Showing papers by "Dragan Poljak published in 2016"

The electromagnetic-thermal dosimetry for the homogeneous human brain model

Abstract: The electromagnetic-thermal dosimetry model of the human brain exposed to electromagnetic (EM) radiation is developed. The EM model based on the surface integral equation (SIE) formulation is derived using the equivalence theorem for the case of a lossy homogeneous dielectric body. The thermal dosimetry model of the brain is based on the form of Pennes׳ equation for heat transfer in biological tissue. The numerical solution of the EM model is carried out using the Method of Moments (MoM) while the bioheat equation is solved using the finite element method (FEM). Developed EM-thermal model has been applied for the internal dosimetry of the human brain to assess the absorbed EM energy and the consequent temperature rise due to the exposure of 900 MHz plane wave. Due to the variability of various parameters, the sensitivity of the maximum, minimum and the average steady-state temperature, on the various thermal parameters have been examined, as well as the influence of the parameters variation on the temperature distribution in case of EM exposure. The proposed model may be found useful in the rapid assessment of the temperature distribution in the human brain, prior to having to deal with a tedious development of a more complex models.

Transcranial magnetic stimulation induced fields in different brain models

Abstract: This paper gives a comparison of the numerical results for the induced electric field, electric current density, and the magnetic flux density inside the adult, 10-years-old, and 5-years-old homogeneous brain models, for three different transcranial magnetic stimulation (TMS) coils. The numerical results are obtained using the TMS model based on the surface integral equation formulation and efficient numerical solution via Method of Moments. The two child brain models are obtained by linearly scaling the adult brain model. The age-dependent parameters of the brain are taken into account as well. The results show that the decrease in the homogeneous brain size results in the increased values of all TMS-induced fields. Implementing the age-related parameters significantly increases the induced current density values while moving the point with half maximum electric field value closer to the surface. The analysis undertaken in this work has underlined the importance of the brain size and the brain tissue con...

A comparison of finite-difference, finite-integration, and integral-equation methods in the time-domain for modelling ground penetrating radar antennas

Abstract: Development of accurate models of GPR antennas is being driven by research into more accurate simulation of amplitude and phase information, improved antenna designs, and better-performing forward simulations for inversion procedures. Models of a simple dipole antenna, as well as more complex models similar to a GSSI 1.5GHz antenna and a MALA Geo-science 1.2GHz antenna were investigated in free space and over lossless and lossy dielectric half-spaces. We present comparisons of simulated data using the Finite-Integration Technique, the Finite-Difference Time-Domain method, and a Time-Domain Integral Equation approach, as well as measured data. For each scenario, phase, amplitude, and the shape of the waveform were compared. Generally we found very good agreement between the different simulation techniques, and good agreement between experimental and simulated data. Differences that were evident highlight the significance of understanding how features such as antenna feeding and material dispersion are modelled. This degree of match between experimental and simulated data cannot be attained by using just an infinitesimal dipole model in a simulation — a model including the structure of the antenna is required. This is important for the many GPR applications which operate in the near-field of the antenna, where the interaction between the antenna, the ground, and targets is important.

A Stochastic Analysis of the Transient Current Induced along the Thin Wire Scatterer Buried in a Lossy Medium

Abstract: The paper deals with the stochastic collocation analysis of a time domain response of a straight thin wire scatterer buried in a lossy half-space. The wire is excited either by a plane wave transmitted through the air-ground interface or by an equivalent current source representing direct lightning strike pulse. Transient current induced at the center of the wire, governed by corresponding Pocklington integrodifferential equation, is determined analytically. This antenna configuration suffers from uncertainties in various parameters, such as ground properties, wire dimensions, and position. The statistical processing of the results yields additional information, thus enabling more accurate and efficient analysis of buried wire configurations.

