Showing papers in "Plasma Sources Science and Technology in 2010"

Global model of low-temperature atmospheric-pressure He + H2O plasmas

Abstract: A detailed global model of atmospheric-pressure He + H2O plasmas is presented in this paper. The model incorporates 46 species and 577 reactions. Based on simulation results obtained with this comprehensive model, the main species and reactions are identified, and simplified models capable of capturing the main physicochemical processes in He + H2O discharges are suggested. The accuracy of the simplified models is quantified and assessed for changes in water concentration, input power and electrode configuration. Simplified models can reduce the number of reactions by a factor of similar to 10 while providing results that are within a factor of two of the detailed model. The simulation results indicate that Penning processes are the main ionization mechanism in this kind of discharge (1-3000 ppm of water), and water clusters of growing size are found to be the dominant charged species when the water concentration is above similar to 100 ppm. Simulation results also predict a growing electronegative character of the discharge with increasing water concentration. The use of He + H2O discharges for the generation of reactive oxygen species of interest in biomedical applications and the green production of hydrogen peroxide are also discussed. Although it would be unrealistic to draw conclusions regarding the efficacy of these processes from a zero-dimensional global model, the results indicate the potential suitability of He + H2O plasmas for these two applications.

On OH production in water containing atmospheric pressure plasmas

Abstract: In this paper radical production in atmospheric pressure water containing plasmas is discussed. As OH is often an important radical in these discharges the paper focuses on OH production.Besides nanosecond pulsed coronas and diffusive glow discharges, several other atmospheric pressure plasmas which are of interest nowadays have a typical electron temperature in the range 1–2 eV and an ionization degree of 10−5–10−4. These properties are quite different from the typical plasma properties known from low pressure gas discharges.In the plasma physics literature OH production is primarily ascribed to be due to electron, metastable induced or thermal dissociation of water, processes which are dominant in (low pressure) gas discharges and in combustion and hot flames. It is shown in this paper that for several atmospheric pressure plasmas also dissociative recombination can be an effective method of OH radical production. Several examples are presented in detail.This paper provides a basic framework for OH production in atmospheric pressure plasmas and shows that accurate knowledge of ne, Te, Tg, the dominant ionic species, radical and neutral species are indispensable to obtain a complete view on the chemical kinetics in these challenging complex atmospheric pressure plasmas. A few relevant plasma diagnostics together with their limitations are also briefly discussed in this context.

Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet

Abstract: Atomic oxygen formation in a radio-frequency driven micro-atmospheric pressure plasma jet is investigated using both advanced optical diagnostics and numerical simulations of the dynamic plasma chemistry. Laser spectroscopic measurements of absolute densities of ground state atomic oxygen reveal steep gradients at the interface between the plasma core and the effluent region. Spatial profiles resolving the interelectrode gap within the core plasma indicate that volume processes dominate over surface reactions. Details of the production and destruction processes are investigated in numerical simulations benchmarked by phase-resolved optical emission spectroscopy. The main production mechanisms are electron induced and hence most efficient in the vicinity of the plasma boundary sheath, where electrons are energized. The destruction is driven through chemical heavy particle reactions. The resulting spatial profile of atomic oxygen is relatively flat. The power dependence of the atomic oxygen density obtained by the numerical simulation is in very good agreement with the laser spectroscopic measurements.

Conversion of carbon dioxide to value-added chemicals in atmospheric pressure dielectric barrier discharges

