Plasma interactions with living matter are presently at the frontiers of plasma research and deve... more Plasma interactions with living matter are presently at the frontiers of plasma research and development. Plasmas contain numerous agents that influence biological activity. They provide essentially two types of biocidal species: reactive species, such as oxygen atoms that lead to lethality of micro-organisms through erosion, and UV radiation that can damage the DNA strands. In this work we investigate a surface wave (2.45 GHz) driven discharge plasma in air, with a small admixture of water vapor, as a source of ground state O(^3P) oxygen atoms, NO molecules and UV radiation. A theoretical model describing both the wave driven discharge zone and its flowing afterglow is used to analyze the performance of this plasma source. The predicted plasma-generated NO(X) and O(^3P) concentrations and NO(γ) radiation intensity along the source are presented and discussed as a function of the microwave power and water vapor percentage in the gas mixture. To validate the theoretical predictions, ...
Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achie... more Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron ...
ABSTRACT A microwave plasma torch operating at 2.45 GHz and atmospheric pressure has been used as... more ABSTRACT A microwave plasma torch operating at 2.45 GHz and atmospheric pressure has been used as a medium and a tool for decomposition of alcohol in order to produce molecular hydrogen. Plasma in a gas mixture of argon and ethanol/methanol, with or without water, has been created using a waveguide surfatron launcher and a microwave generator delivering a power in the range 0.2-2.0 kW. Mass, Fourier Transform Infrared, and optical emission spectrometry have been applied as diagnostic tools. The decomposition yield of methanol was nearly 100 % with H2, CO, CO2, H2O, and solid carbon as the main reaction products. The influence of the fraction of Ar flow through the liquid ethanol/methanol on H2, CO, and CO2 partial pressures has been investigated, as well as the dependence of the produced H2 flow on the total flow and power. The optical emission spectrum in the range 250–700 nm has also been detected. There is a decrease of the OH(A-X) band intensity with the increase of methanol in the mixture. The emission of carbon atoms in the near UV range (240–300 nm) exhibits a significant increase as the amount of alcohol in the mixture grows. The obtained results clearly show that this microwave plasma torch at atmospheric pressure provides an efficient plasma environment for hydrogen production.
We present a review of free electron and field diagnostics in gas discharges, giving a special em... more We present a review of free electron and field diagnostics in gas discharges, giving a special emphasis to spatially resolved measurements. Probes and antennas are the kernel of this work and we mention refinements concerning their use as well as common sources of error.
The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot ant... more The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot antenna excited microwave plasma source operating in Ar and N2–Ar mixtures at 2.45 GHz has been investigated. The discharge source is sustained by the electric field of pure TM surface modes. Optical emission spectroscopy, probe diagnostic techniques and radiophysics methods are applied to measure the emission intensity of radiative states, the electron density and the microwave field intensity, respectively. The spatial distribution of the electron density and the intensity of emission lines belonging to the 2nd positive and 1st negative systems of N2 follow the electric field pattern in the active-discharge zone of the plasma source. On the contrary, the populations in vibrational and rotational levels of the N2(C 3 � u) state are homogeneous irrespective of the azimuthal position.
Microwave H 2 , N 2 , and N 2-Ar discharges driven by traveling surface waves are investigated as... more Microwave H 2 , N 2 , and N 2-Ar discharges driven by traveling surface waves are investigated as sources of ground state N(4 S), H(1s) atoms. The dissociation kinetics is discussed in the framework of theoretical models based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma-wall interactions. It is shown that the number density of hydrogen H(1s) atoms depends heavily on the wall conditions at low pressure conditions. The kinetics of N(4 S) and metastable N(2 D) and N(2 P) atoms and of the molecular N 2 (A 3 6 +) metastable state in a pure N 2 discharge are shown to be strongly coupled. One possible way to control and to enhance nitrogen dissociation is the use of an N 2-Ar mixture. The increase in dissociation degree of N 2 molecules at high Ar fractional concentration can be attributed to dissociative recombination between electrons and N + 2 positive ions. The predicted results for the atomic density are compared with emission spectroscopy data.
View the article online for updates and enhancements. You may also like Continuum double probe th... more View the article online for updates and enhancements. You may also like Continuum double probe theory: effects of electron emission due to metastable species de-excitation and unequal probe arm saturation currents K Keskinen and R M Hobson-The display of logarithmic Langmuir probe characteristics on a cathode-ray oscillograph A R Johnston-Plasma probe characteristics in low density hydrogen pulsed plasmas D I Astakhov, W J Goedheer, C J Lee et al.
