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<p>This thesis contributes significantly to the advanced applications of scattering parameter sensitivity analysis including the design optimization of high-frequency printed structures and in microwave imaging. In both applications, the methods exploit the computational efficiency of the self-adjoint sensitivity analysis (SASA) approach where only one EM simulation suffices to obtain both the responses and their gradients with respect to the optimizable variables.</p> <p>An<em> S</em>-parameter self-adjoint sensitivity formula for multiport planar structures using the method of moments (MoM) current solution is proposed. It can be easily implemented with existing MoM solvers. The shape perturbation which is required in computing the system-matrix derivatives are accommodated by changing the material properties of the local mesh elements. The use of a pre-determined library system matrix further accelerates the design optimization because the writing/reading of the system matrix to/from the disk is avoided. The design optimization of a planar ultra-wide band (UWB) antenna and a double stub tuner are presented as validation examples.</p> <p>In the application of the sensitivity-based imaging, the SASA approach allows for real-time image reconstruction once the field distribution of the reference object (RO) is known. Here, the RO includes the known background medium of the object under test (OUT) and the known antennas. The field distribution can be obtained using simulation or measurement.</p> <p>The spatial resolution is an important measure of the performance of an imaging technique. It represents the smallest detail that can be detected by a given imaging method. The resolution of the sensitivity-based imaging approach has not been studied before. In this thesis, the resolution limits are systematically studied with planar raster scanning and circular array data acquisition. In addition, the method’s robustness to noise is studied. A guideline is presented for an acceptable signal-to-noise ratio (SNR) versus the spatial and frequency sampling rates in designing a data-acquisition system for the method.</p> <p>This thesis validates the sensitivity-based imaging with measured data of human tissue phantoms for the first time. The differences in dielectric properties of the targets are qualitatively reflected in the reconstructed image. A preliminary study of imaging with inexact background information of the OUT is also presented.</p> / Doctor of Philosophy (PhD)

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京都大学 / 新制・課程博士 / 博士(情報学) / 甲第24738号 / 情博第826号 / 新制||情||138(附属図書館) / 京都大学大学院情報学研究科先端数理科学専攻 / (主査)教授 磯 祐介, 准教授 吉川 仁, 准教授 藤原 宏志, 教授 西村 直志(京都大学 名誉教授) / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

Understanding, predicting, and controlling electromagnetic field interactions on and between complex RF platforms requires high fidelity computational electromagnetic (CEM) simulation. The primary CEM tool within NASA is GEMINI, an integral equation based method-of-moments (MoM) code for frequency domain electromagnetic modeling. However, GEMINI is currently limited in the size and complexity of problems that can be effectively handled. To extend GEMINI’S CEM capabilities beyond those currently available, primary research is devoted to integrating the MFDlib library developed at the University of Kentucky with GEMINI for efficient filling, factorization, and solution of large electromagnetic problems formulated using integral equation methods. A secondary research project involves the hybrid parallelization of GEMINI for the efficient speedup of the impedance matrix filling process. This thesis discusses the research, development, and testing of the secondary research project on the High Performance Computing DLX Linux supercomputer cluster. Initial testing of GEMINI’s existing MPI parallelization establishes the benchmark for speedup and reveals performance issues subsequently solved by the NASA CEM Lab. Implementation of hybrid parallelization incorporates GEMINI’s existing course level MPI parallelization with Open MP fine level parallel threading. Simple and nested Open MP threading are compared. Final testing documents the improvements realized by hybrid parallelization.

