Planar Dielectric Barrier Discharge Research Articles

Pulsed Dielectric Barrier Discharges for Gas-Phase Composition Control: A Simulation Model

We present results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular, we addressed the effect of the pulsed operation mode of a planar dielectric barrier discharge. As conjectured, the large difference in the time scales involved in the fast dissociation of molecules in plasmas and their subsequent reactions to produce stable chemical species makes the presence of a continuously repeated plasma production stage unnecessary and a waste of electrical power and efficiency. The results on NOx remediation, ozone production, water vapor and ammonia dissociation are discussed. A few comparisons with experimental findings in a dielectric barrier discharge reactor already used for applications are also briefly addressed. Our results clearly indicate a pattern for the optimization of the discharge using a carefully designed repetition rate and duty cycle.

Energetics of reactions in an atmospheric pressure plasma jet with argon carrier gas and hexamethyldisiloxane reagent

AbstractWe report on a methodology for measuring the energy dissipated per AC high voltage cycle in a cold atmospheric pressure plasma jet (CAPJet). This method is adapted from research by Nisol et al. on plasma polymerization of hexamethyldisiloxane (HMDSO) organosilicon vapor in a large area planar dielectric barrier discharge (DBD) reactor. Here too, we measured ΔEg, the energy difference with and without small HMDSO vapor concentrations in the argon carrier gas flow. From ΔEg we then derived Em, the energy per molecule, and compared values with those of Nisol. Good agreements were found, including in film structures determined from attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra, thus suggesting that realistic Em values can be successfully obtained also for the CAPJet case.

Analysis of ozone generation in a planar atmospheric pressure air dielectric barrier discharge reactor

This work investigates O3 production in a planar atmospheric pressure air dielectric barrier discharge reactor numerically and experimentally. The surface temperature of the reactor is measured by an infrared (IR) thermal imager, and the O3 densities of cases in the reactive zone are measured by ultraviolet absorption spectroscopy. The 1.5D plasma fluid model (PFM) with transverse convection is employed to capture the average properties of a single microdischarge (MD) generated in the reactor. The concept of equivalent reaction is proposed to calculate spatial-cyclic average species sources obtained by the 1.5D PFM and provided to the chemical model of a 3D gas flow model (GFM) for obtaining density distributions of reactive species generated by MDs in the reactive zone. The simulated temperature distribution of the reactor surface is validated by that measured with the IR thermal imager since the gas temperature was reported as a critical discharge parameter for O3 generation. The simulated O3 densities show the same trend as the flow rate changes, which demonstrates the proposed model captures the average discharge dynamics in different operating conditions. In the 1.5D PFM, the simulated results show that the O3 molecules produced in the case of 4 SLM are much more than those produced in the case of 1 SLM though the O atoms produced in the case of 1 SLM are around 20% more than those produced in the case of 4 SLM. In the case of 1 SLM, more than 48% of O3 molecular generated are destructed, while only around 14% of O3 molecules are destructed in the case of 4 SLM. The analysis shows that around 73% of O atoms generated in the 1.5D PFM are consumed in the formation of O3 molecules in the case of 4 SLM, while only 18% of O atoms generated in the case of 1 SLM are consumed in the formation of O3 molecules. In the 3D GFM, the O3 destructed is around 24% of that destructed in the 1.5D PFM in the case of 4 SLM due to the oxidation reaction of NO, while only 11% of O3 molecules destructed as that destructed in the 1.5D PFM in the case of 1 SLM. The amounts of O3 molecules generated in the 3D GFM are minor if they are compared with those generated in the 1.5D PFM in all cases. The overall O3 yield efficiency reaches 97 g kWh−1 with the O3 concentration increasing up to 2700 ppm in the case of 4 SLM, while the O3 yield efficiency decreases to 10 g kWh−1 and O3 concentration drops to 1400 ppm in the case of 1 SLM.

