Time-resolved microscopic ICCD images, emission waveforms and spectra of triggered single-surface coplanar dielectric barrier micro-discharges were acquired with nanosecond time resolution. The micro-discharges were produced in pure argon by applying periodic high-voltage waveforms to a pair of embedded metallic electrodes at atmospheric pressure, and microscopic images of single micro-discharge events complemented with spectrally resolved emission were acquired with a time resolution of a few nanoseconds. Due to the low jitter of the micro-discharge onset with respect to the high-voltage pulse, we succeeded in separating the very weak emission produced during the course of streamer formation from the emission produced during the transition between the streamer and the subsequent transient glow phase. We identified the characteristic spectrometric signatures of the transition between the streamer and glow phases by analysing the lines belonging to the Ar(3s23p54p → 3s23p54s) multiplet. By linking optical and electrical measurements, we estimated the electron density with a high temporal resolution. The electron densities of the streamer discharge, determined spectroscopically, were in the range of 1019–1020 m−3, while an estimate of 1021 m−3 was obtained using macroscopic electrical analysis combined with ICCD imaging. The evolution of selected population ratios of 2p levels during streamer evolution was determined for the first time, and this may be used in the future for advanced streamer diagnostics.

Summary form only given. Electrical discharge plasmas in liquids have been studied for a number of years for applications in different environmental, biological or medical applications. However, the physics underlying the complex phenomena of electric discharge formation in liquid media as well as the chemistry of plasma/liquid interactions induced by these discharges is not fully understood. OH radical is one of the most strongly oxidative species produced by plasma in water and is also building block of H 2 O 2 which is an important agent in the chemical activity of plasma-liquid systems. So far, pulse durations applied for generation of discharge plasmas in liquids were typically in the range of microseconds. The average energy of electrons formed by streamer-like discharges in water was estimated to be 0.5-2 eV This would be sufficient to cause only vibration and rotational excitation rather than electron dissociative reactions of water. Therefore, metastable induced or thermal dissociation of water molecules and electron dissociative recombination of water ions are proposed as more likely pathways of plasmachemical production of OH radicals in water. Recently, a fundamentally different type of discharge generated in water by application of high-voltage pulses with nanosecond duration was reported by several authors. The observed discharge was reported to have a completely different nature from the discharges with microsecond pulse duration. This suggests, that it might be possible to vary electron energy distribution in the discharge and plasmachemical processes in water by pulse duration of pulsed power used for discharge in water.In this work, the formation of OH radicals by a nanosecond pulsed corona discharge in deionized water was studied by means of time resolved optical emission spectroscopy and by chemical methods. FID generator was used for high-voltage pulse generation of pulse length 5ns (FWHM) and amplitude +80-120 kV. Electronically excited radiative state of the OH (A-X) was not detected in emission spectrum of ns discharge in water, however, chemical methods proved formation of OH radicals in water. This is in contradiction to the results obtained for OH radical production in water analyzed under microsecond discharge conditions. These observations were evaluated in more detail and will be discussed with regard to the mechanisms of OH radical formation by plasma in water. * This work was supported by the Czech Science Foundation (project 15-12987S).

Underwater electrical discharge generated by high voltage pulses with nanosecond duration (similar to 110 kV amplitude on the needle, similar to 5 ns duration (FWHM), 1 Hz repetition rate) has been investigated. Time-resolved optical emission spectroscopy showed different emission spectra profile of the ns-pulsed discharge compared to the mu s-pulsed discharges investigated previously. Some characteristic peaks typical for water discharges (H-alpha, O-I) were clearly visible in the spectra. In addition, the integral light emission of the discharge was recorded by photomultiplier tube.

We employed techniques of time-resolved ICCD microscopy and spectroscopy to register basic morphologic and emission fingerprints of micro-discharges produced in deionized water. Fast rise-time high-voltage pulses (duration of 5 ns and amplitude of similar to 110 kV) applied in a point-to-plane electrode geometry produced micro-discharges either periodically or in a single-shot regime. Time resolved ICCD images evidence typical streamer-like branched filamentary morphology while emission spectra show a broad-band continuum evolving during the first few nanoseconds followed by the well-known HI/OI atomic lines (tens of ns).

