High-frequency underwater plasma discharge application in antibacterial activity

Plasma discharge is a novel disinfection and effectual inactivation approach to treat microorganisms in aqueous systems. Inactivation of Gram-negative Escherichia coli (E. coli) by generating high-frequency, high-voltage, oxygen (O₂) injected and hydrogen peroxide (H₂O₂) added discharge in water was achieved. The effect of H₂O₂ dose and oxygen injection rate on electrical characteristics of discharge and E. coli disinfection has been reported. Microbial log reduction dependent on H₂O₂ addition with O₂ injection was observed. The time variation of the inactivation efficiency quantified by the log reduction of the initial E. coli population on the basis of optical density measurement was reported. The analysis of emission spectrum recorded after discharge occurrence illustrated the formation of oxidant species (OH^?, H, and O). Interestingly, the results demonstrated that O₂ injected and H₂O₂ added, underwater plasma discharge had fabulous impact on the E. coli sterilization. The oxygen injection notably reduced the voltage needed for generating breakdown in flowing water and escalated the power of discharge pulses. No impact of hydrogen peroxide addition on breakdown voltage was observed. A significant role of oxidant species in bacterial inactivation also has been identified. Furthermore the E. coli survivability in plasma treated water with oxygen injection and hydrogen peroxide addition drastically reduced to zero. The time course study also showed that the retardant effect on E. coli colony multiplication in plasma treated water was favorable, observed after long time. High-frequency underwater plasma discharge based biological applications is technically relevant and would act as baseline data for the development of novel antibacterial processing strategies.     

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO₂: O₂ and Ar: O₂ mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O ₂ ⁺ ions and plasma decay is determined by dissociative and three-body electron?ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О₂ ⁺ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.     

Nonsputtering impulse magnetron discharge

Experiments with quasi-steady high-current discharges in crossed E × B fields in various gases (Ar, N₂, H₂, and SF₆) and gas mixtures (Ar/SF₆ and Ar/O₂) at pressures from 10⁻³ to 5 Torr in discharge systems with different configurations of electric and magnetic fields revealed a specific type of stable low-voltage discharge that does not transform into an arc. This type of discharge came to be known as a high-current diffuse discharge and, later, a nonsputtering impulse magnetron discharge. This paper presents results from experimental studies of the plasma parameters (the electron temperature, the plasma density, and the temperature of ions and atoms of the plasma-forming gas) of a high-current low-pressure diffuse discharge in crossed E × B fields.     

Effect of a small C3O2 additive on the vibrational distribution function of CO molecules in a low-temperature plasma

The concentration of carbon suboxide (C₃O₂) in the plasmas of sealed-off discharges in mixtures of CO with noble gases is measured for the first time by mass-spectroscopic technique. It is shown that the production of C₃O₂ (and, possibly, more complex carbon oxides) in a gas-discharge plasma significantly boosts the vibrational relaxation of CO molecules and thus greatly affects their vibrational populations. Adding xenon to a He: CO mixture reduces the concentration of C₃O₂. The effect of pulsed UV radiation on the vibrational populations of CO molecules is studied experimentally. It is shown that UV irradiation of the gas mixture after long-term discharge operation increases vibrational populations in the plateau region up to the values observed at the beginning of the discharge. This effect is attributed to the decay of C₃O₂ molecules under the action of UV radiation.     

Glow discharge in singlet oxygen

Currently, there is no experimental data on the plasma balance in gas mixtures with a high content of singlet delta oxygen O₂(¹Δ_g). These data can be obtained by studying the parameters of an electric discharge in singlet oxygen produced by a chemical generator. The O₂(¹Δ_g) molecules significantly change the kinetics of electrons and negative ions in plasma. Hence, the discharge conditions at low and high. O₂(¹Δ_g) concentrations are very different. Here, the parameters of the positive column of a glow discharge in a gas flow from a chemical singlet-oxygen generator are studied. It is experimentally shown that, at an O₂(¹Δ_g) concentration of 50% and at pressures of 1.5 and 2 torr, the electric field required to sustain the discharge is considerably lower than in the case when all of the oxygen molecules are in the ground state. A theoretical model of the glow discharge is proposed whose predictions are in good agreement with the experimental data.     

