Showing papers on "Electric potential published in 1998"

Bipolar electrostatic structures in the shock transition region: Evidence of electron phase space holes

Abstract: We present observations of intense, bipolar, electrostatic structures in the transition region of the terrestrial bow shock from the Wind spacecraft. The electric field signatures are on the order of a tenth of a millisecond in duration and greater than 100 mV/m in amplitude. The measured electric field is generally larger on the smaller dipole antenna, indicating a small spatial size. We compare the potential on the two dipole antennas with a model of antenna response to a Gaussian potential profile. This result agrees with a spatial scale determined by convection and gives a characteristic scale size of 2–7 λd. We interpret the observations as small scale convecting unipolar potential structures, consistent with simulations of electron phase space holes and discuss the results in the context of electron thermalization at strong collisionless shocks.

Intermittent particle transport in two-dimensional edge turbulence

Abstract: Interchange turbulence in two dimensions is investigated in the scrape-off layer (SOL) of fusion devices, when driven by a constant core particle influx. Contrary to the standard gradient-driven approach, density is allowed to fluctuate around its average profile. Transverse transport exhibits some of the features of self-organized critical systems, namely inward and outward avalanches, together with a frequency spectrum decrease in 1/f and f−2 at intermediate and high frequencies, respectively. An avalanche occurs when the local radial density gradient exceeds the critical one. A self-sustained particle flux then follows the large radial structures of the electric potential. As observed experimentally, the radial profile of density relative fluctuations decreases from the wall into the core plasma, while that of electric potential relative fluctuations peaks inside the SOL. Equilibrium density exhibits the experimental exponential decrease. An analytical expression of the SOL width ΔSOL is obtained, whic...

Experimental evidence on the role of the large spatial scale electric field in creating the ring current

Abstract: This paper presents the first simultaneous in situ measurements of the large-scale convection electric field and the ring current induced magnetic field perturbations in the equatorial plane of the inner magnetosphere and compares them to the evolution of major geomagnetic storms as characterized by Dst. The measurements were obtained from the University of California, Berkeley double-probe electric field experiment and the Air Force Geophysics Laboratory fluxgate magnetometer on the CRRES spacecraft. This spacecraft had an apogee near geosynchronous orbit and a perigee near 300 km altitude. We focus on the major geomagnetic storm on March 24, 1991, for which the maximum negative excursion of Dst was about −300 nT. During the main phase of the storm, the large-scale electric field repeatedly penetrated earthward, maximizing between L = 2 and L = 4 with magnitudes of 6 mV/m. These magnitudes were larger than quiet time values of the electric field by a factor of 60 or more. Electric potential drops across the dusk region from L = 2 to L = 4 ranged up to 50–70 kV in concert with increases in Kp up to 9 and dDst/dt (an indicator of the net ring current injection rate) which ranged up to −50 nT/hr. These electric fields lasted for time periods of the order of an hour or more and were capable of injecting ring current ions from L = 8 to L = 2.4 and energizing particles from initial plasma sheet energies of 1–5 keV up to 300 keV through conservation of the first adiabatic invariant. The data obtained during the recovery phase of this storm provide the first direct experimental evidence in the equatorial plane that the electric field is systematically diminished or shielded earthward of the inner edge of the ring current during this phase of the geomagnetic storm. Also observed during the 2-week recovery phase were episodic enhancements in the electric field which coincided and were colocated with enhancements of in situ ring current intensity and which also coincided with decreases in Dst. These enhancements in the electric field and in the ring current magnetic field perturbation occurred at progressively larger radial positions as the recovery phase continued. Evidence for regions of reversed convection near midnight during the recovery phase is provided. An unexpected and important feature of this data set, during both main and recovery phases, near 1800–2100 MLT, is that electric fields are often much stronger earthward of L = 4 or L = 5 than at positions more distant than L = 6. This suggests important features of the interaction between the hot ring current plasma and the large-scale electric field in the inner magnetosphere are not yet understood.

