Study on the Mathematical Model of Dielectric Recovery Characteristics in High Voltage SF6 Circuit Breaker

According to the stream theory,this paper proposes a mathematical model of the dielectric recovery characteristic based on the two-temperature ionization equilibrium equation.Taking the dynamic variation of charged particle's ionization and attachment into account,this model can be used in collaboration with the Coulomb collision model,which gives the relationship of the heavy particle temperature and electron temperature to calculate the electron density and temperature under different pressure and electric field conditions,so as to deliver the breakdown electric field strength under different pressure conditions.Meanwhile an experiment loop of the circuit breaker has been built to measure the breakdown voltage.It is shown that calculated results are in conformity with experiment results on the whole while results based on the stream criterion are larger than experiment results.This indicates that the mathematical model proposed here is more accurate for calculating the dielectric recovery characteristic,it is derived from the stream model with some improvement and refinement and has great significance for increasing the simulation accuracy of circuit breaker's interruption characteristic.     

Effect of Segmented Electrode Length on the Performances of an Aton-Type Hall Thruster

The influences of the low-emissive graphite segmented electrode placed near the channel exit on the discharge characteristics of a Hall thruster are studied using the particlein-cell method.A two-dimensional physical model is established according to the Hall thruster discharge channel configuration.The effects of electrode length on the potential,ion density,electron temperature,ionization rate and discharge current are investigated.It is found that,with the increasing of the segmented electrode length,the equipotential lines bend towards the channel exit,and approximately parallel to the wall at the channel surface,the radial velocity and radial flow of ions are increased,and the electron temperature is also enhanced.Due to the conductive characteristic of electrodes,the radial electric field and the axial electron conductivity near the wall are enhanced,and the probability of the electron-atom ionization is reduced,which leads to the degradation of the ionization rate in the discharge channel.However,the interaction between electrons and the wall enhances the near wall conductivity,therefore the discharge current grows along with the segmented electrode length,and the performance of the thruster is also affected.     

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.     

充气电离室动态特性物理模型研究

针对充气电离室的特点 ,并结合随机信号处理理论 ,在频域内对充气电离室的物理模型进行了描述 ,建立了相关的物理模型 ,并进行了实验验证。该方法为电离室动态特性的研究提供了一种有效手段。     

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7×10^-4 for a neutral projectile to 5.4×10^-4 for a highly charged projectile ion with a total ionization energy of 576 eV.     

Neutron Radiation Effects on MOS Fets: Theory and Experiment

The effects of a pure neutron environment on Metal-Oxide-Semiconductor Field Effect Transistors (MOS-FET) is studied. A model for neutron-produced ionization in the oxide layer is presented. The energy partition between atomic displacement and electronic ionization processes in the nuclear scattering interaction is calculated and compared to the Lindhard model. It is shown that the neutron ionization causes a positive charge buildup in the oxide similar to a gamma ionization charge buildup. It is also shown that fast neutrons produce significant ionization energy which can become the major cause for ionization damage in a mixed neutron-gamma environment such as a nuclear blast. Specific experimental results are given for commercial and experimental MOS-FETS. The experimental technique used in achieving an almost pure neutron environment is described. Finally, an ionization dose equivalence for fast neutrons in SiO2 is given and compared to experimental values. This relation should prove useful in future studies of ionization damage in SiO2 subjected to a neutron-gamma environment.     

Numerical simulation of nanosecond laser ablation and plasma characteristics considering a real gas equation of state

Based on the governing equations which include the heat conduction equation in the target and the fluid equations of the vapor plasma,a two-dimensional axisymmetric model for ns-laser ablation considering the Knudsen layer and plasma shielding effect is developed.The equations of state of the plasma are described by a real gas approximation,which divides the internal energy into the thermal energy of atoms,ions and electrons,ionization energy and the excitation energy of atoms and ions.The dynamic evolution of the silicon target and plasma during laser ablation is studied by using this model,and the distributions of the temperature,plasma density,Mach number related to the evaporation/condensation of the target surface,laser transmissivity as well as internal energy of the plasma are given.It is found that the evolution of the target surface during laser ablation can be divided into three stages:(1)the target surface temperature increases continuously;(2)the sonic and subsonic evaporation;and(3)the subsonic condensation.The result of the internal energy distribution indicates that the ionization and excitation energy plays an important role in the internal energy of the plasma during laser ablation.This model is suitable for the case that the temperature of the target surface is lower than the critical temperature.     

