Theory of a cavity around a large floating sphere in complex (dusty) plasma

In the last experiment with the PK-3 Plus laboratory onboard the International Space Station, interactions of millimeter-size metallic spheres with a complex plasma were studied~[M. Schwabe {\it et al.}, New J. Phys. {\bf 19}, 103019 (2017)]. Among the phenomena observed was the formation of cavities (regions free of microparticles forming a complex plasma) surrounding the spheres. The size of the cavity is governed by the balance of forces experienced by the microparticles at the cavity edge. In this article we develop a detailed theoretical model describing the cavity size and demonstrate that it agrees well with sizes measured experimentally. The model is based on a simple practical expression for the ion drag force, which is constructed to take into account simultaneously the effects of non-linear ion-particle coupling and ion-neutral collisions. The developed model can be useful for describing interactions between a massive body and surrounding complex plasma in a rather wide parameter regime.Comment: 9 pages, 4 figures; to be published (2019

Penetration of a supersonic particle at the interface in a binary complex plasma

The penetration of a supersonic particle at the interface was studied in a binary complex plasma. Inspired by the experiments performed in the PK-3 Plus Laboratory on board the International Space Station, Langevin dynamics simulations were carried out. The evolution of Mach cone at the interface was observed, where a kink of the lateral wake front was observed at the interface. By comparing the evolution of axial and radial velocity, we show that the interface solitary wave is non-linear. The dependence of the background particle dynamics in the vicinity of the interface on the penetration direction reveals that the disparity of the mobility may be the cause of various interface effects

Heat transport in a flowing complex plasma in microgravity conditions

Heat transport in a three-dimensional complex (dusty) plasma was experimentally studied in microgravity conditions using a Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). An extended suspension of microparticles was locally heated by a shear flow created by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed, and from these, using a fluid heat transport equation that takes viscous heating and neutral gas drag into account, the complex plasma’s thermal diffusivity and kinematic viscosity were calculated. Their values are compared with previous results reported in ground-based experiments with complex plasmas

"Zyflex": next generation plasma chamber for complex plasma research in space

Complex plasmas consist of highly charged micrometer-sized grains injected into a low temperature noble gas discharge. Since gravity has a strong influence on the particle system, experiments under microgravity conditions are essential. A novel plasma chamber (the "Zyflex" chamber) has been designed for complex plasma research in a future facility on the International Space Station (ISS). The cylindrical, radiofrequency driven discharge device includes a variety of innovations that for example allow to flexibly adjust plasma parameters and its volume. Compared to former chambers used in space based complex plasma facilities, it also supports much larger particle systems and can be operated at much lower gas pressures, thus reducing the damping of particle motion considerably. Beyond the technical description and particle-incell (PIC) simulation based characterization of the plasma vessel, we show sample results from experiments performed with this device in the lab as well as during parabolic flights. Further, an outlook on the future ISS facility COMPACT with the Zyflex chamber at its core is given. This work is funded by DLR/BMWi (FKZ 50WM1441)

Dissipative solitary wave at the interface of a binary complex plasma

The propagation of a dissipative solitary wave across an interface is studied in a binary complex plasma. The experiments were performed under microgravity conditions in the PK-3 Plus Laboratory on board the International Space Station using microparticles with diameters of 1.55 micrometre and 2.55 micrometre immersed in a low-temperature plasma. The solitary wave was excited at the edge of a particle-free region and propagated from the sub-cloud of small particles into that of big particles. The interfacial effect was observed by measuring the deceleration of particles in the wave crest. The results are compared with a Langevin dynamics simulation, where the waves were excited by a gentle push on the edge of the sub-cloud of small particles. Reflection of the wave at the interface is induced by increasing the strength of the push. By tuning the ion drag force exerted on big particles in the simulation, the effective width of the interface is adjusted. We show that the strength of reflection increases with narrower interfaces

Ekoplasma - The Future of Complex Plasma Research in aboard the International Space Station

