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Manganese dioxide is one of the key materials in battery, adsorption media, and catalytic materials in the environmental field because of its excellent properties. In this investigation, new synthetical technology—the microwave discharge plasma in liquid method was used directly to make manganese dioxide with potassium permanganate (KMnO4) solution. The synthesis was realized in one step by the microwave discharging plasma in potassium permanganate aqueous solution without using any dispersants or stabilizers. The reduction time was very short, it only takes 3 min to complete the reduction of the 0.2 mmol l−1 potassium permanganate solution for the formation of MnO2. The structure characteristics and adsorption-desorption curves were studied by X-ray diffractometer (XRD), transmission electron microscopy and x-ray photoelectron spectroscopy, XRD, etc. it was found that the synthesized manganese dioxide nano-sheet had a layered structure and a large specific surface area (248.89 m2 g−1). The methylene blue was used to validate the effectiveness of the prepared manganese dioxide. The results showed that the synthesized manganese dioxide had good adsorption and photocatalytic properties. In conclusion, the microwave liquid-phase discharge plasma is a very effective reduction method with potential development. [ABSTRACT FROM AUTHOR]

In this work, the reaction mechanism in non-thermal plasma enabled methane conversion driven by different sources, i.e. microwave discharge, nanosecond pulsed dielectric barrier discharge, and spark discharge, is investigated. The behaviour of reactive species is monitored using the optical emission spectroscopy (OES), while the final gaseous products are analysed with gas-chromatograph. A zero-dimensional (0D) reaction kinetics model is established to reveal the underlying reaction processes. By changing pressure, different plasma modes (diffuse and filamentary) are achieved and the relative emission intensity of different species and distribution of gaseous productions are manipulated. It is found that there is a strong correlation between the intensity-ratio of C2A → X and CH A → X, gas temperature, and the relative concentration of C2 products in different discharge forms, i.e. higher intensity ratio of C2/CH is associated with higher gas temperature and more unsaturated hydrocarbon products. Assisted with the 0D reaction kinetics model, the relationship between the intermediate species and final products is illustrated. Based on the investigation in this work, it is proposed that intensity ratio of OES can be widely adapted as a non-intrusive and real-time process monitoring in the plasma-enabled methane conversion. [ABSTRACT FROM AUTHOR]

Strong electromagnetic resonance in cavity resonators can lead to many configurations of resonant field enhancement. Some resonant modes lead to strongly enhanced electric fields, but there also exist modes in which the magnetic field enhancement dominates. The typical fluid model of gas breakdown and electromagnetic interaction does not account for magnetic fields, which can lead to significant differences between observed and numerically computed breakdown thresholds, as well as differences in post-breakdown plasma evolution. Starting from an expansion of the Boltzmann equation for charged particles, we develop an analytical model that captures how a strong oscillating magnetic field can affect plasma transport properties. Certain parameter regimes are shown to have resonance-like behavior of diffusion, drift, and energy deposition to electrons. We develop a model that incorporates the effects of both static and oscillating magnetic fields in terms of a local effective electric field model. The fluid model parameters are compared to electron swarm behavior in a particle computation with Monte–Carlo collisions. [ABSTRACT FROM AUTHOR]

The evolution of species densities along a reactor radially positioned on an O2 surface-wave microwave discharge is investigated by means of modelling with the aim to define the density tuning possibilities. The validity of the models is shown by the comparison of the calculated and measured axial distribution of O-atoms. The calculations revealed that due to the perpendicular injection of the plasma into the reactor, the gas temperature is close to the room temperature in most of it, except for a 5 cm region around the inlet. It is shown that the pressure drop along the discharge tube, which results in the change of pressure in the discharge region with the gas flow rate, induces the variation of the relative density of active species entering the reactor, where the pressure is kept constant. The surface recombination probability of atoms varies along the afterglow tube due to the surface temperature gradient, as well as due to the conditioning of the surface resulting from the continuous operation of the system. The system is shown to be very practical in applications where surfaces/porous materials are to be treated homogeneously by pumping active species through them, since by tuning the gas flow rate equidensity surfaces can be obtained in the case of the two most abundant species, the O-atoms and O2(a) molecules. In the case of O-atoms the densities obtained at the two pressures investigated, i.e. 100 and 50 Pa, are very similar, as well as their evolution along the reactor, while the density of O2(a) molecules decreases considerably with pressure. [ABSTRACT FROM AUTHOR]

