Table of contents

This communication presents an alternative source for the generation of non-thermal plasmas at atmospheric pressure. It is based on a very simple electric diagram that generates, on the plasma electrode, an electric field sufficiently high to ionize various gases (helium, argon or even air), which flow at atmospheric pressure. The dc power supply is lower than 10 V, the frequency of the electric field is in the range 0.1–1 MHz and the plasma power is less than 1 W. The plasma is generated using only one electrode. It has all the characteristics of non-thermal plasmas.

Emission spectroscopy was employed to observe decomposed species of N-isopropylacrylamide (NIPAAm) exposed to He, N₂ and air plasmas, which were generated by an AC discharge at 3.0 Torr pressure. For the He plasma, several strong emissions were observed and assigned to the H_α and H_β atomic lines, CH₃O, CN(B ²Σ–X ²Π), CH(A ²Δ–X ²Π), , N₂(C ³Π_u–B ³Π_g) and C₃H₅, CN, CHO, CH₂O and transitions. These results suggest that plasma etching decomposed the NIPAAm monomer sample, and the resulting species emitted light near the surface.

For the NIPAAm exposed to N₂ plasma, the emission spectrum shows the presence of the following species: , N₂, NH, NH⁺, CO and OH, while when the NIPAAm was exposed to the air plasma, the species observed were , N₂, NH, NH⁺, CO, OH and O. Most of the bands and lines observed in helium plasma were not present in air and nitrogen plasmas. This implies that these species undergo significant chemical conversion in the plasma. The time dependence of the strong emission intensities of the observed species was also investigated. As N₂ and air plasmas were exposed to the NIPAAm monomer, the surface was etched more rapidly than the He plasma.

Spatially resolved optical emission spectroscopy was used to determine the atomic excitation temperature of the capacitively coupled radio-frequency (RF) plasma system. Low pressure plasmas of methane or propane in hydrogen were excited at 13.56 MHz in a parallel plate system. Ar was added as an actinometer. Optical emission lines in the 300–850 nm spectral range were investigated at typical conditions of 100 W RF power, ∼30 mTorr pressure, 20 mm electrode spacing and 50 sccm total flow rate. Two-dimensional intensity profiles of the important species were collected along the vertical and radial axes. The raw radial intensity was transformed into the actual local radiation intensity by Abel inversion. The atomic hydrogen and argon excitation temperature distributions between the power and grounded electrodes were derived from these data and distinct differences were found in methane- and propane-containing plasmas.

A method for the determination of excitation temperatures based on optical emission spectroscopy and Fermi–Dirac distribution was set up and experiments were performed on atmospheric argon dielectric barrier discharges. Local thermodynamic equilibrium was proved to exist in the discharge and the validity of Boltzmann distribution is discussed. The main aim of this paper is to obtain the temperatures of atmospheric Ar II as a function of the discharge voltage, discharge frequency, argon flow rate and the argon fraction. It was found that the excitation temperatures are in the range 3800–4950 K. Besides, an increase in the argon flow rate resulting in a slight growth of the temperature and the add-in of air leading to the decrease in temperature was observed.

An asymmetric electric voltage pulse source (2–20 kHz, V_p–p: 0–20 kV) was applied to stimulate excimer radiation by a dielectric barrier discharge in a binary gas mixture of chlorine (10.8 Torr)/xenon at high pressure (∼460 Torr). When the source frequency increases continuously from 2.0 to 12.0 kHz or in reverse under a fixed output voltage of V_p–p (13.8 kV), it is observed that the discharge modes excited by this pulse source transmit between glow-like and filamentary, and we observe a mixed mode with filaments randomly distributed in the diffuse-like background in the narrow frequency range 7.0–8.0 kHz. It is argued that the reasons for the discharge transition could be the frequency and the time derivative of the voltage waveform of the power source. It is also observed that there is an obvious difference in the emission spectral profiles and the energy efficiency of excimer emission for the two discharge modes.

Dielectric barrier discharges are sources of non-thermal plasmas, in which chemical active particles are generated effectively. One of the most promising applications is chemical processing of gaseous substances, e.g. synthesis or decomposition. The removal of air pollutants such as sulfur dioxide from dry and humid air at atmospheric pressure is investigated experimentally as well as theoretically. The role of chemically active particles, especially of atomic oxygen and of OH radicals, is shown. Optimal conditions of the removal process are determined.

