Table of contents

Recent studies on elementary processes relevant to H₂ plasmas are reviewed, emphasizing, in particular, vibrational state selected electron–molecule and heavy particle–molecule interactions. These data are then discussed in the framework of plasma kinetics for reactors of technological applications. Particular emphasis is given to microwave and parallel-plate RF discharges.

Previously published applications of optical emission spectroscopy as a quantitative plasma diagnostic technique are reviewed. By adding traces of rare gases to the plasma, electron temperatures (T_e) and relative electron and ion densities can be determined from electron impact-induced optical emission. Excitation from both the ground state and metastable states of the rare gases must be considered. At higher rare gas partial pressures, UV radiation trapping and optical cascading must also be taken into account. Absolute species concentrations (e.g. Cl₂, Cl, O, and F) can be derived from their optical emissions, combined with T_e measurements determined from rare gas optical emission. Examples are given of neutral and ion species density measurements in chlorine, oxygen, and fluorocarbon-containing low-pressure, high charge-density plasmas. Typical results of T_e measurements are also presented and compared with Langmuir probe measurements.

This paper seeks to summarize the work that has gone on over the past 20 years in plasmas that has become of great commercial importance but was not studied in depth previously. It points up the differences between conventional electropositive plasmas and those where negative ions dominate. It removes some of the errors and misunderstandings that have occurred and tries to bring the field into a coherent consistent whole.

A study of the strongly coupled dusty plasma was carried out over a wide range of plasma pressures and temperatures. The plasma was investigated under conditions of low pressure dc current and inductively-coupled rf gas discharges and under nuclear induced plasma conditions. The plasma was also formed from positively charged dust grains in the presence of a flux of ultraviolet photons. The results of experimental observations of ordered dust structures are reported, and the characteristic features of the dust structures and the conditions for their appearance are discussed.

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Understanding of cathode phenomena in arc discharges has considerably improved in recent years due to the progress both in experiment and in theory. A review of the present understanding of cathode phenomena in high-pressure arc discharges is given. The effect of processes in the arc column on the cathode part of the discharge in most cases is weak, which is a consequence of the fact that the energy flux coming to the cathode is formed in a thin near-cathode plasma region. A theoretical model of the near-cathode region is described. The model can be used both by itself or as a part of a code modelling the whole system of arc electrodes. In particular, the model results in a simple estimate of the upper limit of the cathode temperature and of the lower limit of the temperature inside a cathode spot. A simple asymptotic solution to the thermal conduction equation in the cathode body, describing a steady-state cathode spot, may be obtained making use of the fact that the energy flux from the plasma to the cathode surface is localized in a narrow temperature range. Approaches to a self-consistent modelling of diffuse and spot modes of current transfer to the cathode are discussed.

This paper gives a summary of recent results about the replacement of mercury in high-pressure discharge lamps by metallic zinc. Actually, this topic is of high relevance for the lighting industry due to the need of more environmentally friendly products. The work presented here is supported by the German government under contract no 13N8072 and 13N8264. Due to upcoming European legislations which are expected for the year 2003, the replacement of mercury in lighting products is a high priority task. For example, mercury-free headlight discharge lamps are requested by the automotive industry.

Pure zinc/argon discharges as well as lamps including zinc or mercury and metal halide additives are investigated. Experimental data are compared with model calculations of the energy balance involving the transport of heat and radiation. Since the excitation energies of relevant zinc transitions are lower than for mercury, axis temperatures of pure zinc lamps are about 300 K below the value of mercury arcs. In addition, the thermal conductivity of zinc including the contribution of radiation diffusion is larger than compared to mercury. From lamp voltage measurements it is found that the cross section for elastical electron scattering by zinc atoms is about the same than for mercury. When adding metal halides to a pure zinc discharge with argon as a starting gas, i.e. NaI, TlI, DyI₃, axis temperatures decrease to about 5100 K due to strong radiation cooling. In order to obtain sufficiently large lamp voltages, wall temperatures of more than 1300 K are adjusted by means of polycrystalline aluminaoxide (Al₂O₃) as a wall material. Electric field strengths of 6.0 and 8.6 V mm⁻¹ are measured for metal halide lamps containing zinc or mercury, respectively. The light technical data of the discharges are very close, since mercury and zinc do not contribute significantly to the radiation in the visible range. Efficacies of up to 93 and 100 lm W⁻¹ are found in metal halide lamps with zinc and mercury, respectively. Consequently, zinc turns out to be an attractive replacer for mercury in this type of lamp not only from an environmental point of view.

