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This contribution summarizes recent experiments in dusty plasmas and puts their results into context. The aim is to discuss the fundamental question whether a dusty plasma can be regarded as a model system for strong coupling and whether the treatment as a one-component plasma is a good and valid approximation. [ABSTRACT FROM AUTHOR]

An enhanced, high-precision, in situ method to determine the radii and refractive indices of single microparticles embedded in a plasma is presented. The particles are confined in the plasma sheath and illuminated with laser light that has a well-defined and adjustable state of polarization. Using an out-of-focus imaging setup, the angle- and polarization-resolved scattering intensities are measured and compared to Lorentz–Mie theory. A two-stage data evaluation process is used to obtain the particle size and the complex refractive index of different particle materials as a function of interaction time with the plasma. [ABSTRACT FROM AUTHOR]

This article presents a method to measure the photophoretic force on microparticles in complex plasmas. A laser heating setup is used to increase the kinetic temperatures of binary mixtures composed of melamine-formaldehyde (MF) and silica (SiO2) particles. The temperature gain of both particle species due to the additional thermal reservoir is compared to a laser force model. Contributions of radiation pressure and photophoretic forces are determined through a variation of neutral gas pressure and application of Lorenz-Mie theory. The photophoretic force is found to be significant for MF particles at typical conditions in complex plasma experiments. [ABSTRACT FROM AUTHOR]

With the ability to trap a single particle in a dusty plasma, it becomes possible to alter the structural properties of the particle assembly or to manipulate its dynamical behavior. Here, we report on a first experiment to excite the eigenmodes of a finite dusty plasma crystal by means of a controlled oscillation of a single particle being trapped in a modified optical tweezer. The eigenmode structure and the excitation process are analyzed. [ABSTRACT FROM PUBLISHER]

The possibility to use diffuse reflections of a laser beam to exert a force on levitating dust particles is studied experimentally. Measurements and theoretical predictions are found to be in good agreement. Further, the method is applied to test the selective excitation of breathing-like modes in finite dust clusters. [ABSTRACT FROM AUTHOR]

Finite size plasma crystals confined in an anisotropic potential well were studied under a rotating and radially unsheared drive in experiment and simulation at moderate rotational frequencies. A radially sheared rotation of these strongly coupled systems is observed for most cluster configurations with a low symmetry. The results show that a differential rotation can be effected by a non-sheared driving force. [ABSTRACT FROM AUTHOR]

The shear viscosity of a finite two-dimensional (2D) Yukawa liquid is calculated using non-equilibrium Langevin simulations. Two counter-propagating shear forces are used to push the particles and cause shear-induced melting of the cluster. The melting temperature of the 2D cluster system is used to derive the effective coupling parameter Γ * of this finite system and allows us to investigate the relationship between shear viscosity and system properties Γ * and κ. The simulation data shows that the value of the shear viscosity rises for coupling parameters Γ * > 20 and follows a simple universal scaling for appropriate normalization. It is further found that the Green–Kubo relation is applicable to determine viscosity in finite non-equilibrium systems with Γ * < 80. [ABSTRACT FROM AUTHOR]

Laser manipulation allows to control the kinetic particle temperature in dusty plasmas. Different methods of laser heating for plasma crystals are benchmarked experimentally. The methods are analyzed with respect to homogeneity and isotropy in a spatial, temporal, and statistical sense. It is shown that it is possible to achieve particle dynamics very close to thermal equilibrium and that laser heating methods allow for a detailed study of phase transitions in finite size systems. [ABSTRACT FROM AUTHOR]

Experiments on dust-acoustic waves (DAW) in a magnetized anodic plasma are presented for the regime of low collisionality. The dust trapping and the self-excited and synchronized DAW dynamics are studied. Based on Langmuir and emissive probe measurements the dust confinement is found to be well described with respect to size, stability, and position of the dust cloud by an effective potential well formed by ion drag and Coulomb forces. Moreover, the measurements indicate the necessity for a kinetic model for the wave dispersion. By means of singular value decomposition the local wavelengths and growth rates of the waves are measured systematically. It is found that the measured mean wave number is well described by kinetic theory, while the theoretical growth rates overestimate the experiments. A novel observation for the DAW is a systematic variation of the wavelength inside the dust cloud. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Experiments on the quantitative determination of the ion drag force of an ion beam on free-falling dust particles in a thermionic filament discharge are presented. The ion beam component of the plasma is oriented perpendicular to the dominating gravitational force. Therefore, the ion drag force is directly given by the observed particle deflection from the vertical direction. The deflection angle is measured as a function of beam energy. The obtained results are compared with theory. Good agreement is found with the Barnes model of the ion drag force, when the linearized Debye-length λD is used. [ABSTRACT FROM AUTHOR]