Experimental study of the effect of dielectric materials on the decomposition of carbon dioxide in a dielectric barrier discharge

The low-temperature nonequilibrium characteristics of dielectric barrier discharge (DBD) make it an important candidate for the decomposition and conversion of CO2. In this study, quartz, alumina and zirconia are selected as the dielectrics to investigate the effect of dielectric materials of planar DBD reactor on the discharge and conversion characteristics of CO2. It is shown that under the same input power conditions, zirconia has the highest CO2 conversion rate when used as a dielectric, followed by alumina, and quartz has the lowest. Combined with the analysis of the measured electrical characteristics, it is shown that zirconia has the highest transferred charge, thus further increasing the discharge efficiency. Experimental results of the single dielectric barrier layer-metal mesh electrode reactor show that the CO2 discharge can operate in the lower power range and the CO2 conversion rate is higher than that of the double dielectric barrier layer reactor. Spectroscopic measurements show that the emission intensity of the discharge in the reactor with high relative permittivity materials is higher than that in the case of low relative permittivity materials, and the emission intensity of the single dielectric layer reactor is also higher than that in the case of the double dielectric layer reactor.

(Invited) Cold Atmospheric Plasma Jet Sources with a Flexible Matrix

Plasma is an ionized gas containing free electrons, ions, neutral species (atoms, molecules, radicals), photons from UV to infrared wavelengths. Its composition makes it a rich environment for reactive process, showing a collective response to external perturbations. Low temperature plasmas are hot electron gas (1-10 eV) which are not in thermal equilibrium with the other cold part of the atomic or molecular species (0.02 eV). These cold plasmas are then suitable for applications to delicate surfaces (skin) without damage, especially since RONS (Reactive Oxygen and Nitrogen Species) show healing properties. Biomedicine is facing new challenges like decontamination, wound healing, and oncotherapy. Plasma medical treatments are selective, instantaneous, tunable in comparison to lasers and chemicals. There is a big need for developing new plasma sources that can treat complex surfaces (with asperities, sharp edges, cavities, complex accessibility)For this purpose, two types of flexible Cold Atmospheric Plasma Jet (CAPJ) sources with different geometries (planar and radial) were made and characterized through: multijet length and flow regime, electrical parameters, Optical Emission Spectroscopy (OES), performance after bending the aerogel. He (99.995%) was used in both configurations. Finite Element Method simulations were run to predict and understand the electromagnetic process in this plasma jet configuration.In planar geometry, DBD (Dielectric Barrier Discharge) was constituted of a silica aerogel placed between two biased electrodes. Silica aerogel has a very low gas permeability and is mechanically stable (highly cross-linked fiber glass network). Plasma jets constituted of 3 aligned jets, a few mm long above the DBD region whose length is proportional to the He flow rate, flowing in laminar regime, were produced by Helium flow (flow rate: 1-4 slm) subjected to sinusoidal voltage of 5kV and 15kHz frequency. Higher flow rate produced turbulent flows. The ratio between gas velocity and discharge voltage can be adjusted: short jets (flow limited operation) are suitable for near surface treatments, longer jets (power limited operation) are appropriate for greater distance treatments. OES showed strong N2 emission. Planar DBD was studied in flat configuration then its flexibility was characterized in bent (concave, convex) configurations. Compliance of the plasma source is limited by aerogel size and thickness. There is possibility to extend the device from 1D to 2D (matrix of plasma jet lines).In radial geometry, DBD was constituted of a soft hollow cylinder (about 5 cm external diameter, 2.5 cm internal diameter and 10 cm long), made of polyethylene (PE) foam, material selected for its flexibility and low permeability. An aluminum/dielectric coated rod electrode was placed in the core of the cylinder, copper circular electrodes were disposed on the external cylinder wall at 15 mm distance from each other surrounding 1mm holes punched in the PE cylinder, where the CAPJs were formed. In this geometry, two different operations modes were apprehended: comb mode was represented by four equidistant axial CAPJs and round brush mode was experienced when three CAPJs were aligned at constant azimuthal angle. In this case, discharges were triggered at 15 kHz AC voltage and a lower voltage than in planar configuration (3.8 kV). The jets were uniform in length and intensity. OES were recorded in the active region (inside the PE housing) and in the afterglow region. In the afterglow region, the spectrum was similar to the one obtained in planar geometry. A preponderance of hydroxyl groups (OH), nitrogen molecules (N2, N2 +), He atoms was found. The source remains stable after deformation of the PE housing.The feasibility of these flexible plasma sources in radial and axial configurations is promising for treating any kind of surface with special topography.

