WO2014016073A1

WO2014016073A1 - Device for concentrating or amplifying a magnetic flux, a method for concentrating or amplifying a magnetic flux, a magnetic operating apparatus, and use of a device for concentrating or amplifying a magnetic flux

Info

Publication number: WO2014016073A1
Application number: PCT/EP2013/063376
Authority: WO
Inventors: Àlvar SÁNCHEZ MORENO; Carles NAVAU ROS; Jordi PRAT CAMPS
Original assignee: Universitat Autonoma De Barcelona
Priority date: 2012-07-27
Filing date: 2013-06-26
Publication date: 2014-01-30

Abstract

The device comprises a plurality of ferromagnetic elements and a plurality of elements which, at least under certain temperature conditions, are at least partially diamagnetic, arranged radially in a shell in alternate layers adjacent to each other or adjacent to intermediate layers of a further material, for: concentrating in an inner volume of said shell the flux of a magnetic field applied or existing in an external part of said shell; or amplifying in said external part of said shell the flux of a magnetic field applied or existing in said inner volume of said shell. The method, magnetic operating apparatus and use of other aspects of the invention have in common the above mentioned feature regarding the arrangement of ferromagnetic and diamagnetic elements.

Description

Device for concentrating or amplifying a magnetic flux, a method for concentrating or amplifying a magnetic flux, a magnetic operating apparatus, and use of a device for concentrating or amplifying a magnetic flux Field of the art

The present invention generally relates, in a first aspect, to a device for concentrating or amplifying a magnetic flux, comprising a magnetic element arranged radially in a shell, and more particularly to a device whose shell comprises a plurality of ferromagnetic elements and a plurality of diamagnetic elements radially arranged.

Second, third and fourth aspects of the invention concerns, respectively, to a method for concentrating or amplifying a magnetic flux, to a magnetic operating apparatus comprising a device for concentrating or amplifying a magnetic flux as per and to the use of a device for concentrating or amplifying a magnetic flux, having all of said aspects in common the above mentioned feature regarding the arrangement of ferromagnetic and diamagnetic elements.

Prior State of the Art

US4806863A discloses an apparatus for testing for flaws in conductive objects which includes a coil and an associated magnetic flux concentrator of electromagnetically active material disposed radially in a cylinder or "spacer" made of a non-electromagnetically active material, occupying only a small part of the length of the cylinder.

The purpose of arranging only one electromagnetically active element is that of locally detecting flaws in a conductive object, internal or external to the cylinder, by concentrating magnetic flux towards the coil which is arranged, respectively, external or internal to the cylinder, thus providing a desired resolution of flaw detection and close coupling between the coil and the conductive object

US4806863A neither teaches or suggests to provide more than one electromagnetically active element nor to extend the dimensions of said element along the length of the cylinder nor to make the spacer of a diamagnetic material nor to achieve a magnetic flux concentrating effect which is not localized in only a small portion of the cylinder circumference.

Transformation optics has pushed the possibilities of controlling light towards previously unexplored limits [1 , 2], including perfect lenses [5] and electromagnetic cloaks [3, 4]. When applied to magnetic fields, transformation optics ideas have allowed unique results such as the experimental realization of an exact cloak [6]. An important application of transformation optics is concentration of electromagnetic energy, which is attempted by using either plasmonics [7, 8] and macroscopic concentrators [9]. The former only works at subwavelenght scales (typically, nanometers for visible or infrared light [10]) and the latter requires filling the concentration space with material [9].

Many applications of magnetism require concentrating the magnetic field in a space region -e. g. in a transformer, a magnetic lens for an electron microscope, or a motor. Magnetic sensors are particularly relevant, with a continuous increase of sensitivity, almost reaching the quantum limit [11 ]. Both ferromagnetic-based, like those based on magnetoresistance, and superconductor-based magnetometers, typically SQUIDs, frequently use magnetic concentration to enhance their sensitivity [12,15].

Conventionally, magnetic concentration is achieved using either ferromagnetic (FMs) materials, attracting magnetic flux, or diamagnetic ones, repelling them, being superconductors (SCs) the optimum diamagnetic materials. Both have been used separately to confine magnetic fields [12-15]. The usual strategy is based on two superconductors -or two ferromagnets- separated by a gap in which flux concentration is produced. Figures 1 a-c show a typical example of concentration of a uniform applied magnetic field in the gap of two superconducting slabs (assumed ideal, with zero permeability μ [6]). The field gets enhanced at the edges of the slabs, not only on the gap region but also on the exterior ones. When reducing the slabs thickness, the contribution from the edges adds up and a rather intense magnetic field develops in the gap (Figure 1 c), although the gap field is not homogeneous but increases towards the strip edges and decreases at the central region. Interestingly, there is a general symmetry between ideal superconductors (μ = 0) and ideal ferromagnets (μ→∞) [16, 17], meaning that the concentration achieved in a vertically applied field by a pair of superconductors is the same as that for a horizontal field by ferromagnetic counterparts (compare Figures 1 b and 1 e). Results in Figures 1 a-c and 1 e illustrate the limits in magnetic concentration using the conventional strategies, which are clearly improvable.

