Three dimensional complex plasma structures in a combined radio frequency and direct current discharge

TL;DR

This study provides a detailed 3D analysis of complex plasma structures in combined rf and dc discharges, revealing coexistence of various phases and introducing new criteria for identifying string-like formations.

Contribution

First detailed 3D analysis of complex plasma structures in combined rf+dc discharges, with novel criteria for string-like structure identification.

Findings

Observation of bowl-shaped particle clouds

Coexistence of solid-like, liquid-like, and string-like phases

Introduction of new criteria for string structure detection

Abstract

We report on the first detailed analysis of large three dimensional (3D) complex plasma structures in experiments performed in pure rf and combined rf+dc discharge modes. Inductively coupled plasma (ICP) is generated by an rf coil wrapped around the vertically positioned cylindrical glass tube at a pressure of 0.3 mbar. In addition, dc plasma can be generated by applying voltage to the electrodes at the ends of the tube far from the rf coil. The injected monodisperse particles are levitated in the plasma below the coil. A scanning laser sheet and a high resolution camera are used to determine the 3D positions of about $10^5$ particles. The observed bowl-shaped particle clouds reveal coexistence of various structures, including well-distinguished solid-like, less ordered liquid-like, and pronounced string-like phases. New criteria to identify string-like structures are proposed.