Electrode Microwave Discharge Research Articles

Chromatographic analysis of a cold plasma jet generated by an electrode microwave discharge in argon flow, interacting with atmospheric air

Application of gas chromatography in analysis of a cold plasma jet generated by an atmospheric pressure microwave discharge in argon flow was considered. Previously developed 2.45-GHz-plasmatron with the external 6-rod-electrode plasma torch was used as a microwave plasma source. The analysis of gaseous samples showed that CO concentration increases by 5-6 times and new gaseous products appear – H2 and CH4 as a result of plasma-gas interaction. The production of CO, H2 and O2 occurs in the processes of dissociation of CO2 and water vapor in the nonequilibrium plasma through the vibrationally excited states.

Emission spectrum analysis of an atmospheric electrode microwave discharge in argon flow and of a cold plasma jet on its base

The measurements and analysis of the emission spectra both of atmospheric-pressure electrode microwave discharge in argon flow and cold plasma jet induced by the discharge are conducted. We used experimental setup based on the previously developed multipurpose 2.45-Hz-plasmatron with the external portable discharge chamber (plasma torch) with the outlet of 2.5 cm in diameter and power of about several hundred watts. Discharge chamber has 6 rod-like electrodes which form a regular hexagon in a cross section of the torch. Discharge channels are formed between the ends of the electrodes and the inner wall of the chamber. Molecular bands of NO, OH, N2, NH and atomic lines of Ar were found in the spectrum of the discharge channels. Based on the analysis of the spectra, it was shown that the gas temperature in the discharge channel was about 1200 K. In the cold plasma jet spectrum, due to its weak luminescence, only the molecular lines OH and N2 were reliably observed.

Эмиссионные спектры аргонового электродного СВЧ-разряда атмосферного давления и холодной плазменной струи на его основе

Эмиссионные спектры аргонового электродного СВЧ-разряда атмосферного давления и холодной плазменной струи на его основе

Design and fabrication of a resonant-cavity electrode microwave discharge device and its performance in argon and xenon plasma

A new configuration of a resonant-cavity electrode microwave discharge device is developed in this work. In the device, a hollow electrode antenna is used for microwave transporting, and a well-designed resonant-cavity could enhance the absorption of the microwave power. A quartz cover is adopted to seal the vacuum and insulate the antenna from plasma, which greatly reduces the heat accumulation on the antenna. A magnetic field coil made by multiple-turn enameled wires is designed which could generate a strong magnetic field based upon a low-current power supply. In the plasma chamber, a distinctive L-shape probe is designed to measure the parameter distribution, and the two-dimensional parameters can be measured with the simple structure. In addition, the characteristic parameters such as electron density and temperature are measured in the chamber with a single probe, and the electron density distribution and spectrum are compared in argon and xenon plasma, which suggests that the microwave plasma device has good adaptability and functionality.

Effect of a DC external electric field on the properties of a nonuniform microwave discharge in hydrogen at reduced pressures

The effect of a dc external electrical field on the properties of a highly nonuniform electrode microwave discharge in hydrogen at a pressure of 1 Torr was studied using optical emission spectroscopy and selfconsistent two-dimensional simulations. It is shown that the negative voltage applied to the antenna electrode with respect to the grounded chamber increases the discharge radiation intensity, while the positive voltage does not affect the discharge properties. The simulation results agree well with the experimental data.

Influence of antenna dielectric cover on the parameters of the electrode microwave discharge

One possibility of reducing the influence of contamination of the plasma volume, when initiating the microwave discharges using antennas, is to cover the antenna with a dielectric. The results of 1-D and 2-D modeling showing the impact of such dielectric cover on the parameters of a non-equilibrium plasma of an electrode microwave discharge in hydrogen at reduced pressure (the change of the electrodynamics of the discharge, charge deposition on the dielectric surface, changing the catalytic properties of the surface of the antenna) are presented in the article.

Spatial distribution of the electron component parameters in the nitrogen plasma of a low-pressure electrode microwave Discharge

Spatial distributions of charged particle concentration, electron temperature, and DC potential in an electrode microwave discharge in nitrogen at a pressure of 1 Torr have been measured using the double electric probe method. It has been shown that, near the electrode/antenna, the charged particle concentration exceeds a critical value. The concentration and heterogeneity of the discharge increase with increasing microwave power.

Effect of small additives of argon on the parameters of a non-uniform microwave discharge in hydrogen at reduced pressures

Results of experiments and two-dimensional self-consistent simulation show that small (1–5%) additives of argon inserted in a hydrogen strongly non-uniform microwave discharge (electrode microwave discharge) reduce the emission intensity of excited particles, power absorbed in plasma, and electron and ion densities. Simulations have shown that this effect is linked with the reduction of the flux of ions to the electrode, which causes a decrease in the microwave field in plasma. These results illustrate the role of discharge non-uniformity in processes of discharge physics. It is shown that the argon admixture disturbs the discharge, and so the known method of optical actinometry cannot be used for plasma diagnostics in the case considered here. On the other hand, the addition of argon can be used to control plasma properties.

