A Probe Method for Determination of time Evolution of Metastable Atoms Density in a Flowing Afterglow Plasma

The Langmuir Probe Measurement of the electron density and electron temperature has been made in the argon flowing afterglow plasma. Furthermore the metastable and charged particle densities evolution in the afterglow plasma are investigated by means of a numerical solution of the set of balance equations. From the fitting of the results of these calculations to the experimental data obtained from Langmuir probe measurements it is possible to determine the evolution of the metastable atoms density in the flowing afterglow plasma.     

A contribution to the study of the influence of metastables in the flowing afterglow plasma

The paper describes an experimental apparatus based on the flowing afterglow technique which enables the study of the role of metastable atoms of the carrier gas by means of titration of the reactant vapour of gas into the flow tube. A study of the behaviour of the electron and ion density and electron distribution of Ar* metastables (Ar as the carrier gas) using Penning ionization of the ethyl alcohol (CH₃CH₂OH) (titration vapour) has been made by means of a described experimental set-up. The method of estimation of the metastables concentration in the decaying flowing afterglow plasma by Langmuir probe technique with use of titrant technique is also studied.     

Pressing to the extremes

In the last decade the study of ion-neutral reactions has been dramatically advanced by the development and energetic exploitation of the steady-state flowing afterglow method by Eldon Ferguson and his colleagues at the National Oceanic and Atmospheric Administration (NOAA), Boulder, Colorado. This new technique has provided a very large number of ion–molecule rate coefficients for reactions at thermal energy.

In this method a fast flow of carrier gas (usually helium) is established in a long flow tube, the reactant ions (A⁺) are generated near the gas input end of the tube (the excitation region) and these ions are carried along the tube in the carrier gas flow. The neutral reactant (B) is introduced into the flowing gas stream near the mid-point of the flow tube so that the reaction A⁺+ B →products, takes place along the remainder of the tube length (the reaction region). The reactant ion A⁺ and any ions produced in the reaction are monitored by a mass spectrometer located at the gas exit end of the tube. The variation of the A⁺ mass spectrometer signal as a function of neutral reactant injection rate B yields the rate coefficient k for the reaction if gas flow rates and tube dimensions are known.

The great advantage and versatility of the steady-state flowing afterglow system lies primarily in the separate control that can be exercised over the ions and the neutrals prior to the reaction and also to the fact that the atomic processes occurring in the afterglow are susceptible to many diverse forms of investigation such as emission and absorption spectroscopy, laser spectroscopy, microwave interferometry and Langmuir probes, as well as mass spectrometry. Furthermore, chemically unstable neutral particles can be readily studied in these systems.     

Non-Maxwellian electron energy distribution function in He,He/Ar,He/Xe/H<Subscript>2</Subscript> and He/Xe/D<Subscript>2</Subscript> low temperature afterglow plasma

Experimental studies of the electron energy distribution function “EEDF” under well defined conditions in flowing afterglow plasma, using a Langmuir probe are reported. The EEDF is measured in He₂ ⁺ and Ar⁺ dominated plasmas and in XeH⁺ and XeD⁺ dominated recombining plasmas. He is used as a buffer gas at medium pressures in all experiments (1600 Pa, 250 K). The deviation of the measured EEDF from Maxwellian distribution is shown to depend on plasma composition and on the processes governing the plasma decay. The influence of energetic electrons produced during the plasma decay on the body and tail of the EEDF is observed. The mechanism of energy balance in afterglow plasma is discussed.     

