Table of contents

The "International Topical Conference on Plasma Physics: Colloidal Plasma Science" was held at the Abdus Salam International Centre for Theoretical Physics (Abdus Salam ICTP), Trieste, Italy during the period 3–7 July 2000. The conference was organized by P. K. Shukla, R. Bingham and L. Stenflo with the help of an international advisory board and a program committee that included well known scientists from all over the world. The conference enters into a series of previous biennial activities that we have held at the Abdus Salam ICTP since 1989.

The purpose of this meeting was to provide an informal forum for scientists who have dealt with various aspects of colloidal and dusty plasma sciences that are relevant to space, astrophysical and laboratory plasmas. The selected topics, which were interdisciplinary, are expected to have a great deal of impact on the development of dusty plasma science which is one of the most rapidly growing areas in physics as demonstrated by the increasing number of published papers in refereed journals and conference proceedings.

The response of the conference was almost overwhelming. It was attended by approximately 110 delegates from Europe, USA, Japan, and the developing countries. A large number of participants were young researchers from both the industrial and developing countries, as the organizers tried to keep a good balance in inviting senior and younger generations of dusty plasma physicists to our Trieste conference.

The scientific program included four review talks (45 minutes) and fortyone invited topical lectures (30 minutes). In addition, there were about sixty poster papers in two sessions. The latter gave opportunities to younger physicists for displaying the results of their recent work and to obtain comments from the other participants. During the five days at the Abdus Salam ICTP, we focused on fundamental aspects of: (i) dust grain charging and its dynamics in a dusty sheath, (ii) waves and instabilities in weakly and strongly coupled dusty and colloidal plasmas, (iii) various nonlinear structures in strongly coupled systems (Mach cones, shocks, and vortices) (iv) formation of Coulomb crystals in space and laboratories, (v) the physics of various attractive forces, (vi) phase transitions, (vii) dust in the Earth's middle atmosphere and the generation of plasma irregularities in expanding ionospheric dust clouds, (viii) coating of particles in a rf discharge, and (ix) exotic dusty plasmas involving busting dust (from cosmic grains to terrestrial microbes). It turns out that colloidal plasma science and dusty plasma physics, which also interacts with condensed matter physics, make plasma physics a truly cross-disciplinary and very fascinating science with wide ranging applications in space and technology. Most of the contributions from the Trieste meeting appear in this Topical Issue of Physica Scripta, which will be distributed to all the participants. It is expected that the papers of the present proceedings, which systematically describe the advancement of the physics of weakly and strongly coupled dusty and colloidal plasma systems, shall be useful for understanding the many complex phenomena that are occurring in space and laboratories.

The organizers are grateful to Professor M. A. Virasoro, the director of the Abdus Salam ICTP, for his generous support and warm hospitality in Trieste. The Editors want to express sincere gratitude to their colleague and co-organizer Professor R. Bingham for his constant and wholehearted support in our endeavours. We highly appreciate the excellent work of the scientific secretary Mr T. Farid, as well as of Mrs A. Lusenti. Thanks are also due to the European Commission for supporting our activity through the Research Training Network entitled "Complex Plasmas: The Science of Colloidal Plasmas and Mesospheric Charged Aerosols".

Finally, we would like to express our gratitude to the Abdus Salam ICTP for providing partial financial support to our conference in Trieste. Besides, the organizers cordial thanks are extended to the speakers and the attendees for their contributions which resulted in the success of the Trieste conference. Specifically, we appreciate the speakers for delivering excellent talks, supplying well prepared manuscripts for publication, and enhancing the plasma physics activity at the Abdus Salam ICTP.

The conference chairman, Professor Padma K. Shukla, was born on July 7, 1950. During the conference dinner we therefore celebrated his 50th birthday. Padma was then honoured by many speakers from the plasma and space physics communities, who stressed Padma's truly outstanding scientific activity (e.g. about 600 publications in the best scientific journals, his international leadership, his many recent papers on various subjects in the journal Phys. Rev. Lett., and his most important discoveries in colloidal plasma science as well as in theoretical space plasma physics and astrophysics). Padma's key role in inspiring and helping bright young scientists, as well as supporting his colleagues in the developing and east European countries was of course also pointed out. But most of all we appreciate Padma because he is such a nice personality with remarkable human qualities.