Development of a miniaturized hydride generation-dielectric barrier discharge atomic absorption spectrometer

A miniaturized atomic absorption spectrometer (AAS) was proposed with a planar dielectric barrier discharge (DBD) microplasma as an atomizer, a charge coupled device (CCD) spectrometer as a spectral detector, and a hydride generation (HG) unit as a sampler, and the potential analytical capability was evaluated through the determination of cadmium. Auxiliary hydrogen was added to enhance the atomic absorption signals and the potential mechanism of enhancement effect was studied by use of various techniques. The HG-DBD-AAS was further applied to the determination of Cd, yielding a 1.7-fold enhancement in AAS response with added hydrogen and a limit of detection (LOD) of 0.3 μg L−1 under optimized conditions. Good agreement with the certified values, and desirable spike recoveries ranging between 98% and 108%, were obtained for two certified reference materials and several real water samples, respectively. It can be useful in field analysis of many trace elements with high detectability.

Mathematical modeling of the electric field in the planar and coaxial dielectric barrier discharge reactor in water treatment application

PurposeTo design a pulse power water treatment system, it is necessary to design a reactor optimally. One of the most essential types of reactors used in water treatment is the dielectric barrier discharge (DBD) reactor. The purpose of this paper is to model the electric field in the two types of planar and coaxial reactors to have an accurate analytical formula for using in the optimal design according to the required electric field of the treatment.Design/methodology/approachThe method proposed in this paper focuses on the voltage of different areas in the reactor and different boundary conditions to obtain the surface charge density. In this regard, parameters of the dielectric and treated material, as well as the reactor dimension, have been affected in the equations. To confirm the analytical results, the finite element method simulation has been performed, and it shows the accuracy of this method.FindingsThe exact analytical equation of the electric field is found within the discharge zone of the planar and coaxial DBD reactors. These equations can predict the values of different parameters of the reactor required to purify the material before each design and it does not even require simulation.Originality/valueThe electric field formula presented in this paper can allow the manufacturers of pulse power water treatment systems to optimize their design easily, cost-effectively and in less time. Also, the formulas provided are completely general and remain effective for all materials.

Ignition-area extension of dielectric barrier discharge under high temperature

We discovered that the ignition area of a planar dielectric barrier discharge gradually extends beyond the area of the opposite electrode when the ambient temperature increases. Even with the opposite electrode the sectional area of which is less than 1 mm2, the discharge eventually ignites over the area of 225 mm2 at ca. 800 K. Several investigations indicate that the creeping discharge occurring around the opposite electrode is not the cause for the extension phenomenon.

Fourier‐transform infrared spectroscopy of ethyl lactate decomposition and thin‐film coating in a filamentary and a glow dielectric barrier discharge

AbstractGlow and filamentary regimes of atmospheric pressure plasma‐enhanced chemical vapor deposition in a planar dielectric barrier discharge configuration were compared for thin‐film deposition from ethyl lactate (EL). EL decomposition in the plasma phase and thin‐film composition were both characterized by Fourier‐transform infrared spectroscopy. EL chemical bonds' concentration along the gas flow decreases progressively in the glow dielectric barrier discharge (GDBD), whereas it drastically oscillates in the filamentary dielectric barrier discharge (FDBD), with values higher than that of the initial mixture. EL decomposition route depends on the discharge regime, as the decrease of the concentration of the different investigated bonds is different for an identical amount of energy provided to EL molecules. CO2 is systematically formed reaching concentrations of 25 and 40 ppm, respectively, in FDBD and GDBD.

New local electrical diagnostic tool for dielectric barrier discharge (DBD)

A new diagnostic tool to study dielectric barrier discharges (DBDs) at atmospheric pressure by local electrical measurements is introduced. The square ground electrode is divided into 64 square segments (3.44mm side length) so as to measure the discharge currents and gas voltages with spatial resolutions, which allows a 2D mapping. The electrical measurement results are validated by a comparison with short exposure time photographs taken from the top view of the discharge cell. For this purpose, we changed the local discharge behavior by varying locally the gas gap and the barrier capacitance and also by using a gas flow. Then, in both situations, the breakdown voltage depends on the position, and the discharge current and gas voltage are different as well. The measurements performed for a planar DBD in nitrogen with admixed nitrous oxide gas show that even if the discharge operates in a diffuse regime, the discharge does not behave exactly homogeneously on the whole surface area. The resulting electrical parameters allow us to refine the understanding of planar DBDs. The discharge activity changes the gas composition and thus, the level of preionization in the direction of the gas flow. This influences the local breakdown voltage and thus, the discharge morphology and local power density on the surface. The use of this new electrical diagnostic tool will allow us to refine the analysis of the spatial development of the discharge. This work gives some clues to improve the spatial resolution of this tool in the future.

