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Multiplication of primary electrons at a dielectrics/gas interface exposed to an external electric field leads to the formation of ultra-fast contracted ionisation waves called surface streamers with high local electric field in their microscopic heads. We study this phenomenon in coplanar barrier discharge in atmospheric pressure air where streamers of both polarities are generated simultaneously. The electric field strength development in the discharge is determined experimentally using time-correlated single photon counting enhanced optical emission spectroscopy with high spatiotemporal resolution. We show that very high values of the electric field of even above 1200 Td (approx. 300 kV cm(-1)) can be a physical reality for positive streamer directly at the dielectric surface. The peak value for negative streamer lies under 200 kV cm(-1). In parallel, a two-dimensional fluid model describes the rapidly changing electric field and the spatiotemporal discharge dynamics in good agreement with the experimental results. We also present the theoretical results of spatiotemporal development of electron density and of charge transfer to the dielectric surface via a collapsing sheath as the discharge structure decays within a very few nanoseconds.

The development of microdischarges and the inception dynamics of subsequent microdischarges in an electrode arrangement consisting of a metal pin and a hemispherical dielectric-covered electrode, operated in air with a small toluene admixture, is studied. The discharge is operated with sinusoidal high voltage. A gated ICCD camera and a current probe enable the recording of images and current pulses of the single microdischarges, respectively, while the spatio-temporally resolved development is measured with a multi-dimensional time-correlated single photon counting technique. The overall discharge dynamics changes significantly if a concentration of 35 ppm toluene is added to dry air. A lower high voltage amplitude than in dry air is needed for stable discharge operation. This can be explained by the lower ionization energy of toluene compared to molecular oxygen and nitrogen. The microdischarge development is the same with or without admixture, i.e. a positive (cathode directed) streamer mechanism is observed. Lower mean power is dissipated into the discharge when toluene is admixed. The main effect caused by toluene admixture is the suppression of high-energy microdischarges in case of the cathodic pin half-cycle of the sinusoidal high voltage. The influence on the inception voltage by additional ionization mechanisms and volume memory effects, the consumption of energetic electrons for toluene decomposition reactions, and the modification of the surface by plasma treatment are discussed as possible reasons.

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 (3.44 mm 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.

Discharge development and streamer propagation on a dielectric surface were investigated in nitrogen-oxygen gas mixtures at atmospheric pressure. A coaxial pin-to-pin arrangement was used to generate single surface discharges driven by positive unipolar square wave high voltages of between 7 and 9 kV at 4.3 kHz. The development of surface discharges was recorded by ICCD and streak cameras. The discharges developed on the surface using pin electrodes attached directly to the dielectric plate. The accumulation over several discharge events showed a propagation front evolving from the electrode tip, while images of single discharges showed a non-uniform and branched structure of discharge channels. The electrode polarity influenced the discharge expansion and propagation velocity. Positive polarity of the metallic electrode (rising slope of HV pulses) led to a cathode-directed streamer with higher propagation velocities (5 × 105 m/s) than for negative polarity (relative to surface charges; falling slope). The increase in the O2 content in N2 from 0.1 to 20 vol% resulted in a decrease in discharge duration and an increase in streamer velocities.

Electrical breakdown in a pulsed asymmetric dielectric barrier discharge between a glass-covered mesh electrode and a grounded metal electrode in the air at atmospheric pressure is investigated. Volume discharge forms between the metal tip and the dielectric surface and spreads over the dielectric surface. Breakdown and discharge behaviors depend on the polarity of the charged electrode covered with glass compared to the metal rod electrode. In the case of the dielectric cathode (covered mesh), volume discharge features a stronger and longer-lasting emission. Volume discharge is weaker with outstretched surface discharge developing on the opposite glass electrode sustained by the embedded mesh when the metal rod functions as a cathode. The development and spatial distribution of the surface discharge depend on the relative polarity of the dielectrics caused by the charge deposition of the preceding discharge and is independent of the polarity of the applied high voltage. The discharge emission is brighter for the metal cathode and dielectric anode than for the metal anode, with a branching discharge developing and spreading in a star-like structure along the embedded grid, while a ring-like structure was observed for the metal anode and dielectric cathode. The duty cycle influences the discharge development and properties through the effects of the gas phase and surface pre-ionization.

