Electrostatic Potential Drop Research Articles

Interaction of O vacancies and domain structures in single crystalBaTiO3: Two-dimensional ferroelectric model

Two-dimensional simulations on the interactions of oxygen vacancies and different domain structures in barium titanate single crystal were carried out using the phase field method. The evolution of the spontaneous polarizations and oxygen vacancies was coupled through Maxwell's equation. The results showed that two barriers near the electrodes existed in both the 90\ifmmode^\circ\else\textdegree\fi{} and 180\ifmmode^\circ\else\textdegree\fi{} domain structures. It has also been observed that while an intrinsic electrostatic potential drop across the 90\ifmmode^\circ\else\textdegree\fi{} domain wall created the electric fields which drove the electrons and oxygen vacancies aggregate on the different sides of the domain wall, the 180\ifmmode^\circ\else\textdegree\fi{} domain wall had insignificant interaction with the potential, and no electron or vacancy accumulation in 180\ifmmode^\circ\else\textdegree\fi{} domain structure was observed. Polarization charge density is believed to be the origin of this difference.

A Simulation of the Effects of Intra‐Cloud Lightning Discharges on the Charges and Electrostatic Potential Distributions in a Thundercloud

AbstractThis paper extends the use of Stochastic Lightning Model in a simulation experiment, in which 12.5 and 250 m resolution simulations of intra‐cloud (IC) lightning discharges within a two dimensional numerical thunderstorm model have been performed, and gives the comparing analyses of the simulated results with the observed data. The results show: (1) During an IC flash discharge, more complex distribution of the space charges in thundercloud is caused by the induced charges along the channels, leading to a sudden decrease in the maxima of electrostatic potential and potential gradient as well as consume of electrostatic energy of thundercloud. The electrostatic potential and field strength drop down respectively to ±30 MV and ±20 kV·m−1 where the flash channels passed, and consumed electrostatic energy of thunderstorm for a positive or negative IC flash is about 107 ~ 1010 J. (2) As contrasted with transient IC discharges, the charge neutralization in thundercloud is a slow process with a relaxation time of about 14~40 secs. The relaxation time is confirmed by turbulence exchanging, advection transporting and gravitational sedimentation, during which the induced charge along leader channels drops down to 50%. There are some excess of induced channel charges to be not neutralized, which contributes to rebuilding new distribution of space charges and electrostatic potential.

On the current sheet model with κ distribution

The present paper (re)derives current sheet equilibrium solutions on the basis of the so-called κ distribution functions for the particles. The present work builds upon a recent paper [W.-Z. Fu and L.-N. Hau, Phys. Plasmas 12, 070701 (2005)], where the authors formulated the equilibrium current sheet model with the κ distribution. According to their work, however, the global temperature profile monotonically increases in the asymptotic regime. In the present paper it is shown that the presence of a finite stationary background population of the particles arrests the unlimited increase of the global temperature profile in the asymptotic limit. The present paper further extends the analysis by considering a current sheet model where the electron current is embedded within a thicker ion current layer, and where there exists a weak electrostatic potential drop across the current sheet.

Investigation of the Electric Field in TiO2/FTO Junctions Used in Dye-Sensitized Solar Cells by Photocurrent Transients

We have investigated the electrostatic potential distribution in compact and nanoporous TiO2 films, deposited on conducting F-doped SnO2 substrate (FTO), which are used in dye-sensitized solar cells. The TiO2 films were immersed into aqueous electrolyte and excited from the FTO side by light pulses of a N2 laser while the current response was measured as a function of time. The measurements were carried out as a function of the pH value of the electrolyte and at different electrostatic potentials. For compact TiO2 films, the sign of the transient current at short response times changed when the applied electrostatic potential or the pH value was decreased. This was not observed for mesoporous TiO2 films directly deposited onto the FTO substrate without a compact TiO2 layer. We interpret the results in terms of a macroscopic electric field across the compact layer which is changed by the applied potential or the pH of the electrolyte. In contrast, measurements on mesoporous TiO2 films indicate that the contact region is mainly field-free, and we explain our results by a very sharp electrostatic potential drop within the first layer of particles at the TiO2/FTO interface.