Two-dimensional simulation and experiments of helium discharge between parallel-plate electrodes in short gaps at atmospheric pressure

ABSTRACT

Helium gas is used as the coolant in high temperature gas-cooled reactor while its insulation property is not good and brings big challenges to insulation design work. In this paper, breakdown voltages of two parallel-plate electrodes in millimeter-scale gaps at atmospheric pressure are calculated numerically with a two-dimensional hydrodynamic model. The Finite-Difference Flux-Corrected Transport method (FD-FCT) is used to avoid spurious oscillation caused by convection and diffusion restrictions. The simulation results are in good agreement with our experimental work. Different discharge characteristics are presented for a breakdown or a non-breakdown voltage. Importantly, when a breakdown is applied, ground-state ionization will make up the most in early stages while later accumulation of metastable atoms and molecules will prompt fractions of penning ionization to increase rapidly. It is predicted that penning ionization will eventually become comparable with ground-state ionization, especially near the anode, as the current density keeps increasing to order of 1 A. Some other simulation results are presented such as time evolutions of the distribution of electrons, ions, field strength and axial potential, etc. Attainment of these breakdown voltages and characteristics of helium discharge is going to help with insulation design work of electric equipment in reactor engineering.     

Two-dimensional self-consistent numerical simulation of the whole discharge region in an atmospheric argon arc

A 2D self-consistent numerical model of the whole argon-arc discharge region that includes electrodes is developed in this work to facilitate analysis of the physical processes occurring in atmospheric arc plasma. The 2D arc column model contains the ionization and thermal non-equilibrium, which is coupled with a 1D electrode sheath model. The influence of plasma-species diffusion near the electrode region is investigated based on Maxwell–Stefan equations and the generalized Ohm's law. The numerical results of argon free-burning arcs at atmospheric pressure are then investigated. The simulation shows that the plasma is obviously in the state of thermal and ionization equilibrium in the arc core region, while it deviates from thermal and ionization equilibrium in the arc fringe region. The actual electron density decreases rapidly in the near-anode and near-cathode regions due to non-equilibrium ionization, resulting in a large electron number gradient in these regions. The results indicate that electron diffusion has an important role in the near-cathode and near-anode regions. When the anode arc root gradually contracts, it is easy to obtain a positive voltage drop of the anode sheath (I = 50 A), while it remains difficult to acquire a positive anode sheath voltage drop (I = 150 A). The current–voltage characteristics predicted by our model are found to be identical to the experimental values.     

电流模式下微型裂变电离室参数对探测性能的影响

裂变电离室在工作时,其内部气体中持续地发生着电离和复合过程。为了进一步了解这些过程对探测性能的影响,需要从等离子体物理的角度来研究裂变电离室。借助BOLSIG+以及电流模式下微型裂变电离室的基本理论模型,计算了电极的几何尺寸、裂变率以及气体电离度等参数对探测器饱和区电压范围的影响,并对结果进行了讨论。此外,根据不同电离度情况下的计算结果,对高温导致探测器饱和区电压范围变小这一现象给出了一种可能的解释。     

An experimental setup for surface ionization and desorption studies

The construction and function of an apparatus intended for negative surface ionization and other desorption studies are described. A molecular beam of the material to be ionized is formed. The number of particles that reaches the ionizer can be measured with a device which is sensitive to the momentum of the particles in the neutral beam. The extraction field is reversible, making it possible to study nonsteady ionization processes and to use the modulated voltage technique, providing a second method to determine the ionization efficiency. The reflection of the neutral beam at the ionizer can be estimated by comparing the two methods. The temperature of the ionizer is measured and controlled by a microcomputer, allowing studies like temperature programmed desorption. Mass analysis and a SEM detector provide a possibility to measure very low ion currents.     

Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide’s measurements of the Cu⁻ population sputtered from a clean Cu-surface, at emission energies below 100 eV.     

Numerical and experimental investigation of plasma plume deflection with MHD flow control

This paper presents a composite magneto hydrodynamics(MHD) method to control the lowtemperature micro-ionized plasma flow generated by injecting alkali salt into the combustion gas to realize the thrust vector of an aeroengine.The principle of plasma flow with MHD control is analyzed.The feasibility of plasma jet deflection is investigated using numerical simulation with MHD control by loading the User-Defined Function model.A test rig with plasma flow controlled by MHD is established.An alkali salt compound with a low ionization energy is injected into combustion gas to obtain the low-temperature plasma flow.Finally,plasma plume deflection is obtained in different working conditions.The results demonstrate that plasma plume deflection with MHD control can be realized via numerical simulation.A low-temperature plasma flow can be obtained by injecting an alkali metal salt compound with low ionization energy into a combustion gas at 1800–2500 K.The vector angle of plasma plume deflection increases with the increase of gas temperature and the magnetic field intensity.It is feasible to realize the aim of the thrust vector of aeroengine by using MHD to control plasma flow deflection.     

