Non-equilibrium Plasma Conditions Research Articles

Validation of non-equilibrium kinetics in CO2–N2 plasmas

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2–O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v= 59 for N2 and up to v 1= 2 and v 2= v 3= 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e–V), vibration-to-translation (V–T) and vibration-to-vibration energy exchange (V–V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2–N2 mixtures, at pressures in the range 0.6–4 Torr (80.00–533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70%, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2–N2 and CO–N2 V–V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Πg) electronically excited state further enhances the CO2 dissociation.

Study of vibrational kinetics of CO2 and CO in CO2–O2 plasmas under non-equilibrium conditions

This work explores the effect of O2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent model, previously validated for pure CO2 discharges, is further extended by adding the vibrational kinetics of CO, including electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The vibrational kinetics considered include levels up to v = 10 for CO and up to v 1 = 2 and v 2 = v 3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and accounts for e-V, V-T in collisions between CO, CO2 and O2 molecules and O atoms and V-V processes involving all possible transfers involving CO2 and CO molecules. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge ignited in pure CO2 and CO2–O2, operating at pressures in the range 0.4–5 Torr (53.33–666.66 Pa). The experimental results show a lower vibrational temperature of the different modes of CO2 and a decreased dissociation fraction of CO2 when O2 is added to the plasma but an increase of the vibrational temperature of CO. On the one hand, the simulations suggest that the former effect is the result of the stronger V-T energy-transfer collisions with O atoms which leads to an increase of the relaxation of the CO2 vibrational modes. On the other hand, two main mechanisms contribute to the lower CO2 dissociation fraction with increased O2 content in the mixture: the back reaction, CO(a3Πr) + O2 → CO2 + O and the recombinative detachment O− + CO → e + CO2.

Influence of N2 on the CO2 vibrational distribution function and dissociation yield in non-equilibrium plasmas

This work explores the effect of nitrogen addition on CO2 dissociation under various non-equilibrium plasma conditions. Experiments are performed in non-thermal plasmas sustained by DC pulsed discharges, for pressure and current ranges of 1 to 5 Torr and 20 to 50 mA, respectively. A self-consistent model, previously validated for pure CO2 discharges, is further extended to take into account e-V, V-T and V–V reactions involving N2. Both model predictions and experimental data reveal a maximum of the asymmetric vibrational temperature T3 at 5 Torr during the discharge around 1 ms, while no such maximum is visible at 1 Torr before the saturation occurs. It is shown that V-T deactivation by O atoms can have a strong influence on the vibrational kinetics, by directly affecting the relaxation of N2 vibrational excited states and, as a consequence, the very important energy transfers between vibrationally excited N2 and CO2 molecules. The experimental results show a twice as large CO2-conversion rate when N2 gas is added to the plasma. The simulations suggest this effect cannot be the result of an increased dissociation by direct electron impact due to modifications in the reduced electric field, but rather of some other contribution to dissociation and/or inhibition of reactions giving back CO2.

Generation of fast electrons in pulsed discharge plasma in helium

The results of experimental studies of the distribution function of fast electrons under conditions of non-equilibrium plasma of a pulsed discharge in helium are presented. It was established by various methods that at the initial stage of the discharge, when the reduced electric field exceeds the critical value, a beam of fast electrons is generated. The beam energy depends on the applied voltage to the discharge gap, and helium pressure.

Determination of water leaks flows and their surface localization in plasma reactors by the ratio of the hydrogen isotopes line intensities

