Observation of the spatial growth of self-excited dust-density waves

The growth of a naturally occurring dust-density wave (DDW) is experimentally observed using high-speed imaging. This low frequency wave (∼25 Hz) grows in amplitude as it propagates downward through a dusty plasma. The wave’s linear growth rate −ki is measured using a phase-sensitive analysis method. For the conditions studied here, the growth rate increases as gas pressure decreases. At a critical gas pressure, which is observed, a balance between an ion-flow instability and dissipation by neutral gas drag determines a threshold for wave propagation. A linear dispersion relation is derived, taking into account the effects of strong-coupling, to compare to the experiment.