Nonlinear regime of the mode-coupling instability in 2D plasma crystals (original) (raw)

Direct Observation of Mode-Coupling Instability in Two-Dimensional Plasma Crystals

Physical Review Letters, 2010

Dedicated experiments on melting of 2D plasma crystals were carried out. The melting was always accompanied by spontaneous growth of the particle kinetic energy, suggesting a universal plasma-driven mechanism underlying the process. By measuring three principal dust-lattice (DL) wave modes simultaneously, it is unambiguously demonstrated that the melting occurs due to the resonance coupling between two of the DL modes. The variation of the wave modes with the experimental conditions, including the emergence of the resonant (hybrid) branch, reveals exceptionally good agreement with the theory of mode-coupling instability.

Synchronization of particle motion induced by mode coupling in a two-dimensional plasma crystal

Physical Review E, 2014

The kinematics of dust particles during the early stage of mode-coupling induced melting in a two-dimensional plasma crystal is explored. The formation of the hybrid mode induces the partial synchronisation of the particle vibrations at the hybrid frequency. A rhythmic pattern of alternating in-phase and anti-phase oscillating chains of particles is observed. The spatial orientation of the synchronisation pattern correlates well with the directions of the maximal increment of the shear-free hybrid mode.

Wave mode coupling due to plasma wakes in two-dimensional plasma crystals: In-depth view

Physics of Plasmas, 2011

Experiments with two-dimensional (2D) plasma crystals are usually carried out in rf plasma sheaths, where the interparticle interactions are modified due to the presence of plasma wakes. The wake-mediated interactions result in the coupling between wave modes in 2D crystals, which can trigger the mode-coupling instability and cause melting. The theory predicts a number of distinct fingerprints to be observed upon the instability onset, such as the emergence of a new hybrid mode, a critical angular dependence, a mixed polarization, and distinct thresholds. In this paper we summarize these key features and provide their detailed discussion, analyze the critical dependence on experimental parameters, and highlight the outstanding issues.

Dynamics of spinning particle pairs in a single-layer complex plasma crystal

Physical Review E, 2017

Spontaneous formation of spinning pairs of particles, or torsions was studied in a single-layer complex plasma crystal by reducing the discharge power at constant neutral gas pressure. At higher gas pressures, torsions spontaneously formed below a certain power threshold. Further reduction of the discharge power led to the formation of multiple torsions. However, at lower gas pressures the torsion formation was preceded by the mode-coupling instability (MCI). The crystal dynamics were studied with the help of the fluctuation spectra of crystal particles' in-plane velocities. Surprisingly, the spectra of the crystal with torsions and MCI are rather similar and contain "hot spots" at similar locations on the (k, ω) plane, despite very different appearances of the respective particle trajectories. The torsion rotation speed was close (slightly below) to the maximum frequency of the in-plane compressional mode. When multiple torsions formed, their rotation speeds were distributed in a narrow range slightly below the maximum frequency.

Synchronization of particle motion in compressed two-dimensional plasma crystals

EPL (Europhysics Letters), 2015

The collective motion of dust particles during the mode-coupling induced melting of a two-dimensional plasma crystal is explored in molecular dynamics simulations. The crystal is compressed horizontally by an anisotropic confinement. This compression leads to an asymmetric triggering of the mode-coupling instability which is accompanied by alternating chains of inphase and anti-phase oscillating particles. A new order parameter is proposed to quantify the synchronization with respect to different directions of the crystal. Depending on the orientation of the confinement anisotropy, mode-coupling instability and synchronized motion are observed in one or two directions. Notably, the synchronization is found to be direction-dependent. The good agreement with experiments suggests that the confinement anisotropy can be used to explain the observed synchronization process.

Coupling of Noncrossing Wave Modes in a Two-Dimensional Plasma Crystal

Physical Review Letters, 2017

We report an experimental observation of coupling of the transverse vertical and longitudinal in-plane dust-lattice wave modes in a two-dimensional complex plasma crystal in the absence of mode crossing. A new large diameter rf plasma chamber was used to suspend the plasma crystal. The observations are confirmed with molecular-dynamics simulations. The coupling manifests itself in traces of the transverse vertical mode appearing in the measured longitudinal spectra and vice versa. We calculate the expected ratio of the trace to the principal mode with a theoretical analysis of the modes in a crystal with finite temperature and find good agreement with the experiment and simulations.

Effect of strong wakes on waves in two-dimensional plasma crystals

Physical Review E, 2014

We study effects of the particle-wake interactions on the dispersion and polarization of dust lattice wave modes in two-dimensional plasma crystals. Most notably, the wake-induced coupling between the modes causes the branches to "attract" each other, and their polarizations become elliptical. Upon the mode hybridization the major axes of the ellipses (remaining mutually orthogonal) rotate by 45 •. To demonstrate importance of the obtained results for experiments, we plot representative particle trajectories and spectral densities of the longitudinal and transverse waves-these characteristics reveal distinct fingerprints of the mixed polarization. Furthermore, we show that at strong coupling the hybrid mode is significantly shifted towards smaller wave numbers, away from the border of the first Brillouin zone (where the hybrid mode is localized for a weak coupling).

The ‘dipole instability’ in complex plasmas and its role in plasma crystal melting

New Journal of Physics, 2006

Using the dipole model for binary dust-dust interactions, the instability of wave perturbations in a one-dimensional particle string oriented along the ion flow (it highlights the major aspect of the multi-layer crystals) is considered. Longitudinal short wavelength perturbations are found to exist below a certain threshold value of gas pressure. They may act as a possible precursor of the melting transition observed in multi-layer plasma crystals. For different discharge conditions, the threshold of this 'dipole instability' is found to be in the range 10-150 Pa.

Nonlinear instability and chaos in plasma wave–wave interactions. I. Introduction

Physics of Plasmas, 1995

Conventional linear stability analyses may fail for fluid systems with an indefinite free-energy functional. When such a system is linearly stable, it is said to possess negative energy modes. Instability may then occur either via dissipation of the negative energy modes, or nonlinearly via resonant wave-wave coupling, leading to explosive growth. In the dissipationless case, it is conjectured that intrinsic chaotic behavior may allow initially nonresonant systems to reach resonance by diffusion in phase space. In this and a companion paper (submitted to Phys. Plasmas), this phenomenon is demonstrated for a simple equilibrium involving cold counterstreaming ions. The system is described in the fluid approximation by a Hamiltonian functional and associated noncanonical Poisson bracket. By Fourier decomposition and appropriate coordinate transformations, the Hamiltonian for the perturbed energy is expressed in action-angle form. The normal modes correspond to Doppler-shifted ion-acoustic waves of positive and negative energy. Nonlinear coupling leads to decay instability via two-wave interactions, and to either decay or explosive instability via three-wave interactions. These instabilities are described for various integrable systems of waves interacting via single nonlinear terms. This discussion provides the foundation for the treatment of nonintegrable systems in the companion paper. 0 199.5 American Institute of Physics.