Star cluster dynamics in a laboratory: Electrons in an ultracold plasma

Abstract

Electrons in a spherical ultracold, quasi-neutral plasma at temperature in the Kelvin range can be created by laser excitation of an ultracold, laser-cooled atomic cloud. The dynamical behaviour of the electrons is similar to the one described by conventional models of star cluster dynamics. The single mass component, the spherical symmetry and no star evolution are here accurate assumptions. The analogue of binary star formations in the cluster case is a three-body recombination in Rydberg atoms in the plasma case with the same Heggie's law: soft binaries get softer and hard binaries get harder. We demonstrate that the evolution of such an ultracold plasma is dominated by Fokker-Planck kinetics equations formally identical to the ones controlling the evolution of a star cluster. The Virial theorem leads to a link between the plasma temperature and the ions and electron numbers. The Fokker-Planck equation is approximate using gaseous and fluid models. We found that the electrons are in a Kramers-Michie-King type quasi-equilibrium distribution as stars in clusters. We suggest that the evaporation rate can be used to determine the temperature. As an example, knowing the electron distribution and using forced fast electron extraction, in a 'violent extraction' way, we are able to determine the plasma temperature knowing the trapping potential depth. © 2005 RAS.