Structure of Coulomb Crystals in Cylindrical Discharge Plasmas Under Gravity and Microgravity

Structures of Coulomb crystals were analyzed in cylindrical discharge plasmas under gravity and microgravity by using an apparatus similar to the PK-4 on the International Space Station (ISS). Under gravity, layers of cylindrical dust clouds were found to locally form the lattices of the face-centered orthorhombic (FCO) structure, which is a deformation from the face-centered-cubic structure. We also observed that the average density increases with the size of dust clouds. Under microgravity, the structure could be regarded as an assembly of linear chains along the cylinder axes. At the same time, the interparticle distance and the plasma parameters measured by a double-probe method indicated that the structure was formed not only from the wake potential but also from mutual interactions between dust particles in neighboring chains.