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The microgravity environment of the Plasmakristall-4 experiment on the International Space Station provides a laboratory for exploring plasma-mediated interactions among charged dust grains in fully three-dimensional space. Away from the strong influence of Earth's gravity, the dust grains can levitate in the bulk of the plasma, where they have been observed to form extended filamentary structures aligned with the discharge tube axis. These structures can be used as a macroscopic analogue for other self-organizing systems, including electrorheological fluids and liquid crystals, and the success of the analogy depends on a thorough understanding of the mechanisms guiding the dust interaction potential. Here we present the results from molecular dynamics simulations of the ion flow past isolated dust chains within the dust cloud and the dust cloud macrostructure. Although dust grains are known to respond on the millisecond timescale, analysis reveals that periodic variations of plasma conditions on the microsecond timescale significantly affect dust structure formation. In addition to the expected formation of filamentary dust chains in the dust cloud macrostructure, dust grains in a large cloud are also observed to organize into ordered positions on the surface of nested cylinders, in agreement with experimental observations. 

The PK-4 system is a micro-gravity dusty plasma experiment currently in operation on-board the International Space Station. The experiment utilizes a long DC discharge in neon or argon gases. We apply our 2D particle-in-cell with Monte Carlo collisions discharge simulation to compute local plasma parameters that serve as input data for future dust dynamics models. The simulation includes electrons, Ne+ ions, and Nem metastable atoms in neon gas and their collisions at solid surfaces including secondary electron emission and glass wall charging. On the time scale of the on-board optical imaging, the positive column appears stable and homogeneous. On the other hand, our simulations show that on microsecond time scales the positive column is highly inhomogeneous: ionization waves with phase velocities in the range between 500 m s−1 and 1200 m s−1 dominate the structure. In these waves, the electric field and charged particle densities can reach amplitudes up to 10 times of their average value. Our experiments on ground-based PK-4 replica systems fully support the numerical findings. In the experiment, the direction of the DC current can be alternated, which has been found to favor dust particle chain formation. We discuss possible mechanisms for how the highly oscillatory plasma environment contributes to the dust particle chain formation. 

Self-organization of dust grains into stable filamentary dust structures (or “chains”) largely depends on dynamic interactions between individual charged dust grains and complex electric potential arising from the distribution of charges within a local plasma environment. Recent studies have shown that the positive column of the gas discharge plasma in the Plasmakristall-4 (PK-4) experiment at the International Space Station supports the presence of fast-moving ionization waves, which lead to variations of plasma parameters by up to an order of magnitude from the average background values. The highly variable environment resulting from ionization waves may have interesting implications for the dynamics and self-organization of dust particles, particularly concerning the formation and stability of dust chains. Here, we investigate the electric potential surrounding dust chains in the PK-4 experiment by employing a molecular dynamics model of the dust and ions with boundary conditions supplied by a particle-in-cell with Monte Carlo collision simulation of the ionization waves. The model is used to examine the effects of the plasma conditions within different regions of the ionization wave and compare the resulting dust structure to that obtained by employing the time-averaged plasma conditions. The comparison between simulated dust chains and experimental data from the PK-4 experiment shows that the time-averaged plasma conditions do not accurately reproduce observed results for dust behavior, indicating that more careful treatment of plasma conditions in the presence of ionization waves is required. It is further shown that commonly used analytic forms of the electric potential do not accurately describe the electric potential near charged dust grains under these plasma conditions.