Erion Gjonaj

High-resolution human anatomy models for advanced electromagnetic field computations

High-resolution anatomy models are used in the computation of induced electromagnetic fields, SAR, and temperature profiles in the human body. In particular, the HUGO model with a voxel resolution of up to 1/spl times/1/spl times/1 mm allows accurate modeling of more than 30 distinct human organs and tissues. The numerical calculations use the finite integration technique, which proves appropriate for the voxel-based human models. Simulation results for the induced electromagnetic fields in the human head due to the high-frequency radiation of a cellular phone are given. In a second application the electromagnetic field pattern and temperature distribution in the human body in regional hyperthermia therapy are calculated.

Accurate modelling of charged particle beams in linear accelerators

A higher order, energy conserving discretization technique for beam dynamics simulations is presented. The method is based on the discontinuous Galerkin (DG) formulation. It utilizes locally refined, non-conforming grids which are designed for high spatial resolution along the path of charged particle beams. Apart from this formulation, the paper introduces a class of general symplectic integrators which conserve discrete energy in a modified sense. Specialized split-operator methods with optimum dispersion properties in the direction of particle motion are, additionally, derived. The application examples given in the paper are performed in a high performance computing environment. They include the self-consistent simulation of the RF electron gun developed by the Photo Injector Test Facility at DESY Zeuthen (PITZ) project and the computation of short range wake fields for ultra-relativistic electron bunches.

Impact of the displacement current on low-frequency electromagnetic fields computed using high-resolution anatomy models

We present a comparison of simulated low-frequency electromagnetic fields in the human body, calculated by means of the electro-quasistatic formulation. The geometrical data in these simulations were provided by an anatomically realistic, high-resolution human body model, while the dielectric properties of the various body tissues were modelled by the parametric Cole-Cole equation. The model was examined under two different excitation sources and various spatial resolutions in a frequency range from 10 Hz to 1 MHz. An analysis of the differences in the computed fields resulting from a neglect of the permittivity was carried out. On this basis, an estimation of the impact of the displacement current on the simulated low-frequency electromagnetic fields in the human body is obtained.

Numerical simulation of coupled transient thermal and electromagnetic fields with the finite integration method

A consistent iterative method for the simultaneous discretization of Maxwells equations and of the heat conduction equation in weakly coupled systems is presented. The algorithm is an implementation of the finite integration method combined with both explicit and implicit FDTD schemes for the transient analysis. The validity of the method is demonstrated in the simple example of a lossy dielectric cylinder subject to microwave and radiant heating in a microwave cavity. In a second example, the implications of nonlinear coupling on the transient field solutions are discussed.

Self-consistent simulations of transient heating effects in electrical devices using the finite integration technique

A newly developed method for coupled electromagnetic and thermal simulations of electrical devices with heat losses is presented. The method is based on the finite integration technique (FIT). Temperature dependent device-material properties, as well as radiant and convective boundary conditions are accommodated in the model. The performance of the method is demonstrated in the simulation of a high precision eddy current welding application and in the calculation of the transient response of a multifinger AlGaAs/GaAs heterojunction bipolar power transistor (HBT). In the latter, a discussion of the effects of thermal shunting in the self-heating process is given.

Time integration methods for particle beam simulations with the finite integration theory

Abstract – In this contribution two novel time integration methods designed for the solution of Maxwell’s equations in the time domain using the Finite Integration Theory (FIT) are presented. This work was motivated by the need for simulating particle beams in electrically long structures, e.g., Linear Particle Accelerators. The extension of such a structure along the beam propagation axis is much larger than the transversal dimensions. The simulation of this structures is usually performed by the Staggered Leap-Frog (SLF) time integration within the framework of the FIT spatial discretization. Unfortunately, the numerical dispersion error of this algorithm is large along the beam propagation axis. Contrary to this, the proposed methods have a minimal error, vanishing for the Courant time step, along this direction. This property allows for a longer simulation time and for more accurate field solutions in accelerator structures.

Conservation Properties of the Discontinuous Galerkin Method for Maxwell Equations

We investigate the conservation properties of the centered DO formulation for Maxwell equations. In particular, we state the problem and derive the criteria for charge conservation. It is shown that the centered scheme guarantees strict charge conservation for Cartesian discretizations with tensor product basis functions of arbitrary order. On unstructured grids, however, the conservation of charge is inherently violated. The reasons for this are of purely topological nature.

A hybrid Finite Integration-Finite Volume Scheme

A hybridized scheme for the numerical solution of transient electromagnetic field problems is presented. The scheme combines the Finite Integration Technique (FIT) and the Finite Volume Method (FVM) in order to profit from the computational efficiency of the FIT while taking advantage of the superior dispersive properties of the FVM. The scheme is based on the longitudinal-transverse (LT) splitting of the discrete curl operator. The FIT is employed for discretizing the two-dimensional subproblem while the one-dimensional problem is discretized according to the FVM. The scheme offers benefits for the simulation of multiscale setups, where the size of the computational domain along one preferred direction is electrically much larger than along the others. In such situations, the accumulation of dispersion errors within hundreds of thousands of time steps usually deteriorates the solution accuracy. The hybrid scheme is applied in combination with adaptive mesh refinement, yielding an efficient scheme for multiscale applications.

Broadband surface impedance boundary conditions for higher order time domain discontinuous Galerkin method

Purpose – The purpose of this paper is to present a time domain discontinuous Galerkin (DG) approach for modeling wideband frequency dependent surface impedance boundary conditions. Design/methodology/approach – The paper solves the Maxwellian initial value problem in a computational domain, which is spatially discretized by the higher order DG method. On the boundary of the computational domain the paper applies a suitable impedance boundary condition (IBC). The frequency dependency of the impedance function is modeled by auxiliary differential equations (ADE). Findings – The authors will study the resonance frequency and the Q factor of different types of cavity resonators including lossy materials. The lossy materials are modeled by means of IBCs. The authors will compare the results with analytical results, as well as numerical results obtained by direct calculations where lossy materials are included explicitly into the numerical model. Several convergence studies are performed which demonstrate the ...

Transient calculation of the induced currents inside the brain during magnetic stimulation

Purpose – Transient calculation of currents in brain tissue induced during a transcranial magnetic stimulation treatment.Design/methodology/approach – Because of the short pulses used in this technique a time‐harmonic approximation is no longer valid, and transient effects have to be considered. We have performed a Fourier analysis of the induced currents calculated in a high‐resolution model of the brain using the extended scalar potential finite differences (Ex‐SPFD) approach.Findings – The peak induced currents in the transient development of the pulse are higher by a factor of approximately seven than the time harmonic solutions at the fundamental frequency. Furthermore, an analysis of the impact of the conductivity dispersion revealed an increase in the peak induced currents by 17.3 percent for white matter and by 20.8 percent for gray matter.Originality/value – Using the numerically efficient Ex‐SPFD approach, along with a high performance cluster, the current densities inside the brain can be calcu...