Showing papers on "Electric field published in 1992"

A thermosphere/ionosphere general circulation model with coupled electrodynamics

Abstract: A new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere. This model calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low-latitude wind, which produces east-west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.

Theoretical modelling of porous oxide growth on aluminium

Abstract: A theoretical model for porous structure formation during electrochemical anodization of aluminium is developed taking into account anodic oxide growth at the oxide/electrolyte and oxide/aluminium interfaces, electrochemical and field-enhanced oxide dissolution, and a three-dimensional configuration of electric field and current in the scalloped barrier oxide layer at pore bottoms. A system of equations ensuring a general description of the porous structure formation is evolved and is further applied to the analysis of stationary pore growth on aluminium. In this particular case analytical expressions are obtained establishing the relations of pore geometry and electric field in oxide with the parameters of anodic process (voltage, temperature, pH). They are found to be in agreement with existing experimental data.

Linear electro-elastic fracture mechanics of piezoelectric materials

Abstract: The concepts of linear elastic fracture mechanics, generalized to treat piezoelectric effects, are employed to study the influence of the electrical fields on the fracture behavior of piezoelectric materials The method of distributed dislocations and electric dipoles, already existing in the literature, is used to calculate the electro-elastic fields and the energy-release rate for a finite crack embedded in an infinite piezoelectric medium which is subjected to both mechanical and electric loads The energy-release rate expressions show that the electric fields generally tend to slow the crack growth It is shown that the stress intensity factor criterion and the energy-release rate criterion differ when the energetics of the electric field is taken into account The study of crack tip singular stress field yields a possible explanation for experimentally observed crack skewing in the presence of a strong electric field

DNA electrophoresis in microlithographic arrays

Abstract: We have used optical microlithography to fabricate capped quasi-two-dimensional obstacle courses in SiO2. We report here observations using epifluorescence microscopy of the electrophoresis and length fractionation of large DNA molecules confined in arrays. Simple reptation theory, based on the work of deGennes, predicts that at low electric fields the electrophoretic mobility of a polymer of length L much greater than the persistence length p scales inversely with L (ref. 2). But elongation of the coil in the matrix at sufficiently strong electric fields results in a length-independent electrophoretic mobility. The application of suitably timed pulsed electric fields restores the fractionating power of gels for long molecules but the protocols of pulsed-field electrophoresis are semi-empirical because the complex and ill-understood gel matrix plays a critical role in fractionation. Microlithographically constructed obstacle arrays, with their low dimensionality, small volume and extremely reproducible topography, will make it possible to understand the motion and fractionation of large polymer molecules in complex but well characterized topologies.

Atomic level simulations on a million particles: The cell multipole method for Coulomb and London nonbond interactions

Abstract: The N2 computations implicit in the Coulomb and other long range interactions remain the critical bottleneck in atomic‐level simulations of the structure and dynamics of large systems. We report here the cell multipole method which scales linearly with N and requires only modest memory. To demonstrate the feasibility of this approach, we report systematic calculations on realistic polymer systems with up to 1.2 million atoms on a laboratory workstation. The method becomes faster than the exact method for systems of 300 atoms, and for a 1.2 million‐atom polymer, it is 2377 times faster. The method treats a class of interactions of the form qiqj/rijp, which includes Coulomb (p=1), London dispersion (p=6), or shielded Coulomb (p=2) interactions. This method is well suited for highly parallel and vector computers.

Uniform magnetic fields and double-wrapped coil systems: Improved techniques for the design of bioelectromagnetic experiments

Abstract: A common mistake in biomagnetic experimentation is the assumption that Helmholtz coils provide uniform magnetic fields; this is true only for a limited volume at their center. Substantial improvements on this design have been made during the past 140 years with systems of three, four, and five coils. Numerical comparisons of the field uniformity generated by these designs are made here, along with a table of construction details and recommendations for their use in experiments in which large volumes of uniform intensity magnetic exposures are needed. Double-wrapping, or systems of bifilar windings, can also help control for the non-magnetic effects of the electric coils used in many experiments. In this design, each coil is wrapped in parallel with two separate, adjacent strands of copper wire, rather than the single strand used normally. If currents are flowing in antiparallel directions, the magnetic fields generated by each strand will cancel and yield virtually no external magnetic field, whereas parallel currents will yield an external field. Both cases will produce similar non-magnetic effects of ohmic heating, and simple measures can reduce the small vibration and electric field differences. Control experiments can then be designed such that the only major difference between treated and untreated groups is the presence or absence of the magnetic field. Double-wrapped coils also facilitate the use of truly double-blind protocol, as the same apparatus can be used either for experimental or control groups.