On deterministic-stochastic time domain study of dipole antenna for GPR applications

Abstract: A deterministic-stochastic transient study of Ground Penetrating Radar (GPR) dipole antenna radiating in a presence of a two-media configuration is carried out in the paper. A deterministic direct time domain formulation is based on the corresponding space-time Hallen integral equation. The numerical solution is carried out via the improved space-time variant of the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM). The Stochastic-Collocation (SC) method is then applied to determine accurate confidence intervals due to the random variations of GPR input parameters. Once obtaining the current along the dipole antenna, it is possible to calculate other parameters of interest for GPR dipole antenna behavior, such as the field reflected from the interface of two media, or the field transmitted into a lower half-space. Some illustrative numerical results for the transient current along the dipole antenna and transient electric field transmitted into the lower half-space are given.

On some applications of stochastic collocation method in computational electromagnetics: Applications in ground penetrating radar, bioelectromagnetics, grounding systems and buried lines

Abstract: The paper reviews the use of deterministic-stochastic models some areas of computational electromagnetics where there is an uncertainty in the input data set, i.e. where some properties of a system are partly or entirely unknown. In such problems a simple stochastic collocation (SC) method is used to properly assess relevant statistics about given responses. The SC approach also provides the assessment of corresponding confidence intervals in the set of obtained numerical results. The expansion of statistical output in terms of mean and variance over a polynomial basis, via SC method, is shown to be robust and efficient approach providing a satisfactory convergence rate. Some illustrative computational examples featuring the specific applications in ground penetrating radar (GPR), human exposure to electromagnetic fields, buried lines and grounding systems are also given in the paper.

EUROfusion Integrated Modelling (EU-IM) Capabilities and Selected Physics Applications

Abstract: Recent developments and achievements of the EUROfusion Code Development for Integrated Modelling project (WPCD, follow-up of EFDA-ITM-TF), which aims at providing a validated integrated modelling suite for the simulation and prediction of complete plasma discharges in any tokamak, are presented. WPCD develops generic complex integrated simulations, workflows, for physics applications, using the standardized EU Integrated Modelling (EU-IM) framework. The integration of codes in EU-IM workflows is besides accompanied by a thorough cross-verification and, recently, by the introduction of rigorous release procedures. Among the achievements, the European TransportSimulator(ETS),hasnowreachedacapabilityequivalenttothestate-of-the-artintegrated modeling transport codes, including interchangeable physics modules for equilibrium (both fixed and free boundary), transport (interpretative analytical, neoclassical, anomalous), impurities (all ionization states), NTM, sawteeth, pellets, neutrals, Heating and Current Drive (HCD) sources including all the heating schemes (EC, NBI, IC, nuclear) and synergy effects. The core ETS has been released and deployed at JET, offering a leading tool for both interpretive transport analysis and predictive modelling of complex scenarios. Selected physics applications are presented, in particular ETS simulations of plasma density control in reactor-scale plasmas fueled with multiple pellets. A MHD stability chain was developed for the analysis of equilibria from any tokamak in the EU-IM platform; it includes a pool of interoperable high-resolution equilibrium and linear MHD stability codes. Having passed a benchmark on core and global ideal kink instabilities, the chain has been released and applied to the predictive analysis of DEMO and JT60-SA scenarios and can be straightforwardly used for interpretive runs on present devices as JET and ASDEX Upgrade. A predictive J-alpha MHD pedestal stability analysis workflow has also been developed. Routine application to sensitivity analysis of DEMO1 scenarios is performed. Furthermore, a workflow including a turbulence code and a synthetic probe was developed and applied to investigate the turbulent transport in the edge and Scrape-Off-Layer (SOL) of ASDEX Upgrade. Finally, a prototype edge workflow integrating the interaction with PFC was demonstrated.

Stochastic Sensitivity in Homogeneous Electromagnetic-Thermal Dosimetry Model of Human Brain

Abstract: In this work we examined how the variability in the brain morphology and the tissue properties affect the assessment of the homogeneous human brain exposed to high frequency electromagnetic (EM) field. Using the deterministic EM-thermal modeling and the stochastic theoretical basis we have studied the effects of these uncertainties on the maximum induced electric field, maximum local Specific Absorption Rate (SAR), average SAR, maximum temperature and the maximum temperature increase, respectively. The results show a good convergence of stochastic technique and an assessment of mean and variance of outputs for the incident plane wave of 900 MHz.

