C. Leys

Direct current plasma jet at atmospheric pressure operating in nitrogen and air

An atmospheric pressure direct current (DC) plasma jet is investigated in N2 and dry air in terms of plasma properties and generation of active species in the active zone and the afterglow. The influence of working gases and the discharge current on plasma parameters and afterglow properties are studied. The electrical diagnostics show that discharge can be sustained in two different operating modes, depending on the current range: a self-pulsing regime at low current and a glow regime at high current. The gas temperature and the N2 vibrational temperature in the active zone of the jet and in the afterglow are determined by means of emission spectroscopy, based on fitting spectra of N2 second positive system (C3Π-B3Π) and the Boltzmann plot method, respectively. The spectra and temperature differences between the N2 and the air plasma jet are presented and analyzed. Space-resolved ozone and nitric oxide density measurements are carried out in the afterglow of the jet. The density of ozone, which is formed...

Penetration of a dielectric barrier discharge plasma into textile structures at medium pressure

Plasma treatment of textiles is becoming more and more popular as a surface modification technique. Plasma treatment changes the outermost layer of a material without interfering with the bulk properties. However, textiles are several millimetres thick and need to be treated homogeneously throughout the entire thickness. To control the penetration depth of the plasma effect, it is necessary to study the influence of operating parameters. Three layers of a 100% polyester non-woven are treated in the medium pressure range (0.3?7?kPa) with a dielectric barrier discharge to study the influence of pressure and treatment time. Current and voltage waveforms and Lichtenberg figures are used to characterize the discharge. Process pressure proved to have an important effect on the penetration of the plasma through the textile layers. This is caused not only by the pressure dependence of diffusive transport of textile modifying particles but also by a different behaviour of the barrier discharge.

The influence of water vapor content on electrical and spectral properties of an atmospheric pressure plasma jet

An atmospheric pressure plasma jet generated in Ar with water vapor is investigated. It is shown that an increase in the water content results in a decrease in the input power and asymmetry of the current waveform on positive and negative half-periods of the applied voltage. Space-resolved spectroscopy with a resolution of 1?mm and an imaging technique are applied for the characterization of the afterglow and investigation of the influence of water content on plasma properties. The rotational temperature of the jet is determined by simulation of the OH radical emission spectrum, transition A?2?+(v = 0) ? X?2?(v = 0). It is revealed that the temperature of the discharge increases from 450?K (Ar) up to 850?K with an increase in the water content up to 7600?ppm. Generation of the discharge in mixtures of argon with water vapor at a concentration of 350?ppm results in a maximal yield of OH radicals that can be useful in plasma jet applications. Preliminary tests of polypropylene surface modification are carried out in order to estimate the influence of water content on the results of treatment.

Electron density measurement in atmospheric pressure plasma jets: Stark broadening of hydrogenated and non-hydrogenated lines

Electron density is one of the key parameters in the physics of a gas discharge. In this contribution the application of the Stark broadening method to determine the electron density in low temperature atmospheric pressure plasma jets is discussed. An overview of the available theoretical Stark broadening calculations of hydrogenated and non-hydrogenated atomic lines is presented. The difficulty in the evaluation of the fine structure splitting of lines, which is important at low electron density, is analysed and recommendations on the applicability of the method for low ionization degree plasmas are given. Different emission line broadening mechanisms under atmospheric pressure conditions are discussed and an experimental line profile fitting procedure for the determination of the Stark broadening contribution is suggested. Available experimental data is carefully analysed for the Stark broadening of lines in plasma jets excited over a wide range of frequencies from dc to MW and pulsed mode. Finally, recommendations are given concerning the application of the Stark broadening technique for the estimation of the electron density under typical conditions of plasma jets.

Influence of capillary geometry and applied voltage on hydrogen peroxide and OH radical formation in ac underwater electrical discharges

The generation of hydroxyl (OH) radicals and hydrogen peroxide (H2O2) in underwater capillary discharges with different geometries is studied. It is found that the efficiency of the production of these active species depends on the input power and on the length of the capillary. A maximal rate of H2O2 generation of 3.6 × 10−3 mmol l s−1 has been measured in a 1 mm capillary at a discharge power level of 77 W. The hydrogen peroxide yield is lower for the 1 mm capillary than for the 5 mm capillary . The kinetics of OH formation is investigated by recording emission spectra in the UV range (280–400 nm). It is shown that the rate of OH formation sharply decreases with increasing capillary length, for capillaries longer than 3 mm. A comparison of the efficiency of different plasma-solution reactors for the generation of active species has been carried out. In the investigated capillary discharge the hydrogen peroxide yield is higher than the values reported for a diaphragm discharge and for a glow discharge with the electrolyte cathode at atmospheric pressure.