Polarization forces and conductivity effects in electrorheological fluids

Abstract: Forces between particles aligned into chains by an applied electric field in an electrorheological (ER) fluid are calculated using finite‐element techniques and, approximately, using a dipole approximation with local‐field effects. Evaluation of the effective dielectric constant is emphasized and the shear modulus is derived from the shear dependence. For high‐frequency (f≳0.1–1 kHz) applied electric fields, the forces and the modulus depend upon the dielectric constants of the suspending fluid and the dispersed particles. For low‐frequency or dc electric fields, the conductivities of the components are dominant. These effects are treated within a Maxwell–Wagner approach. If the ratio of particle‐to‐fluid conductivities substantially exceeds the ratio of dielectric constants, a large enhancement of the modulus is found. Implications for the design of ER fluids are discussed briefly.

A model for the field and temperature dependence of Shockley-Read-Hall lifetimes in silicon

Abstract: We derive a simple analytical model for the field and temperature dependence of Shockley-Read-Hall lifetimes in silicon from a microscopic level, where the capture of carriers at recombination centers is assumed to be a multiphonon process. Strong electric fields, as often present in modern devices, cause trap assisted tunneling, i.e. the multiphonon recombination path is no longer purely vertical in a band diagram, but has a horizontal branch at an effective energy which is given by the maximum of the transition probability. Applying reasonable approximations we calculate this effective recombination path as a function of field strength and temperature. Field enhancement factors of the inverse carrier lifetimes are then presented that require no integration, iteration or higher mathematical functions. The anisotropy and multi-valley nature of the silicon conduction band is carefully taken into account. We discuss all approximations and physical effects by means of the gold acceptor level. The model is able to describe the pre-breakdown behaviour of trap tunneling leakage and is suitable for the implementation into simulation packages.

General relativistic electric potential drops above pulsar polar caps

Abstract: We study the general relativistic electrodynamics of an isolated, rotating, magnetic neutron star. We consider the region of a neutron star magnetosphere with steady, space charge limited flow along open magnetic field lines. The explicit solutions to the Maxwell equations are obtained. Being the simplest, this model enables one to carry out analytically a general relativistic treatment, and to demonstrate the influence of the effects of General Relativity on the creation of an electric field in the afore-mentioned region.

Planar near-field to far-field transformation using an equivalent magnetic current approach

Abstract: An alternative method is presented for computing far-field antenna patterns from near-field measurements. The method utilizes the near-field data to determine equivalent magnetic current sources over a fictitious planar surface that encompasses the antenna, and these currents are used to ascertain the far fields. Under certain approximations, the currents should produce the correct far fields in all regions in front of the antenna regardless of the geometry over which the near-field measurements are made. An electric field integral equation (EFIE) is developed to relate the near fields to the equivalent magnetic currents. The method of moments is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method, and in the case of a rectangular matrix, a least-squares solution for the currents is found without explicitly computing the normal form of the equation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions. Numerical results are presented for several antenna configurations. >

Internal stark effect measurement of the electric field at the amino terminus of an alpha helix.

Abstract: The strengths of electrostatic interactions in biological molecules are difficult to calculate or predict because they occur in complicated, inhomogeneous environments. The electric field at the amino terminus of an alpha helix in water has been determined by measuring the shift in the absorption band for a covalently attached, neutral probe molecule with an electric dipole moment difference between the ground and excited electronic states (an internal Stark effect). The field at the interface between the helix and the solvent is found to be an order of magnitude stronger than expected from the dielectric properties of bulk water. Furthermore, although the total electric dipole moment of the helix increases with length, the electric field at the amino terminus does not.

Ferroelectric liquid crystal element

Abstract: In a ferroelectric liquid crystal element, which is constituted by holding a chiral smectic liquid crystal exhibiting ferroelectricity between electrode substrates, and in which the liquid crystal presents a plurality of surface stabilization states according to an electric field applied through the electrode substrates, and an alternating electric field having a strength equal to or higher than a constant value is applied during a period for inhibiting transition of the surface stabilization state, and to a corresponding liquid crystal portion, within a range causing no transition, the strength of the alternating electric field to be applied during the period for inhibiting transition of the surface stabilization state, and to the corresponding liquid crystal portion is set to be equal to or lower than a predetermined value which does not cause the liquid crystal to flow.

