Published online 27 September 2009 | Nature | doi:10.1038/news.2009.953

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Physicists shrink X-ray source

Laser accelerator almost fits on a tabletop.

laser beamLaser pulses fired into hydrogen produces intense x-rays.Tom Tracy Photography / Alamy

A team of physicists has built a small, powerful X-ray source — a prototype of the sort of machine they hope could replace much larger facilities.

The technology has the potential to revolutionize everything from microbiology to materials science by giving scientists easier access to high-quality images of the things they are studying.

Researchers use X-rays to probe all manner of things — from comet dust to fossilized animals trapped in amber. But making high-quality images requires much brighter and better controlled sources than those available in most institutions. So at the moment, most scientists use large particle accelerators known as synchrotrons, which work by accelerating electrons around a ring. As the electrons bend along the circular path, they naturally emit high-quality X-ray radiation.

Synchrotrons are large, costly and usually in high-demand by scientists, so Matthias Fuchs of the Max-Planck-Institute for Quantum Optics in Garching, Germany, and his colleagues have been working on another way to generate electrons.

Rather than using conventional magnets to guide and accelerate electrons, the team used a powerful laser beam and a small cell of hydrogen gas. They shot a brief, 37-femtosecond (10-15 seconds) pulse into the cell to blow the electrons off the hydrogen atom's nuclei. But electrical attraction causes the electrons to snap back towards the positive ions, so for a brief period after the pulse the electrons vibrate back and forth around the hydrogen atom's positive core, producing a wave. As they do so, a few electrons break loose and ride the crest of the electron wave. "Just like a surfer, the electrons can surf down these waves," Fuchs says.

The electrons then sail through a series of magnetic lenses, which feed them into a second series of magnets that cause them to wiggle back and forth — releasing low-energy 18-nanometre wavelength X-rays as they go.

Short pulse

Because the electric fields between the hydrogen ions and their electrons are so large, the electrons pick up speed much more rapidly than they would in a conventional accelerator. That means a machine the size of a building can be shrunk to the size of a tabletop. Well, almost. Fuchs says that including the laser, the accelerator takes up two fairly large tables. "We came up with the phrase 'banquet tabletop'," he says. The team's research has been published by Nature Physics1.

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Nevertheless, "it is exciting", says Tom Katsouleas, dean of engineering at Duke University in Durham, North Carolina. Other groups had already shown lower-power radiation from similar systems, so it wasn't surprising, he adds. "I don't think anybody really doubted it could be done." "But they've actually shown that the beam quality can be fairly high," he says.

Because relatively few electrons are accelerated, the pulses are bright but short, so the 'tabletop' accelerator is unlikely to replace conventional synchrotrons any time soon. Still, Katsouleas says, there is no reason, in principle, why they could not be made into a workable X-ray source for use in universities. 

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