Novel apparatus for the generation of underwater plasma based on DC diaphragm discharge excited in a vapor bubble has been developed for decontamination and disinfection of conductive water. The apparatus allows deposition of relatively high applied power into the discharge (order of kW) and the treatment of a relatively large volume of liquid (order of L/min). The apparatus is operated at the quasi-pulse regime with self-terminating discharge pulses (with a repetition rate of 15–20 Hz) generated upon the formation of the vapor bubble inside the diaphragm (capillary) and its subsequent breakdown. The effects of input power, solution conductivity and the method of liquid flow through the reactor on the plasmachemical yield of H2O2 production and degradation of phenol have been determined. The biocidal effects of the apparatus were evaluated on inactivation of bacteria E. coli and E. faecalis suspended in aqueous NaCl solutions and on growth inhibition of the cyanobacterium Planktothrix sp. in natural lake water. The apparatus proved to be capable of efficiently reducing biological contamination in water, especially when operated in the plug-flow regime (up to a 5-log reduction in bacteria after 3 passes through the reactor). In the case of cyanobacteria, the growth inhibition further proceeded after exposure to the discharge and one pass of the biomass through the reactor was sufficient to reduce the algae in the water.

We explored basic optical and electrical characteristics of a positive corona-like discharge produced in conductive aqueous solutions by periodic high-voltage pulses. Emission spectra of the discharge were acquired in a needle-to-plate electrode geometry and analysed in the UV–vis–NIR spectral range with nanosecond time resolution for the solution conductivity of 100 and 500 µS cm−1. The most important emission features are due to electronic excitation of HI, OI, OII and OH species. We found evidence of significant time-dependent line-shape broadening of selected HI and OI transitions. The observed broadening is attributed to the dynamic Stark and pressure broadening mechanisms and significantly increases with the aqueous solution conductivity. Electron densities were estimated by fitting a single Voigt peak function to the observed Hα profiles, and can reach as much as ne ≅ 4 × 1018 cm−3 (tD = 300 ns at 100 µS cm−1 solution conductivity) and ne ≅ 5 × 1018 cm−3 (tD = 1 µs at 500 µS cm−1). Temporal evolution of the partially resolved rotational structure of the OH emission reaches a maximum during the discharge decay, with the onset significantly delayed with respect to the streamer ignition.

UV radiation in the 200-300 nm wavelength range with doses of several mJ/cm2 is known to cause lethal damage of cells. Amongst UV effects on bacteria is the dimerization of thymine bases in their DNA strands. This inhibits the ability of bacteria to replicate properly. The results obtained using the emission spectroscopy showed a radiation from the pulsed corona discharge in water in a wide range of wavelengths (200-1000 nm), which is dominated by the spectral lines of hydrogen and oxygen atom and by emission from OH radical. Electron density above 1018 cm-3 in the streamer discharge has been determined from the Halpha spectral line profile. With increasing water conductivity stronger radiation and higher electron density in the streamer discharge have been determined (above 1019 cm-3). Quantitative analysis of ultraviolet radiation from the pulsed corona discharge in water performed by the potassium ferrioxalate actinometry revealed that significant UV emission from the discharge occurs with increasing solution conductivity and the pulse radiant power of the emitted UV radiation could reach levels of the order of tens to hundreds of Watts per pulse in the range of solution conductivity of 100-500 muS/cm. This radiant power corresponds to UV radiation intensity of the order 0.1-10 mW/cm2. This is a significantly higher intensity level than reported for non- equilibrium air plasma where UV radiation does not play a significant direct role in the sterilization process. This indicates that UV radiation may play an important role in the bacterial inactivation by the corona discharge in water. In this work, the effects of pulsed corona discharge in water on the inactivation of Gram-negative bacteria Escherichia coli and Gram-positive bacteria Enterococus faecalis are investigated. The role and contribution of ultraviolet radiation from the discharge in the overall bacterial inactivation by the - lectrical discharge in water of two different solution conductivity (200 and 500 muS/cm) are discussed.

