Numerical Study On Forming Mechanism Of Dust Void In Complex Plasmas

Recently, complex plasma is one of the most highlights of research of plasmas. In a lot ofcomplex plasma experiments, in laboratory as well as under microgravity conditions, peoplehave observed the formation of dust voids. Especially in processing in microelectronics, onesdiscover the spontaneous form of dust voids. In order to ensure microeletronic processing tobe performed effectively, it is needed that the mechanism of formation of dust voids incomplex plasma is researched deeply and in detail.In 1990, Johnson et al at first time observed ionization instability in trandational plasma intheir experiment.In 1999, Samsonov and Goree reaseared experimentally ionization instability in complexplasma. By means of various methods, they researched in detail the production and evolutionof the ionization instability in complex plasma. It was shown that the ionization instability hastwo phases—linear phase and nonlinear phase. In the linear phase, all the harmonics havepossible to be excited and the ionization instability is filament mode. And, in the nonlinearphase, the harmonics excited all drop to zero frequency, and the ionization instability is 'greatvoid' mode. They observed the formation of dust void in complex in their experiments.In 2001, Xiaogang Wang et al analysed theoretically in detail the linear phase andresonance acoustic mode of the ionization instability in complex plasma. They gave thedispertion relation and established the theory of the ionization instability in the linear phase incomplex plasma.In 2003, Avinash et al at first time researched numerically the ionization instability in thenonlinear phase in complex plasma in one-dimensional the Cardesian coordinates. It wasshown that in the nonlinear phase of the ionization instability in complex plasma, the mainforces acting on the dust particles are electric field force and ion drag force. As a result ofboth the forces acting on the dust particles, dust void is formed. But, in their model, theyneglected the dust convective term in the dust momentum equation without justification.The research works in this thesis have three parts.Owing to cylindrical coordinate is more appropriate to actual case in experiment, in thefirst part of this thesis, the nonlinear phase of the ionization instability in complex plasma isresearched numerically in cylindrical coordinate. In the model of this thesis, we consider thedust convective term in the dust momentum equation. In the research of this thesis, both cases of the Cardesian coordinates and cylindrical coordinate and with and without the dustconvective term in the dust momentum equation are compared respectively. The researchresults of this thesis shown that, in cylindrical coordinate, in the nonlinear phase of ionizationinstability in complex plasma, the main forces acting on the dust particle are ion drag force,electric field force and pressure gradation force, and as a result of these three forces actingjointly on the dust particle, dust void is formed. For comparing the numerical simulatingresults, the parameters using in the computing are same with Avinash et al's. Our numericalsimulating results show that the size of dust void obtained in the cylindrical coordinate isbigger than the size obtained in the Cardesian coordinates, and the time of dust void formingin the cylindrical coordinate is shorter than the time in the Cardesian coordinates. Also, thesize of dust void obtained with the dust convective term in the dust momentum equation isbigger than the size obtained without the dust convective term in the dust momentum equation,and the time of dust void forming with the dust convective term in the dust momentumequation is longer than the time without the dust convective term in the dust momentumequation.In the model given by Avinash et al as well as the model in the first part in this thesis, ionproducing (ionizing) and ion disappearing (compositing) are implicit, in the same time, ionconvevtive term is neglected. In the second part of this thesis, the model of nonlinear phase ofionization instability in complex plasma is reestablished. In this model, ion convevtive term isincluded, and ion producing (ionizing) and ion disappearing (compositing) are explicit. Weanalyse numerically these effects on dust void formation. Our numerical results show that theimproving ion moving equation has significant effects on dust void formation and evolutionthan former works. For dust void obtained after improving ion moving equation, the formingtime is longer, and the size is bigger.In the model given by Avinash et al as well as the model in the first and the second parts inthis thesis, there are all assumption of the system with only one kind of dust, that is, in thesystem, every dust particle has same size and same charge. In the third part of this thesis, thecase of system with two various size dust grains is researched numerically. It is found that thesmaller dust grains first form its void structure. This formation process is the same as that inthe single grain size case. When the smaller dust void is saturated, the system reaches a steadystate approximately, with the smaller dust grains form the void structure, and the bigger dustgrains keep moving outwards. As a result, the big dusts are separated for the small dust voidstructure. This phenomenon is due the inertia difference. Corresponding experiments are thenhighly desirable to further study the process.