Table of contents

In this review we discuss the current status of the physics of charged particle swarms, mainly electrons. The whole field is analysed mainly through its relationship to plasma modelling and illustrated by some recent examples developed mainly by our group. The measurements of the swarm coefficients and the availability of the data are briefly discussed. More time is devoted to the development of complete electron–molecule cross section sets along with recent examples such as NO, CF₄ and HBr. We extend the discussion to the availability of ion and fast neutral data and how swarm experiments may serve to provide new data. As a point where new insight into the kinetics of charge particle transport is provided, the role of kinetic phenomena is discussed and recent examples are listed. We focus here on giving two examples on how non-conservative processes make dramatic effects in transport, the negative absolute mobility and the negative differential conductivity for positrons in argon. Finally we discuss the applicability of swarm data in plasma modelling and the relationship to other fields where swarm experiments and analysis make significant contributions.

An overview of the general phenomenology and physical mechanism of large-scale electrical discharges termed 'sprites' observed at high altitude in the Earth's atmosphere above thunderstorms is presented. The primary emphasis is placed on summarizing available experimental data on various emissions documented to date from sprites and interpretation of these emissions in the context of similar data obtained from laboratory discharges, in particular the pulsed corona discharges, which are believed to be the closest pressure-scaled laboratory analogue of sprite discharges at high altitude. We also review some of the recent results on modelling of laboratory and sprite streamers emphasizing the importance of the photoionization effects for the understanding of the observed morphological features of streamers at different pressures in air and provide a comparison of emissions obtained from streamer models with results of recent satellite-based observations of sprites.

State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

The present paper deals with the determination of discharge parameters using N₂(C ³Π_u, v) populations deduced from 2.PG emission spectra, focusing on the influence of N₂(C ³Π_u, v) collision rate coefficients on these determinations. In particular it is shown that the new set of quenching and vibrational relaxation rate coefficients of N₂(C ³Π_u, v = 0–4) vibronic levels recently measured by optical–optical double resonance laser induced fluorescence (LIF) have a large effect on discharge parameter determination in high-pressure discharges. In the present paper we explore this effect, evidencing the differences with respect to the old data set case, in both simulated and real cases of N₂(C ³Π_u, v) vibrational distributions measured at high pressure in a dielectric barrier discharge. Finally we point out the improved potentiality of 2.PG spectroscopy as a diagnostic technique: with the new rate coefficients, and measurement of the N₂(C ³Π_u, v) distribution up to at least v = 3, it is possible to have a quasi-independent evaluation of the electron temperature and of the first level vibrational temperature of the N₂ ground state.

Emission spectroscopy was used to investigate the Balmer-α line broadening in a slot-antenna excited microwave plasma source operating in helium–hydrogen and argon–hydrogen mixtures, and in pure hydrogen at low pressure (p = 0.3 mbar). No evidence was found for excessive broadening of the H_α line under the present operating conditions, even though this line was found to be broader than the helium singlet line at 667.8 nm. The Doppler temperatures corresponding to this helium line (∼650 K) are close to the rotational temperatures (∼500 K) determined from the Q-branch of the Fulcher-α band under the same conditions. The Doppler temperatures corresponding to the Balmer-α broadening are about 3000 K. The H_α line shape does not change as the amount of hydrogen (up to 50%) in the He–H₂ mixture is changed. The results indicate selective heating of H atoms in low-pressure He–H₂, Ar–H₂ and H₂ microwave plasmas.

Magnetic fields are sometimes used to confine the plasma in low-pressure low-temperature gas discharges, for example in magnetron discharges, Hall-effect-thruster discharges, electron-cyclotron-resonance discharges and helicon discharges. We discuss how these magnetized discharges can be modelled by two-dimensional self-consistent models based on electron fluid equations. The magnetized electron flux is described by an anisotropic drift–diffusion equation, where the electron mobility is much smaller perpendicular to the magnetic field than parallel to it. The electric potential is calculated either from Poisson's equation or from the electron equations, assuming quasineutrality. Although these models involve many assumptions, they are appropriate to study the main effects of the magnetic field on the charged particle transport and space charge electric fields in realistic two-dimensional discharge configurations. We demonstrate by new results that these models reproduce known phenomena such as the establishment of the Boltzmann relation along magnetic field lines, the penetration of perpendicular applied electric fields into the plasma bulk and the decrease in magnetic confinement by short-circuit wall currents. We also present an original method to prevent numerical errors arising from the extreme anisotropy of the electron mobility, which tend to invalidate model results from standard numerical methods.

