POLYGON STRUCTURE OF PLASMA CRYSTALS

Stability and lattice oscillations are studied for polygon structures of plasma crystals under the forces of ion drags, gravitational fields , and sheath electric fields. Circular lattices of dust grains, trappe d in the potential minimum of a wake potential produced in the presenc e of ion flow, are levitated horizontally under the balance of forces, Positive ions transfer momentum to a negatively charged dust particul ate either by scattering due to Coulomb force or by direct capturing d ue to the finite size of the dust. Resulting drag force on the dust pa rticulates makes a plasma an effective viscous medium and is found to be responsible for the stability condition of the structure. It is sho wn that the polygon structures of triangle, square, pentagon, or hexag on are stable against spatial perturbations and are accompanied by the oscillations with characteristic low frequency inversely proportional to the effective viscosity. (C) 1998 American Institute of Physics.