Devices for gradient static magnetic field exposure

We describe devices designed for magnetic field exposures in which field amplitude and gradients are controlled simultaneously. Dosimetry based on field continuation of high resolution magnetic field scans and numerical models is compared with validation measurements. The dosimetry variables we consider are based on the assumption that the biological or chemical system under study has field transducers that are spatially isotropic, so that absolute field amplitude and two gradient components fully describe local exposure.     

Spatial distribution of the RF power absorbed in a helicon plasma source

The spatial distributions of the RF power absorbed by plasma electrons in an ion source operating in the helicon mode (ω _ci < ω < ω _ce < ω _pe ) are studied numerically by using a simplified model of an RF plasma source in an external uniform magnetic field. The parameters of the source used in numerical simulations are determined by the necessity of the simultaneous excitation of two types of waves, helicons and Trivelpiece-Gould modes, for which the corresponding transparency diagrams are used. The numerical simulations are carried out for two values of the working gas (helium) pressure and two values of the discharge chamber length under the assumption that symmetric modes are excited. The parameters of the source correspond to those of the injector of the nuclear scanning microprobe operating at the Institute of Applied Physics, National Academy of Sciences of Ukraine. It is assumed that the mechanism of RF power absorption is based on the acceleration of plasma electrons in the field of a Trivelpiece-Gould mode, which is interrupted by pair collisions of plasma electrons with neutral atoms and ions of the working gas. The simulation results show that the total absorbed RF power at a fixed plasma density depends in a resonant manner on the magnetic field. The resonance is found to become smoother with increasing working gas pressure. The distributions of the absorbed RF power in the discharge chamber are presented. The achievable density of the extracted current is estimated using the Bohm criterion.     

On the role of anatomy in learning by the cerebellar cortex

The properties due to the location of neurons, synapses, and possibly even synaptic channels, in neuron networks are still unknown. Our preliminary results suggest that not only the interconnections but also the relative positions of the different elements in the network are of importance in the learning process in the cerebellar cortex. We have used neural field equations to investigate the mechanisms of learning in the hierarchical neural network. The numerical resolution of these equations reveals two important properties: (i) The hierarchical structure of this network has the expected effect on learning because the flow of information at the neuronal level is controlled by the heterosynaptic effect through the synaptic density-connectivity function, i.e. the action potential field variable is controlled by the synaptic efficacy field variable at different points of the neuron. (ii) The geometry of the system involves different velocities of propagation along different fibers, i.e. different delays between cells, and thus has a stabilizing effect on the dynamics, allowing the Purkinje output to reach a given value. The field model proposed should be useful in the study of the spatial properties of hierarchical biological systems.     

Dynamics of a relativistic electron beam in the vicinity of a spherical injecting body in space plasma

The dynamics of a relativistic electron beam in the vicinity of an injector in the form of a spherical conducting body in a space plasma is considered. An equation describing the radial evolution of a steady electron beam with a self-similar density profile in the electric field of the injector is formulated. A method for calculating the radial evolution of a relativistic electron beam in the vicinity of an injector is developed. The method is based on the numerical integration of a set of ordinary differential equations for the beam radius and field potential in the space charge region under the relevant boundary conditions at the injector surface. Results are presented from numerical simulations of the radial dynamics of an electron beam in the vicinity of a spherical screen system for neutralizing the electric charge carried away by the beam. The numerical results show that the electric field of the injector hastens the beam expansion.     

Effect of the electric field of the anode sheath on the growth of aligned carbon nanotubes in a glow discharge

Arrays of aligned carbon nanotubes on silicon substrates were grown in the anode sheath of a dc glow discharge. In order to clarify the role of the electric field in the growth of nanotubes, numerical simulations of charged particle transport in the anode sheath were carried out in the drift-diffusion approximation. The distributions of the charged particle density and electric field are obtained. Possible mechanisms whereby the electric field influences the growth of aligned carbon nanotubes are analyzed. It is found that the nanotubes grow in the region in which the electric field is enhanced due to the depletion of positive ions in the anode sheath.     

