The subatomic dragstrip
Posted: Tuesday, October 28, 2008 5:40 PM by Alan Boyle
SLAC
A technician works inside SLAC's 2-mile-long linear accelerator tunnel.
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Most atom smashers are built like racetracks, with powerful magnets bending subatomic particles into circular routes. The SLAC National Accelerator Laboratory, built in the heart of California's Silicon Valley, is something completely different: It's basically a 2-mile-long dragstrip that whips up electrons to shed light on the structure of matter.
SLAC's straight-shot structure hints at the shape of atom smashers to come - such as the future International Linear Collider. And it makes for one heck of a jogging trail.
"There's actually a race where they go down to the accelerator and back - it's four miles," said SLAC graduate student Chris McGuinness, who is an avid mountain climber as well as a researcher working on the next generation of laser-powered particle accelerators.
Next month's 37th annual SLAC Run and Walk will take place outside the accelerator's housing. But at the same time, 25 feet beneath the surface, electrons and positrons will be running their own races down SLAC's straight track.
At the end of the track, those pumped-up particles can be curled around and smashed together, or they can be captured in a magnetic ring to generate brilliant flashes of X-ray light. The smashing part has supported Nobel-winning discoveries, including this year's physics prize, but the flashing part is pointing the way to SLAC's next frontier.
SLAC
The housing for the SLAC National Accelerator Laboratory's 2-mile-long tunnel
shows up as a straight line in this aerial photo of the Silicon Valley site.
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Transition time
SLAC is definitely in a time of transition, in part evidenced by this month's official name change: For the past 46 years, it's been known as the Stanford Linear Accelerator Center. Today, Stanford University still manages the lab on behalf of the U.S. Department of Energy, but the acronym has been incorporated into a bigger mouthful of a name to recognize SLAC's growing role in photon science and astrophysics.
In particle physics, SLAC's signal accomplishments have focused on figuring out why matter is structured the way it is. "We did the first experiments that showed quarks existed," said Stanford graduate researcher Daniel Ratner.
More recently, the lab's BaBar collaboration has shed light on why matter won out over antimatter, and found the lowest-energy example of weird subatomic stuff known as "bottomonium."
But BaBar's days are numbered: As the Large Hadron Collider takes center stage in particle physics, the detector system at SLAC that yielded such fundamental research (known as the B Factory) is being closed down. That's been one of the reasons why SLAC has laid off 225 of its 1,600 employees over the past year. Budget cuts forced by Congress were another factor.
The good news is that new projects are gaining steam: Scientists at SLAC play a key role in managing the recently launched $690 million Fermi Gamma-ray Space Telescope, and the $400 million Linac Coherent Light Source is due to come online next year.
SLAC
Researcher Dennis Nordlund peers at equipment used for soft X-ray imaging
on a beamline at SLAC's Stanford Synchrotron Radiation Lightsource.
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Inside the accelerator
SLAC's researchers provided a preview of what's ahead on Monday during a tour organized for this week's New Horizons in Science conference, which is presented annually by the Council for the Advancement of Science Writing.
The linear accelerator itself is just the start of the journey, at least as far as electrons are concerned. Although we couldn't walk through the underground accelerator hall itself, McGuinness ushered us through the above-ground Klystron Gallery.
Klystrons are essential elements that generate electromagnetic waves, like glorified microwave ovens. The energy from the barrel-sized klystrons at SLAC is funneled down waveguides to the accelerator, where they push the electrons faster down the track like ocean waves pushing a surfer toward shore.
At the end of the run, magnets divert the drag-racing electrons into the places where they're put to use - for example, the Stanford Synchrotron Radiation Lightsource, or SSRL. The heart of the facility is a warehouse-sized synchrotron ring, somewhat similar to the 17-mile-round ring used for pulsing protons at the Large Hadron Collider.
At SLAC, however, the electrons' energy isn't released by collisions. Instead, they are kept in magnetic captivity and shed their excess energy in the form of X-rays. "The only thing we're using is the energy they shed," researcher John Pople explained.
Flashes of X-rays are focused onto a wide variety of materials, and the patterns made by diffracted X-ray light can show the molecular or even the atomic structure of the material being studied: "Soft" X-rays can probe electronic properties on a scale of less than a micron (a millionth of a meter), while shorter-wavelength, "hard" X-rays can illuminate structures on a scale of less than a nanometer (a billionth of a meter).
"Philosophically speaking, it's a form of microscope," Pople said.
In one of the SSRL's closetlike hutches, Pople and his assistants are using hard X-rays to map the atomic structure of materials that could someday show up in a better breed of artificial corneas. At another beamline, graduate student Eric Verploegen is using soft X-rays to look at the properties of substances being considered for spray-on circuitry. The idea is to come up with flexible, organic-based electronics that could be worked into, say, combat uniforms.
"It's a way to understand how to design the organic transistor material you want to use," Verploegen told us.
Farther down a hallway, at yet another beamline, Swedish-born researcher Dennis Nordlund is working on a tangle of gleaming pipes and metal foil that could do service as an electron spectrometer. The complex plumbing is supposed to create a nearly perfect vacuum inside the instrument.
"It's the same pressure here as it is in space," he said.
The light frontier
If you think that's impressive, you ain't seen nothing yet. The Linac Coherent Light Source, now in the final stages of construction, will pump up electrons to such high energies that the resulting flashes of X-ray light will pack 10 gigawatts of power.
"When it's focused, we have no known material that will stop it from burning through," Ratner said.
New caverns have been burrowed into the sandstone on SLAC's 430-acre campus to accommodate millions of dollars' worth of beamlines and observing istruments. Right now the caverns are empty, but starting next year, the Linac Coherent Light Source will set a new standard for imaging resolution, down to 1 angstrom, or one ten-billionth of a meter. It will also provide the shortest stop-action flash ever known, lasting as little as a femtosecond - that is, one-quadrillionth of a second.
You can think of it as an atomic-scale microscope for materials, or a flash camera capable of seeing atoms. Either way, it's a new frontier for science and technology, as important as the frontier that will be explored at the Large Hadron Collider.
"Instead of being at the energy frontier, we can look at the light frontier," Ratner said.
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