Ar(3P0,2)与H2O传能反应生成OH自由基的机理和初生态转动分布

利用流动余辉技术研究了亚稳态Ar(3P0,2)与H2O碰撞传能反应生成OH自由基的动力学过程,测定了产物的转动态分布,并讨论了OH自由基在实验条件下的转动驰豫情况。     

Kinetic simulations of argon dusty plasma afterglow including metastable atom kinetics

The afterglow of a dusty plasma of rf discharge in argon is simulated by the particle-in-cell-Monte Carlo collision (PIC-MCC) method. The experimental observation that heavy dust contamination of plasma leads to an anomalous increase in the electron density at the beginning of afterglow is explained by release of electrons from the dust surface. Under the assumption that the floating potential of particles is in equilibrium with plasma conditions, the fast cooling of electrons in afterglow plasma due to a rapid escape of hot electrons from the volume leads to a decrease in the magnitude of the floating potential and hence to a loss of charge by dust. The intensive desorption of electrons from nanoparticles is the origin of anomalous behavior of the electron density. At the next stage of afterglow, when the electrons become cool, the plasma decay is defined by ambipolar diffusion. The effect of metastable argon atoms is also considered. Additional ionization due to metastable atom collisions affects the electron temperature but does not change the behavior of the electron density qualitatively.     

Radial distribution of the atom concentration in a nitrogen discharge plasma

The radial distribution profile of the atom concentration in a nitrogen discharge plasma has been determined by means of the Lewis-Rayleigh afterglow radiation. The profile has been investigated in dependence of the pressure and of the discharge current.     

The low-temperature afterglow in N2–ε·CH4 gas mixtures

The influence of the small methane impurities on the pure nitrogen afterglow was studied in DC flowing plasma reactor in wide range of methane concentrations (0.01–400 ppm) at different wall temperatures of the reactor observation part (77–300 K). The relative vibrational distributions of states have been calculated from the recorded spectra in pure nitrogen. We observed strong quenching of the nitrogen pink afterglow at methane concentrations of a few ppm, however the pink afterglow intensity was growing up at the methane concentrations under 1 ppm. Simultaneously, the maximum pink afterglow intensity was observable at later decay times with the increase of the methane concentration. At low wall temperatures, especially at later decay times, we observed extremely high sensitivity of the pure nitrogen to the methane pretence in the discharge. Thus we are able to detect the methane concentrations in order of 0.01 ppm. We also observed the higher transitions of the CN violet system which are usually observed in the spectra of space emission sources. The method detecting hydrocarbon and fluorocarbon impurities in pure nitrogen is based on the results of the above mentioned experiments. We present first results of some experiments studying the polyhydrocarbon destruction rates measured by this new sensitive method. Finally, we designed also the simple kinetic model describing the processes during the afterglow in the N₂–·CH₄ mixture.     

Surface modification of polypropylene with an atmospheric pressure plasma jet sustained in argon and an argon/water vapour mixture

In this paper, an atmospheric pressure plasma jet sustained in pure argon and an argon/water vapour mixture has been used to modify the surface of polypropylene (PP) films. The gas temperature of the plasma jet was found to be 625 K in an active zone between the electrodes and was found to increase in the afterglow. Based on these results, the PP films are placed as close as possible to the edge of the capillary in order to avoid thermal damage to the polymer. XPS results on the untreated and modified PP samples revealed incorporation of a significant amount of oxygen on the polymer surface, however, this oxygen inclusion is more pronounced for the argon/water vapour jet due to the higher radicals density in the jet afterglow. One can therefore conclude that adding water vapour to an argon plasma jet can be a convenient way to increase the efficiency of plasma surface modification.     

The application of time delay method for analysis of processeswhich initiate electrical breakdown in 1.3 mbar nitrogen

The contributions of ions and neutral active states to secondary electron emission in the afterglow has been analyzed of the time delay method. The dependencies of mean value of time delay on relaxation time, overvoltage, and glow current have been discussed. The analysis of validity of Laue distribution for time delay has been performed, becoming the criterion for estimation of the main cause of breakdown. Thus, afterglow can be divided in three regions: one each for the role of charged and neutral species in initiation of subsequent breakdown, and third, as the transitional one     

Photodetachment as a Control Mechanism for Diffuse Discharge Switches

Photodetachment is considered as a control mechanism for diffuse discharge switches. Experiments have been performed on photodetachment of ions in the flowing afterglow of a dc glow discharge in oxygen. Experiments with different laser wavelengths and the dependence of the optogalvanic signal on the laser energy flux indicate that O- is the dominant negative ion. For an energy flux of 35 mJ/cm2, 50percent of the O- ions can be photodetached.     