Summary of the Invention

It is necessary to provide an alternative to the state of the art which provides a device for concentrating or amplifying a magnetic flux which performance goes beyond the one of the already known devices, in terms of, among others, homogeneity of the concentrated or amplified field and intensity of the concentrated or amplified magnetic flux and which can be applied to the concentrating or amplifying of magnetic fluxes of magnetic fields with any direction, and which offer results clearly better compared to those of the above mentioned conventional strategies. To that end, the present invention relates, in a first aspect, to a device for concentrating or amplifying a magnetic flux, comprising a magnetic element arranged radially in a shell for:

- concentrating in an inner volume of said shell the flux of a magnetic field applied or existing in an external part of said shell; or

- amplifying in said external part of said shell the flux of a magnetic field applied or existing in said inner volume of said shell.

In other words, the device of the present invention transfers all the magnetic energy in the shell to the exterior thereof, and vice versa, depending on the location of the magnetic source.

On contrary to the known devices, in the device of the first aspect of the present invention said shell comprises a plurality of said magnetic elements which comprise a plurality of ferromagnetic elements and a plurality of elements which, at least under certain temperature conditions, are at least partially diamagnetic, arranged radially in alternate layers adjacent to each other or adjacent to intermediate layers of a further material.

For a preferred embodiment, said at least partially diamagnetic elements are superconductor elements, whether low temperature (1 .5 - 5K) or high temperature (up to and/or beyond 1 1 OK) superconductors, or even elements which become superconductors near or at ambient temperature, if they are discovered or build in the future, during the term of protection of the present invention.

For clarity sake, in the rest of the present description the mentioned elements which are diamagnetic or partially diamagnetic, even if they are so only under certain temperature conditions, will be referred as diamagnetic or at least partially diamagnetic elements, obviating the temperature conditions, if any, they need for achieving such diamagnetic properties.

Depending on the embodiment, said shell is a hollow body of revolution and/or a tubular body, said inner volume being the hole of said hollow body of revolution and/or of said tubular body. Examples of shapes for said shell are: a hollow sphere, a hollow cylinder with circular or elliptical annular bases or a tube having any type of annular cross section, such as an annular rectangular cross section.

For an embodiment, the shell is constituted only by the ferromagnetic and the at least partially diamagnetic alternate layers arrangement.

For a preferred implementation of said embodiment, and also for other embodiments different thereto, each of the ferromagnetic and at least partially diamagnetic alternate layers has a wedge cross section, and, preferably, having, for at least part of the layers, their respective inclined surfaces adjacent or adjoining to each other.

For an alternative embodiment, the shell is constituted by the ferromagnetic and the at least partially diamagnetic alternate layers and also by said intermediate layers of a further material, said further material having a magnetic permeability equal or substantially equal to 1 , as is the case of air.

For said alternative embodiment, and also for other embodiments different thereto, each of the ferromagnetic and at least partially diamagnetic alternate layers has a rectangular cross section.

Depending on the embodiment, the alternate layers are distributed radially along part or parts of a circumference or along a whole circumference, the edges of the proximal ends of the layers defining the contour of the inner volume and the edges of the distal ends of the layers defining the outer contour of said shell.

Said contour of the inner volume can be constituted directly by the edges of the proximal ends of the layers, arranged side by side, or by an additional supporting envelope onto whose external surface the proximal end edges of the layers are affixed.

In order to achieve the best performance, the number, dimension, composition and configuration of the alternate layers is chosen such that the angular permeability of the shell tends to zero and the radial permeability tends to infinity.

Although the scope of the present invention has been limited, as defined by the appended claims, to the above described radial layers arrangement, said arrangement is a successful attempt to implement a device as close as possible to a device for concentrating or amplifying a magnetic flux whose shell is made of an ideal anisotropic homogeneous "hypothetic" material having a radial permeability μ_Γ arbitrarily high (tending to infinity) and an angular permeability μ₍ arbitrarily small (equal or near to zero) and which meet the condition μ_Γ ^■ μ₍ = 1. In a posterior section a theoretical description of the development of such an ideal anisotropic device will be given.

