Mariana Dalarsson

First-Order Perturbation Approach to Transformer Winding Deformations

An on-line method to detect radial mechanical deforma- tions of power transformer winding turns is presented. First-order perturbation theory is applied to a transformer winding surrounded by the transformer tank wall and the iron core. The transformer wind- ing is modeled as thin conducting cylindrical rings (winding segments or turns) situated within a coaxial waveguide, where the outer con- ducting cylinder represents the transformer tank wall while the inner conducting cylinder represents the iron core. Antennas which radi- ate and measure microwave flelds are proposed inside the transformer tank in order to identify and quantify the mechanical deformations of winding turns. The direct propagation problem is solved using con- ventional waveguide theory with mode-matching and cascading tech- niques. An optimization algorithm is then used to solve the inverse problem whereby a good agreement between the reconstructed and true deformations of the winding segments is obtained.

On the physical limitations for radio frequency absorption in gold nanoparticle suspensions

This paper presents a study on the physical limitations for radio frequency absorption in gold nanoparticle suspensions. A canonical spherical geometry is considered consisting of a spherical suspension of colloidal gold nanoparticles characterized as an arbitrary passive dielectric material which is immersed in an arbitrary lossy medium. A relative heating coefficient and a corresponding optimal near field excitation are defined taking the skin effect of the surrounding medium into account. For small particle suspensions the optimal excitation is an electric dipole field for which explicit asymptotic expressions are readily obtained. It is then proven that the optimal permittivity function yielding a maximal absorption inside the spherical suspension is a conjugate match with respect to the surrounding lossy material. For a surrounding medium consisting of a weak electrolyte solution the optimal conjugate match can then readily be realized at a single frequency, e.g., by tuning the parameters of a Drude model corresponding to the electrophoretic particle acceleration mechanism. As such, the conjugate match can also be regarded to yield an optimal plasmonic resonance. Finally, a convex optimization approach is used to investigate the realizability of a passive material to approximate the desired conjugate match over a finite bandwidth. The relation of the proposed approach to general Mie theory as well as to the approximation of metamaterials are discussed. Numerical examples are included to illustrate the ultimate potential of heating in a realistic scenario in the microwave regime.

A parametric model for the changes in the complex valued conductivity of a lung during tidal breathing

Classical homogenization theory based on the Hashin-Shtrikman coated ellipsoids is used to model the changes in the complex valued conductivity (or admittivity) of a lung during tidal breathing. Here, the lung is modeled as a two-phase composite material where the alveolar air-filling corresponds to the inclusion phase. The theory predicts a linear relationship between the real and the imaginary parts of the change in the complex valued conductivity of a lung during tidal breathing, and where the loss cotangent of the change is approximately the same as of the effective background conductivity and hence easy to estimate. The theory is illustrated with numerical examples based on realistic parameter values and frequency ranges used with electrical impedance tomography (EIT). The theory may be potentially useful for imaging and clinical evaluations in connection with lung EIT for respiratory management and control. (Less)

Parameter studies on optimal absorption and electrophoretic resonances in lossy media

This paper summarizes and elaborates on some new results on the optimal absorption in small spherical suspensions based on electrophoretic (plasmonic) resonances and lossy surrounding media. The main application here is to study the physical limitations for radio frequency absorption in gold nanoparticle (GNP) suspensions and its potential to achieve GNP targeted hyperthermia in cancer therapy. Numerical parameter studies are included to demonstrate the analysis approach.

Absorption and optimal plasmonic resonances for small ellipsoidal particles in lossy media

A new simplified formula is derived for the absorption cross section of small dielectric ellipsoidal particles embedded in lossy media. The new expression leads directly to a closed form solution f ...

Lossy transmision with arbitrary gradient permittivity and permeability and constant impedance throughout the structure

We present a remarkably simple exact analytical solution for the electromagnetic field distribution across an infinite metamaterial composite with an arbitrary graded variation of complex effective permittivity and permeability for the case of constant impedance across the structure. Arbitrary temporal dispersion and losses are allowed and the model is generally applicable to different inhomogeneous and anisotropic media simultaneously containing positive and negative refractive index constituents, as long as the effective medium approximation remains valid. The analytical solution is validated by a dispersive numerical model of lossy metamaterials that uses a transmission line matrix method based on Z-transforms, where a close agreement between the analytic and numerical results is obtained.

Gradient-index infrared metamaterials based on metal-dielectric submicrometer pillar arrays

This paper presents the design and microfabrication of a two-dimensional metal-dielectric metamaterial structure based on an array of pillars with submicrometer diameters and heights. The diameters of pillars periodically vary along one axis in a sawtooth fashion and are constant along the other. The electromagnetic field distribution within this graded metamaterial was considered utilizing an accurate analytical approach. The pillar arrays were fabricated in photoresist and subsequently covered with a sputter-deposited aluminum film. Structures were defined by direct laser writing in photoresist film. Controlled overexposure has been applied in order to make pillar features smaller than the nominal resolution of the equipment. The structures were characterized by optical and atomic force microscopy and by angle-dependent Fourier Transform infrared spectroscopy. The produced graded frequency-selective surfaces may be used e.g. in sensing.