The Experts below are selected from a list of 76806 Experts worldwide ranked by ideXlab platform
R Majeski - One of the best experts on this subject based on the ideXlab platform.
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equilibrium reconstruction with 3d Eddy Currents in the lithium tokamak experiment
Physics of Plasmas, 2017Co-Authors: J C Schmitt, C J Hansen, D P Boyle, R MajeskiAbstract:Axisymmetric free-boundary equilibrium reconstructions of tokamak plasmas in the Lithium Tokamak eXperiment (LTX) are performed using the PSI-Tri equilibrium code. Reconstructions in LTX are complicated by the presence of long-lived non-axisymmetric Eddy Currents generated by a vacuum vessel and first wall structures. To account for this effect, reconstructions are performed with additional toroidal current sources in these conducting regions. The Eddy current sources are fixed in their poloidal distributions, but their magnitude is adjusted as part of the full reconstruction. Eddy distributions are computed by toroidally averaging Currents, generated by coupling to vacuum field coils, from a simplified 3D filament model of important conducting structures. The full 3D Eddy current fields are also used to enable the inclusion of local magnetic field measurements, which have strong 3D Eddy current pick-up, as reconstruction constraints. Using this method, equilibrium reconstruction yields good agreement wit...
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equilibrium reconstruction with 3d Eddy Currents in the lithium tokamak experiment
arXiv: Plasma Physics, 2017Co-Authors: J C Schmitt, C J Hansen, D P Boyle, R MajeskiAbstract:Axisymmetric free-boundary equilibrium reconstructions of tokamak plasmas in the Lithium Tokamak eXperiment (LTX) are performed using the PSI-Tri equilibrium code. Reconstructions in LTX are complicated by the presence of long-lived non-axisymmetric Eddy Currents generated by vacuum vessel and first wall structures. To account for this effect, reconstructions are performed with additional toroidal current sources in these conducting regions. The source distributions are fixed poloidally, but their scale is adjusted as part of the full reconstruction. Eddy distributions are computed by toroidally averaging Currents, generated by coupling to vacuum field coils, from a simplified 3D filament model of important conducting structures. The full 3D Eddy current fields are also used to enable the inclusion of local magnetic field measurements, which have strong 3D Eddy current pick-up, as reconstruction constraints. Using this method, equilibrium reconstruction yields good agreement with all available diagnostic signals. An accompanying field perturbation produced by 3D Eddy Currents on the plasma surface with primarily n=2, m=1 character is also predicted for these equilibria.
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magnetic diagnostics for equilibrium reconstructions with Eddy Currents on the lithium tokamak experimenta
Review of Scientific Instruments, 2014Co-Authors: J C Schmitt, J Bialek, S Lazerson, R MajeskiAbstract:The Lithium Tokamak eXperiment is a spherical tokamak with a close-fitting low-recycling wall composed of thin lithium layers evaporated onto a stainless steel-lined copper shell. Long-lived non-axisymmetric Eddy Currents are induced in the shell and vacuum vessel by transient plasma and coil Currents and these Eddy Currents influence both the plasma and the magnetic diagnostic signals that are used as constraints for equilibrium reconstruction. A newly installed set of re-entrant magnetic diagnostics and internal saddle flux loops, compatible with high-temperatures and lithium environments, is discussed. Details of the axisymmetric (2D) and non-axisymmetric (3D) treatments of the Eddy Currents and the equilibrium reconstruction are presented.
Vincent Baltz - One of the best experts on this subject based on the ideXlab platform.
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Stacking order-dependent sign-change of microwave phase due to Eddy Currents in nanometer-scale NiFe/Cu heterostructures
Applied Physics Letters, 2019Co-Authors: O. Gladii, R Seeger, L. Frangou, G. Forestier, U. Ebels, S. Auffret, Vincent BaltzAbstract:In the field of spintronics, ferromagnetic/non-magnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of Eddy Currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers thick in the sub-skin-depth regime. Asymmetry in the absorption spectrum gradually built up as the excitation frequency and Cu-layer thickness increased. Most significantly, the sign of the asymmetry depended on the stacking order. Experimental data were consistent with a quantitative analysis considering Eddy Currents generated in the Cu layers and the subsequent phaseshift of the feedback magnetic field generated by the Eddy Currents. These results extend our understanding of the impact of Eddy Currents below the microwave magnetic skin-depth and explain the lineshape asymmetry and phase lags reported in stripline experiments.