Commonly used melamine formaldehyde micro-particles exposed to an rf discharge are known to be etched by a plasma as soon as an admixture of oxygen is present. By means of in situ high precision size measurements, the plasma–surface interaction is investigated. A comparison of experimental data, advanced Mie-scattering techniques, and a reaction rate model allows, for the first time, to quantitatively describe the etch process. [ABSTRACT FROM AUTHOR]

The change in entropy of a system that is transferred between two states at different temperatures is measured in a two-dimensional plasma crystal experiment. One- and especially two-component dust clusters are confined in the plasma sheath and heated to different temperatures using laser manipulation. We find that entropies obtained from the phase space yield consistent results for, i.e., the heat capacity which shows excellent agreement with the Dulong-Petit law. The implications for the validity of basic thermodynamical principles in finite size complex (dusty) plasmas are discussed. [ABSTRACT FROM AUTHOR]

Abstract: 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. Graphical abstract: [ABSTRACT FROM AUTHOR]

Single micron-sized melamine-formaldehyde particles are levitated in the sheath of an rf-plasma and exposed to an intense laser beam, while being trapped in optical tweezers. A reversible change in the particles' properties is observed and quantitatively analyzed using reference particles. The investigations indicate a gain in particle charge where the initial charge restores within minutes. Possible reasons for these findings are discussed. [ABSTRACT FROM AUTHOR]

Towards the investigation of polydisperse complex plasmas, the generation of binary mixtures is a first and important step. In this paper, the requirements for the generation of two-dimensional binary mixtures are discussed and first experiments are presented. In particular, the material dependent mass loss of the particles is taken into account by monitoring the charge-to-mass ratio of both species using a resonance method. The charge ratio q1/q2 of the species is determined and the spacial properties of the two-component crystals are examined and compared to numerical simulations. [ABSTRACT FROM AUTHOR]

This paper describes the experimental realization of an optical trap for microparticles levitating in the plasma sheath. Single particles can be trapped in a laser beam comparable to optical tweezers known from colloidal suspensions. The trapping mechanism is discussed and two applications of the system are shown. [ABSTRACT FROM AUTHOR]

The combination of a stereoscopic setup for particle tracking velocimetry of small dust clouds combined with holographic techniques gives several benefits over using either method separately. However, technical limitations and achievable data quality require sophisticated reconstruction algorithms to gain complete and reliable particle trajectories. [ABSTRACT FROM PUBLISHER]

The phenomenon of particle chain formation is studied in a two-particle system. A wake of positive ions leads to an alignment of the negatively charged particles parallel to the ion flow. The dynamic response of this dust system to a small external perturbation is evaluated. It is shown that the eigenfrequency of the downstream particle is reduced compared to an isolated particle. This effect can be identified as a decharging of the particle by the focused ion flow in the wake of the upstream particle. Furthermore, a strong asymmetry of the particle interaction parallel to the ion flow is found. This asymmetry may not be attributed entirely to the interaction forces mediated by the ion wake. [ABSTRACT FROM AUTHOR]

The ability to follow the motion of individual particles in a dusty plasma is essential for most experiments on complex plasmas. However, for 3-D particle clouds, the precise determination of all particle coordinates at once is a sophisticated task. This paper introduces and benchmarks a new diagnostic tool which combines the techniques of digital inline holography and stereoscopy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Chaos and turbulence are often considered as troublesome features of plasma devices. In the general framework of nonlinear dynamical systems, a number of strategies have been developed to achieve active control over complex temporal or spatio-temporal behavior. Many of these techniques apply to plasma instabilities. In the present paper we discuss recent progress in chaos control and taming of turbulence in three different plasma “model” experiments: (1) Chaotic oscillations in simple plasma diodes, (2) ionization wave turbulence in the positive column of glow discharges, and (3) drift wave turbulence in a magnetized plasma column. Depending on the physical mechanism of the specific instability in each case, an appropriate control strategy is chosen out of a variety of different approaches; in particular discrete feedback, continuous feedback, or spatio-temporal open-loop synchronization. Electric control fields are externally applied to the plasma device and the chaotic or turbulent state is stabilized by only weak perturbations of the plasma equilibrium. The success of this approach is demonstrated in both experiment and numerical simulation and the actual effect of the applied control fields is investigated. [ABSTRACT FROM AUTHOR]

