Possible Mechanisms of String Formation in Complex Plasmas at Elevated Pressures

Possible mechanisms of particle attraction providing formation of the field aligned microparticle strings in complex plasmas at elevated gas pressures are theoretically investigated in the light of the Plasmakristall-4 (PK-4) experiment on board the International Space Station. The particle interaction energy is addressed by two different approaches: (i) using the dynamically screened wake potential for small Mach numbers derived by Kompaneets et al., in 2016, and (ii) introducing effect of polarization of the trapped ion cloud by discharge electric fields. Is is found that both approaches yield the particle interaction energy which is independent of the operational discharge mode. In the parameter space of the performed experiments, the first approach can provide onset of the particle attraction and string formation only at gas pressures higher than 40-45 Pa, whilst the mechanism based on the trapped ion effect yields attraction in the experimentally important pressure range 20-40 Pa and may reconcile theory and observations.

Automated summary

This study theoretically investigates possible mechanisms of particle attraction for the formation of field-aligned microparticle strings in complex plasmas at elevated gas pressures. Two different approaches to calculating particle interaction energy are discussed, both of which yield energy independent of the operational discharge mode. One approach requires higher gas pressures for particle attraction and string formation, while the other may reconcile theory and observations in the experimentally important pressure range.