Dietmar Block

Complex plasmas: a laboratory for strong correlations

Strong correlations—cooperative behavior due to many-particle interactions—are omnipresent in nature. They occur in electrolytic solutions, dense plasmas, ultracold ions and atomic gases in traps, complex (dusty) plasmas, electrons and excitons in quantum dots and the quark–gluon plasma. Correlation effects include the emergence of long-range order, of liquid-like or crystalline structures and collective dynamic properties (collective modes). The observation and experimental analysis of strong correlations are often difficult, requiring, in many cases, extreme conditions such as very low temperatures or high densities. An exception is complex plasmas where strong coupling can be easily achieved, even at room temperature. These systems feature the strongest correlations reported so far and experiments allow for an unprecedented precision and full single-particle resolution of the stationary and time-dependent many-particle behavior. The governing role of the interactions in strongly correlated systems gives rise to many universal properties observed in all of them. This makes the analysis of one particular system interesting for many others. This motivates the goal of this paper which is to give an overview on recent experimental and theoretical results in complex plasmas including liquid-like behavior, crystal formation, structural and dynamic properties. It is expected that many of these effects will be of interest also to researchers in other fields where strong correlations play a prominent role. (Some figures in this article are in colour only in the electronic version) This article was invited by Gordon Baym.

Dust confinement and dust-acoustic waves in weakly magnetized anodic plasmas

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.

Structural Properties of Screened Coulomb Balls

Small three-dimensional strongly coupled charged particles in a spherical confinement potential arrange themselves in a nested shell structure. By means of experiments, computer simulations, and theoretical analysis, the sensitivity of their structural properties to the type of interparticle forces is explored. While the normalized shell radii are found to be independent of shielding, the shell occupation numbers are sensitive to screening and are quantitatively explained by an isotropic Yukawa model.

Confinement of Coulomb balls

A model for the confinement of the recently discovered Coulomb balls is proposed. These spherical three-dimensional plasma crystals are trapped inside a rf discharge under gravity conditions and show an unusual structural order in complex plasmas. Measurements of the thermophoretic force acting on the trapped dust particles and simulations of the plasma properties of the discharge are presented. The proposed model of confinement considers thermophoretic, ion-drag, and electric field forces, and shows excellent agreement with the observations. The findings suggest that self-confinement does not significantly contribute to the structural properties of Coulomb balls.

Charging and dynamics of a dust grain in the wake of another grain in flowing plasmas

The charging of a dust grain in supersonic plasma flows in the wake of another grain is studied by numerical simulations. While entering the Mach cone originating from the upstream grain, the grain is discharged by scattered ions. Electrostatic forces acting on the grain in the wake will move it to the stable position in the wake at a distance close to the electron Debye length from the upstream grain. The onset for discharging can be used to estimate the ion flow speed in the system. The simulations are carried out with the DiP3D code, a three-dimensional particle-in-cell code where both electrons and ions are represented as numerical particles [W. J. Miloch et al., Nonlinear Processes Geophys. 14, 575 (2007); New J. Phys. 11, 043005 (2009)].

Chaos Control and Taming of Turbulence in Plasma Devices

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 b...

Classical and quantum Coulomb crystals

Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb (Wigner) crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed.

On the influence of wakefields on three-dimensional particle arrangements

The role of wakefields on the three-dimensional (3D) particle arrangement in finite dust clouds at low neutral gas pressures is investigated experimentally. Using a novel stereoscopic digital in-line holography approach for the instantaneous measurement of the 3D particle positions, the structural properties of finite dust clouds are studied. The competition of a nested shell structure (known from Yukawa balls) and vertical particle chains (known for particle clouds confined in the plasma sheath) results in a markedly different particle arrangement. With experiments studying the dynamical response in a two-particle system for identical plasma and confinement conditions, we show that the presence of an ion focus in the wake of the particle is responsible for the observed structural differences.

Development of a fast impedance probe for absolute electron density measurements in the ionosphere

The design of a digitally controlled impedance probe for the fast absolute measurement of the ionospheric plasma density aboard a sounding rocket is described. Instrument performance is tested in a large plasma chamber. The first results from two flights within the DEOS-campaign are reported, and discussed with respect to spatial resolution and wake effects. The resulting density profiles have the same shape as ground-based ionosonde measurements. The in situ impedance probe measurements yield absolute electron densities that are lower by a factor of 1.5-2.

Laser heating of finite two-dimensional dust clusters: A. Experiments

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.