Vibrational distribution of nitrogen molecules in the C3Πu state in a near-surface microwave plasma in nitrogen at pressures of 1–5 Torr

The radiation of the second positive nitrogen system has been used to study the spatial dependence of the vibrational distribution of nitrogen molecules in the C3Πu state in the near-surface plasma layer of an electrode microwave discharge in nitrogen at pressures of 1–5 Torr. It has been shown that the vibrational distribution changes at a scale of 100 μm. It has been concluded that this state is populated owing to the electron impact from the ground state. The possibility of using the local approximation for the electron energy distribution function to explain the experimental results has been analyzed.

Effect of the dc field on the near-surface plasma of a highly nonuniform microwave discharge

The effect of the dc electric field on the near-surface plasma of an electrode microwave discharge at pressures of 1–5 Torr was studied by the emission spectroscopy method. It is shown that the dc field weakly affects the vibrational distribution of nitrogen molecules in the C3Πu state, but changes the structure of the near-surface plasma (shifting the intensity maxima of the emission bands) and the strength of the microwave field near the electrode surface. It is also found that the ratio between the intensities of bands of different sequences of the second positive system of nitrogen radiated from the same state depends on the position along the discharge axis.

The Influence of Small Hydrogen Admixtures up to 5 % to a Low Pressure Nonuniform Microwave Discharge in Nitrogen

We present the results of spectroscopic and 2D modeling study of the influence of hydrogen addition to non-uniform nitrogen plasma of electrode microwave discharge. The axial intensity distributions of the Hα line and N2 (2+: C3Πu → B3Πg, 1+: B3Πg → A3Σ u + ) and N2 + (1−: B2Σ u + → X2Σ g + ) molecular bands are recorded for different incident microwave power input and hydrogen content. The corona model is used to determine electric field strength in nitrogen discharge from the intensity ratio of 2+ and 1− system of nitrogen bands. By means of 2D modeling spatial distributions of nitrogen molecules in C3Πu state, microwave field strength, electron density, concentrations of N2 +, N4 + are determined in nitrogen and in nitrogen–hydrogen mixtures. The concentration of N2H+ ions in nitrogen–hydrogen mixtures is determined also. The general conclusion of 2D modeling is in agreement with experimental results and shows that the influence of hydrogen addition to discharge is related to the fast conversion reactions of nitrogen ions (N2 +, N4 +) to N2H+ ion. These ion conversions lead to the change of ion plasma components transport properties, to the modification of microwave field strength in plasma, and consequently, to the alternation of all plasma parameters.

The effect of small additives of argon on the parameters of a non-uniform microwave discharge in nitrogen at reduced pressures

The influence of a small addition of argon (2–5%) on the parameters of a strongly non-uniform microwave discharge (with the electrode microwave discharge as an example) in nitrogen at reduced pressures was studied. Experiments showed that the small addition of Ar strongly affected the discharge: it increased in size, and the power absorbed in the plasma and the emission intensities of nitrogen bands reduced. A self-consistent 2D modeling of the discharge was carried out. The model included the Maxwell, Poisson and Boltzmann equations and a set of balance equations for neutral excited and charged plasma species. The processes involving vibrationally excited ground state molecular nitrogen were taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. The results of modeling and experiment were in qualitative agreement. Additional information about the discharge allowed us to explain the experimental results. It was shown that this effect was influenced by a strong spatial non-uniformity of direct electron impact ionization rate, difference in diffusion and mobility coefficients, and difference in diffusion and volume processes of the loss of the main ions. This effect can be observed in all types of discharges if these conditions are satisfied.

3D simulation of electrodynamics of a microwave low pressure discharge

The structure of microwave fields in a discharge chamber used for obtaining electrode microwave discharge is investigated. It is shown that 3D-simulation allows quantitatively determining the optimumal structural parts of the discharge chamber for discharge generation. The represented results of self-consistent simulation of the discharge in nitrogen and hydrogen at a pressure of 1 torr are qualitatively consistent with experimental results.

Nonuniform microwave discharge in a nitrogen-hydrogen mixture

The method of optical emission spectroscopy is used to investigate the influence of additions of hydrogen on the emission characteristics of a highly nonuniform electrode microwave discharge in nitrogen at a pressure of 1 torr. It is demonstrated that the pattern of the influence made by hydrogen addition (zero to 50% with respect to flow rate of hydrogen) on the intensity of emission bands of nitrogen is different in different parts of the discharge. It is further observed that the influence of hydrogen on the vibrational distribution of nitrogen molecules in the C3Πu state is different C3?uin different parts of the discharge. Analysis is made of various processes involving hydrogen, which affect the emission of nitrogen bands. An inference is made that ion conversion is an important mechanism of such influence. One-dimensional simulation is performed in view of these processes, and it is demonstrated that the experimentally observed effects may be associated with this mechanism.

