Dusty plasma cavities: probe-induced and natural

TL;DR

This paper investigates various mechanisms of cavity formation in dust crystals within plasma environments, including probe-induced and natural cavities, through experiments and modeling in ground-based and space conditions.

Contribution

It provides a detailed comparison of cavity formation mechanisms in dust plasma crystals, combining experimental data with theoretical models for the first time.

Findings

Probe-induced cavities are caused by particle repulsion from negatively charged probes.

Natural cavities can form due to ion drag, thermophoresis, and confinement potential effects.

Increasing probe potential can unexpectedly create cavities through different processes.

Abstract

A comprehensive exploration of regional dust evacuation in complex plasma crystals is presented. Voids created in 3D crystals on the International Space Station have provided a rich foundation for experiments, but cavities in dust crystals formed in ground-based experiments have not received as much attention. Inside a modified GEC RF cell, a powered vertical probe was used to clear the central area of a dust crystal, producing a cavity with high cylindrical symmetry. Cavities generated by three mechanisms are examined. First, repulsion of micrometer-sized particles by a negatively charged probe is investigated. A model of this effect developed for a DC plasma is modified and applied to explain new experimental data in RF plasma. Second, the formation of natural cavities is surveyed; a radial ion drag proposed to occur due to a curved sheath is considered in conjunction with thermophoresis and a flattened confinement potential above the center of the electrode. Finally, cavity formation unexpectedly occurs upon increasing the probe potential above the plasma floating potential. The cavities produced by these methods appear similar, but each are shown to be facilitated by fundamentally different processes.