Inspired by said successful radial layers arrangement, the present inventors are testing other different implementations of such an ideal shell by different discretization arrangements thereof using homogeneous and isotropic materials, be said different implementations covered by the present invention, as far as they can be equivalents to said radial layers arrangement.

If in the future, during the term of protection of the present invention, such an "hypothetic" material is discovered or built, its use for concentrating or amplifying a magnetic flux, if arranged as a shell, is here first disclosed. A second aspect of the invention concerns to a method for concentrating or amplifying a magnetic flux, comprising providing a shell with a magnetic element arranged radially therein and using said shell for:

On contrary to the known methods, the method of the second aspect of the present invention comprises providing said shell with a plurality of said magnetic elements by arranging radially in alternate layers a plurality of ferromagnetic elements and a plurality of at least partially diamagnetic elements adjacent to each other or adjacent to intermediate layers of a further material.

The method of the invention comprises providing the device for concentrating or amplifying a magnetic flux of the first aspect of the invention.

In order to provide said shell, the method comprises increasing or decreasing the intensity of the concentrated or amplified magnetic flux by, respectively, increasing or decreasing the thickness of the shell or radial distance between the contour of the inner volume and the distal or free end of the layers.

Regarding the magnetic field, this is at least one of a static magnetic field or a time varying magnetic field.

A third aspect of the invention concerns to a magnetic operating apparatus comprising one or more devices for concentrating or amplifying a magnetic flux as per the first aspect of the invention.

For several embodiments, the magnetic operating device of the third aspect of the present invention comprises or constitutes at least one of a voltage transformer, a motor, a nuclear resonance device, a magnetic energy transmission device, an energy generator, a magnetic coupling device between two separated circuits having respective devices for concentrating or amplifying a magnetic flux, and a magnetic detector comprising at least one magnetic sensor placed inside said inner volume for sensing the intensity and/or direction and/or gradient of a concentrated external uniform or nonuniform magnetic field.

For an embodiment, the magnetic operating device of the third aspect of the invention comprises two or more of said devices for concentrating or amplifying a magnetic flux arranged on a common support.

A fourth aspect of the invention concerns to the use of a device for concentrating or amplifying a magnetic flux as per the first aspect of the invention for at least one of: - concentrating weak magnetic fields for increasing the sensitivity of a magnetic sensor placed inside said inner volume;

- concentrating intense magnetic fields for energy generation;

- amplifying a magnetic field generated by a magnetic field generator placed inside said inner volume, wherein if said magnetic field generator is centred in said inner volume said magnetic field is amplified in magnitude while keeping its shape;

- where the magnetic field is a time varying magnetic field having a predetermined frequency, for wireless power transmission or for energy harvesting, and

- increasing the magnetic coupling between two magnetic circuits, each provided with a respective device for concentrating or amplifying a magnetic flux.

Brief Description of the Drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached, which must be considered in an illustrative and non-limiting manner, in which:

Figure 1 show different graphs representing calculated field lines for different materials when a uniform external field is applied. The first three are ideal superconducting blocks (μ = 0.0001 ) with a thickness of 0.15m separated by a gap of 0.1 m and with decreasing heights of 0.40, 0.05 and 0.01 m (A, B and C respectively). (D) Optimum homogeneous anisotropic concentrating shell (μ_ρ→ °° and μ_θ→ 0) with an inner and outer radius Ri = 0.05m and R₂ = 0.20m respectively. (E) Ideal soft ferromagnetic material (μ = 10000) with the same dimensions of B. (F) Homogeneous isotropic shell made of ideal soft ferromagnetic material with the same dimensions of D.

Figure 2: (Upper) Averaged y-component of the field < B_y >/B_a along the central line between the superconducting blocks depending on the gap ratio (b/W) for different heights (thin lines). The thinner height of the blocks the larger value of averaged y-field, having the analytic upper limit of two superconducting strips (thick dashed line). The thick solid line represents the case of an optimum homogeneous anisotropic concentrating shell showing a remarkable increase of the field for any gap ratio. (Lower) Normalized magnetic flux Φ/Φ₉ (where Φ₉ = l W-/_a4TTl O^"7Wb/m) calculated for the same cases. Using superconducting materials, normalized flux tends to banish as the gap ration tends to 0 even in the case of infinitely thin strips. Differently, using the optimum homogeneous anisotropic concentrating shell the normalized flux keeps a constant value of 1 (fulfilling W = 2R₂ and b = 2R-,). Figure 3a shows, together with magnetic field lines, a cross-section of a hollow cylindrical homogeneous anisotropic concentrator (R₂=0.20m, R-^O.OSm), where shaded region represents a material with μ_ρ=1000 and μ_θ=0.001.