Gas-phase isopropanol degradation by nonthermal plasma combined with Mn-Cu/-Al2O3.

In this study, the dielectric barrier discharge (DBD) induced by nonthermal plasma (NTP) technology was used for isopropanol (IPA) degradation. IPA, intermediate, final product, and ozone concentrations were analyzed using GC-MS, carbon dioxide detector, and ozone detector. The experimental flow rate and concentration were fixed to 1 L/min and 1200 ppm ± 10%, respectively. Different reaction procedures were proposed for self-made metal catalyst combined with a plasma system (plasma alone and γ-Al2O3 combined with plasma, Cu (5 wt%)/γ-Al2O3 combined with plasma, Mn (3 wt%)-Cu (5 wt%)/γ-Al2O3 combined with plasma). In addition, the effect of the carrier gas oxygen content (0%, 20%, and 100%) on IPA conversion and intermediate and carbon dioxide selectivity was also investigated. The results revealed that the Mn (F)-Cu/γ-Al2O3 combined with plasma exhibited more efficient IPA conversion. In the 100% oxygen environment, the IPA conversion rate increased from 79.32 to 99.99%, and carbon dioxide selectivity increased from 3.82 to 50.23%. IPA was completely converted after 60 min of plasma treatment with the acetone selectivity, carbon dioxide selectivity, and tail ozone concentration of 26.71% ± 1.27%, 50.23% ± 0.56%, and 1761 ± 11 ppm, respectively. This study proved that the current single planar DBD configuration is an effective advanced treatment technology for the decomposition of VOCs.

Mass production of plasma activated water by an atmospheric pressure plasma

We report on the mass production of plasma activated water (PAW) by a non-thermal atmospheric pressure plasma. As a non-thermal plasma source, we used planar dielectric barrier discharge (DBD) and two types cylindrical DBDs. The manufacturing characteristics of PAW were investigated for each DBD type. The results show that the radical transfer characteristics from plasma to water have a dominant effect on the PAW properties. Of the two cylindrical DBD reactors, the “direct” type in which the mesh electrode was replaced with the conductor (Al) showed superior characteristics over “indirect” type. DBD reactor with an Al electrode with the length of 1 m long achieved a PAW production capacity of 520 liters per hour and production efficiency of 65 l kWh−1. Such a large-capacity PAW manufacturing device can be applied to various industries including agriculture, sterilization, and water treatment.

Influences of voltage shape and discharge gas on the temporally and spatially resolved emission characteristics of tin in a planar dielectric barrier discharge

Choosing a suitable power supply concerning the shape of the excitation voltage and the type of discharge gas for elemental analysis with a dielectric barrier discharge is not trivial. Both significantly affect the analyte signal. By means of conventional methods such as inductively coupled plasma mass spectrometry or atomic absorption spectrometry optimal conditions can be found, however the reason why the specific parameter set results in ideal signals is hidden in the detailed characteristics of the DBD. These phenomena occur on a narrow time scale of nanoseconds to microseconds and are challenging to observe. Therefore, the emission profiles of tin in a planar dielectric barrier discharge atomizer were studied by means of temporally and spatially resolved optical emission spectroscopy by varying the type of discharge gas and the shape of the excitation voltage. The latter was realized by applying either a sinusoidal or a square wave power supply. The temporally resolved emission profile showed that in case of sinusoidal excitation the power is distributed over a longer time in several discharge events, whereas with square wave excitation the power is coupled to the dielectric barrier discharge in one intensive pulse. The spatially resolved emission signals indicated a different point of excitation of Sn within the dielectric barrier discharge for each power source. The Sn emission in a plasma, which was sustained by the square wave power supply, was not significantly influenced by the change of the discharge gas from argon to helium. However, for the emission signal in a plasma excited by the sinusoidal power supply, helium had an adverse effect.

