On the appearance of ions near the charged surface of an intensely evaporating electrolyte

Two mechanisms behind the appearance of ions in the vapor-gas sheath around a hot electrode immersed in an electrolyte, electrolyte surface instability against the surface charge and field evaporation, are considered. It is shown that the concept of the thermally activated field evaporation of ions provides a better approximation to experimental conditions. A dispersion relation for capillary-barogravity waves at the electrolyte-saturated vapor charged interface is derived. Critical conditions for the instability of these waves are found.     

Ion neutralization model of galvanic cell operation

An ion neutralization model of galvanic cell operation is developed. The electromotive force in this model is formed at the electrode interface owing to the different energies of conduction electrons bonds in electrodes. Constant continuous current is ensured by neutralization of electrolyte ions on the electrode surface. Charge capacity is determined by electrolyte volume and the rate of electrochemical degradation of electrode surfaces in the electrolyte. Potential energy of electrolyte ions serve as the energy source.     

Self-consistent structure of a dc discharge with a closed hall current in crossed electric and magnetic fields

A dc glow discharge with a closed Hall current in crossed electric and magnetic fields in helium is investigated. It is shown that the main features of an unmagnetized dc discharge [1] (such as the separation of the discharge into a space charge sheath and a quasineutral plasma, the formation of a cathode fall region and a negative glow, the appearance of a region with a reversed electric field producing a potential well for low-energy electrons and resulting in the formation of a Faraday dark space, and the formation of three pronounced groups of electrons in the electron distribution function) are also retained in a discharge in crossed fields. It is found that the sheath length is almost independent of the magnetic field, while the length of the negative glow region decreases appreciably with increasing magnetic field. The measured electron distribution function agrees well with the nonlocal theory, according to which the current in the Faraday dark space is carried by the intermediate electrons that are not trapped in the potential well and the energies of which are lower than the first excitation energy.     

Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

Effect of driving frequency on electron heating in capacitively coupled RF argon glow discharges at low pressure

A one-dimensional(1D) fluid model on capacitively coupled radio frequency(RF) argon glow discharge between parallel-plates electrodes at low pressure is established to test the effect of the driving frequency on electron heating. The model is solved numerically by a finite difference method. The numerical results show that the discharge process may be divided into three stages: the growing rapidly stage, the growing slowly stage, and the steady stage. In the steady stage,the maximal electron density increases as the driving frequency increases. The results show that the discharge region has three parts: the powered electrode sheath region, the bulk plasma region and the grounded electrode sheath region. In the growing rapidly stage(at 18 μs), the results of the cycle-averaged electric field, electron temperature, electron density, and electric potentials for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are compared, respectively. Furthermore,the results of cycle-averaged electron pressure cooling, electron ohmic heating, electron heating, and electron energy loss for the driving frequencies of 3.39, 6.78, 13.56, and 27.12 MHz are discussed, respectively. It is also found that the effect of the cycle-averaged electron pressure cooling on the electrons is to "cool" the electrons; the effect of the electron ohmic heating on the electrons is always to "heat" the electrons; the effect of the cycle-averaged electron ohmic heating on the electrons is stronger than the effect of the cycle-averaged electron pressure cooling on the electrons in the discharge region except in the regions near the electrodes. Therefore, the effect of the cycle-averaged electron heating on the electrons is to "heat" the electrons in the discharge region except in the regions near the electrodes. However, in the regions near the electrodes, the effect of the cycle-averaged electron heating on the electron is to "cool" the electrons. Finally, the space distributions of the electron pressure cooling the electron ohmic heating and the electron heating at 1/4 T, 2/4 T, 3/4 T, and 4/4 T in one RF-cycle are presented and compared.     

Modeling of the effect of a dielectric film on the electrode surface upon the discharge glow-to-arc transition

It is shown that when a dielectric film is on the cathode, the current density in the discharge sheath is raised due to the field emission of electrons from the cathode metal substrate under the effect of the electric field generated in the film by the surface charge that accumulates on it. This results in more intense cathode heating and faster glow-arc transitions.     

Electric strength of an accelerating gap in a plasma source of charged particles

An investigation is performed on the electric strength of accelerating gaps of plasma sources of electrons and ions in the presence of beam plasma behind the accelerating electrode. For the bipolar mode, when the gas ionization in the accelerating gas may be ignored, the conditions of bridging the gap of plasma discharge of a source with beam plasma and of disruption of emission current are found.     