Transient bioheat simulation of the laser-tissue interaction in human skin using hybrid finite element formulation

This paper presents a hybrid finite element model for describing quantitatively the thermal responses of skin tissue under laser irradiation. The model is based on the boundary integral-based finite element method and the Pennes bioheat transfer equation. In this study, temporal discretization of the bioheat system is first performed and leads to the well-known modified Helmholtz equation. A radial basis function approach and the boundary integral based finite element method are employed to obtain particular and homogeneous solutions of the laser-tissue interaction problem. In the boundary integral based finite element formulation, two independent fields are assumed: intra-element field and frame field. The intra-element field is approximated through a linear combination of fundamental solutions at a number of source points outside the element domain. The frame temperature field is expressed in terms of nodal temperature and the corresponding shape function. Numerical examples are considered to verify and assess the proposed numerical model. Sensitivity analysis is performed to explore the thermal effects of various control parameters on tissue temperature and to identify the degree of burn injury due to laser heating.     

Corona discharge at the tip of a tall object in the electric field of a thundercloud

Characteristics of a positive transient corona discharge near the tip of a tall solitary grounded object in the electric field of a thundercloud are studied analytically and numerically. The time evolution of the discharge current and the space distribution of the total electric field are simulated for different growth rates of the external field and the dimensions and geometry of the stressed electrode. The effect of aerosol ions is shown to be negligible at a short duration of the corona. The developed simplified analytical approach agrees with numerical simulations.     

Influence of pool geometry on the biological efficiency of vertical slot fishways

The characteristics of the flow in vertical slot fishways depend mainly on the specific pool design, i.e., the geometry of the pool. This paper presents the results of a study on sixteen different designs of vertical slot fishways with two different slopes. The flow field is studied using both a physical-scale model and a numerical model which solves the 2D depth-averaged shallow water equations coupled with a k–? turbulence model. Several hydraulic parameters, such as the average velocity in the slot or the water depth, are considered in order to evaluate their biological efficiency. Based on the results, it appears that the length of the pool is the main geometric dimension that affects the flow in the fishway. The numerical and experimental agreement is very satisfactory.     

Oscillations, shocks, and fine wave structures arising during the coalescence of two force-free current loops

Two-dimensional numerical simulations of the magnetic reconnection of two parallel force-free current loops are carried out using a high-resolution MHD code in which an artificial wind scheme is employed. Two typical cases (namely, co-helicity and counter-helicity reconnection) are investigated. The simulation results show that co-helicity reconnection involves only the reconnection of the poloidal component of the magnetic field, while counter-helicity reconnection involves the reconnection of both the poloidal and axial components of the magnetic field. Therefore, counter-helicity reconnection is much more complicated and violent as compared to co-helicity reconnection. In both cases, jetlike flows are generated. Counter-helicity reconnection is accompanied by oscillations of both the axial magnetic field and the axial component of the velocity. Due to these oscillations, quasi-steady models of a current sheet appear to be inapplicable, because the current sheet structure also changes. The complicated and unsteady structure of the current distribution shows that magnetic reconnection occurs not only in the central sheet between two loops in the earlier stage of the process, but also inside each loop in later stages. Rather complicated flows and waves with fine structures are also generated during reconnection. Some of the waves appear to be shock waves.     

根表面养分吸收通量和根围溶质浓度的近似解析解

该文用Nye-Tinker-Barber模型来研究植物根系表面的养分吸收通量和根围溶质浓度的近似解析解。将根围区域分为远场区域和近场区域, 在远场用相似变量, 在近场用尺度变换, 将远场解在根表面展开并与近场解进行待定函数的匹配, 从而获得对流扩散方程根表面通量和浓度的一阶近似解析解, 该解能够简化到扩散方程的解的形式。对氮、钾、硫、磷、镁、钙的养分吸收通量和氮、钾的浓度分别进行数值模拟, 比较模型的数值解、Roose的近似解析解和该文的近似解析解。结果表明: 在扩散方程中, 6种元素通量的解析解与Roose解析解相近, 但均高于数值解, 钾和磷的通量在短时间内迅速衰减; 钾和氮浓度的全局近似解析解与Roose解析解接近, 并与数值解的变化趋势一致。在对流扩散方程中, 除氮外的5种元素通量的近似解较Roose的解析解更接近于数值解, 且没有奇性。     

