Full-wave Solution Research Articles

Study on human brain protection from electromagnetic radiation of mobile phone by plasma

The propagation characteristics of electromagnetic waves (EMW) in plasma with the plural dielectric properties (real and imaginary parts of the relative dielectric constant) were investigated in the virtual human brain (VHB) model by the full-wave solution. Simulation results indicate that the value of the electric and magnetic field amplitude of EMW with the frequency of 5 GHz and the value of specific absorption ratio within VHB can be reduced by more than 60% and 80%, respectively. After the secondary absorption of the plasma layer, the reflected wave amplitude is attenuated by 33.066 dB. The amplitude attenuation of EMW from 2.45 to 6 GHz can be exceeded 10 dB. Compared to the traditional absorbing materials for the electromagnetic protection, the plasma layer, as an absorbing medium, possesses wide frequency characteristics. These results have important reference value for reducing the high-frequency electromagnetic radiation to VHB.

Analytical Quantum Full-Wave Solutions for a 3-D Circuit Quantum Electrodynamics System

Analytical Quantum Full-Wave Solutions for a 3-D Circuit Quantum Electrodynamics System

Plasma wave propagation conditions analysis using the warm multi-fluid model

Although an accurate description of wave propagation and absorption in plasmas requires complicated full-wave solutions or kinetic simulations, local dispersion analysis can still be helpful to capture the main physics of wave properties. Plasma wave propagation conditions or accessibility informs whether a wave can propagate to a region, which usually depends on the wave frequency, wave vector, the local plasma density, and magnetic field. We demonstrate a warm multi-fluid eigenvalue model and a matrix approach to rapidly calculate plasma wave accessibility diagrams, where thermal effects are also included via an isotropic pressure term. All cold plasma waves, from high-frequency electron cyclotron waves, intermediate-frequency lower hybrid waves, to low-frequency ion cyclotron waves, are presented. By comparing with the kinetic model, it is interesting to find that the warm multi-fluid model, though incapable of reproducing the Bernstein modes, can provide a quick way to determine whether thermal effects are important.

A Full-wave Solution of Deep Sources in the Lossy Human Head to Accurate Electroencephalography and Magnetoencephalography.

Currents in the brain flow inside neurons and across their boundaries into the extracellular medium, create electric and magnetic fields. These fields, which contain suitable information on brain activity, can be measured by electroencephalography (EEG), magnetoencephalography (MEG), and direct neural imaging. In this paper, we employed an electromagnetic model of the neuron activity and human head to derive electric and magnetic fields (brain waves) using a full-wave approach (ie. without any approximation). Currently, the brain waves are only derived using the quasi-static approximation (QSA) of Maxwell's equations in electromagnetic theory. As a result, source localization in brain imaging will produce some errors. So far, the error rate of the QSA on the output results of electric and magnetic fields has not been investigated. This issue has become more noticeable due to the increased sensitivity of modern electroencephalography (EEG) and magnetoencephalography (MEG) devices. This work introduces issues that QSA encounters in this problem and reveals the necessity of a full-wave solution. Then, a full-wave solution of the problem in closed-form format is presented for the first time. This solution is done in two scenarios: the source (active neurons) is in the center of a sphere, and when the source is out of the center but deeply inside the sphere. The first scenario is simpler, but the second one is much more complicated and is solved using a partial-wave series expression. One of the significant achievements of this model is improving the interpretation of EEG and MEG measurements, resulting in more accurate source localization.

Origin of the super-resolution of microsphere-assisted imaging

Theoretical explanation of the super-resolution imaging by contact microspheres created a point of attraction for nanoimaging research during the last decade with many models proposed, yet its origin remains largely elusive. Using a classical double slit object, the key factors responsible for this effect are identified by an ab initio imaging model comprising object illumination, wave scattering, and image reconstruction from the diffracted far fields. The scattering is found by a full-wave solution of the Maxwell equations. The formation of super-resolved images relies on coherent effects, including the light scattering into the waves circulating inside the microsphere and their re-illumination of the object. Achieving the super-resolution of the double slit requires a wide illumination cone as well as a deeply sub-wavelength object-to-microsphere separation. The resultant image has a significantly better resolution as compared to that from the incoherent imaging theory.