Discharge lamps containing mercury at pressures above 100 bar are commercially used in data projectors and television projector systems. Due to their small size, these lamps are difficult to investigate experimentally, but spectral measurements, combined with radiation transport calculations, have provided useful information on the visible spectrum. However, classical spectral line broadening theory is inadequate to describe the UV portion of the spectrum, so self-consistent modelling of these discharges is not possible at present. This paper discusses the differences between discharges containing electrodes and discharges sustained by a microwave (mw) electromagnetic field, on the basis of the experimentally measured temperature profile in an electroded discharge, and a temperature profile computed from a 1D power balance model for a microwave discharge. A model based on the ray-tracing method is employed to simulate the radiation transport in these lamps. The model has been validated by comparing the emission spectrum from dc discharge lamps with those obtained experimentally. The output flux, luminous flux, luminous efficacy, the correlated colour temperature, the chromaticity coordinates and photometric curves of the lamp were then obtained. These results were also compared with those of a theoretically calculated temperature profile for the same lamp, excited by microwave power in the TM010 mode. [ABSTRACT FROM AUTHOR]

This work is devoted to the investigation of the discharge characteristics of high-frequency alternating current (ac) plasma torch working with air medium using electrical and spectroscopic techniques. A simple structure and compact ac plasma torch associated with a resonance power supply allows the generation of low power discharges (lower than 1 kW) with high voltage and low current. The discharge shows a negative resistance characteristic, and its curve shifts up with gas flow increased. The effects of power on the emission intensity of NO (A2Σ+ → X2Π), OH (A2Σ → X2Π, 0–0), N2(C3Πu → B3Πg), Hα and O and their spatial distributions in plasma jet axial direction were investigated. It has been found that the emission intensities of NO, OH, N2, Hα and O rise with an increase in power dissipation. With increasing axial distances of plasma jet from nozzle exit, the emission intensity of OH increases and then decreases, while the emission intensities of other species decrease sharply. The vibrational temperature is much higher than the gas temperature, which demonstrates the ac-driven arc discharge deviation from thermal equilibrium plasma. [ABSTRACT FROM AUTHOR]

Optical emission spectroscopy (OES) and multipole resonance probe (MRP) are adopted to characterize low-pressure microwave (MW) and radio frequency (RF) discharges in oxygen. In this context, both discharges are usually applied for the deposition of permeation barrier SiOx films on plastic foils or the inner surface of plastic bottles. For technological reasons the MW excitation is modulated and a continuous wave (cw) RF bias is used. The RF voltage produces a stationary low-density plasma, whereas the high-density MW discharge is pulsed. For the optimization of deposition process and the quality of the deposited barrier films, plasma conditions are characterized using OES and MRP. To simplify the comparison of applied diagnostics, both MW and RF discharges are studied separately in cw mode. The OES and MRP diagnostic methods complement each other and provide reliable information about electron density and electron temperature. In the MW case, electron density amounts to ne = (1.25 ± 0.26) × 1017 m−3, and kTe to 1.93 ± 0.20 eV, in the RF case ne = (6.8 ± 1.8)×1015 m−3 and kTe = 2.6 ± 0.35 eV. The corresponding gas temperatures are 760±40 K and 440±20 K. [ABSTRACT FROM AUTHOR]

The Darwin approximation is investigated for its possible use in simulation of electromagnetic effects in large size, high-frequency capacitively coupled discharges. The approximation is utilized within the framework of two different fluid models which are applied to typical cases showing pronounced standing wave and skin effects. With the first model it is demonstrated that the Darwin approximation is valid for treatment of such effects in the range of parameters under consideration. The second approach, a reduced nonlinear Darwin approximation-based model, shows that the electromagnetic phenomena persist in a more realistic setting. The Darwin approximation offers a simple and efficient way of carrying out electromagnetic simulations as it removes the Courant condition plaguing explicit electromagnetic algorithms and can be implemented as a straightforward modification of electrostatic algorithms. The algorithm described here avoids iterative schemes needed for the divergence cleaning and represents a fast and efficient solver, which can be used in fluid and kinetic models for self-consistent description of technical plasmas exhibiting certain electromagnetic activity. [ABSTRACT FROM AUTHOR]

