Researchers show plasmas can focus high-energy electron, positron beams. (Accelerators).

A Lawrence Livermore National Laboratory researcher has shown that plasmas can focus high-density, high-energy (30 GeV) electron and positron beams 1,000 times better than the magnetic quadrupoles used in conventional accelerators.

In the E150 experiment on Stanford Linear Accelerator Center's (SLAG) Final Focus Test beam, a plasma could focus an electron beam to a third of its original diameter in just 2 centimeters.

In addition, the researchers demonstrated plasma focusing of high-energy positron beams for the first time.

Technologies have existed for focusing MeV electron beams, but not for the GeV beams. that will be used in future accelerator experiments.

The SLAG work demonstrates a potentially promising technique for focusing those GeV beams. The plasma's focusing effect was anticipated in earlier theoretical and experimental research, but not demonstrated until now.

Competing Forces Influence Beam

How does a plasma focus particle beams so well? To understand this effect, researchers point out that it is important to realize electrons, or other electrically charged particles, in a beam experience two competing forces: a repulsive "Coulomb" force which tries to make the beam blow apart, and magnetic forces which push the electrons together.

As it passes through a plasma, the high energy beam will redistribute the electrons so that the net Coulomb force is decreased but the magnetic force is not affected. This serves to pinch the beam closer together.

Conventional plasmas seem to focus beams very well; so it appears that no exotic plasmas will need to be prepared, the researchers concluded.

Contact: Hector Baldis, Lawrence Livermore National Laboratory, [phone], [e-mail]; or visit the SLAG Web site: http://www.slac.stanford.edu/exv/el150.