Layered Superconductor/Ferromagnet Structures for Magnetic Field Cloaking
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Layered Superconductor/Ferromagnet Structures for Magnetic Field Cloaking Fedor Gömöry, Mykola Solovyov, Ján Šouc Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9 841 04 Bratislava, Slovakia ABSTRACT Combination of superconducting (SC) and ferromagnetic (FM) sheets into layered SC/FM composites allows to obtain metamaterials with unusual magnetic properties: the effective magnetic permeability along the sheets could be much higher than in the perpendicular direction. One can design the magnetic cloak consisting of a shell from SC/FM composite protecting its inner space against the penetration of a static magnetic field. Difference between such a device and a usual SC or FM shield will be in its un-detectability by magnetic sensors checking the field distribution outside the cloak. Thus it could be considered a magnetic invisibility cloak and one can think about using it e.g. for protecting a sensitive electronic circuitry in an electric machine. We have prepared a series of SC/FM cloaks using commercial coated conductors as the superconducting elements and various kinds of ferromagnetic sheets as the ferromagnetic elements. Finite element calculation was utilized to optimize the architecture. Experimental testing of the cloaking ability in static magnetic field was performed by scanning the field distribution in vicinity of the cloak. Detectability in low frequency (< 100 Hz) AC magnetic fields was tested in an AC magnetization set-up allowing to see both screening and dissipation signals. Recording of magnetization loops allowed to analyze in detail the dynamics of field interaction with the cloak. Reduction of the detected magnetic signature due to the cloak was confirmed, however it is still not complete. Tests of various arrangements of superconducting and ferromagnetic materials allowed to identify the main problems hindering to achieve a perfect cloaking. INTRODUCTION The way how a superconductor/ferromagnetic (SC/FM) bilayer could act as a magnetic cloak can be understood from Fig. 1. For sake of simplicity the materials with ideal magnetic
Figure 1: Calculation of the magnetic field distribution in case of applying a DC field in direction perpendicular to the long axis of an object of circular cross-section consisting of two concentric tubes tightly assembled together: inner one from a perfectly diamagnetic material, and the outer one from the ferromagnetic material with the relative permeability µr = 7 (left), µr = 10 (middle), µr = 15 (right).
properties have been considered in the finite element calculation of magnetic field distributions The inner tube is assumed to be perfectly diamagnetic, the outer one is of ferromagnetic material of constant permeability. Then the result does not depend on the strength of applied field. Inner diamagnetic tube screens the bore from penetration of magnetic field securing the magnetic screening of any object that would be placed inside. The magnetic flux expelled by this screening effect is attracted by the outer ferromagnetic materi
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