Numerical Study On The Evolution Of Dusty Plasma Pattern

In a dusty plasma,the dust particles are suspended in the plasma sheath to form a dust cloud.Various phenomena of nonuniform dust cloud are called dust pattern.There are two main types of dust pattern which were observed in a capacitively coupled rf discharge experiment.One type is the circular void,the ring void and their coexistence shaped void formed at the early stage of dust particle growth,and another is the dust crystal formed at the later stage of dust particle growth.In this thesis,the mechanism of the formation and evolution of dust voids and the effect of ion flow on the structure of monolayer dust crystals are studied numerically.We develop a self-consistent nonlinear model based on the early nonlinear theory,to investigate numerically the void evolution under cylindrical coordinates and the effects of the ionization.The ion density profile is solved by an ion continuity equation with the ionization term.It is shown that if the ionization rate is larger than a threshold,an initial steady-state dust-density distribution can evolve into a stable distribution containing a void,as the ionization rate is further increased,the time required for the circular void formation decreases,the radius of the circular void decreases.However,for still larger ionization rates,the stable void cannot be formed.We assume that the ring void is developed from potential instability which was caused by a positive potential center far away from the system center formed by ions gathered.The ring void has been simulated numerically based on this assumption.The simulation results can qualitatively explain the experimental phenomena.The structure of the monolayer dust crystals is studied by Langevin molecular dynamics simulation when the influence of ion flow is taken into account.The "Yukawa/point-wake" model is used to describe the particle-particle interaction and particle-wake interaction in the dust system.We set up two dust particle systems(400 and 100 dust particles respectively)to simulate the monolayer crBystal,and analyze the vertical and horizontal distribution of dust particles.The dust particles move downward from the initial vertical equilibrium position.The monolayer crystal is quasi two-dimensional and presents a "bowl" shaped structure.The dust particles in the large system distribute closely in the horizontal direction and show the ordered structure.The dust particles in the small system are relatively sparse in the horizontal direction,and the dust particles appear the ordered structure only under specific ion flow conditions.The effect of ion wake charge on monolayer dust crystals is stronger than that of the distance between ion wake and the dust particles above it.