Plasma polarization around dust particle in an external electric field

Complete text of publication follows. There are a lot of works devoted to the description of plasma distribution around a single dust particle. Dust particles immersed in a low-density plasma of a glow DC or RF discharge acquire a large negative charge, Qd = Zd e0 ∼ 103-104e0. A cloud of trapped ions with total number Ntr ∼ 0.5Zd can be formed around a dust particle. All these models were made for the case of the absence of an external electric field. However, laboratory dusty plasma often exists in an external electric field, i.e., in cathode layers, striations in a DC glow discharge, etc. In the external electric field, the system of the negatively charged dust particle and the cloud of positive ions can be polarized, and the system acquires a dipole moment. In paper [5], it was shown that for a low-collision case the coefficient of polarizability of the dust particle - a cloud of trapped ions depends non-monotonously on the strength of the external electric field. However, the external electric field breaks the spherical symmetry of plasma distribution around the dust particle and sufficiently complicates the semi-analytical approximations such as in. In this paper, plasma polarization around a dust particle is studied by Monte Carlo simulation. Ion trajectories are calculated with the help of Newton motion equations with high precision. The cross section of charge exchange collisions of ions with neutral atoms was assumed to be independent of the ion energy. The ions free path lengths and new ions velocities after collisions with neutral atoms are simulated by Monte Carlo method. The times those ions spend passing through spatial cells around the dust particle are recalculated into ion density via normalization to the value of ion density in ambient plasma. The equilibrium dust particle charge was obtained from the equality of ion current (Monte Carlo calculations) and electron current (Boltzmann equation for EEDF) to the dust particle surface. It was assumed that the charge of the dust particle is equal to the total volume charge of the surrounding plasma in rather large volume. Calculations were carried out for a wide range of mean free paths, ion Debye lengths, radii of dust particles, and values of the external electric field. It is shown that without the external electric field ion radial distributions agree with the results obtained in [1,2,4,5]. The external electric field leads to the disturbance of a spherical symmetry of plasma parameters around the dust particle. The density of ions, ni(r-bar), becomes anisotropic, which leads to the formation of a dipole moment in the system of the dust particle and ion cloud. The dipole moment is equal to PZ(EZ0) = e integral dr-bar'z(ni(r-bar') - ne(r-bar')). It is shown that not only trapped ions but also the redistribution of free ions in the external electric field influences the process of plasma polarisation around the dust particle. This dynamical process leads to a linear dependence of polarization on the external electric field in contrast to the nonmonotonous dependence of polarization obtained for trapped ions only.