Waves in Dusty Plasmas – Research from Laboratory to Space

Kurzfassung

The research on dusty plasmas can be divided into three areas, natural dusty plasmas, dusty plasmas in processing plasmas and dusty plasmas under controlled lab conditions. Natural dusty plasmas are ubiquitous in space. They can be found in star forming and planet formation regions, around the big planets formed in ring systems, in cometary atmospheres, on planets, moons and asteroids. In processing or fusion plasmas dust particles can appear e.g. due to gas phase polymerisation or ablation by the strong interaction between plasma and walls of the reactors. The third area concerns laboratory dusty plasmas, where dust particles – in most cases monodisperse and spherical microparticles – are introduced into the plasma to study strong coupling effects of fluid or even crystalline behavior of the charged microparticles. This area is also called complex plasma in comparison to complex fluids and to distinguish the classical condensed matter field from natural and processing dusty plasmas. In complex plasmas the microparticles dominate the system due to their size – and therefore charge in the plasma due to the collection of electrons and ions – and mass. Their size allows to observe them individually on the most fundamental – the kinetic – level. In the background medium of plasma particles and neutrals the microparticles’ damping ranges from overdamped to virtually undamped. The low damping range allows dynamical processes to appear like wave and shock propagation, defect motion, etc., the full spectrum of observations in strongly coupled systems. The large mass of the microparticles compared to electrons and ions slows down their motion and enables their easy dynamical observation on the one side. On the other side their mass is responsible for their sedimentation in the gravitational field and therefore the necessity to use gravity compensating forces. In most cases electric fields appearing naturally in the sheath regions of plasma discharges are used to compensate gravity, sometimes thermophoretic force in a strong temperature gradient is used. Both produce either an inhomogeneous environment or strong gas convection influencing the dust cloud. Only under microgravity conditions large 3-dimensional systems can be formed in the homogeneous bulk of the plasma, which are of statistically relevant size. The research under microgravity conditions is performed on parabolic flights, sounding rockets and on the International Space Station ISS allowing 20 sec, 6 min, up to hours, respectively, experiment performance. Since 2001 the ISS is continuously used with the laboratories PKE-Nefedov, PK-3 Plus and PK-4 to perform long-term microgravity research. It allowed a wide spectrum of research topics to be investigated. Phenomena related to crystallisation and melting processes, wave and shock wave propagation, lane formation, transition from laminar to turbulent flow and laser driven shear flows have been studied utilizing large 3D systems. The research also included particle decharging in the plasma afterglow as well as particle sputtering, growth and polymerisation from the gas phase. Even experiments in a plasma free background showing fast agglomeration of oppositely charged particles could be investigated, a process which might be important in early planet formation. The basic knowledge achieved in complex plasma experiments in ground based and microgravity labs is important for the understanding of natural dusty plasmas and dusty plasmas in processing industry. Many of those exist in low-gravity environments due to their environment or gravity is not the major force in the system, like for particles of nanometer sizes, respectively. Especially on planetary bodies, dust and dusty plasma pose challenges for human exploration by its hazardous effect on technical equipment or humans. A better understanding of the physical processes of dust formation, charging, lofting or levitation, their movement and dynamics will not only give insights into the fundamental physics, but can help to improve methods of dust mitigation that will be essential for future exploration missions, especially for the Artemis program to the moon. In this presentation I will specially review the processes involved in dynamical observation of wave propagation in dusty plasmas of linear and non-linear characteristics, observed in ground-based and microgravity environments.