On behalf of the plasma physics community I wish Padma a happy, healthy and prosperous future.

The stress induced microscopic responses of dust particles in strongly coupled dusty plasma liquids suspended in plasma traps are investigated experimentally. Narrow laser beams are used to generate stress on dust particles through optical pressure. Assisted by background thermal fluctuations, chaotic micro-vortices with decaying intensities from the laser beam are generated. The forward transport and transverse diffusion rate are enhanced and show nonlinear reponse to the external stress. Visco-elastic response is observed for an elongated liquid cluster driven by a periodically chopped laser beam. The shear wave is damped.

Dust particle ordering is discussed in various types of low-temperature plasma, such as dc glow discharges, UV- and radioactivity induced plasmas. Nuclear induced plasma was formed in the rare gases by alpha-particles and fission fragments of ²⁵²Cf. The ordered liquid-type structures of dust particles levitating in the interelectrode space have been observed. To remove charged particles from a volume an electrostatic probe has been designed and used. The dynamics of the formation of ordered structures of macroparticles charged by photoemission under the action of solar radiation under microgravitational conditions is studied experimentally and theoretically. An analysis and comparison of tile results of the experimental and theoretical investigations permit drawing a conclusion regarding the possibility of the existence of extended ordered formations of macroparticles charged by photoemission under microgravitational conditions.

Parabolic flights – short time µg-flights of 25 s each – are used to determine the parameter ranges for the upcoming long term experiment on complex plasmas on the International Space Station (ISS) – the Plasma Crystal-Experiment (PKE). The experimental results of approximately 170 parabolic flights conducted in 1999 are summarised. As an example, the interaction of microparticles of two different sizes in an rf-discharge is investigated on parabolic flights under microgravity conditions.

Many recent experiments in dc glow discharge plasmas have shown that clouds of dust particles can be suspended near the biased electrodes. Once formed, the dust clouds have well defined boundaries while particle motion within the clouds can be quite complex. Because the dust particles in the cloud can remain suspended in the plasma for tens of minutes, it implies that the particles have a low diffusive loss rate and follow closed trajectories within the cloud. In the experiments discussed in this paper, direct measurements of the dust particle velocities are made using particle image velocimetry (PIV) techniques. From the velocity measurements, a reconstruction of the three- dimensional transport of the dust particles is performed. A qualitative model is developed for the closed motion of the dust particles in a dc glow discharge dusty plasma.

A theoretical and experimental study of strongly coupled dusty plasma were carried out under thermal plasma conditions (the electron and ion density is in the range of 10⁹–10¹¹ cm^-3, the plasma temperature in the range of 1700–2000 K). The diagnostic techniques for measurements of dust and gas parameters were developed. The diffusion coefficient of the CeO₂ particles was measured by the photon correlation spectroscopy (PCS) method. The dynamics of an ensemble of interacting dust grains under conditions of thermal plasma was studied, and the formation of a liquid-like structure observed in the experiment was investigated.

Formation kinetics and behavior of dust particles below about 10 nm in size, referred to as clusters, in silane capacitively-coupled RF plasmas are studied using double pulse discharge and photon-counting laser-light-scattering methods. Even under so-called device quality conditions, clusters of ∼10¹¹ cm^-3 high compared to a plasma ion density of ∼10^-9 cm³ are found at t ∼ 50 ms after the discharge initiation. Clusters begin to be composed of two size groups at t ∼ 10 ms. The ones in the small size group have an almost constant average size of ∼0.5 nm during the discharge period, while the ones in the large size group grow at a rate of ∼4 nm/s. This result indicates that the large clusters are nucleated by the small ones containing 3–4 Si atoms.