Schumann resonance anomalies associated with two (M~7) Tohoku offshore earthquakes in the spring of 2021

ABSTRACT. We describe the Schumann resonance (SR) anomalies associated with two earthquakes (EQs) observed in Japan in the spring of 2021. SR is the global electromagnetic phenomenon observed in the ELF (extremely low frequency) band, and its resonance peaks are observed in the power spectra on natural radio noise at frequencies of 8, 14, 20, Hz, etc. The natural source of ELF radiation is the global lightning activity occurring in the Earth-ionosphere cavity. The anomalies were observed for the first time in Japan for the EQs in Taiwan when the distance between the observatory and the EQ epicenter was a few Mm (1 Mm = 1000 km). Recently, a new SR anomaly was addressed, related to nearby (a few hundred km) EQs (Hayakawa et al., 2019, 2020a,b). This paper presents the SR anomalies observed in the vicinity of Nagoya-city for two relatively close (~500 km) successive EQs with magnitude around 7 that occurred offshore the Tohoku area in Japan. The anomaly is characterized by the noticeable simultaneous increase or decrease in the amplitudes of three SR modes. This SR unusual behavior was observed prior to and after each of the two EQs that occurred in February and March, 2021. Observational data were interpreted in the model of seismogenic perturbations of the lower ionospheric conductivity profile. Model computations imply the full-wave solution of the ELF electromagnetic problem in the form of the Riccati equation and the 2D (two dimensional) telegraph equations. We show that observed disturbances in the SR power spectra might be attributed to two types of seismogenic modifications in the lower ionospheric profile: the compression or the expansion of the vertical profiles of mesospheric conductivity over the EQ epicenter.

A novel and computationally efficient subgrid technique for modeling scattering and Doppler effects of rough dynamic sea surfaces using the finite-difference time-domain method

Over the last decade, the improvements in computational performance that multiprocessing, solid-state hard drives, and fast memories have brought to desktop workstations have made modeling and simulation methods that were previously inaccessible into the office workspace. These methods were previously restricted to large servers or computer clusters because of their demanding computation and large memory requirements when applied to underwater acoustic simulation scenarios. One such method is the Finite-difference Time-domain (FDTD) technique. The FDTD method is a discrete numeric model that provides the full pressure wave solution solved directly in the time domain. This makes it an ideal model for investigating scattering and Doppler effects of boundaries with complex geometries and motion. A fully developed sea surface generated using the Pierson–Moskowitz wavenumber spectra meets these criteria. A novel subgrid technique has been developed to improve the accuracy of signals scattered from fully developed sea surfaces. This method remains computationally efficient because the subgrid is applied only along the rough sea surface as it varies over time. The FDTD subgrid method has been applied to simulations that investigate the “frozen” sea surface assumption that is utilized by other traditional models that incorporate boundary motion.

Parallelization of a 3D FDTD code and physics studies of electromagnetic wave propagation in fusion plasmas

Numerical codes for electromagnetic wave propagation in magnetized plasmas are mainly based on frequency-domain asymptotic methods, which provide a fast solution and are thus valuable for experiment design and control applications. However, in several cases of practical interest (e.g. mode conversion), these tools run close to their limits of validity and should be compared to full-wave solutions. The code RFFW solves Maxwell's equations with the finite-difference time-domain method in 3D geometry, for scenarios involving high-frequency waves with arbitrary electric field spectrum in plasmas with axisymmetric equilibrium. In fusion-related problems, the code may conduct investigations of wave propagation and absorption relevant to auxiliary plasma heating and current drive, reflectometry and instability control. The code has been parallelized with a hybrid OpenMP-MPI scheme, which has allowed exploiting the much larger processing power and memory of current-day supercomputers. In this work, we present the main aspects of the physics implemented in the code, and also refer shortly to the parallelization scheme. Furthermore, we show results that exhibit the strong scaling performance of the code, and examine cases of electron-cyclotron heating application in medium-sized tokamaks.

Finite-Element A-V Formulation for EMQS Problems via Two-Domain Continuity Gauging

A finite-element A-V formulation is developed to solve EMQS (electromagnetic quasistatic), aka Darwin, problems in the time and frequency domains by making use of a two-domain continuity gauging strategy. The formulation utilizes the magnetic vector and the electric scalar potentials, and an additional scalar in the conducting regions to make the discretized Galerkin system symmetric. The resulting FE (finite element) matrix is LF (low-frequency) stable in the frequency domain. As a result, the time-step size is not limited by the characteristics of the system matrix in the backward Euler time-stepping scheme. The system matrixes are also regular, which allows choosing a direct solver to speed up the temporal solution significantly. The formulation is verified in both the frequency and the time domains by solving benchmark problems, and the results are compared to full-wave and eddy-current solutions.

