C.-R. Du

Agglomeration of microparticles in complex plasmas

Agglomeration of highly charged microparticles was observed and studied in complex plasma experiments carried out in a capacitively coupled rf discharge. The agglomeration was caused by strong waves triggered in a particle cloud by decreasing neutral gas pressure. Using a high-speed camera during this unstable regime, it was possible to resolve the motion of individual microparticles and to show that the relative velocities of some particles were sufficiently high to overcome the mutual Coulomb repulsion and hence to result in agglomeration. After stabilizing the cloud again through the increase of the pressure, we were able to observe the aggregates directly with a long-distance microscope. We show that the agglomeration rate deduced from our experiments is in good agreement with theoretical estimates. In addition, we briefly discuss the mechanisms that can provide binding of highly charged microparticles in a plasma.

Interaction of two-dimensional plasma crystals with upstream charged particles

Two-dimensional plasma crystals are characterized by a strong up-and-down asymmetry not only due to gravity but also due to the presence of plasma flow at the location of particles. We study for the first time the interaction of a single-layer plasma crystal with charged extra particles located above it (upstream of the flow of ions). Upstream extra particles tend to move between the rows of particles in the crystal, accelerate to supersonic speeds, and excite attraction-dominated Mach cones and wakes in the crystal.

Channeling of particles and associated anomalous transport in a two-dimensional complex plasma crystal

Implications of the recently discovered effect of channeling of upstream extra particles for transport phenomena in a two-dimensional plasma crystal are discussed. Upstream particles levitated above the lattice layer and tended to move between the rows of lattice particles. An example of heat transport is considered, where upstream particles act as moving heat sources, which may lead to anomalous heat transport. The average channeling length observed was 15-20 interparticle distances. Other features of the channeling process are also reported.

Model experiment for studying lane formation in binary complex plasmas

We present the first experimental realisation on lane formation in binary complex plasmas under gravity conditions. The amount of penetrating particles can be controlled. The experiment can be operated with stable conditions continuously, which allows to study steady state. The driving force is independent of time and position and the background density is homogeneous and isotropic. This provides an ideal model system for comparison with numerical experiments and observations in colloidal suspensions. The experiment setup is based on PK-3 Plus laboratory operated on the International Space Station. Gravitation is compensated by thermophoretic force and penetrating particles are controlled via a toroidal vortex with poloidal flow. The evolution of lane formation along the penetration direction is illustrated by lane order parameter.

Frequency dependence of microparticle charge in a radio frequency discharge with Margenau electron velocity distribution

rf discharges are widely used in complex plasma experiments. In this paper, we theoretically investigate the dependence of the particle floating potential on the discharge frequency, assuming the model Margenau expression for the electron velocity distribution function. In doing so we use the orbital motion limited cross section to calculate the electron flux to the particle and collision enhanced collection approximation for the ion flux to the particle. The floating potential is then obtained from the flux balance condition. It is shown that for typical plasma conditions in laboratory rf discharges, normalized floating potential grows with increase of the discharge frequency in collisionless regime and decreases in weakly collisional regime. However, variations in the floating potential are usually small when plasma parameters do not depend on the rf frequency.

Microparticles deep in the plasma sheath: Coulomb “explosion”

A cloud of microparticles was trapped deep in the sheath of a radio-frequency (rf) discharge, very close to the lower (grounded) electrode of the plasma chamber. This was achieved by employing a specifically designed rf-driven segment integrated in the lower electrode, which provided an additional confinement compressing the cloud to a very high density. After switching the rf-driven segment off, the cloud “exploded” due to mutual interparticle repulsion. By combining a simple theoretical model with different numerical simulation methods, some basic properties of complex plasmas in this highly non-equilibrium regime were determined.

Lane formation in binary complex plasmas: role of non-additive interactions and initial configurations

In this letter, we study the influence of non-additive interactions on lane formation, using Langevin dynamics simulations. Lane formation and positive non-additivity have recently been observed in binary complex plasmas on board the International Space Station (ISS). Positive non-additivity of particle interactions is known to stimulate phase separation (demixing), but its effect on lane formation is unknown. We show that there is a non-additivity–stimulated crossover from the normal laning mode to a demixing-dominated laning mode. To analyze this crossover on the individual particle level we applied a very sensitive order parameter for lane formation based on anisotropic scaling indices. Extensive numerical simulations enabled us to identify a critical value of the non-additivity parameter Δ for the crossover. In addition the simulations revealed that the dynamics of lane formation is strongly influenced by the exact spatial configurations at the very moment of contact between two different complex plasmas.

Expansion of complex plasma and dust particle charge

Summary form only given. Plasma expansion from a hot dense source to a low pressure environment is relevant to many subjects in science [1], [2]. In this contribution we present the expansion of dusty plasma inside a plasma sheath of an radio-frequency (rf) discharge using specifically designed segmented electrode and estimate the charge of particles at the initial stage of the expansion.

Interaction of 2D plasma crystals with upstream charged particle: Mach cones and channeling effect

Summary form only given. Complex plasmas are composed of a weakly ionized gas and charged microparticles1. In ground-based experiments the particles can levitate above the bottom electrode against gravity by a strong electric field in the (pre)sheath area. Under certain experimental conditions particles can be confined in a single layer and self-organize in a triangular lattice with hexagonal symmetry. Such system is known as 2D plasma crystal.

The charging of dust particles in the range of very high discharge frequencies

Capacitively coupled plasmas are widely used in a variety of thin film etching and deposition applications. The studies and experimental investigations in this field have shown that increasing frequencies above the conventional 13.56 MHz results in the increase of the deposition rate and at the same time minimizes the film damage [1]. On the other hand dust particles are often present in the plasma reactors as a sputtering or nucleation product and may influence the manufacturing. Therefore, controlling the behavior of dust particles is essential for improving of the manufacturing techniques in the low temperature plasma processing.