Wave Modes of Vertical Dust Chains in Complex Plasma

Plasma sheaths are notoriously complicated; however, the recent use of micron size dust particles as in-situ probes of this region is beginning to provide data that can be employed to better understand these phenomena.~In this study, longitudinal and transverse waves are explored for vertically aligned dust particle chains consisting of 3 to 5~particles.~These spherical particles are levitated in the sheath above the powered lower electrode in a GEC reference cell and are confined in~the horizontal direction using a glass box. Under appropriate power and pressure conditions, the horizontal confinement provided by the box is great enough~to create the chains, which can then be perturbed by applying time-varying potentials to a vertical probe attached to a Zyvex S100~nanomanipulator. The probe can be positioned over a range of locations,~allowing both longitudinal and transverse waves to be~driven through the chains. Particles exhibit coupled oscillator motion, individually producing all of the~pure three-particle~longitudinal normal~modes.~Dispersion relations previously derived for particles aligned in the horizontal plane exhibit similar relevant forces; these are adapted~to the vertical direction and compared to experimental results.