Tomáš Hoder

Investigation of the coplanar barrier discharge in synthetic air at atmospheric pressure by cross-correlation spectroscopy

The barrier discharge in the coplanar arrangement operating in a single-filament mode was studied spectroscopically. The evolution of the discharge luminosity was measured by the technique of cross-correlation spectroscopy. The 1D-spatially and temporally resolved luminosities of the first negative (at 391.5 nm) and the second positive (at 337.1 nm) system of molecular nitrogen were recorded using the above-mentioned technique. A cathode-directed ionizing wave (IW) was clearly seen on the plot for radiation intensity at 337.1 nm. In addition to this, also observed was a wave of the enhanced electric field propagating over the anode. In this paper, the propagation of these waves is described and their velocities are determined. The discharge evolution is divided into three phases—the Townsend phase, the phase of the IWs propagation and the extinction phase. Since the above-mentioned luminosity distributions could be interpreted approximately as the electric field (for 391.5 nm) and the electron density (for 337.1 nm) distribution, the qualitative description of the discharge is made accordingly. All these parameters are compared with similar measurements of the volume discharge. Apart from this, an attempt to determine the reduced electric field is made according to the kinetic model. (Some figures in this article are in colour only in the electronic version)

The simplest equivalent circuit of a pulsed dielectric barrier discharge and the determination of the gas gap charge transfer

The concept of the simplest equivalent circuit for a dielectric barrier discharge (DBD) is critically reviewed. It is shown that the approach is consistent with experimental data measured either in large-scale sinusoidal-voltage driven or miniature pulse-voltage driven DBDs. An expression for the charge transferred through the gas gap q(t) is obtained with an accurate account for the displacement current and the values of DBD reactor capacitance. This enables (i) the significant reduction of experimental error in the determination of q(t) in pulsed DBDs, (ii) the verification of the classical electrical theory of ozonizers about maximal transferred charge q(max), and (iii) the development of a graphical method for the determination of q(max) from charge-voltage characteristics (Q-V plots, often referred as Lissajous figures) measured under pulsed excitation. The method of graphical presentation of q(max) is demonstrated with an example of a Q-V plot measured under pulsed excitation. The relations between the discharge current j(R)(t), the transferred charge q(t), and the measurable parameters are presented in new forms, which enable the qualitative interpretation of the measured current and voltage waveforms without the knowledge about the value of the dielectric barrier capacitance C(d). Whereas for quantitative evaluation of electrical measurements, the accurate estimation of the C(d) is important.

On the spatio-temporal development of pulsed barrier discharges: influence of duty cycle variation

The paper presents experiments on the spatial and temporal structure of the breakdown process of microdischarges (MD). For the first time simultaneous streak and iCCD images of individual filaments in a pulsed driven dielectric barrier discharge (DBD) with 1?mm gap in a gas mixture of 0.1?vol% O2 in N2 at atmospheric pressure are recorded. Furthermore current and voltage measurements with fast probes give access to the electrical discharge characteristics such as transferred charge and power. For asymmetric pulses there is a significant difference in the spatial structure as well as in the temporal behaviour of the MDs between the rising and the falling slopes of the high voltage pulse. If the time between rising and falling slopes is reversed all effects reverse as well. For symmetric pulses there are no significant differences between the MD at rising and falling slopes which is in accordance with a sinusoidal DBD operation.

Novel insights into the development of barrier discharges by advanced volume and surface diagnostics

The comprehensive characterization of microdischarges (MDs) requires complementary diagnostics of volume and surface processes at the same discharge configuration under identical conditions. This contribution summarizes the results from optical, spectroscopic and electric investigations as well as the determination of surface charges and metastable nitrogen molecules in filamentary and diffuse barrier discharges. The feasibility of such an approach is demonstrated on selected examples.Fast optical and spectroscopic methods are reviewed for the example of a pulsed driven single filament dielectric barrier MD. It is demonstrated that the methods of streak recording and cross-correlation spectroscopy can complement one another for a comprehensive study of the MD development. Using these techniques it is shown that the so-called prephase is present also in sub-microsecond pulsed barrier discharges. The excitation starts directly with the voltage increase. In the case of diffuse barrier discharges in nitrogen, the combination of spectroscopic and electrical characterization, surface charge measurement by the Pockels effect, and the determination of nitrogen metastables N2(A) by laser-induced fluorescence provides detailed knowledge about the time-integrated surface charge which correlates with the discharge current for each half cycle, whereas the temporal maximum of the metastables of the order of few 1013?cm?3 is delayed in relation to the current maximum. The spatial (axial) maximum of the metastable density is located near the anode like the emission maximum from N2 second positive system at ??=?337?nm. Furthermore, the lifetime of surface charges beyond a typical discharge period has been investigated.