We developed and studied three different extreme ultraviolet (EUV) capillary discharge sources either dedicated to the generation of coherent or incoherent EUV radiation. The CAPELLA source has been developed especially as an EUV source for the metrology at 13.4 nm. With one of these sources, we were able to produce gain on the Balmer-Hα (18.22 nm) and Hβ (13.46 nm) spectral lines in carbon plasma. By injecting 70 GW cm⁻³ we measured gain-length products up to 1.62 and 3.02 for the Hα and Hβ, respectively optimization of the EUV capillary source CAPELLA led to the development of an EUV lamp which emits 2 mJ in the bandwidth of the MoSi mirror, per joule stored, per shot and in full solid angle. The wall-plug efficiency is 0.2%. Stability of this lamp is better than 4% and the lamp can operate at repetition rate of 50 Hz.

In non-uniform plasmas, linear magnetohydrodynamic waves can get into resonant interaction with the background plasma at locations where propagating waves and local modes, existing in regions of pronounced non-uniformity, satisfy the resonant condition of phase synchronism. This leads to a resonant excitation of local modes and growth of their amplitudes. Consequently, the effect of dissipation becomes important which eventually limits the amplitudes to finite values in domains of around resonances. The incident wave then loses its energy and a process known as the resonant wave absorption occurs. In a non-static background plasma, a non-uniform macroscopic mass flow can act as a free energy source, causing phenomena of resonant instability of local modes and over-reflection of incident waves when reflected waves gain energy from the flow. Such resonant processes may appear in magnetic structures in the solar corona, in sunspots, in planetary magnetopauses, bowshocks and magnetotails contributing to energy transfer and transports of other physical quantities. Some of these phenomena can be related to locally enhanced physical quantities as detected by satellites. In this paper, we present a number of analytical and computational approaches in treating the resonant wave behaviour in non-uniform plasma configurations relevant to solar and terrestrial conditions.

A variety of induction-coupled plasma source designs have been characterized for their ability to couple charged species to a lower electrode. Designs can be classified in a spectrum from remote sources, through plasma transport sources, to close-coupled sources. The DC self-bias on a lower electrode at constant bias power is used as a diagnostic of plasma coupling between source and lower electrode.

The plasma–wall interaction region of a fusion device provides the interface between the hot core plasma and the material surfaces. To obtain acceptably low levels of erosion from these surfaces requires most of the power leaving the core to be radiated. This is accomplished in existing devices by encouraging plasma detachment, in which the hot plasma arriving in the region is cooled by volume recombination and ion–neutral momentum transfer with a dense population of neutrals recycled from the surface. The result is a low temperature (1 eV<T_e<5 eV), dense (n_e>10¹⁹ m⁻³) but weakly ionized (n₀>10²⁰ m⁻³, n_e/n₀<0.1) plasma found nowhere else in the fusion environment. This plasma provides many of the conditions found in industrial plasmas exploiting plasma chemistry and the presence of carbon in the region (in the form of carbon-fibre composite used in the plasma facing materials) can result in the formation of deposited hydrocarbon films. The plasma–wall interaction region is therefore among the most difficult in fusion to model, requiring an understanding of atomic, molecular and surface physics issues.

This paper deals with the presentation and discussion of some recent measurements on the interaction of electron and ion swarms in gases, with particular emphasis on flourinated gases. The processes to be discussed are related to electron impact ionization and attachment, including Penning ionization and electron detachment. Electron transport is discussed in mixtures of SF₆ and fluorocarbon gases, where interesting regions of negative differential conductivity have been observed in C₂F₄, and apparent pressure dependences of the drift velocity in C₄F₈. Ion transport is discussed in terms of recent measurements on positive and negative ions in SF₆ and Ar. Finally, the subject of ion–molecule reactions is illustrated with the dissociation and charge transfer processes of daughter ions in nitrogen. Throughout the paper, the advantages and limitations of both the pulsed Townsend technique and the drift tube-mass spectrometer are highlighted.

