J. F. Behnke

Modelling of the homogeneous barrier discharge in helium at atmospheric pressure

Numerical calculations of spatio-temporal characteristics of the homogeneous barrier discharge in helium are performed by means of a one-dimensional fluid model. The influence of the elementary processes on the discharge behaviour is studied by variation of the corresponding rate constants. The simulation and the analytical interpretation are carried out for two basic modes of the homogeneous barrier discharge, i.e. the Townsend and glow modes. The Townsend discharge is characterized by the absence of quasineutral plasma; several current peaks may occur during the half-cycle. The oscillations of the current are caused by a lag between the ion production nearby the anode and the subsequent ion–electron emission on the cathode. The specificity of the glow discharge is the development of a cathode region and a positive column during the breakdown, as well as the presence of quasineutral plasma in subsequent phases. The positive column occurs because the shielding of the external field by the plasma is not instantaneous. The dependence of the discharge behaviour on the external parameters, such as the amplitude and frequency of the applied voltage, discharge gap width, and thickness of dielectric barriers, is analysed. The mode of the discharge is governed mostly by the gap width and barrier thickness and depends weakly on the amplitude and frequency of the applied voltage. As the barriers are thin and the discharge gap is sufficiently wide, the glow mode occurs; otherwise, the discharge is Townsend.

Influence of interaction between charged particles and dielectric surface over a homogeneous barrier discharge in nitrogen

A fluid model of the homogeneous barrier discharge is constructed for nitrogen at atmospheric pressure. The primary excitation and ionization processes specific for this discharge are pointed out. The calculations show that, in a wide range of external conditions, the homogeneous barrier discharge in nitrogen has a form of Townsend discharge which is easy to study. The influence of different mechanisms of electron emission from dielectric barriers and surface recombination over the electrical characteristics of a barrier discharge is studied. Introduction of a finite lifetime at the surface for adsorbed electrons allows us to obtain the results qualitatively corresponding to the experimental data.

Deposition Process Based on Organosilicon Precursors in Dielectric Barrier Discharges at Atmospheric Pressure—A Comparison

Dielectric barrier discharges (DBD) at atmospheric pressure are presented as a tool to create organosilicon deposits on technical planar aluminium substrates (up to 15 × 8 cm2) by admixing small amounts of hexamethyldisiloxane (HMDSO) and tetraethoxysilane (TEOS) to the carrier gas of the discharges. Using barrier materials of different specific capacities (2.6 × 104 and 3.2 pF/cm2) in two electrode arrangements operated at less than 1 W, the influence of the filament properties on the deposition is studied. In comparison to these arrangements, a third electrode setup with a barrier of the specific capacity of 2.9 pF/cm2 is operated at approximately 50 W to study the influence of the specific energy of the plasma (energy per molecule) on the deposition process. The plasma chemical process was studied qualitatively by Gas Chromatography, and properties of the plasma-treated substrates were examined by means of X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FTIR) spectroscopy, as well as visually.

Diffuse barrier discharges in nitrogen with small admixtures of oxygen: discharge mechanism and transition to the filamentary regime

Diffuse barrier discharges (BDs) are characterized by the periodicity of their discharge current and by the uniform coverage of the entire electrode surface by the plasma. Up to now the discharge development, their appearance and dynamics cannot be adequately explained by elementary processes. Different processes are discussed in the literature controversially, in particular the importance of volume and surface processes on the pre-ionization (Penning-ionization, secondary (?-) processes, role of surface charges). Diffuse BDs in nitrogen with small admixtures of oxygen are investigated by plasma diagnostics (current/voltage-oscillography, optical emission spectroscopy) and numerical modelling. Special attention is paid to the transition to the usual filamentary mode, characterized by the presence of micro-discharges and caused by the admixture of oxygen in the range of 0?1200?ppm (parts-per-million). This transition starts at low values of O2 (about 450?ppm) and is introduced by an oscillative multi-peak mode. At higher admixtures (about 1000?ppm) the micro-discharges are generated. According to the results of numerical modelling, secondary electron emission by N2(A?3?u) metastable states plays a major role in discharge maintenance. Due to the much more effective quenching of these states by O2 and NO than by N2 the subsequent delivery of electrons will be decreased when the oxygen amount is increased.

On the stability of a homogeneous barrier discharge in nitrogen relative to radial perturbations

The influence of small radial perturbations of the cathode current on the characteristics of a homogeneous barrier discharge in nitrogen is investigated on the basis of a two-dimensional fluid model. In a Townsend discharge, radial fluctuations are substantially suppressed, which is the evidence of its stability. The oscillative mode of the Townsend discharge is also stable with regard to radial perturbations. As the discharge turns into a form controlled by spatial charge (a streamer is developed), disturbances of all radii grow in time. Such a behaviour testifies the instability of a streamer front and may cause the discharge filamentation. Since only the Townsend discharge is stable, it is possible to use a one-dimensional model to determine the domain of existence for a homogeneous discharge. The study of homogeneity domains by means of the one-dimensional model shows that at relatively large values of the voltage growth rate, discharge gap width, or capacitance of dielectric barriers the discharge tends to be filamentary. Calculation of the domain of the discharge homogeneity agrees satisfactory with the experimental data available as the account is taken for the gas heating.

A comparison of kinetic and fluid models of the positive column of discharges in inert gases

The models of a positive column of an inert gas discharge at low pressures and small currents are developed in the frameworks of nonlocal kinetic and local fluid theories. Both models are based on the joint solution of the corresponding kinetic equation, ion-motion equation and Poisson equation that permit one to describe bulk plasma and near-wall sheaths of space charge. The theories differ principally in the descriptions of the electron component of a plasma. In the fluid theory the radial flux of electrons is a combination of field and diffusive fluxes, but in the kinetic model a similar representation is impossible. A comparison of the distribution functions and macroscopic properties calculated by kinetic and fluid theories for the discharge under the same conditions is performed. This comparison is carried out for the models of the positive column developed under the assumption of plasma quasi-neutrality and with regard to the deviations from quasi-neutrality. The use of the fluid theory beyond its application range (low pressures and small currents) does not reflect the physical picture of discharge sustenance and leads to incorrect results with respect to several parameters.