FACET's accelerator revolution
A new test bed for accelerator technology has thrown open its doors, with the goals of making particle accelerators smaller, cheaper and more efficient—and of expanding their role in society.
The Department of Energy's newest user facility—a cutting-edge particle accelerator available to scientists from all over the world—is a radical new chapter in the history of the world's longest, most powerful linear accelerator.
For more than four decades, the two-mile-long linac at SLAC National Accelerator Laboratory fueled Nobel-winning particle-physics research. Now it's been repurposed—one might even say reimagined—in ways that keep it at the forefront of discovery, and not just in particle physics.
The final third of the linac now powers the Linac Coherent Light Source, the world's first hard X-ray laser. Researchers from around the world use LCLS's unique ability to take crisp pictures of atomic motion and changes in chemical bonds to drive applications in energy and environmental sciences, bioscience and materials engineering.
And the remaining two-thirds of the accelerator have been claimed by FACET, the Facility for Advanced Accelerator Experimental Tests, which is revving up the 50-year-old equipment and packing the final section with state-of-the-art instruments. The result is a test bed for technology that promises to improve the power and efficiency of today's particle accelerators and expand their roles in medicine, materials and biological science, high-energy physics and more.
FACET is also a valuable research tool in its own right, supporting materials research and providing some of the strongest, brightest terahertz energy of any accelerator in the world. Terahertz energy comes in wavelengths shorter than microwaves and longer than the far infrared that are extremely useful for chemical and biological imaging.
As a user facility, FACET accepts proposals from scientists all around the world and invites teams with the most important and feasible research to conduct experiments. The first users arrived in April 2012 and are already seeing results.
"FACET is at the forefront of accelerator research; it's a very exciting place to be," says SLAC accelerator physicist Christine Clarke, who also serves as user coordinator at FACET. It's her job to make sure all the research teams who come to use the facility have what they need to get the data they want.
Surf’s up at FACET
Much of the data researchers seek is related to a particle acceleration technique called plasma wakefield acceleration.
To give a bunch of electrons a big boost in a short distance, plasma wakefield acceleration first chops a single bunch of pre-accelerated electrons in two. The first bunch is sent into plasma—a hot gas of charged particles—which creates an intense, short-lived electric field in the form of a wave, or wake, for the second bunch of electrons to surf.
During 2006 experiments at FACET's precursor, the Final Focus Test Beam, or FFTB, researchers used this method to more than double the electrons' energy in less than a meter. That's about a 3000 times shorter distance than SLAC's linac takes to achieve the same result, a revolution in particle acceleration—that is, if researchers can learn to harness the technique.
"This 3000-times-better acceleration only applies to some of the electrons injected into the plasma—not all of them," says Mark Hogan, head of SLAC's Advanced Accelerator Research Department and a member of the SLAC, University of California-Los Angeles and University of Southern California team that undertook the proof-of-concept work at FFTB. "Our job at FACET is to first replicate the FFTB experiment and then develop it from proof of concept into a reliable technology."
A second potentially game-changing acceleration method under development at FACET makes use of a special type of insulator called a dielectric. Dielectric materials, like sapphire, become polarized in an electric field. Researchers create that electric field by sending a first bunch of electrons through a hollow fiber made of a dielectric. Then the field accelerates a second bunch of electrons with extreme efficiency, much as wakefields do in plasma.
The promise of dielectrics lies in the fact that they generate wakefields without the additional energy needed to create a plasma. With FACET, "We can look at the wakefield coming out of the dielectric and see whether it's working as designed," Hogan says. Once they get that information, they will "push it to failure," to determine how great an induced wakefield the structures can stand before they succumb to dielectric breakdown and lose their capacity to be polarized. For that, researchers need FACET, which is the only facility that can provide a powerful-enough beam for these studies.
"It's really exciting to be on the brink of new science that can't be done anywhere else," says Hogan.