A numerical algorithm for simulation of the electric corona discharge in the triode system

A numerical algorithm is described to calculate the charge density, electric field and corona current distribution in the corona triode. The algorithm employs a hybrid technique based on the Boundary and Finite Element Methods (FEM). FEM is used to determine the electric field because of free space charge produced by the corona discharge. The Boundary Element Method (BEM) is applied for calculating the other component of electric filed as a result of the voltage applied to the electrodes. The Method of Characteristics (MOC) is used to update the space charge density distribution. The characteristic lines are traced backwards from points of the analysed domain to the corona wire. The current density, electric field and space charge density distributions can be controlled by changing the configuration of the system. Results of calculations in a few different cases show the influence of different parameters on the work of the corona triode.     

Theory of the negative anode drop in low-pressure discharges

A kinetic model of the anode region with a negative anode potential drop is proposed, which explicitly takes into account the ratio of the directed velocity of electrons in plasma v ₀ to their thermal speed v _T as a parameter of the electron velocity distribution function. A transcendent equation is derived for determination of the negative anode drop as a function of the ratio v ₀/v _T . It is shown that, unlike the known Langmuir formula, the anode drop remains negative for any value of v ₀/v _T . In the case of small values (v ₀/v _T ? 1), the derived expression asymptotically reduces to the Langmuir formula. It is shown that the dependence of electron concentration on the potential differs from the Boltzmann law. The anode regions of the short high-current vacuum-arc discharge and the classical low-pressure discharge are considered as examples. For the vacuum-arc discharge, the current density distribution and the anode drop distribution over the anode surface under strong current contraction are calculated. For the low-pressure discharge, potential drops are calculated across the region of the inhomogeneous near-anode plasma and the space charge layer.     

Two-Dimensional Model of Plasma Cathode with an Open Plasma Boundary

The paper deals with the solution of two problems for which a two-dimensional distribution is found of the current density and of the electric field in the hollow-cathode plasma. In the first one of these problems, the anode surface of the cavity represents two concentric flat anodes separated by an insulator. This system is used to study the problem of switching the current over from the external to internal anode, with the anode drop being a nonlinear function of current density. The distribution of the current density and of the anode voltage drop on the surfaces of both anodes is calculated for different potentials of the external and internal anodes. In the second problem, the function of internal anode is served by the open boundary of plasma with zero anode drop. The shape of the plasma boundary is determined depending on the plasma cathode geometry and parameters. The singularities are discussed that characterize the extraction of current from the open plasma boundary for different forms of electric discharges.     

Acousto-electric attenuation in a layered structure analyzed by the transmission line technique

In general, theoretical determination of attenuation of ultrasonic waves caused by acousto-electric interaction in any structure is cumbersome and difficult. However, by adopting a transmission line model and using the technique of capacitance perturbation, the analytical process can be greatly simplified. In this paper, we start to analyze a simple structure of an infinitesimally thin semiconductor film deposited on a SAW substrate without an isolation layer in-between. Results for a simplified case are obtained. This analysis is then extended to the general layer structure for both an arbitrary thickness semiconductor layer and an isolation layer. The device parameters are chosen such that the charge relaxation time is much smaller than the duration of the wavelength. Therefore, the electric potential accompanying the acoustic wave is thoroughly screened. Thus, the induced electric field beyond a Debye length or at the top of the semiconductor is negligible. Under this condition, the induced potential as well as the space charge density waves possess a simple form. By integrating the space charge across the thickness of the semiconductor, an effective surface charge density is obtained. With the surface charge density known, one can readily calculate the value of the perturbed capacitance and determine the attenuation constant. The results are in agreement with other analyses.     

