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This foundation article discusses the diagnostics of electrons and ions in plasmas and fluxes of charged and neutral species toward plasma-facing surfaces by non-optical methods. The focus is laid on the fundamentals of the most common methods and their application to non-equilibrium low-pressure electropositive plasmas and ion beams. The key topics are Langmuir probes (LPs), Faraday cups (FCs) and energy analyzers, mass spectrometry, as well as novel calorimetric and force probes (FPs). For each method, the concepts are introduced, hints at best practice are given, and a survey of the recently published literature is included. Starting with the sheath–plasma boundary, the principles of plane and cylindrical LPs and the measurement of electron temperature and density are discussed. Beyond standard textbooks, double, triple and emissive probes are described and modulation techniques for obtaining the electron energy distribution function are presented. More technical issues related to probe compensation in radio-frequency plasmas, probe contamination, and the influence of a magnetic field are also included. The presentation is completed by modern radio-frequency techniques, such as impedance probes, multipole resonance probes, and self-excited electron resonance spectroscopy. FCs, retarding field analyzers (RFAs) as well as novel calorimetric and FPs are useful tools for the measurement of overall, not species resolved, ions and neutral species fluxes toward surfaces. RFAs provide overall ion energy distribution functions, whereas calorimetric and FPs can deliver information about fluxes of fast neutrals. The issues related to secondary electron emission, absolute signal calibration, and analysis of plasmas with complex chemistry are discussed. Mass spectrometry diagnostics is capable of mass and energy-resolved detection of ions and neutral species at the plasma-facing surfaces. Detection, identification and absolute density calibration of neutral stable and radical species are treated in detail and peculiarities of ion mass spectrometry, such as detection of negative ions, ion measurements in front of powered electrodes, or chromatic aberration and acceptance angle limitations in energy spectrum measurements are introduced.

The dynamic evolution of the microscopic structure of solid and liquid phases of complex plasmas is studied experimentally and by means of molecular dynamics (MD) simulations. In small finite systems, the cooperative motion can be described in terms of discrete modes. These modes are studied with different experimental approaches. Using diffuse scattered laser light, applying laser tweezer forces to individual particles, and periodic laser pulses, the excitation of modes is investigated. The instantaneous normal mode analysis of experimental data from two-dimensional liquid clusters gives access to the local dynamics of the liquid phase. Our investigations shed light on the role of compressional and shear modes as well as the determination of diffusion constants and melting temperatures in finite systems. Special attention is paid to hydrodynamic situations with a stationary inhomogeneous dust flow. MD simulations allow to study the collective motion in the shell of nearest neighbors, which can be linked to smooth and sudden changes of the macroscopic flow. Finally, the observed micro-motion in all situations above allows to shed light on the preference of shear-like over compressional motion in terms of a minimized potential energy and a dynamic incompressibility.

In a streaming plasma, negatively charged dust particles create complex charge distributions on the downstream side of the particle, which are responsible for attractive forces between the like-charged particles. This wake phenomenon is studied by means of refined linear response theory and molecular dynamics simulations as well as in experiments. Particular attention is paid to non-Maxwellian velocity distributions that are found in the plasma sheath and to situations with strong magnetic fields, which are becoming increasingly important. Non-Maxwellian distributions and strong magnetic fields result in a substantial damping of the oscillatory wake potential. The interaction force in particle pairs is explored with the phase-resolved resonance method, which demonstrates the non-reciprocity of the interparticle forces in unmagnetized and magnetized systems.

Plasmas growing or containing nanometric dust particles are widely used and proposed in plasma technological applications for production of nano-crystals and surface deposition. Here, we give a compact review of in situ methods for the diagnostics of nanodust and nanodusty plasmas, which have been developed in the framework of the SFB-TR24 to fully characterize these systems. The methods include kinetic Mie ellipsometry, angular-resolved Mie scattering, and 2D imaging Mie ellipsometry to get information about particle growth processes, particle sizes and particle size distributions. There, also the role of multiple scattering events is analyzed using radiative transfer simulations. Computed tomography and Abel inversion techniques to get the 3D dust density profiles of the particle cloud will be presented. Diagnostics of the dust dynamics yields fundamental dust and plasma properties like particle charges and electron and ion densities. Since nanodusty plasmas usually form dense dust clouds electron depletion (Havnes effect) is found to be significant.

