A. V. Tatarinov

Electrodynamics of microwaves in a coaxial non-regular waveguide partly filled with plasma

The electrodynamics of microwave fields (f = 2.45?GHz), propagating inside a coaxial non-regular system with a plasma-ball at the tip of a central electrode, have been simulated. For this purpose, the time-dependent Maxwells equations have been solved numerically in a two-dimensional grid. To outline the main properties of distribution of the microwave energy in such a system, various shapes and densities of quasi-neutral uniform plasma have been investigated. The simulations have been carried out for hydrogen in the range of pressures 0.5?8?Torr and densities 0.2?20nc. It was found that for under-dense plasma of any given configuration the absorbed power mostly concentrated near the tip of the internal electrode. For over-dense plasma, the microwave power is located both near the electrode and inside the surface layer of the plasma-ball. This distribution of absorbed microwave power is similar to the plasma light emission distribution observed in experiments. Transformation of the TEM wave into the surface wave has been demonstrated.

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.

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.

Spatial Structure of Emission from an Electrode Microwave Discharge in Hydrogen

The structure of electrode microwave (2.45 GHz) discharges in hydrogen with electrodes of various shapes and sizes at pressures of 1–8 torr and incident powers of 2–150 W is studied. It is found that the discharges exhibit a common feature that is independent of the antenna-electrode design: near the electrode surface, there is a thin bright sheath surrounded by a less bright, sharply bounded region, which is usually shaped like a sphere. It is suggested that the structure observed arises because the microwave field maintaining the discharge is strongly nonuniform. Near the electrode, there exists a thin dense plasma sheath with a high electron density gradient. A strong dependence of the electron-impact excitation coefficient on the electric field makes the effect even more pronounced. As the electron density decreases due to dissociative recombination, the microwave field gradient decreases and the discharge emission intensity tends to a nearly constant value. Presumably, in the boundary region of the discharge, there exists a surface wave, which increases the emission intensity at the periphery of the discharge.

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.

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.

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.