Various methods for suppressing cluster growth have also been studied using two in situ cluster detection methods. Since species contributing to the initial growth of clusters are principally produced in the radical production region around the plasma/sheath boundary near the rf electrode, the pulse discharge modulation which has the discharge-off period in one modulation cycle longer than the diffusion time of clusters through the radical production region is effective in reducing the growth of clusters. Thermophoretic force due to heating of the grounded electrode drives neutral clusters above a few nm in size toward the cool RF electrode which is at room temperature. Periodical pulse discharge modulation is much more effective in reducing the cluster density when it is combined with grounded electrode heating. Hydrogen dilution of a high H₂/SiH₄ concentration ratio above about 5 is useful for suppressing cluster growth especially in the radical production region near the RF electrode.

Experimental investigations on the interaction between the dust particles in strongly coupled colloidal plasmas by means of laser manipulation are presented. In laboratory experiments, the particles are trapped in the nonequilibrium sheath environment, where gravity and the electric field introduce a preferred vertical direction. Thus, in the horizontal plane a repulsive Yukawa type interaction is found with a screening length of the order of the interparticle distance, whereas in the vertical direction wakefield attraction and nonreciprocal forces have been measured.

Some attractive forces act on negatively charged particles forming simple hexagonal structures of Coulomb crystals in dusty plasmas. In order to elucidate the origin of the forces, optical manipulations have been applied to analyze the behavior of particles in the crystal. It turned out in the analysis that a primary particle in the ion flow caused an attractive force acting on an another particle located in the lower reaches of the flow. This fact implied that the force was due to the wake potential in the flow. Furthermore, a molecular dynamics simulation was employed to examine structures constructed by particles interacting with Coulomb and wake potentials. It was clear in the numerical analysis that the wake potential could form simple hexagonal structures similar to the structure obtained in the experiment.

A simple two-dimensional strongly coupled one-component plasma system has been developed using charged metal balls. The small stainless steal balls are charged up using high voltage on a silicon wafer and the random kinetic energy of the balls is given by the oscillation of the wafer, which is controlled by the input power from a speaker placed underneath the wafer. The pair correlation function and the diffusion coefficient at several Coulomb parameters are measured to study the transition between the liquid and solid phases. In the liquid phase, a coherent mode in the density fluctuation is observed.

Clustered materials strongly couple with intense laser radiation. The instantaneous conversion of laser energy into electron energy and subsequent conversion into ion energy leads to an efficient rapid laser energy injection into high energy (>10 keV) components of ions. This property of the laser-cluster interaction allows us to suggest compact ways to trigger nuclear fusion. A compact pulsed neutron source as well as a compact fusion reactor are considered.

The nonlinear dynamics is studied of a plasma that consists of hot electrons, cold ions and macroscopic dust particles, in the characteristic frequency range below the ion cyclotron and magnetosonic frequencies. The plasma is immersed in a sheared magnetic field and there exists a sheared plasma flow, in the direction perpendicular to the background magnetic field. In the frequency range above the dust-acoustic and dust charging frequencies, regarding the dust grains as immobile and with constant charges, the plasma is described by the first two moments of the drift-kinetic equation, accounting for the contributions of the stress-tensor and finite mass to the electron dynamics. In the strongly nonlinear regime, the plasma dynamics is governed by the vector nonlinearities arising from the E × B convection and the magnetic field lines' bending. For a linear density profile the standard dipole solution is found, while in the case of parabolic density profiles a coherent localized nonlinear solution is constructed in the form of a tripole, since stable simple monopolar vortices do not exist. Due to the presence of dust grains, the electron and ion diamagnetic currents do not cancel out, which limits the energy of these structures, provides their good spatial localization and increases the stability, compared to their ordinary-plasma counterparts.