PIC Simulation of the Motion of Plasma around Ion Sensitive Probes

The current‐voltage characteristics, the structure of electric potential around an ion sensitive probe and the particle flux on the ion collector have been simulated by the two dimensional particle‐in‐cell code (Berkeley Code). Concerning the separate mechanism of ions and electrons on the probe, the importance of electric potential profile around the electrode was pointed out. It was found that the E × B drift motion of electrons moving along the equipotential surface plays an essential role in the ISP measurement.     

Simulations of plasma formation in the cathode sheath of an efficient excimer lamp

Based on the available experimental data and computer simulations, analytical approximations of the quantities characterizing electron multiplication in the cathode sheath are proposed. The critical electric field is found above which runaway electrons are observed. Using the approximations proposed, the dependences of the plasma parameters (the electron and ion densities and currents and the electric field strength) on the distance from the cathode are analyzed. Simple formulas for the total current, the cathode sheath thickness, and the cathode potential drop as functions of the electric field on the cathode surface are derived.     

Generation of an extraction electric field in an electromembrane ion source

A mechanism of generating an extraction field in an ion source in which a polymer track membrane with nanodimensional channels is used as an environment-vacuum interface is considered. A high electric field necessary for the effective extraction of ions from a liquid on the membrane surface into the gas phase is maintained by charging the vacuum surface of the membrane. Charging is provided by oppositely charged secondary ions resulting from the disintegration of primary cluster ions on the extraction electrode. A decrease in the source current observed when the vacuum surface discharges counts in favor of this mechanism. The extracted ion energy distribution in the neighborhood of the extraction zone is obtained by the retarding potential method. Various aspects of ion beam formation in the membrane ion source are discussed.     

Effects of gas pressure on plasma characteristics in dual frequency argon capacitive glow discharges at low pressure by a self-consistent fluid model

A self-consistent fluid model for dual radio frequency argon capacitive glow discharges at low pressure is established.Numerical results are obtained by using a finite difference method to solve the model numerically, and the results are analyzed to study the effect of gas pressure on the plasma characteristics. It shows that when the gas pressure increases from 0.3 Torr(1 Torr = 1.33322×10~2 Pa) to 1.5 Torr, the cycle-averaged plasma density and the ionization rate increase;the cycle-averaged ion current densities and ion energy densities on the electrodes electrode increase; the cycle-averaged electron temperature decreases. Also, the instantaneous electron density in the powered sheath region is presented and discussed. The cycle-averaged electric field has a complex behavior with the increasing of gas pressure, and its changes take place mainly in the two sheath regions. The cycle-averaged electron pressure heating, electron ohmic heating, electron heating, and electron energy loss are all influenced by the gas pressure. Two peaks of the electron heating appear in the sheath regions and the two peaks become larger and move to electrodes as the gas pressure increases.     

Features of vapor transfer in the evaporation of liquids from capillaries in an inhomogeneous temperature field

The process of vapor transfer in conditions of linear change in temperature along the channel is considered. The excess pressure of the vapor-gas mixture above the liquid meniscus is calculated as a function of the capillary radius and temperature. Expressions are found for the vapor flux and time of evaporation. It is shown that these expressions are in good agreement with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 65–70, May, 1984.     

Fluid Model of a Sheath Formed in Front of an Electron Emitting Electrode Immersed in a Plasma with Two Electron Temperatures

The formation of a sheath in front of a negatively biased electrode (collector) that emits electrons is studied by a one‐dimensional fluid model. Electron and ion emission coefficients are introduced in the model. It is assumed that the electrode is immersed in a plasma that contains energetic electrons. The electron velocity distribution function is assumed to be a sum of two Maxwellian distributions with two different temperatures, while the ions and the emitted electrons are assumed to be monoenergetic. The condition for zero electric field at the collector is derived. Using this equation the dependence of electron and ion critical emission coefficients on various parameters ‐ like the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons ‐ is calculated for a floating collector. A modification of the Bohm criterion due to the presence of hot and emitted electrons is also given. The transition between space charge limited and temperature limited electron emission for a current‐carrying collector is also analyzed. The critical potential, where this transition occurs, is calculated as a function of several parameters like the Richardson emission current, the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The cavitation induced Becquerel effect and the hot spot theory of sonoluminescence

Over 150 years ago, Becquerel discovered the ultraviolet illumination of one of a pair of identical electrodes in liquid water produced an electric current, the phenomenon called the Becquerel effect. Recently, a similar effect was observed if the water surrounding one electrode is made to cavitate by focused acoustic radiation, which by similarity is referred to as the cavitation induced Becquerel effect. The current in the cavitation induced Becquerel effect was found to be semi-logarithmic with the standard electrode potential that is consistent with the oxidation of the electrode surface by the photo-decomposition theory of photoelectrochemistry. But oxidation of the electrode surface usually requires high temperatures, say as in cavitation. Absent high bubble temperatures, cavitation may produce vacuum ultraviolet (VUV) light that excites water molecules in the electrode film to higher H(2)O(*) energy states, the excited states oxidizing the electrode surface by chemical reaction. Solutions of the Rayleigh-Plesset equation during bubble collapse that include the condensation of water vapor show any increase in temperature or pressure of the water vapor by compression heating is compensated by the condensation of vapor to the bubble wall, the bubbles collapsing almost isothermally. Hence, the cavitation induced Becquerel effect is likely caused by cavitation induced VUV light at ambient temperature.     