The conversion of carbon dioxide as one of the main greenhouse gases into carbon monoxide as a chemical feedstock is considered as so-called carbon capture usage technology. Recently it was shown, that the dissociation of carbon dioxide to carbon monoxide in Dielectric Barrier Discharges can be enhanced by the addition of nitrogen gas. Here, the development of microdischarges in CO2 and CO2/N2 gas mixtures is studied. Therefore, a single filament DBD arrangement operated under sinusoidal high-voltage is investigated by means of spectroscopic and electrical diagnostics with high spatial and temporal resolution and sensitivity. The filament development is similar as in air or other nitrogen-oxygen gas mixtures, but the gas composition influences the duration and other parameters. The higher the CO2 content the weaker the filaments and the faster the quenching of excited molecular states. The optimum power dissipation into single discharge is obtained for a CO2 content between 20 and 30 vol.%.

Climate change, environmental pollution control, and resource utilization efficiency, as well as food security, sustainable agriculture, and water supply are among the main challenges facing society today. Expertise across different academic fields, technologies, and disciplines is needed to generate new ideas to meet these challenges. This white paper aims to provide a written summary by describing the main aspects and possibilities of the technology. It shows that plasma science and technology can make significant contributions to address the mentioned issues. The paper also addresses to people in the scientific community (inside and outside plasma science) to give inspiration for further work in these fields.

The Ar( 3 P 2) metastable density in the effluent of the cold atmospheric pressure plasma jet kINPen was investigated as a function of the feed gas humidity, the gas curtain composition, and the distance from the nozzle by means of laser atomic absorption spectroscopy. The filamentary character of the plasma jet was considered as the absorption signals are from single individual events. From the effective lifetime, the quenching coefficient for Ar( 3 P 2) by water was determined. A maximum Ar( 3 P 2) density of ( 6.0 ± 0.7 ) × 10 13 cm − 3 was obtained close to the nozzle for a feed gas humidity of 20 ppm. The densities near the nozzle decreased inversely proportional to the water content in the feed gas leveling off at approximately 1 × 10 13 cm − 3, independently from the gas curtain composition. With the addition of water to the feed gas, the excitation dynamics changed, and the production and lifetime of Ar( 3 P 2) was reduced. The impact of Ar( 3 P 2) on the reactive species composition in the effluent was found to be smaller than within the plasma zone inside the plasma jet device.

A dielectric barrier discharge over distilled water was investigated by means of synchronized, fast electrical and optical diagnostics. For this purpose, a single, alumina-covered electrode was placed above a water surface and a grounded tungsten electrode underwater, while the gas gap was constantly flushed with synthetic air at atmospheric pressure. Discharge events occur at rising and falling slopes or at the plateaus of the applied high-voltage pulse. Due to the synchronized diagnostics, a direct comparison of electrical characteristics and optical features of individual discharge events was performed. In most cases, a single discharge channel develops between the tips of the dielectric and the Taylor cone. Surface discharge channels can also occur in the upper part of the electrodes, which can be explained by a water layer being formed on the dielectric and acting as a virtual electrode. The discharge morphology and dynamics were found to be independent of the voltage polarity, which contradicts to known experiments with bare metal pin electrodes instead of dielectric-covered ones.