The analogy between a dust particle in a plasma and an atomic system is illustrated, by adapting well~known quantum models to describe phenomena associated with dust plasma physics. Three different examples are discussed: (a) the Thomas–Fermi model for nonlinear Debye screening of negatively charged dust particle, when the contribution due to the trapped ions is dominant; (b) the Ising model associated with the mean alignment of an array of elongated dust particles under the influence of an ion flow; (c) the Casimir type of attractive force between two parallel plates immersed in a turbulent plasma.

We use a 3D fully relativistic PIC code to model the interaction of a high-intensity (>10¹⁷ W/cm²) ultra-short (fs) laser pulse with a hydrogen and an argon cluster and investigate the time-resolved ion energy spectra, for different laser intensities. Multi-cluster systems are also studied and the influence of cluster distribution in ionic energy spectra is shown. Results indicate that MeV ions are produced through Coulomb explosion of the atomic clusters.

In a recent experiment a wake was created in a 2D lattice of charged dust grains by a charge moving parallel to the lattice plane. Multiple "Mach cones" were observed in the wake. This paper describes a linear theory of the phonon wake caused by a charge moving relative to a crystalline lattice. The theory predicts multiple structures in the wake that match those observed in the experiments. These structures are caused by constructive interference of compressional phonons excited by the moving charge, combined with the strongly dispersive nature of these phonons. The theoretical wake is matched to an experimental wake in order to determine the plasma Debye length and the charge on the dust grains.

On basis of the kinetic equations for electrons and ions of a dusty plasma where charging collisions are dominant, dispersion relations of waves are analyzed. New modes are found and new damping rates of modes already known for plasmas without dust. Exact kinetic equations for all species are derived from first principles. Approximations are obtained for the equilibrium charge and velocity distributions of the dust particles. The charging mechanism and absorption collisions lead to anomalous heating of dust particles, in qualitative agreement with experimental observation. The recent development of kinetic theory with variable mass is briefly discussed.

We consider various properties of wake field generation in plasmas. In particular the influence of an external magnetic field is studied. It turns out that the magnetic field makes it possible for the wake field to propagate out of the plasma, and thereby provide information about the electron density profile. Furthermore, an example where a dust-acoustic wake field is excited by an ion-acoustic pulse is considered, as a means to illustrate general principles for wake field generation.

An approximate analytical model is developed to study the instability of phase space tubes (holes extended perpendicular to B) in a 2-D magnetized plasma. This instability has been observed in long-duration kinetic simulations of the two-stream instability that produce bipolar electric field structures comparable to those observed in the auroral ionosphere. The tubes are shown to possess stable vibrational modes that become unstable when resonantly coupled to an electrostatic whistler with parallel wavelength long compared to the width of the tube.

We describe a theory of topological phase transitions for dynamical space plasma systems far from equilibrium. The theory is based on the concepts of resonances and coherent structures. When conditions are favorable, the coherent structures may merge, interact and evolve into forced and/or self-organized critical topological states [FSOC]. First and second order phase changes may set in as the dynamical system evolves from one critical state to another, or from a critical state to a non-critical state, or when dynamical instabilities develop. The phenomenon resembles that of the behavior of stirred colloidal suspensions. These ideas are applied to the explanation of the development of broad-band power-law electric field spectral densities of the intermittent low-frequency fluctuations commonly observed in the auroral zone.

The main theoretical and experimental results on ion acoustic shock waves in dusty plasmas are reviewed. Two situations are considered: when the dust charge variation is caused by only the microscopic electron and ion currents at the grains and when it is caused by the microscopic currents and the photoelectric current of electrons. The possibilities of observation of shock waves related to the dust charging process in laboratory experiments, in active geophysical rocket experiments, and in astrophysical phenomena are discussed.