A high sensitive method of multispectral actinometry developed for the measurements of the concentration of minor hydrogen-oxygen products of plasma-chemical transformation of water is presented. It is shown that in presence of deuterium the correlation between H/D density ratio and the densities of other products is practically independent of non-equilibrium plasma conditions. It was studied in the hollow cathode experimental installation and model of the processes was developed for scaling purposes. This method allows to find the sources of water leaks to plasma due to the chamber wall defects and to measure their flow rates. The penetration of water vapors into the model installation was provided with a special localized leak and H/D ratio was measured by Dα and Hα lines intensity. The scaling was performed for the specific conditions of ITER reactor near-wall plasma during testing trials. The flows of the Н2О, О2, Н2, Н, and О particles associated with the emergence of water molecules in plasma can be determined by observing disturbances suffered by the relative intensities of the emission spectral lines of particles in the vicinity of defects. The locality is limited by the spatial resolution of the optical system forming the image of the chamber wall. For testing purposes, discharge in an inert gas (He, 2 · 1016 cm−3) with small additions of D2 (1012–1010) cm−3 is ignited in the near-wall region of the ITER chamber with electron concentrations and temperatures ne = (1010–1013) cm−3, Те = (4–30) eV. For this type of discharge water leaks with flows Q = (10−13–10−9) Pa m3 s−1 can be detected. Sensitivity improves with decreasing the amount of D2 additives. With a complete examination of the internal surfaces of the chamber wall and the design requirement for the total water leakage Q < 10−7 Pa m3 s−1, more than 100 small leaks with flows Q = (10−11–10−9) Pa m3 s−1 can be detected simultaneously.

Zinc Oxide Nanostructures Fabricated under Extremely Non-Equilibrium Plasma Conditions

In the present work, extremely non-equilibrium, high temperature and high density argon plasma is used for producing ions from pellet of zinc oxide (ZnO) fitted on top of anode. These ions along with energetic argon ions move vertically upward in a fountain like structure in post focus phase of plasma dynamics and material ions get deposited on the glass substrates placed at 4.0 cm from anode top. This process of production of material ions from ZnO pellet leads to nucleation and nanostructures formation with one and two bursts of focused plasma. The surface morphology studied using scanning electron microscopy shows the formation of nanostructures with mean size about 8 nm. The structural properties of nanostructures in X-ray diffraction pattern show [100] and [002] planes of hexagonal ZnO. Photoluminescence studies show peaks related to defect transitions. The band-gap of nanostructures found from Tauc plot is smaller than that of the bulk ZnO. The resultant morphological, structural and optical properties of nanostructures suggest the possible applications in visible optoelectronic devices.

Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration

Abstract In a solar flare, a large fraction of the magnetic energy released is converted rapidly to the kinetic energy of non-thermal particles and bulk plasma motion. This will likely result in non-equilibrium particle distributions and turbulent plasma conditions. We investigate this by analyzing the profiles of high temperature extreme ultraviolet emission lines from a major flare (SOL2014-03-29T17:44) observed by the EUV Imaging Spectrometer (EIS) on Hinode. We find that in many locations the line profiles are non-Gaussian, consistent with a kappa distribution of emitting ions with properties that vary in space and time. At the flare footpoints, close to sites of hard X-ray emission from non-thermal electrons, the κ index for the Fe xvi 262.976 Å line at 3 MK takes values of 3–5. In the corona, close to a low-energy HXR source, the Fe xxiii 263.760 Å line at 15 MK shows κ values of typically 4–7. The observed trends in the κ parameter show that we are most likely detecting the properties of the ion population rather than any instrumental effects. We calculate that a non-thermal ion population could exist if locally accelerated on timescales ≤0.1 s. However, observations of net redshifts in the lines also imply the presence of plasma downflows, which could lead to bulk turbulence, with increased non-Gaussianity in cooler regions. Both interpretations have important implications for theories of solar flare particle acceleration.

Особенности электролиза воды в условиях неравновесной низкотемпературной плазмы

Особенности электролиза воды в условиях неравновесной низкотемпературной плазмы

Low-temperature ignition of methane-air mixtures under the action of nonequilibrium plasma

A plasma-chemical kinetic mechanism of the low-temperature (600 < T < 1000 K) oxidation/combustion of methane under conditions of nonequilibrium plasma over a wide pressure range (P = 0.1−100 atm) is developed and verified. The mechanism is comprised of three types of elementary processes: chemical reaction of neutral atoms and molecules, primary plasma-chemical processes involving electrons, and secondary plasma-chemical processes involving atomic and molecular ions and excited species. Application of the developed mechanism to describing the plasma-assisted oxidation of methane shows that this mechanism can describe the experimental results qualitatively and quantitatively.