Piezothermoelastic behavior of a laminated plate

Abstract: The response of a thin composite plate constructed of piezothermoelastic layers and subject to stationary thermal and electric fields is examined. Solutions based on classical lamination theory, extended here to include piezoelectric effects, are obtained for a “free” plate of arbitrary contour and for a simply supported, rectangular plate. Numerical results illustrate that thermally induced deformation of a structural laminate can be reduced significantly through the addition of a layer of piezoelectric material under an applied electric potential.

Numerical dosimetry at power-line frequencies using anatomically based models.

Abstract: We have used the finite-difference time-domain (FDTD) method to calculate induced current densities in a 1.31-cm (nominal 1/2 in) resolution anatomically based model of the human body for exposure to purely electric, purely magnetic, and combined electric and magnetic fields at 60 Hz. This model based on anatomic sectional diagrams consists of 45,024 cubic cells of dimension 1.31 cm for which the volume-averaged tissue properties are prescribed. It is recognized that the conductivities of several tissues (skeletal muscle, bone, etc.) are highly anisotropic for power-line frequencies. This has, however, been neglected in the first instance and will be included in future calculations. Because of the quasi-static nature of coupling at the power-line frequencies, a higher quasi-static frequency f' may be used for irradiation of the model, and the internal fields E' thus calculated can be scaled back to the frequency of interest, e.g., 60 Hz. Since in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically 3-4 time periods), this frequency scaling to 5-10 MHz for f' reduces the needed number of iterations by over 5 orders of magnitude. The data calculated for the induced current and its variation as a function ofmore » height are in excellent agreement with the data published in the literature. The average current densities calculated for the various sections of the body for the magnetic field component (H) are considerably smaller (by a factor of 20-50) than those due to the vertically polarized electric field component when the ratio E/H is 377 ohms. We have also used the previously described impedance method to calculate the induced current densities for the anatomically based model of the human body for the various orientations of the time-varying magnetic fields, namely from side to side, front to back, or from top to bottom of the model, respectively. 34 refs.« less

Deformation of ionic polymer gels by electric fields

Abstract: When an electric field is applied to an ionic gel in a buffer solution, the anode side of the gel shrinks if the gel is contacting with the electrode, while it swells if the gel is placed with sufficient separation from the electrode. The changes of ion concentration profiles under an electric field are calculated, taking account of ion transports and electrochemical reactions. Combining these results with Flory's theory for the swelling of ionic gels, the above phenomena are explained. It is predicted that the swelling behavior is governed by the concentration of the dominant ions and that the swelling speed is proportional to the square of the electric current

Physics of the L-mode to H-mode transition in tokamaks

Abstract: Combined theoretical and experimental work has resulted in the creation of a paradigm which has allowed semi-quantitative understanding of the edge confinement improvement that occurs in the H-mode. Shear in the E*B flow of the fluctuations in the plasma edge can lead to decorrelation of the fluctuations, decreased radial correlation lengths and reduced turbulent transport. Changes in the radial electric field, the density fluctuations and the edge transport consistent with shear stabilization of turbulence have been seen in several tokamaks. The purpose of this paper is to discuss the most recent data in the light of the basic paradigm of electric field shear stabilization and to critically compare the experimental results with various theories.

Confinement and profile changes induced by the presence of positive or negative radial electric fields in the edge of the TEXTOR tokamak

Abstract: Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement (τp) of deuterium and of impurity ions as well as energy confinement (τE) are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio τp/τE is about three times lower in the former case

Dielectrophoretic separation of mammalian cells studied by computerized image analysis

Abstract: A method was developed for studying the dielectrophoretic properties of both homogeneous and mixed populations of mammalian cells. Computerized image analysis was employed to quantify the rate of motion of cells suspended in low conductivity medium as they moved under the influence of a non-uniform alternating electric field produced by an interdigitated electrode array. As expected for dielectrophoresis, cells collected at highly inhomogeneous electric field regions of the array when the electrical polarizability of cells exceeded that of the suspending medium or away from such regions when their polarizability was less than that of their medium. These two types of behaviour are classified as positive or negative dielectrophoresis respectively, and the cell collection patterns agreed well with those computed for the electrode configuration employed. The dielectrophoretic characteristics of normal, leukaemic, and differentiation-induced leukaemic mouse erythrocytes were measured as a function of frequency in the range 5*102-105 Hz and were shown to be significantly different.