A floating hairpin resonator probe is used to measure electron density within the radiofrequency cycle of a parallel plate capacitively coupled plasma at steady state. A time resolution capable of detecting electron density oscillations within the radiofrequency cycle is demonstrated. Electron density oscillations are observed at the drive frequency over a range of operating conditions including 10–250 mTorr (1.3–33 Pa) in argon driven at 13.56 or 27.12 MHz. Localized electron density oscillation amplitudes show moderate agreement with literature and fluid simulations near the powered electrode and disagreement in the plasma bulk. The technique is useful for studying electron heating mechanisms in radiofrequency discharges due to its high time resolution. [ABSTRACT FROM AUTHOR]

The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30–100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4  +  O(1D)  →  CH3  +  OH, and O2(a1Δg)  +  H  →  O  +  OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v)  +  O  →  CH3  +  OH, CH4(v)  +  OH  →  CH3  +  H2O, O2(v)  +  H  →  O  +  OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13)  +  OH  →  H2O  +  CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane. [ABSTRACT FROM AUTHOR]

Efficient CO2 reduction is predicted for CO2 microwave plasma by virtue of predominant excitation of the asymmetric stretch vibration. Although interpretation of ongoing research is generally based on this mechanism, direct measurement of the power partitioning to support the assumed preferential vibrational excitation in CO2 microwave plasma is currently lacking. Here, such measurements are performed on a 100 µs pulsed microwave CO2 discharge. The  <1% duty cycle ensures low gas temperature conditions at the discharge onset. Raman and Rayleigh scattering are employed to reveal vibrational, rotational, and gas temperatures in a spatially and temporally resolved manner. A novelty in the approach is that asymmetric stretch excitation is determined from the bending—symmetric stretch Raman spectrum. During the first 40 µs a significant inter-vibrational non-equilibrium is observed with the symmetric stretch and bending temperature reaching 750 K and the asymmetric stretch temperature reaching 1150 K. A maximum rotational-vibrational non-equilibrium occurs after 60 µs when the rotational temperature is half of the 1150 K vibrational temperature. Rotational and translational modes are measured to be in equilibrium at all times. The power partitioning is analyzed to estimate the power consumed by vibrational excitation, which is used to estimate the reduced electric field in the discharge. This work confirms strong vibrational excitation in CO2 microwave plasma albeit less predominant than often assumed. [ABSTRACT FROM AUTHOR]

The properties of an asymmetric microwave gas discharge that occurs in an argon-filled resonator chamber of a processing plant near the surface of a metal product during the formation on it of a composite structure are experimentally and theoretically investigated. It is shown that with a significant difference between the electrode dimensions, the smaller of which is the product, and the larger are the resonator chamber walls, there is an area of a high energy density of the microwave field near the surface of the product, in which electrons are accelerated to high energies up to hundreds of eV. Diffusing from this area, high-energy electrons are decelerated due to inelastic scattering on the gas molecules that fill the resonator chamber, ionizing them or promoting them to an excited state. We have obtained a differential equation and found its solution which describes the diffusion process accompanied by the loss of energy by high-energy electrons, within the framework of the continuous deceleration model. The estimates of the main physical parameters characterizing the microwave gas discharge plasma are found. The conditions that are most favorable for the energy transfer from the microwave field to the plasma electron subsystem and the product surface treatment are determined. [ABSTRACT FROM AUTHOR]

In many cases, the capacitance of a plasma discharge is determined by the sheath capacitance, and therefore controlling the sheath thickness can result in a novel plasma-tunable capacitor. In this work, experiments were conducted with capacitively-coupled RF discharge in the alpha regime using parallel-plate electrode geometry in air at 1 Torr with an RF amplifier operating at a constant voltage in a wide range of frequencies, 20–120 MHz. The impedance characteristics and the sheath and plasma parameters were inferred from the current and voltage measurements supplemented with optical imaging. The experimental data show that the sheath thickness is approximately inversely proportional to the excitation RF frequency, so that the capacitance is proportional to the frequency, in agreement with a simple theory. In applications, tuning the sheath thickness by varying the excitation RF frequency could be used to control the impedance variation in a different range of probing RF frequencies, including control of the transition from capacitive to inductive behaviour. [ABSTRACT FROM AUTHOR]

