Showing papers on "Electric potential published in 1987"

Importance of space-charge effects in resonant tunneling devices

Abstract: The consideration of space charge in the analysis of resonant tunneling devices leads to a substantial modification of the current‐voltage relationship. The region of negative differential resistance (NDR) is shifted to a higher voltage, and broadened along the voltage axis. Moreover, the peak value of current prior to NDR is reduced, leading to a reduction in the predicted peak‐to‐valley ratio. An approach is presented to include space‐charge effects, and a recently fabricated GaAs‐AlxGa1−xAs structure is analyzed, to underscore the importance of a self‐consistent electrostatic potential in theoretical calculations.

Zero-current persistent potential drop across small-capacitance Josephson junctions.

Abstract: Quantum effects in a small-capacitance Josephson junction coupled capacitively to an external circuit are investigated. Capacitive coupling permits one to control the charge on the junction and to hold the junction in an equilibrium state. The equilibrium state is characterized by a persistent voltage drop without an accompanying dc current. The persistent voltage is a periodic function of the control charge with period 2e if Cooper pairs only are present and with period e if quasiparticles are included. In the capacitively coupled junction, Bloch oscillations are induced by increasing the control charge linearly in time. The presence of a persistent voltage drop is reflected in the small-signal impedance of the circuit which describes the response of the junction to an oscillating control charge superimposed on a static control charge. The impedance reveals features which seem closely related to experimental observations by Lambe and Jaklevic [Phys. Rev. Lett. 22, 1371 (1969)] on a normal array of capacitively coupled junctions.

Separation of Small Particles Suspended in Liquid by Nonuniform Traveling Field

Abstract: A study of size and charge-dependent separation of small particles in liquid using a traveling-field-type electric curtain device is made. The principle of the separation is to make use of the spatial harmonic components of the rotating traveling field produced by such a device, the first harmonic propagating in one direction, which plays a dominant role in the region distant from the electrodes, and the second harmonic propagating in the opposite direction, which becomes dominant near the electrodes. Small particles brought into this field undergo circular motion and, as a result of field nonuniformity, are repelled from the electrodes and drift in the direction of the dominant harmonics. The lighter or more charged particles are strongly repelled from the electrodes and swept by the first harmonic, while the heavier or less charged particles can approach the electrodes and are transported by the second harmonic in the opposite direction, thus enabling separation by mass and charge. First a theoretical investigation of this method is made to clarify the operation conditions for the separation, then the experimental observations of particle motion are made and scaling laws of transport velocity with the applied voltage and frequency are confirmed. Finally, an example of a cell separator design using this method is presented.

The Current-Voltage Relationship in Auroral Current Sheets.

Abstract: The current-voltage relation within narrow auroral current sheets is examined through the use of high-resolution data from the high-altitude Dynamics Explorer 1 satellite. The north-south perpendicular electric field and the east-west magnetic field are shown for three cases in which there are large amplitude, oppositely directed paired electric fields which are confined to a region less than 20 km wide. The magnetic field variations are found to be proportional to the second integral of the high-altitude perpendicular electric field. It is shown that at the small-scale limit, this relationship between ΔB and E is consistent with a linear “Ohm's law” relationship between the current density and the parallel potential drop along the magnetic field line. This linear relationship had previously been verified for large-scale auroral formations greater than 20 km wide at the ionosphere. The evidence shown here extends our knowledge down to the scale size of discrete auroral arcs.

Modeling of corona characteristics in a wire-duct precipitator using the charge simulation technique

Abstract: The charge simulation technique has been adapted to model the electrostatic and the corona characteristics in clean air of a duct-type electrostatic precipitator. The study involves the evaluation of the electric potential, electric field, and charge density in the interelectrode space as a function of corona current. The results show good agreement with published experimental data. The method developed can be applied to other geometries in the presence of space charge. The commonly used assumption that the space charge affects the magnitude but not the direction of the electric field is shown to be inadequate for large values of corona current. Also, the effect of using different values for the mobility of negative ions is presented.

Electric field measurements at subcritical, oblique bow shock crossings

Abstract: ISEE-1 electric field measurements at three oblique, subcritical dispersive bow shock crossings are presented The potential drops across the shock due to the large spatial scale normal component of the electric field were found to vary between 340 and 520 V The measurements provide the first observations in a space plasma of the oscillations in the normal component of the electric field connected with the whistler precursor phase standing at a collisionless shock Intense, rapidly varying electric fields with peak amplitudes ranging up to 100 mV/m were observed at the magnetic ramp of the shock in the high time resolution data

On the noncoplanarity of the magnetic field within a fast collisionless shock

Abstract: Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity. This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman-Teller frame of the reference (HTF), is about 2-6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross-shock electric field.

