On plasma-neutral gas interaction

The paper emphasises the importance of plasma-neutral boundary layer in a wide variety of physical phenomena occurring in laboratory and cosmic plasmas. The interaction of a magnetised plasma stream penetrating a neutral gas cloud is discussed in the light of Alfvén’s critical velocity and Varma’s threshold velocity on the ionising interaction. Interaction of a moving magnetised plasma with a stationary neutral gas has been studied and described. The device comprises of a plasma gun and an interaction region where neutral gas cloud is injected. The interaction region is provided with a transverse magnetic field of upto 1000 G. Several diagnostics deployed at the interaction region to make measurements on the macroscopic parameters of plasma and neutral gas are described. The parameters of discharge circuits are measured with high current and voltage probes. An interaction between a magnetised plasma stream and a neutral gas cloud is demonstrated. It is shown that this interaction does not have Varma’s threshold on their relative velocity. The Alfvén’s critical velocity phenomenon is shown to depend on the integrated column neutral gas density that a plasma stream encounters while penetrating through it and not on the neutral gas density in the range of 10¹⁷–10²¹ m^?2.     

The nature and properties of a plasma

We have good reason to believe that much if not most of the matter in the universe is in the plasma state. This state can be described by comparing it with that of an ordinary gas, which is familiar to all of us, where large numbers of molecules move in all directions with different velocities along criss-cross paths, each sudden change of direction of movement being caused by a collision between two or more particles. But whereas in a gas all particles are electrically neutral, in a plasma charged fragments of formerly neutral molecules, positive and sometimes negative ions and electrons, take part in the random motion together with their neutral companions as well as with light quanta (von Engel 1955). In the gas of many stars, the sun, and certain apparatus in our terrestrial laboratories, charged particles predominate over neutral particles; these conditions approach a state called the fully ionized plasma (Spitzer 1956).     

BEHAVIOR OF Ar PLASMA FORMED IN A HIGH DENSITY PLASMA SOURCE-AN ECR REACTOR

In order to develop the ultra-large scale integration(ULSI), low pressure and high density plasma apparatus are required for etching and deposit of thin films. To understand critical parameters such as the pressure, temperature, electrostatic potential and energy distribution of ions impacting on the wafer, it is necessary to understand how these parameters are influenced by the power input and neutral gas pressure. In the present work, a 2-D hybrid electron fluid-particle ion model has been developed to simulate one of the high density plasma sources-an Electron Cyclotron Resonance (ECR) plasma system with various pressures and power inputs in a non-uniform magnetic field. By means of numerical simulation, the energy distributions of argon ion impacting on the wafer are obtained and the plasma density, electron temperature and plasma electrostatic potential are plotted in 3-D. It is concluded that the plasma density depends mainly on both the power input and neutral gas pressure. However, the plasma potential and electron temperature can hardly be affected by the power input, they seem to be primarily dependent on the neutral gas pressure. The comparison shows that the simulation results are qualitatively in good agreement with the experiment measurements.     

Numerical investigation of the plasma processes for propellant heating in electrothermal plasma thruster for nanosatellites

Numerical investigation of the plasma processes in a cylindrical chamber with small dimensions of a novel microwave electrothermal plasma thruster for nanosatellites has been conducted. The absorbed microwave power from the electrons in the plasma column of the surface wave discharge is included in the computational model as a heat source with Gaussian distribution. The computational model takes into account the elastic and inelastic collisions of the electrons with the atoms in the ground state and two excited states (−s, −p) and the processes of recombination and deactivation of the plasma species in the volume and on the walls of the chamber. The computational model includes the flow of neutral gas and the processes in the plasma for effective heating of neutral particles by collisions not only with electrons but also with ions. Selected combinations of input power and propellant mass flow rates are used as initial parameters for the numerical investigation. The results show that at higher mass flow rates the heating of the neutral gas is more effective and at power levels of 4 W and propellant mass flow rate of 3 mg/s the electrothermal plasma thruster demonstrates effective performance and thrust levels in the order of 1 mN.     