The O(3P) atom loss has been studied in O2 RF plasma in the quartz tube in the intermediate pressure range (10–100 Torr) when the transition from the surface to volume loss is observed. Space- and time-resolved actinometry on Ar and Kr atoms was used to study O(3P) atom loss. The research has shown that such a transition actually takes place. However, it was revealed that the gas temperature plays a significant role in this. Gas temperature was measured spectroscopically using the band emission. It was demonstrated that the gas temperature in the plasma volume was rather high (>1200 K) due to the high values of the specific input RF power which in turn led to a significant O(3P) loss rate decrease in the discharge volume. The atomic oxygen loss is limited by the O(3P) surface recombination as well as by the volume recombination in a thin layer near the wall. It leads to a low integral loss rate and provides a high oxygen dissociation degree. Analysis based on measured [O(3P)]/N, Tgas, O3 spatial profiles and surface loss model including recombination with chemisorbed and physisorbed atoms has revealed the importance of both surface and volume losses. High values of the specific input RF power also lead to increase of the gas temperature near the wall and the temperature of the internal tube surface. As a result, the O atom surface loss rate increases and the volume loss rate near the wall decreases, so overall the contributions of both O(3P) volume and surface recombination are comparable at pressures up to 100 Torr. The O(3P) loss kinetics at intermediate pressures appears to be a rather complex phenomenon and requires at least 1D or even 2D modeling for its correct description. Using global models under similar conditions may lead to dubious results in a detailed study of kinetic mechanisms and processes, but it can be useful for a simple analysis of experimental results. [ABSTRACT FROM AUTHOR]

The application of nanosecond-pulsed high-frequency discharges across a pin-to-pin air gap was investigated to determine the gas temperature evolution over a burst series of 10 pulses with approximately 10 ns duration at a frequency varying from 1 kHz to 250 kHz. The benefits of high pulse frequency (>10 kHz) for combustion applications has been previously noted, and there exists some experimental evidence that the temperature increases with pulse frequency, but the phenomenon has not been explored rigorously. In this study, discharges in quiescent air initially at 99 kPa and 293 K were explored at various pulse frequencies using optical emission spectroscopy. The gas temperature was measured via spectroscopy of the N2(C  →  B) emission during each discharge pulse with the assumption that the N2(C) state rotational distribution maps the ground state N2(X) distribution. The results show within a burst of repetitive pulses, that the remnant pulse energy caused a significant gas temperature increase between the end of the previous pulse and the initiation of the subsequent pulse, if the time between pulses was less than 100 µs. This result indicated that the discharges within the burst were thermally ‘coupled’ for frequencies greater than 10 kHz. Analysis of the temperature evolution after the first pulse in a burst indicated a rapid temperature rise of several thousand Kelvin over a few microseconds. The heating rate increased further if repetitive pulses were applied at higher frequencies. It could be inferred from the data that the temperature rise following the first pulse had peaked by 40 µs and that the air in the discharge zone was cooling by the initiation of the second pulse, even at the highest frequency of 250 kHz, presumably due to a gas recirculation effect associated with the first pulse. [ABSTRACT FROM AUTHOR]

Microwave discharges in liquids are a new direction in the physics and chemistry of low temperature plasma. The article describes the results of experiments on the separation of metals from oil when a microwave discharge is created at atmospheric pressure in its volume. It is shown for the first time that the content of metals Al, Co, Cu, Fe, Mo, Ni, V, and Zn in a tree-like structure deposited on the microwave antenna is up to 10 times higher than their content in the initial substance. [ABSTRACT FROM AUTHOR]

Asymmetrically excited, high frequency cylindrical capacitive discharges are widely used for materials etching and thin film deposition. Two-dimensional (2D) electrostatic particle-in-cell (PIC) simulations show the existence of standing waves and wave-induced hysteresis of the plasma density, i.e. two different steady states for the same driving rf voltage amplitude, when the voltage is increased from a low value or decreased from a high value. The phenomenon is explored over a range of pressures (10–30 mTorr) and frequencies (60–80 MHz). Examined at 73 MHz, with increasing gas pressure, the hysteresis loop gradually shrinks and vanishes. To understand the hysteresis induced by z-symmetric and z-antisymmetric radial wave propagation modes, the PIC results are compared with a nonlinear transmission line model assuming uniform bulk plasma density, to determine the symmetric and antisymmetric voltage amplitudes. The model results are in good agreement with the PIC observations, showing central-low and central-high profiles of the antisymmetric mode voltage at low density and high density, respectively. The results are then used to determine the parameters of a lumped circuit model of the two modes, from which the hysteresis is induced by the density dependence of the symmetric and anti-symmetric wave mode absorbed electron powers. For the low density state, the discharge is sustained mainly by the symmetric mode excitation. At high density, the discharge is sustained by both symmetric and anti-symmetric modes, with the latter partly showing a spatial resonance. The results are also shown to be frequency dependent, with an onset of the hysteresis at about 66 MHz. [ABSTRACT FROM AUTHOR]