On the scalar potential of a point charge associated with a time-harmonic dipole in a layered medium

Abstract: It is demonstrated that one can choose the form of the magnetic vector potential to render the scalar potential of a single point charge associated with a horizontal, time-harmonic dipole in a layered medium identical to that associated with a vertical dipole, provided that the source and observation points are within the same layer. This proves the existence of the so-called mixed-potential electric field integral equation for objects of arbitrary shape in layered media.

Io's interaction with the plasma torus: A self-consistent model

Abstract: The interaction between Io and the plasma torus is simulated by a three-dimensional numerical model which allows the calculation of electric fields, current density distributions, and magnetic fields in Io's vicinity and in the Alfven wings. The model is self-consistent in the sense that the influence of magnetic field disturbances on the electric field is taken into account and vice versa. A two-dimensional elliptic differential equation for the electric potential is derived by using Euler potentials. The current density in Io's vicinity is decomposed into poloidal and toroidal fields. This representation allows an effective integration to get the magnetic field disturbances. Pedersen, Hall, inertial (or polarization) and field-aligned currents are calculated. We show results for different upstream plasma conditions and atmosphere/ionosphere models and discuss the consequences of variations in neutral density, Alfvenic Mach number, and other parameters. A series of simple spherically symmetric atmosphere/ionosphere models with neutral gas densities between 1016 m−3 and 1017 m−3 are investigated. By comparison with Voyager magnetic field measurement we get agreement for a column density of 7.5 × 1021 m−2 (surface density N0 = 5×1016 m−3, scale height H = 150 km).

Electron dynamics and potential jump across slow mode shocks

Abstract: In the de Hoffmann-Teller reference frame, the cross-shock electric field is simply the thermoelectric field responsible for preserving charge neutrality. As such, it gives information regarding the heating and dissipation occurring within the shock. The total cross-shock potential can be determined by integrating a weighted electron pressure gradient through the shock, but this requires knowledge of the density and temperature profiles. Here, a recently proposed alternative approach relying on particle dynamics is exploited to provide an independent estimate of this potential. Both determinations are applied to slow mode shocks which form the plasma sheet boundary in the deep geomagnetic tail as observed by ISEE 3. The two methods correlate well. There is no indication of the expected transition from resistive to viscous shocks, although the highest Mach number shocks show the highest potentials. The implications of these results for the electron dissipation mechanisms and turbulence at the shock are discussed.

Electric field dependent fluids

Abstract: An improved method of inducing a change in the dynamic torque transmission of an electrorheological fluid in response to an electric field at low current comprising the steps of: (a) selecting a nonconductive liquid phase (e.g. high dielectric hydrocarbon oil); (b) dispersing in the nonconductive liquid phase a particulate phase substantially free of adsorbed water (e.g. zeolite): and (c) subjecting the resulting electrorheological fluid to an electric potential in excess of about one kilovolt at a current density of less than about one-third microamp per square inch. Such a method achieves a rate of increase of dynamic transmission of torque per unit voltage, dM/dV, in excess of about 0.034 ft-lbs x 10⁻³/kV and a ratio of dynamic transmission of torque at an electric field strength of at least 2.4 kV to the dynamic transmission of torque at zero field strength, M/Mo, of at leeast about 2.

Proposed model for the sintering of a dielectric in a microwave field

Abstract: The processing of ceramics, glasses, and composites using microwave energy is an interesting new technology. The Los Alamos National Laboratory has done exploratory work in this area over the past several years and some of this work has been reported in the literature [1-9]. The purpose of this letter is to present a model for the heating of these materials in an electromagnetic field and to suggest some possible experiments that may be conducted to test the model. Of principal interest to the development of a heating model which relates the energy of an electromagnetic field to the intensive properties of a dielectric is an understanding of how the electromagnetic field behaves at an interface which separates two dielectric media. For the purpose of this discussion, a porous monolithic ceramic, with one phase being air, is considered. A treatment of this may be found in any introductory text on electromagnetic theory, for example, Hayt [10] provides an excellent discussion of this topic. Fig. 1 shows a boundary between two perfect dielectric media. For simplicity assume that one dielectric media is a monolithic ceramic and the other is air. It is important at this point to introduce the following definitions: V = the applied electric potential, d = the dielectric thickness, E = the vector electric field intensity, E = V/d, k' = dielectric constant, D = the vector electric flux density, and D = k'E. The development of a heating model is begun by using an expression developed in [10] which describes how the vector electric field intensity, El, in one dielectric media, k~, is dependent on a vector electric field intensity, E2, in a second dieletric media k~. For the purpose of this discussion, let k~ be greater than k~, then

Heavy ion beam probe energy analyzer for measurements of plasma potential fluctuations

Abstract: The operation of a Proca and Green type 30° parallel plate electrostatic energy analyzer is modeled in a new manner that permits high‐resolution heavy ion beam probe measurements of fluctuating plasma potential. Systematic calibration procedures permit detection of potential changes smaller than 0.01% of the probing beam energy at frequencies up to a megahertz. Most recent applications of beam probes have made use of this new capability.