Ballistic Transport and Electrostatics in Metallic Carbon Nanotubes

We calculate the current and electrostatic potential drop in metallic carbon nanotube wires self-consistently by solving the Green's function and electrostatics equations in the ballistic case. About one-tenth of the applied voltage drops across the bulk of a nanowire, independent of the lengths considered here. The remaining nine-tenths of the bias drops near the contacts, thereby creating a nonlinear potential drop. The scaling of the electric field at the center of the nanotube with length (L) is faster than 1/L (roughly 1/L/sup 1.25-1.75/). At room temperature, the low bias conductance of larger-diameter nanotubes is larger than 4e/sup 2//h due to occupation of noncrossing subbands. The physics of conductance evolution with bias due to Zener tunneling in noncrossing subbands is discussed.

Self-consistent quasineutral dust structures

Distributions of the dusty plasma parameters (electron, ion, and dust densities; dust grain charge; and ion drift velocity) in quasineutral dust structures whose dimensions are much greater than the mean free path of the ions in their interactions with neutral particles are calculated numerically under conditions such that ionization sources are located outside the structures. Planar, cylindrical, and spherical structures are investigated. It is shown that static equilibrium structures are governed by a single (basic) parameter: the electrostatic potential drop between the center of the structure and its boundary. It is found that the maximum value of the basic parameter (in energy units) does not exceed the electron temperature. The basic parameter also determines the total number of dust grains in the structure and the power of external ionization sources that are necessary to sustain this structure. The fact that the basic parameter varies within a limited range allows one to consider all the possible structures with a given dimensionality (planar, cylindrical, and spherical).

Density Profiles in a Quantum Coulomb Fluid Near a Hard Wall

Equilibrium particle densities near a hard wall are studied for a quantum fluid made of point charges which interact via Coulomb potential without any regularization. In the framework of the grand-canonical ensemble, we use an equivalence with a classical system of loops with random shapes, based on the Feynman-Kac path-integral representation of the quantum Gibbs factor. After systematic resummations of Coulomb divergences in the Mayer fugacity expansions of loop densities, there appears a screened potential $\phi$. It obeys an inhomogeneous Debye-H\"uckel equation with an effective screening length which depends on the distance from the wall. The formal solution for $\phi$ can be expanded in powers of the ratios of the de Broglie thermal wavelengths $\laa$'s of each species $\alpha$ and the limit of the screening length far away from the wall. In a regime of low degeneracy and weak coupling, exact analytical density profiles are calculated at first order in two independent parameters. Because of the vanishing of wave-functions close to the wall, density profiles vanish gaussianly fast in the vicinity of the wall over distances $\laa$'s, with an essential singularity in Planck constant $\hbar$. When species have different masses, this effect is equivalent to the appearance of a quantum surface charge localized on the wall and proportional to $\hbar$ at leading order. Then, density profiles, as well as the electrostatic potential drop created by the charge-density profile, also involve a term linear in $\hbar$ andwhich decays exponentially fast over the classical Debye screening length $\xid$. The corresponding contribution to the global surface charge exactly compensates the charge in the very vicinity of the surface, so that the net electric field vanishes in the bulk, as it should.

