Particle Fluxes and Pressure Acting on the Cathode of a Vacuum Arc in Terms of a Kinetic Model of the Cathode Spot

Trace metal scavenging in the ocean is classically explained by selective sorptive uptake of the metal by freshly produced particles, taking into account surface site and solute speciation, followed by settling of those particles. Kinetic models of oceanic trace metal scavenging rely heavily on the information gained from the application of Th nuclides as tracers of oceanic scavenging. The scavenging model of Honeyman and Santschi [1] links the formalism of coagulation/sedimentation models with that of the uranium/thorium decay series. It is based on the assumptions that scavenging rate constants reflect sorbed trace element removal from the water column via particle coagulation and sedimentation, and that the loss of particles by sedimentation is balanced by supply of colloidal particles to the settleable particle pool. Such model assumptions are verified here by a comparison of measured and calculated, depth-normalized sedimentation rates and by being able to predict the broad correlation of scavenging rate constants with particle fluxes, as observed by Bruland and coworkers. Our modeling results suggest that a physical step may be rate controlling, even though trace metal scavenging, overall, may still be driven by biological processes.

A kinetic model and the associated PIC-MCC numerical technique have been used to simulate the plasma generated by a vacuum arc cathode spot fragment and calculate the current densities and net energy fluxes at the cathode surface. The model is based on a comprehensive description of the plasma generation processes, including a detailed representation of metal vapor particle ionization and plasma-cathode interaction. An important feature of this model is its ability to provide details of the spatial variations of plasma parameters within the near cathode region. Calculations are carried out for nickel, titanium and zirconium cathodes as a function of the surface temperature. It is found that below a threshold surface temperature the energy input to the cathode is negative and the spot fragment can not operate in a self-sustained way. The relative importance of the different mechanisms controlling current and energy transfers at the cathode surface are analyzed in detail.

Recently, it is required to elucidate the movement factor of multiple cathode spots in vacuum arc, which can improve the performance of the VCB (Vacuum Circuit Breaker), surface treatment of metal and physical vapor deposition. Multiple cathode spots in vacuum arc move at high speed with evaporation from cathode and interact with each other, which causes the disappearance and generation of the cathode spots unpredictable. Since the movement and interactions of multiple cathode spots in vacuum arc is complex physical phenomenon, the movement factor of multiple cathode spots has not been clarified. Therefore, it has been expected to elucidate this phenomenon by using numerical simulations. Some researchers have researched vacuum arc using 2-D symmetric MHD simulations. However, it cannot simulate the movement and disappearance of multiple cathode spots because the vacuum arc with multiple cathode spots is the asymmetric physical phenomenon. In this research, 3-D electromagnetic thermal fluid simulation was developed in order to elucidate the movement and disappearance of multiple cathode spots in vacuum arc. In this simulation, the current density distribution of cathode spots was determined by the ionic conductivity for the analysis of movement of cathode spots. The initial placement of cathode spots was fixed, the distance among cathode spots was set in the case of narrow distance and wide distance for analysis of constricted arc and diffused arc. As a result, some cathode spots disappeared and caused the of current density increment of the remain cathode spot, which leads to the increment of temperature over one cathode spot. This is because the increment of the conductivity above one cathode spot with increasing vapor concentration from cathode and the number density of electron. This is caused by the joule heating with increasing the current density of one cathode spot. Therefore, the movement and disappearance of multiple cathode spots in vacuum arc can be analyzed by this 3-D electromagnetic thermal fluid simulation.

A mass and energy analyzer capable of time-resolved analysis of the separate constituents of the flux from vacuum arcs near and through current zero has been constructed. The following results are obtained in looking at the radial flux from 50- and 100-A copper arcs subjected to rapidly forced current zeros: (i) A burst of low-energy ions is produced at the time of arc extinction (within the resolving power of the apparatus, <5 μsec), this burst being attributed to the collapse of a potential hump near the cathode. (ii) The presence of copper ions depresses the level of background gas signal, especially for hydrogen.