The properties of a barrier discharge in nitrogen near the transition from the Townsend mode to the filamentary mode are studied on the basis of a two-dimensional fluid model. The formation of an intermediate mode (multipeak Townsend discharge) is discussed. The surface processes (ion–electron emission and photoemission) and the positive volume charge are proposed as the mechanisms of formation of a multipeak mode. It is shown that the Townsend mode is stable relative to radial fluctuations, whereas the glow mode is unstable and turns into a filamentary mode. The development of a radial fluctuation into a filament is demonstrated. The homogeneity of the barrier discharge depends on the barrier material. In particular, the widening of the stability region for the discharge with low-permittivity barriers is proved.

Some general aspects of the sheath structure in electronegative plasmas are analysed, through an extensive review of the authors' contributions to this subject. An analysis of the positive ion flux as a function of the electric potential, in the region in which neutrality still holds, is shown to be useful to establish the structure of both the sheath and the presheath. Depending on the plasma parameters, the sheath can show the structure of a triple layer in which a region with a net positive charge is followed by a region with net negative charge and finally by the positive ion sheath. However, the electric potential distribution is always a monotonic function in the sheath. In the parameter space region in which the presheath becomes stratified, a separate analysis of the sheath fails to give the correct positive ion flux in the sheath. In any case, the analysis of the sheath gives either the exact value for the positive ion flux or a good approximation and, therefore, a separate analysis of the sheath is also useful in the case of electronegative plasmas. Finally, a criterion to establish when the presheath–sheath structure is governed by the negative ions instead of by the electrons is introduced. This criterion determines when the main properties of the sheath, such as the sheath thickness or the floating potential, begin to be governed by the negative ions.

We have studied charged particle densities and fluxes in a customized industrial etch reactor, running in Ar/O₂/c-C₄F₈ gas mixtures at pressures in the region of 50 mTorr and driven by 2 and 27 MHz RF power, either separately or simultaneously. Independent control of ion flux and ion energy is the aim of using dual frequency plasmas. However, little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and they may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator and the positive ion flux with a RF biased ion flux probe. The ratio of these two quantities, which depends on the negative ion fractions and other factors, is seen to vary strongly with gas chemistry, giving evidence for the presence of negative ions. Our results indicate high electronegativity for high c-C₄F₈ flow rates. We have also examined the effect of varying the 2 and 27.12 MHz RF powers on both the electron density and the positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition, ultra-violet cavity ring-down spectroscopy was used to measure the F⁻ density directly (Booth et al 2006 Appl. Phys. Lett.88 151502). This optical measurement was compared with the probe technique.

The identification of growth precursors in reactive plasmas is of major interest for any optimization of plasma processes. Various diagnostics might be employed to detect important radicals in plasma chemistry. However, the species which produce the highest signal in a specific discharge might not be relevant for film growth since they are not efficiently pumped by the substrate surfaces. This paper describes concepts to characterize the plasma chemistry using reactor parameters, which might be compared with diagnostics such as mass spectrometry. In addition, relevant methods to study surface processes such as the roughness evolution during thin film growth are illustrated. The search for growth precursors and material synthesis is illustrated for two examples: (i) the formation of nanoparticles in low pressure reactive plasmas; (ii) the generation of higher hydrocarbons in microplasmas operated at atmospheric pressure.

We have described that an optical evanescent wave, which is an optical near field appearing on a plane dielectric surface, is very valuable for applications of plasma diagnostics in the vicinity of a dielectric surface. First we developed techniques such as evanescent laser-induced fluorescence and laser-induced evanescent-mode fluorescence (LIEF) in order to quantitatively measure the density of the lowest metastable atoms of Ar in the vicinity of the barrier surface. Recent research works using laser evanescent wave spectroscopy have been further studied. It is found that a cross-beam type LIEF and waveguide type laser absorption technique make it possible to observe the surface dynamics and directly measure the surface density of metastables. In addition, we develop an optical technique using an electro-optic crystal as a dielectric barrier in order to observe spatial and temporal changes of a wall voltage due to accumulated charges. This technique is applied to clarify the surface charges of plasma display panel-like microdischarges.

The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 µs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N₂(X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N₂(A) for the NO formation shows good agreement.

