Presence Of Space Charge Research Articles

Effect of trapped space charge on mechanical deformation induced by electric field

An organic insulating film of polyethylene naphthalene 2,6-dicarboxylate (PEN) was characterized by two complementary techniques: the mechanical deformation of the sample surface, induced by the application of an electric field, was obtained by an image correlation technique named DIC (Digital Image Correlation) and the space charge distributions in the thickness direction were measured by FLIMM (Focused Laser Intensity Modulation Method). The validation of the DIC method, which consists of a following of the successive positions of the surface patterns of the sample, is presented. To validate the algorithms and evaluate the performance of the technique, it was applied to computer-simulated speckle images. In a second step, the experimental system was applied to measure the mechanical deformations of a PEN film previously charged by an electric field in order to have space charge accumulation. This space charge accumulation was quantified by FLIMM. The results show that the mechanical behavior of the sample is modified by the presence of space charge.

Numerical Analysis of the Induced Corona Vibrations on High-Voltage Transmission Lines Affected by Rainfall

Droplets formation on high-voltage transmission lines affected by rainfall amplifies the corona discharges from the conductors. The induced corona vibrations intensify the fatigue of high-voltage conductors and supporting elements. The alternating presence of space charge and the ionic wind associated with the corona discharge are the main causes of this phenomenon. The objective of this paper is to validate a numerical simulation method to investigate the effects of different parameters (electric field strength and polarity, rain intensity, transverse wind velocity) on the amplitude of the induced corona vibrations. The finite element method was used to develop the numerical model of the conductor movement under the action of mechanical and electrical forces, while time discretization was made with the finite difference method. The numerical simulation results were in good agreement with the experimental data available in the literature. The model enables better comprehension of the induced corona vibration mechanism.

Two and three electrostatic lens systems for focusing of charged particles

Design of the focusing lens system has been done by using SIMION computer program which is used to trace and simulate the ion beam during its transport. A parallel beam of singly charged nitrogen ions of diameter 2 mm with energy of 3 keV was started at a distance of 50 mm before entering the lens system. In order to design the system, we studied it with different parameters in the presence of space charge. This study includes design and optimization of the operating parameters for the two-and-three cylinder lens systems. In a two-cylinder lens system, the first electrode was set at zero volts and the voltage applied to the second electrode was varied from 0 to –8 kV. Influence of the negative voltage applied at the second electrode for the–two-lens system on both the beam emittance and the beam diameter has been investigated for singly charged nitrogen ion trajectories. Beam emittance and beam diameter as a function of the tube diameter of the two-cylinder lens system have been studied. Also, beam emittance and beam diameter for a singly charged nitrogen ion as a function of the focusing points at different distances measured directly from the end of the-two-lens system has been investigated. Influence of the atomic masses of the different elements has been studied on both the beam emittance and beam diameter. The same study has been done for the-three cylinder-lens system. Key words: Two-and- three cylinder lens systems, acceleration voltage, beam emittance and beam diameter, SIMION computer program.

Experiment on space charge driven nonlinear resonance crossing in an ion synchrotron

Trapping of particles in nonlinear resonances in the presence of space charge and synchrotron motion may be a source of beam halo generation and beam loss in high intensity synchrotrons, in particular for extended storage times at the injection plateau as planned for the SIS100 synchrotron of the FAIR project. Although extensive simulation studies have theoretically demonstrated this mechanism, experimental evidence was so far limited to demonstration experiments at the CERN Proton Synchrotron (PS) in 2002--2003 using an octupolar resonance. Here we describe new experiments at the SIS18 synchrotron at GSI, where the resonance is driven by a sextupolar field error and horizontal static tune scans are taken across the resonance stop band. The new data significantly extend the previous observations by a complete set of measurements comparing beams with and without rf, both at low and high intensity. The correlation between transverse beam loss and simultaneous bunch length shortening provides strong evidence that the measured emittance and the loss in intensity are indeed caused by periodic resonance crossing, leading to the main effect of scattering but also to a lesser extent to the trapping of particles due to the combined effect of the nonlinear resonance and the space charge.