Figure 3b shows, together with magnetic field lines, a cross-section of the shell of the device for concentrating or amplifying a magnetic flux of the first aspect of the present invention, for an embodiment where it has a hollow cylindrical shape with circular annular bases, discretized into 36 wedges (R₂=0.20m, R-^O.OSm), where shaded regions represent the superconducting wedges with μ=0.001 and dotted regions represent the ferromagnetic wedges with μ=1000.

Figure 3c shows a variant of the embodiment of Figure 3b which differs therefrom in that in this case the shell is discretized into 72 wedges.

Figure 3d is a cross section of the shell of the device of the first aspect of the invention, for an embodiment for which the shell is discretized into 72 rectangles (R₂=0.20m, R-^O.OSm), where shaded regions represent the superconducting rectangles with μ=0.001 and dotted regions represent the ferromagnetic rectangles with μ=1000. Magnetic field lines are also represented.

Figure 4 is a graph which shows plots of the y-component of the field along the central line of the devices of Figures 3a to 3d, obtained from simulations, which confirm the uniformity and the increase of the field in the inner region of their shells.

Figures 5a to 5e show, together with magnetic field lines, respective cross- sections of the shell of the device of the first aspect of the present invention, for an embodiment where it has a hollow cylindrical shape with circular annular bases, discretized into, respectively, 6, 10, 18, 36 and 72 wedges (R₂=0.20m, R^O.OSm), with a relative permeability for the ferromagnetic wedges (dotted regions) of μ™=1000 and for the superconducting ones (shaded regions) of μ^βε=0.001. Superconducting and ferromagnetic wedges are alternated and radially disposed.

Figure 6 is a graph which plots of B_y/B_a along the central line (y=0) from x=- 0.35m to x=0.35m for the different configurations of Figures 5a to 5e, obtained from simulations.

Figure 7 shows a plot of the B_y/B_a in the central point (x=0, y=0) for the previous cases of Figures 5a to 5e.

Figure 8 is a plot which shows the factor B_y/B_a in the central point (x=0, y=0) for the implementation of the shell of the device of the invention, acting as magnetic concentrator, of Figures 3c and 5e, but for permeabilities of the ferromagnetic wedges (μ™) varying from 0 to 200. Figure 9 shows, by means of a perspective view, the shell constituting the device of the first aspect of the present invention for the embodiment of Figure 3b, where the shell is discretized into 36 wedges adjoined side by side and alternately, where black regions represent the superconducting wedges and white regions represent the ferromagnetic wedges.

Figure 10 shows, by means of a perspective view, the shell constituting the device of the first aspect of the present invention for an embodiment where the shell is a cylindrical body, having circular annular bases, discretized into 36 rectangles and also by intermediate layers of a further material having a magnetic permeability equal or substantially equal to 1 , such as air, where black regions represent the superconducting wedges and white regions represent the ferromagnetic wedges.

Figure 11 a is a cross section of the shell of the device of the first aspect of the invention, for an embodiment for which the shell has a hollow cylindrical shape with elliptical annular bases, and is discretized into 36 rectangles, for a first design, or E1 design, where shaded regions represent the superconducting rectangles with μ=0.001 and dotted regions represent the ferromagnetic rectangles with μ=1000, and where rectangles are 0.12m long and they have changing thickness in order to fit in the interior ellipse, and the inner radius Ri at the point nearest to the centre is of 0.1 m and at the point farthest to the centre is of 0.2m. Magnetic field lines are also represented.

Figure 11 b shows a variant of the embodiment of Figure 1 1 a representing a second design, or E2 design, which differs therefrom in that the rectangles have different relative dimensions, particularly they have a length of 0.20m and also with a changing thickness.

Figure 12 is a graph showing plots of By/Ba along the central line (y=0) from x=- 0.4m to x=0.4m for the simulation of the geometries of Figures 11 a and 11 b.

Figure 13 shows schemes of the device of the first aspect of the invention, for an embodiment for which the shell is discretized into 36 rectangles (R1 =0.05m, R2=0.20m, black rectangles being superconducting and white rectangles being ferromagnetic) when some rectangles are substracted.

Figure 14 is a graph showing plots of |B|/Ba along the central line for the cases shown in Figure 13.

Figure 15 is a perspective view of the shell constituting the device of the first aspect of the present invention for an embodiment where the shell has a spherical shape and is discretized into a plurality of wedges adjoined side by side and alternately, where black regions represent the superconducting wedges and white regions represent the ferromagnetic wedges. Figure 16 shows, at its upper view, simulated magnetic field lines in a cross-section of a spherical shell similar to that of Figure 15 but discretized into a greater number of wedges, where shaded regions represent the superconducting wedges with μ=0.001 and dotted regions represent the ferromagnetic wedges with μ=1000, and, at its lower view, a plot of By/Ba along the central line from a 3D simulation of said spherical shell.