Two-dimensional simulation of an alternating current matrix plasma display cell: Cross-talk and other geometric effects

Abstract: A two-dimensional, user-friendly model of the discharge occurring in a plasma display panel cell was developed. This model was used to study the transient discharges in an alternating current plasma display with a matrix electrode configuration. The space and time variations of the charge particle densities, excitation rates, electric potential, and surface charge densities are described. The model is also used to study the conditions of existence of electrical interaction between adjacent cells and the effects of electrode misalignment.

Surface Tension and Surface Potential of Na n-Dodecyl Sulfate at the Air−Solution Interface: Model and Experiment

Abstract: Surface potential vs concentration isotherms of Na n-dodecyl sulfate (SDDS) adsorbed at the air−solution interface, measured using the vibrating plate method at various concentrations of added salt, exhibit a pronounced minimum. The results of surface tension measurements indicate that the minimum occurs within the concentration range that corresponds to the transition from the Henry regime of adsorption for low surface coverages to the one typical for adsorption of amphiphiles at high surface coverages. We proposed a simple model of adsorption of ionic surfactants at air−fluid interfaces based on the assumption that surfactant headgroups and counterions can adsorb in the Stern layer at the same Helmholtz plane. The electric potential in the electric double layer was calculated according to the Gouy−Chapman model for the diffuse part of the double layer and a modified Stern model for the inner layer with corrections for the discrete charge effects. The total potential drop across the interface was assumed...

Modelling geomagnetically induced currents produced by realistic and uniform electric fields

Abstract: The methods used to model geomagnetically induced currents (GIC) on power systems depend on the nature of the electric field used as input. A uniform electric field, often used to simplify the modelling, is shown to have different properties from realistic nonuniform fields. Realistic fields which go to zero at infinity can be uniquely represented by the sum of the gradient of a scalar potential and the curl of a vector function. The scalar potential term is conservative and irrotational, while the vector term is nonconservative and solenoidal. In contrast, a uniform electric field can be represented simply by the gradient of a scalar potential. These different mathematical properties mean that modelling techniques derived for uniform fields may not work for realistic fields. This is examined using, as an example, the modelling of GIC produced in a conducting network at the surface of the Earth. It is shown that uniform electric fields can be represented by voltage sources in the transmission lines or at power system ground points. However, realistic electric fields, such as those produced by the auroral electrojet, cannot be represented by voltage sources at ground points and have to be represented by a voltage source in the transmission lines.

A simple model of low‐latitude electric fields

Abstract: The low-latitude electric fields are generally modeled by numerically solving a two-dimensional potential equation developed from assumptions of equipotential field lines and current continuity. Additional assumptions are used to derive simple formulas for the vertical electric field EL and zonal electric field Eϕ in the plane of the magnetic dip equator. Electric fields obtained from the simple model will be compared against the electric fields of the more complete numerical model to demonstrate the validity of the simple model solution and to identify the range of validity. The simple electric field model is used to directly identify the primary causes of the low-latitude electric field structure. The observed structure has sources that can be divided into components: (1) polarization fields created by divergences in neutral-wind dynamo currents and (2) curl-free effects demanded by ∇ × E = 0. It is shown that the evening prereversal enhancement of Eϕ is caused by “curl-freeness” demands of the rapidly changing EL near sunset. This supports cursory statements of Rishbeth [1971] on the cause of the postsunset enhancement. Finally, the resulting altitude variation of the zonal electric field depends on the type of source. For polarization field sources the altitude variation is approximately proportional to L2. For curl-free sources the altitude variation is related to the rate of change of EL with respect to longitude.

Model of electric charges in thunderstorms and associated lightning

Abstract: An axisymmetrical electrostatic model of charges in a thunderstorm cloud at the mature stage is used to initiate a bidirectional vertical leader that develops into either a cloud-to-ground or an intracloud flash from the regions of maximum electric field. The principal result of this study is the determination of the physical and quantitative relationships among cloud charges, potentials, and electric fields, and the induced charges, currents, and electric field changes of the lightning channel from the numerical solution of the Poisson equation for a cloud charge model with lightning. An important consequence of the model is that the upper part of the cloud-to-ground leader and the lower part of the intracloud leader terminate inside the cloud.