Abstract: The aim of this work consists of the evaluation of atmospheric pressure dielectric barrier discharges for the conversion of greenhouse gases into useful compounds. Therefore, pure CO2 feed flows are administered to the discharge zone at varying discharge frequency, power input, gas temperature and feed flow rates, aiming at the formation of CO and O2. The discharge obtained in CO2 is characterized as a filamentary mode with a microdischarge zone in each half cycle of the applied voltage. It is shown that the most important parameter affecting the CO2-conversion levels is the gas flow rate. At low flow rates, both the conversion and the CO-yield are significantly higher. In addition, also an increase in the gas temperature and the power input give rise to higher conversion levels, although the effect on the CO-yield is limited. The optimum discharge frequency depends on the power input level and it cannot be unambiguously stated that higher frequencies give rise to increased conversion levels. A maximum CO2 conversion of 30% is achieved at a flow rate of 0.05Lmin −1 , a power density of 14.75Wcm −3 and a frequency of 60kHz. The most energy efficient conversions are achieved at a flow rate of 0.2Lmin −1 , a power density of 11Wcm −3 and a discharge frequency of 30kHz. (Some figures in this article are in colour only in the electronic version)

Continuous-flow, atmospheric-pressure microplasmas: a versatile source for metal nanoparticle synthesis in the gas or liquid phase

Abstract: Continuous-flow, atmospheric-pressure microplamas are a unique class of plasmas that are highly suitable for emerging nanomaterials applications. Here, we present two schemes for the preparation of metal nanoparticles based on these plasma sources. Nanoparticles are synthesized in the gas phase by non-thermal dissociation of vapor precursors in a microplasma reactor. Monometallic Ni and Fe nanoparticles, as well as compositionally controlled NiFe bimetallic nanoparticles, can be grown with tunable mean diameters between 1 and 5 nm and narrow size distributions. Alternatively, colloidal metal nanoparticles are produced directly in aqueous solutions. Metal cations generated from anodic dissolution of a bulk metal or present in the form of metal salt are reduced by the microplasma to form nanoparticles and capped by a stabilizer. Both approaches are low cost, scalable and general and should allow a wide range of nanoparticle materials to be synthesized in the gas or liquid phase.

Formation and dynamics of plasma bullets in a non-thermal plasma jet: influence of the high-voltage parameters on the plume characteristics

Abstract: Non-thermal plasma jets in open air are composed of ionization waves commonly known as 'plasma bullets' propagating at high velocities. We present in this paper an experimental study of plasma bullets produced in a dielectric barrier discharge linear-field reactor fed with helium and driven by microsecond high-voltage pulses. Two discharges were produced between electrodes for every pulse (at the rising and falling edge), but only one bullet was generated. Fast intensified charge coupled device camera imaging showed that bullet velocity and diameter increase with applied voltage. Spatially resolved optical emission spectroscopy measurements provided evidence of the hollow structure of the jet and its contraction. It was shown that the pulse amplitude significantly enhances electron energy and production of active species. The plasma bullet appeared to behave like a surface discharge in the tube, and like a positive streamer in air. A kinetics mechanism based on electron impact, Penning effect and charge transfer reactions is proposed to explain the propagation of the ionization front and temporal behavior of the radiative species.

Electronic quenching of OH(A) by water in atmospheric pressure plasmas and its influence on the gas temperature determination by OH(A-X) emission

Abstract: In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A?X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He?water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000?ppm H2O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A?X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J ? 7) are thermalized and representative for the gas temperature.

