Ultracold neutral plasmas (UCPs) are transient laboratory plasmas with many unique properties owing to their creation from the near-threshold photoionization of laser-cooled neutral atoms in an ultra-high vacuum environment. UCPs have equilibrium electron and ion temperatures as low as 1 K, orders of magnitude colder than other neutral plasmas. They are untrapped and expand into vacuum with an inhomogeneous density and changing neutrality. The plasmas are also created in a non-equilibrium state and the establishment of equilibrium results in a neutral plasma on the border of strong coupling. All of these features have made UCPs ideal systems for the study of basic plasma phenomena.

This thesis focuses on the long-time expansion of UCPs created from laser-cooled xenon atoms. We investigate the importance of external electric fields and the changing plasma neutrality which have been largely ignored in previous work. We present a detailed analysis of the electron distribution and the rate of electron loss during expansion. We show the effect of electron loss on plasma oscillations, and have developed a new technique to directly observe electrostatic plasma oscillations by measuring the induced current on a nearby electrode. We also expand on previous measurements of plasma expansion in a uniform magnetic field and consider the prospects for magnetic trapping.