Strongly-coupled dusty plasmas

Summary form only given, as follows. When micron-size particulates are introduced into a plasma, they acquire electric charges due to collecting ions and electrons. Because the charge can be very large, up to hundreds of thousands of electron charges, these particulates are generally strongly coupled. That is to say, their inter-particle potential energy is greater than their thermal kinetic energy, which is typical of liquids and solids, rather than the reverse, which is typical of mast gases and plasmas. The background electrons and ions remain weakly coupled. One application of dusty plasmas is to make a macroscopic model system for liquids and solids. Each micron-size particle represents an atom. Because the particles are large enough to scatter light efficiently and they are slow, it is straight-forward to image the particle structure and dynamics, by eye and by video camera. Frames from a videotape are captured onto a computer to identify particle locations. These are processed using structural analysis techniques developed previously by experimenters working with another model system, aqueous suspensions of microspheres. This yields the inter-particle bonds, Voronoi cells, and translational and bond-angle correlation functions. From these, one can determine the state of matter: liquid, intermediate melting phase, or crystalline. In our experiments, we levitate 6 /spl mu/m diameter plastic microspheres in a low power (/spl ap/1 W) low pressure (/spl ap/1 Torr) krypton discharge formed by capacitively-coupled 13.56 MHz rf power. The microspheres are levitated about 2 mm above a horizontal electrode, near the plasma-sheath boundary. The microspheres cooled to room temperature by drag on the neutral gas. A sheet of laser light passes through the cloud of microspheres to illuminate it for video imaging.