Ekoplasma is a joint German-Russian project, developing the future multi-purpose laboratory for the investigation of complex plasmas under microgravity conditions aboard the International Space Station (ISS). Complex plasmas are low-temperature plasmas, consisting of neutral gas atoms, ions, electrons and micro-meter sized particles as an additional component. The particles become charged in the plasma and as a result of their mutual repulsion form an optically thin cloud that can be studied in its full spatial and dynamical complexity on the granularity scale of each particle by optical cameras. Therefore, complex plasmas allow fundamental investigations down to the kinetic level of individual particles also for a wide field of interdisciplinary topics in classical condensed matter physics. Since gravity prevents the formation of large, homogeneous systems on earth, research on the ISS is essential, and Ekoplasma will follow in a line of successful preceding experiments aboard the ISS: PKE-Nefedov, PK-3 Plus and the currently operating PK-4 facility. Ekoplasma is planned to be launched to the ISS in 2022, and it will cover a wide range of research topics such as solidification and melting, phase separation in binary systems, the transition to turbulence, active matter or electrorheology

Three-dimensional structure of a string-fluid complex plasma

Three-dimensional structure of complex (dusty) plasmas was investigated under long-term microgravity conditions in the International-Space-Station-based Plasmakristall-4 facility. The microparticle suspensions were confined in a polarity-switched dc discharge. The experimental results were compared to the results of the molecular dynamics simulations with the interparticle interaction potential represented as a superposition of isotropic Yukawa and anisotropic quadrupole terms. Both simulated and experimental data exhibited qualitatively similar structural features indicating the bulk liquid-like order with the inclusion of solid-like strings aligned with the axial electric field. Individual strings were identified and their size spectrum was calculated. The decay rate of the size spectrum was found to decrease with the enhancement of string-like structural features

Slowing of acoustic waves in electrorheological and string-fluid complex plasmas

The PK-4 laboratory consists of a direct current plasma tube into which microparticles are injected, forming a complex plasma. The microparticles acquire many electrons from the ambient plasma and are thus highly charged and interact with each other. If ion streams are present, wakes form downstream of the microparticles, which lead to an attractive term in the potential between the microparticles, triggering the appearance of microparticle strings and modifying the complex plasma into an electrorheological form. Here we report on a set of experiments on compressional waves in such a string fluid in the PK-4 laboratory during a parabolic flight and on board the International Space Station. We find a slowing of acoustic waves and hypothesize that the additional attractive interaction term leads to slower wave speeds than in complex plasmas with purely repulsive potentials. We test this hypothesis with simulations, and compare with theory

Effect of Negative Ion Generation on Complex Plasma Structure Properties

We propose a low-density discharge plasma model that takes into account the impact of oxygen admixture in typical conditions of complex (dusty) plasmas. Numerical simulations based on this model show that the concentration of negative ions turns out to be very high, and they play an important role in the overall kinetics in this particular range of plasma conditions. The ambipolar diffusion electric field drags these negative ions into the center of the plasma. The density of negative ions is high enough to push the negatively charged dust component out of the center, both by weakening the radial electric field and by increasing the thermophoretic force. This phenomenon was observed in the published experiment and qualitatively supports the proposed model. Additionally, the proposed model allows an alternative explanation of the experiment

Complex Plasma Research on the International Space Station

Complex Plasma Research on the International Space Station Complex (dusty) plasma research under microgravity conditions complements the research in the laboratory. Due to reduction of the main force on microparticles in the lab, gravity, it is possible to form large, homogeneous 3D complex plasma systems in the bulk region of plasmas and to investigate phenomena other than those accessible on earth in detail. Therefore, our team designed and built different setups for microgravity research. PK-3 Plus, which ended in 2013, already gained a long series of interesting scientific results with about 40 publications. The chambers of this setup has a pair of parallel electrode plates and is therefore able to produce a cylindrical symmetric rf-plasma. Particles of different size and material can be injected into the discharge. The cloud is illuminated by a laser sheet and observed in 2D with cameras with different resolutions and fields of view. Moving this observation system in the direction of sight, tomographic reconstruction of the 3D particle configuration is possible later on. This configuration is ideal to investigate stable liquid and crystalline systems and give interesting insights into a wide range of phenomena like crystallization, phase separation of binary mixtures, instabilities like heart-beat instability or projectile interaction with a strongly coupled complex plasma cloud. This contribution will give an overview over a selection of recent results of the very successful project PK-3 Plus. This work and some of the authors were funded by DLR/BMWi FKZ 50WP0203, 50WM1203 and 50WM1441