The particles, high-energy electrons, were traveling in a linear accelerator at nearly the speed of light.

The discovery opens up the possibility of using high-energy particle beams in ways similar to the way fiber optic cables carry digital information.

USC scientists envision circuits of plasma carrying high-energy streams of electrons for a variety of purposes, some not yet imagined. They say the phenomenon might be used to provide better control for medical radiation devices, for example.

The most immediate application is likely to advance state-of-the-art particle accelerators themselves. In current accelerators, beams of particles are steered by large, bulky and powerful magnets. Bending the beams with "refractive optics would be more powerful, more nimble, more compact and possibly less expensive," said Thomas Katsouleas, professor of electrical engineering and associate dean of the University of Southern California School of Engineering, one of the authors of the paper, published in today's issue of the journal Nature.

"Particles normally don't refract. Until now, that's only been seen with waves," Katsouleas said."This was a wonderful bit of serendipity associated with a desire to solve another problem."

Principal author Patric Muggli, a research associate professor of electrical engineering at USC, said the original goal of the research team was to increase the acceleration of a stream of electrons by sending them through plasma.

Muggli, Katsouleas and Seung Lee, a Ph.D. student in electrical engineering at USC, worked with physicists from UCLA led by Chan Joshi and another group at the Stanford Linear Accelerator (SLAC) led by Robert Siemann.

Particle accelerators speed up particles by passing them through a succession of intense electric fields. At SLAC, the electrons accelerate for three kilometers, roughly two miles, to gain enough energy -- 30 billion electron volts -- to reach nearly the speed of light.

"The energy per meter that the particles can gain is limited," said Muggli, explaining why it takes a distance as great as two miles to reach that speed. "We believe that if we can accelerate them in plasma, we could significantly shorten that distance."

Plasma is gas that is so hot that molecules break down into highly energized positively and negatively charged particles.

Plasma exists only for brief periods on Earth, in the fireball of a nuclear explosion or for microseconds in a laboratory. But in stars like the sun, plasma is the normal state of matter. Muggli designed a narrow 1.4 meter tube to hold the plasma, while a beam of electrons passed through it.

"When we put it into the plasma, the beam kicked around and it tripped off all the safety systems," said Muggli. "We didn't know what was happening because it is supposed to go straight."

When the researchers analyzed their data, they discovered that the beam of electrons had refracted, acting like light waves rather than fast-moving particles. Light waves bend when passing diagonally from one material to another because the speed of light changes slightly according to the density of the material it is traversing. This is why an oar appears bent when dipped into water.

"Not only did we see refraction, but we saw the beam reflect at the edge of the plasma, and we saw it split in two," said Katsouleas. "It is a peculiar property of plasma that this occurs. Even though the beam is intense enough to blast through solid steel, it hit plasma that was about one million times less dense than the air and bounced off."

The researchers are still analyzing their data to determine if they succeeded in using plasma to accelerate particles, the original purpose of the experiment.

"We believe that we have done that and should be able to report on that aspect soon," said Katsouleas.

"Refraction of a Particle Beam" is in the May 3 issue of Nature. - By Bob Calverley

[Contact: Bob Calverley]

03-May-2001