Kinetic theory of the Alfvén wave acceleration of auroral electrons

Recent observations have indicated that in addition to the classical “inverted‐V” type electron acceleration, auroral electrons often have a field‐aligned distribution that is broad in energy and sometimes shows time dispersion indicating acceleration at various altitudes up the field line. Such acceleration is not consistent with a purely electrostatic potential drop and suggests a wave heating of auroral electrons. Alfvén waves have been observed on auroral field lines carrying sufficient Poynting flux to provide energy for such acceleration. Calculations based on the linear kinetic theory of Alfvén waves indicate that Landau damping of these waves can efficiently convert this Poynting flux into field‐aligned acceleration of electrons. At high altitudes along auroral field lines that map into the plasma sheet boundary layer (PSBL), the plasma gradients are relatively weak and the local kinetic theory can describe this wave–particle interaction. At lower altitudes, the gradient in the Alfvén speed becomes significant, and a nonlocal description must be used. A nonlocal theory based on a simplified model of the ionospheric Alfvén resonator (IAR) is presented. For a given field‐aligned current (FAC), the efficiency of the wave–particle interaction increases with the ratio of the thermal velocity of the electrons to the Alfvén speed at high altitudes. These calculations indicate that wave acceleration of electrons should occur at and above the altitude where the quasi‐static potential drops form.

Nonlinear theory for dust voids in plasmas

A nonlinear theory for dust voids in plasmas is presented. Specifically, it is shown that dust voids are caused by a large potential drop associated with the ion holes and double layers in plasmas. For this purpose, we solve cylindrically symmetric Poisson's equation with a Boltzmann electron density distribution, a modified Boltzmann dust density distribution, and an ion response consisting of trapped and untrapped ion velocity distributions in the self-consistent plasma potential. Numerical results exhibit significant holes in the ion distribution function, accompanied by a large electrostatic potential drop and charge density double layers. We discuss the underlying physics of dust voids in terms of our self-consistent steady state solutions of the Vlasov–Poisson system of equations.

Contactless method for studying the parameters of the electrode region of an RF discharge

A nonintrusive contactless method for studying the parameters of the electrode region of a capacitive low-pressure RF discharge is proposed. The method involves the measurements of dc and ac electric voltages at the elements of the discharge circuit with subsequent calculations of both the electrostatic potential drop across the electrode sheath and the sheath thickness by using relations derived in the paper. For a collisionless electrode sheath, the density of the positive-ion current onto the electrode and the charge density at the plasma boundary are determined. It is shown experimentally that the method can be successfully applied to studying capacitive RF discharges with inner or outer electrodes.

Characterizing an implanted Si/Si p–n junction with lower doping level by combined electron holography and focused-ion-beam milling

A Si/Si p–n junction with very low doping level was made via a standard device fabrication process by implanting As ions at 25 keV into a p-type Si substrate with a boron concentration of 1015 cm−3, followed by heat annealing at 1035 °C for 33 s. To characterize this junction, a pair of 45° wedge-shape cross sections was prepared simultaneously by focused-ion-beam milling and examined using off-axis electron holography. The reconstructed phase images clearly show the phase shift induced by the electrostatic potential drop across the p–n junction, indicating that the junction has been mapped successfully. Quantitative measurements from the phase images give the potential values of 12.21±0.40 and 11.50±0.27 V, respectively, for the n- and p-type sides of the junction, 0.71±0.05 V for the potential drop across the junction and 50.10±3.88 nm for the total electric dead layer thickness. This work demonstrates that electron holography is a powerful technique for characterizing low dopant level p–n junctions in practical devices.

On electrostatic acceleration of plasmas with the Hall effect using electrode shaping

Resistive magnetohydrodynamics (MHD) is used to model the electromagnetic acceleration of plasmas in coaxial channels. When the Hall effect is considered, the inclusion of resistivity is necessary to obtain physically meaningful solutions. In resistive MHD with the Hall effect, if and only if the electric current and the plasma flow are orthogonal (J⋅U=0), then there is a conserved quantity, in the form of U2/2+w+eΦ/M, along the flow, where U is the flow velocity, Φ is the electric potential, w is the enthalpy, and M is the ion mass. New solutions suggest that in coaxial geometry the Hall effect along the axial plasma flow can be balanced by proper shaping of conducting electrodes, with acceleration then caused by an electrostatic potential drop along the streamlines of the flow. The Hall effect separation of ion and electron flow then just cancels the electrostatic charge separation. Assuming particle ionization increases with energy density in the system, the resulting particle flow rates (Jp) scales with accelerator bias (Vbias) as Jp∝Vbias2, exceeding the Child–Langmuir limit. The magnitude of the Hall effect (as determined by the Morozov Hall parameter, Ⅺ, which is defined as the ratio of electric current to particle current) is related to the energy needed for the creation of each ion–electron pair.