Short- and long-term plasma phenomena in a HiPIMS discharge

Abstract: Using a time-resolved Langmuir probe the temporal evolution of the bulk plasma parameters in a high-power impulse magnetron sputtering (HiPIMS) discharge was investigated for a number of different discharge conditions. The magnetron was operated in argon between 0.5 and 1.6 Pa with a titanium target and with peak target power densities up to 1000 W cm−2. The pulse width and repetition rate were held constant at 100 µs and 100 Hz, respectively. Using an OML analysis as well as a Druyvesteyn formulation, the electron densities, effective temperatures and energy distribution functions were obtained throughout the pulse period (0–9 ms), including a detailed study of the first 10 µs, which was achieved with a temporal resolution better than 0.5 µs. In the initial phase of the voltage pulse (t ~ 1–4 µs), three distinct groups of electrons (indistinguishable from Maxwellian electrons) were observed, namely 'super-thermal', 'hot' and 'cold' populations with effective temperatures of 70–100 eV, 5–7 eV and 0.8–1 eV, respectively. After 4 µs these groups become energetically indistinguishable from each other to form a single distribution with an electron temperature that decays from about 5 to 3 eV during the rest of the pulse on-time. The presence of the 'super-thermal' electron group pushes the probe floating potential to a very negative value (significantly deeper than −95 V) during the initial period of the pulse. In the off-time, the electron density decays with two-fold characteristic times, revealing initially short-term (30–40 µs) and ultimately long-term (3–4 ms) decay rates. These long decay times lead to a relative high density remnant plasma (2 × 109 cm−3) at the end of the off-time, which serves to seed the next voltage pulse. The electron temperature and plasma potential also exhibit two-fold decay in the off-time, but with typically somewhat faster decays, particularly for the long-term decay (100–500 µs) up to the end of the off-time. The time evolution of the plasma potential shows that for a considerable fraction of the on-time the plasma potential remains negative (down to −12 V) only becoming positive after t ~ 60 µs which corresponds to a time of maximum plasma density (typical values of 2 × 1012 cm−3). The generation of super-thermal electrons in the initial phase of the discharge is argued through the development of a simple magnetized-electron bounce model of the expanding sheath.

The evolution of the plasma potential in a HiPIMS discharge and its relationship to deposition rate

Abstract: An electron-emitting probe has been used to measure the temporal evolution of the plasma potential Vp along a line from target (Ti) to substrate above the racetrack in a high-power impulse magnetron sputtering discharge pulsed at 100?Hz. The 20?ns time-resolution of the probe allowed us to observe the highly dynamic nature of Vp as the discharge voltage waveform develops, with large negative Vp values (?210?V) and strong potential gradients existing in the magnetic trap region in the first 6 to 8??s. After 55 to 60??s, Vp is elevated towards ground potential (0?V) and the bulk electric field collapses. Outside the magnetic trap, i.e. on the open field lines, Vp reveals much smaller axial and temporal variations, similar to those observed in conventional pulsed dc magnetrons.At standard conditions (Ar pressure of 0.54?Pa and 650?W average power), the results show that for over 50% of the 100??s plasma 'on-time' the spatial structure of Vp provides a large potential barrier for the sputtered post-ionized species so impeding their transport and deposition at the substrate. This barrier is reduced markedly (by 50%) through a small reduction in the magnetic field strength (33% at the target) so increasing the deposition rate by a factor of 6 at a typical position of the substrate (z = 100?mm). The structure of Vp is marginally sensitive to changes in pressure (over the range 0.54 to 1.08?Pa), but more strongly dependent on the applied power (over the range 650 to 950?W).

Spatially extended atmospheric plasma arrays

Abstract: This paper reports a systematic study of spatially extended atmospheric plasma (SEAP) arrays employing many parallel plasma jets packed densely and arranged in an honeycomb configuration. The work is motivated by the challenge of using inherently small atmospheric plasmas to address many large-scale processing applications including plasma medicine. The first part of the study considers a capillary–ring electrode configuration as the elemental jet with which to construct a 2D SEAP array. It is shown that its plasma dynamics is characterized by strong interaction between two plasmas initially generated near the two electrodes. Its plume length increases considerably when the plasma evolves into a high-current continuous mode from the usual bullet mode. Its electron density is estimated to be at the order of 3.7 × 1012 cm−3. The second part of the study considers 2D SEAP arrays constructed from parallelization of identical capillary–ring plasma jets with very high jet density of 0.47–0.6. Strong jet–jet interactions of a 7-jet 2D array are found to depend on the excitation frequency, and are effectively mitigated with the jet-array structure that acts as an effective ballast. The impact range of the reaction chemistry of the array exceeds considerably the cross-sectional dimension of the array itself, and the physical reach of reactive species generated by any single jet exceeds significantly the jet–jet distance. As a result, the jet array can treat a large sample surface without relative sample–array movement. A 37-channel SEAP array is used to indicate the scalability with an impact range of up to 48.6 mm in diameter, a step change in capability from previously reported SEAP arrays. 2D SEAP arrays represent one of few current options as large-scale low-temperature atmospheric plasma technologies with distinct capability of directed delivery of reactive species and effective control of the jet–jet and jet–sample interactions.