We report results of the measurement of the Stark parameters for several visible neutral argon lines. For this study wall stabilized arc is used as a plasma source. Electron densities of (0.74–2.9)×10²² m⁻³ are determined from the width of the H_β line and electron temperatures of (9280–10 750) K are deduced from plasma composition data. Precision technique for spectral line shape recordings is applied. The deconvolution procedure for asymmetric line profiles is used to determine the Stark broadening parameters from experimental line shapes. The results are compared with theoretical and other experimental data.

The study of plasma expansion is interesting from a fundamental point of view as well as from a more applied point of view. We here give a short overview of the way properties like density, velocity and temperature behave in an expanding thermal plasma. Experimental data show that the basic phenomena of plasma expansion are to some extent similar to those of the expansion of a hot neutral gas. From the application point of view, we present first results on the use of an expanding thermal plasma in the plasma-activated catalysis of ammonia, from N₂–H₂ mixtures.

Ions and radicals are known to be key players in many plasma processes, including anisotropic etching, film deposition and surface modification. How different plasma species influence the reactions and reactivity of each other is not, however, fully understood, especially with respect to surface interactions. Using our imaging of radicals interacting with surfaces (IRIS) technique, we have measured the effects of ion bombardment on the surface reactions of a number of plasma species. Results from IRIS experiments for SiF and SiF₂ in SiF₄ plasmas, CF₂ radicals in hexafluoropropylene oxide plasmas, and NH and NH₂ radicals in NH₃ plasmas are presented for a variety of substrates. Depending on the molecule, the applied rf power, and the substrate material, these species display a wide variety of surface interactions. Moreover, ion bombardment increases surface scattering in primarily etching regimes (SiF₂, CF₂, and NH₂), decreases surface scattering in one system (NH radicals on polymer substrates), and has no effect on SiF radicals in SiF₄ plasmas. Correlating radical and ion bombardment relationships leads to possible plasma processing mechanisms, which are also discussed.

For 20 years, a large part of ionospheric research has been devoted to high latitudes and in particular to the range 60–70° where an oval of auroras permanently encircles each pole. The auroral light emissions are accompanied by the production of ionization, electric currents and fields. Indeed, the auroral latitudes play a dominant role in the ionospheric electrodynamics because electric fields and currents reach thus at their largest intensities. Observations from low-altitude satellites and from ground-based facilities have contributed to the analysis and modelling of the structure and dynamics of the auroral ionosphere. The results illustrated here are inferred from observations of the European Japanese incoherent scatter radars (EISCAT) based in North Scandinavia.

Recently, the field of view of the EISCAT facilities has been extended toward the pole with two radars built in 1996 and 2000 at Spitzbergen (78°N): the EISCAT Svalbard radars. Other ground-based instruments (magnetometers, photometers, etc) have also been deployed at the same location. At first sight, the ionization production in the polar ionosphere is expected to be weak because of the reduced solar illumination. The first observations reveal, in contrast, the presence of intense and variable structures, which are still under investigation. To develop our understanding of these events, we discuss the theoretical results given by the particle penetration from solar origin, and of its effects into the dayside polar ionosphere.

Pulsed helicon discharges produced through m = 1 or m = 2 helical antennas are investigated. Special attention is paid to the axial asymmetry, which is characteristic for helicon sources with helical antenna coupling. The axial profiles of the RF wave fields as well as the energy deposition profiles reveal that the RF power is predominantly absorbed via helicon modes with positive azimuthal mode numbers m (m = +1, +2) travelling in positive magnetic field direction. This can be attributed to the fact that the helicon modes with negative m are strongly damped or even evanescent if the radial plasma density gradient is steep enough. In particular, we examined the RF power absorption in the m = 1 helicon discharge, that cannot be understood in terms of collisions. Various non-collisional absorption mechanisms as well as absorption via excitation of Trivelpiece–Gould waves are discussed. We also present first measurements carried out on the large-volume plasma injected from the m = 1 helicon source into a diffusion chamber.

We present the experimental and theoretical work done at Orsay on the analysis of MeV/n ion beams interacting with fully ionized and neutral hydrogen targets. The obtained results have demonstrated a strong enhancement in the plasma of the deposition of energy and of the dispersion in the distribution of the beam energy. The recently derived CKLT stopping formula has been compared favourably with the experimental results.

