Simulation Study on the Dynamics of Dusty Plasmas and the Structure Formation

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We have developed a new particle simulation code for dust grains which are dynamically charged by the background plasma. Here, not only the regular attachment effect of the electrons and ions, but also the effects of the secondary electron emission and the coagulation of grains are taken into account.Simulation results show that the all the grains change their charge randomly from negative to positive, or positive to negative in the way of "flio-flop" as the time passes by. The lowest (negative) charge is determined by the balance between the attachment of the electrons and ions, while the highest (positive) charge is determined by the balance between the electron attachment and the secondary electron emission.From the simulation results of various paramenters, it is found that the flip-flop effect is outstanding when the radius of the grains is of the order of 10nm. This is because the attachment of single electron to the grain does not affect much on the surface potential when the ra … More dius of the grain is large enough, say, of the order of the 1 mum. When the radius is small enough, the average probability of the electron attachment becomes small.This fact leads to the another interesting result, namely, the coagulation of a large number of grains, e.g., 1000 grains, make the grain size 10 times larger, so that the flip-flop effect dose not play a major role and grain charge may stay negative.Since the secondary electron emission effect plays a significant role in this simulation, the precise treatment for it is quite important. The secondary electron emission effect would be enhanced more than that given by Sternglas which was employed here, when the grain size is of the order of 10 nm as is treated here.If this enhancement is taken into account, the flip-flop effect shown in the present work becomes more active, even for the larger value of the threshold energy which is assumed here to be a little smaller than the actual one.The dust grains, which are charged in positive or negative, show the coagulation to become larger in size. Because of the above characteristics of the flip-flop effect in the grain size, there exists a probable size of the grains for coagulation. Under the typical parameters of a dust plasma, the most suitable size for the coagulation is found to be of the order of 10nm, which shows a good agreement with the experimental facts. Less