Counterstreaming electrons in the near vicinity of the Moon observed by plasma instruments on board NOZOMI

The NOZOMI spacecraft approached the Moon at an altitude of about 2800 km at 0734 UT on December 18, 1998. Around the time of closest approach, a plasma instrument Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ESA) detected non‐solar‐wind electrons in addition to the normal solar wind component. From the characteristics of the electron distribution function, we can categorize these events into two types: (1) backstreaming electrons exhibiting a velocity distribution similar to that of the solar wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons that are thermalized and have a flux comparable to or dominating that of the solar wind electrons. We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigation of the velocity distribution function of the electrons and the magnetic field orientation, we concluded that their origins are (1) the lunar wake region, where the electrostatic potential drop associated with ambipolar plasma expansion reflects the solar wind electrons and produces backstreaming electrons of category 1, and (2) the terrestrial bow shock, where the electrons are thermalized downstream and escaping electrons are of category 2.

Formation of electrostatic potential drops in the auroral zone

In order to examine the self-consistent formation of large-scale quasi-static parallel electric fields in the auroral zone on a micro/meso scale, a particle in cell simulation has been developed. The code resolves electron Debye length scales so that electron micro-processes are included and a variable grid scheme is used such that the overall length scale of the simulation is of the order of an Earth radii along the magnetic field. The simulation is electrostatic and includes the magnetic mirror force, as well as two types of plasmas, a cold dense ionospheric plasma and a warm tenuous magnetospheric plasma. In order to study the formation of parallel electric fields in the auroral zone, different magnetospheric ion and electron inflow boundary conditions are used to drive the system. It has been found that for conditions in the primary (upward) current region an upward directed quasi-static electric field can form across the system due to magnetic mirroring of the magnetospheric ions and electrons at different altitudes. For conditions in the return (downward) current region it is shown that a quasi-static parallel electric field in the opposite sense of that in the primary current region is formed, i.e., the parallel electric field is directed earthward. The conditions for how these different electric fields can be formed are discussed using satellite observations and numerical simulations.

Auroral electron acceleration by Alfvén waves and electrostatic fields

We present a two‐dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfvén waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfvén waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 μAm−2. We also find that the electrostatic potential drop is proportional to the square of the current density.

Application of Multiple Linear Regression and Extended Principal-Component Analysis to Determination of the Acid Dissociation Constant of 7-Hydroxycoumarin in Water/AOT/Isooctane Reverse Micelles

The apparent pKa of dyes in water-in-oil microemulsions depends on the charge of the acid and base forms of the buffers present in the water pool. Extended principal-component analysis allows the precise determination of the apparent pKa and of the spectra of the acid and base forms of the dye. Combination with multiple linear regression increases the precision. The pKa of 7-hydroxycoumarin (umbelliferone) was spectrophotometrically measured in a water/AOT/isooctane microemulsion in the presence of a series of buffers carrying different charges at various different water/surfactant ratios. The spectra of the acid and base forms of the dye in the microemulsion are very similar to those in bulk water in the presence of Tris and ammonia. The presence of carbonate changes somewhat the spectrum of the acid form. Results are discussed taking into account the profile of the electrostatic potential drop in the water pool and the possible partition of umbelliferone between the aqueous core and the surfactant. The pKa values corrected for these effects are independent of w0 and are close to the value of the pKa in bulk water.

