PARTICLE SIMULATION OF 2-DIMENSIONAL DUST CRYSTAL-FORMATION IN A MESOTHERMAL PLASMA-FLOW

A two-dimensional simulation is used to study the crystallization of s trongly coupled dusty plasmas. The dust grains are simulated as partic les, while the electrons and ion solutions are obtained from fluid equ ations. We model gas discharges with dust particles confined in the sh eath and at the sheath edge. In these regions the ions flow past the d ust grains at a velocity v(0) that typically is mesothermal, v(Ti)much less than v(0) much less than v(Te). A negative charged dust particle will under these conditions focus the ions, creating a local maximum in the plasma potential on the downstream side of the particle. This m aximum means that there is an anisotropic interparticle interaction, w hich can be manifested as an attractive interparticle force along the axis of the plasma flow. Our simulation shows that the equilibrium con figuration of a dust crystal structure can be profoundly influenced by this asymmetry in the ion flow and plasma potential for certain charg e to mass ratios for the dust particles. We initially distribute dust particles randomly in space with zero velocity and integrate their equ ations of motion, tracking their orbits as they settle into equilibriu m positions. We show that there are several stable final equilibria, a lthough they do not all have the same potential energy or probability of occurring. The most likely crystal configuration depends on the cha rge-to-mass ratio and the horizontal particle spacing of the grains. T he vertically aligned columns of particles that have been observed in plasma crystal experiments are shown to be most likely for large dust charge-to-mass ratios. (C) 1996 American Vacuum Society.