The potential and density wake behind a finite-sized object in a magnetized collisionless plasma flow is studied with self-consistent numerical simulations. With increasing magnetization of the plasma, the standard picture of ion focusing in the wake for plasmas with large electron to ion temperature ratios becomes invalid. A strong magnetic field parallel to the flow direction leads to a chain of ion depletions in the wake and enhanced ion density at their envelopes. This is due to a novel mechanism of a dynamic ion shadow, which is not the geometrical shadow of the finite-sized object. It corresponds to a change in topology of the wake potential. Complex ion trajectories resulting from electrostatic collisions with the object can lead to significant variations in electrical charging of other objects in the wake.

The potential and density structure behind a spherical microparticle in a magnetized ion flow are studied by means of molecular dynamics simulations. It is shown that, with increasing magnetization of the flow, the ion accumulation in the wake diminishes. Instead, ion depleted regions (shadows) form and ions accumulate at the edge of the shadows. The change of the ion density distribution also affects the force on other microparticles in the downstream region. For weak magnetization and a short distance, these interparticle forces can be attractive and non-reciprocal, as in the unmagnetized case. For large magnetization and further downstream, the force becomes repulsive. The mechanism of shadow formation is shown to involve a fast Coulomb scattering during a short fraction of the gyroperiod and subsequent trapping of the ions on large-radius gyro-orbits.The potential and density structure behind a spherical microparticle in a magnetized ion flow are studied by means of molecular dynamics simulations. It is shown that, with increasing magnetization of the flow, the ion accumulation in the wake diminishes. Instead, ion depleted regions (shadows) form and ions accumulate at the edge of the shadows. The change of the ion density distribution also affects the force on other microparticles in the downstream region. For weak magnetization and a short distance, these interparticle forces can be attractive and non-reciprocal, as in the unmagnetized case. For large magnetization and further downstream, the force becomes repulsive. The mechanism of shadow formation is shown to involve a fast Coulomb scattering during a short fraction of the gyroperiod and subsequent trapping of the ions on large-radius gyro-orbits.

A cloud of nanodust particles is created in a reactive argon-acetylene plasma. It is then transformed into a dusty magnetized argon plasma. Plasma parameters are obtained with the dust-density wave diagnostic introduced by Tadsen et al. [Phys. Plasmas 22, 113701 (2015)10.1063/1.4934927]. A change from an open to a cylindrically enclosed nanodust cloud, which was observed earlier, can now be explained by a stronger electric confinement if a vertical magnetic field is present. Using two-dimensional extinction measurements and the inverse Abel transform to determine the dust density, a redistribution of the dust with increasing magnetic induction is found. The dust-density profile changes from being peaked around the central void to being peaked at an outer torus ring resulting in a hollow profile. As the plasma parameters cannot explain this behavior, we propose a rotation of the nanodust cloud in the magnetized plasma as the origin of the modified profile.

The growth of nanometer-sized particle populations is studied in a reactive argon-acetylene plasma. Particular attention is paid to the spatio-temporal evolution of nested particle populations. For this purpose, a new non-invasive 2D imaging ellipsometry diagnostic is applied in combination with laser extinction measurements. In this way, the spatio-temporal evolution of particle size and density is obtained. The temporal evolution of the particle clouds during the first and subsequent growth cycles reveals a complex interplay between the different particle generations, the plasma and the resulting dust confinement. It is demonstrated that the electrostatic pressure needs to be considered in the force balance of systems with high dust densities. The relationship between the growth of particles and characteristic transition events visible in the dynamic of the void is discussed.

Dust-density wave fields in radio-frequency discharges under microgravity conditions exhibit complex spatiotemporal patterns. One of the most remarkable features, which reflects this behavior, is the occurrence of so-called frequency clusters, i.e., a stepwise spatial variation of the frequency inside the wave field. These frequencies were found to be incommensurable to each other. Such topological phenomena can be emulated by networks of locally coupled van der Pol oscillators. However, recent investigations showed that, in some situations, clusters with commensurable frequencies are preferred. The previously suggested model cannot explain such a preference. Hence, in this contribution, an extension of the existing model is proposed that, in addition to the local coupling, includes a global coupling mechanism.

Dust particles confined in radio frequency discharges can form 2-D and 3-D dust clouds. In front of a positively biased additional “pixel” electrode, 2-D vertical aligned particle chains as well as 3-D cone-like clouds were observed, that are strongly affected by the bias voltage of the additional electrode. In order to understand which mechanisms lead to the chain formation, a stereoscopic camera system with a high spatial resolution was used to reconstruct the 3-D particle positions and their trajectories.