Noncoplanar magnetic fields, shock potentials, and ion deflection

Abstract: A puzzling aspect of the structure of collisionless shock waves concerns the existence of a component of magnetic field that lies out of the coplanarity plane in the transition layer. It is shown that the existence of this component may be understood and its value calculated from a simple expression derived from first principles. The value of this field component may be derived without reference to the transformation properties of the fields or the changes in energy that the ions and electrons undergo on traversing the electrostatic potential of the shock interface.

Pressure control for the fuel system of an internal combustion engine

Abstract: A fuel system for an internal combustion engine wherein the fuel pressure is maintained within a desired predetermined range. An electric fuel pump supplies fuel at a pressure proportional to the electric power applied to the pump. Control circuitry compares a voltage signal corresponding to the fuel pressure within a predetermined voltage range corresponding to a predetermined fuel pressure range. When the voltage signal is below the predetermined voltage range, the electric power applied to the electric pump is increased; when the voltage signal is above the predetermined voltage range, the electric power applied to the electric pump is decreased. The control circuit may further increase or decrease the electric power at a predetermined rate.

Femtosecond streak tube

Abstract: A new design concept of a streak tube is conceived to obtain femtosecond temporal resolution. It is based on four factors as follows. First, in order to decrease the photoelectron transit time spread, one must investigate how high electric field is practically usable near the photocathode. It is found that the value can be increased up to ∼6 kV/mm and this is used as a design value. Second, as the other method to decrease the photoelectron transit time spread, an electromagnetic focusing method must be adopted because there is no region similar to the focusing section of an electrostatic focusing type where the electric potential is low. Third, the focusing magnetic field should be located in a limited region to obtain a very high sweep speed of ∼2×108 m/s. And fourth, in the operation, a special readjustment of the focusing magnetic field should be performed to compensate the photoelectron beam defocusing caused by the very rapid transient deflection field. Based on the above, a new streak tube has been designed, manufactured, and tested. The limiting temporal resolution of ∼400 fs has been predicted theoretically and the value less than ∼500 fs has been experimentally produced.

The molecular dynamics simulation of water clusters

Abstract: Water clusters containing 15, 27, and 64 molecules have been investigated by molecular dynamics at various temperatures. Profiles for atomic density, the polarization vector, local energy, and the pressure tensor have been obtained. A simple way to calculate the local electric potential as an average Coulomb potential at a sphere of radius r has been offered that resulted in more reliable data. The orientation of water molecules at the surface has been studied in detail. The most probable position of the molecular plane has been shown to be perpendicular and of the dipole vector parallel to the surface. Two models of water molecules with different values of the quadrupole component Qzz along the dipole axis have been considered. The theoretical prediction of the proportionality of the surface potential and Qzz has been confirmed.

Induced electric charge of the neutrino in a dispersive medium

Abstract: It is shown that in a a dispersive medium the neutrino acquires an induced electric charge, due to medium polarization, irrespective of the neutrino rest-mass (mν is arbitrary). The neutrino electric formfactor (calculated in the paper) takes into account the additive contribution of vacuum polarization and determines, in the Poisson equation, the density of electric charge induced in a plasma by a probe neutrino processing in vacuum only a weak charge. According to the Gauss theorem, the total electric charge, called by the electric charge of a probe neutrino, is not zero in a medium and is determined by the vector weak coupling constant GF (1 + 4 sin2θw)/√2 (from the total amplitude of ve scattering) and by the plasma Debye radius. Here GF is the Fermi constant and θw, the Weinberg angle1). Explicitly this charge is eνind=-eGF(1+4 sin2θw)/√2e2r2 D. The global gauge invariance is not violated in this case, and the total electric charge of the electroneutral (on the whole) system is zero. As an application, we consider polarization losses of the zero-mass neutrino in a medium, which are compared with some other kinds of neutrino radiative losses.