Low pressure hydrogen discharges diluted with argon explored using a global model

Abstract: A steady state global (volume averaged) model is developed for a low pressure (1–100 mTorr) high density hydrogen discharge that is diluted with argon. The electron density increases, the dissociation fraction of hydrogen increases and the electron temperature decreases with increased argon dilution. We find that is the dominant positive ion up to roughly 30% argon dilution at 10 mTorr, at which point Ar+ becomes the dominant positive ion. The reaction rates for the creation and destruction of various species are explored versus the discharge pressure. In particular we explore the role of the vibrationally excited levels of the hydrogen molecule in the creation of the negative ion H− through dissociative attachment. The role of the ArH+ ion in the discharge chemistry is discussed and we find that ArH+ plays a significant role in the destruction of the H− ion. Furthermore, the creation and destruction of and ArH+ ions are explored. The electronegativity increases with increasing H2 content and reaches a value of approximately unity in a pure H2 discharge at 100 mTorr. The model is compared with measurements found in the literature and is found to be in agreement with measurements although certain discrepancies are pointed out and discussed.

Plasma techniques for nanostructured carbon materials synthesis. A case study: carbon nanowall growth by low pressure expanding RF plasma

Abstract: A short description of approaches for carbon nanostructures synthesis is made and the advantages of using plasma during the growth are presented. As a particular example of a plasma based technique we detail the process of downstream carbon nanowall (CNW) synthesis by a radiofrequency expanding plasma beam. The technique combines magnetron sputtering for catalyst deposition and plasma enhanced chemical vapor deposition (main gas: argon, active gas: hydrogen, precursor gas: acetylene) for carbon growth in a single reactor. The analysis focuses on the correlation between the material properties and the plasma characteristics measured at different points along the flow axis, aiming to reveal the importance of plasma species in the growth process. The material properties were investigated by scanning and transmission electron microscopy, whereas the plasma data were obtained by optical emission spectroscopy, Langmuir probes and mass spectrometry. CNWs with a large area and well isolated from each other are obtained at an optimum distance from the precursor injection point where the plasma presents an enhanced content of carbon nanoclusters. The possible processes responsible for the growth are discussed.

Dual-frequency capacitive radiofrequency discharges: effect of low-frequency power on electron density and ion flux

Abstract: The dependence of electron density and ion flux on radiofrequency (RF) power has been measured in a 2 + 27 MHz dual-frequency capacitive discharge with silicon electrodes at 6.7 Pa gas pressure. In Ar/O2 mixtures the electron density and the ion flux vary in a very similar way (i.e. their ratio, υ, is constant), in good agreement with the simple electropositive transport theory. Both 27 and 2 MHz RF powers have a significant effect on the plasma density and the ion flux. The effect of the 2 MHz power is likely a combination of enhanced plasma heating by dual-frequency excitation and ionization caused by secondary electron beams, which are known to be produced efficiently at oxidized silicon surfaces. In contrast, in Ar/C4F8/O2 mixtures such as those used for industrial dielectric etching, υ is always bigger than the theoretical electropositive value, and becomes very high when the ratio of 2 to 27 MHz power is high. Under these conditions the electron density is very small, whereas the ion flux remains considerable. We attribute the increased plasma transport to the presence of a significant density of F− negative ions, combined with increased penetration of the 2 MHz electric field into the plasma bulk at high 2/27 MHz power ratios.

