The "trampoline effect" and the distribution of forces inside the void region in complex plasmas under microgravity conditions

Summary form only given. The PKE-Nefedov facility allows the study of complex (dusty) plasmas under micro-gravity conditions onboard the International Space Station (ISS). In the experiments, the micro-particles can form regular structures (e.g., plasma crystals) inside the bulk of a rf discharge plasma. In most of the experiments the particles do not fill the entire plasma volume, but a particle-free region in the central part of the discharge - the so called "void" - is formed. It is believed that the main process responsible for the void formation is the ion drag force, which can exceed the electric force in the limit of weak electric fields and, hence, pushes the dust particles out of the central region of a discharge. In the experiments performed at low gas pressure (p=12 Pa) an instability of the dust cloud-void interface was observed. The instability was accompanied by periodic contractions of the void volume and fast injection of relatively small number dust particles inside the void region (what we call here "trampoline effect"). In the next ("relaxation") stage the injected particles were pushed from the void back into the particle cloud. This relaxation stage was much slower than the injection. The cycle fast injection - slow relaxation then repeated periodically. We were able to perform an accurate analysis of particle trajectories during the relaxation stage. From this analysis the distribution of forces acting on the particles inside the void region was reconstructed. Due to relatively low neutral gas pressure used in the experiments the direct comparison with the recent theoretical models of the ion drag force (in collisionless limit for ions) and of the void formation is possible. Such a comparison was performed and good agreement with theoretical predictions was found. The obtained results provide a clear picture of the nature of the void formation in complex plasmas under microgravity conditions.