Hydride generation atomic absorption spectrometry with a dielectric barrier discharge atomizer: Method optimization and evaluation of analytical performance for tin

Atomization conditions for tin hydride in the planar dielectric barrier discharge (DBD) plasma atomizer were optimized with detection by atomic absorption spectrometry (AAS). The effects of apparatus arrangement such as the shape of a waveform function of the high voltage power supply source, DBD atomizer design as well as presence of a dryer tube filled with NaOH pellets to prevent residual aerosol and moisture transport into the DBD were investigated in detail. The optimal experimental setup consisted of a square wave high voltage power supply source coupled to a DBD with vapor-deposited electrodes in the presence of NaOH dryer upstream the DBD atomizer. Argon was found as the best discharge gas under a flow rate of 120 mL min−1 while the DBD optimum high voltage supply rate was 7 kV. A sensitivity of 0.05 s ng−1 Sn and a limit of detection of 1.1 ng mL−1 Sn were reached under optimized conditions. Optimization of the whole experimental setup resulted in 7-fold improvement of sensitivity compared to the original arrangement consisting of a sinusoidal source coupled to a DBD atomizer with glued electrodes in absence of the dryer. The analytical performance of the DBD atomizer under optimized conditions was compared to that of a commonly used externally heated quartz tube atomizer.

Statistical behavior of a single microdischarge in atmospheric-pressure air dielectric barrier discharges

This work investigates the statistical behavior including the dimensions and electrical properties of a single microdischarge (MD) generated in a planar atmospheric-pressure air dielectric barrier discharge reactor using a kHz sinusoidal power source with a gap of 1.4 mm. The MD diameters and surface wave (SW) dimensions are captured by an intensified charge-coupled device camera with currents and charge transferred being recorded. The average currents measured in positive and negative half periods (HPs) are 58.9 and 50.5 mA, respectively. The average diameters measured are 256 and 258 μm in positive and negative HPs, respectively. Therefore, the average current densities calculated in positive and negative HPs are 1.14 × 106 and 9.66 × 105 A m−2, respectively. The high current density leads to the high density of surface charge reaching up to 51.5 nC cm−2 in the positive HP at the anode, which is one order of magnitude higher than those observed in filamentary discharges using He/N2 mixtures. The gap dimension is adjusted to 2.0 mm to investigate the effect of the gap dimension on MD properties. Larger MD diameters, currents, SW dimensions, and the charge transferred are measured in the gap of 2.0 mm although the average current densities evaluated in positive and negative HPs are lower than those of gap 1.4 mm as 9.39 × 105 and 8.15 × 105 A m−2, respectively. The average density of surface charge evaluated in the positive HP at the anode of gap 2.0 mm is 51.7 nC cm−2, which is almost identical to that evaluated in the gap of 1.4 mm.

Atomization of arsenic hydride in a planar dielectric barrier discharge: Behavior of As atoms studied by temporally and spatially resolved optical emission spectrometry

The excitation mechanism of arsenic atoms in a planar shaped dielectric barrier discharge (DBD) atomizer with sputtered grid shaped electrodes was studied by optical emission spectrometry (OES) with resolution in time and space. Due to the shape of the electrodes, the DBD design provides optical access to the plasma not only in end-on position but also in side-on direction. By means of an iCCD camera with nanosecond time resolution coupled to a monochromator it was shown that the spatial and temporal development of the plasma emission depends on the discharge gas nature (Ar, He). Applying argon, a dense and constricted plasma filament is formed, whereas in helium a much more homogeneous plasma is ignited. The impact of both plasmas on the emission signal of arsenic atoms was studied employing analyte introduction via hydride generation. The excitation of arsenic temporally follows the emission signal of helium or argon and is spread across the whole discharge volume. The basic discharge propagation dynamics showed similarities to previously presented results obtained by OES in a capillary DBD.