Comparison of charge transport models in molecularly doped polymers

Abstract: It is well known that charge carrier mobilities μ in molecularly doped polymers, determined by transient photoconductivity experiments, depend on the temperature T, the mean distance p between dopant molecules and the electric field E. Recently introduced deconvolution procedures have made possible the systematic analysis of the dependence of μ on T, p and E. Two theories which have been proposed to account for the data are the polaron theory and the disorder theory of Bassler and co-workers. The current status of these and other theories in explaining the data is reviewed.

The electrohydrodynamic deformation of drops suspended in liquids in steady and oscillatory electric fields

Abstract: When an electric field is applied to a drop suspended in another liquid the drop deforms. The relation between the applied field and the mode and magnitude of the deformation have been studied extensively. Nevertheless, Torza, Cox & Mason (1971) found that quantitative agreement between the leaky dielectric theory (Taylor 1966) and experiment is quite poor. Here we describe results from a new series of experiments. Drops suspended in weakly conducting liquids were deformed into spheroids with both steady and oscillatory fields. Drop deformation, interfacial tension, and the electrical properties of the fluids were measured for each system to provide a definitive test of the theory. The agreement between the leaky dielectric model and our results for drop deformations in steady fields is much improved over previous results, although discrepancies remain for some systems. Drop deformations in oscillatory fields consist of steady and oscillatory parts because of the quadratic dependence on the field strength. Measurements of the steady part at 60 Hz, where the oscillatory deformation is negligible, are in excellent agreement with the theory. The effects of frequency on the steady deformation were studied by measuring oblate deformations at a series of frequencies and field strengths; the agreement with theory is good. Finally, the time-dependent total deformation was measured under conditions where both parts of the deformation are commensurate. Good agreement was found between the measured and predicted maximum and minimum deformations. Nevertheless, only a small range of fluid properties could be studied owing to the need to avoid droplet sedimentation.

Principles of spatial surface electric field filtering in magnetotellurics; electromagnetic array profiling (EMAP)

Abstract: Electromagnetic Array Profiling (EMAP) is an adaptation of magnetotellurics to overcome spatial aliasing effects associated with the sampling of the surface electric field. Undersampling lateral electric field variations can result in misleading geoelectric interpretations of the subsurface, particularly under the common presence of static distortion. In the EMAP field procedure, electric dipoles are positioned end‐to‐end along a continuous survey path; this configuration, in addition to reducing aliasing effects, lends itself to low‐pass filtering of the lateral electric field variations. We show that lengthening an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length. This modification of the sensor characteristics involves a spatial filtering process in which the cutoff wavenumber is inversely proportional to the length of the dipole. However, excessively long dipoles may not prove appropriate at high frequencies where the objective is to sense...

Bending of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogel in electric fields

Abstract: Bending of poly(vinyl alcohol) hydrogel mixed with poly(sodium acrylate) chains, PVA–PAA gel, under the influence of dc electric fields was studied. The PVA–PAA gel was prepared by repeatedly freezing and thawing a mixture of PVA and polyacrylic acid aqueous solutions. The PVA–PAA gel was a hydrogel with the PAA chains, which were entangled with the PVA polymer network and were fixed in the gel. The PVA–PAA gel bent toward the negative electrode in electrolyte solutions under dc electric fields as did the polyelectrolyte gel with negatively charged polyions. The PVA gel, free of PAA, was insensitive to dc electric fields. The deflection of the bending and the bending speed were influenced by the filed intensity, the concentration of the polyion in the gel, and the thickness of the gel. The bending of the PVA–PAA gel was qualitatively explained by a bending theory of polyelectrolyte gel, based upon the change of the osmotic pressure due to the ion concentration difference between the inside and the outside of the gel.

Terahertz emission in single quantum wells after coherent optical excitation of light hole and heavy hole excitons.

Abstract: We report on the first observation of coherent terahertz radiation, tunable from 1.4 to 2.6 THz, emerging from GaAs/Al 0.3 Ga 0.7 As single quantum wells after the coherent optical excitation of both light hole and heavy hole excitons. We attribute the radiation to charge oscillations following the coherent excitation of the excitons in an electric field. Terahertz radiation is also emitted when no electric field is present in the sample suggesting that valence band mixing leads to a significant far-infrared transition dipole moment between the light hole and heavy hole subbands