Complex (dusty) plasmas: Current status and perspectives — G. Morfill
25 Oct 2012
Gregor Morfill, Ph.D., prof., The head of the Max Planck Institute for Extraterrestrial Physics
“Dusty”, or “complex” plasmas are composed of a weakly ionized gas and charged microparticles. Dust and dusty plasmas are ubiquitous in space – they are present in planetary rings, cometary tails, interplanetary and interstellar clouds, the mesosphere, thunderclouds, they are found in the vicinity of artificial satellites and space stations, etc. Furthermore, the presence of dust particles plays a critical role in many important industrial processes (e.g., plasma vapor deposition, microchip production, etching, where growth of dust occurs as a matter of course during the production process) as well as in plasma fusion (where the possibility of producing radioactive and toxic dust in the plasma-wall interactions is an important design issue). Apart from that, plasmas containing microparticles individually visible under optical microscopy are actively investigated in many laboratories (and term “complex plasmas“ is used to distinguish systems specially designed for such investigations from naturally occurring dusty plasmas). After almost a century of study – the first observations of dust in discharges have been reported by Langmuir in 1924 – the current interest in complex plasmas began in the mid 1990’s, triggered by the laboratory discovery of plasma crystals.
The presence of charged microparticles in complex plasmas is essential for collective processes. Ensembles of microparticles give rise to new very-low-frequency wave modes which represent the oscillations of particles against the quasiequilibrium background of electrons and ions. Overall dynamical time scales associated with the dust component are in the range 10–100 Hz. However, microparticles embedded in a plasma do not only change the charge composition, they also introduce new physical processes into the system, e.g., effects associated with dissipation and plasma recombination on the particle surface, variation of the particle charges, etc. These processes imply new mechanisms of the energy influx into the system. Therefore, properties of complex plasmas can be completely different from those of usual multicomponent plasmas.
Due to large charges carried by microparticles (typically, of the order of thousand elementary charges for a micron-size particle), the electrostatic energy of the mutual interaction is remarkably high. Therefore, in complex plasmas, one can observe transitions from a disordered gaseous-like phase to a liquid-like phase and the formation of ordered structures of microparticles – plasma crystals, as illustrated in the fugure. In addition, the shape of the interparticle interaction can be “designed“ (by tuning external conditions). These unique features distinguish complex plasmas from many other laboratory plasmas, where the ion charges are low, the interaction potentials are fixed, and the coupling strength is relatively weak.
Gaseous (left), liquid (center), and crystalline (right) states of complex plasmas.
Today, the physics of complex plasmas is a rapidly growing field of research which covers various fundamental aspects of the plasma physics, physics of (classical) liquids and solids, nonlinear physics, etc. More and more research groups throughout the world have become involved in the field, and the number of scientific publications is growing exponentially. In this lecture, I will try to provide a balanced picture of the current status of the field, by covering the latest development in the most important directions of the experimental and theoretical complex plasmas, and will also outline the perspective issues to pursue in future.