Detailed Description of Several Embodiments

As stated in a previous section, the present invention is a successful attempt to implement a device as close as possible to a device for concentrating or amplifying a magnetic flux whose shell is made of an ideal anisotropic homogeneous "hypothetic" material having a radial permeability μ_Γ arbitrarily high (tending to infinity) and an angular permeability μ₍ arbitrarily small (equal or near to zero) and which meet the condition μ_Γ ·

Regarding said ideal anisotropic device and in order to provide support for explaining the principles on which the present invention is supported, here transformation optics ideas are applied to achieve unprecedented concentration of magnetic fields. It is analytically show that an applied magnetic field can be arbitrarily enhanced in a given region of free space by surrounding this region with a specially designed magnetic material, and this occurs when the external field is not distorted. The required material is, ideally, homogeneous and anisotropic but a very accurate discretized version can be readily implemented in practice, according to the present invention, using only commercially available materials: superconductors and ferromagnets. Because the magnetic field can be concentrated in free space, different from the electromagnetic waves case, a magnetic sensor placed in the concentration region may greatly enhance its sensitivity or a magnetic device - e. g. transformer, motor- its performance.

One of the goals of the present invention is a design for achieving the maximum magnetic field concentration in free space, so that a magnetic sensor or a part of a magnetic device -e. g. a transformer coil- can be placed in the increased field region, and using only available materials. The here used strategy is not concentrating in the gap between two magnetic objects, but instead to surround a region in free space with a specially designed magnetic material to make a maximum magnetic concentration in the hole.

Next how to design a magnetic shell that fulfils said goal is described. It is considered an infinitely long (along z-direction) cylindrical magnetic shell of interior and exterior radii Ri and R₂, respectively, which cross-section is shown in Figure 1 d and 3a, which is placed in a uniform externally applied magnetic field H₀ directed along the y- direction. Below the results will be generalized to the cases of a spherical volume and non-homogeneous fields. It is assumed that the magnetic material in the shell is linear and homogeneous, but can be anisotropic, characterized by an angular and a radial relative permeabilities μ_ρ→∞ and μβ→0, respectively.

To study the maximum flux that can be concentrated in the cylindrical hole (p<Ri) and the material configuration that provides this maximum, the general expressions for the magnetic field distributions in all space for the homogeneous anisotropic cylinder in uniform applied field have been analytically derived, and some of the most interesting results are:

i) The field outside is not disturbed when the permeabilities satisfy μ_ρ=1/μ_θ; this is here called a conjugate case, as in [18].

ii) The magnetic field inside the hole (p<Ri) is always uniform and has the direction of the applied field, and, for a given cylinder with a certain R₂/Ri ratio, it increases when increasing μ_ρ (for fixed μ_θ) and also when decreasing μ_θ (for fixed μ_ρ). The absolute maximum for magnetic field concentration is achieved in the limit μ_ρ→∞ and μβ→0.

In this case the field in the hole is

which corresponds to a case in which the external field is not distorted (μ_Ρ->°° and μβ->0 is a conjugate case). Similar results can be obtained for a spherical shell.

Actually, all the conjugates cases that yield no external field distortion can be alternatively interpreted using the transformation optics technique. Using this approach in the magnetostatic case a homogeneous and anisotropic cylindrical shell with an empty region inside can be interpreted as arising from two particular space transformations. The field in the hole H^HOL is homogeneous and

where Ι<=μ_θ=1/μ_ρ. The maximum H^HOL is for k→0, when it reaches the maximum value of Eq. (1 ). When the same space transformation is used for electromagnetic waves, material with particular properties has to be placed in the interior, preventing the placement of a sensor. The interpretation of these conjugate cases as space transformations allows to analytically determining the fields in all space, even for nonuniform applied fields, which shall be used below.

The magnetic response of the cylindrical shell with μ_ρ=∞ and μ_θ=0 is shown in Figure 1 d. A large magnetic flux concentration [Eq. (1 )] is achieved in the hole, and the external field is not distorted. The field in the hole is homogeneous, so a magnetic sensor placed in the hole would detect a much larger field that that applied -increasing its sensitivity by a large known factor. By comparing with the case of the concentration in the gap of two superconductors we see that not only the average field in the hole is always larger in the design of the ideal anisotropic concentrator shell (Figure 2 upper view), but also the magnetic flux in said ideal case is always constant whereas for the gap strategy the flux tends to zero (Figure 2 lower view) [19], because field is mainly diverted toward the exterior edges when the gap narrows. The present concentration design is optimum because all the magnetic flux enclosed in the material region is totally transferred to the hole owing to the infinite radial permeability and the zero angular one; since the hole area can be made arbitrarily small the inner field can be arbitrarily increased.

