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This study examines the structure and stability of filamentary dusty plasmas using data from the Plasmakristall-4 (PK-4) facility on board the International Space Station. Under the action of a polarity-switched DC electric field, the dust particles in the PK-4 discharge have been found to organize into field-aligned extended filaments, which has been compared to the filamentary state in electrorheological (ER) fluids. Here we discuss how, in addition to an ER-type structural transition, the PK-4 dusty plasmas exhibit structural states reminiscent of those observed in liquid crystals (LCs) with rod-shaped molecules. We find that dust particles within the filaments are strongly coupled in a crystalline-like structure, while the coupling of particles across filaments is liquid-like. In addition to a common orientation along a director axis (nematic behavior), the dust filaments also appear to align in large-scale nested structures, or shells (smectic behavior). Finally, these filaments are found to further arrange in hexagonal patterns within the plane orthogonal to the director axis, suggesting the possibility for smectic-B and smectic-C structural states. As the observed ER and LC features of the filamentary dusty plasma states are sensitive to variations in the PK-4 discharge conditions, we argue that these dusty plasmas can provide a controlled analogous system for the study of fundamental phenomena in soft matter, such as the origins of pattern formation and universality of phase transitions. 

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. 

This report is a summary of the mini-conference on Workforce Development Through Research-Based, Plasma-Focused Science Education and Public Engagement held during the 2022 American Physical Society Division of Plasma Physics annual meeting. The motivation for organizing this mini-conference originates from recent studies and community-based reports highlighting important issues with the current state of the plasma workforce. Here, we summarize the main findings presented in the two speaker sessions of the mini-conference, the challenges, and recommendations identified in the discussion sessions and the results from a post-conference survey. We further provide information on initiatives and studies presented at the mini-conference, along with references to further resources. 

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.