Contribution of space charges to the polarization of ferroelectric superlattices and its effect on dielectric properties

A theoretical model is developed for ferroelectric bilayers and multilayer heterostructures that employs a nonlinear Landau-Devonshire formalism coupled with a detailed analysis of the depolarizing fields arising from the polarization mismatch across interlayer interfaces and the electrical fields of localized space charges at such interfaces. We first present how space charges alter the free-energy curves of ferroelectrics and then proceed with a numerical analysis for heteroepitaxial (001) ${\text{PbTiO}}_{3}{\text{-SrTiO}}_{3}$ (PTO-STO) bilayers and (001) superlattice structures on (001) STO substrates. The switchable (ferroelectric) and nonswitchable (built-in) polarizations and the dielectric properties of PTO-STO bilayers and superlattices are calculated as a function of the planar space-charge density and the volume fraction of the PTO layer. Similar to the temperature dependence of a monolithic ferroelectric, there exists a critical volume fraction PTO below which the bilayer or the superlattice is in the paraelectric state. This critical volume fraction is strongly dependent on the density of trapped charges at the interlayer interfaces. For charge-free (001) PTO-STO heteroepitaxial bilayer and superlattices, the critical fraction is 0.40 for both constructs but increases to 0.6 and 0.72, for the bilayer and the superlattice, respectively, for a planar space-charge density of $0.05\text{ }\text{C}/{\text{m}}^{2}$. Furthermore, our results show that close to the vicinity of ferroelectric-paraelectric phase transition, there is a recovery in ferroelectric polarization. The dielectric-response calculations verify that there is sharp ferroelectric phase transformation for charge-free bilayers and superlattices whereas it is progressively smeared out with an increase in the charge density. Furthermore, our analysis shows that the dielectric constant of these multilayers at a given volume fraction of PTO decreases significantly in the presence of space charges.

Influence of needle tip distance on barrier discharge and ozone generation for multiple needle‐and‐plane electrode configuration

AbstractThe relationship between the barrier discharge characteristics and ozone generation on application of an AC voltage in a triple needle‐plane configuration has been investigated for various distances among the triple needle tips (d=0 to 7.0 mm) with a constant distance between the needle tip and the plane (g=3.0 mm) in dry air. The characteristics of the barrier discharge and ozone generation depend on the needle tip distance. It is considered that the influence is caused by the presence of space charge and accumulated charge, as suggested by discharge images taken by a still camera and a CCD camera. The ozone generation efficiency is also estimated from the power consumption and the ozone concentration. It is found that when the distance among the triple needle tips is small, the above influence is strengthened. In this case, the ozone generation efficiency is improved. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(7): 32–41, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10289

Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees. </para>

Thermal-Pulse Tomography of Space-charge and Polarization Distributions in Electret Polymers

A new, non-destructive technique for the analysis of electret materials is presented. Thermal-pulse tomography produces three-dimensional images of space-charge and polarization distributions with a lateral resolution of better than 50 μm and a depth resolution of less than 0.5 μm. A focused-pulsed laser heats a circular spot on the opaque upper electrode. While diffusing through the sample, the thermal pulse causes local changes in the sample geometry or dielectric properties, resulting in a short-circuit current in the presence of space charge or electric dipoles. From the transient current, the distribution of the internal electric field can be reconstructed by means of scale transformation or regularization methods.

Characterisation of gamma‐irradiated polyetherimide films with infrared spectroscopy and thermally stimulated current measurements

AbstractThe performance of insulators is usually affected by the presence of space charges. The evolution of these charges under applied constraints in many cases leads to the degradation of these materials. To evaluate the performance of an insulator it is necessary to study the evolution of the physical and chemical properties in the presence of space charges. In this paper, the effect of gamma irradiation on the space charge behaviour in polyetherimide (PEI; Ultem 1000) has been investigated at different radiation doses. Space charge measurements were made using thermally stimulated currents. Infrared spectroscopy characterisation was performed to investigate the effect of gamma irradiation on the physicochemical properties of PEI. The results obtained show minor differences in the infrared spectra, mainly in the 410 cm−1 region. It is found that irradiation leads to a few structural changes in the material. The thermostimulated depolarisation current (TSDC) results show that β relaxations in PEI are affected by gamma irradiation. The activation energies of dipoles were calculated from the TSDC curves. Copyright © 2006 Society of Chemical Industry