Beam-Plasma Heating Model

A one-dimensional ionization and heating model is applied to results of several electron-beam-plasma interaction experiments. Beam energy is deposited resistively in the plasma at a rate ?j2, where j is the return current density and ? the plasma resistivity both classical and anomalous due to ion acoustic or e-e-mode turbulence. Principal energy losses include ionization, line radiation, inelastic electron impact excitation, bremsstrahlung and radiative recombination. The level of ionization and plasma heating are computed as a function of neutral gas pressure, beam rise time, pulsewidth and current density, and resistivity model. Plasma dynamics and kinetic effects such as expansion and end loss are not explicitly included in the model.     

Modeling of the nanoparticle coagulation in pulsed radio-frequency capacitively coupled C_2H_2 discharges

The role of pulse parameters on nanoparticle property is investigated self-consistently based on a couple of fluid model and aerosol dynamics model in a capacitively coupled parallel-plate acetylene(C2H2) discharge. In this model, the mass continuity equation, momentum balance equation, and energy balance equation for neutral gas are taken into account.Thus, the thermophoretic force arises when a gas temperature gradient exists. The typical results of this model are positive and negative ion densities, electron impact collisions rates, nanoparticle density, and charge distributions. The simulation is performed for duty ratio 0.4/0.7/1.0, as well as pulse modulation frequency from 40 k Hz to 2.7 MHz for pure C2H2 discharges at a pressure of 500 m Torr. We find that the pulse parameters, especially the duty ratio, have a great affect on the dissociative attachment coefficient and the negative density. More importantly, by decreasing the duty ratio, nanoparticles start to diffuse to the wall. Under the action of gas flow, nanoparticle density peak is created in front of the pulse electrode,where the gas temperature is smaller.     

Investigations on the effect of process parameters on the growth of vertically oriented graphene sheet in plasma-enhanced chemical vapour deposition system

A multistage numerical model comprising the plasma kinetics and surface deposition sub-models is developed to study the influence of process parameters, namely, total gas pressure and input plasma power on the plasma chemistry and growth characteristics of vertically oriented graphene sheets (VOGS) grown in the plasma-enhanced chemical vapour deposition system containing the Ar + H₂ + C₂H₂ reactive gas mixture. The spectral and spatial distributions of temperature and number densities, respectively, of plasma species, that is, charged and neutral species in the plasma reactor, are examined using inductively coupled plasma module of COMSOL Multiphysics 5.2 modelling suite. The numerical data from the computational plasma model are fed as the input parameters for the surface deposition model, and from the simulation results, it is found that there is a significant drop in the densities of various plasma species as one goes from the bulk plasma region to the substrate surface. The significant loss of the energetic electrons is observed in the plasma region at high pressure (for constant input power) and low input power (for constant gas pressure). At low pressure, the carbon species generate at higher rates on the catalyst nanoislands surface, thus enhancing the growth and surface density of VOGS. However, it is found that VOGS growth rate increases when input plasma power is raised from 100 to 300 W and decreases with further increase in the plasma power. A good comparison of the model outcomes with the available experimental results confirms the adequacy of the present model.     

Effect of dust particles on the properties of low-temperature plasmas

Parameters of a low-temperature plasma containing dust particles are calculated numerically with the help of a self-consistent solution of the balance equation for production and recombination of electrons and ions, combined with the molecular dynamics method for direct simulation of processes in the vicinity of macroparticles. The relation between the charges of macroparticles and the neutral gas pressure, as well as the dependence of the low-temperature plasma parameters on the volume concentration of dust particles, is analyzed. It is shown that the plasma characteristics and composition may change noticeably relative to the case unperturbed by dust even for comparatively low concentration of dust particles.     