It is shown that formation of dust voids is a general phenomenon in dusty plasmas. Homogeneous dusty plasmas are universally unstable for formation of dust voids and dust structures. Both collision-dominated and collision-less voids can be formed. They are distinguished by the ratio of the void size to the ion-neutral collision mean free path being either large or small, while the mean free path for dust-ion collisions in both cases is small or comparable with the void size. The boundary which separates the dust region from the dust-free region is always sharp as compared to the void size for low ratio of dust temperature T_d to Z_dT_e (T_e is the electron temperature and Z_d is the dust charge in units of electron charge). It is shown that stationary voids are stable for large deviations from equilibrium size. A review is given on theoretical and numerical investigations of both collision-dominated and collision-less voids together with a comparison with experimental results and observations in space. Investigation is performed of voids in presence of floating potential boundaries. Both flat and spherical voids are considered. Dusty plasma experiments in micro-gravity are computer modeled and necessary conditions for the dusty plasma structure formation are found. A condition is that the structure should be separated from the floating potential wall by a void. Presence of a void region between the dust structure and the wall is shown to be necessary for structures to be formed. These voids are different both from the voids in absence of boundaries and from collisional sheaths in absence of dust. Properties of these voids are investigated. Computer experiments for micro-gravity dusty plasmas shows the possibility of creation of almost homogeneous self-confined dust structures surrounded by voids with properties determined by the dust structure.

We show for the first time that a dust-plasma crystal can be formed in the absence of a confining potential. Numerical simulations are carried out in two- and three dimensions to illustrate that a spherical dust-crystal with free boundaries can exist. We show that this is possible due to attraction between the dust particles caused by the asymmetrical bombardment of the plasma flux on the surface of the dust particles.

Dust particulates trapped in the sheath region around the rf or above the negative dc electrode in plasma discharges frequently form regular lattice structures. Stable structures have been formed in the laboratory and their phase transitions have been studied under neutral gas pressures varying over three orders of magnitude. In all circumstances, except at the lowest pressures, the phase transitions from a solid state structure to a fluid or gas like state can be induced by reducing the gas pressure, and conversely, a gas or liquid phase can be made to condense by increasing the pressure. The transition is attributed to an increasing (decreasing) effective particulate temperature. A mechanism that enables an understanding of the particulate heating under the variable pressure is presented. The essential feature of this mechanism is the time variation of the external potential well in which the particulates find themselves as the plasma conditions are varied.

For dust grains in stationary plasma, a quantitative assessment is made of the effect of centrifugal potential barriers on ion trajectories near a grain. It is shown that in most situations of interest the barriers are weak and only marginally affect the validity of the orbital-motion-limited (OML) theory. The OML theory is then used to show that the electrostatic interaction between grains is always repulsive. The ion-shadowing force is calculated, and it is shown that this force can lead to a weak net attraction between grains at long range, under certain conditions with large grains, dense plasma, and/or low gas pressure. For grains in streaming plasma at or near the sheath, it is shown that nonlinear effects are weak and the grains can be represented as dressed particles interacting via the dynamically shielded Coulomb interaction, which includes wakefields, Landau damping, and collisional damping. The Dynamically Shielded Dust (DSD) simulation code, which is based on this model, is described and a simulation is shown for strongly coupled grains in flowing plasma. The simulation shows ordering of the grains into rigid strings aligned with the ion flow, and looser glass-like organization of the strings in the transverse plane. The presence of strings with odd and even numbers of grains results in stratification of the grains into planes with an alternating structure.

Face-centered cubic and body-centered tetragonal Coulomb crystals in a dusty plasma were formed by growing fine-particles and observed from the top and the side at the same time with the use of two CCD video cameras. Time and space structural transitions between the two structures were analyzed from video images. It was suggested that the structural transitions occurred by the slip of crystal planes of fcc(111) and bct(110) parallel to the electrode. The structural transitions of three-dimensional Coulomb crystals in dusty plasmas simulate well those of real metallic crystals.

The structures of systems of particles with repulsive Yukawa interactions are analyzed by molecular dynamics simulations with emphasis on the behavior of Yukawa mixtures. To keep the density of particles with mutual repulsion at finite values, we need some confining mechanism and the latter has strong effects on the structure formation. Under the gravity, the confinement is one-dimensional and we have layered structures. In this case, Yukawa mixtures are separated by the difference in the charge to mass ratios. In the case without gravity, we may have three-dimensional confinement and the behavior of the system may be analyzed by assuming spherical symmetry. We show that there also occurs a separation of species according to the magnitude of charges under the condition of no gravity and we have well-organized structures when the temperature is sufficiently low.

