Pairing of particles in complex plasmas

Kurzfassung

Complex (dusty) plasmas are suspensions of charged micron-size solid particles in a plasma. In laboratory experiments, complex plasmas are often produced by suspending polymer microparticles in the (pre)sheath area above the lower electrode in a gas discharge. Particle pairing is a ubiquitous phenomenon in such experiments. It was observed in various settings from isolated pairs and larger strings to pairs in 2D crystals and particle bilayers. An important role in the formation of particle pairs belongs to the plasma wakes that appear behind charged particles in the flow of ions. Fundamental questions are what determines the spatial extent of pairs and their stability. These questions recently received attention, but to the best of our knowledge no systematic investigation was done. In this work, we systematically study the dependence of particle pair size on experimental parameters [1]. We used an experimental setup based on a modified Gaseous Electronics Conference (GEC) radio-frequency (rf) reference cell. Plasma was sustained by a capacitively coupled rf discharge in argon at the pressure of 2 Pa. Melamine-formaldehyde microspheres with a diameter of 9.19±0.09 μm were suspended in the (pre)sheath area above the lower rf electrode. The particle suspension was imaged from the top and side with two video cameras. A specially designed "pair state" of complex plasma was prepared where particles formed multiple pairs that were vertically oriented, had a well-defined size, and levitated at the same height but were weakly coupled with each other and did not form any regular structure. For higher gas pressures (> 0.5 Pa), the pairs were stable in a wide range of discharge voltages. As the discharge voltage was reduced, the pairs shifted downward and extended vertically. The vertical size of pairs was measured as a function of experimental parameters, it was found to closely follow the electron Debye screening length. For lower gas pressures (< 0.5 Pa), the pairs became unstable and broke up. A theoretical model of particle pairing is proposed where the vertical confinement of the lower particle is provided by the non-uniform field of the plasma sheath, while the lateral confinement is due to the horizontal wake field. Disappearing lateral confinement at lower gas pressures is the reason of the particle pairs "instability." Pair breakup is facilitated by the particles' acquiring high kinetic energy due to increased charge fluctuations at lower pressures. Good agreement is found with experimental observations. [1] V. Nosenko, A. V. Ivlev, R. Kompaneets, and G. Morfill, Phys. Plasmas 21, 113701 (2014).