Modeling plasma loudspeakers

This paper deals with the acoustic modeling and measurement of a needle-to-grid plasma loudspeaker using a negative Corona discharge. In the first part, we summarize the model described in previous papers, where the electrode gap is divided into a charged particle production region near the needle and a drift region which occupies most of the inter-electrode gap. In each region, interactions between charged and neutral particles in the ionized gas lead to a perturbation of the surrounding air, and thus generate an acoustic field. In each region, viewed as a separate acoustic source, an acoustical model requiring only a few parameters is proposed. In the second part of the paper, an experimental setup is presented for measuring acoustic pressures and directivities. This setup was developed and used to study the evolution of the parameters with physical properties, such as the geometrical and electrical configuration and the needle material. In the last part of this paper, a study on the electroacoustic efficiency of the plasma loudspeaker is described, and differences with respect to the design parameters are analyzed. Although this work is mainly aimed at understanding transduction phenomena, it may be found useful for the development of an audio loudspeaker.     

Characteristics of single and dual radio-frequency (RF) plasma sheaths

The characteristics of radio-frequency (RF) plasma sheaths have been topics of much scientific study for decades, and have also been of great importance in the manufacture of integrated circuits and fabricating microelectromechanical systems (MEMS), as well as in the study of physical phenomena in dusty plasmas. The sheaths behave special properties under various situations where they can be treated as collisionless or collisional, single-or dual-RF, one-or two-dimensional (1D or 2D) sheaths, etc. This paper reviews our recent progress on the dynamics of RF plasma sheaths using a fluid method that includes the fluid equations and Poission’s equation coupled with an equivalent circuit model and a hybrid method in which the fluid model is combined with the Monte-Carlo (MC) method. The structures of RF sheaths behave differently in various situations and plasma parameters such as the ion density, electron temperature, as well as the external parameters such as the applied frequency, power, gas pressure, magnetic field, are crucial for determining the characteristics of plasma sheaths.     

Velocity distribution of plasma electrons in the negative H2- and He-glow with superimposed longitudinal magnetic field

The negative glow plasma has been found nearly field free in axial direction. Therefore plasma electrons in the stationary glow can thermalize down to the temperature of the neutral gas, whenever their diffusion—and recombination—lifetime is high enough. Applying Boltzmann's equation to this problem, the conditions of thermalization of plasma electrons are derived as a function of the outer parameters of the plasma: vessel diameter 2R, neutral gas pressurep and longitudinal magnetic fieldB. — If plasma electrons have a too short diffusion—and recombination—lifetime to be in thermal equilibrium with the neutral gas, the electron energy increases. For this case the distribution function of plasma electrons is derived using Boltzmann's equation. Approximating the calculated energy distribution by a Maxwellian distribution function, the electron temperature in the glow is obtained as a function of the parameters:R, p, B. OurT _e -measurements carried out in the H₂- and He-glows of different tube diameters, neutral gas pressures and magnetic fields agree closely with the theoretical results. TheT _e -measurements have been performed with Langmuir probes and by the method of reversal of the radial ambipolar electric field in a longitudinal magnetic field.     

Electrodynamic Model of the Gas Discharge Sustained by Azimuthal Surface Waves

Results of studying an electrodynamical model of the plasma producer based on azimuthal surface waves (ASW) propagation both in a cylindrical metal waveguide filled by magnetoactive plasma and along metal cylindrical antenna placed into the plasma are presented. Parameters of these gas discharges are studied in the two frequency ranges where propagation of the ASW is possible and also are compared. The value of plasma density that can be achieved by this producer is calculated. The spatial distribution of the wave electromagnetic fields, dependencies of their penetration depth into plasma and angular discharge length on the discharge parameters are studied analytically and numerically. Advantages of the operation in the both frequency ranges are indicated. Possibility of resonant absorption of the ASW energy due to their conversion into the bulk upper hybrid mode is examined as well.     