Measurements of some reaction rates of importance in KrF lasers

A flowing afterglow method has been used to study a number of reactions of importance in KrF lasers using NF₃ as the fluorine donor. Total rate constants are reported for Kr^*(³ P ₂)+NF₃ and Kr⁺+NF₃. The thermal energy attachment coefficient has been measured for NF₃ and the cross section for the electron excitation process Kr^*(³ P ₂) +e→Kr^*(³ P ₁)+e has been estimated. The relevance of these results to discharge pumped KrF lasers is discussed.     

Monte Carlo Simulations of the Influence of Ion‐Neutral Collisions on the Ion Currents Collected by Electrostatic Probes

We have carried out Monte Carlo simulation of the motion of Ar⁺ ions in the space charge sheath surrounding a cylindrical Langmuir probe. The ion currents to the probe have been calculated from these simulations and the percentages of ions crossing the sheath boundary that are collected by the probe have been determined. It has been shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory at lower helium pressure and decrease this percentage below the OMLC theory prediction at higher helium pressure. It has been shown also that the ion current almost does not depend on probe radius at higher helium pressures. The results of the simulations have been compared with recent Langmuir probe measurements made in flowing afterglow plasmas and with other probe theories.     

Methane Conversion in the Flowing Afterglow of a Dinitrogen Microwave Plasma: Initiation of the Reaction

The reaction of methane in the nitrogen afterglow is studied by combination of direct measurements and computer experiment. The experimental data were obtained by optical emission spectroscopy of the discharge, by gas chromatography of resulting stable products and by probe diagnostics of charged species in afterglow plasma. The simulation was based on a macroscopic kinetic approach covering 24 species and 61 reactions with input data given by the afterglow experiment. In the present stage of the modelling the initiation of methane conversion was studied. It was found that, contrary to active discharge, in the afterglow plasma the active neutral species (mainly excited dinitrogen molecules) are most important for the dissociation of methane into CH_x and H radicals.     

CS2分子Penning电离的流动余辉光谱测量

用PIOS(Penning Ionization Opnieal Spectroseopy)方法首次观察到470～490nm间CS~+(A-X)13-2、14-3的发射带,发现此发射带偏重于在Ne/CS_2中产生.这些谱带的产生可用二次碰撞模型加以解释.     

Peculiarities of plasma decay in the afterglow of a low-pressure pulsed discharge in oxygen

Weakly ionized plasma of a pulsed-discharge afterglow in oxygen at low pressures (0.05–0.15 torr) is investigated using probe diagnostics. The plasma conductivity is measured by supplying an additional probing current pulse at a certain instant during the afterglow. The spectral line intensities are also measured to additionally monitor the densities of charged particles. The measurements of the time behavior of the electron density in an oxygen afterglow plasma confirm the previous conclusion that the electrons escape due to enhanced diffusion, which results in the formation of an ion-ion plasma. The possibility of realizing the opposite ultimate case—the detachment decay regime with an increase in the electron density to the density of positive ions in the first stage and the transition to the electron-ion plasma in the second stage—is demonstrated.     

Recombination in Dense Ion Plasmas

A theory is developed to treat ion recombination in dense ion plasmas. Numerical results are obtained with the method of molecular dynamics. Exponential suppression of recombination is found for the discharge afterglow plasma in sulfur hexafluoride at strong Coulomb nonideality Γ. The reason for suppression is an increase of the recombination activation energy ΔE with increase of Γ. The range ΔE corresponds to the multiparticle fluctuations. Ion solvation is another suppression factor. Its importance is demonstrated for the discharge afterglow plasma in fluorine at weak Coulomb nonideality. The theory developed agrees with the experimental data available. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cluster generation from flowing plasma

The method of generation of cluster beams is analyzed in the regime when large clusters grow in a flow of a dense afterglow plasma and clusters are formed in a narrow region near the axis of this flow. This method gives a high intensity of the cluster beam in comparison with standard methods of cluster generation. Numerical parameters are evaluated for processes involving iridium clusters in an argon plasma.