Influence of surface microwave discharge on ignition of high-speed propane-air flows

Ignition of supersonic propane-air flow under conditions of low-temperature plasma of surface microwave discharge is experimentally studied. We show that both rich and poor mixtures ignite, and the combustion intensity is maximal for the stoichiometric mixture. The dependence of the supersonic propaneair flow ignition time on the reduced electric field, E/n, under conditions of nonequilibrium gas-discharge plasma is experimentally obtained. The induction period is shown to decrease from 1 ms to 5 μs with the increase in E/n from 40 to 200 Td. The propagation velocity of the combustion front boundary (depending on the equivalent mixture ratio and the input microwave power) is maximal for the stoichiometric mixture and reaches 160 m/s at E/n = 150 Td. Under these conditions, the combustion temperature is about 3000 K.

Discharge analysis and electrical modeling for the development of efficient dielectric barrier discharge

Dielectric-barrier discharges (DBDs) are characterized by the presence of at least one insulating layer in contact with the discharge between two planar or cylindrical electrodes connected to an AC/pulse power supply. The dielectric layers covering the electrodes act as current limiters and prevent the transition to an arc discharge. DBDs exist usually in filamentary mode, based on the streamer nature of the discharges. The main advantage of this type of electrical discharges is that nonequilibrium and non-thermal plasma conditions can be established at atmospheric pressure. VUV/UV sources based on DBDs are considered as promising alternatives of conventional mercury-based discharge plasmas, producing highly efficient VUV/UV radiation. The experiments have been performed using two coaxial quartz double barrier DBD tubes, which are filled with Xe/Ar at different pressures. A sinusoidal voltage up to 2.4 kV peak with frequencies from 20 to 100 kHz has been applied to the discharge electrodes for the generation of microdischarges. A stable and uniform discharge is produced in the gas gap between the dielectric barrier electrodes. By comparisons of visual images and electrical waveforms, the filamentary discharges for Ar tube while homogeneous discharge for Xe tube at the same conditions have been confirmed. The electrical modeling has been carried out to understand DBD phenomenon in variation of applied voltage waveforms. The simulated discharge characteristics have been validated by the experimental results.

Analysis of Stark line profiles for non-equilibrium plasma diagnosis

Computer simulations permit one to calculate Stark broadened profiles for non-equilibrium plasma conditions. In this work, the dependences of some line parameters on the unbalance between electrons and ions temperatures in the plasma are shown. This method can be applied in some cases to analyze experimental conditions by comparing experimental and simulated full line profiles.

Nickel plasma produced by 532-nm and 1064-nm pulsed laser ablation

A comparison between laser ablation of nickel in vacuum by using 532-and 1064-nm Nd:YAG (Yttrium Aluminium Garnet) laser wavelengths, with an intensity of 5 × 109 W/cm2, is reported. Nanosecond pulsed ablation produces high nonisotropic emission of neutrals and ionic species. For 532-nm laser irradiation, mass quadrupole spectrometry, coupled to electrostatic ion deflection and time-of-flight measurements, allows estimation of the energy distributions of the emitted species from plasma. For 1064-nm laser ablation, a cylindrical electrostatic ion analyzer permits one to measure the yield and the charge state of the emitted ions and reconstruct the ion energy and charge state distributions. Neutrals show typical Boltzmann-like distributions, while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted study of the ablation threshold and yields of nickel in vacuum versus the laser fluence. The plasma temperature was evaluated using experimental data. Special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at nonequilibrium plasma conditions and the angular distribution of the emitted species.

Discharge characteristics of dielectric barrier discharge (DBD) based VUV/UV sources

Dielectric-barrier discharges (DBDs) are characterized by the presence of at least one insulating layer in contact with the discharge between two planar or cylindrical electrodes connected to an AC/pulse power supply. The dielectric layers covering the electrodes act as current limiters and prevent the transition to an arc discharge. DBDs exist usually in filamentary mode, based on the streamer nature of the discharges. The main advantage of this type of electrical discharges is that nonequilibrium and non-thermal plasma conditions can be established at atmospheric pressure. VUV/UV sources based on DBDs are considered as promising alternatives of conventional mercury-based discharge plasmas, producing highly efficient VUV/UV radiation. The experiments have been performed using coaxial and planar geometry of DBD (gas gap: 1–3 mm) made of quartz with N2/Ar/Xe gas at different pressures. A proper ultra high vacuum system and gas filing system has been made for the processing & characterization of DBD tubes. A RF generator (20-100 kHz, 0-2.4 kV peak) is used for discharges in DBD tube. A stable and uniform discharge is produced in the gas gap between the dielectric barrier electrodes. The discharge characteristics have been analyzed by V-I characteristics & Lissajous figure and found that the spatial discharge processes varies strongly according to the applied voltage waveform, pressure of filled gas and geometry of tube.

