Quasi-periodic vertical structures of dusty plasma crystals

A semi-analytical model is proposed for understanding the quasi-periodic vertical structures in a plasma crystal, which are shown to be sustained by the combined effects of an ion flow, gravity and horizontal bonds of the dust particles. These structures are the manifestations of large-amplitude standing wake fields that lead to strong vertical bonds for the formation of an oriented hexagonal lattice. The orientational symmetry breaking is caused by a gravity-induced thick sheath, which can preferentially accelerate a supersonic ion flow in the vertical direction. The electron and streaming ion densities and the electric potential are found to be all in phase in the plasma crystal, whereas the dust density is 180 degrees out of phase. With each dust layer playing a similar role as the plasma wall, the Bohm sheath criterion is also found to be satisfied. Crystal melting may also be qualitatively described by the present model when the horizontal bonds of the dust particles are sufficiently weakened.