空心阴极自脉冲放电的时空演化特性

本文利用圆筒形空心阴极放电结构,研究了自脉冲放电等离子体参量的时空演化特性。在气压为133 Pa的氩气环境下获得了稳定的自脉冲放电,同时将测量得到的极间电压作为输入电势,利用流体模型进行了空心阴极自脉冲放电的数值模拟研究。模拟得到了放电电流、电势、电子密度、电场和净电荷密度的时空分布特性。结果表明等离子体参量随时间成周期性变化。自脉冲现象为由孔外低电流、低径向电场强度、低电子密度、低净电荷密度放电模式向孔内高电流、高径向电场强度、高电子密度、高净电荷密度放电模式的一个往复转换的过程。自脉冲不同阶段放电模式的转换与平均电流有关。当自脉冲放电处于电流峰值时为具有较强空心阴极效应的正常辉光放电模式。自脉冲处于低电流时的放电模式与平均电流有关。当平均电流较低时,自脉冲放电在阳极和孔内阴极附近往复移动;此状态下瞬时电流最低值时放电处于汤生放电模式。当平均电流较高时,自脉冲放电在阴极孔口和孔内阴极附近往复移动;此状态下瞬时电流最低值时阴极附近的阴极鞘层基本形成。     

锂离子电池负极材料钛酸锂的研究进展

锂离子电池作为一种动力能源, 在电动汽车和各种储能系统中有着良好的应用前景。尖晶石结构的钛酸锂(Li₄Ti₅O₁₂)负极材料具有较高的脱嵌锂电位平台、优异的循环稳定性、以及突出的安全性能, 被认为是一种非常有潜力的锂离子电池负极材料, 在锂离子动力电池中具有巨大的发展潜力。然而, 尖晶石型Li₄Ti₅O₁₂存在着本征导电率低, 理论容量小等缺陷, 极大地限制了其规模化应用, 需要进一步改善和提高。本文总结了尖晶石型Li₄Ti₅O₁₂材料在结构形貌、制备方法和性能方面的研究进展, 深入分析和讨论了离子掺杂、碳表面改性和纳米化等改性方法对尖晶石型Li₄Ti₅O₁₂综合电化学性能的改善效果, 并展望了尖晶石型Li₄Ti₅O₁₂作为锂离子电池负极材料未来的发展方向。     

高电压阳极层霍尔离子源三维数值仿真与设计

对于阳极层霍尔离子源装置本身, 宽能、大束流、低气耗及低污染等指标是新一代装置的研发方向。基于拓宽装置放电电压范围的考虑, 针对装置现有结构, 进行了三维数值仿真建模。在现有装置的结构设计基础上, 对装置磁路及结构设计进行修改, 从而实现了对装置工作性能的提高。改进后的阳极层霍尔离子源放电电压范围在原有基础上扩展了50%以上, 放电电压上限可以达到3000 V。改进后的装置在高电压、低气压、小电流的放电条件下运行稳定, 仿真结果对装置的实际加工制造具有一定参考意义。     

Experimental study of the domain of existence of intensive non-self-sustained discharge in crossed fields

A set of experiments was performed on the breadboard model of the Hall thruster with the anode layer. Use of the thermoemission cathodes as the cathode-compensators made it possible to determine the domain of existence of the non-self-sustained type of discharge in the crossed electric and magnetic fields at different values of the anode voltage, the magnetic field, and the working body mass flow rate. According to the electron current transfer mechanism and the current oscillation character, two modes of the discharge are distinguished burning strictly differing, even visually: the “accelerative” and the “stochastic” ones.     

The effect of axial magnetic filed on the process of anode spot formation in a vacuum-arc discharge with CuCr50 electrodes

An experimental study is made of the effect of an external axial magnetic field on the process of anode spot formation in a pulsed vacuum-arc discharge in the range of currents from 5 to 12 kA in a discharge gap with CuCr50 electrodes. The times and currents, at which an anode spot is formed, are determined for each amplitude value of current depending on the magnitude of magnetic field. The minimal value of magnetic field preventing the anode spot formation is determined for each current. The measured values of diameters of the current channel are used to calculate the anode temperature. It is demonstrated that, under experimental conditions, the heating of anode is insufficient for marked evaporation, and the anode spot formation is associated with the critical flow of fast cathode ions.     

