Kinga Kutasi

Theoretical insight into Ar–O2 surface-wave microwave discharges

A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar–O2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surface-wave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar–O2 mixture at 0.5–3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density for surface-wave propagation, ne = 3.74 × 1011 cm−3.It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O2 to pure Ar, and their creation and destruction processes have been identified. The O2 dissociation degree increases with Ar addition into O2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, but decreases with the atomic surface recombination of O-atoms. The density of O− negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions.

On the reliability of low-pressure dc glow discharge modelling

Modelling approaches used for the description of the cathode region of dc glow discharges are reviewed, with the focus on hybrid models which combine the fluid description of positive ions and bulk electrons with the kinetic simulation of fast electrons. The reliability of the calculated discharge characteristics is analysed by testing the different assumptions of the models and the sensitivity of the calculated characteristics on the input data. The applicability of the particle-in-cell technique (complemented with Monte Carlo simulation of collision processes) for the simulation of dc glow discharges is also discussed.

Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring

Self-consistent theoretical models have been developed in order to investigate the early and remote flowing afterglows of a surface-wave Ar?O2 microwave discharge generated at 2.45?GHz in a 0.5?cm diameter tube at pressures between 1 and 12?mbar. The early afterglow that occurs downstream of the discharge fills up the tube that connects the discharge region with the large-volume processing reactor, where the late afterglow develops. The models provide the time-dependent density profiles of different species along the afterglow and their 3D spatial distribution in the processing reactor. Systematic calculations are performed for all mixture compositions from pure Ar to pure O2 at different pressures.It is shown that the Ar+, and can survive up to 1?10?ms in the early afterglow depending on the mixture composition and pressure. In low O2 content mixtures the ion densities can increase in the early afterglow, depending on the operating conditions, as a result of Penning ionization involving the Ar(4s) states and forming Ar+, followed by charge transfer. In pure Ar the UV emitting resonant state atoms remain up to 0.1?ms in the afterglow, but with O2 addition their lifetime becomes considerably shorter. The oxygen species important for many applications, such as O(3P) atoms and O2(a) metastable molecules, survive up to 100?ms, thus are the main components of the late afterglow. It is shown that the O2 molecules are strongly dissociated in the discharge, dissociation being more efficient in high Ar content mixtures. However, the dissociation degree decreases to a few per cent in the early afterglow in about 10?ms. In the case of O2(a) molecules, yields above the threshold yield for the iodine laser operation are obtained at 12?mbar for afterglow times of up to 10?ms. In the large-volume reactor it has been found that at low pressure the density of O(3P) atoms decreases by about one order of magnitude towards the walls, while that of O2(a) changes about 20%, although with pressure the density decreases become more pronounced. Very similar density distributions are found at different mixture compositions for O(3P) atoms, while the quasi-homogeneous O2(a) distribution found in high Ar content mixtures progressively turns into a more inhomogeneous one with O2 addition.

Comparison of calculated and measured optical emission intensities in a direct current argon-copper glow discharge

q . Based on a collisional)radiative model for argon atoms and copper atoms and Cu ions, which was developed as a part of a comprehensive simulation network, optical emission intensities have been calculated for argon and copper lines in a direct current argon glow discharge with copper cathode. Comparison with experimental data has been made, both with respect to the optical emission spectra and to some selected emission lines as a function of axial position. From this study, information can be obtained about the importance of various plasma processes, like electron, fast ion and fast atom impact excitation, and reabsorption of resonant radiation. Q 2000 Elsevier Science B.V. All rights reserved.

Shear Viscosity and Shear Thinning in Two-Dimensional Yukawa Liquids

A two-dimensional Yukawa liquid is studied using two different nonequilibrium molecular dynamics simulation methods. Shear viscosity values in the limit of small shear rates are reported for a wide range of Coulomb coupling parameter and screening lengths. At high shear rates it is demonstrated that this liquid exhibits shear thinning; i.e., the viscosity eta diminishes with increasing shear rate. It is expected that two-dimensional dusty plasmas will exhibit this effect.

Role of the wall reactor material on the species density distributions in an N2–O2 post-discharge for plasma sterilization

The species density distributions in a large post-discharge reactor placed downstream from a flowing microwave discharge in N2?O2 are calculated using a three-dimensional hydrodynamic model. The effects of surface losses of N(4S) and O(3P) atoms on the density distributions of different species in the reactor are investigated for three different wall materials: (i) Pyrex, (ii) aluminium and (iii) stainless steel. The effects produced by considering different surface loss probabilities corresponding to each one of these materials, as well as by assuming the production of NO from the wall, are evaluated and discussed. The simulation is conducted for the case of a 65 ? 25 ? 25?cm3 post-discharge reactor fed from a 2450?MHz discharge, at 2?Torr and 2 ? 103?sccm flow rate, in an N2?xO2 mixture composition, with x = 0.5?7%.

