Neutral shielding and cloaking of magnetic fields using isotropic media Lars Kroon and Kenneth Järrendahl
The self-archived postprint version of this journal article is available at Linköping University Institutional Repository (DiVA): http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-145069
N.B.: When citing this work, cite the original publication. Kroon, L., Järrendahl, K., (2017), Neutral shielding and cloaking of magnetic fields using isotropic media, Journal of Physics, 29(3), . https://doi.org/10.1088/0953-8984/29/3/035801
Original publication available at: https://doi.org/10.1088/0953-8984/29/3/035801 Copyright: IOP Publishing (Hybrid Open Access) http://www.iop.org/
Neutral shielding and cloaking of magnetic fields using isotropic media
Lars Kroon and Kenneth J¨arrendahl Department of Physics, Chemistry and Biology, Link¨oping University, SE-581 83 Link¨oping, Sweden E-mail: [email protected]
Abstract. A method for designing magnetic shields that do not perturb applied multipole fields in the static regime is developed. Cylindrical core-shell structures with two layers characterized by homogeneous isotropic permeabilities are found to support neutral shielding of multipole fields and unique cloaking solutions of arbitrary multipole order. An extra degree of freedom is provided by every layer added to the structure which may be exploited with an effective design formula for cloaking of additional field terms. The theory is illustrated with numerical simulations.
Keywords: magnetic shielding, neutral inclusion, cloaking, multipole fields
Submitted to: J. Phys.: Condens. Matter Neutral shielding and cloaking of magnetic fields 2
1. Introduction of zero frequency. These specifications were met in an experiment for a cylindrical cloak [18] using a novel Electromagnetic shielding is the practice of reducing design for the required diamagnetic metamaterials [19]. the electromagnetic field in a space with barriers made Also magnetic cloaking of uniform static fields has of conductive or magnetic materials. Passive shields been realized using a superconducting-ferromagnetic can reduce the electromagnetic interference between bilayer [20]. devices and their environments, and they are necessary In this paper, we develop a method for designing for the reliable operation of many electronic devices. magnetic shields made of isotropic materials that do Shields are used to either expel fields from an area or not perturb applied multipole fields in the static to confine fields to a region in, e.g., superconducting regime. The idea is inspired by the concept of neutral quantum interface devices for precision measurements inclusion in the theory of composites with respect to a of magnetic fields [1], magnetic resonance imaging uniform field [21, 22, 23, 24, 25]. Another motivation machines and tokamaks in magnetic confinement comes from the theoretical proposals and experimental fusion [2, 3]. Further more, electromagnetic shields are efforts to realize cloaking by manipulating various commonly used in advanced nanotechnology research materials [17, 26, 27, 28, 29, 30]. Perfect cloaking facilities, biomedical research laboratories, continuous requires that the shield and the object we wish to hide beam accelerators and various facilities such as do not disturb the wave field in the exterior domain transformer vault and switchgear in electrical-power for an incident wave from any source. This means that industry [4, 5]. For frequencies above 100 kHz, the solution to Maxwell’s equations remains exactly the satisfactory shielding performance can be achieved by same in the exterior domain as if we use the vacuum a conductive shell (Faraday shield), which makes use permittivity and permeability everywhere