Non-equilibrium phase space dynamics of a strongly-coupled plasma with steep density gradients*

We present an experimental and computational study of the non-equilibrium phase space dynamics of a strongly-coupled plasma with steep initial density gradients surrounding a gap. The density gradients are formed by ionizing an ultracold neutral calcium gas using a spatially structured laser beam. The ion distribution function f(x,v,t) is derived from a series of time- and spatially-resolved images of laser-induced fluorescence. Starting from the initial f(x,v,0), we run a BGK plasma kinetic model to predict f(x,v,t) and compare with the experimental data. The BGK prediction aids the experimental interpretation by providing access to quantities that the experiment cannot determine, such as electric fields, the electron distribution, and ion transport properties. The steep density gradients generate beams into the gap, leading to inter-penetrating plasma flows. We observe the transition from the spatially-heterogeneous, non-Maxwellian initial distribution to local thermodynamic equilibrium. These controllable beams could be used in plasma stopping power measurements and studies of flow-induced plasma instabilities.