The Ultracold Plasmas and Atoms group recently reported the first experimental observation of kinetic energy oscillations in equilibrating strongly-coupled plasmas. This phenomenon has been studied since the 1990's with computer simulations of plasmas produced by irradiation of solids and clusters with fast-pulsed lasers. Experimental observation is now possible in ultracold neutral plasmas created by photoionizing laser-cooled strontium atoms just above the ionization threshold. Experimenters scatter light off the ions to measure their kinetic energy during equilibration, and the ions have a temperature of about 1 K. At such cold temperature, the ions are strongly-coupled, meaning their electrical interactions with each other dominate over thermal motion. Ions should show liquid-like spatial correlations, which is far different from in normal plasmas in which particles are so hot that they move in an uncorrelated, gaseous fashion.

Kinetic energy oscillations are manifestations of spatial correlations. Ions oscillate around the local potential energy minima that are formed as particles minimize their interaction energy by maximizing the distance from their neighbors. The primary motivation for these experiments is to understand how plasmas behave in this exotic regime. This may shed light on other systems in which strong-coupling plays a role, such as white dwarf stars, inertial confinement fusion systems, and quark-gluon plasmas.

Reference:
Y.C. Chen, C. E. Simien, S. Laha, P. Gupta, Y. N. Martinez, P. G. Mickelson, S. B. Nagel, and T. C. Killian, 'Electron Screening and Kinetic Energy Oscillations".