled bulk high-temperature superconductor (HTS) during pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical sample, the magnetic flux density is measured simultaneously in 16 holes by means of microcoils that are placed across the median plane, i.e. at an equal distance from the top and bottom surfaces, and close to the surface of the sample. We discuss the time evolution of the magnetic flux density in the holes during a pulse and measure the time taken by the external magnetic flux to reach each hole. Our data show that the flux front moves faster in the median plane than on the surface when penetrating the sample edge; it then proceeds faster along the surface than in the bulk as it penetrates the sample further. Once the pulse is over, the trapped flux density inside the central hole is found to be about twice as large in the median plane as on the surface. This ratio is confirmed by modelling. [less ▲] Detailed reference viewed: 69 (42 ULg)Magnetic shielding properties of high- Tc superconducting hollow cylinders: model combining experimental data for axial and transverse magnetic field configurations Fagnard, Jean-François ; ; Ausloos, Marcel et al in Superconductor Science and Technology (2009), 22(10), 10500210 Magnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The ... [more ▼] Magnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce the main features of the field penetration curves by using a minimum number of free parameters suitable for both magnetic field orientations. The field penetration measurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applying linearly increasing magnetic fields with a constant sweep rate ranging between 10 uT s[?]1 and 10 mT s[?]1 for both directions of the applied magnetic field. The experimental curves of the internal field versus the applied field, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs, Blim, is lower for the transverse configuration than for the axial configuration. A power law dependence with large exponent, n', is found between Blim and dBapp/dt. The values of n' are nearly the same for both configurations. We show that the main features of the curves Bin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving a E(J) power law with an n-exponent and a field-dependent critical current density, Jc(B), (following the Kim model: Jc = Jc0(1+B/B1)[?]1). In particular, a linear relationship between the measured n'-exponents and the n-exponent of the E(J) power law is suggested by taking into account the field dependence of the critical current density. Differences between the axial and the transverse shielding properties can be simply attributed to demagnetizing fields. [less ▲] Detailed reference viewed: 111 (48 ULg)Modification of the trapped field in bulk HTS as a result of the drilling of a pattern of artificial columnar holes Lousberg, Grégory Poster (2009, September) Detailed reference viewed: 30 (17 ULg)Characterization of the magnetic flux propagation in a drilled YBCO single‐crystal during a pulsed‐field excitation Lousberg, Grégory Poster (2009, September) Article associé : Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface Detailed reference viewed: 17 (7 ULg)DC and AC Shielding Properties of Bulk High-Tc Superconducting Tubes Fagnard, Jean-François ; ; Lousberg, Grégory et al in IEEE Transactions on Applied Superconductivity (2009), 19(3), 2905-2908 We have studied numerically and experimentally the magnetic flux penetration in high-Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view ... [more ▼] We have studied numerically and experimentally the magnetic flux penetration in high-Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of high-Tc superconducting ceramics. We have measured the field attenuation for applied magnetic fields in the frequency range 5 mHz-0.1 Hz by Hall probe measurements and at audio frequencies using a sensing coil. A simple 1D analysis using the Kim critical state model was found to be able to reproduce the experimental data satisfactorily. We have also determined the phase shift between the internal and the applied field both experimentally and numerically. Finally, we have studied the sweep rate dependence of the magnetic shielding properties, using data recorded either at several constant sweep rates dB/dt or at several AC fields of various amplitudes and frequencies. Both methods agree with each other and lead to a n -value of the E ~ J n law equal to ~ 40 at 77 K. [less ▲] Detailed reference viewed: 159 (63 ULg)Numerical simulation of the magnetization of high-temperature superconductors: a 3D finite element method using a single time-step iteration Lousberg, Grégory ; Ausloos, Marcel ; Geuzaine, Christophe et al in Superconductor Science and Technology (2009), 22 In this paper, we report progress towards a 3D finite element model for the magnetization of a high-temperature superconductor (HTS): we suggest a method that takes into account a power law conductivity ... [more ▼] In this paper, we report progress towards a 3D finite element model for the magnetization of a high-temperature superconductor (HTS): we suggest a method that takes into account a power law conductivity and demagnetization effects, while