Particle simulation of two dimensional dust crystal formation in a mesothermal plasma flow

Abstract

A two-dimensional simulation is used to study the crystallization of strongly coupled dusty plasmas. The dust grains are simulated as particles, while the electrons and ion solutions are obtained from fluid equations. We model gas discharges with dust particles confined in the sheath and at the sheath edge. In these regions the ions flow past the dust grains at a velocity v0 that typically is mesothermal, vTi≪v0≪vTe. 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 maximum means that there is an anisotropic interparticle interaction, which can be manifested as an attractive interparticle force along the axis of the plasma flow. Our simulation shows that the equilibrium configuration of a dust crystal structure can be profoundly influenced by this asymmetry in the ion flow and plasma potential for certain charge to mass ratios for the dust particles. We initially distribute dust particles randomly in space with zero velocity and integrate their equations of motion, tracking their orbits as they settle into equilibrium positions. We show that there are several stable final equilibria, although they do not all have the same potential energy or probability of occurring. The most likely crystal configuration depends on the charge-to-mass ratio and the horizontal particle spacing of the grains. The 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.