The interaction between the interelectrode plasma and macroparticles (droplets) produced by a multitude of cathode spots in a vacuum arc between Cu electrodes is analyzed, using previous experimental measurements of the macroparticle size distribution and erosion rate and a flowing plasma model. The effect of the plasma on the macroparticles is considered by treating the macroparticles as floating probes and calculating the particle, momentum, and energy fluxes to them. It is found that slow macroparticles are significantly deflected form their original trajectories owing to ion bombardment, and that steady-state macroparticle temperatures of 2000–2600 K are obtained from the balance of the energy influx (primarily from ion bombardment) with the evaporative outflux. The effect of the macroparticles on the plasma is considered by calculating the production rate of neutral atoms and ions originating from macroparticle evaporation, by examining the possibility of occlusion of the discharge path by macroparticles, and of cathode-spot formation on detached particles. The calculated neutral density is proportional to J and is in the range 0.02–0.5% of the electron density, depending upon electron temperature and macroparticle velocity. In the 107 A m−2 Cu arc, the calculated neutral density is in the range 1×1017–2.5×1018 m−3. The neutral atoms are eventually ionized and the rate of ion production from the macroparticles can become significant in comparison with that of cathode-spot-produced ions under certain conditions. Occlusion of the discharge path is negligible, and the occurrence of sustained cathode-spot activity on the macroparticles is improbable.

ABSTRACTA gas phase kinetic model of chlorine in an ICP reactor (Inductive Coupled Plasma) combined with a surface model have been developed to study the etching profile evolution of InP material. A gas phase chemistry model is used to predict the main neutral and charged specie fluxes impinging upon etched InP surface. These particle fluxes are then injected as input parameters into both a Monte-Carlo sheath model and a 2D surface model to predict the etch profile topography. The coupling between the gas kinetic model, sheath and surface models allows a direct prediction of the InP etch profile evolution versus reactor parameters (pressure, source power, Cl2 flow rate, DC bias on the substrate‥). A parametric study is carried out to show the role of some plasma parameters on etch rate, anisotropy and adsorbed InP surface state by chlorine.

Retarding field analyzers (RFA) provide an integral of the ion velocity distribution in tokamak edge plasmas, leading, in principle, to an estimate of the ion temperature. However, the presence of the RFA itself perturbs the ambient plasma, such that the measured distribution is distorted with respect to the unperturbed one far from the probe. Here, collisionless kinetic modeling is employed to investigate the modification of the plasma characteristics (temperature, particle flux, density, and electric potential) in the presheath of the RFA. The kinetic equations are solved independently by means of two different numerical methods, which provide a reliable check of their results. Moreover, they are interpreted in light of a simplified kinetic analytical model. Systematic numerical studies are performed for a large range of values of the ion-to-electron temperature ratio and the parallel drift speed. In the same way that a Mach probe measures upstream–downstream asymmetries of ion saturation current in flowing plasmas, RFAs are expected to measure important asymmetries of sheath potential and ion temperature. These asymmetries can be used to estimate accurately the ion temperature in the absence of the probe perturbation.

Island formation in strain-free heteroepitaxial deposition of thin films is analyzed using kinetic Monte Carlo simulations of two minimal lattice models and scaling approaches. The transition from layer-by-layer (LBL) to island (ISL) growth is driven by a weaker binding strength of the substrate, which, in the kinetic model, is equivalent to an increased diffusivity of particles on the substrate compared to particles on the film. The LBL-ISL transition region is characterized by particle fluxes between layers 1 and 2 significantly exceeding the net flux between them, which sets a quasiequilibrium condition. Deposition on top of monolayer islands weakly contributes to the second-layer nucleation, in contrast with the homoepitaxial growth case. A thermodynamic approach for compact islands with one or two layers predicts the minimum size in which the second layer is stable. When this is linked to scaling expressions for submonolayer island deposition, the dependence of the ISL-LBL transition point on the kinetic parameters qualitatively matches the simulation results, with quantitative agreement in some parameter ranges. The transition occurs in the equilibrium regime of partial wetting, and the convergence of the transition point upon reducing the deposition rate is very slow and practically unattainable in experiments.

Simulations of high-density deuterium plasmas in a lower single-null magnetic configuration based on a TCV discharge are presented. We evolve the dynamics of three charged species (electrons, D+ and D2+ ), interacting with two neutrals species ( D and D2 ) through ionization, charge-exchange, recombination and molecular dissociation processes. The plasma is modelled by using the drift-reduced fluid Braginskii equations, while the neutral dynamics is described by a kinetic model. To control the divertor conditions, a D2 puffing is used and the effect of increasing the puffing strength is investigated. The increase in fuelling leads to an increase of density in the scrape-off layer and a decrease of the plasma temperature. At the same time, the particle and heat fluxes to the divertor target decrease and the detachment of the inner target is observed. The analysis of particle and transport balance in the divertor volume shows that the decrease of the particle flux is caused by a decrease of the local neutral ionization together with a decrease of the parallel velocity, caused by the lower plasma temperature and the increase in momentum losses. The relative importance of the different collision terms is assessed, showing the crucial role of molecular interactions, as they are responsible for increasing the atomic neutral density and temperature, since most of the D neutrals are produced by molecular activated recombination and D2 dissociation. The presence of strong electric fields in high-density plasmas is also shown, revealing the role of the E × B drift in setting the asymmetry between the divertor targets. Simulation results are in agreement with experimental observations of increased density decay length, attributed to a decrease of parallel transport, together with an increase of plasma blob size and radial velocity.