In complex plasmas, self-excited dust-density waves (DDWs) are observed below a critical gas pressure. Under microgravity conditions, the dust cloud fills the entire discharge volume so that extended wave fields can be generated. Here, we study the relationship between plasma glow and DDWs. Analyzing the dust density and plasma glow by means of video microscopy reveals self-organized oscillations of the entire plasma. In the region of strongly modulated DDWs, also waves occur in the plasma glow, having wavelengths and frequencies comparable to those of the DDWs.

Molecular dynamic simulations were performed to study the characteristics of particle flows in a toroidal trap driven by the ion drag under the influence of gravity, interparticle forces, and friction with a neutral gas background. This paper is focused on the influence of the driving force and the strength of friction on the dynamics of the dust flow and its different regimes. For small values of the ion drag, the particle flow exhibits instabilities with strong velocity fluctuations, while for high values of the ion drag, a quiescent particle flow is obtained. The transition from unstable flows to quiescent flows is studied in detail.

A novel approach of using microparticles as probes for the sheath structure of radio-frequency discharges is presented. Starting with a heavy (large) particle confined deep in the plasma sheath, the ambient plasma causes a continuous mass loss, which increases the levitation height of the particle. It is shown that this process can be precisely monitored with the phase-resolved resonance method, which allows probing the force balance of the particle with high spatial resolution. The resulting force profile is in reasonable agreement with recent sheath models.

In the previous chapters, the description of the plasma state was refined step by step. In the single-particle model, we were interested in the motion of individual particles in typical magnetic field configurations, but the interaction between the particles and the modification of the fields by the presence and motion of charged particles was neglected. In the fluid model, we had considered the average behavior of particles filling a small volume of space. In this approximation, only moments of a shifted Maxwell distribution, like mean flow velocity or gas temperature, were retained, but, by combining with Maxwell’s equations, the model became self-consistent. The fluid model goes beyond the single-particle model in that pressure effects are now included. This fluid model, and its formulation in terms of MHD-equations, became capable to describe the combined macroscopic motion of plasma and magnetic field lines. A first attempt to deal with non-Maxwellian velocity distributions was the introduction of a beam-plasma system, which generates self-excited electrostatic waves near the electron plasma frequency.

The description of the hot gas of electrons and ions forming a plasma involves a number of non-deterministic or stochastic processes that require a statistical description. The plasma constituents have a wide spread of velocities and perform collisions between the charged particles, or with the gas atoms of the parent gas. The behavior of the plasma as a whole can no longer be reduced to the deterministic motion of individual particles in prescribe felds. Rather, the large number of particles introduces uncertainties that force us to describe the plasma by average quantities. For example, the average motion depends on macroscopic quantities like temperature and density gradients, which generate particle fluxes or electric currents. This Section discusses the stochastic motion of particles and introduces simple statistical concepts to describe typical transport processes in gas discharges. To illustrate the concepts, typical applications are given in gas discharges, in ion thrusters design for spacecrafts, or in the heat balance for nuclear fusion.

The fundamentals of the ‘resonance method’ are presented. The method relies on evaluating the dynamic response of one or more dust particles in the sheath of a laboratory plasma to small external perturbations. It allows one to make in situ high-precision measurements of particle properties. It is shown that the particle mass and charge and the strength of the interaction between two particles can be measured. Technical requirements, limitations and application examples are presented and discussed.

