Interaction of 2D plasma crystals with upstream charged particle: Mach cones and channeling effect

Abstract

Summary form only given. Complex plasmas are composed of a weakly ionized gas and charged microparticles [Morfill, G.E., et al., 2009]. In ground-based experiments the particles can levitate above the bottom electrode against gravity by a strong electric field in the (pre)sheath area. Under certain experimental conditions particles can be confined in a single layer and self-organize in a triangular lattice with hexagonal symmetry. Such system is known as 2D plasma crystal. In the course of the experiments with 2D plasma crystals, one often observes extra particles outside the lattice layer. The extra particles can move either above the lattice layer (upstream of the ion flow) or beneath it (downstream of the ion flow), and disturb the lattice structure by creating wave patterns. When the speed of the extra particle is higher than sound speed of the lattice, the disturbance forms a Mach cone. The latter phenomenon has been well studied with downstream particles in the last decade [Samsonov, D., et al., 2000]. In this contribution, we present for the first time a series of experiments performed in a GEC reference chamber to study the interaction of 2D plasma crystals with upstream charged particle. Interestingly, the Mach cones formed by the upstream particle have very different features from that induced by the downstream particle. At the apex of the Mach cone, the upstream particle exerts attraction-dominated force on the particles in the lattice. We attribute the particle-lattice layer attraction to the positive ion wake formed underneath the extra particle [Lampe, M., et al., 2000] in the (pre)sheath area. Furthermore, the extra upstream particles tend to move between rows of particles in the crystal, which is known as channeling effect [Feldman, L.C., et al., 1982]. As an upstream particle moves through a lattice cell, it deforms the cell by the attractive force. We show in this contribution that the interaction between the extra particle and the lattice particles forming the channel is nonreciprocal.