<p>In this study we propose a fast hybrid spectral-element time-domain (SETD) / finite-element time-domain (FETD) method for transient analysis of multiscale electromagnetic problems, where electrically fine structures with details much smaller than a typical wavelength and electrically coarse structures comparable to or larger than a typical wavelength coexist.</p><p>Simulations of multiscale electromagnetic problems, such as electromagnetic interference (EMI), electromagnetic compatibility (EMC), and electronic packaging, can be very challenging for conventional numerical methods. In terms of spatial discretization, conventional methods use a single mesh for the whole structure, thus a high discretization density required to capture the geometric characteristics of electrically fine structures will inevitably lead to a large number of wasted unknowns in the electrically coarse parts. This issue will become especially severe for orthogonal grids used by the popular finite-difference time-domain (FDTD) method. In terms of temporal integration, dense meshes in electrically fine domains will make the time step size extremely small for numerical methods with explicit time-stepping schemes. Implicit schemes can surpass stability criterion limited by the Courant-Friedrichs-Levy (CFL) condition. However, due to the large system matrices generated by conventional methods, it is almost impossible to employ implicit schemes to the whole structure for time-stepping.</p><p>To address these challenges, we propose an efficient hybrid SETD/FETD method for transient electromagnetic simulations by taking advantages of the strengths of these two methods while avoiding their weaknesses in multiscale problems. More specifically, a multiscale structure is divided into several subdomains based on the electrical size of each part, and a hybrid spectral-element / finite-element scheme is proposed for spatial discretization. The hexahedron-based spectral elements with higher interpolation degrees are efficient in modeling electrically coarse structures, and the tetrahedron-based finite elements with lower interpolation degrees are flexible in discretizing electrically fine structures with complex shapes. A non-spurious finite element method (FEM) as well as a non-spurious spectral element method (SEM) is proposed to make the hybrid SEM/FEM discretization work. For time integration we employ hybrid implicit / explicit (IMEX) time-stepping schemes, where explicit schemes are used for electrically coarse subdomains discretized by coarse spectral element meshes, and implicit schemes are used to overcome the CFL limit for electrically fine subdomains discretized by dense finite element meshes. Numerical examples show that the proposed hybrid SETD/FETD method is free of spurious modes, is flexible in discretizing sophisticated structure, and is more efficient than conventional methods for multiscale electromagnetic simulations.</p> / Dissertation

Efficient electromagnetic solvers based on surface integral equations (SIEs) are developed for the analysis of scattering from large-scale and complex composite structures that consist of piecewise homogeneous magnetodielectric and perfect electrically/magnetically conducting (PEC/PMC) regions. First, a multiple-grid extension of the adaptive integral method (AIM) is presented for multi-region problems. The proposed method accelerates the iterative method-of-moments solution of the pertinent SIEs by employing multiple auxiliary Cartesian grids: If the structure of interest is composed of K homogeneous regions, it introduces K different auxiliary grids. It uses the k^{th} auxiliary grid first to determine near-zones for the basis functions and then to execute AIM projection/anterpolation, propagation, interpolation, and near-zone pre-correction stages in the k^{th} region. Thus, the AIM stages are executed a total of K times using different grids and different groups of basis functions. The proposed multiple-grid AIM scheme requires a total of O(N^{nz,near}+sum({N_k}^Clog{N_k}^C)) operations per iteration, where N^{nz,near} denotes the total number of near-zone interactions in all regions and {N_k}^C denotes the number of nodes of the k^{th} Cartesian grid. Numerical results validate the method’s accuracy and reduced complexity for large-scale canonical structures with large numbers of regions (up to 10^6 degrees of freedom and 10^3 regions). Then, a Green function modification approach and a scheme of Hankel- to Teoplitz-matrix conversions are efficiently incorporated to the multiple-grid AIM method to account for a PEC/PMC plane. Theoretical analysis and numerical examples show that, compared to a brute-force imaging scheme, the Green function modification approach reduces the simulation time and memory requirement by a factor of (almost) two or larger if the structure of interest is terminated on or resides above the plane, respectively. In addition, the SIEs are extended to cover structures composed of metamaterial regions, PEC regions, and PEC-material junctions. Moreover, recently introduced well-conditioned SIEs are adopted to achieve faster iterative solver convergence. Comprehensive numerical tests are performed to evaluate the accuracy, computational complexity, and convergence of the novel formulation which is shown to significantly reduce the number of iterations and the overall computational work. Lastly, the efficiency and capabilities of the proposed solvers are demonstrated by solving complex scattering problems, specifically those pertinent to analysis of wave propagation in natural forested environments, the design of metamaterials, and the application of metamaterials to radar cross section reduction. / text

