Lénaïc Couëdel

Direct observation of mode-coupling instability in two-dimensional plasma crystals.

Dedicated experiments on melting of two-dimensional 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 wave modes simultaneously, it is unambiguously demonstrated that the melting occurs due to the resonance coupling between two of the dust-lattice 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.

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

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.

Influence of the ambipolar-to-free diffusion transition on dust particle charge in a complex plasma afterglow

The influence of diffusive losses on residual dust charge in a complex plasma afterglow has been investigated. The residual charge distribution was measured and exhibits a mean value Qdres∼(−3e−5e) with a tail in the positive region. The experimental results have been compared with simulated charge distributions. The dust residual charges were simulated based on a model developed to describe complex plasma decay. The experimental and simulated data show that the transition from ambipolar to free diffusion in the decaying plasma plays a significant role in determining the residual dust particle charges. The presence of positively charged dust particles is explained by a broadening of the charge distribution function in the afterglow plasma.

Dust particles in low-pressure plasmas: Formation and induced phenomena*

Formation of dust particles is a common mechanism in low-pressure plasmas. These big particles (in comparison with other plasma species) are sometimes the desired final products of the process, but they may also constitute a severe drawback in certain contexts. In either situation, it is necessary to understand growth mechanisms well, in order to control or avoid dust particle formation. One of the problems that has to be overcome is that dust particle growth is usually a continuous mechanism: once started, it can enter into a cyclic regime where new generations of dust particles are succeeding one after the other. This cyclic phenomenon often induces a side effect consisting of instabilities of a few tens of Hz. This paper discusses the main characteristics of dust successive generations, and particularly the importance of dust-free spaces (void) involved in this process. Finally, some aspects related to deposition when several generations coexist will be presented.

First Direct Measurement of Optical Phonons in 2D Plasma Crystals

Spectra of phonons with out-of-plane polarization were studied experimentally in a 2D plasma crystal. The dispersion relation was directly measured for the first time using a novel method of particle imaging. The out-of-plane mode was proven to have negative optical dispersion at small wave numbers, comparison with theory showed good agreement. The effect of the plasma wakes on the dispersion relation is briefly discussed.

Comprehensive experimental study of heartbeat oscillations observed under microgravity conditions in the PK-3 Plus laboratory on board the International Space Station

Heartbeat oscillations in complex plasmas with a broad range of fundamental frequencies are observed and studied. The experiments are performed with monodisperse microparticles of different diameters in argon as well as in neon plasmas. The oscillation frequency increases with increasing rf power and neutral gas pressure. At the lower frequencies, oscillations are strongly nonlinear. The microparticle pulsations, the variation of the electrical discharge parameters and the spatially resolved changes in the plasma glow are proven to be strongly correlated. Heartbeat oscillation dynamics is associated with global confinement modes.

Self-excited void instability during dust particle growth in a dusty plasma

A new kind of void instability in a complex plasma is described. This instability is directly linked to the growth of a new generation of dust particles inside the void. It consists of slow contraction and expansion sequences of the void size of which frequency and amplitude evolve while the new dust particles are growing.

Dust charge distribution in complex plasma afterglow

Dust charge distributions have been measured for the first time in a complex plasma. Experiments were performed late in a discharge afterglow. Residual charges of few hundred particles have been determined. It was found that the mean residual charge is negative but the tail of the distribution extends into the positive-charge region. The experiments were complemented by numerical simulations taking into account discreteness of the charging process and dependence of plasma parameters on the transition from ambipolar to free diffusion. It is shown that the existence of positively charged particles can be explained by the broadening of the dust charge distribution in the late complex plasma afterglow.

Carousel Instability in a Capacitively Coupled RF Dusty Plasma

Rotating plasma spheroids are observed in a capacitively coupled radio-frequency discharge containing grown dust particles. These plasma spheroids are regions of slightly enhanced emission that rotate along the circumference of electrodes. This effect is not observed systematically, and when it occurs, the number and the speed of the spheroids can vary with the discharge parameters. These spheroids have been evidenced by using high-speed imaging.

Plasma Emission Modifications and Instabilities Induced by the Presence of Growing Dust Particles

Formation of dust particles in a plasma can strongly change its properties due to electron attachment on dust surface. An easy way to detect dust formation is to analyze modifications of the plasma emission. In this paper, changes in the plasma emission are related to the growth of dust particles. We particularly show that dust formation induces low-frequency plasma instabilities. Another interesting induced effect is the formation of an enhanced emission region which is dust free and usually named ldquovoidrdquo.