Two-dimensional plasma crystals: waves and instabilities
Abstract: Complex plasmas consist of particles immersed in a weakly ionized plasmas. Due to the absorption of ambient electrons and ions, microparticles acquire negative charges and can form coupled systems. Microparticles injected in capacitively-coupled radio-frequency discharges levitate in the sheath region near the bottom electrode, where the electric field can balance gravity. Under certain conditions the particles can form a monolayer and arrange themselves into ordered structures: 2D plasma crystals. In such crystals, two in-plane wave modes with an acoustic dispersion can be sustained (longitudinal and transverse modes). Since the strength of the vertical confinement is finite, there is a third fundamental wave mode associated with the out-of- plane oscillations that has a negative optical dispersion. Due to the strong electric field in the sheath region, every particle is influenced by a strong ion flow. The ions tend to focus downstream of the particles making the system highly polarized (plasma wake). In 2D plasma crystals, wake-mediated interactions result in the coupling of the crystal in-plane and out-of-plane modes into a shear-free hybrid mode of the lattice layer and trigger the mode-coupling instability (MCI) which can melt the crystal. Localized “hot spots" in the lattice phonon spectra are a typical signature of this mode. MCI induced melting can only be triggered if (i) the modes intersect, and (ii) the neutral gas damping is sufficiently low. In this presentation, experimental measurements of the spectra of phonons with out-of-plane polarization in 2D plasma crystals are presented. The dispersion relation was directly measured using a method of particle imaging that allowed us to resolve the particle motion in the 3 dimensions. We observed experimentally the coupling between the out-of-plane mode and the in-plane longitudinal mode which under certain conditions can form hybridized modes and trigger the MCI. The kinematics of dust particles during the early stage of MCI revealed that the formation of the hybrid mode induces the partial synchronization of the particle oscillations at the hybrid frequency. Phase-and frequency-locked hybrid particle motion in both vertical and horizontal directions was evidenced. The full melting of a two-dimensional plasma crystal induced in a principally stable monolayer by localized laser stimulation is also presented. Two distinct behaviors of the crystal after laser stimulation were observed depending on the amount of injected energy: (i) below a well-defined threshold, the laser melted area recrystallized; (ii) above the threshold, it expanded outwards in a similar fashion to mode-coupling instability-induced melting. The reported experimental observations are due to the fluid mode-coupling instability, which can pump energy into the particle monolayer at a rate surpassing the heat transport and damping rates in the energetic localized melted spot. This behavior exhibits remarkable similarities with impulsive spot heating in ordinary reactive matter
Bio: Dr Lenaic Couedel is since March 2018 Associate Professor at the University of Saskatchewan (U of S), Saskatoon. He obtained his PhD from the University of Sydney, Australia in 2008. He was from 2009 to 2011 a postdoctoral fellow at the Max-Planck Institute for extraterrestrial Physics in the group of Professor Gregor Morfill. Prior to joining the University of Saskatchewan, Dr Couedel was a CNRS research scientist in the PIIM laboratory at Aix-Marseille University (2011-2018). Dr Couedel is an experimental plasma physicist. His current research interests are complex (dusty) plasmas (especially nanoparticle growth in low temperature plasmas (magnetized and unmagnetized), and instabilities in complex plasma crystals) and low temperature plasma diagnostics (especially sheath diagnostics).
