Alexander D. Usachev

PK-4: Complex Plasmas in Space—The Next Generation

PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc discharge is produced in a glass tube with a length of 35 cm and a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them

The project “Plasmakristall — 4” (PK-4) — a dusty plasma experiment in a combined dc/rf(i) discharge plasma under microgravity conditions

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and ISS (International Space Station). The aim of the PK-4 experiment is an investigation of physical processes in complex (dusty) plasmas under microgravity conditions in a combined dc/rf discharge plasma.

Particle flows in a dc discharge in laboratory and microgravity conditions

We describe a series of experiments on dust particles flows in a positive column of a horizontal dc discharge operating in laboratory and microgravity conditions. The main observation is that the particle flow velocities in laboratory experiments are systematically higher than in microgravity experiments for otherwise identical discharge conditions. The paper provides an explanation for this interesting and unexpected observation. The explanation is based on a physical model, which properly takes into account main plasma-particle interaction mechanisms relevant to the described experimental study. A comparison of experimentally measured particle velocities and those calculated using the proposed model demonstrates reasonable agreement, both in laboratory and microgravity conditions, in the entire range of discharge parameters investigated.

Cooperative phenomena in laminar fluids: observation of streamlines

Complex plasmas are an ideal model system to investigate laminar fluids as they allow to study fluids at the kinetic level. At this level we are able to identify streamlines particle by particle. This gives us the ability to research the behaviour of these streamlines as well as the behaviour of each individual particle of the streamline.We carried out our experiments in a modified GEC‐RF‐Reference cell. We trapped the particles within two glass rings and forced them to form a circular flow by using several stripe electrodes. In this flow the particles behave like an ideal fluid and form streamlines. By putting an obstacle into the flow we reduce the cross‐section. To pass through this constricted cross‐section some streamlines have to reconnect. After the obstacle the streamlines split up again. An analysis how streamlines split up and reconnect as result of external pressure on the fluid in our system is presented here.Streamlines also occur if two clouds of particles penetrate each other. We call this ...

Recent Complex Plasma Experiments in a DC Discharge

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

Assessing particle kinematics via template matching algorithms

Template matching algorithms represent a viable tool to locate particles in optical images. A crucial factor of the performance of these methods is the choice of the similarity measure. Recently, it was shown in [Gao and Helgeson, Opt. Express 22 (2014)] that the correlation coefficient (CC) leads to good results. Here, we introduce the mutual information (MI) as a nonlinear similarity measure and compare the performance of the MI and the CC for different noise scenarios. It turns out that the mutual information leads to superior results in the case of signal dependent noise. We propose a novel approach to estimate the velocity of particles which is applicable in imaging scenarios where the particles appear elongated due to their movement. By designing a bank of anisotropic templates supposed to fit the elongation of the particles we are able to reliably estimate their velocity and direction of motion out of a single image.

Self-organization in complex plasmas — lane formation and beyond

Complex plasmas consist of micron-sized particles inside a low temperature plasma usually generated by a rf or dc discharge in a gas at a pressure around 1 mbar. The particles collect negative charges of several thousand electrons and interact with themselves, the plasma, and (confining) electric fields. By adjusting the plasma parameters complex plasmas can be put in nearly any state from crystalline (plasma crystal) to super-critical. This can be visualized by cameras and allows to study the behavior of matter on the kinetic level of individual particles. To get rid of the disturbing influence of gravity complex plasma experiments were performed onboard the space station ISS and on parabolic flights. The PK-4 setup uses a dc discharge in a glass tube to create complex plasmas to study fluids on the particle level. Experiments on parabolic flights exhibit effects of self- organization in complex plasma flows, such as lane formation. Particles in interpenetrating clouds form lanes to minimize the energy to pass through each other. These effects are also known from granular media and even moving crowds. The behavior of complex plasma flows in PK-4 is compared to these cases.

Structural and Dynamical Properties of Microrod Dusty Plasma in a Uniform DC Discharge under Microgravity.

In present experiment an ordering and dynamics of monodisperse nylon rods (Du2009=u200910u2009μm, Lu2009=u2009300u2009μm) in a uniform DC gas discharge plasma under microgravity have been investigated. Ordered rod structures were registered in DC discharge with a rod concentration of 400–8000u2009cm−3 and a neon pressure range of 20–50 Pa. The structures revealed orientationally ordered hexagonal structures. DC discharge became unstable at rod number density more then 8000u2009cm−3. Rod drift velocities in a permanent electric field were measured for the neon pressure range. Dust acoustic instability (ν∼0.4±0.1u2009Hz, λ∼1.1±0.4u2009cm, CDAW∼0.5u2009cm/s) in rod cloud was observed at a neon pressure of 25 Pa and a rod number density of 1500u2009cm−3. Using the “low” frequency approximation of the linearized DAW dispersion relation and the measured rod drift velocity a rod electric charge had been estimated as ZR∼150000e.