Speaker: Dr. Uwe Kokopka, Max Planck Institute for Extraterrestrial Physics, Garching
Abstract: Dust particles, embedded in a plasma, are getting charged due to a variety of mechanisms like electron-/ion- bombardment or secondary electron emission. At low densities the dust in the plasma can be treated as an impurity that mainly probes the background plasma without significantly altering it. At higher densities a change from dust in plasma to a "dusty plasma" takes place. The dust species begin to alter the overall behavior of the other plasma components, and vice versa, so that the dust has to be treated as an integral component. Both situations can be frequently observed in astrophysical and plasma processing environments. At sufficient high dust densities, the dynamics of the whole system eventually become dominated by the dust species, since most of the inertia in the system is bound to it. In this situation the charged dust particles are often treated similarly to a one-component system of directly interacting charged particles that are "somehow" screened by the background plasma. Due to the high particle mass, the time and spatial scales of the dynamics are shifted towards macroscopic scales. Typically, very fast plasma dynamics such as waves, diffusion, or gyro motion of particles can then be observed simply with the naked eye. This so-called "complex plasma", named in analogy to complex fluids (e.g. colloidal particles in a liquid suspension), also shows strongly coupled phenomena like crystallization that are least expected to be present in a plasma environment. All of this is visible on the fundamental, "atomic" scale of single (dust) particles for the systems that can consist of just a few or up-to billions of particles. This makes complex plasmas a very attractive research field which heavily supports and strongly benefits from interdisciplinary research efforts. The high mass of the dust particles that shifts the dynamics towards macroscopic scales has a drawback however. In a typical laboratory complex plasma setup the dust sediments, forming compressed layers close to the lower sheath boundaries where the weight of the particles is compensated by the effect of the sheath/pre-sheath electric field. Only small/light particles (~ 1 um) can easily fill the bulk of the plasma. Alternatively, experiments in u-gravity must be performed. Since the discovery in 1994 of the crystalline state of a complex plasma, the "plasma crystal", by my colleague H. Thomas, I have studied a variety of topics related to complex plasmas. I have looked at fundamental themes like single particle interaction, instabilities and the role of a magnetic fields in a complex plasmas. Additionally, I studied interdisciplinary topics such as crystallization and flow dynamics. Some experiments were aimed at applications for dust removal, or dust growth, that have relevance to plasma processing and fusion. Experiments on dust coagulation and runaway growth provided unintended yet significant results regarding the astrophysical question of planet formation. The presentation contains an introduction to fundamental complex plasma research. I will discuss the charging and "somehow" screening of microparticles in a plasma environment as well as the role of gravity. Examples of laboratory and u-gravity experiments will be shown, demonstrating the interdisciplinary potential of complex plasmas research. Also two experimental setups that I have designed and built to perform the shown experiments will be introduced: a unique high magnetic field plasma facility and a flexible plasma device, the "PlasmaLab", that is foreseen to be operated as part of a future experiment aboard the international space station (ISS).