Heating and phase transitions of dust-plasma crystals in a flowing plasma

A three dimensional particle simulation is used to study hexagonal dust crystals in flowing plasmas. The flowing velocity ${\mathrm{v}}_0$ is mesothermal (${\mathrm{v}}_{\mathrm{Ti}}$${\mathrm{v}}_0$${\mathrm{v}}_{\mathrm{Te}}$), which is a typical situation for dust particles confined in industrial plasmas and plasma experiments. Unlike for the Yukawa system (Debye-Hückel shielded dust), we find parameter regimes where a hexagonal dust crystal is stable. This is due to the wakefield generated around the individual particles which causes attractive interparticle forces along the axis of the plasma flow. These attractive forces cause a rather strong binding of the crystal since the simulation shows that solid-fluid transitions occur at a much lower Γ value (higher dust temperature) than for the Yukawa system. Stability of a hexagonal dust crystal is found to depend strongly upon the gas neutral pressure since stable crystal structure is obtained for dust-neutral collision frequencies ${\gamma }_{\mathrm{d}}$ above some threshold value ${\gamma }_{\mathrm{d}0\mathrm{}}$. For ${\gamma }_{\mathrm{d}}$${\gamma }_{\mathrm{d}0\mathrm{}}$ the ion flow will excite crystal waves (phonons) where the amplitude of these waves grows as a function of time. This instability will either saturate due to nonlinear lattice waves or continue until the dust crystal structure eventually melts.