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

Coulomb crystallization of monodisperse 9.4-mu m-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 vertically aligned particles. (C) 1996 American Vacuum Society.