Effect of solution conductivity on production of H2O2, H2 and O2 generated by the pulsed corona discharge in water has been investigated. Hydrogen was dominant product of plasmachemical activity of the discharge when its formation significantly increased with increasing solution conductivity. Compared to H2O2 its production rose from ratio of 2:1 at 100 μS/cm up to 5:1 at 500 μS/cm. Possible routes of electron impact dissociation of water induced by plasma in water in dependence on the solution conductivity was proposed.

Non-equilibrium plasma generated by electrical discharges in liquids initiate various chemical and physical processes that can be potentially utilized in different environmental, biological or medical applications. These processes include high electric field, ultraviolet radiation, overpressure shock waves and, of particular importance, formation of various reactive chemical species such as radicals (OH, H, O) and molecular species (H2O2, H2, O3), among which OH radicals and hydrogen peroxide are the most important for oxidation processes. The magnitude of the contributions of the individual effects in the decontamination or microbial inactivation processes strongly depends upon the energy of the discharge and also on the solution conductivity. An increase in the solution conductivity is connected with a higher concentration of ions in the liquid, which strongly alter the propagation of the streamer channel in water by compensating the space charge electric field on the streamer head. Thus, higher conductivity results, on the one hand, in a larger discharge current due to lower resistivity of liquid media, and, on the other hand, in a shortening of the streamer channel length due to faster compensation of space charge electric field on the streamer head by ions in the liquid. This results in a higher power density in the discharge channel and a higher plasma temperature and higher UV radiation. Hydrogen peroxide as the most abundant chemical species directly produced by the discharge in water is often utilized in Fenton's reaction to increase the plasmachemical efficiency of the removal of organic compounds by addition of ferrous salts into treated water. There is also evidence about contribution of H2O2 in bacterial inactivation by electrical discharge in water. In the present study, the role of solution conductivity in the hydrogen peroxide production by the pulsed corona discharge in water generated using point to plane geometry of electrodes is investigated. The free radical scavenging property of DMSO and phenol is used to determine the initial rate of formation of H2O2 by the pulsed corona discharge. The influence of UV radiation of the discharge and the effect of H2 and O2 on the production of hydrogen peroxide will be discussed.

The decontamination effect of underwater pulsed corona discharge on gram-negative bacteria Escherichia coli and gram-positive bacteria Enterococus faecalis has been investigated. The reactor of needle-plate electrode geometry was used. The concentration of both types of bacteria decreased exponentially with increasing discharge treatment time and supplied energy input. The contribution of UV radiation and hydrogen peroxide in the overall bacterial inactivation efficiency of the discharge has been determined. UV light emitted from the discharge plasma channels was found to have significant effect on the bacterial inactivation and its contribution was estimated to be about 30%. Effect of hydrogen peroxide was small, however, under influence of high electric field its lethal activity was greatly enhanced.

Abstract Streamer discharges in atmospheric gases are currently receiving increased attention in connection with environmental issues or new material treatment technologies. The chemical reactivity of streamer-produced plasma, created by various active atomic and molecular species, is a critical parameter for most application areas. Recently, advanced diagnostics based on optical and energy transfer methods have been applied to study dynamics and spatial distribution of several important streamer-produced species. This paper presents an overview of the most recently reported experimental achievements to monitor and detect reactive species, such as, e.g., NO, OH, and O3 radicals, nitrogen and oxygen atoms, or N2(A3Σ) metastables.