A model for steady-state large-volume plasma generation

A simple scheme for generating a uniform, steady-state, large-volume plasma is presented. The weakly magnetized plasma is created by direct ionization of the background gas by low-energy electrons generated from thermionic filaments. An annular arrangement of the filaments ensures a uniform plasma density in the radial direction as predicted by theory. Experiments have been performed to characterize the plasma generated in such a configuration. In order to explain the experimental observation, a bulk plasma theory based on plasma transport by means of cross-field diffusion is developed. As assumed in the theoretical model, the experimental measurements indicate a uniform plasma density along the axis. Both the theory and experiment indicate that the plasma density is a function of the square of the external magnetic field. The theory also predicts the plasma density to be proportional to the neutral density to the two-thirds power, in agreement with the experimental data. The experimental data agree well with theoretical prediction for a broad range of system parameters     

Influence of ambient gas ionization on the deposition of clusters formed in an ablation plume

The effect of inert gas ionization on the dynamics of a laser ablation plume expanding through a background inert gas is studied. Charge transfer reactions between ablated ions and neutral background gas atoms yield to the formation of a charged layer on the plume expansion front. The energy lost by ablated ions when the plume is slowed down is calculated. The observed microstructure differences between carbon films prepared by pulsed laser deposition in helium, where the ionization mechanism is absent and respectively in argon, where it is present, are well correlated to model predictions.     

Bilanzierung der Ozonsynthese im nichtthermischen Plasma

A simple model of a non-thermal oxygen plasma is used to calculate the ozon concentration as a function of the plasma parameters pressure, gas and electron temperature and electron concentration. In particular it is shown that the ozon yield depends strongly on the electron temperature and concentration except for certain intervals of parameters where a distinct plateau exists and the ozon concentration reaches its maximum value.     

充中性气体相对论返波振荡器的粒子模拟研究

 用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制，成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程，在电子束传输的路径上形成离子通道，有效中和电子束径向空间电荷力，有利于电子束的传输及束波相互作用产生微波。增加中性气体密度，返波管的输出频率明显上移，其辐射的功率和效率比相同的真空器件也有明显的提高。     

充中性气体相对论返波振荡器的粒子模拟研究

用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制,成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程,在电子束传输的路径上形成离子通道,有效中和电子束径向空间电荷力,有利于电子束的传输及束波相互作用产生微波。增加中性气体密度,返波管的输出频率明显上移,其辐射的功率和效率比相同的真空器件也有明显的提高。     

Modeling of electron cyclotron resonance discharges

A simple model to predict the spatially-averaged plasma characteristics of electron cyclotron resonance (ECR) reactors is presented. The model consists of global conservation equations for species concentration, electron density and energy. A gas energy balance is used to predict the neutral temperature self-consistently. The model is demonstrated for an ECR argon discharge. The predicted behavior of the discharge as a function of system variables agrees well with experimental observations     

Particle‐in‐cell/Monte Carlo simulation of electron and ion currents to cylindrical Langmuir probe

Electron and ion currents to a cylindrical Langmuir (electrostatic) probe were calculated using the particle‐in‐cell/Monte Carlo (PIC/MC) self‐consistent simulation for a neutral gas in the pressure range 2–3,000 Pa. The simulation enables us to calculate the probe currents even at high neutral gas pressures when the collisions of collected charged particles with neutral gas particles near the probe are important. The main aim of this paper is the calculation of probe currents at such high gas pressures and the comparison of the results with experimentally measured probe currents. Simulations were performed for two cases: (a) probes with varying radii in a non‐thermal plasma with high electron temperature at low neutral gas pressure of 2 Pa (in order to verify the correctness of our simulations), and (b) probe with the radius of 10 μm in the afterglow plasma with low electron temperature and a higher neutral gas pressure (up to 3,000 Pa). The electron probe currents obtained in case (a) show good agreement with those predicted by the orbital motion limited current (OMLC) theory for probes with radii up to 100 μm for the given plasma conditions. At larger probe radii and/or at higher probe voltages, the OMLC theory incorrectly predicts too high an electron probe current for the plasma parameters studied. Additionally, a formula describing the spatial dependence of the electron density in the presheath in the collisionless case is derived. The simulation at higher neutral gas pressures, i.e. case (b), shows a decrease of the electron probe current with increasing gas pressure and the creation of a large presheath around the probe. The simulated electron probe currents are compared with those of measurements by other authors, and the differences are discussed.