The Balmer line spectra emitted by different types of DC glow and RF discharges have typical mult... more The Balmer line spectra emitted by different types of DC glow and RF discharges have typical multimode behaviour, with widely broadened “wings” (“fast” hydrogen) and a sharp top (“slow” hydrogen), while the spectra emitted by microwave discharges usually exhibit single mode behaviour. As is well known, these discharges are free from strong DC fields. This report presents spectroscopic measurements of Balmer lines emitted by a microwave H2 plasma at low pressures (p = 0.01 – 0.2 mbar). A classical surface wave sustained discharge has been used as a plasma source [1]. The microwave power is provided by a 500 MHz generator (P = 40 – 250 W). The discharge takes place inside a Pyrex tube with internal and external radii of 2.25 and 2.5 cm. Under the present conditions, the electron density decreases from about (1 – 1.5) × 10 10 cm -3 at the beginning of the plasma column to about (2 – 3) × Ncr (Ncr ≈ 7 × 10 9 cm -3 ) at the end. The effective value of the microwave electric field intensi...
Langmuir probe diagnostics in tokamaks are very challenging, namely due to plasma fluctuations wh... more Langmuir probe diagnostics in tokamaks are very challenging, namely due to plasma fluctuations whose characteristic period is not much shorter than probe scanning-time. As a result, plasma parameters evaluation is usually constrained by the isotropic, Maxwellian electron energy distribution function (EEDF) assumption, probe diagnostics have been considered unreliable, EEDFs are not usually measured, and a huge amount of probe data has never been processed due to the lack of an appropriate numerical tool. While results from all diagnostics are not cross-linked in order to gather complete information on how EEDFs change in time and space, a particular diagnostic cannot do much more than looking for “averaged” values. Yet, it is imperative that each diagnostic does not lose the precious time-dependence so that a complete, overall picture is achievable whenever possible. In this paper we present results of a numerical procedure developed to minimize the effect of the above plasma fluctu...
In the present work, a surface-wave driven microwave plasma at atmospheric pressure conditions ha... more In the present work, a surface-wave driven microwave plasma at atmospheric pressure conditions has been applied for synthesis of carbon nanostructures. Three different types of nanostructures have been selectively synthesized by controlling the temperature in the assembling zone of the plasma reactor. The method is based on introduction of ethanol vapors through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. Gas-phase carbon atoms diffuse into colder zones both in radial and axial direc-tions, nucleate and transform into solid carbon. A part of the solid carbon is deposited on the discharge tube wall while the main part is gradually withdrawn from the hot region of the plasma in the outlet gas stream. We suggest that the formation of carbon structures can be described by following chain of processes: 1) formation of active nucleus; 2) process of growing and transformation of the active nucleus into spherical clusters; 3) process of growin...
Micro Electrical Discharge Machining (micro-EDM) is a plasma-assisted process for the manufacturi... more Micro Electrical Discharge Machining (micro-EDM) is a plasma-assisted process for the manufacturing of micro-components in high-hardness conductive materials. The removal of material is the result of a sum of dc discharges, produced within a point to plane system of electrodes immersed in a dielectric fluid, whose electric disruption with the development of plasma-currents can be induced by imposing a threshold voltage. To better understand the interaction between the micro-plasma and the material, we have designed and build an experimental setup for the production of single-discharges, characterized by a constant voltage-current operation point. The device allows the ignition of resistive plasmas in air/water at different pulse-times (∼100-3200 µs), currents (∼1-20 A for ∼75-250 V), polarities and gap-sizes (∼0,5-25 µm), and the changes in these work conditions can be correlated to the type and size of the craters produced. The micro-plasma electron density n e is measured using the Stark broadening of the H α atomic line. First results give n e ∼ 10 16-10 17 cm 3 , in agreement with estimates obtained from an electrical circuit model of the discharge.
An experimental investigation on the degree of molecular dissociation in a surface wave-driven di... more An experimental investigation on the degree of molecular dissociation in a surface wave-driven discharge (ω/2π=2.45GHz) operating in N2–H2 mixture at pressure p=(66.5–266Pa) is performed by means of optical emission spectroscopy. The dissociation degree of nitrogen molecules [N(4S)]/[N2] increases with the amount of hydrogen (up to 50%) in the mixture in correlation with the increase in electron temperature, when the electron density and the pressure are kept nearly constant. The relative number of hydrogen atoms keeps approximately constant with increasing H2 percentage (between 10% and 50%) in the mixture. The degree of molecular dissociation decreases along the plasma column length following the decrease in the electron density.
The spatial variations in the relative emission intensity of N 2 and N 2 + radiative states and o... more The spatial variations in the relative emission intensity of N 2 and N 2 + radiative states and of the NO(g) band in a large volume N 2-O 2 microwave plasma source have been investigated by means of optical emission spectroscopy. These variations have been correlated with the spatial distribution of the total electric field intensity of a TM 140 surface mode.