The emissions of airborne contaminants from various industrial processes are an important source of air pollution and presents problems for human health and the environment in general. The removal of gaseous pollutants, such of nitrogen and sulphur oxides (NOx and SOx), from exhaust gases, or volatile organic compounds (VOC) emitted from various industrial processes is of continuous challenge in the environmental engineering. Plasma-based pollutant decomposition methods emerge as promising techniques, but little is known about their overall environmental performance. We compared two plasma based technologies (Electron Beam Flue Gas Treatment (EBFGT) and Dielectric Barrier Discharge (DBD) against the conventional methods (Wet Flue Gas Desulphurization with Selective Catalytic Reduction, WFGD+SCR, biofiltration and adsorption) using life cycle analysis (LCA), which took into account the usage of materials, waste generation and energy consumption. Five main categories (Global Warming, Ozone Layer Depletion, Acidification, Euthrophication and Human Toxicity, based on the CML2001 method) were used for the environmental impact evaluation. Based on CML2001 IKP Experts weighting, WFGD+SCR technology is marginally more environmentally friendly in comparison to EBFGT (59.25 to 73.74 CML2001 IKP Experts score). The generation of byproducts (in this case gypsum waste) is the main disadvantage of the WFGD+SCR. At the same time, EBFGT enables formation of useful byproducts, which are suitable to be utilized as fertilizers in the agriculture sector. On the other hand, DBD was six times more favorable to the environment than the adsorbtion and more than two times more environmentally friendly than the biofiltration technology (4.23, 27.78 and 9.29 relative units, respectively). With respect to the adsorption/thermal incineration of VOCs, both the electricity and the material consumption, as well as the management of byproducts caused the highest impact. In case of biofiltration, the management of remaining filter material waste (in this case, landfilling) was the most significant. Relatively high electrical energy demand causes lower positioning of plasma based technologies in cases where no other materials are utilized and major waste is formed. In turn, many traditional end-of pipe technologies are associated with high amounts of process waste, which provides plasma technologies with an opportunity to establish them in the market as more efficient and in many occasions, more environment-friendly ones.

Surface dielectric barrier discharges are investigated in order to explore the combined effects of barrier thickness and microstructure of the exposed electrode on the ion wind generation. Actuators with straight and structured high voltage electrodes with characteristic sizes of 200 and 250 µm and dielectric thicknesses of 0.5, 1 and 2 mm are compared. It is observed that: i) actuator efficiency of ion wind generation strongly depends on the applied voltage amplitude; ii) operation voltage depends on the dielectric thickness logarithmically; iii) electrode microstructure slightly increases the dynamic pressure (few percent in maximum), however the effect decreases with thicker dielectrics and smaller electrode structures; iv) the pattern of the most intensive discharge parts as well as the dielectric erosion repeats the regular structure of the electrodes down to 200 µm. Several identical samples are tested during different days to estimate the impact of the air humidity and the degradation of the dielectric. The microscale precision of the sample manufacture was accomplished by a commercial facility for printed circuit boards.

The phenomenology and breakdown mechanism of dielectric barrier discharges are strongly determined by volume and surface memory effects. In particular, the pre-ionization provided by residual species in the volume or surface charges on the dielectrics influences the breakdown behavior of filamentary and diffuse discharges. This was investigated by advanced diagnostics such as streak camera imaging, laser photodetachment of negative ions and laser photodesorption of electrons from dielectric surfaces in correlation with 1D fluid modeling. The streak camera images show that an increasing number of residual charges in the volume changes the microdischarge breakdown in air-like gas mixtures from a cathode-directed streamer to a simultaneous propagation of cathode- and anode-directed streamers. In contrast, seed electrons are important for the pre-ionization if the density of residual charges in the volume is low. One source of seed electrons are negative ions, whose density exceeds the electron density during the pre-phase of diffuse helium–oxygen barrier discharges as indicated by the laser photodetachment experiments. Electrons desorbed from the cathodic dielectric have an even larger influence. They induce a transition from the glow-like to the Townsend-like discharge mode in nominally pure helium. Apart from analyzing the importance of the pre-ionization for the breakdown mechanism, the opportunities for manipulating the lateral structure and discharge modes are discussed. For this purpose, the intensity and diameter of a diffuse discharge in helium are controlled by an illuminated semiconducting barrier.