Erosion of needle electrodes in the pulsed corona discharge in water with a pulse energy of ∼ 2÷; 3 J was investigated in dependence on the electrode material (platinum, tungsten and stainless-steel) and the solution conductivity (100 and 500 µS/cm). Erosion of electrodes remarkable increased with the higher solution conductivity for all three tested metals. The highest erosion rates were determined for tungsten while platinum was the least eroded material. In addition to the dominant melting effect, release of anode material by the electrolysis significantly contributed to the total erosion of needle electrodes. The highest contribution of electrolysis was determined for stainless-steel electrodes that released up to 40–50% of eroded metal in the form of iron ions. Peculiar protrusions were observed on the surface of eroded tungsten electrodes.

Ozone formation by a pulse positive corona discharge generated in the gas phase between a planar high voltage electrode made from reticulated vitreous carbon and a water surface with an immersed ground stainless steel plate electrode was investigated under various operating conditions. The effects of gas flow rate (0.5-3 litre min(-1)), discharge gap spacing (2.5-10 mm), applied input power (2-45 W) and gas composition (oxygen containing argon or nitrogen) on ozone production were determined. Ozone concentration increased with increasing power input and with increasing discharge gap. The production of ozone was significantly affected by the presence of water vapour formed through vaporization of water at the gas-liquid interface by the action of the gas phase discharge. The highest energy efficiency for ozone production was obtained using high voltage pulses of approximately 150 ns duration in Ar/O-2 mixtures with the maximum efficiency (energy yield) of 23 g kWh(-1) for 40% argon content.

Summary form only given. Pulsed high voltage discharges generated directly in water initiate a variety of chemical and physical effects including the high electric field, intense ultraviolet radiation, overpressure shock waves, and, of particular importance, formation of various chemically active species (OH, H and O radicals, H2O2). These reactive species and physical conditions in turn have been shown to be effective at degrading a variety of organic compounds; in the destruction and inactivation of microorganisms; and also in the modification of surface properties of polymeric materials. Recent research has showed that plasma chemical activity of electrical discharge in water can be enhanced by the addition of solid particles into the discharge reactor. Several types of materials have been tested such as activated carbon, silica gel, alumina, titanium oxide or zeolites. However, despite observed synergistic effects there is only limited knowledge about the role of these materials in the plasma assisted processes in water. It is apparent, that presence of these materials can affect chemical activity of the discharge in various ways including in addition to the simple adsorption processes also plasma induced catalytical reactions on their surface. Consequently, plasma-surface interactions can also alter the electrical discharge properties through the formation of non-equilibrium surface layers and local electric field at the solid surfaces. These processes were determined important particularly during generation of pulsed electrical discharge in water using ceramic-coated electrodes prepared by thermal plasma spraying technology. It has been found that characteristics of these discharges strongly depend on type of ceramic material and also chemical composition of the aqueous solution. In the present study, comparison is made for two types of ceramics-oxide (corundum) and silicates (almandine). The possible mechanisms of plasma-surface interactions will be discussed with regard to the polarity of applied power and chemical composition of the aqueous solution.

Generator of two successive shock waves focused to a common focal point has been developed. Cylindrical pressure waves created by multichannel electrical discharges on two cylindrical composite anodes are focused by a metallic parabolic reflector and near the focus they are transformed to strong shock waves. The anodes are energized from separate power supplies and time delay between the discharges can be varied. Schlieren photos of the focal region demonstrated that interaction of the two waves results in creation of a large number of secondary short wavelength shocks. Strong attenuation of the second wave has bee shown by measurements of the shock waveforms at the focus. Localized injury of rabbit's liver induced by the shock waves has been demonstrated by the method of magnetic resonance imaging. Histological analysis of the liver samples taken from the injured region revealed that the boundary between the injured and health tissues is very sharp.