Plasma interactions with living matter are presently at the frontiers of plasma research and deve... more Plasma interactions with living matter are presently at the frontiers of plasma research and development. Plasmas contain numerous agents that influence biological activity. They provide essentially two types of biocidal species: reactive species, such as oxygen atoms that lead to lethality of micro-organisms through erosion, and UV radiation that can damage the DNA strands. In this work we investigate a surface wave (2.45 GHz) driven discharge plasma in air, with a small admixture of water vapor, as a source of ground state O(^3P) oxygen atoms, NO molecules and UV radiation. A theoretical model describing both the wave driven discharge zone and its flowing afterglow is used to analyze the performance of this plasma source. The predicted plasma-generated NO(X) and O(^3P) concentrations and NO(γ) radiation intensity along the source are presented and discussed as a function of the microwave power and water vapor percentage in the gas mixture. To validate the theoretical predictions, ...
Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achie... more Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron ...
ABSTRACT A microwave plasma torch operating at 2.45 GHz and atmospheric pressure has been used as... more ABSTRACT A microwave plasma torch operating at 2.45 GHz and atmospheric pressure has been used as a medium and a tool for decomposition of alcohol in order to produce molecular hydrogen. Plasma in a gas mixture of argon and ethanol/methanol, with or without water, has been created using a waveguide surfatron launcher and a microwave generator delivering a power in the range 0.2-2.0 kW. Mass, Fourier Transform Infrared, and optical emission spectrometry have been applied as diagnostic tools. The decomposition yield of methanol was nearly 100 % with H2, CO, CO2, H2O, and solid carbon as the main reaction products. The influence of the fraction of Ar flow through the liquid ethanol/methanol on H2, CO, and CO2 partial pressures has been investigated, as well as the dependence of the produced H2 flow on the total flow and power. The optical emission spectrum in the range 250–700 nm has also been detected. There is a decrease of the OH(A-X) band intensity with the increase of methanol in the mixture. The emission of carbon atoms in the near UV range (240–300 nm) exhibits a significant increase as the amount of alcohol in the mixture grows. The obtained results clearly show that this microwave plasma torch at atmospheric pressure provides an efficient plasma environment for hydrogen production.
We present a review of free electron and field diagnostics in gas discharges, giving a special em... more We present a review of free electron and field diagnostics in gas discharges, giving a special emphasis to spatially resolved measurements. Probes and antennas are the kernel of this work and we mention refinements concerning their use as well as common sources of error.
The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot ant... more The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot antenna excited microwave plasma source operating in Ar and N2–Ar mixtures at 2.45 GHz has been investigated. The discharge source is sustained by the electric field of pure TM surface modes. Optical emission spectroscopy, probe diagnostic techniques and radiophysics methods are applied to measure the emission intensity of radiative states, the electron density and the microwave field intensity, respectively. The spatial distribution of the electron density and the intensity of emission lines belonging to the 2nd positive and 1st negative systems of N2 follow the electric field pattern in the active-discharge zone of the plasma source. On the contrary, the populations in vibrational and rotational levels of the N2(C 3 � u) state are homogeneous irrespective of the azimuthal position.
Microwave H 2 , N 2 , and N 2-Ar discharges driven by traveling surface waves are investigated as... more Microwave H 2 , N 2 , and N 2-Ar discharges driven by traveling surface waves are investigated as sources of ground state N(4 S), H(1s) atoms. The dissociation kinetics is discussed in the framework of theoretical models based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma-wall interactions. It is shown that the number density of hydrogen H(1s) atoms depends heavily on the wall conditions at low pressure conditions. The kinetics of N(4 S) and metastable N(2 D) and N(2 P) atoms and of the molecular N 2 (A 3 6 +) metastable state in a pure N 2 discharge are shown to be strongly coupled. One possible way to control and to enhance nitrogen dissociation is the use of an N 2-Ar mixture. The increase in dissociation degree of N 2 molecules at high Ar fractional concentration can be attributed to dissociative recombination between electrons and N + 2 positive ions. The predicted results for the atomic density are compared with emission spectroscopy data.
View the article online for updates and enhancements. You may also like Continuum double probe th... more View the article online for updates and enhancements. You may also like Continuum double probe theory: effects of electron emission due to metastable species de-excitation and unequal probe arm saturation currents K Keskinen and R M Hobson-The display of logarithmic Langmuir probe characteristics on a cathode-ray oscillograph A R Johnston-Plasma probe characteristics in low density hydrogen pulsed plasmas D I Astakhov, W J Goedheer, C J Lee et al.