The achieved results can be applied to increasing the sensitivity of magnetic sensors. If the field to be detected is spatially homogeneous, the simple placing of the sensor inside the cylindrical concentrator of inner and outer radius Ri and R₂, respectively, with permeabilities μ_ρ→∞ and μβ→0 will increase magnetic flux in the hole- and thus sensor sensitivity by a factor R₂/Ri (Eq. (1 )). Dimensions of many sensors, such as SQUIDs [20] or Hall sensors [21 ] are typically small so enclosing them with a shell to enhance their sensitivity may be practical in many applications. Another case of interest is for sensing non-uniform fields from nearby magnetic sources as in biosensors [22], in measurements of human brain response in magnetoencephalography [23, 24] or for detecting single magnetic microbeads [21 ]. In some of these situations it is measured not the field but its gradient, in order to separate the signal of the source from other distant noise sources. Because of transformation optics ideas, the magnetic field in any point of the hole and its gradient can be analytically obtained by a simple space transformation. The gradient becomes scaled by a higher power of the radius relation, (R₂/RI)^2"2m9, which makes the here described ideal or optimum concentrator, and also the discretized implementation of the present invention, particularly useful for magnetic gradiometers.

Next, the practical implementations of different designs are commented. The optimum concentrator requires μ_ρ→∞ and μβ→0; actual materials with such anisotropy do not exist. But a discretized version yielding very satisfactory results can be made using available superconductor (SC) and ferromagnetic (FM) materials. An alternation of radially displaced FM and SC wedges will constitute a natural discretization of the required material as the FM will give the large radial permeability and the alternated SC will prevent the appearance of angular components of field, leading to an effective μ_θ=0, as demonstrated in Figure 4. For a large N -easily achieved in practice with thin sheets- the field in the hole is very homogeneous and approaches the exact limit.

Embodiments of Figures 13 and 14 show that a partial concentration can be achieved even for open geometries.

Such superconductor and ferromagnetic materials are readily available, even commercially. In [6] the present inventors fabricated a magnetic cloak using these two materials, and their ideal behaviour was experimentally confirmed for fields as large as 40mT and liquid nitrogen temperatures (in the present case the permeability of the magnetic layer should be larger than that in [6], so permalloy or similar alloys could be used for the magnetic layers). Based on said experimentally confirmed results achieved by the present inventors for said magnetic cloak, it is expected that the experiments which will be done on an implementation of the device of the present invention will also confirm the simulation results here included, using the same materials than for said magnetic cloak.

Finally, the device of the present invention not only works as a magnetic flux concentrator but also works in reverse: the field of a dipolar source centred in the hole keeps the shape in the exterior with its magnitude enhanced, i.e. as a magnetic flux amplifier, which may lead to further applications.

Different embodiments of the device of the first aspect of the invention, for different geometries and number and shapes of layers, have been simulated, and are next described with reference to Figures 3 to 16, together with the most relevant obtained results. The simulations have been performed assigning to the simulated materials permeabilities similar to the ones of commercial materials

Figure 3a shows, together with magnetic field lines, a cross-section of a hollow cylindrical homogeneous anisotropic concentrator (R₂=0.20m, R^O.OSm), where shaded region represents a material with μ_ρ=1000 and μ_θ=0.001.

The cross-sections of different discretizations of the ideal anisotropic shell are shown in Figures 3b to 3d, where the shell is, respectively, discretized into 36 and 72 wedges, and into 72 rectangles. Simulation results shown in Figure 4 confirm the uniformity and the increase of the field in the inner region of said shells.

Figures 9 and 10 show, by means of respective perspective views, the shell of Figure 3b, where the 36 wedges or layers are adjoined side by side and alternately, and another embodiment of the device of the invention, in which the shell is a cylindrical body, having circular annular bases, discretized into 36 rectangles and also by intermediate layers of a further material having a magnetic permeability equal or substantially equal to 1 , such as air. In both Figures, black regions represent the superconducting parts and white regions represent the ferromagnetic ones.

At both embodiments, of Figures 9 and 10, the contour of the inner volume, i.e. of the hole of the cylinder, is constituted directly by the edges of the proximal ends of the layers, arranged side by side, whether they are wedges or rectangles, while the edges of the distal ends of the layers define the outer contour of the shell, whether a continuous outer contour, for the embodiment of Figure 9, or a discontinuous outer contour, for the embodiment of Figure 10.

