Structure and dynamics of dust in streaming plasma: dust molecules, strings, and Crystals

In a plasma with ions streaming at a uniform velocity ∼c_s, dust grains can be accurately modeled as particles interacting via the dynamically-screened Coulomb interaction, calculated from linear response theory for the plasma. This force is nonreciprocal, i.e., action does not equal reaction, which has remarkable dynamical consequences. We show that up to four grains can form a stable self-bound molecule, which propels itself upstream against the ion flow. Stable equilibria are also found for pairs of grains confined in harmonic or quartic external potentials. For two grains in an anharmonic potential, or for three or more grains in any potential, there is no conserved quantity and self-excited oscillations can occur. In general, there are multiple equilibria, hysteresis occurs as parameters are varied, and it is not possible to distinguish ground and excited states. We show how the organizational and dynamical principles that govern the behavior of few-grain and low-dimensional systems also elucidate the more complex dynamics of crystals.