Theoretical analysis combined with computer simulation is proposed for continued investigation of the properties of strongly coupled complex (dusty) plasmas. Complex (dusty) plasmas consist of mesoscopic grains immersed in the background of gaseous plasma of electrons, ions and neutral atoms. Such systems occur in a variety of situations. The Proposal addresses problems associated with collective behavior in such systems: i.e. phenomena resulting from the cooperative participation of many particles. Such a collective behavior is the hallmark of strongly coupled systems. Six major areas are proposed for study: (i) magnetic interaction between grains carrying a magnetic dipole moment; (ii) propagation of waves in complex plasmas in two dimensional and three-dimensional configurations; (iii) micro-instabilities generated by beams of ions or grains penetrating into the complex plasma; (iv) stochastic and disordered behavior in complex plasmas; (v) phase transitions in complex plasmas; (vi) feasibility study for the cryogenic generation of complex plasmas on the surface of liquid helium. One of the principal approaches to be used in the proposed investigations relies on an analytic method referred to as the Quasi Localized Charge Approximation that has been successfully used previously. Collaboration with a research group at the Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences in Budapest, Hungary on computer simulation will enhance the research program. Broader impacts of the research will contribute to improved understanding of problems of fundamental importance in plasma and condensed matter physics. Establishing techniques suitable for the manipulation of mesoscopic structures will also have impact on engineering applications. Scientific exchanges with several major theoretical and experimental research groups both in the U. S. and overseas will be facilitated. These principles and techniques of strongly coupled plasma physics will be part of a graduate program in the physical and engineering sciences.
Plasmas are many particle systems of charged particles, occurring in Nature as well as in laboratory settings. A macroscopic body formed by a plasma can be in the gaseous, liquid or solid state. The interaction between the charged particles is via the electrostatic (Coulomb) force. As a result of this interaction the particles acquire a potential energy. At the same time, the thermal motion of the particles creates a kinetic energy for the system. It is the value of the ratio of the two, the potential energy/kinetic energy, usually referred to as the coupling parameter that governs the behavior of the system. In particular, plasmas with low values of the coupling parameter are in the gaseous state, while those with higher values (usually referred to as strongly coupled plasmas) form a liquid or solid state. Examples for such systems are electrons in low dimensional semiconductor structures, stellar interiors, ionic liquids, laser produced plasmas, etc. A very special kind of strongly plasmas are the dusty (complex) plasmas created in laboratory discharges. These plasmas are systems containing small (micron to submicron) sized dust grains that become electrically charged in the plasma owing to various processes including the collection of plasma electrons and ions. In low-temperature laboratory or microgravity experiments the dust is negatively charged because of the higher mobility of the electrons. In these environments the ensemble of grains can often be strongly coupled, to the extent that the plasma is either in a liquid phase or crystallizes into a floating lattice structure. It is by this behavior that the dusty plasma becomes a model for condensed matter processes, where, however, the constituent particles are microscopic electrons and ions. Because the particles constituting the dusty plasma are of mesoscopic size, their individual behavior can be made visible by laser light scattering and thus can provide an "enlarged" picture of fundamental condensed matter processes. Dusty plasma systems are usually modeled as Yukawa systems, that is, as a system of charged particles interacting via a screened Coulomb interaction, with the screening provided by the background plasma. Our principal research effort has been directed at modeling and understanding the dynamical behavior of such plasmas. The most prominent manifestation of the dynamical behavior is the excitation and propagation of waves. In our earlier NSF supported work we developed an approximation method, called the Quasi-Localized Charge Approximation (QLCA), which is based on a physical picture that characterizes strongly coupled plasmas: this is the phenomenon of localization, when particles in the attempt to avoiding each other settle down in a small spatial domain (the extreme of this scenario is the formation of a crystal lattice). Using the resulting mathematical formalism we have been able to describe wave excitations and wave propagation in strongly coupled dusty plasmas analytically. Our current research efforts have focused on problems in areas where fundamental physical problems can be addressed by future experimental efforts. (i) In order to study the magnetic interaction between particles carrying, in addition to charges, a magnetic dipole moment as well, we have developed a general theory for wave propagation in the presence of an external magnetic field in a dusty plasma consisting of such particles. (ii) It is well known that in "normal", i. e. weakly coupled gaseous plasmas the interaction of an injected beam of particles can interact with the plasma waves and generate an exponentially growing instability. We have shown that the strong coupling importantly modifies this process. (iii) In most condensed matter systems different kinds of particles (electrons and ions, e. g.) coexist, To model such systems, we have developed the theory for waves in two-component (two differently charged and of different masses) Yukawa plasmas. Our analytic work was supported by a close collaboration with a research group at the Wigner Centre for Physics of the Hungarian Academy of Sciences in Budapest, Hungary. This research team is recognized for its expertise in computer simulations of strongly coupled plasmas. As a result of our collaboration our theoretical models were thoroughly checked and corroborated. Mutual visits by the Budapest group to Boston College and members of our group to the Wigner Centre took place regularly. We also had an ongoing collaboration with the research group of the Institute of Theoretical Physics and Astrophysics in Kiel, Germany: two members of the group had an extended stay at Boston College. Intellectual merit of the outcome. We have applied the earlier created approximation scheme to a novel physical system, namely dusty plasmas, and thus have provided a basis for experiments anchored in fundamental physical principles. Broader impact or the outcome. The broad impact of the project is due to the role of dusty plasmas as mesoscopic model for microscopic processes. Thus any findings in this area have immediate repercussions in astrophysics and condensed matter physics.