The development of a microwave discharge is accompanied by noticeable scattering of the incident electromagnetic field. The scattered radiation includes information about the time evolution of the discharge formation and its integral parameters. In this work a method of diagnostics using scattered radiation to study a single thin plasma channel (plasmoid, plasma dipole) and corresponding experimental results in air within the pressure range P  =  70–150 Torr are presented. A simple analytical model for analysis of the experimental results of microwave scattering from a plasmoid is proposed. Information concerning the average elongation velocity of the plasma channel in the direction parallel to the incident electric field and the induced dipole moment of the plasmoid is obtained. Based on information about the dipole moment as well as the form and sizes of the plasma dipole the effective cross section, the total charge, the current averaged over the channel length, the plasma density and conductivity averaged over the channel volume, the electric field amplitude in the central region of the channel and the absorbed power are estimated. The estimated value of the plasma density is shown to agree with experimental results obtained using spectroscopic methods. [ABSTRACT FROM AUTHOR]

An overview of the recent progress on plasma-assisted CO2 conversion in microwave discharges is given. Special attention is devoted to the results obtained using plasma catalysis, which are compared to the plasma-only CO2 decomposition cases. The effects of plasma operating conditions, catalyst preparation methods, nature of plasma activation gas, gas mixture, as well as the NiO content on the TiO2 surface on CO2 conversion and its energy efficiency are discussed. A significant improvement in CO2 conversion is obtained with a NiO/TiO2 catalyst activated in Ar plasma, when the NiO content is about 10 wt.%. The catalyst characterization data show that Ar plasma treatment results in a higher density of oxygen vacancies and a comparatively more uniform distribution of NiO on the TiO2 surface, which strongly influence CO2 conversion and its energy efficiency. The dissociative electron attachment of CO2 at the catalyst surface enhanced by the oxygen vacancies and by plasma electrons may also explain the increase in conversion and energy efficiencies. A mechanism for the plasma-catalytic CO2 conversion at the surface of an Ar plasma-threated catalyst is proposed. [ABSTRACT FROM AUTHOR]

A capacitively coupled RF oxygen discharge is studied by means of mass spectroscopy. Mass spectra of neutral and positive species are measured in the mid plane between the electrodes at different distances between plasma and mass-spectrometer orifice. In the case of positive ions, as expected, the largest flux originates from . However, a significant number of impurities are detected, especially for low input powers and larger distances. The most abundant positive ions (besides ) are , , and . In particular, for the case of hydrated hydronium ions (n  =  1, 2) a surprisingly large flux (for low pressure plasma conditions) is detected. Another interesting fact concerns the ions. Despite the relatively high ammount of water impurities ions are present only in traces. The reaction mechanisms leading to the production of the observed ions, especially the hydrated hydronium ions are discussed. [ABSTRACT FROM AUTHOR]

A new mechanism for producing negative ions in low density and low power hydrogen plasmas was proposed recently. It refers to anion formation due to collisions between hydrogen atoms being in the first excited state. The proposed mechanism was indirectly supported by the quadratic relation observed between the extracted negative ion current and Lyman-α radiation of a filament-driven arc discharge, when borrowed data from the literature were combined. The present work provides experimental data comparing directly the absolute negative ion density and Lyman-α radiation in an ECR-driven hydrogen plasma source. The previously mentioned quadratic relation is not observed in the specific source studied, underlying the difficulty of distinguishing between the proposed mechanism and other negative ion production paths. [ABSTRACT FROM AUTHOR]

