Modeling the alignment of grains in a dusty plasma

Summary form only given, as follows. Dusty plasmas provide new challenges for traditional computational plasma physics methods. For example, in the last few years it has been shown that dust grains in a plasma tend to form into regular crystalline arrays, known as "plasma crystals". The grains are trapped in the sheath region of rf discharges and tend to become aligned in the direction of ion flow toward the electrode, which can lead to crystal structures different from those predicted by the theory of strongly coupled plasmas. The alignment process has been studied with molecular dynamics simulations that treat the dust grains as discrete charged particles interacting via screened potentials. An asymmetric addition to the interaction is included through a dipole force. The addition of a very weak dipole is sufficient to modify the structure of the crystal, making it more consistent with experiments, and phase transitions to different configurations occur as the dipole contribution is increased. Experiments at Los Alamos have studied the particle spacing in chains of dust grains as a function of the discharge parameters and suggest that wake effects from the flowing ions can provide a dipole of correct magnitude to explain the observations. The detailed properties of the wake potential surrounding a single grain or a linear array of grains are studied with particle-in-cell simulations that treat both dust grains and the flowing plasma ions as discrete particles that are collisionally coupled to the background neutral gas. The simulations address the dipole approximation, nonlinear modifications to the standard test particle approach, shielding from ions orbiting the dust grains, and wake damping due to phase-mixing and collisional effects.