For other embodiments, not shown, the inner contour of the shell is defined by an additional supporting envelope onto whose external surface the proximal end edges of the layers are affixed. Magnetic field concentration depending on the permeability of the ferromagnetic wedges:

Plot of Figure 8 represents the calculated factor B_y/B_a in the central point (x=0, y=0) which has been obtained by simulating the shell of the device of the invention for the geometries of Figures 3c and 5e, but for permeabilities of the ferromagnetic wedges (μ™) varying from 0 to 200, calculating the factor B_y/B_a in the central point (x=0, y=0), where dotted line corresponds to the simulated case while solid line corresponds to the maximum value of By/Ba obtained with this concentrator (geometry of Figures 3c and 5e) for μ™=1000.

In all the simulations the magnetic field in the interior keeps homogeneous and the change in the μ™ only affects the modulus of the field as seen in said Figure 8, which shows that for a permeability μ™ superior to 5-10 the external magnetic field applied Bo is concentrated more than three times in the inner volume of the shell (when

Magnetic field concentration depending on the number of wedges: In order to assess the performance in terms of magnetic field concentrator of the device of the invention, the different implementations thereof illustrated in Figures 5a to 5e have been simulated, and the results, in terms of factor B_y/B_a versus radial distance (x) to the axis of the cylinder are depicted in Figure 6, and in terms of factor B_y/B_a at the central point (x=0, y=0) versus number of wedges are depicted in Figure 7.

Taking into account that

the maximum factor By/Ba that we would achieve with an ideal homogeneous anisotropic concentrator with μ_ρ=∞ and μ_θ=0 would be 4. The finer discretization into a higher number of wedges, the larger the field in the interior is, approaching to the theoretical maximum of 4. In addition, the higher the number of wedges, the more homogeneous the field in the interior is.

Even with only 6 wedges, the results are good enough for some industrial applications.

Magnetic field concentration using rectangular pieces elliptically arranged:

Figures 1 1a and 1 1 b show two different geometries for the shell of the device of the first aspect of the invention, which provide, in both cases, an elliptical shell formed with 36 rectangular layers of ferromagnetic (dotted) and superconductor (shaded) elements arranged alternately, extending from the inner volume or hole, and which differ in that for the arrangement of Figure 1 1a the rectangles are 0.12m long while in Figure 1 1 b they are 0.20 long.

These two geometries have been simulated interacting with a uniform applied magnetic field, using a relative permeability for the ferromagnetic rectangles of

. Results are shown in Figure 12, where |By|/Ba is plotted along the central line for the design E1 , corresponding to that of Figure 11 a and also for the design E2, corresponding to the one of Figure 11 b.

From said Figures, it can be observed that the higher length of the rectangles, the larger field concentration in the interior region. However the field inside is not homogeneous and decreases in the centre of the ellipse (where the distance between the upper and lower halves of material is larger).

Magnetic field concentration using open geometries:

With the intention of probing that the device of the present invention also works well enough even if subtracting some of the layers of the shell, some simulations have been performed for the shells schemed in Figure 13, where the shell formed by 36 rectangles is shown together with six other views for which 1 , 3, 5, 7, 9 and 1 1 rectangular pieces (per side) starting from the central ones have been subtracted, with the purpose, for example, of providing a through opening for communicating the inner volume with the exterior of the shell to pass wires of a possible magnetic detector place inside the inner volume.

The simulation results are depicted in Figure 14, by a series of plot of B_y/B_a along the central line, for all the geometries of Figure 13 and also for the ideal anisotropic shell.

These results show that the homogeneity and the concentration degree is affected as a function of the number of pieces subtracted, but the external magnetic field Bo is still concentrated at the centre of the shell. Figure 15 shows another embodiment of the device of the first aspect of the invention, which shell in this case adopts the shape of a sphere discretized into a plurality of wedges adjoined side by side and alternately, where black regions represent the superconducting wedges and white regions represent the ferromagnetic wedges.

The results of a tridimensional simulation of a spherical shell similar to that of Figure 15 but with a greater number of wedges, are shown in Figure 16. In the upper view the simulated field lines are plotted when a uniform field is applied (shaded regions represent superconducting wedges and dotted regions ferromagnetic wedges). In the lower view the factor |By|/Ba along the central line is plotted, showing the field concentration inside the spherical shell.

There must be pointed out that the peaks of the different plots shown in Figures 4, 6, 12, 14 and 16 tend to infinite, having been depicted as having a limited high value due to the discrete limitations of the simulation tools used for obtaining them.