A model is used to study the afterglow of a flowing microwave discharge at ω/(2π) = 2450 MHz, p = 667 Pa (5 Torr), in the mixture N₂–xO₂, with x = 0.7–7% of O₂. This model considers a self-consistent kinetic description of the discharge and early-afterglow regions, followed by a 3D hydrodynamic analysis of the post-discharge chamber. The behaviour of NO(B) molecules and O(³P) atoms is discussed in detail, since these two species play an important role in the sterilization process, respectively, due to the UV emission associated with the NO_β bands and due to erosion effects. The present work shows that a maximum in the UV emission intensity from NO_β occurs in the range 0.7–2% of O₂ added to the mixture, which is in agreement with the survival curves of spores presented by Philip et al (2002 IEEE Trans. Plasma Sci.30 1429). In general, the oxygen atoms concentration is more important as the added O₂ percentage increases. The interplay of N(⁴S), O(³P), NO(X), N₂(X, v) and NO(B) species in the overall kinetics both in the discharge and in the early-afterglow region is discussed. Particular attention is devoted to the density of NO(B) and O(³P) in the sterilization vessel at different spatial planes and for various mixture compositions.

The paper discusses the deposition of protective coatings ranging from organosilicon plasma polymers to SiO₂-like films and hard diamond-like carbon/silicon oxide (DLC : SiO_x) coatings in radio frequency capacitively coupled discharges using hexamethyldisiloxane (HMDSO). As a result of the optimization of the deposition conditions it was possible to obtain high performance protective coatings. In the HMDSO/O₂ mixture, it was shown that rather than the SiO₂-like film a hard cross-linked SiO_xC_yH_z polymer film can be used as a protective coating for polycarbonate. The optimum conditions for the deposition of an almost stress-free film were 17% of HMDSO and dc bias voltage of −240 V. The film hardness and elastic modulus were 10 GPa and 75 GPa, respectively. The refractive index at 600 nm was 1.5 and the extinction coefficient decreased from 0.02 at 240 nm down to zero at 600 nm.

The films deposited from HMDSO/CH₄ and HMDSO/CH₄/H₂ mixtures exhibited the attractive properties of DLC films with the partial elimination of some of their drawbacks, such as absorption in the visible and a high intrinsic stress. The optimum concentration of the HMDSO was approximately 21%. Under these conditions the concentration of SiO_x in the films was approximately 9 at.%. The film hardness and elastic modulus were above 22 GPa and 120 GPa, respectively.

This work presents an experimental study of copper cathode erosion in magnetically driven arcs as a function of electrode temperature. The experiments were carried out in air at atmospheric pressure. A critical temperature of about 500–600 K for the transition from micro- to macro-erosion was found for magnetic fields in the range 0.01–0.35 T. We show that the electrode temperature is the main parameter that determines erosion, especially in the macro-erosion regime when it strongly increases with temperature. The obtained experimental data can be represented as an exponential function of the time for the electrode surface to reach the fusion temperature underneath the arc root.

Plasma scaling up can be achieved by distributing elementary microwave plasma sources over two or tri-dimensional networks. This concept is applied to a planar reactor comprising 4 × 3 microwave plasma sources distributed according to a square lattice matrix configuration. In each elementary plasma source, the plasma is produced at the end of a coaxial applicator implemented perpendicularly to the planar source. An argon plasma can be sustained in the medium pressure range from 7.5 to 750 Pa. The sheet of plasma thus obtained becomes uniform at a distance from the source plane of 15 mm, i.e. less than half the 40 mm lattice mesh. Using a cylindrical Langmuir probe, plasma density and electron temperature have been investigated as functions of pressure and microwave power. Results show that the plasma can reach densities between 10¹² and 10¹³ cm⁻³ with a uniformity better than ±3.5%.

The thermal behaviour of Hall effect thrusters was investigated by means of calibrated infrared thermal imaging performed in the 8–9 µm spectral domain. Study on the variation of the steady state temperature of Hall thruster elements like discharge chamber (channel) walls and anodes along with discharge voltage and propellant (xenon) mass flow rate confirms that energy loss mechanisms, which are responsible for the heating of the thrusters, are a direct consequence of interactions between charged particles and surfaces. In order to obtain new insights into plasma surface interactions inside a thruster, the channel wall temperature was monitored over a broad range of electrical power stretching from 400 W to 5.5 kW for three types of thrusters with different designs, dimensions and operation domains, namely SPT100-ML, PPS ® 1350-G and PPSX000-ML. Note that over the range of thruster operating conditions the facility backpressure varies from 10⁻⁵ to 6 × 10⁻⁵ mbar. In addition, the effect of discharge chamber wall material on temperature field was also investigated using dielectric BN–SiO₂ and AlN walls as well as conducting graphite walls. For a given thruster geometry and material, a simple relationship between the mean wall temperature and the input electrical power can be established, in contradiction to the complex dynamics of such a magnetized plasma medium. Besides, thruster thermal history and degree of wear do not have a strong impact on power losses inside the channel.