Investigating the charge states of atoms and the directed movement of electrons in a metal

Research into the interaction of ions with a surface and the formation of contact potential difference, galvanic electricity, and alternating current shows that the directed movement of electrons (an electric current) occurs in a metal in the absence of an electric field of the poles of the source of electrical energy in the metal and against the field of charged electrodes. It is assumed that the movement of electrons in the metal occurs under the action of the Coulomb interactions of valence electrons associated with atoms.     

Particle simulations of radio-frequency glow discharges

Particle-in-cell (PIC) simulations are used to study the structure of radio-frequency (RF) glow discharges in helium between parallel-plate electrodes. The authors have examined a range of conditions and report on a variety of observed phenomena. Comparisons to experiment and analytical models are made, when possible. The differences between discharges in which secondary electrons play a key role in sustaining the discharge and those in which secondary electrons are unimportant are examined in three cases which illustrate the importance of the discharge-sustaining mechanisms. Electron-energy distributions are found to be, in general, non-Maxwellian, with shapes that depend in complex ways on discharge conditions. In the absence of secondary electron emission, electron heating in the sheath regions of the discharge is enhanced at higher voltages compared to ohmic heating in the bulk of the plasma. Fast electrons accelerated by the advancing sheath can carry a substantial fraction of the conduction current in the bulk of the discharge, reducing the effective bulk ohmic heating of electrons. Ion-energy distributions at electrode surfaces have been predicted and are compared to experimental measurements     

Formation and dispersion of an electrolyte film on an ice electrode melting as a result of Joule heat evolution

The thickness of a liquid film (≈3.6 μm) forming on an ice electrode is determined by solving the Stefan problem. The electrode melts as a result of Joule heat evolution when the current passes through it. The temperature distributions in the film and ice substrate are found. The radius of curvature of emitting asperities formed as a result of film instability against the surface charge is found to be ≈40 nm. This value provides the intense field evaporation of individual ions and ionic clusters from the top of the asperities at a potential difference of ≈100 V.     

Onset of breakdown and formation of cathode spots

When an increasing diode voltage is applied, enhanced field emission of electrons begins from a growing number of small spots or whiskers on the cathode surface. This stimulates desorption of weakly bound adsorbates from the surface of a whisker. As the diode voltage increases, the 100-V equipotential surface moving toward the cathode is met by the desorbed neutrals moving away from the cathode, resulting in sharp risetime for the onset of ionization of desorbed neutrals by field-emitted electrons. Positive ions produced in the ionization region a few microns from the electron emitting spot are accelerated back to it. This bombardment leads to surface heating of the spot. The onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot leads to pressure and electric field distributions that ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions result in the explosive formation of cathode spot plasma     

Structure of the direct-current magnetron discharge in neon at different polarities of the electrode system

A dc magnetron discharge in neon is studied at different polarities of the electrode system. It is found that the electron energy distribution function is composed of three groups of electrons: fast electrons accelerated by the strong field of a cathode sheath, slow electrons confined in a potential well due to the space-charge field, and intermediate-energy electrons. It is shown that the energy distribution of the confined electrons is a Maxwell-Boltzmann distribution function, whereas the energy distribution of the intermediate electrons is typical of electron diffusion at a constant total energy. The measured values of the cathode sheath thickness depend on the source polarity.     

负离子对等离子体鞘层特性影响的数值研究

构造强电负性等离子体中平板电极处鞘层形成的模型,使用流体动力学方程组研究等离子体鞘层.得到空间电势、空间静电荷分布和鞘层宽度作为距离函数的数值结果.结果表明,强电负性鞘层中,在等离子体区和鞘层区之间几乎没有过渡的预鞘区,在靠近极板附近鞘层里的电子、负离子和正离子的分布形成一个纯正离子鞘区.在靠近鞘边附近,空间静电荷密度分布有一个很尖的峰.发现同电正性情形相比,强电负性鞘层的宽度要窄很多,空间电势下降得快得多.