Although the device of the invention is mainly applied to static fields, extension to other frequency ranges such as those used in power wireless transmission would be possible if materials with the required permeability values at such non-zero frequencies are found, possibly using metamaterials [27].

A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims. Particularly, as stated above, a person skilled in the art could think of shapes for the shell and layer arrangement different from the ones explicitly disclosed herein which would fall under the scope of the here claimed invention. References

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Claims

1 . - Device for concentrating or amplifying a magnetic flux, comprising a magnetic element arranged radially in a shell for:

- amplifying in said external part of said shell the flux of a magnetic field applied or existing in said inner volume of said shell;

characterized in that said shell comprises a plurality of said magnetic elements which comprise a plurality of ferromagnetic elements and a plurality of elements which, at least under certain temperature conditions, are at least partially diamagnetic, arranged radially in alternate layers adjacent to each other or adjacent to intermediate layers of a further material.

2. - Device for concentrating or amplifying a magnetic flux as per claim 1 , wherein said shell is a hollow body of revolution and/or a tubular body, said inner volume being the hole of said hollow body of revolution and/or of said tubular body.

3. - Device for concentrating or amplifying a magnetic flux as per any of the previous claims, wherein said shell is constituted only by said ferromagnetic and at least partially diamagnetic alternate layers arrangement.

4. - Device for concentrating or amplifying a magnetic flux as per any of claims 1 to 2, wherein said shell is constituted by said ferromagnetic and at least partially diamagnetic alternate layers and also by said intermediate layers of a further material, said further material having a magnetic permeability equal or substantially equal to 1.

5. - Device for concentrating or amplifying a magnetic flux as per any of the previous claims, wherein each of said ferromagnetic and at least partially diamagnetic alternate layers has one of a wedge cross section or a rectangular cross section.

6. - Device for concentrating or amplifying a magnetic flux as per any of the previous claims, wherein said alternate layers are distributed radially along part or parts of a circumference or along a whole circumference, the edges of the proximal ends of said layers defining the contour of said inner volume and the edges of the distal ends of said layers defining the outer contour of said shell.

7. - Device for concentrating or amplifying a magnetic flux as per claim 1 , wherein said at least partially diamagnetic elements are superconductor elements.

8. - Device for concentrating or amplifying a magnetic flux as per any of the previous claims, wherein the number, dimension, composition and configuration of said alternate layers is chosen such that the angular permeability of the shell tends to zero and the radial permeability tends to infinity.

9. - A method for concentrating or amplifying a magnetic flux, comprising providing a shell with a magnetic element arranged radially therein and using said shell for:

wherein the method is characterized in that it comprises providing said shell with a plurality of said magnetic elements by arranging radially in alternate layers a plurality of ferromagnetic elements and a plurality of at least partially diamagnetic elements adjacent to each other or adjacent to intermediate layers of a further material.

10. - The method of claim 9, comprising providing the device for concentrating or amplifying a magnetic flux of any of claims 1 to 8.

11. - The method of claim 9 or 10, comprising, in order to provide said shell, increasing or decreasing the intensity of the concentrated or amplified magnetic flux by, respectively, increasing or decreasing the thickness of said shell or radial distance between the contour of the inner volume and the distal or free end of said layers.

12. - The method of any of claims 9, 10 or 11 , wherein said magnetic field is at least one of a static magnetic field or a time varying magnetic field.

13.- Magnetic operating apparatus, comprising at least one device for concentrating or amplifying a magnetic flux as per any of claims 1 to 8.

14. - The magnetic operating device of claim 13, comprising or constituting at least one of a voltage transformer, a motor, a nuclear resonance device, a magnetic energy transmission device, an energy generator, a magnetic coupling device between two separated circuits having respective devices for concentrating or amplifying a magnetic flux, and a magnetic detector comprising at least one magnetic sensor placed inside said inner volume for sensing the intensity and/or direction and/or gradient of a concentrated external uniform or non-uniform magnetic field.

15. - The magnetic operating device of claim 13 or 14, comprising at least two of said devices for concentrating or amplifying a magnetic flux arranged on a common support.

16. - Use of a device for concentrating or amplifying a magnetic flux as per any of claims 1 to 8 for at least one of:

- concentrating weak magnetic fields for increasing the sensitivity of a magnetic sensor placed inside said inner volume;

- concentrating intense magnetic fields for energy generation; - amplifying a magnetic field generated by a magnetic field generator placed inside said inner volume, wherein if said magnetic field generator is centred in said inner volume said magnetic field is amplified in magnitude while keeping its shape;