Low-pressure glow discharge in a Xenon/Chlorine mixture

The spatial, electrical, and optical characteristics of a transverse glow discharge and a volume discharge with a spherical anode and plane cathode in low-pressure Xe/Cl₂ mixtures are studied. It is shown that the transverse glow discharge in mixtures with a low chlorine content occupies most of the interelectrode gap and exists in the form of strata. As the total pressure (P≥300 Pa) and the partial chlorine pressure (P(Cl₂)≥80 Pa) increase, a solitary plasma domain with a volume of 1–2 cm³ forms in the discharge gap. It acts as a selective source of UV radiation in the XeCl(D-X) 236-nm, Cl₂ (D′-A′) 257-nm, and XeCl(B-X) 308-nm bands. In certain Xe/Cl₂ mixtures, plasma self-oscillations in the frequency range 1–100 kHz are observed. The current of a low-pressure volume discharge with a spherical anode and plane cathode and the emission from it have both a dc and an ac component. The pressure and composition of the working mixture, as well as the average current of the volume discharge are optimized to attain the maximum emission intensity of the XeCl(D,B-X) bands. Low-pressure volume discharges in xenon/chlorine mixtures can be used as active media in low-pressure large-aperture planar or cylindrical excimer-halogen lamps emitting modulated or repetitive pulsed UV radiation.     

Surface Modification of Melamine-Formaldehyde (MF-R) Macroparticles in Complex Plasma

The surface modification of melamine-formaldehyde (MF-R) macroparticles (4.12 ± 0.09 μm in diameter) in dc glow discharges in neon, argon, and an argon–oxygen mixture (90% Ar, 10% O₂) was studied experimentally. The macroparticles were treated in the discharge plasma for 10, 20, 40, and 60 min. The macroparticles were placed in ordered plasma–dust structures and then extracted from them. The results of atomic force microscopy of the surface profile are presented. Quantitative data on destruction of the surface layer and aspects of its modification are discussed. The amount of substance removed from the particle surface for the exposure time was calculated using the fractal analysis method.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Improved Sterilization of Sensitive Biomaterials with Supercritical Carbon Dioxide at Low Temperature

The development of bio-resorbable implant materials is rapidly going on. Sterilization of those materials is inevitable to assure the hygienic requirements for critical medical devices according to the medical device directive (MDD, 93/42/EG). Biopolymer-containing biomaterials are often highly sensitive towards classical sterilization procedures like steam, ethylene oxide treatment or gamma irradiation. Supercritical CO₂ (scCO₂) treatment is a promising strategy for the terminal sterilization of sensitive biomaterials at low temperature. In combination with low amounts of additives scCO₂ treatment effectively inactivates microorganisms including bacterial spores. We established a scCO₂ sterilization procedure under addition of 0.25% water, 0.15% hydrogen peroxide and 0.5% acetic anhydride. The procedure was successfully tested for the inactivation of a wide panel of microorganisms including endospores of different bacterial species, vegetative cells of gram positive and negative bacteria including mycobacteria, fungi including yeast, and bacteriophages. For robust testing of the sterilization effect with regard to later application of implant materials sterilization all microorganisms were embedded in alginate/agarose cylinders that were used as Process Challenge Devices (PCD). These PCD served as surrogate models for bioresorbable 3D scaffolds. Furthermore, the impact of scCO₂ sterilization on mechanical properties of polysaccharide-based hydrogels and collagen-based scaffolds was analyzed. The procedure was shown to be less compromising on mechanical and rheological properties compared to established low-temperature sterilization methods like gamma irradiation and ethylene oxide exposure as well as conventional steam sterilization. Cytocompatibility of alginate gels and scaffolds from mineralized collagen was compared after sterilization with ethylene oxide, gamma irradiation, steam sterilization and scCO₂ treatment. Human mesenchymal stem cell viability and proliferation were not compromised by scCO₂ treatment of these materials and scaffolds. We conclude that scCO₂ sterilization under addition of water, hydrogen peroxide and acetic anhydride is a very effective, gentle, non-cytotoxic and thus a promising alternative sterilization method especially for biomaterials.     

Decomposition of toluene in a steady-state atmospheric-pressure glow discharge

Results are presented from experimental studies of decomposition of toluene (C₆H₅CH₃) in a polluted air flow by means of a steady-state atmospheric pressure glow discharge at different water vapor contents in the working gas. The experimental results on the degree of C₆H₅CH₃ removal are compared with the results of computer simulations conducted in the framework of the developed kinetic model of plasma chemical decomposition of toluene in the N₂: O₂: H₂O gas mixture. A substantial influence of the gas flow humidity on toluene decomposition in the atmospheric pressure glow discharge is demonstrated. The main mechanisms of the influence of humidity on C₆H₅CH₃ decomposition are determined. The existence of two stages in the process of toluene removal, which differ in their duration and the intensity of plasma chemical decomposition of C₆H₅CH₃ is established. Based on the results of computer simulations, the composition of the products of plasma chemical reactions at the output of the reactor is analyzed as a function of the specific energy deposition and gas flow humidity. The existence of a catalytic cycle in which hydroxyl radical OH acts a catalyst and which substantially accelerates the recombination of oxygen atoms and suppression of ozone generation when the plasma-forming gas contains water vapor is established.     

