R. A. Quinn

Experimental studies of two‐dimensional and three‐dimensional structure in a crystallized dusty plasma

Coulomb crystallization of monodisperse 9.4‐μm‐diam spheres confined in a plasma is investigated in a modified GEC rf Reference Cell using various gases and electrode topographies. For some plasma conditions, planar electrodes confine particles radially in a few horizontal layers due to the curvature of the sheath boundary, and a two‐dimensional (2D) hexagonal lattice is observed which structural analysis shows to be consistent with the intermediate ‘‘hexatic’’ phase of KTHNY 2D melting theory. A depression in the electrode surface causes a corresponding depression in the sheath and allows trapping of more layers in a three‐dimensional (3D) structure, which is viewed in cross section by video imaging of a plane illuminated by a horizontal laser sheet. To synthesize 3D images, a stack of 2D images is made by moving the laser sheet and camera focal plane vertically through the particle cloud. This reveals regions of two stable 3D configurations within the cloud: body‐centered‐cubic and simple hexagonal with...

Experimental investigation of particle heating in a strongly coupled dusty plasma

Highly chargeddust particles in a plasma can be strongly coupled when their kinetic temperature is low. This temperature is determined by a balance of heating and gas cooling. The heating is believed to be electrostatic, although its exact nature is still under investigation. Experiments in a multiple-layer plasma crystal were conducted to test proposed heating mechanisms. A method for measuring small-amplitude, low-frequency fluctuations in ion density was developed and, using this, very low-frequency electrostaticfluctuations were found upstream of the particles. These fluctuations should propagate with the ions towards the particles and heat them. However, the fluctuations were uncorrelated with, and too weak to account for, the observed particle temperatures. In the experiment, the temperature increased and then decreased with gas pressure; this result is only partly consistent with an ion wake heating mechanism. These negative findings help narrow the range of possible explanations for the observed temperatures.

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.

A model of particle temperature in dusty plasmas

An overview of a new analytic model of the macroscopic particle kinetic temperature in dusty plasmas is presented. The model is based on a Langevin analysis of the single particle equation of motion and includes electrostatic heating effects. Electrostatic heating of the particles is thought to be necessary to explain anomalously high particle kinetic temperatures observed in recent laboratory dusty plasma experiments.