Direct, Transfer-Free Growth of Large-Area Hexagonal Boron Nitride Films by Plasma-Enhanced Chemical Film Conversion (PECFC) of Printable, Solution-Processed Ammonia Borane

Synthesis of large-area hexagonal boron nitride (h-BN) films for two-dimensional (2D) electronic applications typically requires high temperatures (∼1000 °C) and catalytic metal substrates which necessitate transfer. Here, analogous to plasma-enhanced chemical vapor deposition, a nonthermal plasma is employed to create energetic and chemically reactive states such as atomic hydrogen and convert a molecular precursor film to h-BN at temperatures as low as 500 °C directly on metal-free substrates-a process we term plasma-enhanced chemical film conversion (PECFC). Films containing ammonia borane as a precursor are prepared by a variety of solution processing methods including spray deposition, spin coating, and inkjet printing and reacted in a cold-wall reactor with a planar dielectric barrier discharge operated at atmospheric pressure in a background of argon or a mixture of argon and hydrogen. Systematic characterization of the converted h-BN films by micro-Raman spectroscopy shows that the minimum temperature for nucleation on silicon-based substrates can be decreased from 800 to 500 °C by the addition of a plasma. Furthermore, the crystalline domain size, as reflected by the full width at half-maximum, increased by more than 3 times. To demonstrate the potential of the h-BN films as a gate dielectric in 2D electronic devices, molybdenum disulfide field effect transistors were fabricated, and the field effect mobility was found to be improved by up to 4 times over silicon dioxide. Overall, PECFC allows h-BN films to be grown at lower temperatures and with improved crystallinity than CVD, directly on substrates suitable for electronic device fabrication.

Behavior of selenium hydride in heated quartz tube and dielectric barrier discharge atomizers

Atomization of SeH2 in an externally heated multiple microflame quartz tube atomizer (MMQTA) as well as planar dielectric barrier discharge (DBD) atomizer was investigated using a variety of probes. Deposits of Se on inner surfaces of the atomizers were quantified and their distribution visualized by autoradiography with 75Se radiotracer. The gas phase fraction of Se transported beyond the confines of the atomizers was also determined. In the MMQTA, a 15% mass fraction of Se was deposited in a narrow zone at both colder ends of the optical arm (100–400 °C). By contrast, a 25-40% mass fraction of Se was deposited homogeneously along the entire length of the optical arm of the DBD, depending on detection technique employed. The fraction of Se transported outside the MMQTA approached 90%, whereas it was 50–70% in the DBD. The presence of H2 was essential for atomization of selenium hydride in both atomizers. The gaseous effluent arising from the hydride generator as well as the atomizers was investigated by direct analysis in real time (DART) coupled to an Orbitrap-mass spectrometer, enabling identification of major gas phase species of Se.

Novel designs of dielectric barrier discharge hydride atomizers for atomic spectrometry

Eight designs of dielectric barrier discharge (DBD) atomizers have been constructed and optimized. Subsequently, their performance with atomic absorption (AAS), atomic fluorescence (AFS) and atomic emission (AES) detectors was investigated employing arsane as a model analyte. Although being all based on a planar shaped quartz body, the individual DBD designs differed from each other in the style of electrode attachment, electrode shape and area as well as electrode distance. All the designs studied were found compatible with AAS detection giving comparable sensitivity (between 0.38 and 0.47 s ng−1 As) and detection limit (around 0.2 ng ml−1 As) under optimum atomization conditions. However, significant differences in optimum conditions were found among the designs in terms of the applied voltage that strongly depends on the style of electrode attachment and electrode distance. The design with metal strip electrodes glued to a quartz body requires >14 kV to be operated. The design with sputtered electrodes of the same shape can reach the same sensitivity with 8.5 kV. These values can be further reduced to 6.5 kV, when decreasing the electrode distance from 3 to 1 mm. Selected DBD designs tested have been proven to be compatible with other spectrometric detectors such as AFS reaching detection limit 0.05 ng ml−1 As or AES with detection limit of 30 ng ml−1 As showing the universality of the planar DBD designs and its potential to further applications as well as mechanistic studies.

Plasma dynamics in a packed bed dielectric barrier discharge (DBD) operated in helium

Packed bed dielectric barrier discharges (DBDs) are very promising for several applications including remediation of environmental pollutants and greenhouse gas conversion. In this work, we have investigated the space and time-resolved emission from a packed bed DBD operated in helium, to understand the plasma dynamics. We have chosen a simple planar DBD arrangement with a patterned dielectric, which mimics the spherical boundaries between the dielectric pellets and allows the optical access to the plasma. The results show that plasma is sustained in a packed bed DBD by three mechanisms: filamentary discharge in the void (between the center of dielectric structures and the opposite electrode), microdischarges at the contact points and surface ionization waves over the dielectric surface. It is observed that for most of the duration plasma is generated at the contact points between the dielectric structures.