Production of singlet oxygen in a non-self-sustained discharge

The production of O₂(a¹Δ_g) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O₂(a¹Δ_g production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2–1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O₂ molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O₂(a¹Δ_g and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O₂(a¹Δ_g and the dynamics of its concentration. It is shown that, in the dynamics of O₂(a¹Δ_g molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O(³P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar: O₂ mixture is analyzed based on the results obtained using the model developed. It is shown that, for actual electron beam current densities, a significant energy deposition in a non-self-sustained discharge in the mixtures under study can be achieved due to the high rate of electron detachment from negative ions. In this case, however, significant heating of the mixture can lead to a rapid quenching of O₂(a¹Δ_g molecules by atomic hydrogen.     

Characteristics of a transverse RF discharge in Xe/Cl<Subscript>2</Subscript> mixtures

Results are presented from the study of the electrical and optical characteristics of a transverse RF discharge in Xe/Cl₂ mixtures at pressures of p≤400 Pa. The working mixture was excited by a modulated RF discharge (f=1.76 MHz) with a transverse electrode configuration (L≤17 cm). The emission spectrum in the spectral range of 210–600 nm and the waveforms of the discharge current, discharge voltage, and plasma emission intensity were investigated. The UV emission power from the discharge was studied as a function of the pressure and composition of a Xe/Cl₂ mixture. It is shown that a discharge in a xenon-chlorine mixture acts as planar excimer-halogen lamp operating in the spectral range of 220–450 nm, which contains a system of overlapping XeCl(D, B-X; B, C-A) and Cl₂(D′-A′) bands. Transverse RF discharges in Xe/Cl₂ mixtures can be used to create a wideband lamp with two 50-cm² planar apertures and the low circulation rate of the working mixture.     

Simulation of internal transport barriers by means of the canonical profile transport model

Models with critical gradients are widely used to describe energy balance in L-mode discharges. The so-called first critical gradient can be found from the canonical temperature profile. Here, it is suggested that discharge regimes with transport barriers can be described based on the idea of the second critical gradient. If, in a certain plasma region, the pressure gradient exceeds the second critical gradient, then the plasma bifurcates into a new state and a transport barrier forms in this region. This idea was implemented in a modified canonical profile transport model that makes it possible to describe the energy and particle balance in tokamak plasmas with arbitrary cross sections and aspect ratios. The magnitude of the second critical gradient was chosen by comparing the results calculated for several tokamak discharges with the experimental data. It is found that the second critical gradient is related to the magnetic shear s. The criterion of the transport barrier formation has the form (a ²/r)d/drln(p/p _c ) > z ₀ (r), where r is the radial coordinate, a is the plasma minor radius, p is the plasma pressure, p _c is the canonical pressure profile, and the dimensionless function z _O(r) = C _O + C ₁ s (with C _0i ～1, C _0e ～3, and C _1i,e ～2) describes the difference between the first and second critical gradients. Simulations show that this criterion is close to that obtained experimentally in JET. The model constructed here is used to simulate internal transport barriers in the JET, TFTR, DIII-D, and MAST tokamaks. The possible dependence of the second critical gradient on the plasma parameters is discussed.     

The atmosphere of the primitive Earth and the prebiotic synthesis of amino acids

The atmosphere of the Earth at the time of its formation is now generally believed to have been reducing, an idea proposed by Oparin and extensively discussed by Urey. This atmosphere would have contained CH₄, N₂ with traces of NH₃, water and hydrogen. Only traces of NH₃ would have been present because of its solubility in water. UV light and electric discharges were the major sources of energy for amino acid synthesis, with electric discharges being the most efficient, although most other sources of energy also give amino acids.The first prebiotic electric discharge synthesis of amino acids showed that surprisingly high yields of amino acids were synthesized. Eleven amino acids were identified, four of which occur in proteins. Hydroxy acids, simple aliphatic acids and urea were also identified. These experiments have been repeated recently, and 33 amino acids were identified, ten of which occur in proteins, including all of the hydrophobic amino acids.Methionine can be synthesized by electric discharges if H₂S or CH₃SH is added to the reduced gases. The prebiotic synthesis of phenylalanine, tyrosine and trytophan involves pyrolysis reactions combined with plausible solution reactions.Eighteen amino acids have been identified in the Murchison meteorite, a type II carbonaceous chondrite, of which six occur in proteins. All of the amino acids found in the Murchison meteorite have been found among the electric discharge products. Furthermore, the ratios of amino acids in the meteorite show a close correspondence to the ratios from the electric discharge synthesis, indicating that the amino acids on the parent body of the carbonaceous chondrites were synthesized by electric discharges or by an analogous process.     