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stacking order dependent sign change of microwave phase due to Eddy Currents in nanometer scale nife cu heterostructures
Applied Physics Letters, 2019Co-Authors: O. Gladii, R Seeger, L. Frangou, G. Forestier, U. Ebels, S. Auffret, Vincent BaltzAbstract:In the field of spintronics, ferromagnetic/non-magnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of Eddy Currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers thick in the sub-skin-depth regime. Asymmetry in the absorption spectrum gradually built up as the excitation frequency and Cu-layer thickness increased. Most significantly, the sign of the asymmetry depended on the stacking order. Experimental data were consistent with a quantitative analysis considering Eddy Currents generated in the Cu layers and the subsequent phaseshift of the feedback magnetic field generated by the Eddy Currents. These results extend our understanding of the impact of Eddy Currents below the microwave magnetic skin-depth and explain the lineshape asymmetry and phase lags reported in stripline experiments.
Stuart Crozier - One of the best experts on this subject based on the ideXlab platform.
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multilayer integral method for simulation of Eddy Currents in thin volumes of arbitrary geometry produced by mri gradient coils
Magnetic Resonance in Medicine, 2014Co-Authors: Hector Sanchez Lopez, Adnan Trakic, Fabio Freschi, Elliot Smith, Jeremy Herbert, Miguel Fuentes, Stephen Wilson, Limei Liu, Maurizio Repetto, Stuart CrozierAbstract:Purpose This article aims to present a fast, efficient and accurate multi-layer integral method (MIM) for the evaluation of complex spatiotemporal Eddy Currents in nonmagnetic and thin volumes of irregular geometries induced by arbitrary arrangements of gradient coils. Methods The volume of interest is divided into a number of layers, wherein the thickness of each layer is assumed to be smaller than the skin depth and where one of the linear dimensions is much smaller than the remaining two dimensions. The diffusion equation of the current density is solved both in time-harmonic and transient domain. Results The experimentally measured magnetic fields produced by the coil and the induced Eddy Currents as well as the corresponding time-decay constants were in close agreement with the results produced by the MIM. Relevant parameters such as power loss and force induced by the Eddy Currents in a split cryostat were simulated using the MIM. Conclusion The proposed method is capable of accurately simulating the current diffusion process inside thin volumes, such as the magnet cryostat. The method permits the priori-calculation of optimal pre-emphasis parameters. The MIM enables unified designs of gradient coil - magnet structures for an optimal mitigation of deleterious Eddy current effects.
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analysis of transient Eddy Currents in mri using a cylindrical fdtd method
IEEE Transactions on Applied Superconductivity, 2006Co-Authors: Adnan Trakic, Hua Wang, Feng Liu, Hector Sanchez Lopez, Stuart CrozierAbstract:Most magnetic resonance imaging (MRI) spatial encoding techniques employ low-frequency pulsed magnetic field gradients that undesirably induce multiexponentially decaying Eddy Currents in nearby conducting structures of the MRI system. The Eddy Currents degrade the switching performance of the gradient system, distort the MRI image, and introduce thermal loads in the cryostat vessel and superconducting MRI components. Heating of superconducting magnets due to induced Eddy Currents is particularly problematic as it offsets the superconducting operating point, which can cause a system quench. A numerical characterization of transient Eddy current effects is vital for their compensation/control and further advancement of the MRI technology as a whole. However, transient Eddy current calculations are particularly computationally intensive. In large-scale problems, such as gradient switching in MRI, conventional finite-element method (FEM)-based routines impose very large computational loads during generation/solving of the system equations. Therefore, other computational alternatives need to be explored. This paper outlines a three-dimensional finite-difference time-domain (FDTD) method in cylindrical coordinates for the modeling of low-frequency transient Eddy Currents in MRI, as an extension to the recently proposed time-harmonic scheme. The weakly coupled Maxwell's equations are adapted to the low-frequency regime by downscaling the speed of light constant, which permits the use of larger FDTD time steps while maintaining the validity of the Courant-Friedrich-Levy stability condition. The principal hypothesis of this work is that the modified FDTD routine can be employed to analyze pulsed-gradient-induced, transient Eddy Currents in superconducting MRI system models. The hypothesis is supported through a verification of the numerical scheme on a canonical problem and by analyzing undesired temporal Eddy current effects such as the B0-shift caused by actively shielded symmetric/asymmetric transverse x-gradient head and unshielded z-gradient whole-body coils operating in proximity to a superconducting MRI magnet
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An FDTD model for calculation of gradient-induced Eddy Currents in MRI system
IEEE Transactions on Applied Superconductivity, 2004Co-Authors: Stuart CrozierAbstract:In most magnetic resonance imaging (MRI) systems, pulsed magnetic gradient fields induce Eddy Currents in the conducting structures of the superconducting magnet. The Eddy Currents induced in structures within the cryostat are particularly problematic as they are characterized by long time constants by virtue of the low resistivity of the conductors. This paper presents a three-dimensional (3-D) finite-difference time-domain (FDTD) scheme in cylindrical coordinates for Eddy-current calculation in conductors. This model is intended to be part of a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The singularity apparent in the governing equations is removed by using a series expansion method and the conductor-air boundary condition is handled using a variant of the surface impedance concept. The numerical difficulty due to the "asymmetry" of Maxwell equations for low-frequency Eddy-current problems is circumvented by taking advantage of the known penetration behavior of the Eddy-current fields. A perfectly matched layer absorbing boundary condition in 3-D cylindrical coordinates is also incorporated. The numerical method has been verified against analytical solutions for simple cases. Finally, the algorithm is illustrated by modeling a pulsed field gradient coil system within an MRI magnet system. The results demonstrate that the proposed FDTD scheme can be used to calculate large-scale Eddy-current problems in materials with high conductivity at low frequencies.