The equilibrium, stability, and trapping of dust particles associated with vertical motions in the plasma sheath and pre-sheath are calculated, taking into account the dependence of the variables (such as dust charge, ion flow velocity, etc.) on the local position in the sheath/pre-sheath region of a collisional plasma with an ionization source. Vibrations and waves associated with vertical motions of the grains in arrays and lattices are analysed, and the frequencies of the modes are calculated.

The wave dispersion relations in the fluid phase of Yukawa systems and classical one-component plasmas (OCPs) are obtained from molecular dynamics (MD) simulations. The Yukawa system is a collection of particles interacting through Yukawa (i.e., screened Coulomb) potentials, which can serve as a simple model for dusty plasmas or colloidal suspensions. The classical OCP is the Yukawa system in the no screening limit. We have demonstrated that the transverse wave dispersion has a cut-off at a long wavelength even in the case of weak or no screening.

We describe a proposed Dust Charge Probe (DCP) experiment to characterize the dust loaded photoelectron sheath on the surface of Mars. DCP can provide measurements of the photoelectric sheath, as well as the charge, size and velocity distribution of the wind-entrained dust particles. Here we discuss the expected physical properties of the photoelectron sheath, the detection technique and the engineering design of the experiment.

We discuss the few existing in situ rocket observations of dust and plasma during the radar backscatter phenomenon Polar Mesospheric Summer Echoes (PMSE). We show from one recent observation, where the rocket flew directly through the radar beam during observation of a PMSE, that there is a very good correspondence between the charge density of dust layer and the PMSE. A surprising observation is that only a small amount of charged dust can be associated with a considerable PMSE signal strength. By estimating the relationship between radar backscatter and dust charge density for other rocket measurements we have found a total relationship between radar backscatter strength and dust charge density. This shows a rapid increase of radar backscatter strength at low N_dZ_d, a more or less constant level for a considerable range in N_dZ_d and a decrease to low values for large values of N_dZ_d where most of the negative charge will be on the dust.

The theory of dust-acoustic waves in plasmas with strongly coupled dusts is reviewed. Physical effects needed for a self-consistent description, such as dust-charge variation, collisional dissipation, ionization and recombination, which are usually on the same time scale, are included. The strong dust coupling is represented by a viscoelastic relaxation model with a wavevector dependent relaxation time. It is found that the dispersion properties of the dust acoustic waves can be considerably affected by the strong coupling and relaxation processes.

A numerical model is described that may be used to investigate important effects of dust charging on ion waves and instabilities in dusty plasmas. The model considers a three species system with fluid electrons and ions and Particle-In-Cell PIC charged dust grains in which the dust charge on the grains varies in time according to the standard charging model. Waves in magnetized or unmagnetized dusty plasmas can be considered. Fundamental effects of dust charging on wave modes which include the dust charge fluctuation mode and damping of ion waves are considered. Also important effects of dust charging on waves produced by the expansion of dust into a background plasma are discussed. Finally an ion acoustic instability that is due to dust charging is investigated. These examples show that there is good agreement with past theoretical predictions on dust charge fluctuation effects and also numerical simulations provide useful additional insight into the physical processes involved.

Knowledge of the formation and distribution of interstellar, interplanetary, and planetary dust grains, and their physical, chemical and optical characteristics provide valuable information about many issues dealing with the origin and formation of the solar system bodies, interplanetary and interstellar environments as well as various industrial processes. Understanding the microphysics of individual grains and their interaction with the surrounding environment is a key to properly model various conditions and interpret existing data. The theory and models of individual dust grains are well developed for environments that vary from dense planetary atmospheres to dusty plasmas to diffuse environments such as interplanetary space. However, experimental investigations of individual dust grains in equilibrium are less common, perhaps due to the difficulty of these experiments. Laboratory measurements of dust grains have primarily measured ensemble properties or transient properties of single grains.