The self–pulsing regime of a microhollow cathode discharge in argon is reported. The plasma is generated inside the hole drilled in an anode–dielectric–cathode device. The hole dimension ranges from 200 to 400 µm and the gas pressure ranges from 40 to 200 Torr. It is shown by optical spectroscopy and fast CCD imaging that the current pulse is related to a fast expansion of the plasma outside the microhole on the cathode backside. The pulse current duration ranges from 0.4 to 2 µs depending on the gas pressure. The self-pulsing regime occurs at medium current range (0.1–1 mA). At lower current the discharge is steady and the plasma is confined inside the hole (abnormal regime); at higher current, the plasma is steady and the plasma expands outside the hole on the cathode backside. The self-pulsing frequency is a linear function of the averaged discharge current and decreases with the device capacitance. The dependence of the self-pulsing characteristics (frequency, light emission, power deposition, etc) on the gas pressure follows a Paschen-like law; this is interpreted in considering that the fast expansion of the plasma outside the hole is similar to a gas breakdown. A simple electrical model, using a bistable voltage-controlled variable resistor to simulate the evolution of the plasma impedance, provides qualitative results in good agreement with the experiments.

Steady-state modelling and experimental results are given on the electric arc attachment on cold carbon cathodes working at low pressure. The modelling results are compared with the case of copper cathodes and with experimental data on vacuum arc erosion characteristics for graphite materials. A region of existence of a physically meaningful solution for self-sustained operation of the steady-state cathode spot is given in the electron temperature–cathode spot plasma pressure space. A solution domain comprised between T_e ≈ 1.2–1.5 eV and p≈ 2–45 atm corresponding to carbon surface temperatures in the range 4200–4900 K is found. Values of the local heat flux to the cathode surface are evaluated in the range 1–20 × 10¹⁰ W m⁻², and ratios of the various contributions to this flux and current density are given. Also given are the cathode spot radii and upper/lower limits for the erosion rate through vapourization, these being compared with experimental data. It is shown that the cathode spot pressure conditions can provide a mechanism for the control of macroparticle emission on carbon. This effect is used experimentally through cathode spot plasma confinement for the reduction of the microdroplet emission in arc sources used for diamondlike film deposition. Experimental data obtained on graphite materials are in agreement with the model-based design guidelines.

The chemistry associated with atmospheric dielectric barrier discharges (DBDs) in air has been studied. Laboratory and industrial DBD systems have been investigated. In this work, we have emphasized the use of aqueous extractions of treated surfaces, followed by analyses by ion chromatography to study the DBD chemistry. A range of DBD factors including dose, humidity, airflow and electrode configuration (one versus two dielectric barriers) is found to influence the levels of acids, notably nitrous, nitric and oxalic acids, on the treated surfaces. A mechanistic rationale involving the primary formation of the nitrous and nitric acids in the gas phase is proposed.

Time transients of the number densities of Rb in the ground electronic state were measured in a positive column of moderate-current hydrogen-pulsed discharge. Atomic rubidium was formed by dissociation of RbCl. The population of Rb was monitored by high resolution laser absorption. The laser was tuned to one of the D₂ transitions near 780 nm. Experiments were done at hydrogen number densities from 6.4 × 10¹⁵ to 9.6 × 10¹⁶ cm⁻³ and current densities ranging from 0.1 to 0.5 A cm⁻². Number densities of RbCl varied from 4.0 × 10⁵ to 3.0 × 10⁷ cm⁻³. The kinetic model in the form of rate equations reproduces closely measured time transients of the Rb population. The model assumes that the primary mechanism for the production of atomic rubidium is the chemical reaction RbCl + H → Rb + HCl.

In this paper the results of theoretical and experimental studies of the component content of active particles formed in a plasma-chemical reactor composed of a multiple-cell generator of active particles, based on volume barrier discharge, and a working chamber are presented. For calculation of the content of uncharged plasma components an approach is proposed which is based on averaging of the power introduced over the entire volume. Advantages of such an approach lie in an absence of fitting parameters, such as the dimensions of microdischarges, their surface density and rate of breakdown. The calculation and the experiment were accomplished with the use of dry air (20% relative humidity) as the plasma generating medium. Concentrations of O₃, HNO₃, HNO₂, N₂ O₅ and NO₃ were measured experimentally in the discharge volume and working chamber for the residence time of particles on a discharge of 0.3 s and more and discharge specific power of 1.5 W cm⁻³. It has been determined that the best agreement between the calculation and the experiment occurs at calculated gas medium temperatures in the discharge plasma of about 400–425 K, which correspond to the experimentally measured rotational temperature of nitrogen. In most cases the calculated concentrations of O₃, HNO₃, HNO₂, N₂O₅ and NO₃ for the barrier discharge and the working chamber are in fairly good agreement with the respective measured values.

