Dust particle alignments and transitions in a plasma sheath

Summary form only given. It was shown experimentally that a system of two dust particles can have stable orientations with the particles aligned either parallel or perpendicular with respect to the direction of plasma flow, with stability determined by the ratio of the confinement strengths in the horizontal and vertical directions and the interparticle forces. The horizontal alignment corresponds to a regime dominated by symmetric particle interactions. In the vertical alignment, the lower particle strongly interacts with the wake of the upper particle. Here, analyses of transitions in a two-particle dust structure in a plasma sheath under various discharge conditions are presented. The transitions occur in two stages including discontinuous and continuous ones. The two-stage character can be attributed to the existence of multiple equilibria for the vertically aligned dust system and the system hopping from one equilibrium to another. The dynamics of the transition from the vertical to the horizontal state is explored in detail. It is shown that the alignment transition involves an additional first-order critical phenomenon followed by a second order symmetry-breaking transition. Two dust particles suspended in the sheath engage in a symmetrical Debye-type interaction as well as an asymmetric attractive interaction due to ion focusing of the vertically streaming plasma. Furthermore, in addition to the linear vertical sheath electric field, which stabilises the particles against the force of gravity, a linear radial electric field is typically applied to trap the particles within the centre of the discharge. The combination of perpendicular electric fields breaks the spherical symmetry of the plasma potential to cylindrical symmetry about the vertical axis with a perpendicular plane of reflection. We report a new model developed and tested for dust particle interactions in a plasma sheath. The model has revealed new features on the static behaviour of the vertical - - dust particles in terms of existence of previously unknown equilibria and also on the dynamics of the vertical to horizontal transition. The model can be used to explain experimental behaviour of dust particles which previously was without theoretical underpinning. Quantitative comparison has also been performed, giving new insight into the wake parameters.