SCIENCE / MEDICINE : Stanford’s Linear Collider Faces a Final Few Technical Hurdles

One of the boldest experiments in modern science, designed to probe some of the innermost secrets of the atom, has passed a major milestone, but several technological hurdles are proving elusive, according to scientists with the Stanford Linear Collider.

All along, scientists and engineers who have been building the $115-million facility have been saying that the radical design of the collider will push technology to the limits, but that if it succeeds, it should accomplish feats rivaling a European collider that is due to begin operating soon. That one will cost 10 times as much as the Stanford facility.

The Stanford collider is the brainchild of its director, Nobel laureate Burton Richter, who hopes it will be able to capture millions of subatomic “z particles” believed to be the bearers of the “weak force” which allows atoms to decay in a process that releases radioactivity.

Remaining Problems

Richter had hoped to have the collider in full operation early last year, but some of the technological challenges have proved substantial. If the remaining problems can be resolved quickly, success could come as early as late next month, according to physicist Michael Riordan, spokesman for the collider.

“We’re getting ready to do physics,” Riordan said.

Other modern colliders are circular in design, and subatomic particles are propelled at nearly the speed of light in opposite directions around the “race track.” The particle beams cross at several points and each time they cross some particles coming from one direction collide with those coming the other way. The debris from those collisions tells high energy physicists about the nature of the universe’s smallest bits of matter.

However, the size of the facility determines the amount of energy in such collisions--and thus the size of the smallest particles that can be produced. And as physicists have sought higher and higher levels of energy the price tag has escalated enormously, as evidenced by the $6-billion supercollider the federal government is considering building.

Richter concluded some years ago that similar levels of energy might be possible at far less cost if the particles could be sent down a straight course and then guided around a circle at the end where they would collide. And thus the 2-mile-long Stanford Linear Collider, funded by the U.S. Department of Energy, was born.

Beam Narrowed

However, since the beams will meet only once in the Stanford collider, each beam must be extremely narrow--about one-tenth the thickness of a human hair--and the particles must be highly concentrated within each beam.

For the first time last week, Stanford physicists managed to narrow the beams to about 8 microns, still twice the size they should be, but a substantial improvement nonetheless. The beams were focused so accurately that they met in the right spot.

It was only a test, and scientists were not surprised when no particles collided. The beams were too broad and the particles were not as concentrated as they should be. The particles move through the system on electromagnetic pulses, sort of like surfers riding a wave, and each bunch had only about 2 billion particles instead of the desired intensity of 10 billion, Riordan said.

To succeed, the number of particles in each bunch must increase dramatically, but when the intensity is increased, he added, the beams spread out to unacceptable levels.

Scientists are struggling now to increase the intensity and narrow the beams even further. Those goals are essential if the facility is to succeed.