ULTRACOLD NEUTRAL PLASMAS CREATED AT NIST.

On Earth, most material exists as either a solid, liquid, or gas, made up of electrons and ions bound to each other in neutral atoms and molecules. However, most matter in the universe exists as plasma, in which electrons and ions move as free charged particles. Familiar plasmas are relatively hot, such as in the solar corona (1 000 000 K), fluorescent light bulbs (10 000 K), or the ionosphere around the Earth (1000 K). NIST researchers have created ultracold plasmas at temperatures as low as 1 K. They use lasers to trap and cool (to less than 0.00001 K) a few million neutral xenon atoms, and then illuminate the cloud with a pulse of laser light containing just enough energy to unbind electrons and ions and create the plasma. Little is known about plasmas in this new regime. The researchers have measured how the plasma freely expands and discovered an unexpected source of expansion energy that they believe is tied to the recombination of the electrons and ions into neutral atoms--the lowest energy state of th e system. At high temperatures, this recombination takes a long time and plasmas can be long-lived. Recent experiments reveal that, as the ultracold plasma expands, the rate of recombination is much larger than expected; the results differ so greatly from behavior in hot plasmas that no current theory can describe the results. Further study is motivated not only by fundamental interest but also because experiments may shed light on the physics of dense cores of gas giant planets like Jupiter, or guide efforts to create anti-hydrogen through the recombination of positrons and anti-protons.