Showing papers on "Electric field published in 1996"

Local Control of Microdomain Orientation in Diblock Copolymer Thin Films with Electric Fields

Abstract: Local control of the domain orientation in diblock copolymer thin films can be obtained by the application of electric fields on micrometer-length scales. Thin films of an asymmetric polystyrene-polymethylmethacrylate diblock copolymer, with cylindrical polymethylmethacrylate microdomains, were spin-coated onto substrates previously patterned with planar electrodes. The substrates, 100-nanometer-thick silicon nitride membranes, allow direct observation of the electrodes and the copolymer domain structure by transmission electron microscopy. The cylinders aligned parallel to the electric field lines for fields exceeding 30 kilovolts per centimeter, after annealing at 250°C in an inert atmosphere for 24 hours. This technique could find application in nanostructure fabrication.

Optical ablation by high-power short-pulse lasers

Abstract: Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Electrorheology : mechanisms and models

Abstract: Electrorheological (ER) suspensions, typically composed of nonconducting or weakly conducting particles dispersed in an insulating liquid, undergo dramatic, reversible changes when exposed to an external electric field. Apparent suspension viscosities can increase several orders of magnitude for electric field strengths of the order of 1 kV mm −1 , with simultaneous ordering of the microstructure into particulate columns. While this electronic control of momentum transport and structure has many applications, development is severely inhibited by a lack of suitable materials and an incomplete understanding of the underlying mechanisms. This review focuses on the current understanding of the microscopic phenomena believed to control ER and the models used to describe macroscopic behavior. Particular emphasis is placed upon comparing model predictions with experimental observations, relating macroscopic behavior to microscopic mechanisms, and demonstrating the utility of mechanistic models for furthering our understanding of electrorheology.

Nitrogen containing hydrogenated amorphous carbon for thin-film field emission cathodes

Abstract: Field emission measurements using 0.3 μm thick nitrogen containing hydrogenated amorphous carbon films (a‐C:H:N) on n++‐Si cathodes are reported. Onset emission fields as low as 4 V μm−1 have been obtained using a flat plate anode configuration. Uniform emission is observed over the entire cathode area at current densities below 7×10−2 mA cm−2. At higher current density preferential emission from spots is observed. The spot emission is imaged using the ITO coated plate anode. A model based on the a‐C:H:N acting as a space charge interlayer on the n++‐Si is proposed to explain the emission at low electric fields.

Interferometric measurements of electric field-induced displacements in piezoelectric thin films

Abstract: Interferometric measurements of electric field‐induced displacements in piezoelectric thin films using single‐beam and double‐beam optical detection schemes are reported. It is shown that vibrational response measured with a single‐beam interferometer includes a large contribution of the bending motion of substrate. Therefore, it is difficult to apply single‐beam technique for piezoelectric measurements in thin films. To suppress the bending effect a high‐resolution double‐beam interferometer is proposed. The sensitivity of the interferometer is significantly improved in comparison with previously reported system. The interferometer is shown to resolve small displacements without using a lock‐in technique. An example of the interferometric capabilities is demonstrated with experimental results on electric field, frequency, and time dependences of piezoelectric response for quartz and Pb(Zr,Ti)O3 thin film.

An approximate formula for the calculation of the horizontal electric field from lightning at close, intermediate, and long range

Abstract: We present an approximate formula to calculate the horizontal electric field from lightning that is applicable for close, intermediate, and long distances to the lightning, at ground level and at a height above ground. The formula is analytically simple and can be readily implemented for numerical calculations in the frequency and in the time domains. The formula can be particularly useful for lightning induced voltage calculations. A test of the formula by comparison with the results obtained using good approximations to Sommerfeld's integrals is presented. The results compare favorably for a wide range of distances. Theoretical waveforms obtained with a return stroke model and the formula predict that, for negative ground lightning, the horizontal component of the electric field at close range and at a height of a few meters above the ground starts with a sharp pulse directed toward the lightning channel and is followed by a slower field change of opposite polarity.

Efficient implementation of the coupled-wave method for metallic lamellar gratings in TM polarization

Abstract: A new implementation of the coupled-wave method for TM polarization is proposed. We use a second-order differential operator established by Neviere together with a scattering-matrix approach. Thus we obtain for metallic gratings a convergence rate as quick as that in TE polarization.

Evidence of strong correlation between space-charge buildup and breakdown in cable insulation

Abstract: Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a nondestructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8/spl times/ the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

Breakdown threshold and plasma formation in femtosecond laser-solid interaction

Abstract: Combining femtosecond pump–probe techniques with optical microscopy, we have studied laser-induced optical breakdown in optically transparent solids with high temporal and spatial resolution. The threshold of plasma formation has been determined from measurements of the changes of the optical reflectivity associated with the developing plasma. It is shown that plasma generation occurs at the surface. We have observed a remarkable resistance to optical breakdown and material damage in the interaction of femtosecond laser pulses with bulk optical materials.