Unsteady loading rates can affect the proper operation of a biofilter. In addition, biofiltration is known to be less effective when dealing with poorly soluble substances, like some volatile organic compounds (VOCs). A non-thermal plasma (NTP) produced with a dielectric barrier discharge (DBD) was studied as an option to pre-treat an air stream contaminated by a mixture of VOCs. Therefore, the plasma reactor was operated upstream of a laboratory-scale biofilter. Air admixed with toluene, n -heptane, p -xylene, ethylbenzene and benzene with average concentrations of 95.6, 49.4, 60.8, 47.3 and 36.6 ppm, respectively, was used as a model polluted gas, as these contaminants represent the air stripped by an oil-refinery wastewater treatment plant. Peaks of loading rates at the inlet of the biofilter were simulated by the increase of the flow rates of VOCs. The operation of NTP, with specific energy densities between 92 J L −1 and 256 J L −1 allowed reducing the VOC concentrations down to the level of optimal biofilter operation. In addition, non-water soluble VOCs were converted to more soluble compounds by the plasma treatment. In this first attempt to investigate the synergies between NTP and biofiltration, NTP reveals as a promising option to pre-treat effluents upstream of biofilters for optimized operation.

Dielectric barrier discharges (DBDs) are plasmas generated in configurations with an insulating (dielectric) material between the electrodes which is responsible for a self-pulsing operation. DBDs are a typical example of nonthermal atmospheric or normal pressure gas discharges. Initially used for the generation of ozone, they have opened up many other fields of application. Therefore DBDs are a relevant tool in current plasma technology as well as an object for fundamental studies. Another motivation for further research is the fact that so-called partial discharges in insulated high voltage systems are special types of DBDs. The breakdown processes, the formation of structures, and the role of surface processes are currently under investigation. This review is intended to give an update to the already existing literature on DBDs considering the research and development within the last two decades. The main principles and different modes of discharge generation are summarized. A collection of known as well as special electrode configurations and reactor designs will be presented. This shall demonstrate the different and broad possibilities, but also the similarities and common aspects of devices for different fields of applications explored within the last years. The main part is devoted to the progress on the investigation of different aspects of breakdown and plasma formation with the focus on single filaments or microdischarges. This includes a summary of the current knowledge on the electrical characterization of filamentary DBDs. In particular, the recent new insights on the elementary volume and surface memory mechanisms in these discharges will be discussed. An outlook for the forthcoming challenges on research and development will be given.

We investigate single microdischarges (MDs) in a sinusoidally operated barrier corona discharge in air. For the voltage amplitude being applied, two subsequent MDs appear in the anodic pin half-cycle. The developments of these subsequent MDs were studied and presented in detail in a previous contribution [Jahanbakhsh et al., Plasma Sources Sci. Technol. 27, 115011 (2018)]. In the present study, the reduced electric field strength ( E / n) values of the MDs are determined. In addition, the current pulses are synchronized, with a subnanosecond time resolution, to the spatiotemporally resolved light emission and E / n development of the MDs. It is proposed that the current pulse derivative maximum corresponds to the streamer head arrival on the cathode surface. Therefore, the derivatives of the current pulses are used to synchronize the light emission and current measurements. Based on this synchronization, spatiotemporally resolved light emissions at different positions are compared to the averaged current pulses. Considering the observed correlations, it is proposed that after the arrival of the streamer head on the dielectric (cathode) surface and bulk plasma formation, the ionization processes near the dielectric surface are the dominant source of electron current production. The determination of the E / n is based on the analysis of the time-correlated single photon counting results for the molecular states of the first negative and the second positive systems of nitrogen. The E / n increases during the streamer propagation in the gap, reaching its maximum value at the impact of the streamer on the cathode. The E / n values for the second group MDs are lower only in the vicinity of the dielectric surface, which can be attributed to the positive residual surface charges from the first group MDs.

