I. L. Epstein

Spectroscopy of microwave discharge in liquid C7–C16 hydrocarbons

Emission spectra of in-liquid microwave plasma in C7–C16 hydrocarbons were studied in the range of wavelengths from 200 to 800 nm. It was shown that spectra are similar for all studied hydrocarbons. Swan-bands only were observed in the plasma emission. Model of Swan-bands emission was designed for experimental spectra processing. Rotational and vibrational temperatures determined from sequences with v = −1, 0, +1 of C2-bands were 1600 ± 200 K and 7000 ± 2000 K correspondingly. Addition of Ar in the plasma decreased the rotational up to 700 K but did not change the vibrational temperature. It was shown that studied in-liquid microwave plasma is non-equilibrium. Results of electrodynamic modeling of microwave discharge apparatus and some information on the solid phase generated in hydrocarbon plasma processing were presented.

The Formation of Gas Bubbles by Processing of Liquid n-Heptane in the Microwave Discharge

Numerical modeling of the process of formation of gas bubbles during initiation of the microwave discharge in liquid n-heptane at atmospheric pressure has been performed. The developed model has an axial symmetry. The model is based on joint solution of the Maxwell equations, Navier–Stokes equation, heat equation, continuity equations for electrons (written in the ambipolar diffusion approximation) and the n-heptane concentration (including its thermal decomposition and dissociation by electron impact) and the Boltzmann equation for free electrons of the plasma. The calculations allowed to describe the dynamics of the formation of gas bubbles in the liquid, to evaluate the role of electron impact in the decomposition of n-heptane, and to estimate the characteristic times of various processes in the system. The results of new experiments are compared with the simulation results. On the basis of this comparison one could explain the presence in the spectra of the discharge only bands of C2.

Influence of a DC field on the near-electrode zone of nonuniform microwave discharge in hydrogen

We investigate, by the emission spectroscopy, the influence of the DC electric field on the nearsur-face plasma of the electrode microwave hydrogen discharge (EMD) at pressures of 1 to 5 Torr. We obtain the DC current-voltage characteristics of the EMD, the strength values of the microwave field, and its spatial distributions in the EMD near-electrode zone. Under a positive voltage on the electrode as against the chamber, the variation in the discharge structure are minor, and, at the particular voltages depending on the pressure and the microwave power, the spatial charge layer breakdown takes place near the discharge chamber surface. Under a negative voltage on the electrode (the plasma cathode microwave mode), the discharge structure and dimensions vary and, at high currents, the discharge cancels.

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

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 Maxwells 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.

Simulation of microwave-induced formation of gas bubbles in liquid n -heptane

A model has been built and the formation of gas bubbles by exciting an atmospheric-pressure microwave discharge in liquid n-heptane has been numerically simulated in the approximation of axial symmetry. The model is based on the simultaneous solution of Maxwell’s equations, Navier–Stokes equations, the heat conduction equation, a balance equation for the electron number density (using the ambipolar diffusion approximation), Boltzmann’s equation for free plasma electrons, and the overall equation for the thermal degradation of n-heptane. The two-phase medium has been described using the phase field method. The calculation has made it possible to describe both the dynamics of the formation of gas bubbles in the liquid and the thermal processes in the system. The calculated gas temperature in the gas bubble with the plasma is in agreement with the measurement results.

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.

Quasistatic simulation of microwave discharges in a spherically symmetric electrode system in nitrogen

Results are presented from one-dimensional quasistatic simulations of steady microwave discharges in a spherically symmetric electrode system in nitrogen at pressures of 1–8 Torr. The computational model includes the equation for calculating the electric field strength in the quasistatic approximation, Poisson’s equation, 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 particles are taken into account by the familiar analytic expression for the vibrational distribution of molecules in the diffusion approximation. It is shown that, because of the electric field nonuniformity, the physical properties (in particular, the plasma ion composition) are different in different discharge regions.

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.

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.