An overview on recent developments in the field of complex plasmas is presented. The focus is directed on the activities of research groups participating in the Collaborative Research Centre 24 “Fundamentals of Complex Plasmas” at the Universities in Greifswald and Kiel as well as at the Leibniz-Institute for Plasma Research and Technology. This article summarizes results on strongly correlated dusty plasmas, reactive as well as electronegative plasmas and plasma-surface interaction, and gives an introduction to the following articles of this special issue. Important developments are highlighted and future research directions are outlined (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The formation of wake fields downstream of an objects in flowing plasmas has strong implications on structure, stability and dynamics of complex plasmas. This paper aims at putting recent experimental investigations and different theoretical approaches on charging of dust grains, stability of dust grain arrangements and the dynamical properties of complex plasmas into perspective. Further, the combination of wake fields and grain confinement is discussed, which is the generic situation in complex plasmas. It is shown that in spherical traps the resulting competition of trap geometry and vertical alignment leads to a sophisticated mixture of nested shell and string order (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase transitions of finite systems of charged microspheres trapped in a gaseous discharge plasma in twodimensional and three-dimensional arrangements are investigated. Novel experimental techniques to drive phase transitions in these dust clusters as well as advanced theoretical concepts to quantitatively characterize these transitions are presented (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An overview is given of the confinement and dynamical phenomena observed in experiments and simulations of magnetized dust tori. Due to the presence of gravity, a strongly inhomogeneous velocity field is found along the circumference of the torus. The simulations show that the dust flow, which is unsheared and nearly incompressible, exhibits a distinct shell structure, which can be understood by rapid frictional cooling and strong Coulomb coupling. At lower frictional damping, the symmetry of the flow can be spontaneously broken, leading to a region of strong velocity shear and excitation of Kelvin-Helmholtz instabilities. New experimental evidence of counterflows is found (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of negative ions on the drift wave instability has been studied in detail in a linear device by means of Langmuir probes and cross-correlation analysis. Drift waves are excited in low-density (5 × 1014 m–3) and strongly magnetized (0.5 T) pure argon plasmas and in the presence of an oxygen admixture. The radial density profile of negative ions is hollow. For increasing concentration of negative ions the wave frequency decreases by about 25%. Despite of an axial density gradient, a global wave frequency is established for the entire column length. While for the noble gas case the drift wave frequency is given by the equilibrium plasma parameters in the mid-plane, there is no such relationship for the argon plasma with oxygen admixture. This different finding is attributed to the inhomogeneous distribution of the negative ions (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of velocity spread and ion-neutral collisions on the wake of a microparticle in a collisional magnetized ion flow is explored by means of molecular dynamics simulations. The ion flow is described in the constant-mean-free-path limit. A constant electric field is superimposed that maintains the ion drift at the Bohm speed and approximates conditions in the plasma sheath. The contribution of ion Landau damping to the wake structure is separated by simulations with a collisionless drift distribution. It is found that ion Landau damping and collisions have a counteracting effect on the ion density in the focus region. The dynamic shadows that are a typical feature of collisionless magnetized wakes with cold ion beams are damped by the velocity spread and vanish by a collision-enhanced ion density in the wake. Dynamic shadows reappear only at very high magnetic fields, B ≈ 10 T. In two-particle arrangements, the full collisional model shows that horizontal attractive forces persist up to B = 4 T but become repulsive for higher magnetization.

The wake behind a spherical microparticle in a magnetized ion flow is studied experimentally by analyzing the arrangement of a pair of particles. It is shown that there are two stable particle arrangements at intermediate magnetic inductions, whereas only oblique (horizontal) particle configurations are found at the highest magnetic field. Self-consistent collisional molecular dynamics simulations of the particle system show that the underlying mechanism of these arrangements is the weakening of attractive wake forces by the increasing magnetic field. Plasma instabilities provide a trigger for the onset of the transition between the two different arrangements. Furthermore, the course of the transition is qualitatively explained by the charge variation of the downstream particle in the wake of the upstream particle. In addition, a thorough analysis of the sheath by means of particle-in-cell simulations in combination with particle resonance measurements yields consistent values of the particle mass and charge, as well as the levitating electric field and ion flow velocity.

The charging process of micrometer-sized irregularly shaped particles in the sheath of a radio frequency discharge is measured using a combination of long-distance microscopy and the phase-resolved resonance method. The applicability of the method is shown for a cylindrical zinc oxide particle by measuring its mass density. A particle with more complex geometry is compared to a spherical polymethyl methacrylate particle to investigate the charging of irregularly shaped particles in detail. The results are similar to the charging of the smallest enclosing sphere suggesting that the charging process is independent of the shape of the particle. Furthermore, molecular dynamics simulations were performed, which support the experimental results.

Dusty-plasma experiments with flat dust clusters are often performed in radio frequency discharges at typical gas pressures of 1 to 100 Pa. For the understanding of the structure and the dynamical behavior of dust clusters, the grain charge and the effective Debye length are of key interest. Direct measurements of these quantities are a challenging task because the dust grains are confined in the plasma-boundary layer, where plasma diagnostics is difficult. Here, we present a new approach to determine the grain charge and the horizontal screening length, which does not require prior knowledge of the plasma parameters. The method is based on a slow and controlled rotation of the neutral-gas column due to a rotating electrode. This leads to centrifugal forces to the dust grains without changing the plasma conditions and allows studying the interparticle distance of a dust cluster as a function of its rotation frequency, which directly depends on the particle charge, the screening length, and the strength of the confining potential.