Reconstruction of the Charge Density Wave State Under the Applied Electric Field

Charge density wave (CDW) under an applied electric field in constraint geometry experience stresses, which can easily exceed a plastic threshold. The stress is resolved by the ground state reconstruction which proceed via creation of topological defects like solitons and dislocations??the CDW vortices. These states can be observed experimentally either in average at macroscopic scales of X-ray and multijunction space resolved studies, at mesoscopic scales of coherent X-ray micro-diffraction and nano-junctions or individually as by the STM. Here, we report numerical modeling taking into account multiple fields in their mutual nonlinear interactions: the phase and the amplitude of the CDW order parameter, distributions of the electric field, of the density and the current of normal carriers. Following events of creation and the subsequent evolution of dislocations, we find that vortices are formed in the junction when the voltage across, or the current through, exceed a threshold. The number of vortices remnant in the reconstructed ground state increases stepwise??in agreement with experiments. The vortex core concentrates the voltage drop across the junction giving rise to observed peaks of the interlayer tunneling. The studied reconstruction in junctions of CDWs may be relevant to modern efforts of the field-effect transformations in other correlated electronic systems.     

Visualization of the spatial distribution of current densities in a photographic system with a semiconductor photodetector

Abstract

This paper studies the visualization of the spatial distribution pattern of current density in a semiconductor photographic system with a gallium arsenide semiconductor photodetector. The spatial distribution of the current in the filaments was determined by photometric analysis of the gas discharge light emission when a current was passed through a photographic cell. This method ensured spatial resolution of ~ 10/mm and made it possible to describe quantitatively the distributions involving a drop in current density of ? 10². Transformation of the profile and amplitude of the current density of the filaments in the different regions of the current-voltage characteristic (CVC) has been studied. The filamentation (i.e. an inhomogencous distribution of the current density) was primarily due to the formation of a space charge of positive ions in the discharge gap between the photodetector and a transparent anode plate that changed the discharge from the Townsend type to the glow type.     

Using open atmospheric electric discharge for water purification from surface contamination with oil products

The characteristics of open atmospheric dc discharge between a liquid nonmetal cathode (tap water layer) and a metal anode have been studied. The effect of discharge on a layer of oil products (diesel fuel, lubricant oils) contaminating the liquid cathode surface was determined. The discharge current-voltage characteristics and the dependence of the cathode current density on the discharge current I were measured in the interval 20 mA ≤ I ≤ 300 mA for the discharge gap width varied within h = 2–10 mm. For h ≥ 4 mm and I ≥ 120 mA, the cathode current density and the interelectrode voltage are independent of the discharge current, which is characteristic of the normal glow discharge. Under the action of discharge, oil products in the contamination layer on the liquid cathode surface are partly decomposed and partly converted, after which the conversion products can be readily removed from the surface by mechanical methods. The efficiency of contaminant removal can reach 98%. Analysis of the conversion products showed that they are composed of polymer chains with variable length and structure involving oxygen-containing groups.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

A multichannel discharge between a stream electrolytic cathode and a metal anode at atmospheric pressure

An experimental investigation is performed of the structure and current-voltage characteristics (CVC) of a multichannel discharge (MD) between a stream electrolytic cathode (SEC) and a metal anode, as well as of the density of SEC and anode current in a wide range of values of current I = 0.02–10 A, interelectrode spacing l = 5–50 mm, and anode diameter d _a = 5–40 mm. The development of cathode spots on the SEC surface is studied. It is found that a cathode spot of SEC may have various shapes and structures (regular circle, horseshoes, filamentary spots, and so on). The CVC of a multichannel discharge between SEC and metal anode depends significantly on I, l, and d _a , as well as on the material of the anode and on the composition and concentration of electrolyte. The dependence of the density of SEC current on discharge current is nonmonotonic. It is for the first time that a bundle-like MD between SEC and metal anode and multichannel and diffuse discharges between the plasma region and stream electrolytic cathode are observed, and their characteristics are investigated.     