In this paper measurements of short wavelength electron density fluctuations using collective scattering of infrared light are presented. The Wendelstein 7-AS (W7-AS) stellarator (Renner H et al 1989 Plasma Phys. Control. Fusion31 1579) and the diagnostic are briefly described. A series of plasma discharges with reproducible confinement transitions was created by ramping up the plasma current. Utilizing the fact that the density fluctuation wavenumber κ is anisotropic in the directions parallel and perpendicular to the local magnetic field, the diagnostic can provide a radial profile of the turbulence during both normal and degraded confinement. The found profiles display an increase of core turbulence for the reduced confinement state. The results are discussed and compared to similar tokamak measurements.

This paper describes the developments of laser Thomson scattering (LTS) for measuring electron density and temperature, or more generally electron energy distribution function (eedf) in glow discharge plasmas with electron densities below 10¹⁸ m⁻³. A brief description of the method for overcoming the sparcity of scattered photons by data accumulation techniques is followed by examples of applications, conducted at the authors' laboratory, to the measurement of the shape of the eedf and temperature anisotropy in various discharges including reactive plasmas, and some as close as 100 μm from a material surface. Measurements of the latter sort should permit the elucidation of micro-discharges and electrode effects such as cathode fall phenomena in particular.

Production of N and O atoms has been investigated by NO titration in flowing reactors for research study (5 litre) and for industrial applications (900 litre). The two reactors differ by the chamber volumes and compositions: pyrex glass for the small and steel for the big one. In the same conditions of pressure, power density and post-discharge time, it is measured about the same values of N-atom density inside the two reactors but the O-atom density is nearly one order of magnitude lower in the industrial one. Such a difference is explained from variations of destruction probability of N and O atoms on the reactor walls and by consideration of scaling up parameters.

Such flowing reactors are applied to metal surface cleaning, polymer activation by O atoms and to sterilization processes by a synergetic effect of O atoms and UV emission of NOβ bands.

A three-dimensional Particle-In-Cell simulation describing the interaction of an intense laser beam with a plasma slab is presented. It is observed that the laser-generated electron current decays into magnetically isolated filaments. The filaments grow in scale and magnitude by a merging process in the course of which the field topology changes. The opposite process also takes place occasionally. The laser-driven charge and energy flows and the reconnecting magnetic field mutually interact. At the end of the merging process flows and magnetic fields are large close to the laser irradiated surface of the plasma slab. Both decrease rapidly in the bulk plasma. Due to the magnetic pressure in the filaments hollow density channels in the electron density are formed. The simulation reveals that net charge flows in these channels can exceed the classical Alfvén current.

This paper deals with a number of studies, carried out in Russia, on investigation of plasma technologies for environment protection. Physical processes in low temperature plasma generators (AC plasmatrons), which are the basic units of these systems are described. Plasma technologies on treatment of medical waste and liquid Cl–F carbons are given. The results of pulse discharges of low energy for bactericidal water purification are presented.

Laser absorption spectroscopy in the vacuum ultraviolet range (VUV-LAS) has been applied for the quantitative measurement of light atomic species and halogen atoms, which are of importance in various plasma processing technologies. So far, measurements of F and C atoms have been made with this method. For the generation of a VUV laser, a two-photon resonance four-wave mixing method has been used in the sum frequency generation scheme or difference frequency generation scheme. Wavelength tunability is essential when the background absorption exists due to the parent molecules and other species produced in plasmas. The absolute density of target atoms can be derived from the integral over the line profile converted into the absorption coefficient. Since the VUV-LAS measurement can yield only the values averaged over the line-of-sight, some complementary methods such as two-photon absorption laser induced fluorescence (TALIF) and optical emission actinometry are used for the spatial resolution. Examples of experimental data are given for F and C (VUV-LAS) and for O, N and H (TALIF) with a brief discussion.

The main goal of the studies presented is to investigate properties of the ion emission from the plasma produced with the use of high-intensity (up to 8×10¹⁶ W cm⁻²) 1-ps laser. The terawatt Nd : glass CPA laser system delivering up to 1 J energy in 1.2 ps pulse at the wavelength of 1.053 μm irradiated solid targets. The parameters of ion streams emitted from the plasma were measured with the use of an electrostatic ion-energy analyser and ion collectors. The charge state, the ion energy, the ion current density together with its angular distribution were the basic parameters of our interest. The results obtained demonstrate that the interaction of the 1-ps laser pulses with different targets provides favourable conditions for the generation of a large amount of highly charged (e.g. Ta³⁸⁺, Au³³⁺) fast ions (energy about 1 MeV for Au ions) driven by hot electrons.