Breakdown characteristics in pulsed-driven dielectric barrier discharges: influence of the pre-breakdown phase due to volume memory effects

The pre-phase of the breakdown of pulsed-driven dielectric barrier discharges (DBDs) was investigated by fast optical and electrical measurements on double-sided DBDs with a 1 mm gap in a gas mixture of 0.1 vol% O2 in N2 at atmospheric pressure. Depending on the pulse width (the pause time between subsequent DBDs), four different breakdown regimes of the following discharge were observed. By systematically reducing the pulse width, the breakdown characteristics could be changed from a single cathode-directed propagation (positive streamer) to simultaneous cathode- and anode-directed propagations (positive and negative streamer) and no propagation at all for sub-μs pulse times. For all cases, different spatio-temporal emission structures in the pre-phase were observed. The experimental results were compared with time-dependent, spatially one-dimensional fluid model calculations. The modelling results confirmed that different pre-ionisation conditions, i.e. considerably high space charges in the volume created by the residual electrons and ions from the previous discharge, are the reason for the observed phenomena.

Influence of the capillary on the ignition of the transient spark discharge

A self-pulsing negative dc discharge in argon generated in a needle-to-plane geometry at open atmosphere is investigated. Additionally, the needle electrode can be surrounded by a quartz capillary. It is shown that the relative position of the capillary end to the needle tip strongly influences the discharge inception and its spatio-temporal dynamics. Without the capillary for the selected working parameters a streamer corona is ignited, but when the capillary surrounds the needle, the transient spark (TS) discharge is ignited after a pre-streamer (PS) occurs. The time between PS and TS discharge depends on the relative capillary end position. The existence of the PS is confirmed by electro-optical characterization. Furthermore, spectrally and spatio-temporally resolved cross-correlation spectroscopy is applied to show the most active region of pre-phase emission activity as indicators for high local electric field strength. The results indicate that with a capillary in place, the necessary energy input of the pre-phase into the system is mainly reduced by additional electrical fields at the capillary edge. Even such a small change as a shift of dielectric surface close to the plasma largely changes the energy balance in the system.

Barrier discharges driven by sub-microsecond pulses at atmospheric pressure: Breakdown manipulation by pulse width

Barrier discharges at atmospheric pressure in nitrogen-oxygen mixture powered by high voltage pulses of widths between 10 μs and 200 ns were investigated. The development of the microdischarges on rising and falling slopes was recorded by streak and intensified CCD cameras simultaneously. The breakdown on the falling slope strongly depends on the pulse width. As a result of pulse width variation the starting point of ignition changes and positive and negative streamers occur simultaneously in the falling slope. The observed effect is caused by the electric field rearrangement in the gap due to the different positive ion densities related to their gap crossing times.

Plasma-Liquid Interactions: Chemistry and Antimicrobial Effects

Plasma-induced inactivation of bacteria in aqueous liquids is supported by acidic pH and accompanied by generation of low-molecular chemical species which are detected as nitrate, nitrite and hydrogen peroxide. To get more insight into mechanisms of change of liquid composition by plasma treatment as well as transmission of bactericidal plasma effects into aqueous liquids, bactericidal effects under different plasma-treatment conditions and liquid analytics are combined with theoretical considerations to focus possible reaction channels of plasma-water interactions.

Volume Effects of Atmospheric-Pressure Plasma in Liquids

Generation of chemical species by atmospheric-pressure plasma treatment of aqueous liquids is a result of reactions at the plasma/gas-liquid interface and subsequent diffusion and convection processes into the liquid volume. Using color forming reactions, acidification as well as generation of nitrite is visualized in the water treated by a surface dielectric barrier discharge under atmospheric conditions in ambient air.

Two-Dimensional Spatially Resolved Cross-Correlational Spectroscopy of the Microdischarge Development in Barrier Discharges in Air

The technique of cross-correlation spectroscopy enables the spatio-temporally, spectrally and phase-resolved measurement of light intensity of statistically occurring microdischarges (MDs) of filamentary barrier discharges in synthetic air at atmospheric pressure. The results visualize the development of a single MD, consisting of the Townsend prephase, the cathode-directed ionization wave, anode glow, and outward-propagating discharges on the dielectric surfaces. These results contribute to a much deeper understanding of the breakdown mechanism.