Physical properties and chemical efficiency of an underwater dc discharge generated in He, Ar, N2 and air bubbles

A dc excited discharge generated in bubbles (He, Ar, Air, N2) in liquid phase is investigated in this work. Voltage/current characteristics and emission spectra of the discharge are recorded in the current range 10?30?mA. Electron density in the discharge is measured from Stark broadening of the H? line and is of the order of 2?6 ? 1020?m?3, depending on the feed gas. Estimation of electron temperature is carried out based on the balance of charged particles. Gas temperature is estimated by the slope of the Boltzmann plot and by the simulation of the OH band with different , and Tvib. Rotation temperature in the He discharge is 1200?K at I = 10?mA and linearly increases with current up to 1600?K. In the plasma of molecular gases the temperature is higher and almost constant at different currents. Chemical efficiency of the plasma is measured by the production of H2O2 and by the destruction of Direct Blue 106 dye. The highest energy consumption of H2O2 generation is achieved in the air discharge and it decreases up to 50% in the He plasma. Maximal efficiency of dye destruction is observed in the N2 plasma characterized by an energy consumption of dye decomposition of 0.86?g?kWh?1.

Dissociation levels in fast-axial-flow CO2-lasers - A quantitative model.

A plasma chemistry model is presented that explains the observed CO2 dissociation levels in a closed‐cycle fast‐axial‐flow CO2 laser. The model includes reactions between the neutral species CO2, CO, O, O2, O3, H2O, and OH, and the negative ions O−, O−2, CO−3, and CO−4. Dissociation rates are computed by solving the electron Boltzmann equation for experimental values of the reduced field E/N. It is found that gas replenishment and the neutral recombination reaction between CO and the OH radical are the most effective mechanisms to suppress the CO concentration in the gas circuit. The influence of CO2 dissociation on the laser output power level is discussed.

OH radical production in an atmospheric pressure surface micro-discharge array

The generation of OH radicals from an array of surface micro-discharges working in atmospheric pressure He/Ar/H2O mixtures is investigated. The absolute OH density and its temporal-and-spatial dynamics are detected by UV broadband absorption spectroscopy (UV-BAS) and laser-induced fluorescence (LIF) spectroscopy. The measured absolute density of OH(X) state is about 1021 m−3 in Ar/H2O mixture reaching a peak at 0.05% of H2O. In the case of He/H2O mixtures however, the peaking at ~1019 m−3 is approximately two orders of magnitude lower and decreases monotonously with increasing H2O content. From a control standpoint, the ratio of the Ar/He mixture may be adjusted to tune the OH density over two orders of magnitude and to modulate the H2O content dependence of the OH density. The capability of modulating the OH radical production over a large density range is of practical interest for many applications such as atmospheric chemistry and biochemistry. With the array of atmospheric micro-discharges sustained over a large electrode area, a uniform distribution of its OH density can be achieved in a plane parallel to the electrodes thus enabling spatially controlled surface treatment of large samples.

Visualization of the Penetration Depth of Plasma in Three-Dimensional Porous PCL Scaffolds

A dielectric barrier discharge (DBD) discharge is used to modify the surface properties of 3-D porous polycaprolactone (PCL) scaffolds. After plasma treatment, the penetration of blue ink into the samples was used to determine the effectiveness of the plasma treatment inside the structures. It was found that the ink could penetrate deeper into the scaffolds after plasma treatment.

Ionization equilibrium in flowing laser mixtures

The reduced electrical field strength is a key physical input to plasma chemistry and vibrational kinetics models of laser discharges. In this paper the different processes that influence the ionization equilibrium are studied and a tractable model is constructed from which the self-sustaining electrical field in the positive column of a flowing laser mixture is derived. Apart from the charged particle losses related to the turbulent flow, a number of plasma-chemical effects such as dissociation and water contamination are included in the analysis. The total negative-ion density is denoted as a critical quantity in explaining experimental reduced electrical field values.