Computational electromagnetic modeling involves generating system matrices by discretizing integral equations and solving the resulting system of linear equations. Many methods of solving the system of linear equations exist and one such method is the factorization of the matrix using the so called local-global solution (LOGOS) modes. Computer codes to perform the discretization of the integral equations, filling of the matrix, and the subsequent LOGOS factorization have previously been developed by others. However, these codes are limited to complex double precision arithmetic only. This thesis extends and expands the existing computer by creating a more general implementation that is able to analyze a problem not only in complex double precision but also in real double precision and both complex and real single precision. The existing code is expanded using "templates" in Fortran 90 and the resulting generic code is used test the performance of the LOGOS (both OL- and NL-LOGOS) factorization on matrices generated by discretization of the volume integral equation. As part of this effort, we demonstrate for the first time that the LOGOS factorization provides an O(N log N) complexity solution to the volume integral equation formulation of low-frequency electromagnetic problems.

The frequency demands of radiating systems are moving into the terahertz band with potential applications that include sensing, imaging, and extremely broadband communication. One commonly used method for generating and detecting terahertz waves is to excite a voltage-biased photoconductive antenna with an extremely short laser pulse. The pulsed laser generates charge carriers in a photoconductive substrate which are swept onto the metallic antenna traces to produce an electric current that radiates or detects a terahertz band signal. Therefore, analysis of a photoconductive antenna requires simultaneous solutions of both semiconductor physics equations (including drift-diffusion and continuity relations) and Maxwell’s equations. A multi-physics analysis scheme based on the Discontinuous-Galerkin Finite-Element Time-Domain (DGFETD) is presented that couples the semiconductor drift-diffusion equations with the electromagnetic Maxwell’s equations. A simple port model is discussed that efficiently couples the two equation sets. Various photoconductive antennas were fabricated using TiAu metallization on a GaAs substrate and the fabrication process is detailed. Computed emission intensities are compared with measured data. Optimized antenna designs based on the analysis are presented for a variety of antenna configurations.

La connaissance précise des champs électromagnétiques rayonnés par les dispositifs hyperfréquences nécessite des outils instrumentaux permettant la mesure directe ou indirecte de ces champs. La technique du scan champ proche fait partie de ces outils. Ce manuscrit décrit les travaux de caractérisation des sondes électromagnétiques pour une plate-forme de scan champ proche développée au laboratoire LAPLACE. L’accent a été mis sur la simulation électromagnétique des dispositifs de test utilisés dans le travail de déconvolution du facteur d’antenne des sondes de champs magnétiques ou électriques. Ces dispositifs comprennent aussi bien des structures planaires telles que des interconnexions en ligne microruban que des composants en guide d’ondes tels que des guides ouverts de formes rectangulaire ou circulaire. Pour analyser ces structures des logiciels commerciaux basés sur la méthode des différences finis ont été utilisés. Dans le cas des structures rayonnantes un programme basé sur la méthode de l’opérateur transverse a été développé, permettant la détermination de l’admittance de rayonnement et les champs rayonnés en zones proche et lointaine. Les résultats obtenus ont été validés par des simulations avec des outils commerciaux, et par des mesures réalisés au laboratoire. / The accurate knowledge of electromagnetic fields radiated by microwave devices requires instrumental tools for direct or indirect measurement of these fields. Near-field scan technique is one of those tools. This manuscript describes the work done to characterize electromagnetic probes using a near field scan platform developed in the laboratory LAPLACE. We focused our work on the electromagnetic simulation of test devices that are used in the deconvolution of antenna factor of magnetic or electric probes. These devices include both planar structures such as microstrip line and also waveguide components such as rectangular or circular open-ended waveguides. To analyze these structures, commercial software based on finite element method was used. In case of radiating structures, a program based on transverse operator method was developed. It allows the determination of the admittance of radiation and the radiated electromagnetic fields in near-field and far-field regions. The results were validated by simulations with commercial tools, and measurements made in the laboratory.