Direct penetration of the polar electric field to the equator during a DP 2 event as detected by the auroral and equatorial magnetometer chains and the EISCAT radar

Abstract: The quasi-periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.

Collisional losses of ring current ions

Abstract: The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce-averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce-averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time-dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low-energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low-energy ion precipitation inside the plasmasphere.

Theoretical and Experimental Investigations of Stochastic Boundaries in Tokamaks

Abstract: This review paper addresses the physics of stochastic boundaries. Although it is focused on the tokamak configuration many features are common to the stochastic boundaries of stellarators. The stochastic properties of magnetic field lines are recalled and related to the spectrum of the radial magnetic perturbation. The stochastic region, referred to as the divertor volume, is shown to be bounded to the edge plasma. Furthermore, the stochastic features discriminate two regions. On short scales, the stochasticity is not effective and parallel transport dominates, this defines the laminar region. On the long scales one recovers the proper stochastic features which characterize the ergodic regime. Theoretical predictions for the transport of energy, current and particles in the divertor volume are analysed for both the laminar and ergodic regimes. A strong increase in electron transport is expected which should lead to a strong increase in the heat diffusivity, a strong increase in the resistivity in the toroidal direction and generally a decrease in the free electron lifetime in the divertor volume. Ambipolarity of particle transport is ensured by a radial electric field. The ion transport, i.e. particle transport, is then more difficult to analyse since one has to consider the strong coupling to the electron temperature field and to the electric potential field. The perturbation level is such that the particle transport induced by the stochasticity remains comparable to the anomalous transport. The experimental data show good agreement with the predictions on electron transport. This translates into a flattening of the edge temperature gradient, a narrowing of the current channel, which probably governs the observed stabilization of MHD activity, and a strong decrease in the lifetime of the runaway electrons. Regarding particle transport, the response is larger than expected and stochastic boundaries are characterized by significant screening properties compared to limiter shots. This property is shown to be a signature of a pumping capability combined with a change of transport properties. Indeed the transport of neutrals is changed since the ratio of the ionization scales to the distance between the recycling surfaces and the separatrix is reduced. Furthermore, a decreased lifetime of the ions at the very edge of the plasma is expected. Screening effects are thus observed for species exhibiting large wall pumping capability, while He and Ne are weakly affected by the stochastic boundary. The plasma properties in the ergodic volume, namely a reduced edge temperature, an increased impurity radiation, an efficient particle screening and the stabilization of MHD activity, have opened the way to radiating layer investigations on Tore Supra. Stable operation has been achieved with 80% of radiated power and radio frequency heating up to 6 MW.

Auxiliary optics for a twisting ball display

Abstract: A gyricon or rotating-particle display having an auxiliary optical structure. The display includes a substrate with an optically transmissive window, a plurality of particles disposed in the substrate, and an optical focusing element optically coupled to the window. Each particle has an anisotropy for providing an electrical dipole moment, the electrical dipole moment rendering the particle electrically responsive such that when the particle is rotatably disposed in an electric field while the electrical dipole moment of the particle is provided, the particle tends to rotate to an orientation in which the electrical dipole moment aligns with the field. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate; when the particle is in this rotatable disposition, it is not attached to the substrate. Each particle, when rotatably disposed in the substrate, is disposable in first and second rotational orientations with respect to the optically transmissive window. Each particle provides a first optical modulation characteristic when disposed in its first orientation with respect to a flux of optical energy through the window, and further provides a second optical modulation characteristic when disposed in its second orientation with respect to a flux of optical energy through the window. The optical focusing element can be optically refractive; for example, it can include an array of converging lenses, such as a "fly's-eye" array of microlenses. In this case, the particles can be disposed in an array that is registered with the lens array.

Electrokinetic Remediation. II: Theoretical Model

Abstract: A mathematical model is presented for multicomponent species transport under coupled hydraulic, electric, and chemical potential differences. Mass balance of species and pore fluid together with charge balance across the medium result in a set of differential equations. Sorption, aqueous phase, and precipitation reactions are accounted by a set of algebraic equations. Instantaneous chemical equilibrium conditions are assumed. Transport of H{sup +}, OH{sup {minus}}, Pb{sup 2+}, NO{sub 3}{sup {minus}}, the associated chemical reactions, electric potential, and pore pressure distribution across the electrodes in electrokinetic remediation are modeled. Model predictions of acid transport, lead transport, and pore pressure distribution display very good agreement with the pilot-scale test results validating the formalisms offered for multicomponent transport of reactive species under an electric field. The model also bridges the gap between the electrochemistry and mechanics in electroosmotic consolidation of soils.

