Observation of dust Coulomb clusters in a plasma trap

The formation of ordered quasi-two-dimensional Coulomb clusters of negatively charged dust particles with particle number N, from a few to 791 in a cylindrical plasma trap, are demonstrated. For small N, particles are alternately packed in concentric shells with periodically oscillating occupation numbers as N increases. The thermal induced collective excitations are dominated by the intershell angular motions. For large N, the large triangular core surrounded by a few outer circular shells appears and supports vortexlike excitations, which induce uniform and isotropic motions. The observed behaviors are generic and similar to the predictions by other more ideal models with the same symmetry.