The Scintillation Process in Alkali Halide Crystals

The scintillation process in impurity-activated alkali halide crystals is considered by analogy to that in binary solid organic solutions. A general relation is proposed for the differential scintillation efficiency dL/dE as the product of the efficiencies of the primary and secondary scintillation processes. The primary efficiency is influenced by electron-hole recombination and ionization quenching and is a function of the specific energy loss dE/dr. The secondary efficiency is influenced by host-activator energy transfer and concentration quenching and is a function of the activator concentration c . Experimental data in support of the model are cited. An extension of the model is proposed to account for the temperature dependence of the scintillation efficiencies and decay times of pure and impurity-activated alkali halides, and this is compared with experimental data.     

Predictions of inner-shell Coulomb ionization by heavy charged particles

Probabilities for K-, L-, and M-shell ionization by impinging protons are presented as functions of the impact parameter. The results are based on the straight-line version of the semiclassical approximation model. Values for total Coulomb ionization cross sections are given as well. A method for extension to other bare projectiles is indicated.     

Analyses on the Ionization Instability of Non-Equilibrium Seeded Plasma in an MHD Generator

Recently,closed cycle magnetohydrodynamic power generation system research has been focused on improving the isentropic efficiency and the enthalpy extraction ratio.By reducing the cross-section area ratio of the disk magnetohydrodynamic generator,it is believed that a high isentropic efficiency can be achieved with the same enthalpy extraction.In this study,the result relating to a plasma state which takes into account the ionization instability of non-equilibrium seeded plasma is added to the theoretical prediction of the relationship between enthalpy extraction and isentropic efficiency.As a result,the electron temperature which reaches the seed complete ionization state without the growth of ionization instability can be realized at a relatively high seed fraction condition.However,the upper limit of the power generation performance is suggested to remain lower than the value expected in the low seed fraction condition.It is also suggested that a higher power generation performance may be obtained by implementing the electron temperature range which reaches the seed complete ionization state at a low seed fraction.     

Influence of Ionization Degrees on Conversion of CO and CO_2 in Atmospheric Plasma near the Ground

A zero-dimensional model is used to study the processes of physical and chemical reactions in atmospheric plasma with different ionization degrees near the ground （0 km）. The temporal evolutions of CO, C02 and other main reactants （namely OH and O2）, which affect the conversion of CO and C02, are obtained for afterglow plasma with different initial values. The results show that the consumption rate of CO is largest when the initiM electron number density neo=1012 cm-3, i.e. the ionization degree is 0.000004%. The number density of CO2 is relatively small when neo=1016 cm-3, i.e. the ionization degree is 0.04%, whereas they are very close under the condition of other ionization degrees. Considering the total number densities of CO and C02 and the consumption rate of CO comprehensively, the best condition is neo=1013 cm-3, i.e. the ionization degree is 0.00004% for reducing the densities of CO and CO2 in the atmospheric plasma. The temporal evolutions of N＋, Ar＋, CO＋ and CO＋ are also shown, and the influences on the temporal evolutions of CO and C02 are analyzed with increasing ionization degree.     

Numerical simulation and analysis of lithium plasma during low-pressure DC arc discharge

Alkali metal DC arc discharge has the characteristics of easy ionization, low power consumption, high plasma temperature and ionization degree, etc, which can be applied in aerospace vehicles in various ways. In this paper, we calculate the physical property parameters of lithium vapor, one of the major alkali metals, and analyze the discharge characteristics of lithium plasma with the magnetohydrodynamic (MHD) model. The discharge effects between constant current and voltage sources are also compared. It is shown that the lithium plasma of DC arc discharge has relatively high temperature and current density. The peak temperature can reach tens of thousands of K, and the current density reaches 6×10 ⁷ A⁻² . Under the same rated power, the plasma parameters of the constant voltage source discharge are much higher than those of the constant current source discharge, which can be used as the preferred discharge mode for aerospace applications.     

表面电离的计算与测量

通过对电离度和表面下离子流密度的计算以及对靶电流和负离子的测量，对诸如钽，铁，钨，镍，铼和铂等高功函数金属表面的电离效率进行比较研究后表明：铁作为一种普通金属同样是较好的电离材料，并对影响表面电离的温度，铯蒸汽压力以及表面功函数进行了讨论。