Measurements of current and voltage are the basic diagnostics for electrical discharges. However, in the case of dielectric barrier discharges (DBDs), the measured current and voltage waveforms are influenced by the discharge reactor geometry, and thus, interpretation of measured quantities is required to determine the discharge properties. This contribution presents the main stages of the development of electrical diagnostics of DBDs, which are based on lumped electrical elements. The compilation and revision of the contributions to the equivalent circuit approach are targeted to indicate: (1) the interconnection between the stage of development, (2) its applicability, and (3) the current state-of-the-art of this approach.

Summary Background Over the past few years, plasma medicine has become an important field in medical science. Cold plasma has proven anti-inflammatory, antimicrobial and antineoplastic effects. Aim To test the decontamination power of two cold plasma sources [low-temperature atmospheric pressure plasma jet (APPJ) and dielectric barrier discharge plasma (DBD)] in vivo on human fingertips. Methods After 3, 15, 30, 60, 90, 120, 150, 180, 210 and 240 s of spot treatment with the APPJ and DBD, the log reduction factors (RFs) of physiological (PF) and artificially (AF) contaminated flora ( Staphylococcus epidermidis and Micrococcus luteus ) were calculated. The bacterial load was determined after counting. Tolerance (paresthesia, pain and heat) was measured using a numerical rating scale. Findings Both plasma devices led to a significant reduction in PF and AF. The maximum log reduction factors for PF were 1.3 for the DBD at 210 s and 0.8 for the APPJ at 60 s. For AF, the maximum log reduction factors were 1.7 for the DBD at 90 s and 1.4 for the APPJ at 120 s. Treatment with both devices was well tolerated. Conclusion Both the APPJ and DBD were highly effective in eradicating PF and AF from the fingertips of healthy volunteers. No plasma-resistant isolates were observed. Cold plasma appears to have potential for skin disinfection. For hand hygiene purposes, plasma exposure times would need to be reduced significantly by technical means.

Summary Background: Cold plasma, a new treatment principle in dermatology based on ionic discharge delivering reactive molecular species and UV-light, exhibits strong antimicrobial efficacy in vitro and in vivo. Before implementing plasma as new medical treatment tool, its safety must be proven, as well as assessing skin tolerance and patient acceptance. Patients and Methods: We investigated the plasma effects of three different plasma sources (pulsed, non-pulsed atmospheric pressure plasma jet (APPJ) and a dielectric barrier discharge (DBD)) on the transepidermal water loss (TEWL) and skin moisture after treating the fingertips of four healthy male volunteers. Results: TEWL values were reduced by pulsed APPJ and DBD by about 20% but increased after non-pulsed APPJ by 5–20%. TEWL values normalized 30 min after all forms of plasma treatment. Skin moisture was increased immediately and 30 min after treatment with pulsed APPJ but was not affected by non-pulsed APPJ and DBD. Conclusions: All plasma treatments were well-tolerated and did not damage the skin barrier nor cause skin dryness. Cold plasma fulfils basic recommendations for safe use on human skin and as future option may serve as the first physical skin antiseptic.

A non-thermal plasma source based on a surface dielectric barrier discharge (DBD) is developed for purification of recirculating air in operating theatres in hospitals. This is a challenging application due to high flow rates, short treatment times and the low threshold for ozone in the ventilated air. Therefore, the surface DBD was enhanced in order to generate an ionic wind, which can deflect and thus, filter out airborne microorganisms. Electrical and gas diagnostics as well as microbiological experiments were performed in a downscaled plasma source under variation of various electrical parameters, but application-oriented airflow velocity and humidity. The dependence of electrical power and ozone concentration as well as charged particles in the plasma treated air on frequency, voltage and relative humidity is presented and discussed. The presence of humidity causes a more conductive dielectric surface and thus a weaker plasma formation, especially at low frequency. The airborne test bacteria, Escherichia coli, showed significant effect to plasma treatment (up to 20% reduction) and to plasma with ionic wind (up to 90% removal); especially a configuration with 70% removal and an accompanying ozone concentration of only 360 ppb is promising for future application.