The paper is a review of the current understanding for the operation of the non-magnetized large-area planar microwave plasma sources with emphasis on the high-density surface-wave plasmas. The paper starts with a review of the various plasma source designs currently in use and short comparison of microwave and lower-frequency RF plasmas. Then the most important phenomena in such plasmas (occurrence of standing-wave eigenmode resonance densities, density dependence of the impedance, energy balance, stability, multi-valued power-density dependence, mode jumps, memory and hysteresis loops, self-tuning ability, local plasma resonance, non-collisional heating) are revisited with simple two-dimensional models. For each phenomenon references to more detailed theoretical treatment and experimental studies are given and a few key experimental curves are reproduced. At the end some unresolved problems are listed.

A study of the interactions of energetic ions with various surfaces using molecular dynamics simulations is reported. Silicon atoms in the amorphous region are readily mixed by argon ions. Limited mixing in the crystalline layer is observed. Fluorine adsorbed on the silicon surface does not mix into the layer with argon ion impact. When an energetic F⁺ impacts a silicon surface, the uptake and apparent sub-surface mixing of F is much greater than Ar⁺-induced mixing. However, a closer examination shows that the F impacts have primarily increased the Si surface area by creating crevices and cracks, and that the F remains mainly on the surface of this layer.

Experiments and numerical model calculations on non-thermal plasma treatment of lean combustion exhaust gases were reviewed. It was found that because of the oxygen concentration of several per cent, oxidation of noxious compounds is the prevailing non-thermal plasma-induced process. Therefore nitric oxides cannot be reduced directly, but hybrid processes combining non-thermal plasma pre-treatment with catalytic reduction using either hydrocarbons or ammonia-based reducing agents have to be applied.

Plasma-enhanced selective catalytic reduction (PE-SCR) of the nitric oxides emitted from a modern car's diesel engine for values of more than 60% was demonstrated in test bench experiments. For these experiments, a compact dielectric barrier discharge reactor with a flow cross section of 15 cm² excited by a semiconductor switched pulse voltage source and a urea-based selective catalytic reduction system were applied. The average fuel penalty for this process under urban driving conditions was estimated to be around 2%.

Thus PE-SCR has the potential to reduce the NO_x emission of diesel cars to values well below future emission standards to be set in force in 2007. A number of investigations on the non-thermal plasma-induced oxidation of diesel soot showed very encouraging results.

To investigate the mechanism of highly selective SiO₂ contact hole etching, we analysed surfaces exposed to C₄F₈/Ar/O₂ plasma in a dual-frequency parallel-plate etching system. The thickness and composition of the fluorocarbon layer on large areas and at the bottom of contact holes were quantitatively analysed, in conjunction with the flux of CF_x chemical species and the etch rates. In a highly selective etch processes, the thickness of the fluorocarbon layer on the SiO₂ surface is less than 1 nm, while that on the Si surface is 4–6 nm. We found that the etch rate is strongly affected by the thickness of the fluorocarbon film on the SiO₂ and Si surfaces at the bottom of the contact holes, as well as on the large areas. However, a small increase of C₄F₈ gas flow causes a larger increase in the fluorocarbon film thickness at an aspect ratio of 4 than that on the large flat area surface. Furthermore, we observed a slightly thinner and C-rich film at an aspect ratio of 10. This was probably caused by the decrease in the radical flux passing through the small hole. This aspect-ratio dependence is the cause that the process window for highly selective hole etching tends to be narrow.

The active matrix display industry is rapidly presented. The most challenging aspect for plasma source design lies in the substrate size which is now entering the 1–2 m range. This paper focuses on the conventional planar RF capacitor at 13.56 MHz. A detailed analysis of the local perturbation due to a hole in the metal susceptor illustrates the process effects of various plasma microscopic parameters. It appears that not only the process rate must be kept uniform, but also the ion bombardment. It is also shown that very wide capacitive reactors no longer follow some classical rules: (1) plasma RF conductivity is limited, its propagation is shown to be described by a telegraph equation, in agreement with numerical modelling and electrostatic measurements, (2) the RF wavelength is no longer infinite compared to the dimensions, this leads to standing waves. The presence of the plasma is shown to worsen the effect by shortening the RF propagation wavelength.