We present both the theoretical analysis and proof-of-principle experimental results of a novel transmission-line microwave interferometer for measurements of plasma electron density. The principle of this technique is the same as conventional microwave interferometers except that the sensing microwave propagates along a transmission-line structure. For this study, the transmission-line is a circular coaxial dielectric waveguide operated at 2.4 GHz. A microwave module consisting of a microwave source and a phase detector has also been developed for detecting the phase of the microwave propagating through the transmission-line. Good agreement of phase measurements between the microwave module and a microwave network analyser has been demonstrated. The electron density measured by the interferometer is also consistent with the results from a Langmuir probe.

Optimization of the working conditions of the magnetoplasma compressor (MPC) has been performed through analysing discharge and compression plasma flow parameters in hydrogen, nitrogen and argon at different pressures. Energy conversion rate, volt–ampere curve exponent and plasma flow velocities have been studied to optimize the efficiency of energy transfer from the supply source to the plasma. It has been found that the most effective energy transfer from the supply to the plasma is in hydrogen as a working gas at 1000 Pa pressure. It was found that the accelerating regime exists for hydrogen up to 3000 Pa pressures, in nitrogen up to 2000 Pa and in argon up to 1000 Pa pressure. At higher pressures MPC in all the gases works in the decelerating regime. At pressures lower than 200 Pa, high cathode erosion is observed. MPC plasma flow parameter optimization is very important because this plasma accelerating system may be of special interest for solid surface modification and other technology applications.

In this work, the microstructure transition from amorphous to microcrystalline silicon is defined in terms of the silane concentration in the plasma as opposed to the silane concentration in the input gas flow. In situ Fourier transform infrared absorption spectroscopy combined with ex situ Raman spectroscopy has been used to calibrate and validate this approach. Results show that a relevant parameter to obtain μc-Si : H from SiH₄/H₂ mixtures is the plasma composition, which is determined not only by the gas dilution ratio but also by the silane depletion fraction. It is also shown that μc-Si : H can only be deposited efficiently, in terms of gas utilization, at a high rate by using high input concentration and depletion of silane.

A particle-in-cell simulation with Monte Carlo collisions was developed to study ion extraction from a capacitively-coupled argon plasma through a grid. A one-dimensional simulation of the plasma reactor was coupled with a two-dimensional simulation of the sheath region over a grid hole. The 13.56 MHz power applied to the plasma was pulsed 50 µs ON and 50 µs OFF. A dc bias voltage was applied in the afterglow (power OFF fraction of the cycle), to raise the plasma potential and expel ions out of the plasma through the grid. The electron temperature decayed rapidly in the afterglow, minimizing spatial variations of the plasma potential and allowing a nearly mono-energetic ion beam to be extracted. Due to the absence of plasma moulding over the grid holes, the beam was highly directional with an angular spread decreasing with increasing dc bias. The applied dc bias also set the beam energy. Simulation predictions were in good quantitative agreement with measurements. The ion beam could be neutralized by grazing angle collisions on a set of parallel neutralization plates downstream of the extraction grid, to produce a highly directional and nearly mono-energetic neutral beam.

Finite-difference-time-domain arithmetic is applied to simulate the propagation of an electromagnetic (EM) wave in a two-dimensional atmospheric pressure plasma (APP) and a metal layer with strong electron-neutral collisions. The dependences of the EM wave attenuation on the parameters of the APP are provided. The two-dimensional numerical results indicate that when the profile of the electron density is given, the attenuation of an EM wave in APP is strongly affected by (a) the polarization mode (TM mode or TE mode), (b) the incident angle of the EM wave, (c) the EM wave frequency, (d) the width of the plasma layer, and (e) the collision frequency between electrons and neutrals. In this paper, the behaviour of the propagation of an EM wave inside the plasma layer is explained by the principle of wave interference. The relationship between the attenuation property and the above-mentioned parameters is also studied.

