[Paper Review] Spontaneous dust pulse formation in the afterglow of complex plasmas under microgravity conditions

M. Chaudhuri, Lénaïc Couëdel+5 more|arXiv (Cornell University)|Mar 17, 2021

Dust and Plasma Wave Phenomena60 references4 citations

TL;DR

This study reports spontaneous formation of oblique, symmetric dust pulses in the afterglow of microgravity complex plasmas on the ISS, triggered by a low-frequency external electric field after plasma extinction. The pulses arise from a spontaneously developing double layer that induces asymmetric microparticle charging, leading to uni- or bi-directional particle flow, with molecular dynamics simulations supporting the double layer hypothesis as the origin of the anisotropic forces.

ABSTRACT

A new type of nonlinear dust pulse structures has been observed in afterglow complex plasma under microgravity condition on board the International Space Station (ISS). The dust pulses are triggered spontaneously as the plasma is switched off and the particles start to flow through each other (uni-directional or counter-streaming) in the presence of a low-frequency external electric excitation. The pulses are oblique with respect to the microparticle cloud and appear to be symmetric with respect to the central axis. A possible explanation of this observation with the spontaneous development of a double layer in the afterglow of complex plasma is described.

Motivation & Objective

• To investigate the formation of nonlinear dust pulse structures in the afterglow phase of complex plasmas under microgravity conditions.
• To understand the role of residual electric fields and particle charging dynamics in triggering spontaneous particle flow post-plasma extinction.
• To determine the mechanism behind the emergence of symmetric, oblique, and uni- or bi-directional dust pulses in the absence of external driving forces.
• To explore the influence of a low-frequency external electric excitation on the development of double layers and particle motion in the afterglow.
• To validate the observed phenomena through molecular dynamics simulations and experimental data from the PK-3 Plus facility on the ISS.

Proposed method

• Conducted experiments on the International Space Station (ISS) using the PK-3 Plus laboratory to study complex plasmas under microgravity.
• Applied a direct current (DC) electric field during the afterglow phase to induce particle motion and observe pulse formation.
• Used high-speed video microscopy to capture particle dynamics and time-space plots (periodgrams) to analyze pulse propagation and directionality.
• Employed molecular dynamics simulations to model the effect of a double layer on particle charging and force anisotropy.
• Applied the drift motion limited ion flux model to estimate particle charge in the weakly collisional plasma regime, replacing standard OML theory.
• Used Epstein drag theory to model neutral gas drag forces on microparticles, assuming diffuse scattering and negligible gas flow.

Experimental results

Research questions

• RQ1 What causes the spontaneous formation of oblique, symmetric dust pulses in the afterglow of complex plasmas under microgravity?
• RQ2 How does the presence of a low-frequency external electric field influence particle flow and pulse directionality after plasma extinction?
• RQ3 What is the role of a spontaneously developing double layer in generating asymmetric forces on microparticles during the afterglow?
• RQ4 Why do uni-directional and bi-directional pulses coexist under similar conditions, and what determines their formation?
• RQ5 How do residual particle charges and ion drag forces sustain particle motion beyond the expected damping timescale in the afterglow?

Key findings

• Oblique, symmetric dust pulses formed spontaneously in the afterglow phase of complex plasmas under microgravity, appearing as bright lines in video observations due to high particle density.
• Pulses were observed in both uni-directional and bi-directional configurations, with time-space plots showing one or two propagation arms, respectively.
• The pulses were triggered by a low-frequency external electric excitation applied during plasma extinction, which induced asymmetric microparticle charging.
• Molecular dynamics simulations supported the hypothesis that a double layer formed in the afterglow, creating strong spatial anisotropy in particle forces.
• The bi-directional pulse formation was attributed to the presence of two oppositely charged particle layers due to the double layer's impact on decharging dynamics.
• The persistence of particle motion beyond expected damping rates suggests an additional energy source, possibly from residual ion flows, though this remains unexplained.