Laser light can cool particle beams in 3 dimensions

17 Jun 2002

Proton-antiproton collision experiments at CERN and Fermilab depend on cooling techniques that allow swarms of unruly antiprotons, created by smashing protons into a small target, to be gathered into tidy bunches. In such accelerator experiments, the goal is to coax particles to travel down the beam pipe with the same longitudinal velocity and very little motion perpendicular to the beam axis.

"Cooling" the beam-- getting rid of unwanted motion-- can be achieved by injecting a co-moving stream of electrons, which interact with and absorb excess energy in the beam particles. Laser light can also be used to soak up unwanted motion. In atom traps, lasers have cooled populations of atoms to within a fraction of a kelvin.

At accelerators so far, lasers have only succeeded in cooling longitudinal motion. Now a group of scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany has used lasers to indirectly cool transverse motions as well through laser-induced intrabeam interactions among 7-MeV beryllium ions circulating around the Heidelberg Test Storage Ring. The effectiveness of the process can be characterized by specifying an equivalent temperature for the beam particles. Starting longitudinal and transverse "temperatures" for the ions are 1500 and 15,000 K, respectively.

Electron cooling reduces these values to 300 and 3500 K. The result with lasers is even better: 10 and 2500 K. Recently, still lower temperatures have been obtained. Team leader Rudolf says that it may soon be possible to cool a beam to such an extent that it "liquefies."