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, trapped in the potential minimum of a wake potential produced in the presence of ion flow, are levitated horizontally under the balance of forces. Positive ions transfer momentum to a negatively charged dust particulate either by scattering due to Coulomb force or by direct capturing due to the finite size of the dust. Resulting drag force on the dust particulates makes a plasma an effective viscous medium and is found to be responsible for the stability condition of the structure. It is shown that the polygon structures of triangle, square, pentagon, or hexagon are stable against spatial perturbations and are accompanied by the oscillations with characteristic low frequency inversely proportional to the effective viscosity.