Plasma–laser characterization by electrostatic mass quadrupole analyzer

A study of laser ablation of different metals (aluminium, zinc, tantalum and lead), in vacuum, by using 3ns Nd:YAG laser radiation, at 532nm wavelength, is reported. Laser pulse, at intensities of the order of 109W/cm2, produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, associated to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb–Boltzmann-shifted distributions. The plasma characterization is rationalized in terms of kinetic energies of ejected particles, ion, electron and neutral temperatures, ion charge states, and plasma density. A special regard is given to the parameters which regulate the plasma temperature: the boiling point, the electron density and the ionization potentials of the ablated elements. The ion acceleration processes occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions are presented and discussed.

Characterization of laser-generated silicon plasma

A study of visible laser ablation of silicon, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation, at an intensity of the order of 10 10 W/cm 2, produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species from plasma. Neutrals show typical Boltzmann-like distributions while ions show Coulomb–Boltzmann-shifted distributions depending on their charge state. Time-of-flight measurements were also performed by using an ion collector consisting of a collimated Faraday cup placed along the normal to the target surface. Surface profiles of the craters, created by the laser radiation absorption, permitted to study the ablation threshold and ablation yields of silicon in vacuum. The plasma fractional ionization, temperature and density were evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the non-equilibrium plasma conditions. The angular distribution of the neutral and ion species is discussed.

Particle emission from tantalum plasma produced by 532nm laser pulse ablation

A study of visible laser ablation of tantalum in vacuum by using 3ns Nd:YAG laser radiation at high pulse energy is reported. Nanosecond pulsed ablation, at an intensity on the order of 109W∕cm2, produces high nonisotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold, ablation yields, and deposition rates of thin films on silicon substrates. The multicomponent structure of the plume emission is rationalized in terms of charge state, ion and neutral equivalent temperatures, and plasma density. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the nonequilibrium plasma conditions. The angular distributions of the neutral and ion species are also presented and discussed.

Carbon-plasma produced in vacuum by 532 nm–3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm 2, produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.

Polymer-Like C:H Thin Film Coating of Nanopowders in Capacitively Coupled RF Discharge

Nanopowders of amorphous SiO2, with typical particle sizes of 30–80 nm, were treated under the non-equilibrium plasma conditions, created by a capacitively coupled (CC) RF discharge formed in pure methane or ethane. The plasma gas flow rate varied between 0.02 and 0.06 slpm, with reactor pressures between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the rotational temperature, as derived from the C2 5160 A Swan band and N2 second positive 3577 A band, and the atomic hydrogen excitation temperature, from the Hβ Hγ and Hδ lines during the powder treatment process. The compositions of the gases that passed through the plasma were analyzed by mass spectrometry. In spite of the evidence for the presence of CnH2n+2 and CnH2n (n=1–3) species and acetylene in the discharge, the homogeneous formation of soot was not observed. At the same time, the introduced nanoparticles were observed to act as centers for the inception and growth of C:H thin coatings in the form of a polymer-like hydrocarbon layers, of thickness between <5 and 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analysis performed on the treated powders provide evidence to the formation of an amorphous, high density C:H matrix on the particles' surfaces.

Filamentary, patterned, and diffuse barrier discharges

Barrier discharges, also known as dielectric-barrier discharges or silent discharges, provide a simple technology to establish nonequilibrium plasma conditions in atmospheric-pressure gases. This property has led to a number of industrial applications, including ozone generation, surface modification, pollution control, CO/sub 2/ lasers, excimer lamps, and flat plasma-display panels. Depending on a variety of gas properties, operating parameters, and boundary conditions, the discharge can exhibit pronounced filamentary character, self-organized regular-discharge patterns, or completely diffuse appearance. The literature on these different types of barrier discharges is reviewed, and the underlying physical phenomena are discussed. Relative recent investigations on low-current density diffuse barrier discharges suggest novel applications of fairly "mild" plasmas for sterilization and disinfection purposes and utilizing their selective influence on biological cells.