Dust particles confined in a magnetized anodic plasma can form different cloud shapes. In a certain parameter regime, torus-shaped dust clouds with a dust-free region (void) in the center are found. One can observe that most of the particles perform a rotational motion about the major axis of the torus. In order to understand the responsible mechanism, a model was developed to describe the confinement of dust particles by means of a force balance of electric field force and ion drag force derived from electric measurements of the plasma potential and the plasma density. Furthermore, a condition for the onset of the particle rotation and, with it, the formation of a torus-shaped cloud is developed. It is also possible to estimate the rotation velocity of the particles. The resulting values are compared with the data observed by particle image velocimetry.

Dust density waves (DDWs) are compressional modes that are often excited by subsonic ion flows in dusty plasmas. Previous experiments relying on imaging of only the dust revealed that they can propagate parallel to the ion flow direction or at an oblique angle. An experiment was performed using microgravity conditions on parabolic flights with video imaging of both the dust and the plasma glow. Glow arises from electron-impact excitation of neutral gas atoms, and it serves as a signature of energetic electrons. Averaging over time, it was found that the presence of dust enhances the glow brightness everywhere in the plasma. Resolving the time variation, a spontaneously excited DDW was observed at 3.9 Hz. It was characterized not only by a compression of the dust number density but also by a modulation of the glow intensity. The correlation between the wave and the glow is analyzed by Fourier methods. We found an unexpected phase relation between the plasma glow and the DDW of 118o. A glow maximum is followed by a dust density maximum.

In dusty plasmas, overlapping Debye spheres around dust grains could produce an attractive force between them. Its effects on static structures of two-dimensional (2D) dusty plasmas are studied here by using molecular dynamics simulations. Results, in terms of the equilibrium radial distribution function, are compared with those deduced from purely repulsive Debye–Huckel or Yukawa potential for different Coulomb-coupling and screening parameters. The effect of the attractive force is found quite noticeable for usual experimental conditions, and becomes more pronounced for larger screening parameter κ. In particular, it is observed that for large κ the attractive force is dominant, and dust grains tend to aggregate and form patterns with scattering voids.

To study the structural and dynamical properties of finite 3D dust clouds (Yukawa balls) new diagnostic tools have been developed. This contribution describes the progress towards 3D diagnostics for measuring the particle positions. It is shown that these diagnostics are capable of investigating the structural and dynamical properties of Yukawa balls and gaining insight into their basic construction principles.

Experiments on plasma turbulence and related transport processes generally require diagnostics with high spatio-temporal resolution. Standard tools for this purpose are statistical methods like cross-correlation analysis or conditional averaging. High spatial resolution is achieved at the expense of an asynchronous recording and a subsequent averaging process. Recently, we had shown that diagnostics arrays in combination with a super-resolution algorithm can achieve a comparably high spatial resolution [1]. This paper compares the results of both techniques applied to identical experimental conditions. It addresses the question of whether the higher efforts for spatio-temporal diagnostics are needed to study the dynamics of large-scale structures in a turbulent system and to what extent the results of statistical approaches are reliable.

Experiments on the laser manipulation of dust particles in a 3-D dust cloud under microgravity conditions are presented. The experiments have been performed on two parabolic flight campaigns in 2005 and 2006 to study the formation of the dust-free region ("void") in the center of the cloud. The manipulation laser is used to drive particles from the void edge into the void. From the particle trajectories, the forces on the particles are derived. The measured forces are compared to models of the ion drag force

Conditional averaging is one of the standard methods to detect large-scale structures in turbulent flows. By means of synthetic and experimental data it is shown that the usual trigger conditions are not sufficient to obtain reliable structure amplitudes. Our analysis shows that the amplitude reduction is caused by trigger jitter and a finite probability for trigger errors to occur. Using an improved conditional averaging scheme based upon a cross-correlation analysis, the possibility to remove these errors and to obtain correct amplitudes is shown.

The phase separation in binary mixtures of charged particles has been investigated in a dusty plasma under microgravity on parabolic flights. A method based on the use of fluorescent dust particles was developed that allows us to distinguish between particles of slightly different size. A clear trend towards phase separation even for smallest size (charge) disparities is observed. The diffusion flux is directly measured from the experiment and uphill diffusion coefficients have been determined.