It is well known that propylene plays a very important role in decreasing the energy consumption of plasma abatement of flue gas containing NO_x, but how to further improve the utilization efficiency of C₃H₆ needs to be studied. This paper presents an improved wire-plate type discharge reactor for plasma remediation of flue gas of utility or boiler facilities, in which nozzle electrodes for injection of additives such as C₃H₆ and NH₃ were installed in the front of wire electrodes. This paper mainly focused on the study of advancing initiation of C₃H₆ oxidation chemistry and thus NO abatement by a pulsed and dc corona radical shower. The experimental results indicate that the NO removal rate was increased by over 10% with a pulsed corona radical shower; a dc corona C₃H₆ radical shower could increase the NO removal rate by 7%–24% under an energy-cost of 0.09–0.355 Wh Nm⁻³. Water vapour or ammonia injection decreased the NO abatement with the corona C₃H₆ radical shower, but the NO_x removal rate was increased.

A large area pulsed electron beam is produced by a high-voltage barrier discharge. We compare the properties of the x-rays generated by stopping this beam of electrons in a thin metal foil with those generated by stopping the electrons directly in various gases. The generation of x-rays was investigated in He, Ne and Ar with and without an Al foil in the discharge chamber. It appears that, for voltages of up to 15 kV used in our experiments, x-rays are produced by the 'bremsstrahlung' mechanism and that characteristic x-ray radiation does not play an important role. The x-ray intensity strongly depends on the parameters of the electron beam (electron energy and current density) and the stopping material properties (Z-number). The energy of the x-ray photons is comparable to the applied voltage. The highest obtained energy in the x-ray spectrum depends on the electron energy (∼10 keV in the investigated case) and the lowest energy is determined by the transmittance of the output window and the window of the detector.

This paper presents a 4-phase radiative plasma focus model, where the dynamics of the current sheath is represented using Lee's model. The model is based on the snowplow model in the axial phase and the slug model in the radial phase, complemented with sensible estimations made for the plasma parameters. The x-ray emission characteristics are investigated using a corona plasma equilibrium model. A refinement to the code was made, firstly by taking into account the tapering of the anode in the axial phase and secondly by including the energy loss due to recombination radiation in the slow compression (radiative) phase. Our improved code was calibrated for the NX2, a 3 kJ plasma focus device, operated in neon at a pressure range of 4–7 mbar with a tapered copper anode. An additional macro was programmed to the code in order to automate the curve fitting of the simulated current traces with those obtained experimentally. The resulting theoretical x-ray yield predictions are compared against experimental data, showing good agreement in terms of pressure dependence trends. The model, however, appears to consistently underestimate the absolute x-ray yield when compared with the experimentally obtained values.

Shock adiabats and exact stationary solutions of non-viscous one-dimensional gas dynamic equation for a gas with sustained steady-state nonequilibrium are constructed. Shock waves and a solitary pulse are investigated in negative-dispersion nonequilibrium media.

The suitability of the electrical discharge technique for application in plasma-induced ignition and plasma-assisted combustion in high-speed flow is reviewed. Nonequilibrium, unsteady and nonuniform modes are under analysis to demonstrate the advantage of such a technique over heating. A reduction in the required power deposition is possible due to unsteady operation and non-homogeneous spatial distribution. Mixing intensification in non-premixed flow could be achieved by nonuniform electrical discharges. The scheme of fuel ignition behind the wallstep and in the cavity is under consideration. Experimental results on multi-electrode discharge maintenance in the separation zone of supersonic flow are presented. The model test on hydrogen and ethylene ignition is demonstrated at direct fuel injection. An energetic threshold of fuel ignition under separation and in the shear layer is measured under the experimental conditions.

At Laboratoire d'Aérothermique low pressure stationary arc jets are used to simulate the plasma flow around a space probe during a hypersonic entry into a planetary atmosphere. This paper focuses on the experimental investigation of the species present in the flow produced at the exit of the supersonic nozzle in CO₂–N₂ plasma mixtures similar to the Mars atmosphere. The thermal nonequilibrium of the internal modes of some radicals is examined together with the electron density and temperature.

An experimental study of plasma assisted combustion has been initiated at ONERA in order to evaluate the potential application to supersonic combustion with various fuels. For this first step, we choose an easier and more versatile configuration where a coaxial dielectric barrier discharge is applied to a methane diffusion flame. The effects of this discharge on the flame structure and especially the reduction in its detachment height have been investigated. The role of the large scale motion of the flame on its anchoring by the plasma has then been revealed, along with the presence of a plasma channel linking the flame to the burner. Electrical and optical investigations demonstrated the pulsed nature of the light emission of the plasma and the spectroscopic study of OH, CH and exhibits a 10 mm overlap between the plasma and the flame.

