Long-time evolution of charged grains in plasma under harmonic external force and after being withdrawn

Evolution of the charged grains in a two-dimensional dusty plasma under a spatially harmonic external force, in particular, their long-time behaviors after the force has been withdrawn, is studied by using molecular dynamics simulation. Under an external force and a grain-grain interaction force, initially homogeneously distributed grains can reach a quasi-stationary state in the form of a disk crystal. After the external force is withdrawn, the disk moves initially with its size and shape nearly unchanged until it rapidly stops moving, and eventually the disk grain rotates like a vortex. The time needed to reach the final state increases with the strength of the initial external force increasing.