E. A. Bogdanov

Simulation of pulsed dielectric barrier discharge xenon excimer lamp

Recently, it has been shown that the efficiency of excimer lamps can be drastically increased in a pulsed regime. A one-dimensional simulation of pulsed excimer lamps has been performed by Carman and Mildren (2003 J. Phys. D: Appl. Phys. 36 19) (C&M). However, some computational results of the work of C&M are questionable and need to be revisited. In this paper, a dielectric barrier discharge (DBD) in xenon has been simulated for operating conditions similar to those of C&M to better understand plasma dynamics in a pulsed regime. Our simulation results differ considerably from the computational results of C&M. Although these differences do not affect profoundly the plasma macro parameters measured in the C&M experiments, they offer a better understanding of plasma dynamics in pulsed DBDs and form a solid foundation for computational optimization of excimer lamps. It was found that the dynamics of breakdown and the current pulse depend significantly on the initial densities of species after a previous pulse, and so it is important to accurately simulate the plasma evolution in both the afterglow and active stages. It seems possible to modify the power deposition in the plasma by varying external discharge parameters such as the amplitude and the rise time of the applied voltage, and to modify the plasma composition by changing the pulse repetition rate and plasma decay in the afterglow stage.

Influence of the transverse dimension on the structure and properties of dc glow discharges

Two–dimensional (2D) simulations of a dc glow discharge with a cold cathode in argon have been performed for various radii of the discharge tube. It is shown that the loss of the charged particles to the walls can significantly affect plasma parameters as well as properties of the cathode sheath. The longitude dimensions of the negative glow and Faraday dark space depend on the transverse loss of the charge particles and are not consistently predicted with a 1D model. The common assumption that the cathode sheath can be analyzed independently of the plasma also may not be valid. The transverse inhomogeneity of the plasma leads to a change in the current density distribution over the cathode surface. The thickness of the cathode sheath can vary with radial distance from the discharge axis, even for the case of negligible radial loss of the charge particles. The 2D model results provide an analysis of the conditions of applicability of the 1D model.

The conditions for realization of the Boltzmann distribution of negative ions in a plasma

The self-consistent electric field acting in a plasma retards the most mobile charged particles, which usually leads to a Boltzmann distribution of electrons. If negative ions cross the discharge volume several times during their lifetime in the volume processes, these particles also obey the Boltzmann distribution. It is demonstrated that this condition is usually satisfied when the characteristic time of electron attachment is small as compared to the time of ambipolar diffusion of the negative ions (ion diffusion at an electron temperature). In the opposite case, the profiles of electrons and negative ions are similar.

Substantiation of the two-temperature kinetic model by comparing calculations within the kinetic and fluid models of the positive column plasma of a dc oxygen discharge

Results from kinetic and fluid simulations of the positive column plasma of a dc oxygen discharge are compared using commercial CFDRC software (http://www.cfdrc.com/˜cfdplasma), which enables one to perform numerical simulations in an arbitrary 3D geometry with the use of both the fluid equations for all the components (fluid model) and the kinetic equation for the electron energy distribution function (kinetic model). It is shown that, for both the local and nonlocal regimes of the formation of the electron energy distribution function (EEDF), the non-Maxwellian EEDF can satisfactorily be approximated by two groups of electrons. This allows one to take into account kinetic effects within the conventional fluid model in the simplest way by using the proposed two-temperature approximation of the nonequilibrium and nonlocal EEDF (2T fluid model).

Scaling laws for the spatial distributions of the plasma parameters in the positive column of a dc oxygen discharge

Comprehensive self-consistent simulations of the positive column plasma of a dc oxygen discharge are performed with the help of commercial CFDRC software (http://www.cfdrc.com/~cfdplasma), which enables one to carry out computations in an arbitrary 3D geometry using fluid equations for heavy components and a kinetic equation for electrons. The main scaling laws for the spatial distributions of charged particles are determined. These scaling laws are found to be quite different in the parameter ranges that are dominated by different physical processes. At low pressures, both the electrons and negative ions in the inner discharge region obey a Boltzmann distribution; as a result, a flat profile of the electron density and a parabolic profile of the ion density are established there. In the ion balance, transport processes prevail, so that ion heating in an electric field dramatically affects the spatial distribution of the charged particles. At elevated pressures, the volume processes prevail in the balance of negative ions and the profiles of the charged particle densities in the inner region turn out to be similar to each other.

Different approaches to fluid simulation of the longitudinal structure of the atmospheric-pressure microdischarge in helium

Numerical results for different versions of the fluid model of an atmospheric-pressure glow discharge in helium are compared. It is shown that efforts to improve the fluid model are to a large extent prospectless and often even impair previous results. This is because the fluid model has fundamental limitations when describing heavily nonequilibrium media, such as the gas discharge. In such systems, the properties of an ensemble of electrons cannot be reduced to the behavior of an “averaged particle,” which is characterized by the averaged concentration, averaged directional velocity, and averaged energy (temperature). In particular, the values of the electron temperature in the near-cathode plasma obtained by fluid simulation far exceed both the available experimental data and physical estimates. It is therefore necessary to develop consistent kinetic techniques to correctly describe the behavior of electrons in the near-cathode plasma.

Scaling Laws for Oxygen Discharge Plasmas

A fluid model is used to simulate ICP discharges in oxygen for a wide range of conditions under which commercial plasma-chemical reactors typically operate. Simple scaling laws are constructed with which different parameters of discharge plasmas in electronegative gases can be readily estimated from the given external parameters—the specific input power W and the product pL of the gas pressure and the characteristic plasma dimension.

Is the negative glow plasma of a direct current glow discharge negatively charged

A classic problem in gas discharge physics is discussed: what is the sign of charge density in the negative glow region of a glow discharge? It is shown that traditional interpretations in text-books on gas discharge physics that states a negative charge of the negative glow plasma are based on analogies with a simple one-dimensional model of discharge. Because the real glow discharges with a positive column are always two-dimensional, the transversal (radial) term in divergence with the electric field can provide a non-monotonic axial profile of charge density in the plasma, while maintaining a positive sign. The numerical calculation of glow discharge is presented, showing a positive space charge in the negative glow under conditions, where a one-dimensional model of the discharge would predict a negative space charge.

Spatial Distribution of Parameters in Normal Micro-DC Glow Discharge in Air

Design of devices based on glow microdischarges requires a detailed knowledge of spatial distribution of the main plasma parameters, which is almost impossible to obtain experimentally because of the small size of such discharges. This paper presents the results of simulation for flat microdischarge in air using an extended hydrodynamic model. Images of cathode spots for different discharge currents are presented, as well as current-voltage characteristics.

Modeling a short dc discharge with thermionic cathode and auxiliary anode

A short dc discharge with a thermionic cathode can be used as a current and voltage stabilizer, but is subject to current oscillation. If instead of one anode two anodes are used, the current oscillations can be reduced. We have developed a kinetic model of such a discharge with two anodes, where the primary anode has a small opening for passing a fraction of the discharge current to an auxiliary anode. The model demonstrates that the current-voltage relationship of the discharge with two anodes is characterized everywhere by positive slope, i.e., positive differential resistance. Therefore, the discharge with two anodes is expected to be stable to the spontaneous oscillation in current that is induced by negative differential resistance. As a result, such a discharge can be used in an engineering application that requires stable plasma, such as a current and voltage stabilizer.