A technique developed in the 1950's for ion spectroscopy, generally referred to as a quadrupole trap has recently been employed as an electrodynamic balance to investigate single micron-sized dust grains and for atmospheric aerosol research. A description of the theoretical basis and the experimental setup of the electrodynamic balance being developed in our laboratory are given. This laboratory technique lends itself to many applications that relate to planetary atmospheres, heliospheric environments, pre-stellar and pre-planetary conditions, and industrial settings. We present results from some recent experiments carried out to investigate the equilibrium potential of dust grains exposed to far ultraviolet light or to an electron beam. Some future experiments using an electrodynamic balance to investigate the optical characteristics, and condensation process involving dust grains in various astrophysical environments are discussed.

A new electro-acoustic effect related to the propagation of an acoustic mode driven by electric field "pressure" in dusty plasmas is described. In this mode the dust space charge is screened due to adiabatic dust charge fluctuations, while the inertia is due to the dust mass. The mode exists for moderately large value of the dust density.

A new feature of the magnetic field dynamics in the interstellar medium is discussed. Due to the presence of charged grains strong Hall electric fields arise to support the quasi-neutrality. Magnetized electrons and ions drifting in the Hall electric field transfer the frozen-in magnetic flux and the magnetic energy. The drift velocity is much greater than the current velocity and particle displacement in this motion could be of astrophysical scale. As a result the local "frozen-in" law of the magnetic field in matter is broken.

Electrode sheath profile measurements are reported, with dust particles in suspension and without. With the shape of the unperturbed potential profile, basic characteristics can be predicted as the surface potential, the charge and potential energy of a dust particle in function of its position in the sheath. The dust particle screening length is also estimated analyzing binary collisions in a gas phase cloud. In the presence of dust particles, an average increase of ion drift velocities is measured showing that micron-sized particles whose charge is due to the electron and ion fluxes produce a self-consistent redistribution of the plasma particle fluxes.

This paper presents several aspects of applied dusty plasma research. New applications of dust particles are emerging; there is growing demand for particles with special properties, and for particle-seeded composite materials. Low-pressure plasmas offer a unique possibility of confinement, control and fine tailoring of particle properties. The role of low-pressure technology in surface modification (coating) of dust grains is discussed and illustrated with examples. Fundamental research related to industrial particle treatment aims at the understanding the surface chemistry of processing. More specifically, the nature of plasma-particle interactions must be resolved. Heat exchange and particle temperature, discussed in this paper, are of major interest in the applied dusty plasma studies.

Electrostatic charging can have important consequences for both the growth and disruption of microparticulates immersed in a plasma. In this topical review, my emphasis is on the latter process, while I extend the term microparticulates not only to include ordinary inanimate cosmic or terrestrial dust but also to include terrestrial microbes whose sizes range from tens of nanometers (viruses) to tens of micrometers (bacteria). Following a description of the basic mechanism of electrostatic disruption of a solid body, I will discuss the role of size, shape and surface irregularity on the process. I will also consider the mitigating role of electric field emission of electrons on the disruption process of a negatively charged grain as its size falls below a critical size. I will conclude by reviewing some early evidence for the electrostatic disruption of cosmic grains, and the very recent evidence for the electrostatic disruption of the bacterial cell membranes in terrestrial sterilization experiments.

