Cold traps for ion crystals, solid plasmas.

Cold traps for ion crystals, solid plasmas

Taking an electron away from an atom produces a positively charged ion. Such atomic ions repel each other. When they are cooled to temperatures near absolute zero and held in electromagnetic traps, the ions settle into distinctive patterns -- from a few ions strung out like beads in a necklace to thousands of ions arranged as the surfaces of concentric shells.

Observations of such regular arrangements provide vivid demonstrations of the collective behavior of charged particles, says David J. Wineland of the National Institute of Standards and Technology in Boulder, Colo. This information helps physicists understand how charged particles interact in plasmas and other systems in which the behavior of each particle is strongly influenced by its neighbors. Recent progress in studying trapped atomic ions was the topic of Wineland's presentation last week in Baltimore at the Conference on Lasers and Electro-Optics.

Wineland and his colleagues cool singly charged mercury or beryllium ions to temperatures below 10 millikelvins, just a fraction of a degree above absolute zero. The particles sit in an electromagnetic trap that keeps them from escaping. The trap itself is about the length of the word "LIBERTY" on a penny.

The cooled, confined atomic ions move so slowly that they have insufficient energy to overcome the repulsive electrical force between them. They stay as far apart as possible and settle into patterns in which they are evenly spaced. The large spacings between ions -- on the order of a few microns -- allow detection of the positions of individual ions. Depending on the electric and magnetic fields shaping the trap, he researchers see these ions spread out in rings or lines.

Such orderly arrangements of trapped ions can be interpreted as crystals, clusters or even pseudomolecules. For example, Wineland and his group have studied the vibrations of a pair of trapped ions as one way of understanding the motions of atoms within a molecule, even though atoms in a molecule are much closer together. "Heating" produced by the application of an external electromagnetic field causes the structures to become disordered, or to "melt."

Assembling thousands of ions in a cold trap produces the equivalent of a one-component plasma -- a collection of charged particles, all with the same mass and charge, embedded in a uniform, oppositely charged background. In this case, the ions appear to organize themselves into a number of concentric, spherical shells centered on the trap's mid-point.

Wineland and his colleagues have observed these plasmas as solids, in which the ions stay fixed in place, and as liquids, in which ions diffuse from one region to another. They have also seen a "mixed" phase in which diffusion continues within a shell but stops between shells.

Now the researchers want to increase substantially the number of particles they can hold in a trap. For very large numbers of ions, theoretical predictions suggest the shell structure should wash out and the ions settle into a type of cubic lattice.