Diffuse dielectric barrier discharges in neon and helium at atmospheric pressure were studied. The discharges were generated between two metal electrodes, both covered by an alumina layer and driven by ac voltage of frequency 10 kHz. The discharge gap was 2.2 mm and 5 mm, respectively. The discharges were investigated by electrical measurements and by temporally and spatially resolved optical emission spectroscopy. The experimental results revealed similar discharge behaviour in both gases being considered. Although the discharges were ignited at slightly different electric field strengths, their evolutions were found to be similar. At maximum discharge current the spatial light intensity distribution was characterized by the formation of a cathode fall. A difference was observed in the magnitudes of the current density only. In addition to the regime with a single discharge pulse per voltage half period T/2, a discharge mode with two and more subsequent current pulses per T/2 (also referred to as the pseudoglow discharge regime in the literature) was obtained due to an increase in the voltage amplitude or an admixture of nitrogen.

Non-equilibrium plasmas have been intensively studied over the past century in the context of material processing, environmental remediation, ozone generation, excimer lamps and plasma display panels. Research on atmospheric pressure non-equilibrium plasmas intensified over the last two decades leading to a large variety of plasma sources that have been developed for an extended application range including chemical conversion, medicine, chemical analysis and disinfection. The fundamental understanding of these discharges is emerging but there remain a lot of unexplained phenomena in these intrinsically complex plasmas. The properties of nonequilibrium plasmas at atmospheric pressure span over a huge range of electron densities as well as heavy particle and electron temperatures. This paper provides an overview of the key underlying processes that are important for the generation and stabilization of atmospheric pressure non-equilibrium plasmas. The unique physical and chemical properties of theses discharges are also summarized.

The coplanar barrier discharge in synthetic air at 30 kPa pressure was studied by time-correlated single photon counting enhanced optical emission spectroscopy, far-field microscopy enhanced intensified CCD camera and sensitive current measurements. The discharge operated in a regime where two subsequent microdischarges appeared within the same voltage half-period. The electrical analysis of the barrier discharge setup enabled us to quantify charge transfer and the effective electric field development. During the second microdischarge the positive surface streamers follow the interface (triple-line) between the area of deposited charge from the previous one and the area of uncharged dielectric surface. It is shown that additional branching and flashes of surface streamers are responsible for the increased spatial complexity of the deposited surface charges at high overvoltage. A suppressed streamer propagating over the area of deposited surface charge was tracked and the evidence of surface streamer reconnection is presented. A spatiotemporal distribution (resolution of 120 ps and 100 um) of the reduced electric field strength was obtained for both microdischarges from the recorded luminosities of the molecular nitrogen. The reduced electric field of positive streamers in the first microdischarge reached 1200 Td. For the second one, the electric field values for the streamer at the triple-line are slightly lower than that, while for the suppressed streamers are even higher.

The characterization of electrical power distribution in an atmospheric pressure plasma jet operated at six different frequencies is investigated by measurement of input power and voltage-charge plots for the core plasma and the plasma in the effluent. The frequency change is further characterized by measuring the temperature in the effluent at different positions and input power levels. The power input into the core plasma is limited to about 1 W for a wide range of frequencies while the total input power and thus the power input into the effluent increases continuously with frequency.

A streamer-to-spark transition in a self-pulsing transient spark (TS) discharge of positive polarity in air was investigated using cross-correlation spectroscopy. The entire temporal evolution of the TS was recorded for several spectral bands and lines: the second positive system of N2 (337.1 nm), the first negative system of N2+ (391.4 nm), and atomic oxygen (777.1 nm). The results enable the visualization of the different phases of discharge development including the primary streamer, the secondary streamer, and the transition to the spark. The spatio-temporal distribution of the reduced electric field strength during the primary streamer phase of the TS was determined and discussed. The transition from the streamer to the spark proceeds very fast within about 10 ns for the TS with a current pulse repetition rate in the range 8-10 kHz. This is attributed to memory effects, leading to a low net electron attachment rate and faster propagation of the secondary streamer. Gas heating, accumulation of species such as oxygen atoms from the previous TS pulses, as well as generation of charged particles by stepwise ionization seem to play important roles contributing to this fast streamer-to-spark transition.