C J Hansen - One of the best experts on this subject based on the ideXlab platform.
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equilibrium reconstruction with 3d Eddy Currents in the lithium tokamak experiment
Physics of Plasmas, 2017Co-Authors: J C Schmitt, C J Hansen, D P Boyle, R MajeskiAbstract:Axisymmetric free-boundary equilibrium reconstructions of tokamak plasmas in the Lithium Tokamak eXperiment (LTX) are performed using the PSI-Tri equilibrium code. Reconstructions in LTX are complicated by the presence of long-lived non-axisymmetric Eddy Currents generated by a vacuum vessel and first wall structures. To account for this effect, reconstructions are performed with additional toroidal current sources in these conducting regions. The Eddy current sources are fixed in their poloidal distributions, but their magnitude is adjusted as part of the full reconstruction. Eddy distributions are computed by toroidally averaging Currents, generated by coupling to vacuum field coils, from a simplified 3D filament model of important conducting structures. The full 3D Eddy current fields are also used to enable the inclusion of local magnetic field measurements, which have strong 3D Eddy current pick-up, as reconstruction constraints. Using this method, equilibrium reconstruction yields good agreement wit...
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equilibrium reconstruction with 3d Eddy Currents in the lithium tokamak experiment
arXiv: Plasma Physics, 2017Co-Authors: J C Schmitt, C J Hansen, D P Boyle, R MajeskiAbstract:Axisymmetric free-boundary equilibrium reconstructions of tokamak plasmas in the Lithium Tokamak eXperiment (LTX) are performed using the PSI-Tri equilibrium code. Reconstructions in LTX are complicated by the presence of long-lived non-axisymmetric Eddy Currents generated by vacuum vessel and first wall structures. To account for this effect, reconstructions are performed with additional toroidal current sources in these conducting regions. The source distributions are fixed poloidally, but their scale is adjusted as part of the full reconstruction. Eddy distributions are computed by toroidally averaging Currents, generated by coupling to vacuum field coils, from a simplified 3D filament model of important conducting structures. The full 3D Eddy current fields are also used to enable the inclusion of local magnetic field measurements, which have strong 3D Eddy current pick-up, as reconstruction constraints. Using this method, equilibrium reconstruction yields good agreement with all available diagnostic signals. An accompanying field perturbation produced by 3D Eddy Currents on the plasma surface with primarily n=2, m=1 character is also predicted for these equilibria.
Kim Butts Pauly - One of the best experts on this subject based on the ideXlab platform.
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improved half rf slice selectivity in the presence of Eddy Currents with out of slice saturation
Magnetic Resonance in Medicine, 2009Co-Authors: Sonal Josan, Elena Kaye, John M Pauly, Bruce L Daniel, Kim Butts PaulyAbstract:Ultrashort echo time imaging with half RF pulse excitation is sensitive to Eddy Currents induced by the slice-select gradient that distort the half pulse slice profile. This work demonstrates improvements to the half pulse profile by using spatial saturation on both sides of the imaged slice to suppress the out-of-slice magnetization. This effectively improves the selectivity of the half pulse excitation profile. A quadratic phase RF pulse with high bandwidth and selectivity is used to achieve a wide saturation band with sharp edges. Experimental results demonstrate substantially improved slice selectivity and R2* quantitation accuracy obtained with the out-of-slice saturation. This approach is effective in making short T2 imaging and quantitation with half pulses less sensitive to Eddy Currents.