The Balmer line spectra emitted by different types of DC glow and RF discharges have typical mult... more The Balmer line spectra emitted by different types of DC glow and RF discharges have typical multimode behaviour, with widely broadened “wings” (“fast” hydrogen) and a sharp top (“slow” hydrogen), while the spectra emitted by microwave discharges usually exhibit single mode behaviour. As is well known, these discharges are free from strong DC fields. This report presents spectroscopic measurements of Balmer lines emitted by a microwave H2 plasma at low pressures (p = 0.01 – 0.2 mbar). A classical surface wave sustained discharge has been used as a plasma source [1]. The microwave power is provided by a 500 MHz generator (P = 40 – 250 W). The discharge takes place inside a Pyrex tube with internal and external radii of 2.25 and 2.5 cm. Under the present conditions, the electron density decreases from about (1 – 1.5) × 10 10 cm -3 at the beginning of the plasma column to about (2 – 3) × Ncr (Ncr ≈ 7 × 10 9 cm -3 ) at the end. The effective value of the microwave electric field intensi...
Langmuir probe diagnostics in tokamaks are very challenging, namely due to plasma fluctuations wh... more Langmuir probe diagnostics in tokamaks are very challenging, namely due to plasma fluctuations whose characteristic period is not much shorter than probe scanning-time. As a result, plasma parameters evaluation is usually constrained by the isotropic, Maxwellian electron energy distribution function (EEDF) assumption, probe diagnostics have been considered unreliable, EEDFs are not usually measured, and a huge amount of probe data has never been processed due to the lack of an appropriate numerical tool. While results from all diagnostics are not cross-linked in order to gather complete information on how EEDFs change in time and space, a particular diagnostic cannot do much more than looking for “averaged” values. Yet, it is imperative that each diagnostic does not lose the precious time-dependence so that a complete, overall picture is achievable whenever possible. In this paper we present results of a numerical procedure developed to minimize the effect of the above plasma fluctu...
In the present work, a surface-wave driven microwave plasma at atmospheric pressure conditions ha... more In the present work, a surface-wave driven microwave plasma at atmospheric pressure conditions has been applied for synthesis of carbon nanostructures. Three different types of nanostructures have been selectively synthesized by controlling the temperature in the assembling zone of the plasma reactor. The method is based on introduction of ethanol vapors through a microwave argon plasma environment, where decomposition of ethanol molecules takes place. Gas-phase carbon atoms diffuse into colder zones both in radial and axial direc-tions, nucleate and transform into solid carbon. A part of the solid carbon is deposited on the discharge tube wall while the main part is gradually withdrawn from the hot region of the plasma in the outlet gas stream. We suggest that the formation of carbon structures can be described by following chain of processes: 1) formation of active nucleus; 2) process of growing and transformation of the active nucleus into spherical clusters; 3) process of growin...
Micro Electrical Discharge Machining (micro-EDM) is a plasma-assisted process for the manufacturi... more Micro Electrical Discharge Machining (micro-EDM) is a plasma-assisted process for the manufacturing of micro-components in high-hardness conductive materials. The removal of material is the result of a sum of dc discharges, produced within a point to plane system of electrodes immersed in a dielectric fluid, whose electric disruption with the development of plasma-currents can be induced by imposing a threshold voltage. To better understand the interaction between the micro-plasma and the material, we have designed and build an experimental setup for the production of single-discharges, characterized by a constant voltage-current operation point. The device allows the ignition of resistive plasmas in air/water at different pulse-times (∼100-3200 µs), currents (∼1-20 A for ∼75-250 V), polarities and gap-sizes (∼0,5-25 µm), and the changes in these work conditions can be correlated to the type and size of the craters produced. The micro-plasma electron density n e is measured using the Stark broadening of the H α atomic line. First results give n e ∼ 10 16-10 17 cm 3 , in agreement with estimates obtained from an electrical circuit model of the discharge.
An experimental investigation on the degree of molecular dissociation in a surface wave-driven di... more An experimental investigation on the degree of molecular dissociation in a surface wave-driven discharge (ω/2π=2.45GHz) operating in N2–H2 mixture at pressure p=(66.5–266Pa) is performed by means of optical emission spectroscopy. The dissociation degree of nitrogen molecules [N(4S)]/[N2] increases with the amount of hydrogen (up to 50%) in the mixture in correlation with the increase in electron temperature, when the electron density and the pressure are kept nearly constant. The relative number of hydrogen atoms keeps approximately constant with increasing H2 percentage (between 10% and 50%) in the mixture. The degree of molecular dissociation decreases along the plasma column length following the decrease in the electron density.
The spatial variations in the relative emission intensity of N 2 and N 2 + radiative states and o... more The spatial variations in the relative emission intensity of N 2 and N 2 + radiative states and of the NO(g) band in a large volume N 2-O 2 microwave plasma source have been investigated by means of optical emission spectroscopy. These variations have been correlated with the spatial distribution of the total electric field intensity of a TM 140 surface mode.
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