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International audience; Solid dust particles from a few nanometers to centimeters can often be found in plasmas. This new species is at the origin of a wide variety of new plasma phenomena. These plasmas containing dust particles are called dusty or complex plasmas. They are encountered in many environments such as astrophysics, industrial processes, and fusion devices. Dust particles can come from regions in the plasma vicinity or can be formed in the plasma due to the presence of molecular precursors. These precursors can result from the presence of reactive gases (silane, methane, acetylene) or from material sputtering at the molecular level. Then, they initiate a complex succession of chemical and physical reactions, leading to the growth of dust particles. As long as precursors are available in the plasma phase, dust particle formation can be a cyclic phenomenon. Furthermore, as the dust particles are growing, they attach plasma free electrons, leading to a disturbance of plasma equilibrium when the dust particle density is high. New phenomena, like the occurrence of various instabilities are then observed. These behaviors seem to be present in many dusty plasmas created in different chemistries, experimental setups and conditions. In this presentation, we will discuss about these nearly universal phenomena and will principally focus on experiments related to dust successive generations and instabilities.

Poster; Poussières et plasmas cohabitent fréquemment. Les poussières sont souvent créées par le plasma lui-même, du fait de la présence de gaz réactifs ou par l'érosion des surfaces exposées au plasma. Ces plasmas poussiéreux se rencontrent à l'état naturel en astrophysique et dans les plasmas artificiels utilisés pour les applications industrielles ou pour la fusion thermonucléaire. Dans les plasmas de laboratoire (plasmas froids, basse pression), les poussières forment un nuage qui, sous l'effet de diverses forces, est généralement piégé dans la décharge. Ce nuage n'est pas homogène et une zone vide de poussières (void) apparaît au centre de la décharge. L'étude de cette zone est fondamentale car son existence soulève de nombreux problèmes expérimentaux et théoriques. Sa présence empêche tout traitement homogène du plasma et du nuage dans les modèles théoriques ou les simulations. Le void peut conduire l'expérimentateur à diagnostiquer l'absence de poussières s'il utilise par exemple des techniques de diffusion de la lumière dans cette zone. Le dépôt de poussières pour les applications industrielles est également modifié, le void entraînant des inhomogénéités. Le void est également le siège de nombreux phénomènes complexes: générations successives de poussières, instabilités non-linéaires, interfaces, ... Ces aspects seront abordés en insistant plus particulièrement sur les oscillations non-linéaires du void.

Oral; In a plasma, dust particles acquire a negative charge. Thus, a high density of dust particles strongly reduces the free electron density and can drastically alter the plasma equilibrium leading to a wide variety of instabilities (as in electronegative plasmas). One of these instabilities concerns the dust-free region (void) often appearing in the center of the discharge. This void is maintained by two forces of opposite directions and a break in this equilibrium can lead to strongly nonlinear oscillations of the void size. In this presentation we analyze these low-frequency oscillations on the dust cloud and the plasma thanks to high-speed imaging. Correlations are made with the evolution of the discharge current which shows a nonlinear behavior similar to mixed-mode oscillations (MMOs) well- known in other fields like chemistry or neuronal science. We perform an analogy between MMOs in these fields and the ones we obtained. These MMOs are also highly studied through dynamical system theories which can provide a new approach for studying plasma instabilities.

Oral; International audience; A dust cloud trapped in a plasma often contains a dust-free region ("void") near the plasma center. This void has important effects: it induces a spatial inhomogeneity of the dust particle distribution and is at the origin of many intricate unstable phenomena. One of this behavior is the heartbeat instability consisting of successive contractions and expansions of the void. This instability is characterized by a strong nonlinear dynamics which can reveal the occurrence of incomplete sequences corresponding to failed contractions. Experimental results based on high-speed imaging are presented for the first time and underline this threshold effect in both the dust cloud motion and the evolution of the plasma light emission.