The micro-discharge properties and evolution in a 2D array of integrated coaxial microhollow dielectric barrier discharges are studied by using highly time-resolved electrical and optical diagnostics. The study is focused on the effect of the gas flow rate and gas residence time on discharge properties. The investigated integrated coaxial microhollow discharge geometry allows operating the discharge at exceptionally small residence times, which can be equal to or even smaller than the discharge period, at reasonable gas flow rates. The gas flow has an impact on gas heating, residual humidity, pre-ionization density and the densities of excited and reactive species produced by previous discharges. A unique voltage–charge plot is obtained with elongated periods without discharge activity. A very significant effect of flow on NO emission is observed that relates to the impact of flow on the NO production in these micro-discharges. Using the emission intensities of molecular bands of the second positive system of nitrogen and the first negative system of the nitrogen ion, effective reduced electric field strengths are obtained with a maximum equal to 870 Td. The reduced electric field decreases with increasing gas flow rate. This behavior is consistent with the reduction of the overall discharge intensity due to a reduced amount of charges present in the discharge gap. Both the flow rate and a reduction in water impurity changing the ion mobility can be responsible for the different effective electric field distributions at the highest and no flow conditions.

It is demonstrated that a four-wave mixing technique based on coherent anti-Stokes Raman spectroscopy (CARS) can determine the electric field strength of a pulsed-driven filamentary dielectric barrier discharge (DBD) of 1 mm gap, using hydrogen as a tracer medium in nitrogen at atmospheric pressure. The measurements are presented for a hydrogen admixture of 10%, but even 5% H2 admixture delivers sufficient infrared signals. The lasers do not affect the discharge by photoionization or by other radiation-induced processes. The absolute values of the electric field strength can be determined by the calibration of the CARS setup with high voltage amplitudes below the ignition threshold of the arrangement. This procedure also enables the determination of the applied breakdown voltage. The alteration of the electric field is observed during the internal polarity reversal and the breakdown process. One advantage of the CARS technique over emission-based methods is that it can be used independently of emission, e.g. in the pre-phase and in between two consecutive discharges, where no emission occurs at all.

To face the worldwide problem of anthropogenic carbon dioxide (CO 2 ) emission new techniques have to be developed. One approach for carbon capture utilization (CCU) is the conversion of CO 2 to more valuable chemicals, e.g., carbon monoxide (CO) by means of non-thermal plasma generated at ambient conditions and supplied by excess energy from renewable sources. This paper reports about the effect of the admixture of inert gases, namely nitrogen or argon to CO 2 in a coplanar dielectric barrier discharge (DBD). Systematic experiments were conducted to investigate the effects of applied voltage, frequency, flowrate and CO 2 concentration in the influent. The composition of products, energy efficiency and yield were determined. Within the investigated parameter ranges, the maximum conversion of CO 2 to CO efficiency of 1% was achieved when the specific input energy was 190 J L -1 , whereas the maximum CO yield of 0.7% was achieved when the specific input energy was 210 J L -1 . In conclusion, the energy efficiency can be significantly increased by operating the plasma in a diluted CO 2 gas.

