Analysis of Fault Arc in High-Speed Switch Applied in Hybrid Circuit Breaker

The behavior of fault arc in a high-speed switch(HSS) has been studied theoretically and experimentally.A simplified HSS setup is designed to support this work.A two-dimensional arc model is developed to analyze the characteristics of fault arc based on magnetic-hydrodynamic(MHD) theory.The advantage of such a model is that the thermal transfer coefficient can be determined by depending on the numerical method alone.The influence of net emission coefficients(NEC) radiation model and P1 model on fault arc is analyzed in detail.Results show that NEC model predicts more radiation energy and less pressure rise without the re-absorption effect considered.As a consequence,P1 model is more suitable to calculate the pressure rise caused by fault arc.Finally,the pressure rise during longer arcing time for different arc currents is predicted.     

Simulation of Fault Arc Based on Different Radiation Models in a Closed Tank

This paper focuses on the simulation of a fault arc in a closed tank based on the magneto-hydrodynamic(MHD) method,in which a comparative study of three radiation models,including net emission coefficients(NEC),semi-empirical model based on NEC as well as the P1 model,is developed.The pressure rise calculated by the three radiation models are compared to the measured results.Particularly when the semi-empirical model is used,the effect of different boundary temperatures of the re-absorption layer in the semi-empirical model on pressure rise is concentrated on.The results show that the re-absorption effect in the low-temperature region affects radiation transfer of fault arcs evidently,and thus the internal pressure rise.Compared with the NEC model,P1 and the semi-empirical model with 0.7 α 0.83 are more suitable to calculate the pressure rise of the fault arc,where is an adjusted parameter involving the boundary temperature of the re-absorption region in the semi-empirical model.     

Evolution simulation of lightning discharge based on a magnetohydrodynamics method

In order to solve the load problem for aircraft lightning strikes,lightning channel evolution is simulated under the key physical parameters for aircraft lightning current component C.A numerical model of the discharge channel is established,based on magnetohydrodynamics (MHD) and performed by FLUENT software.With the aid of user-defined functions and a userdefined scalar,the Lorentz force,Joule heating and material parameters of an air thermal plasma are added.A three-dimensional lightning arc channel is simulated and the arc evolution in space is obtained.The results show that the temperature distribution of the lightning channel is symmetrical and that the hottest region occurs at the center of the lightning channel.The distributions of potential and current density are obtained,showing that the difference in electric potential or energy between two points tends to make the arc channel develop downwards.The arc channel comes into expansion on the anode surface due to stagnation of the thermal plasma and there exists impingement on the copper plate when the arc channel comes into contact with the anode plate.     

Modelling Study to Compare the Flow and Heat Transfer Characteristics of Low-Power Hydrogen, Nitrogen and Argon Arc-Heated Thrusters

A modelling study is performed to compare the plasma flow and heat transfer characteristics of low-power arc-heated thrusters (arcjets) for three different propellants: hydrogen, nitrogen and argon. The all-speed SIMPLE algorithm is employed to solve the governing equations, which take into account the effects of compressibility, Lorentz force and Joule heating, as well as the temperature- and pressure-dependence of the gas properties. The temperature, velocity and Mach number distributions calculated within the thruster nozzle obtained with different propellant gases are compared for the same thruster structure, dimensions, inlet-gas stagnant pressure and arc currents. The temperature distributions in the solid region of the anode-nozzle wall are also given. It is found that the flow and energy conversion processes in the thruster nozzle show many similar features for all three propellants. For example, the propellant is heated mainly in the near-cathode and constrictor region, with the highest plasma temperature appearing near the cathode tip; the flow transition from the subsonic to supersonic regime occurs within the constrictor region; the highest axial velocity appears inside the nozzle; and most of the input propellant flows towards the thruster exit through the cooler gas region near the anode-nozzle wall. However, since the properties of hydrogen, nitrogen and argon, especially their molecular weights, specific enthalpies and thermal conductivities, are different, there are appreciable differences in arcjet performance. For example, compared to the other two propellants, the hydrogen arcjet thruster shows a higher plasma temperature in the arc region, and higher axial velocity but lower temperature at the thruster exit. Correspondingly, the hydrogen arcjet thruster has the highest specific impulse and arc voltage for the same inlet stagnant pressure and arc current. The predictions of the modelling are compared favourably with available experimental results.     