Rational Design of Molecules with Large Hyperpolarizabilities. Electric Field, Solvent Polarity, and Bond Length Alternation Effects on Merocyanine Dye Linear and Nonlinear Optical Properties

Abstract: The effects of applied electric fields and solvent polarity on the linear and nonlinear optical (NLO) properties of three prototypical merocyanine (donor−conjugated pathway−acceptor) chromophores of differing architectural types are analyzed using semiempirical INDO/1 calculations in the presence of imposed static electric fields, using finite-field self-consistent field (FFSCF), and in the presence of solvent dielectrics, self-consistent reaction field (SCRF), models. The NLO properties are computed using the computationally efficient correction vector approach. Both applied electric fields and solvent are found to affect the extent of charge separation induced in the ground states of these molecules. This charge separation leads to a geometric distortion, measured by the bond-length alternation (BLA) parameter, which indexes the geometrical evolution of the molecular architecture from a neutral polyenic structure to a partially ionic cyanine-like structure to an ionic polymethine-like structure. These g...

Electrohydrodynamic instability and motion induced by injected space charge in insulating liquids

Abstract: A survey is given of different aspects of fluid motion induced by the Coulomb force exerted by the electric field on an injected space charge. The instability problem in highly symmetrical electrode configurations is considered with positive coupling between velocity and charge perturbations, linear and nonlinear criteria and the phenomena during the transient regime of unipolar injection. Then the features of electroconvection are described for both strong and weak injection and also in the case of coaxial cylinders. The last part deals with the liquid motion induced by injection from very restricted areas, as occurs in configurations of low symmetry with particular attention devoted to charge plumes.

Electrorheological Dampers, Part I: Analysis and Design

Abstract: Electrorheological (ER) materials are suspensions of specialized, micron-sized particles in nonconducting oils. When electric fields are applied to ER materials, they exhibit dramatic changes (within milli-seconds) in material properties. Pre-yield, yielding, and post-yield mechanisms are all influenced by the electric field. Namely, an applied electric field dramatically increases the stiffness and energy dissipation properties of these materials. A previously known cubic equation which describes the flow of fluids with a yield stress through a rectangular duct can be applied to annular flow, provided that certain conditions on the material properties are satisfied. An analytic solution and a uniform approximation to the solution, for the rectangular duct Poiseuille flow case is presented. A numerical method is required to solve the flow in annular geometries. The approximation for rectangular ducts is extended to deal with the annular duct case.

Effect of the Electric Field Generated by the Helix Dipole on Photoinduced Intramolecular Electron Transfer in Dichromophoric α-Helical Peptides

Abstract: In this work, we have investigated the effect of the position of probe chromophores relative to the direction of the electric field generated by the helix on the rate of intramolecular electron transfer reactions. Helical oligopeptides 1 and 2 with pendant electron donor (D) and acceptor (A) chromophores differ only by the positional reversal of the donor-acceptor (DA) pair along the dipolar helix. Hence we anticipate that the alignment of the electric field in 1 against the direction of photoinduced electron transfer should induce a faster rate of electron transfer in 1 than in 2. Our observations suggest that the orientation of a donor-acceptor pair with respect to the molecular electric field generated by the helix dipole of a peptide affects the electron transfer rates in the expected manner. 21 refs., 1 fig., 1 tab.

Vacuum instability in external fields.

Abstract: We study particle creation from the vacuum by external electric fields, in particular, by fields, which are acting for a finite time, in the frame of QED in arbitrary space-time dimensions. In all the cases special sets of exact solutions of the Dirac equation (in and out solutions) are constructed. Using them, the characteristics of the effect are calculated. The time and dimensional analyses of the vacuum instability are presented. It is shown that the distributions of particles created by quasiconstant electric fields can be written in a form which has a thermal character and which seems to be universal, i.e., is valid for any theory with quasiconstant external fields. Its application, for example, to particle creation in an external constant gravitational field reproduces the Hawking temperature exactly. \textcopyright{} 1996 The American Physical Society.

Electric Field Scaling at a B=0 Metal-Insulator Transition in Two Dimensions.

Abstract: The nonlinear (electric field-dependent) resistivity of the 2D electron system in silicon exhibits scaling as a function of electric field and electron density in both the metallic and insulating phases, providing further evidence for a true metal-insulator transition in this 2D system at $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. Comparison with the temperature scaling yields separate determinations of the correlation length exponent, $\ensuremath{ u}\ensuremath{\approx}1.5$, and the dynamical exponent, $z\ensuremath{\approx}0.8$, close to the theoretical value $z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$.