Optical coupled-mode theory for dielectric solids of revolution

We propose a single resonance coupled-mode approach to light scattering by dielectric solids of revolution. By using a biorthogonal decomposition of the $S$ matrix found with the extended boundary condition method we derived all parameters required for application of the temporal coupled-mode theory in a closed form. The proposed approach allows for constructing a frequency-dependent Fano response due to a single resonance after the full-wave solution has been found at a single incident frequency.

Efficient prediction of airborne noise propagation in a non-turbulent urban environment using Gaussian beam tracing method

This paper presents a noise propagation approach based on the Gaussian beam tracing (GBT) method that accounts for multiple reflections over three-dimensional terrain topology and atmospheric refraction due to horizontal and vertical variability in wind velocity. A semi-empirical formulation is derived to reduce truncation error in the beam summation for receivers on the terrain surfaces. The reliability of the present GBT approach is assessed with an acoustic solver based on the finite element method (FEM) solutions of the convected wave equation. The predicted wavefields with the two methods are compared for different source-receiver geometries, urban settings, and wind conditions. When the beam summation is performed without the empirical formulation, the maximum difference is more than 40 dB; it drops below 8 dB with the empirical formulation. In the presence of wind, the direct and reflected waves can have different ray paths than those in a quiescent atmosphere, which results in less apparent diffraction patterns. A 17-fold reduction in computation time is achieved compared to the FEM solver. The results suggest that the present GBT acoustic propagation model can be applied to high-frequency noise propagation in urban environments with acceptable accuracy and better computational efficiency than full-wave solutions.

A Low-Frequency Approximation PEEC Model for Thin-Wire Grounding Systems in Half-Space

This paper presents a PEEC model of thin-wire structures for lightning grounding analysis based on low-frequency approximation of dyadic Green's functions. To avoid the time-consuming numerical Sommerfeld integrals, the primary and quasi-dynamic terms are extracted from the spectral-domain Green's function and used as the low-frequency approximation closed-form of the full-wave solution, which is appropriate in the near-field region. The corresponding equivalent circuit is constructed with this closed-form approximated Green's function and the formulas for the partial inductance and coefficient of potential is given. Comparison in terms of the Green's function and circuit parameters between the proposed method and the conventional modified-image method is made. The validity of the proposed method is confirmed with experimental results gathered from published literature and MOM-calculated results from other researchers. A systematic parametric analysis is conducted to evaluate the accuracy and applicability of the different approximate models, which clearly shows the superiority of the proposed method over the traditional modified-image method.

Spacecraft charging with EMA3D Charge

Spacecraft are exposed to harsh radiation environments. These dynamic radiation environments present significant threat to mission performance when charge accumulation and dissipation is not adequately mitigated. EMA3D Charge is a new simulation platform tailored to the study of surface charging, deep dielectric charging, and radiation hardening of spacecraft and their components. Charge implements multiple solvers capable of co-simulating: finite element electromagnetics, boundary element method surface charging, particle transport and particle-in-cell. The multiple solvers facilitate model reuse and enable powerful physical analysis not possible with separate, individual solvers. The solvers include a three-dimensional particle transport tool, a surface charge balance solver, and a finite-element-method electromagnetic solver that supports both quasi-static and full-wave (or fully electrodynamic) solutions. This article summarizes the solvers, example applications, and validation compared to accepted numerical solutions for spacecraft in the space plasma environment.