Characteristics of Electrode-Water-Electrode Discharge and its Application to Water Treatment

Atmospheric air discharge above the surface of water is an effective method for water treatment.The leakage current and Joule heating of water are reduced by the air gap,which raises the energy efficiency of the water treatment.However,the application of this kind of discharge is limited by a pair of conflicting factors：the chemical efficiency grows as the discharge gap distance decreases,while the spark breakdown voltage decreases as the gap distance decreases.To raise the spark breakdown voltage and the chemical efficiency of atmospheric pressure water surface discharge,both the high-voltage electrode and the ground electrode are suspended above the water surface to form an electrode-water-electrode discharge system.For this system,there are two potential discharge directions：from one electrode to another directly,and from the electrodes to the water surface.The first step in utilizing the electrode-water-electrode discharge is to find out the discharge direction transition criterion.In this paper,the discharge direction transition criterions of spark discharge and streamer discharge are presented.By comparing the discharge characteristics and the chemical efficiencies,the discharge propagating from the electrodes to the water surface is proved to be more suitable for water treatment than that propagating directly between the electrodes.     

Simulation of the Effect of a Metal Vapor Arc on Electrode Erosion in Liquid Metal Current Limiting Device

The effect of arc plasma on electrode erosion in a liquid metal current limiter(LMCL)is studied.Based on a simplified two-dimensional magnetohydrodynamic model,the elongated GaInSn metal vapor arc and its contraction process in a liquid metal current limiter are simulated.The distributions of temperature,pressure and velocity of the arc plasma are calculated.The simulation results indicate that the electrode erosion is mainly caused by two high temperature gas jet flows arising from the pressure gradient,which is a result of the non-uniform arc temperature distribution.The gas flows,which act as jets onto the electrode surface,lead to the evaporation of the electrode material form the surface.A redesign structure of the electrode is proposed and implemented according to the analysis,which greatly increased the service life of the electrode.     

Discharge characteristics of a needle-toplate electrode at a micro-scale gap

To understand the discharge characteristics under a gap of micrometers,the breakdown voltage and current–voltage curve are measured experimentally in a needle-to-plate electrode at a microscale gap of 3–50 μm in air.The effect of the needle radius and the gas pressure on the discharge characteristics are tested.The results show that when the gap is larger than 10 μm,the relation between the breakdown voltage and the gap looks like the Paschen curve;while below 10 μm,the breakdown voltage is nearly constant in the range of the tested gap.However,at the same gap distance,the breakdown voltage is still affected by the pressure and shows a trend similar to Paschen's law.The current–voltage characteristic in all the gaps is similar and follows the trend of a typical Townsend-to-glow discharge.A simple model is used to explain the non-normality of breakdown in the micro-gaps.The Townsend mechanism is suggested to control the breakdown process in this configuration before the gap reduces much smaller in air.     

Numerical Experiments on Radiative Cooling and Collapse in Plasma Focus Operated in Krypton

The Plasma Focus has wide-ranging applications due to its intense radiation of SXR, XR, electron and ion beams and fusion neutrons when operated in deuterium. The 5-phase Lee Model code has been developed for the focus operated in various gases including D, D–T, He, Ne, N, O, Ar, Kr and Xe. Radiation-coupled motion is included in the modelling. In this paper we look at the effect of radiation cooling and radiation collapse in krypton. The Pease–Braginskii current is that current flowing in a hydrogen pinch which is just large enough for the Bremsstrahlung to balance Joule heating. This radiation-cooled threshold current for a hydrogen pinch is 1.6 MA. It is known that in gases undergoing line radiation strongly the radiation-cooled threshold current is considerably lowered. We show that the equations of the Lee Model code may be used to compute this lowering. The code also shows the effect of radiation cooling leading to radiative collapse. Numerical experiments based on experimentally fitted model parameters are run to demonstrate a regime in which radiation collapse is observed in Kr at a pinch current of 50–100 kA.     