Combined electrical, optical, and spectroscopic measurements have been used to determine the total flux, ion flux, particle flux, and neutral atom flux emitted from the cathode spot region of a copper vacuum arc of 80 A total current and 2-sec duration. It is found that the cathode erosion products consist predominantly of ions and particles, the emission of neutral atomic flux from the cathode being less than 1% of the total flux. The total and particle flux distributions are peaked in the direction of the cathode plane, whereas the ion-flux distribution is forward peaked. However, both ions and particles are detected in the cathode shadow, a result which is contrary to the hypothesis of purely collisionless transport of cathode erosion products. The neutral atom density measurements are consistent with a model assuming the source of the vapor to be evaporation in flight from the hot particles emitted from the cathode spot region. The size distribution of the particles has a maximum for particles of diameter in the range 0–1 μ.

The kinetics of the formation of hydrogen peroxide, nitrate and nitrite ions and the pH of the solution, which served as the cathode, were studied under the action of a direct current discharge at atmospheric pressure in air. A 0D kinetic model has been developed that describes the reactions occurring in solution. The model includes 28 components, 119 reactions between them, as well as fluxes of particles coming to the surface of the solution from the discharge. The particle fluxes were determined from the 0D model of a discharge in air based on the self-consistent solution of the Boltzmann equation, the equations of vibrational kinetics for the ground states of N2, O2, H2O, NO molecules, and the equations of chemical kinetics. The proposed model is semi-empirical, since it includes not only known experimental kinetic data, but also some assumptions that were made to match the calculation results with experiment. An analysis of the experimental data and calculations by the model showed that the main factors initiating reactions in solution are the bombardment of the surface by a flux of positive ions accelerated in the cathode potential drop and the flux of NO molecules from the discharge. Ion bombardment leads to the formation of hydrogen peroxide, during the decomposition of which OH radicals are formed, the subsequent reactions of which determine the composition of the particles of the solution. The source of nitrogen-containing particles is the flux of NO molecules from the discharge. Data are given on the kinetics of the concentrations of the main particles of the solution and the analysis of the mechanisms of the processes of their formation and decay. The results of calculations of the concentrations of H2O2, NO2 −, NO3 −, and pH agree with the experiment within the limits of the latter’s accuracy.

Vacuum arc is a special metal vapor discharge phenomenon, because its discharge medium totally comes from the evaporation and ionization of electrode materials. In the case of low current, the vacuum arc is completely composed of plasma jets emitted from discrete cathode spots on the cathode surface and the current carried by each spot depends on the cathode material. When the arc current exceeds a certain value, a certain number of cathode spot plasma jets will appear. Vacuum arcs play a very important role in some industrial applications such as vacuum circuit breakers, vacuum coatings and electric thrusters. As an important plasma control method, the external axial magnetic field (AMF) has an important influence on the macroscopic morphology and microscopic parameter distribution of the vacuum arc. Various studies of vacuum arc under AMF have been carried out and some progress has been made. However, the existing literature about the simulation research of vacuum arc is mostly concentrated in the case of large current, and less attention is paid to the case of small current. The reason is that the traditional methods, magneto-hydrodynamics or particle-in-cell, are limited by either accuracy or efficiency, and cannot be effectively applied to the low current vacuum arc plasma jet simulations. In this paper, we develop a fully three-dimensional hybrid plasma simulation algorithm to study the single cathode spot vacuum arc plasma jet under AMF. In this model, ions are modelled as particles while electrons are treated as massless fluid, and the self-generated magnetic field is also considered. To simplify the condition, the cathode spot in our model only exists as a plasma jet source, thus the detailed mechanism of producing plasmas is neglected. And the movement of the cathode spot is not considered either. The results show that the single cathode spot plasma jet diffuses into the interelectrode in a cone shape after leaving the cathode spot, and the ion density drops rapidly from cathode to anode. Under the simulation conditions in this paper (<i>I</i> ≤ 150 A), the self-generated magnetic field will not have a significant influence on the plasma jet itself in the case of low current. The external AMF has a compressive effect on the diffusion of the vacuum arc plasma jet. Under the AMF, the radial movement of the ions is suppressed, and the decrease of the ion radial velocity leads to a smaller diffusion radius of the jet. This compression effect of the AMF on the plasma jet is related to both the intensity of the external AMF and the magnitude of the arc current. In the case of a constant arc current magnitude, the compression effect gradually increases as the value of the AMF intensity gradually increases; in the case of a constant value of the external AMF, the compression effect gradually decreases as the current gradually becomes larger.