An Efficient Deterministic-Stochastic Model of the Human Body Exposed to ELF Electric Field

Abstract: The paper deals with the deterministic-stochastic model of the human body represented as cylindrical antenna illuminated by a low frequency electric field. Both analytical and numerical (Galerkin-Bubnov scheme of Boundary Element Method) deterministic solutions of the problem are outlined. This contribution introduces the new perspective of the problem: the variability inherent to input parameters, such as the height of the body, the shape of the body, and the conductivity of body tissue, is propagated to the output of interest (induced axial current). The stochastic approach is based on the stochastic collocation (SC) method. Computational examples show the mean trend of both analytically and numerically computed axial current with the confidence margins for different set of input random variables. The results point out the possibility of improving the efficiency in calculation of basic restriction parameter values in electromagnetic dosimetry.

Transient Electromagnetic Field Coupling to Buried Thin Wire Configurations: Antenna Model versus Transmission Line Approach in the Time Domain

Abstract: The paper examines the antenna model for the transient analysis of electromagnetic field coupling to straight wire configurations buried in a lossy half-space. The wire antenna theory (AT) model is implemented directly in the time domain and it is based on the corresponding space-time Pocklington integrodifferential equation. The solution of the Pocklington equation is carried out analytically. The obtained results are compared against the results calculated via the transmission line (TL) approach. The TL approach is based on the telegrapher’s equations, which are solved using the modified transmission line method (MTLM) and Finite Difference Time Domain (FDTD) technique, respectively. Some illustrative computational examples for buried straight wire scatterer and horizontal grounding electrode are given throughout this work.

Time domain and frequency domain integral equation method for the analysis of ground penetrating radar (GPR) antenna

Abstract: The paper deals with the comparison between the frequency domain and time domain analysis of transient electric field generated by the ground penetrating radar (GPR) dipole antenna and transmitted into the dielectric half-space for GPR antenna. The time domain integral equation (TDIE) approach is based on the Hallen integral equation for half-space problems. The numerical solution is carried out via the space-time scheme of the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM). The frequency domain formulation is based on the Pocklington's integro-differential equation which is solved by using the frequency domain scheme of the Galerkin Bubnov Indirect Boundary Element Method. The related transient response is obtained by means of the Inverse Fast Fourier Transform (IFFT). The Integral Equation combined with the transmission coefficient arising from modified image theory (MIT) represents the novel method for obtaining the transmitted electric field of a GPR antenna. This paper brings the comparison of the results obtained by different approaches in solving the Integral Equation. The calculated results agree satisfactorily.

Impulse impedance of the horizontal grounding electrode: Experimental analysis versus full-wave computational model

Abstract: The paper deals with the comparison of novel experimental and computational method for obtaining the impulse impedance of the horizontal grounding electrode defined as a ratio of maximum values of induced voltage and current at the injection point on the electrode. Impedance is obtained in-situ based on injection of the 1.5 A current pulses (4/10 μs and 8/20 μs) and measurement of corresponding voltage drop. On the other hand, the current distribution along the electrode is calculated using the antenna theory based model featuring the thin-wire approximation and corresponding Pocklington integro-differential equation in the frequency domain. The equation is solved using analytical technique. The results obtained via different methods agree satisfactorily, thus validating antenna model of the grounding electrode. Obtained results are applicable for any grounding system based on horizontal grounding electrode.

Myelinated nerve fiber antenna model activation

Abstract: A myelinated nerve fiber is modeled as a straight thin wire antenna and stimulated by a current generator at the fiber beginning. The model is based on the corresponding homogenous Pocklington integro-differential equation for the perfectly conducting wire in a lossy unbounded homogenous space, where the activated node of Ranvier is modeled as a thin wire junction with an additional current source. The additional current source represents the ionic current of the activated Ranvier's node. The intracellular current results are obtained for the nerve fibers of two different diameters, stimulated by different rectangular pulses. The thresholds are determined by related strength-duration curves.