An in-situ method to measure the radii of single microparticles in plasmas with high precision is presented. The particles are trapped in the plasma sheath and illuminated with laser light. Using out-of-focus imaging and polarizing optics, the angle- and polarization-resolved scattering intensities are measured and compared to Lorenz-Mie theory. A two-stage fit procedure is introduced to obtain the complex refractive index in addition to the particle radius. Complementary long-distance microscopy measurements are performed to compare with fit results. The method is applied to particles of different materials used in complex plasma research to measure etch rates due to plasma inherent processes. [ABSTRACT FROM AUTHOR]

The spectral properties of three-dimensional dust clusters confined in gaseous discharges are investigated using both a fluid mode description and the normal mode analysis (NMA). The modes are analysed for crystalline clusters as well as for laser-heated fluid-like clusters. It is shown that even for clusters with low particle numbers and under presence of damping fluid modes can be identified. Laser-heating leads to the excitation of several, mainly transverse, modes. The mode frequencies are found to be nearly independent of the coupling parameter and support the predictions of the underlying theory. The NMA and the fluid mode spectra demonstrate that the wakefield attraction is present for the experimentally observed Yukawa balls at low pressure. Both methods complement each other, since NMA is more suitable for crystalline clusters, whereas the fluid modes allow to explore even fluid-like dust clouds. [ABSTRACT FROM AUTHOR]

The quantities entropy and diffusion are measured for two- and three-dimensional (3D) dust clusters in the fluid state. Entropy and diffusion are predicted to be closely linked via unstable modes. The method of instantaneous normal modes is applied for various laser-heated clusters to determine these unstable modes and the corresponding diffusive properties. The configurational entropy is measured for 2D and 3D clusters from structural rearrangements. The entropy shows a threshold behavior at a critical temperature for the 2D clusters, allowing us to estimate a configurational melting temperature. Further, the entropie disorder increases for larger clusters. Finally, the predicted relation between entropy and unstable modes has been confirmed from our experiments for 2D systems, whereas 3D systems do not show such a clear correlation. [ABSTRACT FROM AUTHOR]

The liquid state and the freezing transition of finite two-dimensional dust systems are studied using the instantaneous normal mode (INM) analysis. This technique measures the instantaneous unstable modes of a cluster configuration and relates them to the liquid properties of the system. Here, the INM analysis has been applied to experiments on laser-heated dust clusters. From the experiments, diffusion constants and melting temperatures for clusters of different size have been derived. The INM diffusion constants have been compared to those derived from other standard approaches. The scatter of the diffusion constant retrieved by the INM is smaller than that retrieved by other methods, allowing a more reliable determination of melting temperatures. Moreover, the behavior of double-well and escape modes, which reflect certain topological properties of unstable modes, correlates very well with the behavior of the diffusion constant. Further, the dynamic nature of the unstable modes has been determined as mostly shearlike. Finally, the INM results on the experiments are checked against those from Langevin simulations. [ABSTRACT FROM AUTHOR]

The experimental melting transition of finite two-dimensional dust clusters in a dusty plasma is analyzed using the method of instantaneous normal modes. In the experiment, dust clusters are heated in a thermodynamic equilibrium from a solid to a liquid state using a four-axis laser manipulation system. The fluid properties of the dust cluster, such as the diffusion constant, are measured from the instantaneous normal mode analysis. Thereby, the phase transition of these finite clusters is approached from the liquid phase. From the diffusion constants, unique melting temperatures have been assigned to dust clusters of various sizes that very well reflect their dynamical stability properties. [ABSTRACT FROM AUTHOR]

Laser heating of monolayer dusty plasmas is investigated theoretically by Langevin dynamics simulations. The laser radiation pressure is used to externally control the dust temperature without changing the plasma properties. We show that the laser scanning pattern has a major influence on both the velocity distribution function and the stationary structure of the cluster. Furthermore, the heating effect is found to be enhanced when the laser spots move with slightly higher frequencies than the trap frequency. The simulations confirm that a proper thermodynamic excitation of the dust particles is possible. [ABSTRACT FROM AUTHOR]

The melting transition of finite three-dimensional dust clouds (Yukawa balls) from a solid-like to a liquid-like state is systematically studied with high spatial and temporal resolution of the individual grains by means of stereoscopy. Two different melting scenarios are reported: Melting is induced first by an increase of plasma power, and second by laser-induced heating. The experiments confirm that melting starts with a loss of orientational correlation, followed by a loss of the radial order upon further heating. While the plasma-power melting is driven via the ion wakefield, laser heating provides a more equilibrium scenario. The internal loss of correlations is well captured by the triple correlation function (TCF) which is insensitive to particle exchanges and the rotation of the cluster as a whole. The critical Coulomb coupling parameter for N = 35 is determined as Γcrit ≈ 570. The experimental findings are in good agreement with thermodynamic Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Abstract: 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. Graphical abstract: [ABSTRACT FROM AUTHOR]