Critical state analysis of orthogonal flux interactions in pinned superconductors

We show that, based on the critical state model for flux-line pinning in hard superconductors, one can assess the magnetic moment relaxation induced by the oscillations of a perpendicular magnetic field. Our theory follows a recent proposal of using phenomenological 2D modeling for the description of crossed field dynamics in high-T$_c$ superconductors [{\tt arXiv:cond-mat/0703330}]. Stationary regimes with either saturation to metastable configurations, or complete decay to the thermodynamic equilibrium are obtained. The transition between both types of response is related to the disappearance of a flux free core within the sample. As a common feature, a step-like dependence in the time relaxation is predicted for both cases. The theory may be applied to long bars of arbitrary and non homogeneous cross section, under in-plane magnetic field processes.Comment: 11 figures, submitte

Inversion mechanism for the transport current in type-II superconductors

The longitudinal transport problem (the current is applied parallel to some bias magnetic field) in type-II superconductors is analyzed theoretically. Based on analytical results for simplified configurations, and relying on numerical studies for general scenarios, it is shown that a remarkable inversion of the current flow in a surface layer may be predicted under a wide set of experimental conditions. Strongly inhomogeneous current density profiles, characterized by enhanced transport toward the center and reduced, or even negative, values at the periphery of the conductor, are expected when the physical mechanisms of flux depinning and consumption (via line cutting) are recalled. A number of striking collateral effects, such as local and global paramagnetic behavior, are predicted. Our geometrical description of the macroscopic material laws allows a pictorial interpretation of the physical phenomena underlying the transport backflow.Comment: 8 pages, 6 figures (Best quality pictures are available by author's contact

Exotic magnetic response of superconducting wires subject to synchronous and asynchronous oscillating excitations

Under the terms of the Creative Commons Attribution 3.0 Unported License.The time-dependent local electromagnetic properties of a type-II superconducting wire subject to the concomitant action of an ac transport current and an oscillating transverse magnetic field in synchronous and asynchronous regimes are thoroughly studied under the critical state approach. Relative double frequency effects between the electromagnetic excitations have been explored for the asynchronous cases. Outstandingly, the occurrence of this event can drastically alter the efficiency of the superconducting wire by increasing the ac losses, contrary to the prediction of a reduction in the ac losses when a relative phase shifting is considered. Likewise, striking magnetization loops and remarkable differences to the ac losses predicted by simplified analytical approaches are reported. For cyclic regime, the time-dependent distribution of local current density, the density of power dissipation, and the components of the magnetic flux density are shown in a wide number of cases. Multiply connected domains are revealed for the flux front profiles via the time-dependent consumption of the magnetization currents by effect of the injected transport current lines. Finally, we have shown that the strong localization of the power density and the recently envisaged low pass filtering effect in the wire's magnetic response is only affordable when both electromagnetic excitations evolve synchronous. © 2013 AIP Publishing LLC.This work was supported by the Spanish MINECO and the FEDER program (MAT2011-22719 and ENE2011-29741 projects), DGA Grant T12/2011. Funding provided by the Spanish CSIC JAE program is gratefully acknowledged.Peer Reviewe

Modelling toolkit for simulation of maglev devices

arXiv:1612.09266v1A stand-alone App has been developed, focused on obtaining information about relevant engineering properties of magnetic levitation systems. Our modelling toolkit provides real time simulations of 2D magneto-mechanical quantities for superconductor (SC)/permanent magnet structures. The source code is open and may be customised for a variety of configurations. Ultimately, it relies on the variational statement of the critical state model for the superconducting component and has been verified against experimental data for YBaCuO/NdFeB assemblies. On a quantitative basis, the values of the arising forces, induced superconducting currents, as well as a plot of the magnetic field lines are displayed upon selection of an arbitrary trajectory of the magnet in the vicinity of the SC. The stability issues related to the cooling process, as well as the maximum attainable forces for a given material and geometry are immediately observed. Due to the complexity of the problem, a strategy based on cluster computing, database compression, and real-time post-processing on the device has been implemented.Funding of this research by Spanish MINECO and FEDER programme (Project ENE2014-52105-R) and by Gobierno de Aragón (Research group T-12) is gratefully acknowledged. J Peña-Roche acknowledges a Research Grant by ICMA (PI2 programme 2015).Peer Reviewe