We investigate the potential use of line ratio diagnostics to evaluate electron temperature in either helium or helium seeded argon plasmas. Plasmas are produced in a helicon plasma source. A rf compensated Langmuir probe is used to measure both the electron temperature and plasma density while a spectrometer is used to measure He I line intensities from the plasma. For all plasma densities where the electron temperature remains at 5 ± 1 eV, three He line ratios are measured. Each experimental ratio is compared with the prediction of three different collisional radiative models. One of these models makes uses of recent R-matrix with pseudo-states calculations for collisional rate coefficients. A discussion related to the different observations and model predictions is presented.

A self-consistent model describing a microwave discharge with radial energy input is proposed to simulate electromagnetic field, radiative transfer, photoionization processes, plasma chemical reactions, heat transfer and hydrodynamics. The model is implemented for the case of discharges in neon and argon; the relevant kinetic data for these gases are collected and supplemented. Self-consistent simulation has shown that the electromagnetic field and the plasma configuration do not correspond to the widely used surface-wave approach.

We present a systematic study of electrical characteristics and Langmuir probe measurements on plane-parallel electrode helium glow discharges operated in normal and abnormal glow regimes. The experimental studies are complemented with numerical modelling based on a hybrid approach combining fluid and particle descriptions of the discharge plasma. A comparison between the experimental data and the two-dimensional modelling results is presented. The temperature of slow Maxwellian electrons is found to be ≈0.055 eV in the negative-glow region nearly independent of the discharge conditions. This temperature value is used as an input parameter of the hybrid model. Good agreement is obtained between the calculated spatial density distribution of electron impact excitation events and the CCD image of the light emission from the discharge. The calculated electron densities are 1.3–2 times higher than the corresponding Langmuir probe data; the possible reason for these differences is discussed.

This paper is focused on experimental studies of a high power pulsed magnetron discharge stabilized by low current pre-ionization. Time resolved studies were performed for a Cu target by optical emission spectroscopy and electrical measurements for different pressures of Ar buffer gas. Due to the elimination of the statistical delay time and a fast discharge current rise the quasi-stationary state was reached in 6 µs. The quasi-stationary state is characterized by an extremely high and pressure independent discharge current density of ∼10 A cm⁻² and stable Cu⁺ and Cu⁺⁺ emissions. Such fast discharge dynamics permits the magnetron cathode current to be driven with a pulse of duration of the order of a few µs, significantly shorter than in other devices. During this short time, the plasma does not have time to undergo the transition from the glow to the arc discharge even at the extremely high cathode loads met in our case. Different stages of the fast discharge development are identified and the composition of the magnetized plasma as a function of the pressure is discussed in detail.

This paper deals with the measurement of metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical–optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-γ emissions, and of NO density by LIF. A metastable density of the order of 10¹³ cm⁻³ was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O₂ and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Radial distributions of the parameters in oxygen plasmas generated by the slot antenna (µSLAN) type microwave plasma source (P = 1000 W at frequency 2.45 GHz) are obtained by the single Langmuir probe technique at a neutral gas pressure of 100 Pa. Results show that the electron temperature and the plasma potential have a weak dependence on the radius, except in the region close to the wall. The electron concentration strongly depends on the microwave electric field strength and it increases with the radius. Radial distributions of the plasma parameters are discussed on the basis of the numerical simulations of the electromagnetic field distribution in the discharge. A new type of filter system for a passive compensation of the effect of the microwave field on the probe characteristics is applied.

The anode region of a high intensity argon arc has been studied experimentally. As the cold ambient gas enters, the arc-anode attachment can appear in four different modes. Besides the well-known diffuse and constricted anode attachment modes, two transition modes have been observed, namely a lift-up mode and a multiple-attachment mode. The electron temperature and the electron density distributions for these modes have been obtained with a laser Thomson scattering system, which can probe the plasma 50 µm in front of the anode surface. The measurement results show good agreement with previous Langmuir probe results. The electric field distributions for these modes have been obtained for the first time by solving the charge continuity equation. The results indicate that the entrainment of the ambient cold gas first generates a ring in the fringes of the arc, which has low electron temperatures, low electron densities and an increased electric field strength accelerating electrons towards the anode. These characteristics encourage the electron overheating instability to develop, and lead to the formation of the multiple-attachment mode. One of the constricted multiple attachments grows and takes over most of the current as more ambient gas enters to form an anode jet and a constricted mode. Moreover, directly from the electric field distributions, the anode potential fall has been calculated. A negative anode fall exists in the centres of diffuse attachments, while a positive anode fall exists in the centres of constricted attachments.