in space. of the eddy current to absorb the electromagnetic Such a requirement is rather stringent but can be met waves [6]. For a low-frequency electromagnetic wave or by using materials with their physical moduli equal static electric or magnetic field, shields made of high to negative numbers, zero or infinite, which might be magnetic permeability metal alloys can be used, such realized by metamaterials. An approximate cylindrical as sheets of µ-metal, to channel flux lines around the cloak for microwaves has been realized from a reduced shielded region [7, 8, 9, 10]. The shielding effectiveness set of effective anisotropic and inhomogeneous material depends on the permeability, which generally drops off parameters [31]. Relaxing the requirement of perfect at both very low magnetic field strengths and at high cloaking one can use plasmonic covers to cancel the field strengths where the material becomes saturated. dominant scattering from an object [32]. In this So to achieve low residual fields, magnetic shields often approach there is no need for anisotropy and several consist of several enclosures one inside the other, each layers can provide a discrete inhomogeneity to obtain of which successively reduces the inner field [11, 12, multifrequency solutions [33]. In the quasi-static 13, 14]. Also, superconducting materials can expel limit the dipole case is nothing but the transparency magnetic fields by the Meissner effect. However, a condition for a uniform field [34]. single shield with relative permeability of 0 ≤ µr < 1 We present a formula for making cylindrical core- (diamagnetic) or µr > 1 (paramagnetic, ferromagnetic) shell structures with concentric layers, characterized disturbs the external magnetic field in free space. by isotropic and homogeneous permeabilities, neutral There are many situations in which it may be to applied multipole fields (section 3). In the limit of desirable to hide a sensitive object from a magnetic a perfect diamagnetic (or an infinite µ-metal) response field, but not to disturb the applied field. In this way, of the innermost layer we find exact cloaking solutions external functions of the field, such as resolving objects for every multipole order. This is an extensions of the in a magnetic resonance imaging scan, are preserved. double-layer dipole cloak [20] and every layer added to A direct current magnetic cloak that shields the inner this structure constitutes an extra degree of freedom region without affecting the exterior space requires an to obtain simultaneous cloaking solutions. The theory inhomogeneous and anisotropic medium characterized is illustrated with numerical simulations for a few-layer by a response that is diamagnetic in one direction and shields (section 4) and their significance in the problem paramagnetic in the perpendicular directions [15, 16], of cloaking is discussed as we conclude (section 5). as demanded by transformation optics [17] in the limit Next we describe the methodology (section 2). Neutral shielding and cloaking of magnetic fields 3
The coefficients are related by the transfer matrix −2n 1 1 ΓjRj Tj = 2n , (5) 1+Γ ΓjR 1 j j where we have defined the mismatch factor µj − µj+1 Γj = . (6) µj + µj+1