Comparison of generalized telegrapher equations approach and circuit model for wireless power transfer

Abstract: In this paper, we apply generalized telegrapher's equations in analysis of to two coupled wires. By the constant current approximation, a lumped circuit model of wireless power transfer between the two is derived from the full-wave model. In order to test its applicability, estimations of mutual inductivity and system input impedance provided by the circuit model are compared to the full-wave model ones.

On the Use of Analytical Methods in Electromagnetic Compatibility and Magnetohydrodynamics

Abstract: The paper deals with the use of analytical methods for solving various integro-differential equations in electromagnetic compatibility, with the emphasis on the frequency and time domain solutions of the thin wire configurations buried in a lossy ground. Solutions in the frequency domain are carried out via certain mathematical manipulations with the current function appearing in corresponding integral equations. On the other hand, analytical solutions in the time domain are undertaken using the Laplace transform and Cauchy residue theorem . Obtained analytical results are compared to those calculated using the numerical solution of the frequency domain Pocklington equation, where applicable. Also, an overview of analytical solutions to the Grad–Shafranov equation for tokamak plasma is given.

Some computational aspects of calculation of integrals arising within the framework of Method of Moments - application to bioelectromagnetics

Abstract: Formulation involving the electric field integral equation (EFIE) and the related numerical solution via Method of Moments (MoM) variants requires tedious calculation of various double surface integrals arising from the use of vector triangular basis functions. The approach to evaluate such an integral strongly depends on the distance between source and observation points, respectively. In the case of a rather close interaction between the elements, due to a strong singularity, a purely numerical approach could fail resulting in the incorrect, spurious results. Hence, a combined analytical-numerical approach to extract the mentioned singularities is necessary. The present paper deals with an implementation of such an approach to the calculation of certain integrals types arising in the EFIE formulation for electromagnetic scattering from dielectric objects.

Frequency Domain and Time Domain Response of the Horizontal Grounding Electrode Using the Antenna Theory Approach

Abstract: The analysis of horizontal grounding electrode has been carried out using the antenna theory (AT) approach in the frequency and time domain , respectively. The formulation is based on the corresponding space-frequency and space-time Pocklington integro-differential equations. The integro-differential relationships are numerically handled via the Galerkin–Bubnov scheme of the Indirect Boundary Element Method (GB-IBEM). Some illustrative computational examples related to frequency domain (FD) and time domain (TD) analysis are given in the paper.

Integral equation models in some biomedical applications of electromagnetic fields: Transcranial magnetic stimulation (TMS), nerve fiber stimulation

Abstract: The paper reviews certain integral equation formulations and related numerical solution methods used in studies of biomedical applications of electromagnetic fields related to transcranial magnetic stimulation (TMS) and nerve fiber stimulation. TMS is modeled via the set of coupled surface integral equations (SIEs), while the numerical solution of governing equations is undertaken via an efficient Method of Moments (MoM) scheme. A myelinated nerve fiber, stimulated by a current generator, is represented by a straight thin wire antenna. The model is based on the corresponding homogeneous Pocklington integro-diferential equation numerically solved via the Galerkin Bubnov Indirect Boundary Element Method (GB-IBEM). Some illustrative numerical results for the TMS induced fields and intracellular current distribution along the myelinated nerve fiber (active and passive), respectively, is obtained.

The Electromagnetic–Thermal Dosimetry Model of the Human Brain

Abstract: The electromagnetic–thermal dosimetry model for the human brain exposed to EM radiation is developed. The electromagnetic (EM) model based on the surface integral equation (SIE) formulation is derived using the equivalence theorem for the case of a lossy homogeneous dielectric body. The thermal dosimetry model of the brain is based on the form of Pennes’ equation of heat transfer in biological tissue. The numerical solution of the EM model is carried using the Method of Moments (MoM) while the bioheat equation is solved using the finite element method. Developed electromagnetic thermal model has been applied in internal dosimetry of the human brain to assess the absorbed electromagnetic energy and consequent temperature rise due to exposure of 900 MHz plane wave.