neglecting the effects associated with currents that are not perpendicular to the local magnetic induction. We consider samples that are subjected to a uniform magnetic field varying linearly with time. Their magnetization is calculated by means of a weak formulation in the magnetostatic approximation of the Maxwell equations (A–φ formulation). An implicit method is used for the temporal resolution (backward Euler scheme) and is solved with the open source solver GetDP. Fixed point iterations are used to deal with the power law conductivity of HTS. The finite element formulation is validated for an HTS tube with large n value by comparing with results obtained with other well-established methods. We show that carrying out the calculations with a single time-step (as opposed to many small time-steps) produces results with excellent accuracy in a drastically reduced simulation time. The numerical method is extended to the study of the trapped magnetization of cylinders that are drilled with different arrays of columnar holes arranged parallel to the cylinder axis. [less ▲] Detailed reference viewed: 170 (57 ULg)Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor Lousberg, Grégory ; Fagnard, Jean-François ; et al in Superconductor Science and Technology (2009), 22 We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 ... [more ▼] We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 to 130 mT and the flux in the superconductor is probed by miniature coils inserted in the holes. In a given hole, three different penetration regimes can be observed: (i) the shielded regime, where no magnetic flux threads the hole; (ii) the gradual penetration regime, where the waveform of the magnetic field has a clipped sine shape whose fundamental component scales with the applied field; and (iii) the flux concentration regime, where the waveform of the magnetic field is nearly a sine wave, with an amplitude exceeding that of the applied field by up to a factor of two. The distribution of the penetration regimes in the holes is compared with that of the magnetic flux density at the top and bottom surfaces of the sample, and is interpreted with the help of optical polarized light micrographs of these surfaces. We show that the measurement of the magnetic field inside the holes can be used as a local characterization of the bulk magnetic properties of the sample. [less ▲] Detailed reference viewed: 120 (82 ULg)Modeling of ferromagnetic/high‐Tc superconductor structures for shielding applications Lousberg, Grégory Poster (2008, August) Detailed reference viewed: 28 (16 ULg)3D finite‐element modeling for the magnetization of bulk high‐Tc drilled superconductors Lousberg, Grégory Poster (2008, August) Article associé : Numerical simulation of the magnetization of high-temperature superconductors: a 3D finite element method using a single time-step iteration Detailed reference viewed: 28 (9 ULg)Backward Euler method for single time step finite element simulation of the magnetization of high-temperature bulk superconductors with non linear conductivity Lousberg, Grégory Conference (2008, May) We consider a three dimensional (3D) finite element modeling of high temperature superconductors (HTS) submitted to a uniform magnetic field varying linearly with time. HTS are characterized by a non ... [more ▼] We consider a three dimensional (3D) finite element modeling of high temperature superconductors (HTS) submitted to a uniform magnetic field varying linearly with time. HTS are characterized by a non linear conductivity described by a power law with the critical exponent, n. We implement an A- phi weak formulation into the opensource solver GetDP. Backward Euler method is used for the temporal resolution and is coupled to a non linear Picard iteration scheme for dealing with the non linear resistivity of HTS.We consider single- and multiple-time step methods and show that the single time step method produces an accurate result in a drastically reduced calculation time for HTS with large pinning strength (or similarly, with large values of n). The finite element formulation is validated by comparing the magnetization of a HTS tube calculated either with our finite element formulation or with other well-known techniques. The numerical method is then applied for comparing the magnetization of cylinders that are drilled by different periodic arrangements of columnar holes parallel to the cylinder axis. [less ▲] Detailed reference viewed: 274 (24 ULg)Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux? Lousberg, Grégory ; Ausloos, Marcel ; Vanderbemden, Philippe et al in Superconductor Science and Technology (2008), 21 Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be ... [more ▼] Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be trapped in the sample. In this paper, we use the Bean model to study the magnetization and the current line distribution in drilled samples, as a function of the hole positions. A single hole perturbs the critical current flow over an extended region that is bounded by a discontinuity line, where the direction of the current density changes abruptly. We demonstrate that the trapped magnetic flux is maximized if the centre