Ion holes in the hydrodynamic regime in ultracold neutral plasmas

Abstract

We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma. V C 2013 AIP Publishing LLC.

Emergence of kinetic behavior in streaming ultracold neutral plasmas

“…For short mean free path, the streams cannot penetrate, and a density enhancement emerges at the confluence. The gap splits and two density depletions propagate away from plasma center as seen in [30]. There is indication of this behavior for T e (0) = 25 K (Figs.…”

“…The emergence of kinetic effects is heralded by the absence of local thermal equilibrium for ions and interpenetrating streaming plasma populations. This compliments recent work on acoustic-wave phenomena in ultracold neutral plasmas in the purely hydrodynamic regime [28][29][30]. Results are compared with a two-fluid hydrodynamic code [31,32].…”

“…Streaming plasmas are precursors to complex phenomena such as instabilities, shock waves, and solitons [9,16,37,38], and they have been studied in a wide variety of environments such as the solar wind [39], supernovae [23], double plasma devices [40][41][42][43], and laser-produced plasmas [2-6, 9, 16]. A partially transmitting strip, instead of an opaque wire, was recently used to create a smaller amplitude density depletion in the plasma center [30]. This led to hydrodynamic phenomena such as gap splitting and propagation of localized density depletions at the ion acoustic wave speed.…”

“…These two caveats indicate that the colliding streams in the T e (0) = 25 K plasma are not fully in the hydrodynamic regime and kinetic effects are important. Experiments in [30] accessed and characterized the purely hydrodynamic regime for T e (0) = 25 K by starting with a very small density depletion and approximately five times greater density in the gap. In this case, the central feature rose to full height, and the amplitude of the traveling density depletions showed no significant attenuation.…”

“…It is the first observation in ultracold neutral plasmas of ion transport [26][27][28][29][30] that can not be explained with a hydrodynamic treatment. The emergence of kinetic effects is heralded by the absence of local thermal equilibrium for ions and interpenetrating streaming plasma populations.…”