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1、精品论文Double layers and ion-acoustic waves: a fully particle-in-cell simulationGUO Jun1, LI Bo25(1. School of Mathematics and Physics, Qingdao University of Science and Technology, ShanDong QingDao 266061;2. School of Space science and Physics, Shandong University at Weihai,ShanDong WeiHai 264209)Abst
2、ract: Double layers and ion-acoustic waves are investigated by using a one-dimensional10electrostatic particle-in-cell simulation code. Our results show that double layers can be formed even when the drift velocity between electrons and ions is less than the electron thermal velocity. The electron a
3、nd ion density depressions are clearly seen. Electrons gradually develop a distributioncomprising both background and beam components. In fact, as the initial electron-ion drift velocity isless than the electron thermal velocity, intense ion-acoustic waves can be found only where the electron15beam
4、is located, suggesting that they are excited by the self-consistently developed electron beam.Besides the Langmuir waves and ion-acoustic waves, the beam mode excited by electron beam produced in our simulation has been found clearly.Keywords: electron-ion streaming instability; Langmuir instability
5、; particle-in-cell200IntroductionDouble layers have long been of interest to both laboratory and space plasma communities1-7. The definitive double layer structures were observed by the Viking satellite in themagnetosphere, which suggests that double layers are small. These structures have an extent
6、 of100m, amounting to a few tens of Debye lengths, and the plasma density therein is reduced by up8- 9 -25to 50%. The necessary condition for the formation of double layers obtained from the previousworks1-4is that the electron drift velocity exceeds the electron thermal velocity, i.e., doublelayers
7、 are a result of the Buneman instability. If the opposite is true, i.e., the relative streaming velocity is sufficiently less than electron thermal speed, the instability is called the ion-acoustic9-12(IA) instability. Both the Buneman16-17and IA instabilities13-15have been investigated carefully.30
8、However, there are observationsthat support the existence of double layers along auroralfield lines where the electron drift speed is much less than the electron thermal speed. Sato and18Okudapoint out that an IA instability can result in the formation of double layers if a system issufficiently lon
9、g. The initial drift velocity used in their simulation is less than the electron thermal velocity. They also pointed out that the anomalous resistivity generated by the IA instability35causes the buildup of a DC potential which in turn accelerates electrons further to enhance the original instabilit
10、y, leading to the formation of double layers. But a low ion-to-electrons mass ratio (which is equal to 100) is used in their simulation. What consequences will a realistic mass ratio (1836) bring forth?Computer simulations are the most powerful tool to study this dynamical process. For13-15, 19-2340
11、instance, Vlasov simulationshave been in use to study the electron dynamics, and theanomalous resistivity produced therein via the IA instability. Many authors use one- or24-27two-dimensional particle-in-cell codes to examine electrostatic instabilities. To explore theFoundations: National Natural S
12、cience Foundation of China (Nos. 40974097, 41204115), Research Fund for theDoctoral Program of Higher Education of China(No.20110131110058)Brief author introduction:GUO Jun, (1973-), Female, Associate Professor, Space Plasma Physics.Correspondance author: LI Bo, (1976-), Male, Professor, Space Plasm
13、a Physics. E-mail: bblformation of double layers in general, and the wave and particle dynamics in particular, we use a1D treatment in this study to help isolate the physics we are interested in, by excluding45complications such as oblique wave modes. In this paper, using one-dimensional electrostat
14、ic particle simulation, we will present the results of kinetic simulations designed to study thedevelopment and evolution of double layers in a current-carrying plasma. A relatively lowvelocity u /v0the=0.6 is used. The excitation and evolution of ion-acoustic waves will also bediscussed.Figure 1. E
15、volutionary history of various parameters. (a) The electron drift speed ue versus normalized time, (b)the evolution of electric energy density E2.501SimulationWe have performed one-dimensional electrostatic particle simulation with the system length L=1024D, where D=vthe/pe is the electron Debye len
16、gth, vthe= (Te/me) 1/2 is the initial electron thermal velocity, pe is the electron plasma frequency. Besides, Te and me are the electron55temperature and mass, respectively. Initially the density is uniform and the electron velocity distribution is a drifting Maxwellian. The ion drift velocity is z
17、ero. We normalize velocities by-1initial electron thermal velocity vthe, and time by pe. Periodic boundary conditions are used. Theelectrostatic fields are defined on grids 25-27. Electric fields and potentials are obtained by solving the Poisson equation, while particles move in the electric field.
18、 The grid spacing is x=1.0D, and60the time step is pet=0.02. We use u0/vthe =0.6 in the simulation, which means that the initialrelative streaming velocity between ions and electrons is less than the electron thermal speed. The numbers of particles employed for both species are both 409600.Figure 2.
19、 Various parameters at . (a) and (c) the electron and ion phase-space distributions; (b)and (d) the electron and ion density profiles; (e) the electric field distribution.A few remarks are necessary on the following parameters we adopt. First, consider the65ion-to-electron mass ratio. We use an mi/m
20、e of 1600, which is close to the realistic value (1836).Our simulations practically yield the same result as what one may find with the realistic ratio. Second, let us discuss our choice of electron-to-ion temperature ratio Te/Ti , on which the growth rate of the ion-acoustic instability sensitively
21、 depends. The ion-acoustic instability dominates when the electron temperature exceeds the ion temperature TeTi. Note that reference 18, the70most relevant to the present study, adopted a temperature ratio Te/Ti of 20 and a mass ratio of 100.To find out what the differences in adopted mass ratios br
22、ing out, we adopt a very similar temperature ratio, 16 to be specific.2Simulation ResultsFigure 1 (a) shows the electron drift speed ue versus normalized time. Figure 1 (b) shows the75evolution of electric energy density. In the early stage of wave evolution (pet=0-2500) the total field energy incre
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