A crystal-like structure can be formed out of charged dust grains in a low-temperature plasma discharge and in space plasmas such as Saturn's rings. Sergey Vladimirov and Neil Cramer have found that the grains in the crystal vibrate with definite frequencies. This will aid in the experimental diagnostics of the crystal and its background plasma.
Plasmas containing small solid particles ranging in size from nanometers to micrometers are called `dusty plasmas'. The particles acquire negative charge by a flow of fast plasma electrons onto the grains. Dusty plasmas are common in space, occurring in such diverse environments as interstellar clouds, interplanetary dust, comets, planetary rings, and the Earth's magnetosphere. In the laboratory, dusty plasmas can occur naturally in processing plasmas, such as those used for etching semiconductors. In addition, the study of the influence of dust in environmental research, such as in the Earth's ionosphere and atmosphere, is important. Dust particles can form a crystal structure known as a Coulomb solid. The crystal spacing can be large (of the order of millimeters), so the dust crystal provides a macroscopic, classical model of a solid state crystal.
Sergey Vladimirov and Neil Cramer have carried out investigations of the mechanical-electrostatic modes of vibration of a dust-plasma crystal. They have discovered transverse modes of a horizontal line of grains (where the ions flow vertically downward to a plane horizontal cathode), and of two such lines of grains, and the modes of a vertical string of grains. The last two arrangements have the unique feature that the effect of the background plasma on the mutual grain interaction is asymmetric because of the ``wake" downstream of the grains. The characteristic frequencies of the vibrations are dependent on the parameters of the plasma and the dust grains, such as the grain charge, and so measurement of the frequencies could provide diagnostics of these quantities. This theory will be experimentally tested in the Plasma Physics Department of the School of Physics with the aid of an ARC Large Grant awarded for 1999-2001.