Optical emission measurements of electron energy distributions in low-pressure argon inductively coupled plasmas

Abstract: Optical modeling of emissions from low-temperature plasmas provides a non-invasive technique to measure the electron energy distribution function (EEDF) of the plasma. While many models assume the EEDF has a Maxwell?Boltzmann distribution, the EEDFs of numerous plasma systems deviate significantly from the Maxwellian form. In this paper, we present an optical emission model for the Ar(3p54p ? 3p54s) emission array which is capable of capturing details of non-Maxwellian distributions. Our model combines previously measured electron-impact excitation cross sections with Ar(3p54s) number density measurements and emission spectra. The model also includes corrections for radiation trapping of the Ar(3p54p ? 3p54s) emission lines. Results obtained with this optical technique are compared with corresponding Langmuir probe measurements of the EEDF for Ar and Ar/N2 inductively coupled plasma systems operating under a wide variety of source conditions (1?25?mTorr, 20?1000?W, %N2 admixture). Both the optical emission method and probe measurements indicate the EEDF shapes are Maxwellian for low electron energies, but with depleted high energy tails.

A global (volume averaged) model of a chlorine discharge

Abstract: A steady state global (volume averaged) model is developed for the chlorine discharge using a revised reaction set. Various calculated plasma parameters are compared with measurements found in the literature, showing a good overall agreement. The reaction rates for the various reactions are evaluated in the pressure range 1–100 mTorr. In particular, we explore the dissociation process as well as the creation and destruction of the negative ions Cl−. The discharge is highly dissociated throughout the pressure range explored, 1–100 mTorr, even when the absorbed power is low. The mechanism for Cl creation is complex. Although electron impact dissociation dominates with roughly 60–65% contribution, mutual neutralization of positive and negative ions and dissociative electron attachment are important contributors to the production of Cl atoms at high pressure. The electronegativity increases rapidly with decreasing dissociation fraction since the Cl− ions are created entirely by dissociative electron attachment, predominantly from Cl2(v = 0), but also up to 14% from Cl2(v > 0) at 100 mTorr. The negative ion Cl− is lost almost entirely through mutual neutralization with at high pressure while Cl+ has a significant contribution at low pressure.

Studies on scalability and scaling laws for the plasma focus: similarities and differences in devices from 1 MJ to 0.1 J

Abstract: A comprehensive analysis of scaling laws for plasma focus devices producing neutrons is presented. Similarities and differences in plasma focus devices working with stored energies ranging from 1 MJ to 0.1 J are found. First, a brief review listing the most important results achieved by the Thermonuclear Plasma Department of the Chilean Nuclear Energy Commission, CCHEN, is presented. The aim of the work at CCHEN has been to characterize the physics of dense plasma foci and also to carry out the design and construction of smaller devices—in terms of both input energy and size—capable of providing dense hot plasmas. Certain scaling rules have been found from this research. These rules combined with other scaling laws have been applied to design and construct plasma focus devices with storage energy in a region never explored before (tens of joules and less than 1 J). Thus, a comprehensive analysis also including results from other groups is presented. In particular, all the devices, from the largest to the smallest, maintain the same value of ion density, magnetic field, plasma sheath velocity, Alfven speed and the quantity of energy per particle. Therefore, fusion reactions are even possible to obtain in ultraminiature devices (driven by generators of 0.1 J for example), as they are in the larger devices (driven by generators of 1 MJ). However, the stability of the plasma pinch highly depends on the size and energy of the device.

A non-equilibrium plasma source: magnetically stabilized gliding arc discharge: I. Design and diagnostics

Abstract: Previous studies of the gliding arc (GA) plasma have shown that the highest degree of non-equilibrium is obtained at the maximum length of elongation of the discharge. In order to better understand the properties of the discharge in this mode of operation, a system was developed wherein the GA is stabilized by a magnetic field. The GA is driven as a single unit in the direction governed by the Lorentz force. The discharge reactor is designed in such a way that the plasma moves continuously without further elongation or extinction, while electrical parameters can be carefully controlled. This paper presents the experimentally measured effects of plasma current on (a) the motion of plasma, (b) the presence of an 'overshooting' regime, (c) voltage characteristics and (d) rotational and vibrational temperatures, when operating in air at atmospheric pressure.