Mode transition induced by gas pressure in dusty acetylene microdischarges: two-dimensional simulation

Radio-frequency microdischarge in acetylene is investigated by use of a fluid model and an aerosol dynamics model in a cylindrical discharge chamber. In this article, the results at a pressure of 100–500 Torr, a voltage of 80–150 V, and an electrode gap of 400–1000 μm are carefully analyzed and discussed. It is shown that two electron heating modes α and γ appear in the microdischarge, and the pressure-dependent transition from α to γ was accompanied by the abrupt decrease of electron density and electron temperature. The mode transition phenomenon is further confirmed by the variation of the electron temperature axial profiles, the profiles vary continuously from a center high at the pressure of 100 Torr to an edge high at the pressure of500 Torr. Furthermore, in the α mode(100 Torr) the plasma density increases linearly with the increase of electrode gap, but decreases sharply with the increase of electrode gap in the γ mode(100 Torr). The gas pressure and applied voltage effects on the nanoparticle density and degree of nonuniformity are also investigated. It has been shown that the gas pressure greatly influences the axial profiles of nanoparticle density and the values of the degree of nonuniformity, while the values of the plasma parameters(electron density and nanoparticle density) strongly depend on the applied voltage.     

同心环形管内沸腾两相流动与传热实验研究

对窄缝为2．1mm的同心环形管,试验研究了外管加热条件下水的沸腾两相流动阻力与传热特性,得到了以下结果：窄缝环形管内两相流动的阻力较普通圆管内大,沸腾换热得到了较明显的强化,换热系数弓压力、热平衡干度、工质流量、加热负荷均有关系,且与缝隙宽度和加热方式有关;提出了环形管强化传热的微液膜蒸发机理与汽泡扰动机理的物理解释;得到了环形管内流动摩擦阻力系数与传热系数的实验关联式。     

Study of Shock Wave and Magnetic Pressure Effects on the Rail Gap Switch Surface Used at the APF Plasma Focus Device

Whereas high voltage and current create a rough environment for switch electrodes in pulse power technology, the switch requires the most maintenance or replacement after a short time. In this paper we investigate the effects of magnetic and shock pressures created by high power electric arc between a rail gap switch with copper electrodes at the APF plasma focus device. As studied by others, the shock pressure is some order of magnitude higher than the magnetic pressure after electric arc generation. We calculated the magnetic pressure, electric arc radius, time dependent arc velocity, and also time dependent shock pressure created by an oscillating current discharge applied across the rail gap electrodes surface. Modeling included a MathCAD analysis of the diverging wave front through the electrode and the results show that the shock wave pressure induced after the electric arc has a serious destructive effect on our switch surface.     

低能高束流氩离子源的结构及性能

离子源技术是等离子体研究中的一项重要内容,而低能大束流源则是离子源技术研究中的一个重要方向,因为这样的源在离子束刻蚀、离子束溅射镀膜以及荷能粒子与物质相互作用方面都有广泛的应用;本文采用空心阴极空心阳极结构,用热阴极电子发射弧放电驱动并用磁场约束产生等离子体,用曲面发射引出离子束,研制成了氩气放电溅射离子源;研究了灯丝加热电流、弧压对弧流的影响和弧流与工作气体压力对离子束引出的影响规律.离子源的引出电压在0-4.0 kV之间连续可调,最大引出束流为100 mA,束斑面积为φ6.0 cm,以Ti为溅射靶时的最大溅射沉积率为0.45 nm/s,离子源可连续工作160 h.     

Development of a silver ion source using nanosecond pulses of a Nd:YAG laser at different wavelengths

A silver ion source was designed by focusing the fundamental and harmonics of Q-switched Nd:YAG laser pulses onto a silver target and simultaneously applying an electric potential in an argon environment. The silver ions were detected at a distance of 2 cm from the target surface using a Faraday cup ion probe after letting them pass through a retarding mesh grid (copper electrode). We aim to produce and characterize the silver ions generated by the laser radiation of different wavelengths and pulse energy, ambient gas pressure and the electrode spacing under applied electric field. In addition to this, the effect of laser radiation on plasma under vacuum and at different argon gas pressures was investigated. The velocity distribution function of the plasma emitted from the silver target was investigated under argon discharge. These measurements demonstrated clearly that the velocity distribution function and current signals depend on laser power, laser wavelength and argon pressure. We observed a ten fold increase in the plume current with increase in the applied voltage and ion velocity in the presence of a laser field. The surface morphology of the laser irradiated samples was investigated using reflection optical microscopy.     

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.     

Effects of background pressure on the arc characteristics of gas spark gap

Gas spark gap is widely used in any pulsed power system as the key element which directly determines its repetitive performance and output characteristics. Among many factors of threeelectrode gas spark gap, background pressure is of much importance in determining the gap performance parameters such as the delay and jitter, and relevant studies have been rarely performed. A magneto-hydrodynamic model of the arc in gas spark gap is built and the effects of background pressure on the arc characteristics are discussed in this paper. It is demonstrated that a higher background pressure may result in radial compression of the arc column, a higher arc voltage, and a lower declination rate of arc resistance in the first quarter cycle. Relevant simulation data would be helpful for the optimization of the design of gas spark gap.     