Material laws and related uncommon phenomena in the electromagnetic response of type-II superconductors in longitudinal geometry

14 páginas, 12 figuras.-- El pdf del artículo es la versión pre-print: arXiv:1108.0658v1Relying on our theoretical approach for the superconducting critical state problem in 3D magnetic field configurations, we present an exhaustive analysis of the electrodynamic response for the so-called longitudinal transport problem in the slab geometry. A wide set of experimental conditions have been considered, including modulation of the applied magnetic field either perpendicular or parallel (longitudinal) to the transport current density. The main objective of our work was to characterize the role of the macroscopic material law that should properly account for the underlying mechanisms of flux cutting and depinning. The intriguing occurrence of negative current patterns and the enhancement of the transport current flow along the center of the superconducting sample are reproduced as a straightforward consequence of the magnetically induced internal anisotropy. Moreover, we show that, related to a maximal projection of the current density vector onto the local magnetic field, a maximal transport current density occurs somewhere within the sample. The elusive measurement of the flux cutting threshold (critical value of such parallel component Jc||) is suggested on the basis of local measurements of the transport current density. Finally, we show that a high correlation exists between the evolution of the transport current density and the appearance of paramagnetic peak structures in terms of the applied longitudinal magnetic field.This work was supported by the Spanish CICyT and FEDER program (project MAT2008-05983-C03-01) and by the DGA grant T12/2011. H S Ruiz acknowledges the financial support provided by the Spanish CSIC JAE program.Peer reviewe

Vortex pinning by intrinsic correlated defects in Fe1-ySe

We present a study on the transport and magnetic properties of superconducting Fe1-ySe single crystals. In the superconducting state, the in-plane electrical resistivity of the crystal is measured for fields up to 16 T and as a function of field direction, in order to understand how the vortex dynamics is affected by the presence of defects. A strong deviation from the slightly anisotropic crystal (electronic anisotropy constant 7 ~ 1.08) is observed as a steep angular dependence, which is interpreted as a signature of the presence of correlated defects. The influence of the correlated defects on the critical current is studied through the angular dependence of the magnetization, and compared to numerical simulations

Trade-off modeling of superconducting levitation machines: theory and experiment

Based on the critical state model for the superconducting components, we develop a set of theoretical tools that allow to extract relevant engineering parameters of a superconducting levitation machine. We provide a number of analytical and numerical expressions for the evaluation of the electromagnetic quantities, energies and forces in 2D problems. This assumption includes: (i) rotational symmetric systems as those in bearings and motors, and also the case of (ii) translational symmetry as in long transportation lines. The theory, that trades off simplicity and predictive power builds on the vector potential/current density formulation of the Maxwell equations (A, J) and is validated by comparison against experimental tension-compression data in our universal test machine. As shown, very simple computer coding is required to implement the method.Funding of this research by Spanish MINECO and the European FEDER Program (Projects MAT2011-22719 and ENE2011-29741) and by Gobierno de Aragon (Research group T12) is gratefully acknowledged.Peer Reviewe

General critical states in type-II superconductors

The magnetic flux dynamics of type-II superconductors within the critical state regime is posed in a generalized framework, by using a variational theory supported by well established physical principles. The equivalence between the variational statement and more conventional treatments, based on the solution of the differential Maxwell equations together with appropriate conductivity laws is shown. Advantages of the variational method are emphasized, focusing on its numerical performance, that allows to explore new physical scenarios. In particular, we present the extension of the so-called double critical state model to three dimensional configurations in which only flux transport (T-states), cutting (C-states) or both mechanisms (CT-states) occur. The theory is applied to several problems. First, we show the features of the transition from T to CT states. Second, we give a generalized expression for the flux cutting threshold in 3-D and show its relevance in the slab geometry. In addition, several models that allow to treat flux depinning and cutting mechanisms are compared. Finally, the longitudinal transport problem (current is applied parallel to the external magnetic field) is analyzed both under T and CT conditions. The complex interaction between shielding and transport is solved.Comment: 21 figures, submitted for publicatio