Experimental and modelling studies of ion formation during combustion of propane/air mixtures are presented. The positive and negative ions mass/charge spectra in propane/air stoichiometric flame at atmospheric pressure are recorded in the range from 0 to 512 atomic mass units. The C₂H₃O⁺ and ions are found to be the most abundant ionic species in the flame front region. By increasing the distance from the flame front the ion composition changes significantly. In the burnt gas region the H₃O⁺, NO⁺, , ions are found to be the major charged species. To explain the experimental results the extended kinetic model describing the ion formation in flame and in the extraction system of the mass-spectrometer as well as ion–soot interaction is developed. It is shown that the ionic clusters, which are observed experimentally, form during the adiabatic expansion in the extraction system, and the presence of soot particles may change the total positive and negative ion concentrations in the gas phase.

The surfaguide is a waveguide-based electromagnetic-surface-wave launcher that allows sustaining long plasma columns using microwaves. Its electrodynamic characteristics are examined experimentally and theoretically in the perspective of achieving an efficient plasma source without any need for impedance matching retuning as operating conditions are varied over a broad range. The plasma source design and its modelling using equivalent-circuit theory are described and a simple procedure is provided to determine the optimum dimensions of the surfaguide that maximize the transfer of microwave power to plasma. As an example, with an optimized surfaguide, the reflected power in an N₂ discharge at atmospheric pressure stays below 3% for powers in the 2–6 kW range and gas flow rates in the 30–150 l min⁻¹ domain under varying concentrations (< 2%) of admixed gases such as SF₆, O₂ and argon.

A new compact plasma torch associated with a resonance power supply allows the generation of low power discharges (typically 100 W–1 kW) under high voltage (>1 kV) low current (<1 A) conditions. The resonance power supply allows continuous control of the discharge current, which is a major improvement with respect to the traditional dc power source based on a high voltage transformer. In addition, this system is characterized by a high conversion efficiency that is crucial when it comes to industrial applications. It has been shown that different regimes ranging from streamer over gliding arc to continuous discharges were obtained depending on the operating conditions. The objective of this paper is a better understanding of the different observed behaviour through the determination of the main torch and power supply parameters.

The spatial distribution of neutral gas temperature and total pressure have been measured for pure N₂, He/5%N₂ and Ar/5%N₂ in an inductively coupled plasma (ICP) reactor, and a significant rise in the neutral gas temperature has been observed. When thermal transpiration is used to correct total pressure measurements, the total pressure remains constant regardless of the plasma condition. Neutral pressure is depleted due to the pressure balance when the plasma pressure (mainly electron pressure) becomes comparable to the neutral pressure in high density plasma. Since the neutral gas follows the ideal gas law, the neutral gas density profile was obtained from the neutral gas temperature and the corrected neutral pressure measurements. The results show that the neutral gas density at the centre of the plasma chamber (factor of 2–4 ×) decreases significantly in the presence of a plasma discharge. Significant spatial variation in neutral gas uniformity occurs in such plasmas due to neutral gas heating and pressure balance.

Low-current (∼250 mA), high-voltage (∼700 V), dc discharges are observed to operate in air at atmospheric pressure when a closed loop is included for current regulation on the power supply. A dynamic process might control the discharge steadiness more efficiently than the conventional stability criterion that compares the slope of the static volt-ampere characteristic to the value of the external ballast resistor. To check the validity of the inferred stabilization process, a typical 4.7 cm long plasma filament operates in ambient air. Optical and electrical diagnostics are performed to investigate the discharge properties. Measurements then reveal most typical features of an actual arc discharge in air. Consequently, a numerical simulation based on a time-dependent Elenbaas–Heller equation allows calculation of the time-evolution of the plasma in the discharge. Finally, electron density measurements using a specific microwave absorption device confirm the high rate of ionization of the plasma: almost two orders of magnitude higher than for a typical glow discharge in free air.

In this first issue of Plasma Sources Science and Technology (PSST) of 2007, I would like to take the opportunity to reflect upon the achievements of 2006 and to look forward to notable events and features planned for the journal over the coming year.

The past year has seen a significant increase in submissions to the journal, with a resulting jump of 33\% in the number of articles published compared to 2005. This rapid growth underlines the popularity of PSST amongst members of the community as an excellent means of communicating their latest research to their peers. The latest ISI data show that the journal's Impact Factor remains high at 1.798, comparing favourably with other titles in the field.

Another noteworthy feature of 2006 has been the continuing improvement in publication times, largely as a result of the recent adoption of IOP Publishing's electronic article management system which has simplified the processes of submission and reviewing for both authors and referees. The trend of shorter receipt-to-publication times is one that we fully expect will continue over the coming twelve months.