Propagation of electrostatic dust modes has been reviewed in the light of the concept of dust fugacity defined by f ≡ 4πn_d0 λ_D² R, where n_d0 and R are the dust number density and the grain size (radius) while the plasma Debye length (λ_D) is given through λ_D^-2 = λ_De^-2 + λ_Di^-2. Dusty plasmas are defined to be tenuous, dilute or dense when f ≪ 1, ∼1, or ≫ 1, respectively. Attention is focused on "Dust–Acoustic Waves" (DAWs) and "Dust–Coulomb Waves" (DCWs) which exist in the tenuous (f ≪ 1) and the dense (f ≫ 1) regimes, respectively. A simple physical picture of the DCWs has been proposed in terms of an effective pressure called "Coulomb Pressure defined by P_C ≡ n_d0q_d0²/R, where q_d0 is the grain charge. In the lowest order, the DCW phase speed is given by ω/k = (P_C/ρ_d^δ)^1/2, where ρ_d ≡ n_d0m_d is the dust mass density and δ ≡ ω₂/ω₁ is the ratio of charging frequencies. Thus, DCWs which are driven by the Coulomb pressure can be considered as the electrostatic analogue of hydromagnetic (Alfvén or magnetoacoustic) modes which are driven by magnetic field pressure. In the dilute regime, the two waves loose their identities and merge into a single mode, which may be called "Dust Charge–Density Wave" (DCDW). When the grains are closest, DCW dispersion relation is identical with that of "Dust–Lattice Waves" (DLWs). Dense dusty plasmas are governed by a new scale-length defined by λ_R ≡ 1/(4πn_d0Rδ)^1/2, which characterizes the effective shielding length due to grain collective interactions. The scale-length λ_R plays a fundamental role in dense dusty plasmas, which is very similar to that of the Debye length λ_D in the tenuous regime. The two scale-lengths are related to the fugacity through fδ ≡ λ_D²/λ_R². The frequency spectrum as well as the damping rates for various dust modes have been analytically obtained, and compared with the numerical solutions of the kinetic (Vlasov) dispersion relation.

A new dispersion relation for low-frequency waves in an unmagnetized dusty plasma is obtained when the plasma contains electrons, ions and non-spherical (elongated) rotating charged dust grains. Since the dipole moments of the latter are non-zero, significant modifications of the dusty plasma wave spectra emerge. Analyses of the newly found dispersion relation reveals instabilities of both the electromagnetic and electrostatic waves. Specifically, waves with frequencies close to the angular rotation frequency of the dust grains are found to be unstable. The relevance of our investigation to enhanced fluctuations in space and laboratory dusty plasmas has been pointed out.

We review our results on the lattice structure and the lattice dynamics of dusty plasma Coulomb crystals formed in rectangular conductive grooves. The basic structure appears to be made of mutually repulsive columns of grains confined by the walls of the groove. The columns are oriented along the direction of the electrode sheath electric field. Inter-grain coupling as a function of plasma temperature and density were investigated by measurement of these parameters. A simple phenomenological model wherein the inter-grain spacing along the column results from an attractive electric field induced dipole-dipole force balanced by a repulsive monopole Coulomb force is consistent with observed features of the Coulomb crystal. In addition, here we present some preliminary measurements of the vibration and rotation dynamics of the individual grains in the Coulomb crystal. The thermal energy of the dust grain thus calculated is much less than the inter-grain Coulomb potential energy as required for the formation of stable structures. Also the observed rotational frequency is consistent with the assumption of thermal equilibrium between the dust grains and the neutral gas.

The wake-field excited by a test charge moving through a multicomponent dusty plasma is calculated by using the fluid as well as the Vlasov–Poisson model. It is shown that the ion-wake field may be oscillatory (V_t < C_DA), constant (V_t ∼ C_DA), or exponentially decreasing (V_t > C_DA), depending upon the test charge velocity V_t relative to the dust acoustic speed C_DA. Numerical results for the wake-field are presented for different test charge velocities. The work has relevance in space and astrophysical plasmas.

The propagation of nonlinear low-frequency waves in a dusty plasma cylinder along its axis is investigated. Nonlinear waves are considered in two limiting cases according to whether the dusty cylinder radius is larger or smaller than the plasma Debye length. Spatial restrictions of a dusty plasma lead to an increase in the dispersion in the long wavelength limit, to substantial modifications of the longitudinal modulational instability, and to the possible formation of nonlinear Schrödinger type solitons. In both cases there is a threshold for solitary wave formation.