How Low Can We Go?  Measuring the Lowest Temperatures that can be Achieved in Ultracold Plasmas

Jacob Roberts, Michael Lim, and Steve Rolston

By combining the techniques of laser cooling and photoionization, we have created a reasonably dense (up to 1010 cm-3), stable, neutral plasma at ultracold temperatures from a sample of metastable Xenon atoms.  Altering the frequency and intensity of the photoionzing laser allows us to control the initial temperature (Telectron = 1-1000 K) and density of the plasma.  The plasma is not confined, and so after its initial creation it rapidly expands over the course of its ~100 ms lifetime in response to the pressure of the electron component.  As it expands, the plasma is predicted to cool.  If it cools enough, then liquid-like spatial correlations should develop as the plasma enters a strongly coupled regime that has not been directly observed in two-component laboratory experiments.  The goal of this work is to trace the temperature evolution of the ultracold plasma to see if the strongly coupled regime can be achieved, and to provide additional data to be used in understanding the heating mechanisms, such as Rydberg formation, that are present in our system.