Effect of Blood Vessel Segmentation on the Outcome of Electroporation-Based Treatments of Liver Tumors

Electroporation-based treatments rely on increasing the permeability of the cell membrane by high voltage electric pulses applied to tissue via electrodes. To ensure that the whole tumor is covered with sufficiently high electric field, accurate numerical models are built based on individual patient anatomy. Extraction of patient''s anatomy through segmentation of medical images inevitably produces some errors. In order to ensure the robustness of treatment planning, it is necessary to evaluate the potential effect of such errors on the electric field distribution. In this work we focus on determining the effect of errors in automatic segmentation of hepatic vessels on the electric field distribution in electroporation-based treatments in the liver. First, a numerical analysis was performed on a simple ''sphere and cylinder'' model for tumors and vessels of different sizes and relative positions. Second, an analysis of two models extracted from medical images of real patients in which we introduced variations of an error of the automatic vessel segmentation method was performed. The results obtained from a simple model indicate that ignoring the vessels when calculating the electric field distribution can cause insufficient coverage of the tumor with electric fields. Results of this study indicate that this effect happens for small (10 mm) and medium-sized (30 mm) tumors, especially in the absence of a central electrode inserted in the tumor. The results obtained from the real-case models also show higher negative impact of automatic vessel segmentation errors on the electric field distribution when the central electrode is absent. However, the average error of the automatic vessel segmentation did not have an impact on the electric field distribution if the central electrode was present. This suggests the algorithm is robust enough to be used in creating a model for treatment parameter optimization, but with a central electrode.     

Two-dimensional electrodynamic structure of the normal glow discharge in an axial magnetic field

Results are presented from numerical simulations of an axisymmetric normal glow discharge in molecular hydrogen and molecular nitrogen in an axial magnetic field. The charged particle densities and averaged azimuthal rotation velocities of electrons and ions are studied as functions of the gas pressure in the range of 1–5 Torr, electric field strength in the range of 100–600 V/cm, and magnetic field in the range of 0.01–0.3 T. It is found that the axial magnetic field does not disturb the normal current density law.     

Particle deposition onto a human head: Influence of electrostatic and wind fields

This study investigates electrostatic fields surrounding the human head and particle deposition onto facial skin and eyes caused by the combined effect of electrostatic and wind fields. The electrostatic fields are calculated by a three-dimensional numerical model calculating the field strength between a field source and a human head. The deposition velocity can be viewed as determined by the sum of two contributions: that of an electrostatic field and that of a wind field. Deposition velocities are calculated by a semiempirical particle deposition model that considers particle transport from the free stream to the human face. The particle deposition model uses the electrostatic field model results as input parameters and is applied to the forehead and eyes of two facial shapes for two different turbulence conditions and aerosol charge distributions. The results of different practical working conditions, under which the potential difference between head (person) and source ranges from 5.6 to 15.0 kV, indicates that the presence of electrostatic fields always increases particle deposition for industrial aerosols. For aged aerosols an effect is only present for submicron particles. Bioelectromagnetics 19:246–258, 1998. © 1998 Wiley-Liss, Inc.     

Determination of the induced ELF electric field distribution in a two layer in vitro system simulating biological cells in nutrient solution

In-vitro studies of biological effects of electromagnetic fields are often conducted with cultured cells either in suspension or grown in a monolayer. In the former case, the exposed medium can be assumed to be homogeneous; however, eventually the cells settle to the bottom of the container forming a two layer system with different dielectric and conductive properties. In the present work the effect of this separation on the electric field distribution is calculated and experimentally measured at selected positions for a commonly used exposure configuration. The settled cell suspension is modeled by a well-defined two layer system placed in a rectangular container with the base of the container parallel to the direction of the magnetic field. Theoretical calculations based on numerical techniques are done for various two layer systems with different conductivities in each layer. The agreement between the theoretical calculations and the experimental measurements is within ± 1.5 mV/m, or 10% of the maximum induced field when the conductivity of the lower layer is ten times that of the upper layer. This result is well within experimental error. When the thickness of one of the layers is small compared to the thickness of the other layer, it is found that the electric field distribution is essentially that of the homogeneous case. The latter situation corresponds to a typical cell exposure condition. © 1993 Wiley-Liss, Inc.     