Bipolar high voltage pulse power generator is a device that generates alternately positive and negative pulses of the same amplitude and waveform. It consists of a variable high voltage DC source, a rotating spark gap and a low inductance capacitor. The spark gap allows switching both the charging and discharging phases of the capacitor into the load. The time delay between positive and negative pulses is one half of the repetition period, which is controlled by revolutions of the rotating spark gap. The amplitude of output voltage pulses can be varied up to 35 kV with the repetition frequency up to 100 Hz. Shape of output pulse is determined by used capacitor and the load value. For typical parameters (capacitor 14 nF, average load value ~100 Omega) pulses with exponential slope, rise time 40 ns and pulse duration ~3 mus are generated. The potential applications of such system include in particular generation of the pulse electrical discharges in the liquid phase. This paper describes design and performance of the bipolar high voltage pulse power generator. The high voltage measurements of the generator output pulsed into resistive load are presented and compared with the results of circuit simulations.

Production of ozone generated in oxygen by the gas phase discharge a bove aqueous solution was studied for di fferent applied pulse high voltage and discharge gap heights between gas p hase high voltage electrode and water surface. The ozone production increased with higher gap and higher applied voltage, while the e fficiency decreased with higher voltage for fixed gap height and increase d with higher gap height. The appearance of the gas phase discharge occ urring above the water significantly differed with the change of discharge gap height. The formation of ozone wa s affected by the presence of water vapor formed through the vaporization of water sur face by the gas phase discharge, which was in direct contact with aqueous solution.

A novel method for generation of focused shock waves has been developed. A cylindrical pressure wave, created by a high-voltage multichannel discharge in water with an increased electrical conductivity, is focused by a metallic parabolic reflector. The discharge is formed on a composite anode consisting of a cylindrical stainless steel electrode covered by a thin porous ceramic layer. In such an arrangement at the applied voltage of 30 kV, a large number of short discharge channels distributed homogeneously on the anode surface is initiated. Each discharge channel creates a semi-spherical pressure wave, and by superposition of all of the waves, a cylindrical pressure wave propagating from the anode is formed. The cylindrical pressure wave is focused by a metallic parabolic reflector (cathode) and near the focus it is transformed into a strong shock wave. The focal volume is 2.5 mm in diameter and 32 mm long. We have demonstrated that the focused shock waves destroy human red blood cells (erythrocytes) very efficiently. Interaction of the focused shock waves with fresh potatoes that have high water content have been used to assess the mechanical effects of the waves. The 6-cm-thick slab of potato was placed next to the focal region and exposed to 10 shocks at 30 kV. The experiment demonstrated that only the focal region inside the potato had been damaged and no damage was seen between the potato surface and the focal region. Local injury of animal liver tissues exposed to the shock waves were observed in vitro. Livers of three rabbits have been exposed in vivo. Histological analysis of the exposed tissues revealed injuries and hemorrhages at the focal region of the shock waves.

The aim of this work was to investigate the possibility of applying a high voltage pulsed electrical discharges for dye wastewater treatment. Commercial organic monochlorotriazine reactive dye of the anthraquinone type C.I. Reactive Blue 49 (RB49) was chosen as a representative of persistent and recalcitrant wastewater pollutant. The modified pinhole discharge flow-through reactor was used to treat such type of contaminant. Applying HV pulses 30 kV, 3.15 J/pulse, 50 Hz repetition rate, complete decolorisation and partial mineralization of RB49 has been reached and demonstrated by means of UV/VIS absorption, TOC and AOX measurements.

A modified pinhole discharge with coaxial electrodes has been developed. An inner tubular stainless steel anode (O 12 mm) is coaxial with a cylindrical outer cathode (O 100 mm). The anode is covered by a polyethylene insulator and one or three small holes (O 0.7 mm) are drilled in the central part of the anode. Treated liquids are pumped into the inner volume of the anode; it flows into the interelectrode volume through the holes and circulates back to the anode volume. Pulse power supply consists of a 7 or 14 nF storage capacitor charged up to 30 kV and rotating spark gap switch. Pulse repetition frequency was 50 Hz and the solution conductivity was varied in the range of 0.3-1 mS/cm. Production of hydrogen peroxide at a similar rate as in the needle-plate reactors has been measured. Decolorizing of the organic dye "reactive blue 137" and reduction of the number of living bacteria by four orders of magnitude have been demonstrated.