Implicit and electrostatic particle-in-cell/Monte Carlo model in two-dimensional and axisymmetric geometry: I. Analysis of numerical techniques

Abstract: We developed an implicit particle-in-cell/Monte Carlo model in two-dimensional and axisymmetric geometry for simulations of radio-frequency discharges, by introducing several numerical schemes which include variable weights and a multigrid field solver Compared with the standard explicit models, we found that the computational efficiency is significantly increased and the accuracy is maintained Numerical schemes are discussed and benchmark results are presented The code can be used to simulate practical reactors

Spectroscopic study of an atmospheric pressure dc glow discharge with a water electrode in atomic and molecular gases

Abstract: The positive column of dc excited atmospheric pressure glow discharges in a metal pin–water electrode system is investigated in air, N2, He, Ar, N2O, CO2 and He–N2 mixtures. The electric field and the plasma temperatures in the positive column are measured and the effect of the filling gas on the optical emission is examined. Estimates of the electron temperature and density and the observed emission properties indicate that the formation processes of the excited dissociation fragments of H2O are due to recombination processes rather than direct electron excitation. Deviation from a Boltzmann rotational population distribution of OH(A) is observed in all gases due to the formation process of OH(A). In the case of He the non-Boltzmann behaviour is more pronounced which can be caused by the lower gas temperature as a higher gas temperature leads to more thermalization of the rotational population distribution of OH(A). The results in this paper suggest that vibrational energy transfer contributes to the non-Boltzmann distribution of OH(A). The temperature corresponding to the small rotational numbers of OH(A) can be used as an estimate of the gas temperature.

Modeling of dual frequency capacitively coupled plasma sources utilizing a full-wave Maxwell solver: I. Scaling with high frequency

Abstract: Dual frequency capacitively coupled plasma (DF-CCP) tools for etching and deposition for microelectronics fabrication typically use a high frequency (HF, tens to hundreds of MHz) to sustain the plasma and a low frequency (LF, a few to 10MHz) for ion acceleration into the wafer. With an increase in both the HF and wafer size, electromagnetic wave effects (i.e. propagation, constructive and destructive interference) can affect the spatial distribution of power deposition and reactive fluxes to the wafer. In this paper, results from a two-dimensional computational investigation of a DF-CCP reactor, incorporating a full-wave solution of Maxwell’s equations, are discussed. As in single frequency CCPs, the electron density transitions from edge high to center high with increasing HF. This transition is analyzed by correlating the spatial variation of the phase, magnitude and wavelength of the HF electric field to the spatial variation of the electron energy distributions and ionization sources. This transition is sensitive to the gas mixture, particularly those containing electronegative gases due to the accompany change in conductivity. The consequences of these wave effects on the ion energy distributions incident onto the wafer are also discussed. (Some figures in this article are in colour only in the electronic version)

A phenomenological equilibrium model applicable to high-power pulsed magnetron sputtering

Abstract: We present a phenomenological equilibrium model applicable to high-power pulsed dc magnetron sputtering with relatively long steady-state discharge regimes established during pulses. The model makes it possible to calculate the fraction of ionized sputtered atoms directed back to the target, σ, the degree of ionization of sputtered atoms in front of the target, β, the normalized rate coefficient, α (determining the deposition rate of films per target power density), and the ionized fraction of target material atoms in the flux onto the substrate, Θ, as functions of the magnetron voltage, Ud, and the fraction of target material ions in the total ion flux onto the target, mt (being related to an applied target power density). We used this model to clarify the large differences between the corresponding deposition characteristics (such as the deposition rate of films per average target power density in a period and the ionized fraction of target material atoms in the flux onto the substrate) of copper and titanium measured by us during high-power pulsed dc magnetron sputtering of the two materials. We investigated the effects of higher losses of the target material ions to the chamber walls and of reduced additional ionization in the plasma bulk in the magnetron system with a weaker magnetic confinement. For total self-sputtering of copper, when mt = 1, we obtained values of σ in the range from 0.54 at Ud = 600 V to 0.32 at Ud = 1000 V, in excellent agreement with the recent measurements of Andersson and Anders.