Surface dielectric barrier discharges with conductive water-solution electrodes were generated at atmospheric pressure air, nitrogen, oxygen, and argon. The discharges were studied by conventional and high-speed camera photography. Plasma rotational and vibrational temperatures and the electron number density were estimated using optical emission spectroscopy. Surprisingly, especially for oxygen, the discharge was found to generate visually diffuse strongly non-isothermal plasma. This observation indicates the interesting application potential of the discharge for surface plasma treatments of, i.e. the inner and outer surfaces of hollow dielectric bodies. [ABSTRACT FROM AUTHOR]

In this paper we report studies of basic properties of breakdown, low-current Townsend discharge and high-current discharge regimes in water vapour. Paschen curves and the corresponding distributions of emission intensities at low current were recorded in the range of pd (pressure x electrode gap) from 0.1 to 10 Torrcm covering the region of Paschen minimum. From the experimental profiles we obtained effective ionization coefficient of water vapour for the E/N range 650 Td–7 kTd and fitted the results by using the extended Townsend analytical formula. Using the obtained ionization coefficient, we calculated the effective yield of secondary electrons from the copper cathode. Results of the measurements of Volt-Ampere characteristics in water vapour were presented together with the images of the axial structure of the discharge in a wide range of discharge currents for two pd values. Recorded profiles showed development of the spatial structure of the discharge in different operating regimes. We were able to identify conditions where processes induced by heavy particles, probably fast hydrogen atoms, are dominant in inducing emission from the discharge. Finally, standard scaling laws were tested for low current and glow discharges in water vapour. [ABSTRACT FROM AUTHOR]

This work presents a numerical model of hydrogen plasma in a microwave coaxial discharge at low pressure (25–250 Pa). The model is a mixed two-dimensional (2D)/three-dimensional (3D) model in that it combines three-dimensional geometry for the electromagnetic field and two-dimensional geometry for the transport equations. The model is validated against experimental results available in the literature and, where possible, simulations of comparable discharges. The model shows reasonable agreement in the relevant pressure range. A parametric study with respect to pressure is carried out and it is observed that the plasma contracts towards the quartz tube with increasing pressure. Increasing the pressure also influences the abundance of H+ ions but on the other hand it has little impact on hydrogen dissociation degree and electron temperature. Furthermore, the uniformity of the plasma above the substrate holder is analyzed. It is observed that at pressures over 150 Pa, the plasma gets non-uniform in the direction parallel to the plasma lines. Finally, the uniformity of particle and energy fluxes to the substrate holder are analyzed. Knowing the fluxes is especially useful for the material applications of the device. [ABSTRACT FROM AUTHOR]

Gas breakdown at microscale dimensions has been of great interest to the microelectromechanical systems (MEMS) and plasma communities for nearly 15 years as the first reports of deviations from traditional theory began to emerge. Since those first reports, a significant amount of work has investigated why gas breakdown deviates from the classic Paschen’s Law when the dimensions are in the range of 1–10 µm. Nearly universally, these deviations that form the so-called modified Paschen’s curve have been attributed to electron field emission, where electrons directly tunnel from the cathode into the gas due to the very high electric fields at microscale dimensions. Furthermore, because of ionization in the gas gap, field emission is enhanced by positive ions and thus is inherently coupled to the gas and discharge dynamics. Progress in understanding the mechanisms and physics of this process has in turn led to new ideas and devices that capitalize on the high surface-to-volume ratio in microscale dimensions and take advantage of cathode emission processes. This topical review summarizes and analyzes the numerous experimental, computational and analytical works on breakdown at microscale dimensions, discusses implications and new areas emerging in microscale devices that take advantage of field emission and presents perspectives looking ahead at new opportunities for field emission-driven microplasmas. [ABSTRACT FROM AUTHOR]

This study is devoted to the characterization of a 40 MHz RF discharge at 2 Torr pressure in argon–oxygen mixtures (0–5% O2 fraction). The electrical characteristics were used to determine the electron density ne and reduced electric field E/N in the plasma. The gas temperature Tg and number densities of Ar atoms in 1s states were determined from spectral measurements. The spectral measurements were also used to obtain the relative populations of argon 2p and atomic oxygen 35P and 33P states as a function of ne and O2 fraction.It was shown that the number density of Ar atoms in 1s states increased with the growth of O2 fraction, and the density of Ar 1s states was sufficiently high to cause the trapping of radiation originating from Ar atoms in 2p states. Argon atoms in 2p states were predominantly populated from 1s states; population from the Ar ground state was found to be important only for the 2p1 state at higher O2 fraction. The excitation of the O 5P state was caused by electron impact, while in the case of O 3P state excitation the role of electrons was negligible. [ABSTRACT FROM AUTHOR]