The cathode spot (CS), as an intense source of inter-electrode arc plasma, plays a predominant role in maintaining the burning of vacuum arc, especially with an inactive anode. Consequently, the dynamics of CSs have a significant influence on the characteristics of the vacuum arc. Many experimental investigations have been devoted to better understanding of the motion of CSs, especially the initial expansion process of cathodes spots in high-current triggered vacuum arc. It has been indicated that the motion characteristic of cathode spots is greatly influenced by external axial magnetic field (AMF). In this work, a method is established to simulate the initial expansion process of CSs in high-current triggered vacuum arc, based on the proposed stepwise model of the motion of a single CS1. In this method, every new CS can be ignited in any direction around the old one with certain probability, which is connected with the magnetic field around the position of the old CS. With this approach, the initial expansion processes of CSs in free-burning and AMF-stabilized high-current triggered vacuum arc are simulated numerically with CSs initially uniformly distributed on a ring. The self-generated transverse magnetic field at the position of every CS is calculated by commercial software ANSYS with the current distribution in contact plate taken into account2. Simulation results agree well with relevant experiment results. Simulation results show that CSs expand faster without external AMF than that under AMF, and external AMF has significant influence on the distribution of CSs on the cathode surface, e.g., more and more CSs appear inside the ring when external AMF is present. Furthermore, the results also indicate that the expanding ring structure of CSs is unstable, and AMF can accelerate the breaking of the ring.

A dc low-pressure discharge in a helium-xenon mixture with the cathode spot on a flat oxide cathode has been investigated. The temperature of the cathode surface in the vicinity of the spot was determined experimentally. Furthermore, the gas temperature and the spatial density profile of the xenon 1s <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> metastable atoms were measured in front of the spot. A fluid model was applied to determine the particle densities and fluxes in the cathode region. This model comprises the particle balance equations for the charged particles and metastable atoms, Poisson’s equation, the energy balance equation for the electrons and a lookup table for the electron transport coefficients and collision rates determined from a kinetic treatment.

Summary form only given. The vacuum arc is a high current, low voltage electrical discharge in which a conducting medium is supplied in the form of vaporized electrode material. The plasma production is localized at one or several minute locations on the cathode (dependent on the arc current), known as cathode spots. The vacuum arc plasma originates from the cathode spots is a supersonic highly ionized plasma jet. Due to superior properties vacuum arc plasma jets have various applications. Historically high current interrupters were the first significant commercial application of the vacuum arc. The metal plasma jet produced by the vacuum arc is an attractive source of high-energy metal ion flux for the deposition of thin metallic films and multi-layers. A vacuum arc is excellent source of multiply charged metal ions and is used to create intense ion beams. Recently several other applications of the vacuum arc were proposed, such as plasma immersion ion implantation and plasma propulsion. Vacuum arc phenomena occur in unipolar arcs during current rise in Tokamaks and in exploding wires. The vacuum arc jet consists of ions, electrons, neutral vapor, and macroparticles (MP). MPs usually limit vacuum arc applications and considered as the major disadvantage. The MP's are molten droplets or solid particles (0.1-100 /spl mu/m in size), which are generated in the cathode spot region by action of the plasma pressure. In this work state-of-art of the vacuum arc plasma jet simulation will be reviewed. The principles of aforementioned applications will be also discussed. Several mechanisms of macroparticle elimination based on the fact that macroparticles are charged in the plasma will be considered. The vacuum arc plasma jet flow will be analyzed using the 2D hydrodynamic free plasma jet boundary expansion model. Several effects will be considered in this contest, such as multiply charged ions transport and separation, plasma jet behavior in a magnetic field and effects associated with electrode geometry. The diffuse vacuum arc in an axial magnetic field and V-shape arc voltage characteristics in a magnetic field will be discussed.