Effects of a p–n junction on heterojunction far infrared detectors

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) far infrared detectors based on the GaAs/AlGaAs material system have shown promise for operation at wavelengths up to a few hundred microns. HEIWIP detectors with GaAs emitters have been shown to operate out to 92μm. Recent modifications to use AlGaAs emitters have extended the zero response threshold out to 128μm. Extension to longer wavelengths will require reducing the dark current in the devices. An approach using the addition of a p–n junction in the detector, which was shown to work in QWIP and homojunction detectors is considered here. Differences between the predicted and observed threshold behavior could be explained by the presence of space charge within the device. The band bending from this space charge produces the observed variation in the threshold. The space charge can also be used to explain anomalous conduction observed at low biases. When the device is forward biased, the current is expected, to be small until the bias voltage is similar to the bandgap of 1.4eV, above which the current should increase rapidly. Dark current was observed for biases significantly less than the bandgap. The threshold bias decreased with temperature, and was as low as 0.25V for a temperature of 300K. This is much lower than could be explained by thermal effects alone.

Validation of electro-optic sensors for measurement of DC fields in the presence of space charge

The paper presents theoretical analysis and experimental verification of an electro-optic sensor for measurements of DC electric fields in space charge environment. Effects of both unipolar and bipolar charge were investigated. It was shown that in both cases the sensor output is linearly dependent on the intensity of measured electric field and independent of the space charge density. The achieved measurement resolution is 100 V/m with a dynamic range of 80 dB and a temperature stability of 0.1%/°C for temperatures near 20 °C.

Effect of current density on electron beam induced charging in MgO

It is well known that the presence of space charge in an insulator is correlated with an electric breakdown. Many studies have been carried out on the experimental characterization of space charges. In this paper, we outline the dependence on the current density of the charge-trapping phenomenon in magnesium oxide. Our study was performed with a dedicated scanning electron microscope (SEM) on the electrical property evolution of surface of magnesium oxide (100) (MgO) single crystal, during a 1.1, 5 and 30keV electron irradiation. The types of charges trapped on the irradiated areas and the charging kinetics are determined by measuring the total secondary electron emission (SEE) σ during the injection process by means of two complementary detectors. At low energies 1.1 and 5keV, two different kinds of self-regulated regime (σ=1) were observed as a function of current density. At 30keV energy, the electron emission appears to be stimulated by the current density, due to the Poole–Frenkel effect.

Finite element modelling of ionized field quantities around a monopolar HVDC transmission line

In this paper, the Poisson's equation describing the ionized field around an HVDC line is solved using an improved finite element based technique. First order isoparametric quadrilateral elements, together with a modified updating criterion for the space charge distribution, are implemented in the iterative procedure. A novel technique is presented for mesh generation in the presence of space charges. Electric field lines and equipotential lines have been computed using the proposed technique. Total corona current at different applied voltages above corona onset voltage, electric field at the ground plane with and without the presence of space charges and current density at the ground plane have also been computed. The results are in agreement with the experimental values available in the published literature.

Low crystallization temperature and unusual switching properties of ferroelectric Nb-doped Bi4Ti3O12 thin films prepared by rapid thermal annealing

Nb-doped Bi4Ti3O12 (Nb-BIT) ferroelectric thin films were prepared in the presence of a nonionic surfactant (pluronic P123) added as an additive to the sol solution and by rapid thermal annealing (RTA). The film annealed at the relatively low temperature of 600 °C was well crystallized and showed good ferroelectricity. The switching charge of capacitors with polarization reversal rapidly increased with a large amplitude and low frequency of the applied pulse, and gradually decreased with a small amplitude and high frequency. The remanent polarization (2Pr) after subjecting the Nb-BIT capacitors to 108 read/write cycles was 46 μC/cm2, which is remarkably higher than 20 μC/cm2 observed in the initial state. These phenomena seem to appear by the presence of space charges trapped after heat treatment by the RTA process.

