Accelerators load some new ammo: Crystals

Particle-accelerator specialists are forever trying to squeeze speeding particles into denser beams. That means more particle collisions within the accelerators, yielding more data and quickening the pace of discovery.

The girth of an ion beam (top) shrinks as the beam crystallizes (bottom). Schätz et al./Nature

But corralling the particles into narrow streams presents a challenge because the particles, which typically are all either positively or negatively charged, repel each other. Now, a German team has shown a way to minimize this problem: Freeze particles in the beams. Instead of individual particles whipping around a ring, cold, dense crystals make the rounds.

“It’s the most stable and brilliant beam one can imagine,” says Ulrich Schramm, a member of the team at the Ludwig-Maximilians-Universität (LMU) Munich in Garching, Germany.

In the Aug. 16 Nature, he, Tobias Schätz, and Dietrich Habs report accelerating a single-stranded crystal of magnesium ions–like “a string of pearls,” Schramm says. In experiments described at a plasma-physics conference earlier this summer, the team also accelerated multiple-stranded versions.

The LMU results are “a very good first step toward bright sources of the future,” says Max Zolotorev of Lawrence Berkeley (Calif.) National Laboratory.

The researchers used dual lasers to chill the magnesium ions to a temperature approaching absolute zero, thereby bringing them to a near standstill. The same lasers then accelerated the resulting crystals–containing up to 96,000 ions–around the ring.

So far, the procedure has worked only in a doughnut-size, circular accelerator, which the scientists developed over several years explicitly to determine whether crystal beams were possible. Their device neither accelerates the crystals fast enough to produce interesting reactions nor provides ways to collide the beam into targets as larger accelerators do.

Habs’ group had previously tried to create crystalline beams in large accelerators, but the machines they used broke the delicate structures apart, says Schramm.

The new results indicate that beams of ionic crystals should be possible to maintain even in large, high-energy ion accelerators, as long as the beam trajectories are smooth. However, in scaling up to large sizes and energies, “there are many effects that can destroy such a crystal,” Schramm concedes.

Moreover, the approach is unsuited for some of the world’s top accelerators because they host particles, such as protons and electrons, that can’t be chilled and crystallized with laser beams.

The next step, says Schramm, is to design a big ion machine that will produce crystallized beams. The Gesellschaft für Schwerionenforschung, a German heavy-ion research center in Darmstadt, is making plans to build just such an accelerator, “that would fit nicely the requirements for a crystalline beam,” he adds. If funding for the 300-meter-circumference device comes through, the scientists may find out by around 2010 just what this newly demonstrated strategy can deliver.

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