What do you do with the longest building in the world once it's fulfilled the purpose you've built it for? In the case of the SLAC National Accelerator Laboratory, you find other ways of getting useful work out of it and the hardware it contains.

The main accelerator at SLAC was built to characterize the W and Z bosons, discovered earlier at CERN. By colliding electrons and their antiparticles (positrons) at a carefully chosen energy, SLAC produced these bosons in large numbers, allowing their detailed characterization. (The same approach is being planned for studying the Higgs Boson.)

SLAC has since been reinventing the giant accelerator. And the center itself has also diversified away from its roots as an accelerator facility, as we got to see in a recent visit.

Accelerating, but not colliding

The giant hall, which is 1.9 miles (3km) long, houses what are called klystrons. This equipment ingests energy in the form of electricity and imparts it to the electrons, which are accelerated through a tunnel that runs below the building. Originally, electrons and positrons were sent to collide inside a giant detector. That device has since been disassembled, although the giant room where it was located and serviced is still there (and being used for storage). Once the W and Z particles were sufficiently studied, SLAC's accelerator was used to collide electrons in order to make particles called B mesons. These were studied in a detector called BABAR in the hope that it would shed some light on the reason the Universe is filled with matter.

BABAR wasn't part of our tour—unfortunate, since it's one of the coolest looking detectors around. That use came to an end as well, in part because the LHCb detector would study the same issue more efficiently.

But SLAC has found a new use for its accelerated electrons, turning them into one of the highest energy lasers around. When charged particles are forced to take curved paths, it changes their momentum. To balance the energetic books, the particles emit energy in the form of photons. Particle accelerators that follow a curved path, called synchrotrons, have been converted into sources of X-ray light based on this principle.

SLAC's Linac Coherent Light Source takes a different approach. It sends the electrons through a series of what are called "undulators," which push them back and forth. The bundles are spaced so that each undulation is additive—more photons get added to the pulse with each pass through an undulator. And SLAC has 32 of them, spread over roughly 100 meters. By the time it comes out, the pulse of X-rays is about a billion times brighter than what a synchrotron could provide. And the entire pulse lasts roughly 100 femtoseconds. There's nothing else in the world that's anywhere near that powerful. And the Linac Coherent Light Source can provide 100 pulses a second.

What do you do with all that power? 100 femtoseconds is on the order of the time involved in the motion of atoms within a molecule. X-rays scatter off those atoms, creating a pattern that can be used to identify the position of those atoms. The combination means you can essentially take snapshots of a molecule in action—this is especially useful for proteins. In fact, the laser is so intense, it's possible to take snapshots of single molecules; these are destroyed in the process, something the researchers term "diffraction by destruction." Lots of individual snapshots of single molecules can then be built into a coherent picture of the molecule.

John Timmer



John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer



John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

John Timmer

To get the most out of the laser, there are multiple stations in two experiment halls (the beam is also split in two partway through the facility). In many of these, other lasers, housed on the floor above, are sent down into the experiment. These can place molecules in a specific energy state prior to their imaging by the X-ray laser.

Beyond imaging

There are now a lot of biologists that spend time at SLAC, a significant change from its particle physics days. SLAC has also expanded its focus in other ways. Some of these are obvious; it's testing new ways of accelerating particles as part of ongoing research efforts. (While positrons aren't used in generating the X-ray laser, they're still produced at SLAC for these tests.) The site's computers are also used as part of the analysis of data from the ATLAS detector at the LHC.

But SLAC scientists are also looking at energetic events well beyond the range of particle accelerators. They built the Large Area Telescope of the Fermi Gamma Ray Space Telescope. This instrument scans they sky to identify unusual events. Typically, gamma rays are associated with events that accelerate particles to extreme energies, such as supernovae and feeding black holes. The BICEP2 detector, which made news for the apparent discovery and later de-discovery of primordial gravitational waves, was also built here.

That's not the only astronomy work going on. SLAC is involved in the construction of the Large Synoptic Survey Telescope, which is being built above the Atacama desert in Chile. This is an 8.4 meter telescope with a 3 billion pixel camera that will survey the entire southern sky once a week for 10 years, generating 30TB of data each night. This will pick up everything that changes over time, from near-Earth asteroids to distant supernovae. The hope is that with a large enough catalog of supernovae, we'll be able to pin down some of the details of the Universe's accelerating expansion, providing a clearer picture of the nature of dark energy.

In keeping with the interest in astronomy and astrophysics, several of the SLAC faculty are part of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University. (One of them, Tom Abel, showed us some of the 3D simulations of the Universe's evolution he's developed; we're trying to get copies of those movies to share.)

Ars' visit to the facility was short. But we were able to see that there's been a lot going on at SLAC, even after the particle collisions stopped.