Analyses of attractive forces between particles in Coulomb crystal of dusty plasmas by optical manipulations

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Abstract
Optical manipulations have been attempted to analyze attractive forces between particles in a simple hexagonal Coulomb crystal of a dusty plasma. This technique can manipulate particles in a plasma without contact with them directly, using the forces of radiation pressure from laser light. In the Coulomb crystal, particle rows, which are found in the direction perpendicular to an electrode, are formed due to not only Coulomb repulsive force but also some attractive forces. The manipulation method showed that a particle located in the upper stream of ion flows caused an attractive force on another particle located in the lower reaches of them. The fact implied that the force was caused by a wake potential related to ion flows in the presheath or sheath region. In this paper, the mechanisms of simple hexagonal Coulomb crystalline formations are discussed, with the wake potential taken into account.