A model of Martian surface chemistry

Summary Alkaline earth and alkali metal superoxides and peroxides,-Fe₂O₃ and carbon suboxide polymer are proposed to be constituents of the Martian surface material. These reactive substances explain the water modified reactions and thermal behaviors of the Martian samples demonstrated by all of the Viking Biology Experiments. It is also proposed that the syntheses of these substances result mainly from electrical discharges between wind-mobilized particles at Martian pressures; plasmas are initiated and maintained by these discharges. Active species in the plasma either combine to form or react with inorganic surfaces to create the reactive constitutents.     

Dynamics of a high-temperature pinch in the presence of dust

Results are presented from experimental studies of the formation of a pinch in a plasma focus discharge in the presence of Al₂O₃ dust grains. Considerable attention is given to an analysis of the method for creating a dust target and determining its parameters. Phase transitions in a dust medium have been analyzed. It is found that the phase state of the dust component can be substantially affected by the radiation of the imploding plasma shell even in the preimplosion phase of the discharge. It is shown that a pinch produced in the presence of dust is more stable against MHD instabilities.     

Kinetics of Energetic O<Superscript>−</Superscript> Ions in the Discharge Plasmas of Water Vapor and H<Subscript>2</Subscript>O-Containing Mixtures

The process of relaxation of energetic O^– ions formed via dissociative attachment of electrons to molecules in the discharge plasmas of water vapor and H₂O: O₂ mixtures in a strong electric field is studied by the Monte Carlo method. The probability of energetic ions being involved in threshold ion–molecular processes is calculated. It is shown that several percent of energetic O^– ions formed via electron attachment to H₂O molecules in the course of plasma thermalization transform into OH^– ions via charge exchange or are destroyed with the formation of free electrons. The probabilities of charge exchange of O^– ions and electron detachment from them increase significantly (up to 90%) when O^– ions are formed via electron attachment to O₂ molecules in water vapor with an oxygen additive. This effect decreases with increasing oxygen fraction in the mixture but remains appreciable even when the fraction of H₂O molecules in the H₂O: O₂ mixture does not exceed several percent.     

Anodization of aluminum and silicon in plasma of a non-self-sustained glow discharge

The results of anodization of aluminum and silicon in an oxygen plasma are presented. The plasma was generated by a non-self-sustained glow discharge with a hollow cathode excited by an electron beam at the oxygen pressure of 20 Pa. The density of the current flowing through the anodized specimen did not exceed 1.5 mA/cm², and its temperature was 200–250°C. Continuous Al₂O₃ and SiO₂ films were formed on the aluminum and silicon surfaces. The growth rate of the oxide layers was 150–200 nm/h for Al₂O₃ and 400–800 nm/h for SiO₂.     

Scanning calorimetry measurements of the gas temperature in an RF-discharge fluorine-containing plasma

The neutral-gas temperature in a low-pressure (50 Pa) capacitive RF discharge in a CF₄+O₂ mixture is determined from the heating kinetics of a gallium arsenide single crystal, which is chemically inert to any radicals in a fluorine-containing plasma. Experimental methods are discussed that make it possible to confirm the absence of heat sources capable of additional heating of the calorimeter in the discharge. The features and applicability limits of the method of non-steady-state gas thermometry in a weakly ionized nonequilibrium plasma are discussed. The method proposed is compared with conventional steady-state methods based on measurements of the established temperature of a thermal probe in the discharge. Temperature scanning makes it possible to study dependences that cannot be investigated by steady-state methods, in particular, the temperature dependence of the calorimeter heating power, which is very important for diagnosing the processes of plasma-surface heat transfer.     

Scaling of a fast spherical discharge

The influence of the discharge cavity dimensions on the properties of the spherical plasma formed in a fast discharge was studied experimentally. The passage of a current pulse with an amplitude of 30–40 kA and a rise rate of ~10¹² A/s (a fast discharge) through a spherical ceramic (Al₂O₃) cavity with an inner diameter of 11 mm filled with argon at a pressure of 80 Pa results in the formation of a 1- to 2-mm-diameter spherical plasma with an electron temperature of several tens of electronvolts and a density of 10¹⁸–10¹⁹ cm^–3. It is shown that an increase in the inner diameter of the discharge cavity from 11 to 21 mm leads to the fourfold increase in the formation time of the spherical plasma and a decrease in the average ion charge. A decrease in the cavity diameter to 7 mm makes the spherical plasma unstable.