Dust coupling parameter of radio-frequency-discharge complex plasma under microgravity conditions

TL;DR

This study investigates particle oscillations in a dust crystal formed in a microgravity plasma, revealing high isotropy, significant kinetic overheating, and a low coupling parameter that explains phase transitions.

Contribution

It introduces a novel method to determine the dust particle coupling parameter using oscillation data and provides a theoretical interpretation of large oscillation amplitudes under microgravity.

Findings

Coupling parameter is around 100, much lower than expected from gas temperature.

Particles exhibit significant kinetic overheating under stationary conditions.

Theoretical estimates match experimental oscillation data.

Abstract

Oscillation of particles in a dust crystal formed in a low-pressure radio-frequency gas discharge under microgravity conditions is studied. Analysis of experimental data obtained in our previous study shows that the oscillations are highly isotropic and nearly homogeneous in the bulk of a dust crystal; oscillations of the neighboring particles are significantly correlated. We demonstrate that the standard deviation of the particle radius-vector along with the local particle number density fully define the coupling parameter of the particle subsystem. The latter proves to be of the order of 100, which is two orders of magnitude lower than the coupling parameter estimated for the Brownian diffusion of particles with the gas temperature. This means significant kinetic overheating of particles under stationary conditions. A theoretical interpretation of the large amplitude of oscillation implies the increase of particle charge fluctuations in the dust crystal. The theoretical estimates are based on the ionization equation of state for the complex plasma and the equation for the plasma perturbation evolution. They are shown to match the results of experimental data processing. Estimated order of magnitude of the coupling parameter accounts for the existence of the solid-liquid phase transition observed for similar systems in experiments.