Spatial structures of plasma parameters in a radio-frequency inductively coupled magnetic neutral loop discharge are investigated under various parameter variations using spatially resolved Langmuir probe measurements. A strong coupling between the plasma production region, in the neutral loop (NL) plane, and the axially remote substrate region is observed. The two regions are connected through the separatrices and therefore, spatial profiles in the substrate region are strongly influenced by the plasma production region and the structure of the separatrices. The electron temperature in the plasma production region peaks in the centre of the NL while the maximum in electron density is shifted radially inwards due to diffusion. Details of the structures in both regions, the production region and the substrate region, are determined through the position of the NL and the gradient of the inhomogeneous magnetic field around the NL confinement region. Parameter combinations are found providing higher plasma densities and better uniformity than in common inductively coupled plasmas without applying an additional magnetic field. The uniformity can be further improved using temporal variations of the magnetic field structure.

A plasma needle is a radio-frequency (rf) micro-discharge operated in a mixture of helium and air at atmospheric pressure. This source is designed for medical treatment of living tissues. Therapeutic effects of plasma treatment depend on generation of short-living active radicals: reactive oxygen and nitrogen species (ROS/RNS). In this work we determine the concentration of several ROS and RNS (atomic oxygen, nitrogen and hydroxyl radical) by means of threshold ionization mass spectrometry. It is shown that molecular oxygen and nitrogen are substantially dissociated in the plasma. Atomic nitrogen and oxygen are the most abundant radicals: the densities are on average few times 10⁻⁴. Hydroxyl radicals are less abundant (10⁻⁵ fraction of the total gas density). As expected, the densities of active species increase with increasing plasma power. Spatial (axial) distributions have been determined; the radical density reaches a maximum at 2.5 mm away from the rf powered electrode, and it decreases at distances larger than 3.5 mm. The amount of active radical species is reasonably high, which explains the effectiveness of plasma in bacterial inactivation and tissue treatment.

This paper presents a tutorial on the fluid modelling of the positive column of direct-current cylindrical glow discharges. The model writes the continuity and momentum-transfer equations for electrons and ions, coupled with Poisson's equation and the electron mean energy transport equations. The model is solved for helium at pressures p ≃ 0.1–10 Torr, gas temperature T_g = 500 K, discharge currents I_dc = 5–200 mA, and tube radius R = 1 cm. The electron transport parameters and rate coefficients are calculated using the local mean energy approximation in articulation with a two-term Boltzmann solver. Model equations are integrated from the discharge axis to the wall, passing through the space-charge sheath and monitoring the charge separation within it. The results obtained allow a systematic characterization of discharge energy deposition and space-charge sheath formation, as a function of pressure and axial current. A general good agreement is found between model results and experimental measurements available in the literature.

Analysis of Langmuir probe characteristics contains a paradox in that it is unknown a priori which theory is applicable before it is applied. Often theories are assumed to be correct when certain criteria are met although they may not validate the approach used. We have analysed the Langmuir probe data from cylindrical double and single probes acquired from a dc discharge plasma over a wide variety of conditions. This discharge contains a dual-temperature distribution and hence fitting a theoretically generated curve is impractical. To determine the densities, an examination of the current theories was necessary. For the conditions where the probe radius is the same order of magnitude as the Debye length, the gradient expected for orbital-motion limited (OML) is approximately the same as the radial-motion gradients. An analysis of the 'gradients' from the radial-motion theory was able to resolve the differences from the OML gradient value of two. The method was also able to determine whether radial or OML theories applied without knowledge of the electron temperature, or separation of the ion and electron contributions. Only the value of the space potential is necessary to determine the applicable theory.

A microwave torch is designed to produce fan-shaped plasma, which extends about 140 mm laterally. This torch produces an abundance of reactive atomic oxygen in the plasma effluent as evidenced by its emission spectroscopy. The results of the spectral intensity measurements show that the produced atomic oxygen outside the microwave cavity distributes quite uniformly over a width of about 80 mm and reaches out more than 10 mm. An experiment applying this plasma to kill Bacillus cereus contained in an envelope has been performed. The kill rate is presented.

Correlation of x-ray emission with pinch energy from a 1.5 kJ Mather-type plasma focus device for Ag and Sn inserts at the Cu tapered anode tip is reported. The space and time resolved x-ray emission characteristics are investigated by using a simple pinhole camera with appropriate filters and a multichannel pin-diode spectrometer. High voltage probe and Rogowski coil signals are used to estimate the pinch energy. At optimum conditions, the maximum x-ray yield in 4π-geometry is found to be 9 and 8 J/shot with efficiency of 0.6% and 0.5% for Sn and Ag inserted anodes. This is despite the fact that input energy converted to pinch energy is lower at 8% for Sn insert compared with 15% for the Ag insert. An increase in x-ray yield with an increase in pinch energy is observed for Sn as well as Ag. Pinhole images reveal that x-rays of energy less than 5 keV are emitted from the focus region and the high-energy x-rays are emanated from the anode tip.