of each hole is positioned on one of the discontinuity lines produced by the neighbouring holes. For a cylindrical sample, we construct a polar triangular hole pattern that exploits this principle; in such a lattice, the trapped field is ∼20% higher than in a squared lattice, for which the holes do not lie on discontinuity lines. This result indicates that one can simultaneously enhance the oxygen annealing, the heat transfer and maximize the trapped field. [less ▲] Detailed reference viewed: 65 (27 ULg)Hole lattice influence on the magnetization of drilled superconductors Lousberg, Grégory Poster (2007, September) Article associé : Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux? Detailed reference viewed: 15 (7 ULg)Measurement of the magnetic flux threading holes of drilled superconductors Lousberg, Grégory Poster (2007, September) Article associé : Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor Detailed reference viewed: 11 (4 ULg)Schottky-Barrier height lowering by an increase of the substrate doping in PtSi Schottky barrier source/drain FETs Lousberg, Grégory ; ; et al in IEEE Electron Device Letters (2007), 28(2), 123-125 In this letter, the Schottky-barrier height (SBH) lowering in Pt silicide/n-Si junctions and its implications to Schottky-barrier source/drain p-field-effect transistors (p-SBFETs) are studied ... [more ▼] In this letter, the Schottky-barrier height (SBH) lowering in Pt silicide/n-Si junctions and its implications to Schottky-barrier source/drain p-field-effect transistors (p-SBFETs) are studied experimentally and numerically. We demonstrate that the increase of the n-Si substrate doping is responsible for a larger hole SBH lowering through an image-force mechanism, which leads to a substantial gain of the drive current in the long-channel bulk p-SBFETs. Numerical simulations. show that the channel doping concentration is also critical for short-channel p/n-silicon-on-insulator SBFET performance. [less ▲] Detailed reference viewed: 63 (3 ULg)Spatial coherence measurements in arrays of coupled vertical cavity surface emitting lasers ; Lousberg, Grégory ; et al in Applied Physics Letters (2007), 90 The spatial coherence and the optical phase distribution across a two-dimensional 2D photonic crystal implemented with coupled arrays of vertical cavity surface emitting lasers VCSELs are experimentally ... [more ▼] The spatial coherence and the optical phase distribution across a two-dimensional 2D photonic crystal implemented with coupled arrays of vertical cavity surface emitting lasers VCSELs are experimentally characterized. This is achieved by performing Young’s interference experiments between pairs of array elements using a spatial light modulator arrangement. In contrast to far-field measurements that provide information only on the global spatial coherence, this approach can yield full mapping of the complex degree of spatial coherence. Examples of such analysis are presented for nominally uniform one-dimensional and 2D arrays of coupled VCSELs and possible mechanisms of the observed coherence degradation are discussed. [less ▲] Detailed reference viewed: 16 (1 ULg)Experimental and simulation study of the Schottky barrier lowering by substrate doping variation for PtSi Source/Drain SBFETs Lousberg, Grégory ; ; et al in Proceedings of ESSDERC 2006 (2006, September) In this paper, the authors study experimentally and numerically the Schottky barrier height (SBH) lowering of Pt silicide/n-Si diodes and its implications to Schottky-barrier (SB) source/drain p-FETs. The ... [more ▼] In this paper, the authors study experimentally and numerically the Schottky barrier height (SBH) lowering of Pt silicide/n-Si diodes and its implications to Schottky-barrier (SB) source/drain p-FETs. The authors demonstrate that hole SBH can be lowered through an image-force mechanism by increasing the n-Si substrate doping, which leads to a substantial gain of the drive current in the long-channel bulk p-SBFETs. Numerical simulations show that the channel doping concentration is also critical for short-channel n- & p-SOI SBFETs performance [less ▲] Detailed reference viewed: 30 (0 ULg)Space-domain lock-in amplifier based on a liquid-crystal spatial light modulator Lousberg, Grégory ; ; et al in Optics Letters (2006), 31(7), 990 We present a two-dimensional (2D) spatial lock-in amplifier that provides a contrast ratio of more than 10,000:1 for transmitted and blocked intensity patterns using a conventional liquid-crystal spatial ... [more ▼] We present a two-dimensional (2D) spatial lock-in amplifier that provides a contrast ratio of more than 10,000:1 for transmitted and blocked intensity patterns using a conventional liquid-crystal spatial light modulator. The device is based on spatial-domain modulation–demodulation of intensity patterns under coherent imaging conditions. The operation of the 2D lock-in amplifier is illustrated by implementing Young’s double-slit arrangement for measurements of the mutual coherence between individual emitters of a 2D phase-coupled array of vertical cavity surface emitting lasers. [less ▲] Detailed reference viewed: 40 (3 ULg) |
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