The Brownian motion of a single particle in the plasma sheath is studied to separate the effect of stochastic heating by charge fluctuations from heating by collective effects. By measuring the particle velocities in the ballistic regime and by carefully determining the particle mass from the Epstein drag it is shown that for a pressure of 10 Pa, which is typical of many experiments, the proper kinetic temperature of the Brownian particle remains close to the gas temperature and rises only slightly with particle size. This weak effect is confirmed by a detailed model for charging and charge fluctuations in the sheath. A substantial temperature rise is found for decreasing pressure, which approximately shows the expected scaling with p(-2). The system under study is an example for non-equilibrium Brownian motion under the influence of white noise without corresponding dissipation.

The plasma parameters in the IMPF (International Microgravity Plasma Facility) device for studying complex plasmas are determined with a 2D-scanning Langmuir probe system, which is suitable for operation on the International Space Station. The probe characteristics in low density, low pressure radio frequency (rf) discharges are evaluated in the framework of the radial-motion theory with corrections for collisions. A critical comparison between experimental values for plasma potential and ion density with SIGLO simulations (Kinema Software) is made, which yields good overall agreement. In particular, the shaping of the plasma profile by different rf power in the plasma centre and in an outer ring is well described. Small amounts of dust particles are used as a novel method to mark the boundary, where ion-drag force and electric field force are balanced. Again close agreement with simulation is found and demonstrates the applicability of the tracer technique as a quantitative diagnostic means.

To study turbulence, high spatial and temporal resolution are required. In experiments, plasma diagnostics are characterized by low spatial and high temporal resolution. An adapted super-resolution algorithm is proposed to improve the spatial resolution by using temporal information. The performance of the super-resolution algorithm is compared with simple interpolation techniques. By means of synthetic data its performance is benchmarked.

The detection of large-scale structures in broadband turbulence is a recent task in neutral fluids and plasma physics. Conditional averaging is one of the most commonly used methods for this purpose. In this paper, the prospects and limitations of conditional averaging are systematically investigated. By means of an analytic model, synthetic and experimental data, the reliability of the conditional averaging method to resolve amplitudes and phase shifts of density and potential structures embedded in a noisy background are discussed.

Experiments on the quantitative determination of the ion drag force on free-falling dust particles in a collisionless regime are presented. The ion drag forces are measured for ion energies up to 40 eV and the obtained results are compared in detail with theories. Good agreement is found with the Barnes model [Barnes et al., Phys. Rev. Lett. 68, 313 (1992)] of the ion drag force for high ion energies (E>3 eV). At lower ion energies the model of Khrapak [Khrapak et al., Phys. Rev. Lett. 90, 225002 (2003)] is found to give a better description of the capture of slowly streaming ions by highly charged particles.

The spatio-temporal dynamics of large-scale structures in curved magnetic field of simple magnetized torus (SMT) are presented. The experiments are performed in the SMT BLAAMANN and employ the techniques of conditional averaging (CA) and cross-correlation (CCF) of Langmuir probe data. By means of a detailed comparison of radio-frequency and thermionic discharges, it is shown that general features of the observed large-scale structures are independent of the discharge mechanism. Further, the observed large scale structures have a pronounced spatio-temporal periodicity, which suggests a description as mode-like. The conjectured relationship between the observed coherent structures and the inverse energy cascade is therefore questioned again.

This contribution introduces a new experiment for the investigation of magnetized complex plasmas. Its basic features are discussed and first experiments on free-falling dust particles are presented. By means of Langmuir probes and the resonance cone technique, the influence of dust particles on the free electron density is studied. With both methods, significant electron density reductions are observed.

A new diagnostic method for magnetized dusty plasmas, the excitation of lower hybrid resonance cones, is investigated experimentally. The resonance cone is excited with a small antenna, and the angular distribution of the wave field with respect to the magnetic field shows a resonant enhancement, which shifts according to the free electron density. It is demonstrated that dust reduces the free electron density in agreement with Langmuir probe results. Wave damping by scattering effects is found negligible.

Experiments on the quantitative determination of the weaker forces (ion drag, thermophoresis, and electric field force) on free-falling dust particles in a rf discharge tube are presented. The strongest force, gravity, is balanced by gas friction and the weaker forces are investigated in the radial (horizontal) plane. Under most discharge conditions, the particles are found to be expelled from the central plasma region. A transition to a situation where the falling particles are focused into the plasma center is observed at low gas pressures using small particles. These investigations allow a quantitative understanding of the mechanism of unwanted dust-free areas (so-called voids) in dusty plasmas under microgravity. Good quantitative agreement with standard models of the ion drag is found.