Spectroscopic study of an electrode microwave discharge in argon and argon–hydrogen mixtures

The results of an optical emission spectroscopy study of low-pressure microwave induced discharge in argon, argon–hydrogen (0.9% H 2) and hydrogen–argon (5% Ar) mixture are reported. At four different pressures the Boltzman plot of relative ArI line intensities is used to measure electron excitation temperature, which was close to 3000 K in argon and in argon–hydrogen mixture discharges. The spatial distributions of light emission from argon and hydrogen–argon discharges are recorded with CCD camera and compared with spectroscopically observed spatial distributions of the ArI 415.8 nm line intensity in argon and the H β line intensity in hydrogen–argon mixture at three different pressures. The variation of light intensity with pressure for argon lines ArI 842.5 nm, ArI 750.3 nm and ArI 419.8 nm as well as for hydrogen H β line is also studied. For high- and low-lying energy levels in argon and in argon–hydrogen mixture, a different dependence of spectral line intensity upon gas pressure is detected. In hydrogen–argon mixture, a non-linear decrease of argon and hydrogen spectral line intensity with an increase of gas pressure is observed.

Electrode microwave discharge: Areas of application and recent results of discharge physics

The first paper on the electrode microwave discharge (EMD) appeared in 1996. Presently many problems of EMD physics and applications have already been solved. Several examples of EMD application are discussed: diamond growth, deposition of CNx films and nanotubes, deposition of metal films (Cu, Al), deposition of TiN and TiO2 films, generation of O2(a1Δ), and EMD as a plasma cathode. Results of EMD experiments and modeling give rise to the assumption that an EMD consists of a self-sustained domain (near-electrode plasma region with overcritical plasma density) which is surrounded by a region of a non-self-sustained discharge (ball shaped region with undercritical plasma density). We assumed that the layer of charge separation and of induced electrostatic field originated at the outer EMD boundary was one of the reasons for the abrupt decrease of the plasma density which leads to the formation of a compact plasma structure. Recent modeling results of the strongly nonuniform electrode microwave plasma based on a quasi static, 1D spherically symmetric model showed that such a layer can be generated at the point where a sudden increase of the total ionization rate takes place.

Physics and microstructure of electrode microwave discharge

The general features and peculiarities of the stationary electrode microwave discharge (EMD) are presented. EMD is ignited near the tip of the powered electrode (antenna) and its luminous plasma region is less than the dimensions of the discharge chamber. The possible areas of EMD application are demonstrated. Results of spatially resolved optical emission spectroscopy of nitrogen EMD are presented. The joint experimental–simulation method is used to process the 2+ nitrogen band emission for measurements of electron density and electric field strength near the electrode plasma sheath of the nitrogen discharge. The concept of a ‘self-maintained–non-self-maintained’ mechanism of EMD is substantiated.

Microwave electrode discharge in nitrogen: Structure and characteristics of the electrode region

The parameters of the electrode region of an electrode microwave discharge in nitrogen are studied by emission spectroscopy. The radial and axial distributions of the intensities of the bands of the second (N2(C3Πu → B3Πg)) and first (N2(B3Πg → A3Σu+)) positive systems of molecular nitrogen and the first negative system of nitrogen ions (N2+ (B2Σu+ → X2Σg+)), the radial profiles of the electric field E and the electron density Ne, and the absolute populations of the vibrational levels vC = 0–4 of the C3Πu excited state of N2 and the vibrational level vBi = 0 of the B2Σu+ excited state of a molecular nitrogen ion are determined. The population temperature of the first vibrational level TV of the ground electronic state X1Σg+ of N2 and the excitation temperature TC of the C3Πu state in the electrode region of the discharge are measured. The radius of the spherical region and the spatially integrated plasma emission spectra are studied as functions of the incident microwave power and gas pressure. A method for determining the electron density and the microwave field strength from the plasma emission characteristics is described in detail.

Modeling of the electrode microwave discharge in nitrogen

Self-consistent two-dimensional modeling of a steady microwave discharge initiated at the end of the central electrode in nitrogen is presented. The discharge parameters are calculated at a gas pressure of 1 Torr and incident power of 30 W. The computational model includes the time-dependent Maxwell's equations, the balance equations describing the kinetics of charged and neutral plasma particles and the time-independent homogeneous Boltzmann equation for electrons. The processes involving vibrationally excited ground state molecular nitrogen are taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. It is shown that the spatially non-uniform microwave field causes the difference in plasma particles kinetics in different parts of the discharge. Results of numerical simulations for space distribution of electronically excited molecules have been found in good qualitative agreement with those taken from spectral measurements of first and second positive systems of nitrogen. Results confirm the concept according to which such a discharge comprises a self-sustained and a non-self-sustained discharge.

An electrode microwave discharge in a static field

The effect of an external static electric field on an electrode microwave discharge in nitrogen is investigated at pressures of 1 to 5 Torr and incident power of 20 to 170 W. It is demonstrated that the current-voltage characteristics (CVC) of the discharge with respect to direct current are nonlinear; the nonlinearity defining the CVC is the plasma layer at the surface of the discharge chamber. Qualitative agreement is obtained between the measured CVC and those calculated using the self-consistent one-dimensional quasi-static model of microwave discharge in a system of electrodes with spherical symmetry. It is demonstrated that the applied dc voltage affects only slightly the discharge characteristics; however, it enables one to control flows of charged particles to the surface of the discharge chamber.