Study on Structural Change of Coulomb Crystals in Plasmas

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Structural change of Coulomb crystals, which were formed by growing carbon particles in a methane plasma, have been analyzed with the use of 2 CCD video cameras and Mie-scattering ellipsometry. Three dimensional structures of the crystals were observed from the top and the side at the same time and at the same position. As a result, it has been understood that the structure was three-dimensional like face-centered or body-centered for smaller particles (1.2-1.5 mu m) while two-dimensional of simple hexagonal one for larger particles (>1.8 mu m). It has bee also found that the three-dimensional crystal once became in the liquid phase before changing to two-dimensional one and that transition to three-dimensional structure occurred from the lower region while the particle alignment , 1. e., transition to two-dimensional one from the upper.The three-dimensional structure of a three-dimensional Coulomb crystal was precisely analyzed and proved to be strictly face-centered orthorohmbic. It was understood that the structure was formed from the slip of (111) plane of a face-centered cubic structure and the reconstruction of particles for minimum Coulomb energy in the plane.The force of aligning particles in the direction perpendicular to an electrode in the two-dimensional Coulomb crystal was investigated by the application of radiation pressure or photophoretic force of laser light and the decrease of particle density. Consequently it has been found that the particle alignment is caused by the resultant of an attractive and the Coulomb repulsive forces.