Comparison between the PEA method and the PWP method for space charge measurement in solid dielectrics

Abstract: The principles of both pulsed electro acoustic (PEA) and pressure wave propagation (PWP) methods for measuring the space charge distributions in solid dielectrics are summarized. The relationship between the charge distribution in a specimen and voltage signal of the PEA method or the current signal of the PWP method are expressed in the time and frequency domains. Furthermore, the calibration method of the amount of accumulated charge density using deconvolution techniques is described for both methods. To compare these two methods, PMMA samples with and without space charge are measured using both methods. In addition, the electric field and electric potential distributions are calculated and discussed. The experimental results obtained using both methods are almost the same.

Ultrafast Quantum Kinetics of Time-Dependent RPA-Screened Coulomb Scattering

Abstract: We study the Coulomb quantum kinetics of relaxation and dephasing of an electron-hole gas in asemiconductor excited by a coherent femtosecond laser pulse. Our theory employs the full two-time-dependent RPA-screened Coulomb potential. The self-consistently calculated potential evolves froma bare potential to the well-known dynamically screened RPA potential for long times. The timedependence of the particle distributions, the interband polarization, and the potential are calculated. Asa first application we study Rabi flopping with resonant femtosecond pulses. [S0031-9007(98)06707-6]

Transient Response of a Piezothermoelastic Circular Disk Under Axisymmetric Heating

Abstract: Transient temperature, displacement, stress and electric field intensities in a finite circular piezothermoelastic disk undergoing axisymmetric surface heating are examined. Exact solutions to the equations of equilibrium and electrostatics are obtained using a potential function approach based upon two piezothermoelastic potential functions, three piezoelastic potential functions and a piezoelectric potential function. The disk under consideration is assumed to exhibit hexagonal material symmetry of class 6 mm. The initial temperature of the disk is zero; thereafter one face is subjected to linear heat transfer from an adjacent medium (Newton's law of cooling), while the temperature of the other face remains constant. Both faces are taken to be free of traction. The cylindrical boundary of the disk is thermally insulated, electrically charge-free, and constrained against radial deformation. Numerical results are obtained for the stress and the electric potential distributions in a cadmium selenide disk.

Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules

Abstract: A numerical model is presented to determine the Keldysh adiabaticity parameter for the interaction of an intense laser with a polyatomic molecule. The adiabaticity parameter is a guide to determining whether the ionization process is in the field or multiphoton ionization regime. The adiabaticity parameters are compared for potentials including the simple zero-range potential, the Coulomb potential, an atomic potential (Xe), a diatomic (N2) molecular potential, and a polyatomic (C6H6) molecular potential. It is demonstrated that the Coulomb potential is approximately equal to the atomic and diatomic potentials and differs from the zero-range potential employed in the Keldysh model in a way which is predominantly dependent upon the ionization potential. Both simple models substantially overestimate the adiabaticity parameter for C6H6 at all field strengths and at fields above 1.25 V/A both simple models become completely unphysical. This is because barrier suppression ionization is predicted to occur for b...

Current‐voltage relationship in the downward auroral current region

Abstract: In the auroral zone of the earth currents flow along magnetic field lines. In the downward current region currents are mainly carried by upflowing electrons from the ionosphere. Because of the low plasma density along auroral field lines, substantial currents in the range of microamps per square meter require substantial potential drops parallel to the magnetic field in the range of a few hundred to a few thousand volts. The current-voltage relation along such magnetic field lines can be determined for simple profiles of the background ion density by invoking the condition of charge neutrality. For typical parameters, the current density is found to be a few times larger in the downward current region compared to currents in the upward current region for similar potential drops. Thus potential drops up to a few thousand volts and the consequent acceleration of ionospheric electrons up to keV energies, such as has been observed by the FAST satellite, are a necessary consequence of the observed current densities in the downward auroral current region.