Electric field calculations for needle-plane geometry and space charge in polyethylene

Electric fields in polymeric insulation are calculated with the boundary element method (BEM) in the presence and absence of space charge. A 3-D version of the BEM software is employed to determine the profiles of divergent electric fields produced by metallic and semicon needles embedded in the polymeric insulation. It is shown that the value of the electric field in the insulation decreases rapidly and within 5 /spl mu/m from the tip it is less than 50% of its initial value. The BEM can also provide 3D profiles of the electric field distributions in the presence of space charge.

INFLUENCE OF DENSITY, OCCUPANCY AND LOCATION OF ELECTRON TRAPS ON SHOT NOISE IN NONDEGENERATE QUASIBALLISTIC TRANSPORT

An ensemble Monte Carlo simulator self-consistently coupled with a Poisson solver is used to study the influence of the density, occupancy and location of electrons traps on the current fluctuations in nondegenerate quasiballistic structures. Transport and noise are found to depend significantly on the density and location of the traps. In particular, the presence of traps increases the noise level as compared with the purely ballistic case. The nonuniformity of the trap occupancy profiles along the active region of the structures affects markedly (specially for low trap densities) the dependence of shot noise on the applied voltage. The location of the traps with respect to the potential minimum appearing in the structures because of the presence of space charge determines their influence on the noise level, fact which can be of help to identify the trap position.

Step voltage in metal/polyimide/organic molecule/polyimide/metal junction

Abstract A step structure, very similar to a Coulomb staircase, has been successfully observed in the current–voltage ( I – V ) characteristic of metal/polyimide (PI)/rhodamine-dendorimer (Rh-G2)/PI/metal junctions prepared by the Langmuir–Blodgett technique. Assuming Rh-G2 molecule functions like a metallic sphere and PI works as tunneling barriers, the I – V characteristic of the junctions was analyzed based on Coulomb blockade theory. Taking into account the presence of space charge at the PI-metal interface, which has been revealed by the surface potential measurement, the equation of threshold voltage of the step structure is derived. It was found that the derived equation is satisfactory to explain the I – V characteristic.

Electro-optic sensor for measurement of DC fields in the presence of space charge

This paper presents a theoretical analysis and experimental verification of electro-optic sensors used to measure DC electric fields in space charge environments. Both unipolar and bipolar charge environments were analyzed. The results suggest that in both cases the sensor output is linearly dependent on the measured electric field and independent of the density of space charge. To conduct the measurement, however, the prior knowledge of the type of charge present (unipolar-bipolar) is essential.

Dependence on crystallographic orientation of trapping properties in a polished MgO single crystal

It is well known that the presence of space charges in an insulator is correlated with electric breakdown. Many studies have been carried out on the experimental characterization of space charges and on phenomenological models of trapping and detrapping. In this paper, we outline the dependence on crystallographic orientations of the charge-trapping phenomenon in polished MgO. The charging phenomenon was characterized during and after electron injection by using a scanning electron microscope (SEM). It was shown that the trapping ability depends on the dislocation network of different crystallographic orientations. PACS No.: 72.20Jv

Molecular modeling of electron traps in polymer insulators: Chemical defects and impurities

The presence of space charge in the polymeric insulation of high-voltage cables is thought to be correlated with electric breakdown. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. It is clear that both physical (e.g., conformational disorder) and chemical defects (e.g., broken bonds and impurities) are present in insulating materials and that both may trap electrons. We have shown that by defining the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the material. In a recent paper [M. Meunier and N. Quirke, J. Chem. Phys. 113, 369 (2000)] we proposed methods that have made it possible to provide estimates of the energy, number and residence times of electrons in conformational traps in polyethylene. Typical physical trap energies are of the order of 0.15 eV and all are less than 0.3 eV. In the present paper we focus on the role of chemical defects, where we expect much deeper traps but at very low concentrations. Following the methodology used in our previous paper we have used density-functional theory to calculate trap energies for a set of chemical impurities and additives commonly found in polyethylene used for high-voltage cable insulation. In an extension of our approach we have estimated the effect of neighboring molecules on the trap energies of such defects. The resulting trap energy-trap density distribution reveals some very deep (&amp;gt;1 eV) traps presumably implicated in the formation of long-lived space charge in polymeric insulators and consequently to changes in the dielectric properties of the material.