In the last twelve years the research on dusty plasmas was at first required to understand the dust-induced problems in low-pressure plasma tools and surface processing technologies. Meanwhile, the research on physics and chemistry of dusty plasmas led to the discovery of a fascinating domain. The scientific knowledge acquired on dusty plasmas leads to new ideas for applications and the aim of this contribution is to present examples of such connections. An exhaustive review of the matter would be clearly impossible and only three aspects are emphasized. The first concerns plasmas where nanosized particles are grown and their use for deposition of thin films with new properties. The second concerns situations where a significant amount of dust is stored in a discharge, inducing high electron energies, with strong impact on plasma chemistries. Finally, the last one will concern the well-known confinement of dust particles by electrostatic sheaths and some examples of its actual or potential applications.

While the study of dust–plasma interactions is by no means new, early progress in the field was slow and uneven. It received a major boost in the early 1980s with the Voyager spacecraft observations of peculiar features in the Saturnian ring system (e.g. the `radial spokes') which could not be explained by gravitation alone and led to the development of the gravito-electrodynamic theory of dust dynamics. This theory scored another major success more recently in providing the only possible explanation of collimated high-speed beams of fine dust particles observed to sporadically emanate from Jupiter by the Ulysses and Galileo spacecrafts.

These dynamical studies were complimented in the early 1990s by the study of collective processes in dusty plasmas. Not only has this led to the discovery of a whole slew of new wave modes and instabilities with wide ranging consequences for the space environment, it also spurred laboratory studies leading to the observation of several of them, including the very low frequency dust acoustic mode, which can be made strikingly visual by laser light scattering off the dust.

The most fascinating new development in dusty plasmas, which occurred about 7 years ago, was the crystallization of dusty plasmas in several laboratories. In these so-called `plasma crystals', micrometre-sized dust, which are either externally introduced or internally grown in the plasma, acquire large negative charges and form Coulomb lattices as was theoretically anticipated for some time. This entirely new material, whose crystalline structure is so strikingly observed by laser light scattering, could be a valuable tool for studying physical processes in condensed matter, such as melting, annealing and lattice defects. Recognizing the crucial role of gravity on the crystal structure, microgravity experiments have already been performed in aircraft, sounding rockets, the Mir Space Station, and most recently in the International Space Station, leading to interesting new phenomena and insights.

Dust–plasma interactions are also important in the industrial laboratory. The nuisances and hazards related to dust charging in mills, granaries and mines have been known for a long time. At the same time dust charging has been effectively used in electrostatic precipitation, separation and spraying as well as in the sterilization and electrophoresis of biological materials. Also, plasma processing is now used in the semiconductor manufacturing industry. The condensation and transport of fine dust particles in these high-density, low-temperature plasmas leading to product yield loss, is a significant problem facing this multibillion dollar industry, and is expected to become more important as feature sizes decrease in the future. The role of dust–plasma interactions in magnetic fusion devices, where dust could grow in the edge of the fusion plasma discharge itself and be subsequently transported elsewhere, is also beginning to receive attention, lately.

I expect this rapid growth will continue well into the foreseeable future. While the richness and complexity of the field, with numerous unresolved and challenging problems, will continue to stimulate the field, several forthcoming space missions (e.g. Cassini and Rosetta) will provide important observational boosts. The fascinating high-resolution Hubble Space Telescope photographs of dusty cosmic regions, such as those where new stellar and planetary formation are taking place in plain sight (the so-called cosmic nurseries), is only now beginning to underscore the importance of dusty plasma phenomena in these vital astrophysical regions. Finally, the proposed International Microgravity Plasma Facility, dedicated to dusty plasma studies in the International Space Station within the next few years, will provide a major boost to this fascinating field.