D-c electric-potential method applied to thermal/mechanical fatigue crack growth

Abstract: The paper describes a d-c electric potential system for measuring crack length under thermal/mechanical fatigue-crack-growth (TMFCG) test conditions. A programmable d-c current supply and precision multimeter produce reliable electric-potential readings. H.H. Johnson's formula is used to calculate crack length from electric potential for the center-crack-tensionM(T) geometry. Calibration constants for the formula are determined from an initial optical crack-length measurement. The resolution of the system is 1.0 microvolt which corresponds to a crack extension of approximately 0.002 mm for the center-crack-tension geometry using a current of 10.00 amps. Good crack-length accuracy and low data scatter are achieved by taking special precautions to minimize or eliminate errors in potential measurement due to thermal effects. Material resistivity changes are identified as the cause of short and long term changes in the measured electric potential for uncracked specimens. Crack-length accuracy is discussed in terms of short-term scatter and longterm drift.

Ferroelectric liquid crystal cells

Abstract: Relaxation effects of a uniform mode ferroelectric liquid crystal cell with obliquely evaporated alignment layers, characterised by the reduction in apparent tilt angle consequent upon removal of a switching stimulus, are eliminated by a poling treatment that typically comprises the application of a relatively low frequency (c. 500Hz) relatively large amplitude (c. 60 volts peak-to-peak) electric potential difference across the thickness of the liquid crystal layer.

Magnetic field‐aligned electric field acceleration and the characteristics of the optical aurora

Abstract: The long-recognized association of brighter aurora with more deeply penetrating, and hence more energetic, electrons is examined. Using the Knight (1973) relation between the magnetic-field-aligned current density and potential drop (derived from the theory of single-particle motion in the presence of a magnetic-field-aligned electric field), an approximate expression relating the energy flux of the precipitating electrons over discrete aurora and the mean particle energy is derived. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red/blue ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora.

Photo ion spectrometer

Abstract: A method and apparatus for extracting for quantitative analysis ions of selected atomic components of a sample A lens system is configured to provide a slowly diminishing field region for a volume containing the selected atomic components, enabling accurate energy analysis of ions generated in the slowly diminishing field region The lens system also enables focusing on a sample of a charged particle beam, such as an ion beam, along a path length perpendicular to the sample and extraction of the charged particles along a path length also perpendicular to the sample Improvement of signal to noise ratio is achieved by laser excitation of ions to selected autoionization states before carrying out quantitative analysis Accurate energy analysis of energetic charged particles is assured by using a preselected resistive thick film configuration disposed on an insulator substrate for generating predetermined electric field boundary conditions to achieve for analysis the required electric field potential The spectrometer also is applicable in the fields of SIMS, ISS and electron spectroscopy

Interaction of the plasma sheet with the lobes of the Earth's magnetotail

Abstract: Simulations with a two-dimensional electromagnetic particle code in the Darwin approximation are used to investigate mechanisms for the generation of potential differences across magnetic field lines in the magnetotail region earthward of the neutral line. Two mechanisms are found. One is the electrostatic ion cyclotron wave generated by counterstreaming ion beams. Another and more powerful process depends on the generation of magnetic fields, By, normal to the noon-midnight meridian plane of the magnetosphere. Currents carried primarily by electrons generate a By. This causes the magnetic field to loop out of the meridian plane in a direction such that electrons lose potential energy and gain kinetic energy in the convection electric field. Much of the kinetic energy goes into motion in the x direction in the region of the neutral sheet. The current carried by the electrons is in a direction to intensify the By field. The electrons are also compressed, resulting in a negative space charge in the region of the electron current jet. This process therefore generates electrostatic potential variations across magnetic field lines. Contrary to prior expectations, there is no evidence that tearing mode instabilities or conducting boundaries play any prominent role in the generation of the potentials.

Microfields in stroboscopic voltage measurements via electron emission. I. Response function of the potential energy

Abstract: It is shown that the response of the electrostatic potential on voltage changes at a conducting line in a microscopic structure is fully described by a pure geometry function. This function is determined by the special microscopic character of the conducting line and by the screening due to neighboring conductors. It already allows a qualitative description of transit‐time and cross‐talk effects in voltage measurements via electron emission or electro‐optical sampling, and provides a basis for the quantitative calculation of these effects. The geometry dependence of this function will be analyzed in detail.

Spiky parallel dc electric fields in the aurora

Abstract: A series of computer simulations is reported on the detailed shape and distribution of the dc electric fields that result when an external potential drop is applied to a plasma with strong low-frequency turbulence. The structures are shown to depend on the size of the potential drop and upon the shape of the ion turbulence. For the type of turbulence expected for current driven electrostatic ion cyclotron waves the dc fields seen in the simulations are in good agreement with those observed in the aurora.