Electron beam treatment of non-conducting materials by a fore-pump-pressure plasma-cathode electron beam source

Abstract: In the irradiation of an insulated target by an electron beam produced by a plasma-cathode electron beam source operating in the fore-vacuum pressure range (5‐15 Pa), the target potential is much lower than the electron beam energy, offering the possibility of direct electron treatment of insulating materials. It is found that in the electron beam irradiation of a non-conducting target in a moderately high pressure range, the electron charge on the target surface is neutralized mainly by ions from a volume discharge established between the negatively charged target surface and the grounded walls of the vacuum chamber. This allows the possibility of direct electron beam treatment (heating, melting, welding) of ceramics and other non-conducting and semiconductor materials.

Non-thermal atmospheric pressure HF plasma source: generation of nitric oxide and ozone for bio-medical applications

Abstract: A new miniature high-frequency (HF) plasma source intended for bio-medical applications is studied using nitrogen/oxygen mixture at atmospheric pressure. This plasma source can be used as an element of a plasma source array for applications in dermatology and surgery. Nitric oxide and ozone which are produced in this plasma source are well-known agents for proliferation of the cells, inhalation therapy for newborn infants, disinfection of wounds and blood ozonation.Using optical emission spectroscopy, microphotography and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined for varied nitrogen/oxygen flows. The influence of the gas flows on the plasma conditions is studied. Ozone and nitric oxide concentrations in the effluent of the plasma source are measured using absorption spectroscopy and electro-chemical NO-detector at variable gas flows. Correlations between plasma parameters and concentrations of the particles in the effluent of the plasma source are discussed. By varying the gas flows, the HF plasma source can be optimized for nitric oxide or ozone production. Maximum concentrations of 2750 ppm and 400 ppm of NO and O3, correspondingly, are generated.

The challenge of revealing and tailoring the dynamics of radio-frequency plasmas

Abstract: The complex dynamics of ionization and excitation mechanisms in capacitively coupled radio-frequency plasmas is discussed for single- and dual-frequency operations in low-pressure and atmospheric pressure plasmas. Electrons are energized through the dynamics of electric fields in the vicinity of the plasma boundary sheaths. Distinctly different power dissipation mechanisms can either co-exist or initiate mode transitions exhibiting characteristic spatio-temporal ionization structures. Phase resolved optical emission spectroscopy, in combination with adequate modelling of the population dynamics of excited states, and numerical simulations reveal dissipation associated with sheath expansion, sheath collapse, transient electron avalanches and wave–particle interactions. In dual-frequency systems the relative phase between the two frequency components provides additional strategies to tailor the plasma dynamics.

Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

Abstract: A complex chemistry in Titan’s atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N2–CH4 mixtures (from 0% to 10% of CH4) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 10 15 m −3 in pure N2 plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH4 is present in the discharge, even for a proportion as small as 2% of CH4. An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N2 bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH4 in the gas mixture. (Some figures in this article are in colour only in the electronic version)

Influence of the pre-ionization background and simulation of the optical emission of a streamer discharge in preheated air at atmospheric pressure between two point electrodes

Abstract: This paper presents simulations of positive and negative streamers propagating between two point electrodes in preheated air at atmospheric pressure. As many discharges have occurred before the simulated one, seed charges are taken into account in the interelectrode gap. First, for a pre-ionization background of 109?cm?3, we have studied the influence of the data set used for transport parameters and reaction rates for air on the simulation results. We have compared results obtained in 1997 using input parameters from Morrow and Lowke and from Kulikovsky. Deviations as large as 20% of streamer characteristics (i.e. electric field in the streamer head and body, streamer velocity, streamer radius, streamer electron density) have been observed for this point-to-point configuration. Second, we have studied the influence of the pulsed voltage frequency on the discharge structure. For the studied discharge regime, a change in the applied voltage frequency corresponds to a change in the pre-ionization background. In this work, we have considered a wide range of pre-ionization values from 104 and up to 109?cm?3. We have noted that the value of the pre-ionization background has a small influence on the electron density, electric field and location of the negative streamer head. Conversely, it has a significant influence on the positive streamer characteristics. Finally, we have compared instantaneous and time-averaged optical emissions of the three band systems of N2 and (1PN2, 2PN2 and ) during the discharge propagation. We have shown that the emission of the 2PN2 is the strongest of the three bands, in agreement with experimental observations. It is interesting to note that even with a short time averaging of a few nanoseconds, which corresponds to currently used instruments, the structure of the time-averaged emission of the 2PN2 is different from the instantaneous one and shows negative and positive streamers with smaller radial expansions and more diffuse streamer heads.