Design and optimization of nanooptical couplers based on photonic crystals involving dielectric rods of varying lengths

This study presents design and optimization of compact and efficient nanooptical couplers involving photonic crystals. Nanooptical couplers that have single and double input ports are designed to obtain efficient transmission of electromagnetic waves in desired directions. In addition, these nanooptical couplers are cascaded by adding one after another to realize electromagnetic transmission systems. In the design and optimization of all these nanooptical couplers, the multilevel fast multipole algorithm, which is an efficient full-wave solution method, is used to perform electromagnetic analyses and simulations. A heuristic optimization method based on genetic algorithms is employed to obtain effective designs that provide the highest efficiency values. Two types of optimization strategies are applied using nanorods with a fixed length and using nanorods with varying lengths. This way, photonic crystals consisting of irregular arrays of both identical and nonidentical dielectric elements are designed for the realization of nanooptical couplers. The designs and their numerical results show that it is possible to design and further improve efficient nanooptical couplers with simple and compact geometries based on the principles of photonic crystals. Using relatively simple geometries and a single material, the designed nanooptical couplers are more preferable than the available designs in the literature.

Transient overvoltage analysis in the medium voltage substations based on full-wave modeling of two-layer grounding system

In this paper, a comprehensive investigation of substation grounding soil structure effects on switching/lightning transients is presented. The full-wave computational solution Method of Moments (MoM) is adopted to solve Maxwell’s equations. The significant advantage of this technique is its ability to deal effectively with substation grounding systems (SGSs) at high frequency. This evaluation is carried out considering two soil structures: (1) a uniform soil and (2) soil with a layered structure. The impacts of both soil structures on the peak transient overvoltage values are compared. To begin with, the harmonic impedance matrix (HIM) of the layered multi-terminal SGS is computed. Then, a vector fitting approach is employed to incorporate the accurate SGS behavior by relying on the obtained HIM into time domain simulations. Finally, the transient overvoltages and maximum ground potential rise (GPR) are computed. The chief contribution of this paper is the transient assessment of the two-layer multi-terminal soil structure of the large SGS taking into account the frequency dependence model of the electrical parameters of soil, which is not generally investigated and instead, treated simplistically. According to the results of the study, the lack of an exact grounding system models may lead to underestimations/overestimations of the GPR and the overvoltage values. The results demonstrate the need for the grounding modeling of SGS considering layered soil structure to be incorporated into power system transient analysis in a wide range of frequencies.

Sensitivity of a tapered fiber refractive index sensor at diameters comparable to wavelength

While there is an established literature on the effect of geometric parameters and material properties on the sensitivity of tapered fiber refractive index sensors, the impact of these parameters has been largely overlooked at sensor at diameters comparable to wavelength. Here, we investigate the effect of geometric parameters and materials properties at dimensions comparable to wavelength using full-wave solutions of Maxwell’s equations. Our results indicate that to achieve the maximum sensitivity for the tapered fiber sensors, the diameter should be closer to operational wavelength. For diameters less than the operational wavelength, sensitivity reduces instead of further increasing, a phenomenon opposite to what was predicted by ray-tracing based tapered fiber models. Another important finding of this study is the effect of optical loss constant on the sensitivity, a parameter often overlooked in the literature. As the optical loss constant was varied from 0 to 0.04 for each iteration of RI variation range, a linear decrease in sensitivity from 0.12 trans. (A.U)/RIU to 0.05 trans. (A.U)/RIU.

Scattering analyses of arbitrary roughness from 2-D perfectly conductiveperiodic surfaces with moments method

In this paper, a periodic-MoM-based code with high accuracy performance is developed to calculate electromagnetic scattering from a periodic conductive surface in two dimensions with any degree of roughness. Firstly, the existing separate methods in the literature are reviewed step by step to compose a periodic-MoM solution for 2-D periodic surfaces. Then, the dynamic selection of optimal formulation of the periodic-MoM solutions created using these existing methods is evaluated to reduce solution time and obtain high accuracy. In this study, the performance parameters of the existing methods are investigated in solving a real 3-D scattering problem by a periodic-MoM for the first time in the literature. Eventually, a commercial EM solver with a similar numerical approach is employed for comparison, validation, and accuracy achievements of these different periodic-MoM formulations. Furthermore, an analytical solution named Floquet mode-matching method (FMMM) is developed based on the Rayleigh hypothesis for slightly rough surfaces, and the accuracy of the code is tested using this analytical solution. This study is entirely compatible with analytical solutions and gives better results than this commercial EM solver in terms of accuracy. Also, the FMMM formulation derived for comparison provides a more straightforward solution than the small perturbation method (SPM), a classical solution method, and it is adapted to two-dimensional surface roughness for the first time in this paper. However, this full-wave solution has no restriction on surface roughness, unlike the analytical solutions.