Effects of Power Terms and Thermodynamics on the Contraction of Pinch Radius in Plasma Focus Devices Using the Lee Model

The influence of the power terms Joule heating and radiative losses on the pinch radius in plasma focus devices is studied. Numerical experiments were carried out using the Lee model on three plasma focus devices spanning a large range of storage energy (PF400, INTI PF, PF1000) with different filling gases (N, O, Ne, Ar, Kr, Xe). Six possible regimes each characterized by a combination of significant power terms affecting plasma focus dynamics are found and discussed. These six possible regimes are further moderated by thermodynamic effects related to the specific heat ratio SHR of the plasma. In PF1000, the thermodynamic compression effects are clearly apparent in the radius ratio versus pressure curve for nitrogen which with atomic number Z_n = 7 is less radiative than neon with Z_n = 10, the dominant line radiation being proportional to Z _n ⁴ . In neon radiative compression at optimum pressure is so dominant that it masks thermodynamic compression in the compression versus pressure graph. Results show that plasma radiation losses enhance the contraction of the plasma focus pinch radius within suitable pressure ranges characteristic of each machine for each gas discussed in this paper. The radiation enhancement of compression increases with the atomic number of the gas.     

Evaluation of Gap Heat Transfer between Boron Carbide Pellet and Cladding in Control Rod of FBR

Heat transfer across a gas gap between a boron carbide pellet and a cladding in FBR control rod has been experimentally investigated. The main purpose of this investigation is to present a calculational method for the gap heat transfer in order to improve accuracy of thermal design for the control rod with 0.5 mm gap width at a beginning of reactor operation.

Two types of tests have been carried out using simulated control rods. One is low temperature tests below 200°C. The test results indicated that the convective heat transfer has a negligible effect on the gap conductance when the Rayleigh number using the gap width as the characteristic length is below 0.1. The other is high temperature tests up to 600°C.

The results showed +10 to — 5% variations in the gap conductance data due to eccentricity between the pellet and the cladding. The prediction based on conduction and radiation heat transfers considering a thermal expansion and an eccentricity gave better results of gap conductance having a maximum difference of only 17% from the measured ones. Calculation in the radiation heat transfer used thermal emissivities. 0.85 for the boron carbide and 0.15 for the cladding, measured by infrared thermography.     

大气压下大功率等离子体炬的数值模拟

主要研究直流等离子体炬的数值模拟方法,通过求解等离子体弧社区域的能量守恒,动量守恒,质量守恒及电流连续性方程,得到不同边界条件下温度,速度,电流密度分布,计算了大气压下２００Ａ自由氩弧的温度分布,电势和电流分布,并与实验数据及现有的理论计算进行了比较,得到了较好的结果,在此基础上结合本实验室５０００Ａ大功率等离子体炬,讨论了有阴极喷口存在下的弧柱部分的数值计算,以及能量守恒方程中辐射项,焦耳热项的     

Characteristic analysis of CO2 switching arcs under a DC current

The current interruption capability of a gas,when used in high voltage gas-blast circuit breakers,depends not only on its material properties but also the flow field since turbulence plays a dominant role in arc cooling during the interruption process.Based on available experimental results,a study of CO2 switching arcs under a DC (direct current) current in the model circuit breaker has been conducted to calibrate CO2 arc model and to analyse its electric and thermal property.Through detailed analysis of the results mechanisms responsible for the temperature distribution are identified and the domain energy transportation process of different region discussed.The present work provides significant coefficients for CO2 switching arc simulation and gives a better understanding of CO2 arc burning mechanisms.     

Study of Distortion of a Vacuum Arc at a Long-Gap Distance

Experimental study of the long-gap distance vacuum arc distortion for three types of axial magnetic field (AMF) contacts, by using high-speed charge coupled device (CCD), is presented. The arc current was of a half-cycle sine wave with a frequency of 50Hz, produced by an L-C discharging circuit. The time of appearance and duration of vacuum arc distortion under three conditions were studied. It was found that the gap distance, current and diameter of the electrode affected the characteristics of vacuum distortion at a long-gap distance. Some characteristics of the vacuum arc at a long-gap distance was revealed and the experience and data for further investigation were provided.