High Resolution Measurement and Modeling of Ion Dynamics in an Ultracold Neutral Plasma

Abstract

Using high-resolution laser-induced fluorescence spectroscopy, ion dynamics in an expanding ultracold neutral plasma (UNP) have been studied in unprecedented detail. The evolution of the ion temperature is of great interest because the ions in an ultracold neutral plasma are strongly coupled, meaning their Coulomb interaction energy exceeds the thermal energy. This leads to novel plasma properties that are hard to describe theoretically. Understanding all the factors that contribute to ion temperature evolution is critical for designing schemes to cool the ions and achieve even stronger Coulomb coupling. This work has also observed several phenomena that have not previously been studied in ultracold plasmas. Ion adiabatic cooling has been observed in a UNP for the first time, resulting in ion temperatures as low as 100mK and up to a tripling of the Coulomb coupling parameter. The importance of electron-ion energy transfer is demonstrated with the first observations of electron-ion collisional heating. Inclusion of this effect into existing numerical models provides better agreement with experimental results. Results of molecular dynamics simulations of equilibrating plasmas have been compared to early time experimental data, which provides a new method of density determination that is much less sensitive to experimental systematics and is accurate to within 10%. This improved certainty in the density allows accurate comparison between data and a theoretical model of the plasma dynamics. The agreement is excellent, but on a hydrodynamic time scale a small amount of extra heat (on order of 100mK), not ascribable to electron-ion collisions or other sources in the model, is detected in the ion component. Potential sources are discussed within as well as proposals for further studies and improvements.