Full-wave anelastic and compressible Fourier methods for gravity waves in the thermosphere

A general method for computing solutions of systems of linear gravity-wave governing equations, allowing for vertically varying background parameters, is introduced and applied to two sets of governing equations for gravity waves in the thermosphere, both of which include molecular viscosity and thermal diffusion. The first set is fully compressible and the second set is anelastic. In the latter case, a new set of viscous and diffusive anelastic governing equations is given, and an anelastic dispersion relation, previously obtained only as a limiting case of a fully compressible dispersion relation, is shown to follow from the new anelastic governing equations. The new full-wave method is an extension of a numerical multilayer method described previously by Knight et al. (2019). Here, the term “numerical multilayer method” refers to a multilayer method that converges to a solution of an underlying vertical structure equation or system of equations in the limit of infinitesimal layer width. The previously described method was primarily suitable for dispersion-relation partial differential equations (PDEs), solutions of which only give approximate solutions of the underlying governing equations when the background parameters vary. Much of the theory developed for the previous method is still applicable to the new method, with some modifications, including the definition of upgoing and downgoing modes in terms of roots of the dispersion relation and the use of imaginary frequency shifts to make this classification of modes possible in all cases. The practical advantage of solving the anelastic equations instead of the compressible equations is that the needed imaginary frequency shifts are easier to determine in the anelastic case. To simplify the discussion, the application of the new full-wave method to the two sets of governing equations is only worked out for two-dimensional waves in the situation where kinematic viscosity varies but background temperature, scale height (for density), and horizontal wind are constant. Full-wave anelastic and compressible solutions are given for three idealized examples and compared with each other and with anelastic dispersion-relation PDE solutions. It is seen that full-wave anelastic solutions give good approximations of full-wave compressible solutions, while dispersion-relation PDE solutions become increasingly inaccurate with increasing kinematic viscosity for large horizontal wavelengths.

Estimating Fields in Spacecraft Cavities: Experimental Validation and Optimization of Finite-Difference Time-Domain and Power Balance Computational Tools

Electromagnetic fields in representative spacecraft cavities were successfully predicted using finite-difference time-domain and power balance computational tools. Results were validated with measurements of two test articles, showing excellent correlation in shielding effectiveness from 300 MHz to 18 GHz. The validated tools were then extended to predict fields inside representative, to-scale payload fairings including common systems and components like satellite payloads, antennas, acoustic blankets, and a cable harness. Various computational techniques were used to compare their speed and accuracy. Ultimately, we conclude that a multi-fidelity approach – combining full-wave, statistical, and hybrid solutions – is beneficial and necessary for complex and large cavity problems. The tools and techniques presented here can serve as part of a toolkit to rapidly estimate shielding effectiveness, the impact of payloads, and overall fields in spacecraft cavities.

Group velocity dispersion in terahertz frequency combs within a generalized Maxwell-Bloch framework

As many molecules have their rotovibrational resonance frequencies in the mid-infrared or terahertz regime, efficient generation of corresponding frequency combs may lead to large progress in gas spectroscopy and sensing. Quantum cascade lasers (QCLs) are among the most promising candidates for a compact and cheap radiation source in this frequency range. This contribution presents a full-wave numerical solution of the Maxwell-Liouville-von Neumann equations, thus avoiding the limited applicability of the rotating wave approximation to moderate field strengths and spectral bandwidths. We include losses and chromatic dispersion of the optically active material in the QCL. The semiclassical approach uses the finite-difference time-domain (FDTD) method to derive update equations for the electric field, starting from the one-dimensional Maxwell equations. There, the optical full-wave propagation is coupled to the electronic quantum system via a polarization term that arises from the evolution of the density matrix. Furthermore, dispersion effects are considered through a classical polarization term and losses are introduced by a finite material conductivity. This work mainly focuses on the integration of group velocity dispersion (GVD) due to the bulk material and, if applicable, the waveguide geometry into the update equations. It is known to be one of the main degradation mechanisms of terahertz frequency combs, but has not yet been added to the existing full-wave solver. The implementation is carried out as Lorentz model and is applied to an experimentally investigated QCL frequency comb setup from the literature. The reported results are in good agreement with the experimental data. Especially, they confirm the need for dispersion compensation for the generation of terahertz frequency combs in QCLs.