Dynamic phenomena in complex plasmas
Mercredi 16 mai 2012
à 11h en salle C. Brot
Complex (or dusty) plasmas consist of microspheres (or dust grains) immersed into ordinary ionelectron plasmas. Charged by the interaction with the electrons and ions, the microparticles interact with each other electrostatically, can be levitated and confined in a gas discharge. Due to the high charge of the grains, collective interactions become important, giving rise to ordered crystal-like structures. Similar to colloids, complex plasmas can exist in solid, liquid or gaseous states and exhibit phase transitions. They can be observed in real time at the kinetic level. Due to low damping of the grain motion by neutral gas (several orders of magnitude lower than in colloids), complex plasmas sustain a range of dynamic phenomena such as linear and nonlinear waves, shocks, melting, crystallization, diffusion... This makes them ideal model systems for studying dynamic phenomena. Complex plasmas can be found in space environments. Examples range from comet tails to planetary rings. Practical applications include satellite protection from charging and dust impacts, as well as removal of abrasive dust from spacesuits and machinery. Complex plasmas can also be obtained in laboratory by adding grains to a gas discharge. Dust was observed to spontaneously grow in ultraclean etching reactors and in fusion devices, where it causes contamination. Applications include growth of powders for ceramics and catalysts. Knowledge of complex plasma properties will help optimize production processes and increase yields. I will report on experimental observations and numerical simulations of dynamic phenomena in complex plasmas. Our three-dimensional molecular dynamics code solves the equations of motion for every grain taking into account interactions between all grains. Our experiments utilising high quality monodisperse microspheres are performed in a radio-frequency capacitively coupled gas discharge. External excitation forces in the simulations and electric pulses injected through metallic wires in the experiments are used to perturb the system. Their propagations in the complex plasma lattices are then analysed. Structural properties and nonlinear characteristics are examined as waves propagate across the complex plasmas. It was found that two-counterpropagating solutions experience a delay after their collision. The amplitude of a soliton can grow as it propagates in an inhomogeneous lattice even in the presence of damping, which is similar to a “tsunami effect”. Our recent results include dynamics of lattice defects in the presence of solitons and under compressional stress.
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