This factor takes values in the range −1 ≤ Γj ≤ 1. The relation between the coefficients interior and exterior to a (k − 1)-layer shield is found by iteration of (4) as (1) k (k+1) An An (1) = Tj (k+1) . (7) " Bn # j=1 " Bn # Y (k+1) Figure 1. Schematic picture of a uniform magnetic field We consider An as given in terms of an applied H1 over concentric cylindrical domains with isotropic relative (k+1) n−1 (k+1) field strength Hn = |∇ψn | = nr |An | in the permeabilities µ1,µ2,...,µk in free space (µk+1 = 1) and radii R1,R2,...,Rk centered at the origin. limit r → ∞, originating from a magnetic multipole of order n at infinity. The monopole (n = 0) term (k+1) is zero implying that A0 is an unphysical constant 2. Methodology taken to be zero, which is why we consider n ≥ 1 in the potential (2). The dipole (n = 1) case corresponds We consider cylindrical core-shell structures with to the uniform field in figure 1. Due to the fact that concentric layers made of materials with homogeneous the potential must be finite within the core r 1 1 Γ 3 Γ 0.8 0.5 2 -1 0.6 0 0.4 Mismatch Factor -0.5 Shielding Factor 0.2 n = 1 n = 2 -1 0 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 Γ Γ 1 1 Figure 2. The mismatch factors Γ3 and Γ2 as functions of Γ1 Figure 3. The inverse shielding factor (13) for n = 1, 2 as giving neutral shielding (10) of dipole (n = 1) and quadrupole functions of Γ1 using R1/R2 = 0.75. The mismatch factors Γ2 (n = 2) fields for R1/R2 = 0.75 and R2/R3 = 0.8. and Γ3 are those in figure 2 giving neutrality to both dipole and quadrupole fields. 3.2. Double-layer shields (1) of the core coefficient An to the exterior coefficient (4) For two-layer shields the neutrality condition applied An . Removing the shield we get the ratio to equation (7) with k = 3 using (5) reads (1) An 2µ4 2n = (12) R1 (4) Γ2 +Γ1 2n µ1 + µ4 R2 R2 An Γ3 = − 2n . (10) e (1) R1 R3 of the core coefficient An to the exterior coefficient 1+Γ2Γ1 R2 (4) An . (Formally this follows from the coefficient The allowed values of the mismatch factors Γ1, Γ2 and relation (11) by puttingeµ2 = µ3 = µ4 so that 1 − Γ1 = Γ3 are determined by requiring that the ratios R1/R2 2µ4/(µ1 + µ4) and Γ2 =Γ3 = 0.) Without a shield the and R2/R3 are positive and less than unity. For a core scatters the applied field unless µ1 = µ4 in which given geometry the mismatch factors Γ2 and Γ3 can case (12) is unity. The inverse of the shielding factor be uniquely determined as functions of Γ1; equating can be expressed as the mismatch factor Γ3 (n = 1) to Γ3 (n = 2) in (10) (1) −1 An 1+Γ2Γ1 +Γ3(Γ1 +Γ2) yields a quadratic equation in Γ2, which admits one S = = , (13) (1) 2n acceptable (|Γ2|≤ 1) analytical solution. This solution An 2 R1 (1 − Γ3) 1+Γ2Γ1 R2 and the corresponding mismatch factor Γ3 are plotted in figure 2 as functions of Γ1 for the thickness ratios taking thee value 1 for complete transparency and 0 R1/R2 = 0.75 and R2/R3 = 0.8. We observe that Γ2 is for perfect screening. This expression follows from a decreasing function of Γ1, whilst Γ3 is an increasing equations (11) and (12) with repeated use of the − function of Γ1, and that they have opposite signs. A mismatch factors (6). In figure 3 we plot S 1 for magnetic shield satisfying these conditions is neutral n = 1, 2 as functions of Γ1 using R1/R2 = 0.75 and to both dipole (n = 1) and quadrupole (n = 2) fields. the mismatch factors in figure 2. The inverse shielding The shielding effectiveness is commonly quantified by factor (13) clearly decreases with increasing order n n the shielding factor, S, which is defined by the ratio of since Γ1 and Γ2 have opposite signs and (R1/R2) the magnetic field without the shield to the magnetic decreases with increasing value of n. The larger the field with the shield at the same point. The shielding absolute value of Γ1 the better the shielding. factor is unity for complete transparency and turns By expanding Γ2 and Γ3 in equation (10) using (6), to infinity in the limit of perfect screening. Here, we we find the permeability relation for the two-layer consider the inverse of the shielding factor in order to shield to be get values in the unit interval. Solving equation (7) for 2n R2 (1) (4) 2µ (µ⋆ − µ ) B = B = 0 using (10) we find the ratio 3 2 3 R3 n n µ = µ + , (14) 4 3 2n (1) ⋆ R2 An (1 − Γ1)(1 − Γ2) 2µ +(µ − µ ) 1 − = (11) 3 2 3 R3 (4) 2n An R1 (1+Γ3) 1+Γ2Γ1 R2 Neutral shielding and cloaking of magnetic fields 5 5 1 µ / µ (n = 1) (a) 3 4 µ / µ (n = 2) 4 3 4 0.8 µ / µ (n = 3) 3 4 -1 µ / µ 2 4 3 0.6 n = 1 n = 2 2 0.4 n = 3 Permeability Ratio Shielding Factor 1 0.2 0 0 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 Γ Γ 1 1 5 µ / µ (n = 1) Figure 5. The inverse shielding factor (13) with µ1/µ4 = 1 3 4 (b) and R1/R2 = 0.75 for n = 1, 2, 3 as functions of Γ1. The µ / µ (n = 2) 4 3 4 permeabilities µ2 and µ3 are those in figure 4(b) giving neutral µ / µ (n = 3) 3 4 shielding of each multipole field separately. µ / µ 3 2 4 2 The permeability relation (14) means that a multipole Permeability Ratio field in the exterior region (r>R3) is not perturbed 1 by the core-shell structure (0 0 solutions at a specific permeability ratio µ3/µ4 > 1. -1 -0.5 0 0.5 1 When µ1/µ4 = 1 (figure 4(b)) all curves intersect Γ 1 each other at Γ1 = 0, corresponding to the trivial solution µ1 = µ2 = µ3 = µ4. However, separate Figure 4. The permeability ratios µ2/µ4 and µ3/µ4 for neutral multipole solutions for neutral shielding of the core shielding of dipole (n = 1), quadrupole (n = 2), and octupole (n = 3) fields as functions of Γ1 in the case (a) µ1/µ4 = 0.3782, exist for nontrivial material parameters. In this case (b) µ1/µ4 = 1 and (c) µ1/µ4 = 2.6444. The thickness ratios are there is no scattering when the shield is removed and R1/R2 = 0.75 and R2/R3 = 0.8. consequently the inverse of the corresponding shielding factors become zero in the limits Γ1 → 1 (Γ2 → −1) and Γ1 → −1 (Γ2 → 1) as shown in figure 5. In these where limits we can place any object in the central region 2n without affecting the shielding effectiveness. R1 2µ Γ ⋆ 2 1 R2 In the limit µ → 0 (µ → 0), the permeability µ⋆ = µ + (15) 2 2 2 2 2n relation (14) reduces to R1 1 − Γ1 +Γ1 1 − R2 2n 1+ R2 µ R3 denotes the effective permeability (9) of the single-layer 3 = . (17) µ 2n shield, expressed in terms of the mismatch factor Γ , 4 R2 1 1 − R3 the ratio R /R , and the relative permeability 1 2 This gives perfect screening of the core and thus defines 1 − Γ1 a cloaking solution for the applied multipole field of µ2 = µ1 . (16) 1+Γ1 order n ≥ 1, depending on the ratios µ3/µ4 and R2/R3. Neutral shielding and cloaking of magnetic fields 6 10 5 n = 1 n = 1 n = 2 n = 2 8 4 n = 3 n = 3 6 5 3 4 µ µ / / 4 3 µ µ 4 2 2 1 0 0 -2 -1 0 1 2 0 0.2 0.4 0.6 0.8 1 10 10 10 10 10 R / R µ / µ 2 3 3 4 Figure 6. The permeability ratio µ3/µ4 as a function of R2/R3 Figure 7. The permeability ratio µ4/µ5 as a function of µ3/µ4 in the case µ2 = 0 for cloaking of dipole (n = 1), quadrupole for cloaking of dipole (n = 1), quadrupole (n = 2) and octupole (n = 2) and octupole (n = 3) fields, respectively. (n = 3) fields. The thickness ratios are R2/R3 = R3/R4 = 0.8. Figure 6 shows the ratio (17) as a function of R2/R3 of the core-shell structure for k ≥ 2. Here µk is for cloaking of dipole (n = 1), quadrupole (n = 2) the permeability of the kth domain, Rk−1/Rk is the ⋆ and octupole (n = 3) fields, respectively. This formula corresponding thickness ratio and µk−1 is the effective ⋆ is an extension of the dipole case [20] to multipolar permeability of the (k − 2)-layer shield, taking µ1 to fields. For R2/R3 = 0.8 the values (17) can also be be the core permeability µ1. This is an extension of a read at Γ1 = 1 in figure 4 and both µ3/µ4 = 4.5556 neutral cylindrical coated inclusion [23] from uniform (n = 1) and µ3/µ4 = 2.3875 (n = 2) will be used to multipolar fields. As an example, we consider the in the numerical simulations. A perfect diamagnetic three-layer shield defined by confining µ4 to the layer response of the inner layer (µ2 = 0) corresponds to R3