Results of experiments and two-dimensional self-consistent simulation show that small (1–5%) additives of argon inserted in a hydrogen strongly non-uniform microwave discharge (electrode microwave discharge) reduce the emission intensity of excited particles, power absorbed in plasma, and electron and ion densities. Simulations have shown that this effect is linked with the reduction of the flux of ions to the electrode, which causes a decrease in the microwave field in plasma. These results illustrate the role of discharge non-uniformity in processes of discharge physics. It is shown that the argon admixture disturbs the discharge, and so the known method of optical actinometry cannot be used for plasma diagnostics in the case considered here. On the other hand, the addition of argon can be used to control plasma properties. [ABSTRACT FROM AUTHOR]

A radio-frequency (rf) atmospheric-pressure discharge in He–O2 mixture is studied using a fluid model for its wall fluxes and their dependence on electron and chemical kinetics in the sheath region. It is shown that ground-state O, and O− are the dominant wall fluxes of neutral species, cations and anions, respectively. Detailed analysis of particle transport shows that wall fluxes are supplied from a boundary layer of 3–300 µm immediately next to an electrode, a fraction of the thickness of the sheath region. The width of the boundary layer mirrors the effective excursion distance during lifetime of plasma species, and is a result of much reduced length scale of particle transport at elevated gas pressures. As a result, plasma species supplying their wall fluxes are produced locally within the boundary layer and the chemical composition of the overall wall flux depends critically on spatio-temporal characteristics of electron temperature and density within the sheath. Wall fluxes of cations and ions are found to consist of a train of nanosecond pulses, whereas wall fluxes of neutral species are largely time-invariant. [ABSTRACT FROM AUTHOR]

A PIC-MCC/fluid hybrid model was employed to study a parallel-plate capacitively coupled radio-frequency discharge in hydrogen, under the application of a dc bias voltage. When a negative dc voltage was applied to one of the electrodes of a continuous wave (cw) plasma, a ‘beam’ of secondary electrons was formed that struck the substrate counter-electrode at nearly normal incidence. The energy distribution of the electrons striking the substrate extended all the way to VRF + |Vdc|, the sum of the peak RF voltage and the absolute value of the applied dc bias. Such directional, energetic electrons may be useful for ameliorating charging damage in etching of high aspect ratio nano-features. The vibrational distribution function of molecular hydrogen was calculated self-consistently, and was found to have a characteristic plateau for intermediate values of the vibrational quantum number, v. When a positive dc bias voltage was applied synchronously during a specified time window in the afterglow of a pulsed plasma, the ion energy distributions (IEDs) of positive ions acquired an extra peak at an energy equivalent of the applied dc voltage. The electron energy distribution function was slightly and temporarily heated during the application of the dc bias pulse. The calculated IEDs of and ions in a cw plasma without dc bias were found to be in good agreement with published experimental data. [ABSTRACT FROM AUTHOR]

This study focuses on the generation and loss of reactive oxygen species (ROS) in low-temperature atmospheric-pressure RF (13.56 MHz) He + O2 + H2O plasmas, which are of interest for many biomedical applications. These plasmas create cocktails of ROS containing ozone, singlet oxygen, atomic oxygen, hydroxyl radicals, hydrogen peroxide and hydroperoxyl radicals, i.e. ROS of great significance as recognized by the free-radical biology community. By means of one-dimensional fluid simulations (61 species, 878 reactions), the key ROS and their generation and loss mechanisms are identified as a function of the oxygen and water content in the feed gas. Identification of the main chemical pathways can guide the optimization of He + O2 + H2O plasmas for the production of particular ROS. It is found that for a given oxygen concentration, the presence of water in the feed gas decreases the net production of oxygen-derived ROS, while for a given water concentration, the presence of oxygen enhances the net production of water-derived ROS. Although most ROS can be generated in a wide range of oxygen and water admixtures, the chemical pathways leading to their generation change significantly as a function of the feed gas composition. Therefore, care must be taken when selecting reduced chemical sets to study these plasmas. [ABSTRACT FROM AUTHOR]