Hybrid model of a plane-parallel hollow-cathode discharge

The development of the hollow-cathode effect in a plane-parallel hollow cathode dc argon glow discharge was investigated experimentally and by means of a two-dimensional self-consistent hybrid model, combining the fluid description of positive ions and slow electrons with a particle simulation of fast electrons. In the experiments the discharge was formed between two flat disc copper electrodes (of 3.14 cm diameter and separated by a L = 2 cm gap) serving as cathodes and a metal tube surrounding these electrodes which served as the anode. The electrical characteristics of the discharge and the spatial intensity distribution of selected spectral lines (Ar I 750.3 nm, 811.5 nm and Ar II 476.5 nm) were recorded at current densities 0.1 mA cm-2 j 0.5 mA cm-2 and for gas pressures 0.2 mbar p 1 mbar. While at pressures of ~1 mbar the cathode regions are developed separately for both cathodes, the light intensity distribution measurements demonstrated the gradual merging of the negative glows with decreasing pressure. At pL 0.8 mbar cm, a common negative glow is formed in the discharge. Complementing the experimental observations, the simulations made it possible to determine various discharge characteristics (e.g. spatial distribution of electric potential, ionization source, and ion density). At low pL values the simulations also indicated the existence of oscillating electrons. The spatial distribution of light intensity calculated for different pressures shows good qualitative agreement with the experimentally observed distributions.

Application of extremely non-equilibrium plasmas in the processing of nano and biomedical materials

Some applications of extremely non-equilibrium oxygen plasma for tailoring the surface properties of organic as well as inorganic materials are presented. Plasma of low or moderate ionization fraction and very high dissociation fraction is created by high frequency electrodeless discharges created in chambers made from a material of low recombination coefficient. The O atom density often exceeds 1021 m−3 which allows for rapid functionalization of carbon-containing materials. Surface saturation with polar oxygen-rich groups is achieved in a fraction of a second and further exposure leads to etching. The etching is often non-uniform and results in nano-structuring of surface morphology. A combination of rich morphology and saturation with polar functional groups allows for a super-hydrophilic character of originally hydrophobic materials. Polymer composites are etched selectively so the polymer component is removed from the sample surface, leading to modified surface properties. Furthermore, such a treatment allows for distinguishing the distribution and orientation of fillers inside the polymer matrix. The exposure of inorganic materials to non-equilibrium oxygen plasma causes one-dimensional growth of metal oxide nanoparticles, thus representing a unique technique for the rapid catalyser-free growth of nanowires.

Self-consistent modelling of helium discharges: investigation of the role of He2+ ions

Helium glow discharges in the p = 6-60 mbar pressure range have been investigated experimentally and by means of a one-dimensional self-consistent hybrid model. Unlike most of the hybrid models developed previously for noble gas discharges, our model also includes He2+ molecular ions. The electrical parameters of the discharges, recorded in the experiments, as well as the electron temperature for the different discharge conditions (determined spectroscopically) have been used as input data for the hybrid model. Our studies show that He2+ ions are present in the discharge even at 6 mbar with a concentration comparable to that of He+ ions. Conversion of atomic to molecular ions and the associative ionization process are identified as main sources of molecular ions. At low pressures the He2+ ions are principally lost at the electrodes, while at higher pressures the recombination processes become their dominant losses. As the rates of recombination processes are strongly pressure-dependent, the optical emission spectrum changes with increasing pressure and shows significant intensity of He2 molecular bands. The contribution of He2+ ions to the ion current at the cathode as well as their contribution to maintenance of the discharge is found to be around 10%, even at the lowest pressure of 6 mbar, where He2+ formation in most of the discharge models is neglected.

Controlling the oxygen species density distributions in the flowing afterglow of O2/Ar–O2 surface-wave microwave discharges

The evolution of species densities along a reactor radially positioned on an O2 surface-wave microwave discharge is investigated by means of modelling with the aim to define the density tuning possibilities. The validity of the models is shown by the comparison of the calculated and measured axial distribution of O-atoms. The calculations revealed that due to the perpendicular injection of the plasma into the reactor, the gas temperature is close to the room temperature in most of it, except for a 5?cm region around the inlet. It is shown that the pressure drop along the discharge tube, which results in the change of pressure in the discharge region with the gas flow rate, induces the variation of the relative density of active species entering the reactor, where the pressure is kept constant. The surface recombination probability of atoms varies along the afterglow tube due to the surface temperature gradient, as well as due to the conditioning of the surface resulting from the continuous operation of the system. The system is shown to be very practical in applications where surfaces/porous materials are to be treated homogeneously by pumping active species through them, since by tuning the gas flow rate equidensity surfaces can be obtained in the case of the two most abundant species, the O-atoms and O2(a) molecules. In the case of O-atoms the densities obtained at the two pressures investigated, i.e. 100 and 50?Pa, are very similar, as well as their evolution along the reactor, while the density of O2(a) molecules decreases considerably with pressure.