Image: NASA/JPL/Univ. of Arizona
Physicists in the U.S. and France have found a new technique for whipping up instant gases of ionized atoms at temperatures nearing absolute zero. These ultracold plasmas are useful for study in that they probably resemble the plasmas churning within white dwarf stars and massive planets, such as Jupiter (see image). Until recently, researchers were unable to create plasmas so cold--making this latest feat, reported in yesterday's issue of Physical Review Letters, all the more remarkable. These ultracold plasmas differ from their warmer, though still officially "cold," cousins in that they are held together solely by the attraction between the ions and electrons they contain.
Thomas Gallagher of the University of Virginia, Pierre Pillet of the National Center for Scientific Research in Orsay, France, and their colleagues happened on the new instant plasma recipe while working with so-called Rydberg atoms. Researchers can create these atoms by exciting an outer electron to a distant orbital. So swollen with energy, a Rydberg atom becomes enormous; the diameter of a cesium Rydberg atom, for instance, can reach 1,600 angstroms. Gallagher and Pillet cooled rubidium or cesium atoms using lasers and trapped them, then exciting their electrons to a Rydberg state. They discovered that when the density of these atoms was high enough, the cloud spontaneously turned into a plasma. In other words, the highly excited electrons lost their weak affiliations with individual ions.
Trapped cold atoms don't normally undergo such transformations. Thus, Gallagher believes that the small percentage of hot Rydberg atoms found in their trap played a key part--by colliding with the cold atoms and by increasing the rate of ionization, caused by thermal photons. Still, the researchers estimate that the collisions and ionization amount to only 10 percent of the energy needed to produce the plasma. So their next round of experiments will look for the rest.