Nucleation and subsequent growth processes of clusters, i.e. particles below a few nm in size, have been studied in capacitively coupled high-frequency SiH₄ discharges. Time evolution of size and density of clusters has clearly shown the existence of bottleneck in the size distribution at around Si₄H_x's. After the nucleation, the clusters of the large size group, Si_nH_x's (n>4), grow mainly due to influxes of those of the small size group, Si_nH_x's (2⩽n⩽4) and other molecular species SiH_x's (0⩽x⩽3). These results have led to the growth model of clusters, which can reasonably explain the results obtained until now. Furthermore, effects of gas flow, discharge modulation, gas temperature gradient and hydrogen dilution on growth and behaviour of clusters have been studied to control their growth. The reactor, which suppresses the growth of clusters both by thermophoretic force and by gas flow and evacuation without stagnation, has been newly developed to make a-Si:H films of high qualities. The qualities of films deposited using the reactor are much better than those of conventional high-quality films.

In this Special Issue of Plasma Sources, Science and Technology, we are pleased to present papers which we hope will provide a broad view of the current work in plasmas and discharges and their applications.

Papers in this Special Issue are based on invited talks presented at the 25th International Conference on Phenomena in Ionized Gases (ICPIG). The issue is not a conference proceedings and all contributions were treated as regular papers. They were written and reviewed according to the usual high standards of the journal. Each was sent to two referees, all required some revision and not all talks resulted in papers published in this issue.

We hope that readers will appreciate this presentation of the current state-of-the-art of the subject.

The International Conference on Phenomena in Ionized Gases (ICPIG) is an established international conference with a long and proud history. It covers not only the fundamental fields of discharges and plasmas but also their many important applications such as semiconductor processing, surface treatments, pollution control, light source and gaseous lasers. There is no other international conference which includes such a wide range of topics.

Ulrich Kogelschatz has been awarded the A. H. von Engel prize for his `Important contributions to the principles and applications of dielectric barrier discharges'.

Ulrich Kogelschatz was born in 1937 in Germany. He obtained his academic education from 1957 to 1967 at Kiel University, one of the early centres of plasma physics in Germany. After finishing his doctoral thesis he worked at the NASA Langley Research Center at Hampton, Virginia, USA, and later joined the Brown Boveri Corporate Research Center at Baden, Switzerland.

Originally U. Kogelschatz investigated the generation of ozone in so-called `silent discharges'. To begin with, he clarified experimentally the fundamental properties of the individual filaments. In a subsequent step he and his collaborators supplemented the experiments by a detailed numerical modelling of the reaction kinetics in these pulsed high-pressure glow discharges. They used the insight gained from these investigations to improve existing technical ozonizers enormously. Nobody had expected that basic research on a discharge more than 100 years old could have such far-reaching technical and economic consequences.

With this detailed knowledge of the properties of this discharge Kogelschatz realized that this type of highly anisothermic plasma could be used for the generation of incoherent excimer radiation. Once again he made several important contributions, both in terms of fundamental research and in transferring this knowledge to technical applications.

The outcome was the development of high-power excimer lamps. It should also be noted that the development of plasma displays which use Xe₂-excimer radiation from microscopic dielectric barrier discharges benefited from his research.

The success of the dielectric barrier discharge, and the simplicity of its realization, stimulated a large amount of research into the potential of this transient high-pressure glow discharge for use in plasma chemistry. This is another outcome of the work done by Kogelschatz. Moreover, he also contributed personally to the development of this field. Such research may be important in the future for the cleaning of the exhaust gases of combustion engines, and for the production of hydrogen from hydrocarbons.

In addition to his research work on dielectric barrier discharges, he has made important contributions to the plasma physics of high-voltage circuit breakers and corona discharges, and to their application in electrostatic precipitators for power plants.

Ulrich Kogelschatz has undertaken scientific work on dielectric barrier discharges as a senior scientist at the ASEA-Brown Boveri Research Centre in Baden, Switzerland. He has shown with his outstanding scientific work that basic research can be combined with investigation of applications, leading to economically important technical devices. This is a difficult task, especially in an industrial laboratory, and it is a shining example for research workers in academic institutions.

Industrial innovation frequently results from an improved understanding of basic physics. Scientific discoveries quite often lead to engineering inventions that have not been the target of the original investigations. Examples are given from the field of plasma physics and plasma technology. High voltage circuit breakers, ozone generators, high power CO₂ lasers, excimer lamps, and plasma display panels have profited substantially from initially purely scientific investigations. In the meantime they have reached multi-billion dollar market shares.