Observable Electric Potential and Electrostatic Potential in Electrochemical Systems

The role of the electric potential in the description of transport processes in electrochemical systems is critically analyzed. Since the electrostatic potential drop between two parts of a system which differ in temperature, pressure, or concentration cannot be measured, an observable electric potential (OEP) is defined from the potential difference measured between the electrodes reversible to one of the ion constituents in equilibrium with the system. The OEP formalism is compared to the Nernst−Planck equations (NPE) formalism. The comparative study is applied to the mass transport process that occurs in a liquid junction of a binary electrolyte solution when the electric current density is zero. The potential profiles are shown for the OEP and the electrostatic potential, the latter being calculated for different definitions of the activity coefficient γ-. It is concluded that the OEP formalism presents clear benefits with respect to the NPE formalism.

Analysis of the ionospheric cross polar cap potential drop using DMSP data during the National Space Weather Program study period

During the National Space Weather Program (NSWP) event study period of November 2–11, 1993 there were three operational DMSP meteorological satellites (F8, F10, and F11) in orbit, each carrying the Special Sensor for Ions, Electrons, and Scintillation (SSIES) plasma instrument package. Ion flow data from these instruments are used to determine the electrostatic potential drop across both the northern and southern polar caps. The magnitude and distribution of the potential are used to characterize the convection patterns present in the polar ionospheres. The results from all three satellites ate presented to show an overall observational history of the potential drop and the convection pattern during the study period. These observational parameters provide crucial inputs and checks to several ionospheric and magnetospheric models being used in this study. Evidence is presented of an unambiguous difference in the cross polar cap potential drop between the two hemispheres for an extended period of time.

Relationship of topside ionospheric ion outflows to auroral forms and precipitation, plasma waves, and convection observed by Polar

The POLAR satellite often observes upflowing ionospheric ions (UFIs) in and near the auroral oval on southern perigee (∼5000 km altitude) passes. We present the UFI features observed by the thermal ion dynamics experiment (TIDE) and the toroidal imaging mass angle spectrograph (TIMAS) in the dusk‐dawn sector under two different geomagnetic activity conditions in order to elicit their relationships with auroral forms, wave emissions, and convection pattern from additional POLAR instruments. During the active interval, the ultraviolet imager (UVI) observed a bright discrete aurora on the duskside after the substorm onset and then observed a small isolated aurora form and diffuse auroras on the dawnside during the recovery phase. The UFIs showed clear conic distributions when the plasma wave instrument (PWI) detected strong broadband wave emissions below ∼ 10 kHz, while no significant auroral activities were observed by UVI. At higher latitudes, the low‐energy UFI conies gradually changed to the polar wind component with decreasing intensity of the broadband emissions. V‐shaped auroral kilometric radiation (AKR) signatures observed above ∼200 kHz by PWI coincided with the region where the discrete aurora and the UFI beams were detected. The latitude of these features was lower than that of the UFI conies. During the observations of the UFI beams and conies, the lower‐frequency fluctuations observed by the electric field instrument were also enhanced, and the convection directions exhibited large fluctuations. It is evident that large electrostatic potential drops produced the precipitating electrons and discrete auroras, the UFI beams, and the AKR, which is also supported by the energetic plasma data from HYDRA. Since the intense broadband emissions were also observed with the UFIs, the ionospheric ions could be energized transversely before or during the parallel acceleration due to the potential drops.

Energetic ion sputtering effects at Ganymede

The energy spectra of magnetospheric ion measurements from the Energetic Particle Detectors (EPD) experiment on Galileo during the G2 encounter with Ganymede were used to estimate the ion sputtering rate from the surface of this largest Galilean moon. A combination of the ion composition and the mass‐dependent sputtering yield shows that a total production rate of about 1026 ‐ 4.6 × 1026 H2O molecules/s can be expected from the precipitation of the energetic ions at the polar region. The emission of the sputtered neutral particles will contribute to the formation of an extended exosphere as well as a distributed gas cloud in the Jovian system. Additional sputtering might be possible if an electrostatic potential drop of a few kV forms leading to the acceleration of ionospheric (oxygen) ions towards Ganymede's surface.