An Ultracold Neutral Plasma

Created by Photoionizing Laser-Cooled Xenon

Thomas Killian, Simon Kulin, and Steve Rolston

We create an ultracold neutral plasma by photoionization of laser-cooled xenon atoms. The charge carrier density is as high as 2 x10^{9} cm^{-3}, and the temperatures of electrons and ions are as low as 100 mK and 10 uK, respectively. Such low temperatures are unprecedented in neutral plasma physics. Plasma behavior is evident in the trapping of electrons by the positive ion cloud when the Debye screening length becomes smaller than the size of the sample. We produce strongly coupled plasmas with parameters such that the kinetic energy is small compared to the Coulomb interaction energy between nearest neighbors. This is the reverse of the usual situation for classical Debye plasmas. Plasma oscillations are observed and serve as a probe of the electron density distribution.