Helicon discharges characteristically exhibit a sharp jump from low to high density as the radiofrequency power is raised. This is usually explained by the transition from an inductively coupled plasma mode to a helicon mode when the dispersion relation for helicon wave propagation is satisfied at a critical power or magnetic field. Experiments have suggested a different mechanism for the sudden jump, a mechanism that depends on overcoming the parasitic circuit losses. This effect is analysed computationally, and agreement with measurements is obtained.

In the present paper, O-atom kinetics is investigated in low pressure (around 0.7 Pa) O₂/hexamethyldisiloxane pulsed plasmas by time-resolved optical emission spectroscopy. In order to distinguish direct O-atom excitation by electron impact from dissociative excitation of O₂, CO, CO₂ and H₂O molecules formed in O₂/HMDSO plasmas, measurements were first carried out in pure O₂, CO₂ and H₂O plasmas. From these measurements it was concluded that direct O-atom excitation by electron impact was the dominant mechanism and that actinometry could be used to monitor O-atom kinetics provided molecular oxygen dissociative excitation is taken into account. Second, the surface O-atom loss coefficient measured in oxygen pulsed plasmas is in agreement with the literature (10⁻¹) and is five times higher than the one measured in CO₂ pulsed plasma (2 × 10⁻²). This latter result is attributed to competition effect at surface sites between CO and O. Finally, we point out the influence of water molecule wall adsorption on O-atom kinetics in H₂O and O₂/HMDSO pulsed plasmas. During the post-discharge, H₂O and/or OH molecules adsorbed at the reactor walls strongly reduce the O-atom loss coefficient. During the discharge, ion bombardment enhances desorption of molecules leading to an increase in the O-atom loss coefficient.

Imaging Laser Absorption Spectroscopy (ILAS) was performed on a metal-halide lamp under hyper-gravity conditions in a centrifuge (acceleration ranging from 1 to 10g). Diffusive and convective processes in the arc discharge lamp cause an unwanted non-uniform distribution of the radiating metal additive, which results in colour separation. Convection is induced by gravity, and measuring under different apparent gravity conditions aids the understanding of the flow processes in the lamp. The centrifuge was built to investigate the lamp under varying apparent gravity conditions. The metal additive density distribution in the lamp is measured by ILAS. In this novel diagnostic technique the laser beam is expanded, so the absorption in the complete plasma volume is imaged simultaneously.

The reflection of an electromagnetic wave in a thin plasma layer attached to a metal plate at high pressure is investigated with the finite-difference time-domain method. The effects of the plasma thickness, the plasma density distribution function, the collision frequency between electrons and neutrals and the frequency of incident wave on the reflection coefficient of the electromagnetic wave are discussed. Numerical results indicate that the reflection coefficient of the wave depends on its frequency, the plasma thickness, the plasma density distribution and the collision frequency. The reflection coefficient is low only at the low band of the calculated frequency for different plasma distribution functions if the plasma layer is very thin, e.g. 10 mm. Plasmas with an excess of 20 mm for a high collision frequency such as 103 GHz are capable of absorbing microwave radiation over a wider frequency range for different plasma distributions.

Numerical simulations and experimental studies were conducted to characterize direct current (dc) hydrogen discharge for a pin plate electrode configuration having an inter-electrode separation distance of 400 µm. A self-consistent two-dimensional hybrid model was developed to simulate the atmospheric pressure dc hydrogen micro-discharges. The discharge simulation model considered consists of momentum and energy conservation equations for a multi-component gas mixture, conservation equations for each component of the mixture (electrons, ions, excited species and neutrals) and state relations. The model uses a drift–diffusion approximation for the electron and the ion fluxes. The species considered include H, H₂, H⁺, , , , H_{2 v=1} and the electrons. The electric field is obtained from the solution of Poisson's equation. Numerical simulations and experimental measurements indicated some of the key features of a normal glow discharge: flat voltage–current characteristics and constant cathode current density. Basic plasma properties such as electron number density, gas temperature, electric field and electron temperature were studied. The model predicted a constant current density of ∼22 A cm⁻² in the normal glow regime. The normal current density was found to be a temperature scaled value of a low pressure normal current density. The ion Joule heating and Frank–Condon heating were found to be the dominant gas heating mechanisms. The peak gas temperature of ∼500 K indicated the discharge to be a non-thermal non-equilibrium discharge. Predictions from the model compares favorably well with the experimental measurements.