Using theoretical and experimental methods, the electric field and the electron multiplication in direct vicinity of a sharp cathode is analysed. The development of the electric field in the pre-breakdown phase of the atmospheric pressure air negative DC corona discharge in the Trichel pulse regime is determined. During the following ultra-fast electrical breakdown, the emission of selected spectral bands of the nitrogen molecule is recorded with high spatiotemporal resolution using the time-correlated single photon counting method. The emission of a Townsend discharge is used to calibrate the setup for the quantitative determination of electric field. Therefore, the Trichel pulse corona and Townsend discharge cell are arranged in the same single-table setup. This direct calibration procedure is described step-by-step including the discussion of known limitations. Finally, the electric field development of the positive streamer passing the 160 microns distance in less than two nanoseconds is determined. Due to the high spatiotemporal gradients of the electric field strength within the streamer breakdown, the local field approximation of the electron component is analysed by investigating numerically the temporal and spatial electron relaxation by means of the solution of the electron Boltzmann equation and Monte Carlo simulation. Results of these computations are given for several reduced electric field values and prove that the electrons are in a hydrodynamic equilibrium state for experimentally given space and time scales for reduced elds above 100 Td.

A time-correlated single-photon counting technique was used to verify the formation of a cathode-directed streamer inside the narrow cathode region following the interpulse phase of regular negative corona Trichel pulses in ambient air. A purely experimental approach was used to determine the spatiotemporal development of the electric field during the Trichel pulse rise with an extremely high resolution of 10 μm and tens of picoseconds. The results confirm the positive-streamer mechanism for Trichel pulse formation and provide supportive evidence for the hypothesis that the formation of a primary cathode-directed streamer occurs always in any streamer-initiated breakdown and prebreakdown phenomena associated with cathode spot formation.

International audience; Microdischarges in a surface barrier discharge with special asymmetric needle-needle arrangement were investigated by means of cross-correlation spectroscopy (with sub-ns and sub-mm resolution) and an intensified CCD camera equipped with a far field microscope. The surface barrier discharge was driven at conditions (overvoltage) resulting in several microdischarges per half period of the applied sinusoidal voltage. At these conditions and in this arrangement regular patterns of microdischarges between the two electrodes points are formed due to local charging of the dielectric surface. The cross-correlation spectroscopy setup enables the recording of microdischarge development for different phases of the applied voltage. Distinct differences in the microdischarge development between the subsequent phase channels in the positive half period can be observed, while the first channel shows similar behavior as in the single-microdischarge mode which was conducted recently.

Microdischarges in a barrier discharge with an asymmetric electrode arrangement ('metal?dielectric') were investigated with fine temporal (down to 10?ps) and spatial (down to 10??m) resolution by the technique of cross-correlation spectroscopy. The discharge was operated in dry air at atmospheric pressure. The spatio-temporal distributions of the light intensities of the 0?0 transitions of the second positive (? = 337.1?nm) and first negative (? = 391.5?nm) systems of molecular nitrogen were compared with the corresponding experimental results for a symmetric electrode arrangement ('dielectric?dielectric') and for the coplanar barrier discharge. For all the discharge types being considered, the mechanism of electrical breakdown was found to consist of the Townsend pre-breakdown phase, the phase of ionizing wave propagation and the decay phase. Despite the qualitative similarity of the microdischarge development in different electrode arrangements, a detailed comparison of certain discharge characteristics (ionizing wave velocity, spatially or temporally integrated light intensity) enables us to reveal the influence of the electrode configurations on the process of electrical breakdown.

The aim of this work is to study the atmospheric pressure glow discharge (APG discharge) in neon. The discharge was generated between two metal electrodes, which were covered with an alumina layer. A small admixture of hydrogen could be added to neon. The type of the discharge (as concerning filamentary or glow discharge) was mainly derived from electrical measurements. The light emitted by the discharge was studied by means of optical emission spectroscopy (OES) with a temporal and spatial resolution. The results confirm that in neon the same type of diffuse dielectric-barrier discharge can be generated as in helium, namely a transient sub-normal glow discharge. This regime was also observed if up to 1% of hydrogen was added to neon. In general the recorded spectra consisted of atomic spectral lines of neon, as well as hydrogen atomic lines and molecular bands of N2 and OH due to external admixture or impurities in the carrier gas. When the amount of impurities was low enough, a continuous spectrum probably belonging to bremsstrahlung of electrons on neutral atoms was observed.