Our success is of course principally due to the authors who have contributed such consistently high-quality papers to the journal. Particular thanks must also go to the many referees whose hard work often goes unacknowledged. Their valuable suggestions and constructive criticism frequently lead authors to make significant improvements in their papers as well as encouraging the healthy scientific debate which is an essential part of the publication process.

The journal has recently seen the departure of a number of long-standing Editorial Board members. I would particularly like to mention Daan Schram, who retired from his post as Regional Editor for Europe early last year. Daan has been involved with PSST from its inception and his contribution to the journal's development over the years has been invaluable. He will continue to work with us as a member of the Board and his position as European Editor has now been assumed by M C M (Richard) van de Sanden. As ever, the Board have done a great job of driving the scientific direction of the journal and their work is greatly appreciated.

A key objective for the year to come will be to publish a greater number of quality review articles focused on high-interest topics and we encourage potential authors to submit their proposals for consideration by the Editorial Board. We will also continue to support the plasma community through the sponsorship of relevant conferences and publication of special topical issues.

Finally, I would like to announce that I will be stepping down as Editor-in-Chief in mid-2007. Since founding PSST in 1992, I have spent many enjoyable and stimulating years in the role. During this time, it has been a great privilege to have been a part of such a dynamic community and my thanks go to all of those who I have worked with who have made it such a rewarding experience. The best part has been getting to know the people! Further details of my retirement and my successor will follow in the next few months. I intend to remain an active member of the Board after my retirement from the Editorship and I have no doubt that the journal will maintain its high standards and continue to enjoy success for many years to come.

Noah Hershkovitz, University of Wisconsin-Madison, Editor-in-Chief

The 18th European Sectional Conference on Atomic and Molecular Physics of Ionized Gases (ESCAMPIG XVIII) was organised in Lecce (Italy) from 12 to 16 July 2006 by the Institute of Inorganic Methodologies and Plasmas (Bari, Rome, Potenza) of the Italian National Research Council (CNR–IMIP) and the Chemistry Department of Bari University.

The ESCAMPIG conference is a regular biennial Europhysics Conference of European Physical Society focusing on collisional and radiative aspects of atomic and molecular physics in partially ionised gases as well as on plasma surface interaction.

The conference is an important European meeting favouring discussions and updates in low temperature plasma sciences through general and topical invited lectures, selected oral presentations (hot topics) and posters on the following topics:

• Atomic and molecular processes in plasmas • Particle energy distribution functions • Discharge physics: sheaths, transport processes, and modelling • Plasma diagnostics • Laser and particle beam assisted plasma processes • Physical basis of plasma chemistry and plasma surface interactions

During ESCAMPIG XVIII special sessions were dedicated to workshops on:

• Plasmas and nanomaterials, organized by Professors G Bruno and R d'Agostino • Fundamental processes in laboratory and natural plasmas, organized by Professor A Laganà.

ESCAMPIG XVIII was attended by 215 scientists from 28 countries. Despite the European character of the ESCAMPIG conference, the participation of scientists from universities and research institutions of non-European countries has also been interesting. A special mention is due to the significant participation of scientists from the low temperature plasma community of Japan.

The conference topics were the focus of fruitful discussions involving 16 invited lectures, 7 selected hot topics, and 200 poster presentations. The workshop topics were addressed by 12 selected contributions.

An important issue of ESCAMPIG is to fertilize the topics of the conference though the publication of peer-reviewed articles based on invited lectures and hot topic contributions discussed at the conference in the journal Plasma Source Sciences and Technology (PSST). Most of the authors agreed to prepare in due course articles suitable for the journal in the form of reviews, critical overviews of their own published results with latest new developments, and original works. We would like to thank the authors for their efforts in preparing stimulating lectures and interesting articles for the readers of PSST, and the scientific community dealing with ionized gases sources and chemical physics of low temperature plasmas also thanks the editors of PSST for peer reviewing this special issue collection.

We would like to thank the International Scientific Committee, chaired by Professor W G Graham, for the selection of the appealing lectures and hot topic contributions as well as the organizers of the workshops for having enriched the discussion in the ESCAMPIG XVIII conference.

Thanks are also due to the organizing institutions, CNR–IMIP and the Department of Chemistry, and to the sponsors who, with their financial support, enable the significant participation of PhD students and scientist with needs, and who offered to the LOC the opportunity to arrange a pleasant stay in Lecce for the participants of ESCAMPIG XVIII.

Mario Cacciatore and Santolo De Benedictis, CNR–Institute of Inorganic Methodologies and Plasmas, Bari, Italy

Guest Editors