DUSTY DISCHARGES WITH SECONDARY ELECTRON EMISSION

INTRODUCTION There are two main regimes of RF discharges [1,2] called the α – regime and γ regime. They differ by the role of a secondary emission from electrodes or walls. RF discharges are supported mainly by a volume ionization in the α – regime unlike the γ regime where the secondary emission is important for the discharge support. The α – regime is realized at a relatively high pressure, the γ regime at a low pressure. Although RF discharges with secondary emission were investigated earlier, the role of the secondary emission in the discharges is not yet clear. The main difficulty arises from the necessary to take into account (self-consistently with discharge parameters) all kinds of secondary electrons emitted from the electrodes (walls) by ion, electron, fast atom, or metastable impact as well as by ultraviolet radiation from the discharge. Recently, models of the effective secondary-emission yield γ per ion were developed for the breakdown [3] and DC glow discharges [4] in argon. The models take into account all kinds of secondary electrons and give the dependence of the effective yield γ on the cathode electric field reduced by the argon atom density. These models can be useful for RF discharges. Dust particles can appear in RF discharges as the product of the plasma-wall interaction with their subsequent transport into an interelectrode space or can be created due to coagulation of various components in chemically active plasmas. It is known [5,6] that the dust particles can essentially influence the parameters of the RF discharges due to a continuous selective collection of background electrons and ions that can essentially influence their energy distribution functions. Dusty RF discharges with secondary electrons were not investigated earlier, although it is obvious that secondary emission has to influence the properties of dusty discharges especially at low pressures when the role of secondary electrons is growing. The computer simulation of dusty RF discharges with secondary electrons is the aim of the work. MODEL A one-dimensional RF discharge is considered between two plane electrodes separated by a gap of d = 2.0 cm which is filled with Ar at various pressures. Immobile dust particles of a given radius Rd are distributed uniformly in the interelectrode gap with a density Nd. The dust particles collect and scatter electrons and ions distributed in the discharge with density ne and ni, respectively. A harmonic external voltage Ve(t)=Vosin(wt) at a frequency f=13,56 MHz and various amplitudes Vo sustains the RF discharge. The discharge is grounded at x=d. The PIC/MCC method described in detail earlier for discharges without dust particles is developed for computer simulations of the RF discharge with dust particles. The Monte Carlo technique is used to describe electron and ion collisions. The collisions include elastic collisions of electrons and ions with atoms, ionization and excitation of atoms by electrons, charge exchange between ions and atoms, Coulomb collisions of electrons and ions with dust particles, as well as the electron and ion collection and scattering by dust particles. In addition to a usual PIC/MCC scheme, the weighting procedure is used also for the determination of a superparticle charge part, which is interacting with a dust particle. The electron-argon collision cross-sections used in the model are the same as those used in [7]. The Coulomb cross-section for electron and ion scattering by immobile dust particles is taken from [8]. The secondary emission is taken into account in the framework of the models of [3,4] or at given various constant yields γ of the effective secondary emission. The simulation starts at an initial uniform distribution of electrons and ions with given densities and is prolonged by iterations up to a moment when a change of discharge parameters is less a given limit. Simulation shows that 400-1000 cycles are enough to obtain the periodically steady state of RF discharges. RESULTS Obtained spatial distributions of the electron ne and ion ni densities across the interelectrode gap are shown in Fig. 1 for various phases φ of the sustaining external voltage