Cathode layer characteristics of a low-pressure glow discharge in argon and nitrogen

Cathode layer characteristics were experimentally determined for a glow discharge in argon and nitrogen in a broad range of gas pressures. A normal glow discharge regime is observed only provided that the gas ionization takes place in the anode layer (on the right of the inflection point in the glow discharge initiation curve). Importance of using a correct method for the normal cathode voltage drop is emphasized.     

The role of fast electrons in the formation of a pulsed volume discharge at elevated gas pressures

A volume electric gas discharge was obtained using a pulsed inhomogeneous electric field without a preionization source in various gases (nitrogen, air, helium, neon, argon, krypton) at elevated pressures. In air at atmospheric pressure and nanosecond voltage pulses, the specific energy deposited in the gas amounted up to ∼1 J/cm³. The mechanism of the volume discharge formation is related to the appearance of fast (keV-energy) electrons emitted from plasma formations at the cathode. Fast electrons provide for the effective preionization of the gas in the interelectrode space and favor the formation of volume discharge. Under these conditions, the maximum voltage drop across the discharge gap is achieved in the quasi-stationary stage of discharge.     

Functionally strain-graded nanoscoops for high power Li-ion battery anodes

Lithium-ion batteries show poor performance for high power applications involving ultrafast charging/discharging rates. Here we report a functionally strain-graded carbon-aluminum-silicon anode architecture that overcomes this drawback. It consists of an array of nanostructures each comprising an amorphous carbon nanorod with an intermediate layer of aluminum that is finally capped by a silicon nanoscoop on the very top. The gradation in strain arises from graded levels of volumetric expansion in these three materials on alloying with lithium. The introduction of aluminum as an intermediate layer enables the gradual transition of strain from carbon to silicon, thereby minimizing the mismatch at interfaces between differentially strained materials and enabling stable operation of the electrode under high-rate charge/discharge conditions. At an accelerated current density of ～51.2 A/g (i.e., charge/discharge rate of ～40C), the strain-graded carbon-aluminum-silicon nanoscoop anode provides average capacities of ～412 mAh/g with a power output of ～100 kW/kg(electrode) continuously over 100 charge/discharge cycles.     

Screening effect of a-C:H alignment layer in nematic liquid crystal cell

The electrical and optical characteristics of nematic liquid crystal (NLC) cells and their dependence on the thickness of an amorphous hydrogenated carbon (a-C:H) alignment layer have been studied. An increase in the a-C:H layer thickness favors enhancement of the screening of a bias voltage applied to this layer, which is manifested by an increase in the threshold voltage of the electrooptical splay effect. This is accompanied by a decrease in the initial (pretilt) director angle, which is evidence for an increase in the anchoring energy due to the field of space charge localized at the interface.     

The formation of a multichannel structure of surface discharge in noble gases

An experimental investigation is performed of the spatial structure of highly conducting channels and of the volt-second characteristics of high-current pulsed surface discharge in Ne, Ar, and Xe for the purpose of checking the model of formation of multichannel mode of such discharge suggested in [1]. The experiments are performed in a setup with two versions which differ by the electric supply circuit of the discharge by a factor of 27 as regards the impedance and by a factor of 250 as regards the discharge energy. The time dependence of voltage across the discharge gap is obtained, as well as the dependence of the number of channels in the gap up to the mode of quasi-homogeneous filling of the gap on the discharge voltage at a gas pressure of 30 and 100 kPa. Also given are the results of model calculations of these parameters. The experimental data are compared with the calculation results and demonstrate good agreement; this supports the validity of the adopted model of formation of multichannel mode of the discharge.