Self-consistent dusty sheaths in edge plasmas

Electrostatic charged sheaths are appeared in front of electrodes and walls facing plasma due to a difference between fluxes of adsorbed charged particles from plasma. The sheaths counterbalance the fluxes and separate electrodes or walls from quasineutral plasma. Magnetic fields can essentially modify sheaths which determine particle and energy fluxes to walls from magnetised plasmas. In particular, sheaths in oblique magnetic fields are appeared in controlled fusion devices around diverter plates and limiters facing edge plasmas. Dust particles can appear in sheaths as the product of the plasma-wall interaction in various technological devices including controlled fusion devices. The dust particles can be created also due to coagulation of various components in chemically active plasmas with their subsequent transport into sheaths. Besides, dust particles can be immersed from outside and be trapped in sheaths creating plasma crystals, which are intensively investigated now including dusty plasma oscillations and waves. The dust particles can essentially influence sheath properties due to the continuous selective collection of background electrons and ions that can cause an essential change of both electron and ion energy distribution functions as well as an ion flux in sheaths. As a result, spatial distributions of plasma parameters in sheaths can be essentially changed including a potential profile that causes properties of dusty plasma oscillations and waves. Electrodes or walls disturb a plasma far from sheaths creating quasineutral nonuniform plasma regions with slow electric fields and slow gradients of plasma density which are called presheaths. The presheaths provide electron and ion fluxes through the sheaths and therefore properties of sheaths and presheaths are interrelated and have to be considered commonly as self-consistent plasma regions. However the common consideration of both self-consistent sheaths and presheaths is a very complex problem due to the strong difference of space scales in both regions as well as due to non-equilibrium of electron and ion energy distribution functions. The problem is especially complex for dusty sheaths. Fortunately, there are several possibilities to consider the self-consistent sheaths separately from presheaths. The possibilities include a separated consideration of sheaths with self-consistent boundary conditions, finding of asymptotic behaviour of rarefaction waves created by electrodes or walls immersed into plasma, and a general consideration of entire plasma with given boundary conditions on electrodes and walls. Using possibilities indicated above, computer simulations of self-consistent dusty sheaths are carried out in various cases by the PIC/MC method. In particular, it is considered sheaths in non-equilibrium plasmas including two-temperature plasmas and plasmas with an electron beams, sheaths in oblique magnetic fields, and sheaths in low-pressure argon RF and DC discharges with given boundary conditions on electrodes. Obtained results show that dust particles can modify boundary conditions at the sheath edge. Spatial distributions of plasma parameters in dusty sheaths can be essentially differed from the distributions in usual sheaths, in particular the distributions indicate a possibility to create in dusty plasmas the double sheaths separated by a quasi-neutral plasma. Spatial distributions of a dust particle charge are non-trivial due to a non-equilibrium of an electron energy distribution function caused by a selective collection of electrons and ions by dust particles and intensive mixing of fast electrons in non-uniform quasi-neutral dusty plasmas. The spatial distributions of a dust particle charge show that charged dust particles can protect walls from an intensive ion sputtering decreasing the ion energy. (orig.)