Computer simulation of ion conductance in membrane channels

Abstract: We present a method for calculation of electric forces in biological channels, which facilitates microscopic modeling of ion transport in channels using computer simulation. The method is based on solving Poisson's equation on a grid and storing the electric potential and field for various configurations in a table. During simulations, the potential and field at any point are calculated by interpolating between table entries rather than solving Poisson's equation. This speeds up computer simulations by orders of magnitude with minimal loss in accuracy. With this method, one can run simulations long enough to determine the channel conductance, which can be compared directly with experimental data. Since conductance is the most important observable quantity in description of membrane channels, this method will be very useful in future simulation studies of channels.

Calculation of the field enhancement on laser-illuminated scanning probe tips by the boundary element method

Abstract: In the near field of an illuminated scanning probe tip with a dimension much smaller than a light wavelength a tremendous field enhancement is possible. This effect is similar to field enhancement on small metal particles first in- vestigated in SERS (surface-enhanced Raman spectroscopy). In this article we propose to apply the boundary element method (BEM) to calculate the electric potential and field for an arbitrary tip-sample geometry. BEM works especially well for the Laplace problem with piecewise constant mate- rial properties. We developed a simple and fast program and carried out calculations for some tip-sample configurations.

New Thin Piezoelectric Plate Models

Abstract: Early investigations on piezoelectric plates were based on a priori mechanical and experimental considerations. They assume plane stress and consider only transverse components of electric displace...

Nodal sets for the groundstate of the Schroedinger operator with zero magnetic field in a non simply connected domain

Abstract: We investigate nodal sets of magnetic Schroedinger operators with zero magnetic field, acting on a non simply connected domain in $\r^2$. For the case of circulation 1/2 of the magnetic vector potential around each hole in the region, we obtain a charactisation of the nodal set, and use this to obtain bounds on the multiplicity of the groundstate. For the case of one hole and a fixed electric potential, we show that the first eigenvalue takes its highest value for circulation 1/2.

The cylindrical DC magnetron discharge: I. Particle-in-cell simulation

Abstract: The radial structure of a low-pressure cylindrical post-cathode direct-current magnetron discharge is investigated using a one-dimensional, electrostatic particle-in-cell code, incorporating non-periodic boundary conditions and an external circuit. Electron and ion collisions with a background gas of argon are modelled using Monte Carlo techniques. The radial structure of the discharge is examined for a range of operating conditions. Profiles of the electric potential, electric field and space charge density are found to vary systematically with the pressure p and magnetic field strength B, in a way which corresponds to a transition from the usual positive space charge mode at low values of B/p to a higher impedance negative space charge mode at higher values of B/p. This is consistent with a continuous and considerable reduction in the ratio of the electron-to-ion classical cross-field transport coefficients with increasing B/p. Results of a fluid model also predict the transition to the negative space charge mode.

A Steady-State Quantum Euler–Poisson System for Potential Flows

Abstract: A potential flow formulation of the hydrodynamic equations with the quantum Bohm potential for the particle density and the current density is given. The equations are selfconsistently coupled to Poisson's equation for the electric potential. The stationary model consists of nonlinear elliptic equations of degenerate type with a quadratic growth of the gradient. Physically motivated Dirichlet boundary conditions are prescribed. The existence of solutions is proved under the assumption that the electric energy is small compared to the thermal energy. The proof is based on Leray-Schauder's fixed point theorem and a truncation method. The main difficulty is to find a uniform lower bound for the density. For sufficiently large electric energy, there exists a generalized solution (of a simplified system), where the density vanishes at some point. Finally, uniqueness of the solution is shown for a sufficiently large scaled Planck constant.

Method of power supply for electronic systems and device therefor

Abstract: This invention describes a method and a device for self-contained power supply of electronic systems by conversion of energy from non-electric sources into electric energy by means of charge generators. Charge generators used in such devices may be piezoelectric or triboelectric elements or radioactive sources of charged particles which, unlike traditional electric supply sources, do not require periodical replacement or recharging. When a charge generator (1) is activated, it generates a series of electrical charges q having a high electric potential Uin which are inputted in the charge energy converter (2). The converter (2) is used to increase the initial value q of the charges up to a value Q and simultaneously to reduce their potential to a value Uout which is lower than Uin. An electric charge accumulator (3) arranged at the output of the device accumulates the charges Q resulting from conversion and which are used to supply electronic systems.