Ring current development during storm main phase

Abstract: The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L < 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

Slow beam extraction by a transverse RF field with AM and FM

Abstract: A beam extraction method using a transverse rf electric field with amplitude and frequency modulation has been studied in order to develop an irradiation method which is synchronized with the breathing of a patient for high-quality charged particle therapy. The dependence of the extracted beam intensity on the voltage, the frequency band width and the center frequency of the transverse rf electric field has been investigated. The extracted beam intensity was slightly increased from that of the ordinary slow extraction method with a third order resonance. The response of the extracted beam intensity to the applied transverse rf electric field was as prompt as within 1 ms. The horizontal emittance of beams extracted by the present method was reduced by about 70% compared with that by the ordinary one due to utilizing a constant separatrix. Amplitude modulation can control the global beam spill structure. The frequency modulation reduced the effect of the current ripple of the main quadrupole magnets.

Trapping of micrometre and sub-micrometre particles by high-frequency electric fields and hydrodynamic forces

Abstract: We demonstrate that micrometre and sub-micrometre particles can be trapped, aggregated and concentrated in planar quadrupole electrode configurations by positive and negative dielectrophoresis. For particles less than in diameter, concentration is driven by thermal gradients, hydrodynamic effects and sedimentation forces. Liquid streaming is induced by the AC field itself via local heating and results, under special conditions, in vortices which improve the trapping efficiency. Microstructures were fabricated by electron-beam lithography and modified by UV laser ablation. They had typical gap dimensions between 500 nm and several micrometres. The theoretical and experimental results illustrate the basic principles of particle behaviour in ultra-miniaturized field traps filled with aqueous solutions. The smallest single particle that we could stably trap was a Latex bead of 650 nm. The smallest particles which were concentrated in the central part of the field trap were 14 nm in diameter. At high frequencies (in the megahertz range), field strengths up to 56 MV can be applied in the narrow gaps of 500 nm. Further perspectives for microparticle and macromolecular trapping are discussed.

The effects of simple objects on the electric field of apteronotus

Abstract: How might electric fish determine, from patterns of transdermal voltage changes, the size, shape, location, and impedance of a nearby object? I have investigated this question by measuring and simulating electric images of spheres and ellipsoids near an Apteronotus leptorhynchus. Previous studies have shown that this fish's electric field magnitude, and perturbations of the field due to objects, are complicated nonliner functions of distance from the fish. These functions become much simpler when distance is measured from the axes of symmetry of the fish and the object, instead of their respective edges. My analysis suggests the following characteristics of high frequency electric sense and electric images. 1. The shape of electric images on the fish's body is relatively independent of a spherical object's radius, conductivity, and rostrocaudal location. 2. An image's relative width increases linearly with lateral distance, and might therefore unambiguously encode object distance. 3. Only objects with very large dielectric constants cause appreciable phase shifts, and the degree of shift depends strongly on water conductivity. 4. Several parameters, such as the range of electric sense, may depend on the rostrocaudal location of an object. Large objects may be detectable further from the head than the tail, and conversely, small objects may be detectable further from the tail than head. 5. Asymmetrical objects produce different electric images, correlated with their cross-sections, for different orientations and phases of the electric field. 6. The steep attenuation with distance of the field magnitude causes spatial distortions in electric images, somewhat analogous to the perspective distortion inherent in wide angle optical lenses.

Polarizable point‐charge model for water: Results under normal and extreme conditions

Abstract: Molecular dynamics simulations of liquid water under normal and extreme conditions are performed using the polarizable point‐charge (PPC) model. This efficient three‐site model explicitly incorporates results from ab initio studies of the water molecule in applied electric fields. The structural, thermodynamic, and dielectric properties, and the self‐diffusion coefficient are examined at a number of temperatures ranging from 263 to 573 K. These simulation results are compared with available experimental data along the liquid–vapor coexistence line; the agreement is very good for all properties studied. The temperature of maximum density for the PPC model is found to coincide with the experimentally observed value of 277 K. The spatial coordination of water molecules in the liquid and the anisotropy of the self‐diffusion tensor are analyzed at various state points. Increased directional anisotropy in the local translational diffusion, suggestive of prenucleation phenomena, can be observed at T=263 K. Above T=473 K the local translational anisotropy becomes rather insensitive to temperature variation indicating a weakening of the correlations between water molecules. Rototranslational dynamics and nonlinear polarization effects arising in polarizable models for water are discussed along with their phenomenological implications. The dimer properties for the PPC potential are also reported.