In this work the authors present Fabry–Pérot measurements of Ar atom spectral line profiles originating from the expansion of an Ar–N₂ thermal plasma into a low pressure environment. The plasma is characterized by strong density, temperature and velocity gradients and it offers a high degree of light absorption for some gas mixtures. First, it is demonstrated that the Abel inversion method cannot be applied under such conditions even if absorption is absent. Second, two approaches are examined to obtain plasma parameters in the case of an optically thick expanding plasma. It is shown that velocity and temperature development along the jet radius can be inferred from experimental lineshapes when radiation production and absorption are taken into account in the core and in the surroundings of the jet.

This paper discusses the mechanisms of gas breakdown at low values of pressure and inter-electrode gap, i.e. in the vicinity of the Paschen minimum. In this area of pressure and inter-electrode gap values, breakdown occurs either through gas or vacuum mechanisms, and also the so called anomalous Paschen effect appears. Electrical breakdown of electropositive, electronegative and noble gases has been investigated theoretically, experimentally and numerically. Based on the results obtained, regions in which particular breakdown mechanisms appear have been demarcated. Special attention has been devoted to the anomalous Paschen effect as well as to the avalanche vacuum breakdown mechanism.

This paper presents formulation of computationally efficient models of photoionization produced by non-thermal gas discharges in air based on three-group Eddington and improved Eddington (SP₃) approximations to the radiative transfer equation, and on effective representation of the classic integral model for photoionization in air developed by Zheleznyak et al (1982) by a set of three Helmholtz differential equations. The reported formulations represent extensions of ideas advanced recently by Ségur et al (2006) and Luque et al (2007), and allow fast and accurate solution of photoionization problems at different air pressures for the range , where is the partial pressure of molecular oxygen in air in units of Torr ( at atmospheric pressure) and R in cm is an effective geometrical size of the physical system of interest. The presented formulations can be extended to other gases and gas mixtures subject to availability of related emission, absorption and photoionization coefficients. The validity of the developed models is demonstrated by performing direct comparisons of the results from these models and results obtained from the classic integral model. Specific validation comparisons are presented for a set of artificial sources of photoionizing radiation with different Gaussian dimensions, and for a realistic problem involving development of a double-headed streamer at ground pressure. The reported results demonstrate the importance of accurate definition of the boundary conditions for the photoionization production rate for the solution of second order partial differential equations involved in the Eddington, SP₃ and the Helmholtz formulations. The specific algorithms derived from the classic photoionization model of Zheleznyak et al (1982), allowing accurate calculations of boundary conditions for differential equations involved in all three new models described in this paper, are presented. It is noted that the accurate formulation of boundary conditions represents an important task needed for a successful extension of the proposed formulations to two- and three-dimensional physical systems with obstacles of complex geometry (i.e. electrodes, dust particles, aerosols, etc), which are opaque for the photoionizing UV photons.

As was announced at the beginning of the year, Professor Noah Hershkowitz is soon to step down as Editor-in-Chief of Plasma Sources Science and Technology (PSST). As Founding Editor, Professor Hershkowitz has overseen the development of the journal from its initial inception in 1990 and foundation two years later, to the well established and internationally recognised publication it is today. It is a tribute to his dedication to the journal, and to the community that it serves, that this ambitious project has been such a success, with the journal receiving its highest ever impact factor this year. Professor Hershkowitz will remain actively involved with PSST as an Associate Editor and will undoubtedly continue to contribute his unique knowledge and expertise to the journal for many years to come. We look forward to working with Noah in his new role and would like to thank him for his dedication, guidance, hard work and patience working with the editorial team here in Bristol during his time as Editor-in-Chief. Thank You Noah.

We are delighted to announce that Professor Hershkowitz will be handing over the position of Editor-in-Chief to the very capable hands of Professor Mark J Kushner who has had a long-standing involvement with PSST as an Editorial Board member. Professor Kushner has been very active in working with Noah and the rest of the team to manage the growth and development of the journal. Also joining Mark and Noah on the senior Editorial Team will be Professor M C M (Richard) Van de Sanden and Professor W G (Bill) Graham who have agreed to serve as Associate Editors. We look forward to working with this new team and would like to take this opportunity to welcome Mark, Richard, Bill and Noah to their new roles.

PSST Publishing Team: Nicola Gulley (Group Publisher), Caroline Wilkinson, Adrian Corrigan, Kathryn Anslow and Christopher Mooney