The low pressure Cl2/O2 discharge and the role of ClO

Abstract: A global (volume averaged) model is applied to a low pressure (1?100?mTorr) high density chlorine discharge that is diluted with oxygen. The influence of oxygen dilution on the particle densities and the electron temperature is explored. The electronegativity is found to increase strongly with increased pressure in a chlorine-rich mixture, whereas it is nearly pressure independent in an oxygen-rich mixture. The chlorine dissociation fraction increases with increased oxygen dilution, although the increase is neither pronounced nor sharp at low oxygen content. We explore the role of the ClO molecule in the discharge and confirm that the ClO molecule is mainly created through recombination of Cl and O atoms at the chamber wall, which in turn significantly increases the loss of Cl atoms in oxygen-rich mixtures. The most important loss process for ClO is electron impact dissociation in the plasma bulk. The molecular ion ClO+ is almost entirely created by charge transfer for oxygen dilution below 67%, while electron impact ionization becomes the dominant creation process for ClO+ at higher dilution.

Mixing rules for thermal plasma properties in mixtures of argon, air and metallic vapours

Abstract: Modelling of electric arcs and thermal plasmas in mixtures of gases and vapours needs prior knowledge of rather large data banks corresponding to thermodynamic functions, transport coefficients and radiation properties. For a given pressure these data are functions of temperature and gas proportions in the mixture. In order to reduce the memory or because some properties of the mixtures are not known, some mixing laws can be useful. These mixing rules allow estimation of the properties of the mixtures when only the corresponding properties of the pure gases or vapours are known. In this paper we study several mixing rules for mixtures of argon or air with metallic vapours. Simple laws such as linear interpolations are compared with relations deduced from physical consideration and some general behaviour is given in conclusion: mixing rules for electrical conductivity, viscosity and net emission coefficient are good. For other properties the use of a mixing law may produce rather large errors.

Efficient production of microwave bubble plasma in water for plasma processing in liquid

Abstract: Microwave bubble plasma in water is a novel plasma applicable to the processing of materials in liquid. An electromagnetic simulation of slot excitation of microwaves reveals that the electric field at a slot antenna is significantly influenced by the size of the bubble existing in front of the antenna. To improve the power efficiency and the plasma stability, a bubble control plate is installed adjacent to the antenna, the effect of which on the electric field enhancement is confirmed in the simulation. Furthermore, three slot antennas are newly developed. According to these modifications of the microwave excitation system, a dramatic increase in the decomposition efficiency of an organic solute by a factor of 20 is found in the experiment.

Effect of humidity on gas temperature in the afterglow of pulsed positive corona discharge

Abstract: The effects of humidity on gas temperature in the afterglow of a pulsed positive corona discharge are studied. The gas temperature is measured using the laser-induced fluorescence (LIF) of NO molecules. The discharge occurs in a 13 mm point-to-plane gap under atmospheric pressure. When the water vapor concentration in air is increased from 0.5% to 2.4%, the temperature increases from 550 to 850 K near the anode tip, and from 350 to 650 K at a position 2.5 mm from the anode tip. The gas heating in the humid environment is due to the fast vibration-to-vibration processes of the O2–H2O and N2–H2O systems and the extremely rapid vibration-to-translation process of the H2O–H2O system. These processes accelerate the transfer of energy from O2(v) and N2(v) to translational energy. Measurements of the LIF of O2(v = 6) show that the decay rate of O2(v) density is increased by humidification.