New physical effects in formation of non-linear equilibrium of dust structures in complex plasmas

摘要

The result that homogeneous complex/dusty plasmas are universally unstable for structurization was first obtained in [1,2]. Subsequent efforts have been to describe the non-linear self-consistent structures as possible final state of structurization [3]. These theoretical efforts have been based on simplified description of processes responsible for confinement of all components in structures (such as linear screening, quasi-neutrality, negligible diffusion etc.). The further developments of experiments indicate the necessity to create compact dust structures for observing plasma crystals in pure boundary free conditions. For large dust charges necessary for crystal formation the dust screening is strongly nonlinear which substantially modifies the ion drag force being crucial for structure formation. The present report gives the main physical and computational results taking into account three new effects: nonlinear screening, ion diffusion due to high rate of collisions with non-linearly screened grains and neutrals, nonlinear ion pressure force due to ion-grain and ion-neutral collisions. All 3 effects have been investigated for the first time. They strongly depend on dust density (that is high inside the dust structures) and on parameter of non-linear screening β = Z_d~e~2/λ_DT_i (λ_D) being the Debye radius Z_d the dust charge and T_i the ion temperature) which is about 30 in laboratory experiment. The results of numerical investigations predict existence of equilibrium and stable structures for parameters of planned experiments. 2.Physics of dust and ion/electron confinement in the structures. Numerical investigations of dust structures was accompanied with calculations that have been devoted to clarify the physics of confinement of dusty plasmas components in the structures. They are the following: the grain drag force by plasma flux is directed inward the structure; to balance the drag an electric field force is excited which is for grains directed outwards and since the grains are charged negatively the electric field is directed inwards and drive ions inwards; the ions have a potential well at the structure center that substantially exceeds their temperature and the electrons are adiabatically following the potential distribution to compensate partially the charge separation created by drag force. This picture is not possible in absence of dust where the polarization forces can create Coulomb explosion. The degree of quasi-neutrality inside the dust structure is regulated by the strength of the drag force but not by polarization due to diffusion. In the de- scribed picture the maximum of ion and, dust densities are located at the center of structure although the distributions can be complicated with several higher maxima inside the structures. For certain large distance which can be named as structure size the dust density vanishes and the for larger distances in some range the balance is absent. Such structures are surrounded by dust voids. 3.Nonlinear drag force. The new numerical calculations of grain non-linear screening have been made using exact expressions for screening [4]. It was found that the total screening of a grain charge occurs at about 10λ_D and the cross-section of ion scattering on,large angles on grain potential can ex- ceed about 4 times the previously known cross-sections both for approximate nonlinear screening [5] and for linear screening [6]. These cross-sections nave been found numerically as functions of normalized velocity of impact ion y = v_i√2vTi and non-linear parameter β in the range 0.05 < y < 6;3 < β < 90 both by the backscattering model and directly by integrating on impact parameters taking into account the potential barriers.