Ion holes in the hydrodynamic regime in ultracold neutral plasmas

We describe the creation of localized density perturbations, or ion holes, in an ultracold neutralplasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wavespeed. We also observe the process of hole splitting, which results from the formation of a densitydepletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulationsdescribe the results well. Measurements of the ion velocity distribution also show the effects of theion hole and confirm the hydrodynamic conditions in the plasma