Magnetic induction at 60 Hz in the human heart: a comparison between the in situ and isolated scenarios

Numerical modelling is used to estimate the electric fields and currents induced in the human heart and associated major blood vessels by 60 Hz external magnetic fields. The modelling is accomplished using a scalar-potential finite-difference code applied to a 3.6-mm resolution voxel-based model of the whole human body. The main goal of the present work is a comparison between the induced field levels in the heart located in situ and in isolation. This information is of value in assessing any health risks due to such fields, given that some existing protection standards consider the heart as an isolated conducting body. It is shown that the field levels differ significantly between these two scenarios. Consequently, data from more realistic and detailed numerical studies are required for the development of reliable standards.     

Effect of Magnetic Field on the Spectral Characteristics of Thermal Motion of Charged Particles in an Isotropic Trap

Plasma Physics Reports - Results are presented from analytical and numerical studies of the effect of a dc magnetic field on the spectral characteristics of thermal motion of charged particles in...     

Approximate analytical solutions of root surface nutrient uptake flux and rhizosphere solute concentrations

该文用Nye-Tinker-Barber模型来研究植物根系表面的养分吸收通量和根围溶质浓度的近似解析解。将根围区域分为远场区域和近场区域, 在远场用相似变量, 在近场用尺度变换, 将远场解在根表面展开并与近场解进行待定函数的匹配, 从而获得对流扩散方程根表面通量和浓度的一阶近似解析解, 该解能够简化到扩散方程的解的形式。对氮、钾、硫、磷、镁、钙的养分吸收通量和氮、钾的浓度分别进行数值模拟, 比较模型的数值解、Roose的近似解析解和该文的近似解析解。结果表明: 在扩散方程中, 6种元素通量的解析解与Roose解析解相近, 但均高于数值解, 钾和磷的通量在短时间内迅速衰减; 钾和氮浓度的全局近似解析解与Roose解析解接近, 并与数值解的变化趋势一致。在对流扩散方程中, 除氮外的5种元素通量的近似解较Roose的解析解更接近于数值解, 且没有奇性。     

Dosimetric analysis of the carousel setup for the exposure of rats at 1.62 GHz

The so-called carousel setup has been widely utilized for testing the hypotheses of adverse health effects on the central nervous system (CNS) due to mobile phone exposures in the frequency bands 800-900 MHz. The objectives of this article were to analyze the suitability of the setup for the upper mobile frequency range, i.e., 1.4-2 GHz, and to conduct a detailed experimental and numerical dosimetry for the setup at the IRIDIUM frequency band of 1.62 GHz. The setup consists of a plastic base on which ten rats, restrained in radially positioned tubes, are exposed to the electromagnetic field emanating from a sleeved dipole antenna at the center. Latest generation miniaturized dosimetric E field and temperature probes were used to measure the specific absorption rate (SAR) inside the brain of three rat cadavers of the Lewis strain and two rat cadavers of the Fisher 344 strain. A numerical analysis was conducted on the basis of three numerical rat phantoms with voxel sizes between 1.5 and 0.125 mm3 that are based on high resolution MRI scans of a 300 g male Wistar rat and a 370 g male Sprague-Dawley rat. The average of the assessed SAR values in the brain was 2.8 mW/g per W antenna input power for adult rats with masses between 220 and 350 g and 5.3 mW/g per W antenna input power for a juvenile rat with a mass of 95 g. The strong increase of the SAR in the brain with decreasing animal size was verified by simulations of the absorption in numerical phantoms scaled to sizes between 100 and 500 g with three different scaling methods. The study also demonstrated that current rat phantom models do not provide sufficient spatial resolution to perform absolute SAR assessment for the brain tissue. The variation of the SAR(brain)(av) due to changes in position was assessed to be in the range from +15% to -30%. A study on the dependence of the performance of the carousel setup on the frequency revealed that efficiency, defined as SAR(brain)(av) per W antenna input power, and the ratio between SAR(brain)(av) and SAR(body)(av) are optimal in the mobile communications frequency range, i.e., 0.8-3 GHz.