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.
Meanwhile, across the bay
That's not to say that FACET is the only game in town. Lawrence Berkeley National Laboratory, just an hour north of SLAC, is working on complementary research that focuses on laser wakefield acceleration using the Berkeley Lab Laser Accelerator, or BELLA.
Like FACET, BELLA will produce high-quality, multi-billion-electronvolt electron beams via plasma acceleration. But BELLA will use extremely intense laser pulses—instead of electron bunches—to stir up both plasma and plasma waves.
FACET's and BELLA's complementary approaches will probe different aspects of the physics of plasma wave acceleration and together will provide the insight needed to overcome the technical hurdles that still stand in the way of smaller, faster, cheaper accelerators.
"One thing is certain: Research done at FACET—and across the bay at BELLA—will benefit accelerator science and will one day lead to powerful accelerators that benefit just about every field imaginable," says Eric Colby, head of SLAC's Accelerator Research Division.
An impact beyond R&D
Searching for better ways to accelerate particles is a critical task because accelerated particles have a surprisingly wide variety of uses. The Department of Energy report Accelerators for America's Future lays it out:
"A beam of the right particles with the right energy at the right intensity can shrink a tumor, produce cleaner energy, spot suspicious cargo, make a better radial tire, clean up dirty drinking water, map a protein, study a nuclear explosion, design a new drug, make a heat-resistant automotive cable, diagnose a disease, reduce nuclear waste, detect an art forgery, implant ions in a semiconductor, prospect for oil, date an archaeological find, package a Thanksgiving turkey or discover the secrets of the universe."
According to Colby, it all has to do with economies of scale—only scaling down, instead of up. With today's technology, the next generation of colliders will have to be much bigger to achieve higher energies. "With proposals for an International Linear Collider measuring up to 31 kilometers in length, there is no doubt it will be much larger than SLAC's linac," Colby says. "The next-generation light sources—the X-ray free electron lasers like SLAC's Linac Coherent Light Source, which rely on linacs about 1 kilometer in length—are not 'compact' scientific instruments either."
This, Colby says, has driven research and development efforts to look for ways to make linacs much more compact and lower-cost. The work not only benefits the large-scale research accelerators but will also have an impact on the more than 1000 particle accelerators manufactured in the U.S. every year for use as cancer treatment machines, cargo scanners, sterilization equipment and more. Reducing the cost and the size of these systems will provide the dual benefit of lowering the cost of using accelerators and widening their application.
Improving today's science
As a working linear accelerator already capable of delivering extremely short, high-energy electron bunches, FACET can contribute to materials science studies today. For example, at the FFTB researchers found that speeding bunches of electrons interact with magnetic materials in intriguing ways, causing them to flip their polarity without need of a magnet. Why? How? And can this phenomenon be used to improve data storage? The FFTB team is now at FACET, searching for the answers to those questions. If they succeed, the result could be computer memory that's faster and literally cooler, generating far less heat than the electromagnets in use now.
As the brightest source of terahertz radiation in the world, FACET will also open up a promising research area. Terahertz radiation is of growing interest to researchers because it's non-ionizing, and thus less damaging to biological samples than X-rays, and the terahertz wavelengths are just the right size to probe many molecular bonds.
FACET researchers are using this first run to learn how to tune the accelerator to provide the strongest, most focused terahertz energy fields possible, with plans in place to direct the radiation to experimental hutches next year.
Looking to the future
In its first few months of operation, members of the FACET team spent long hours in the tight confines of the accelerator tunnel, 25 feet below ground, putting in place most of the tools needed to make FACET both a groundbreaking accelerator research facility and a valuable terahertz light source.
And plans are already under way to improve the facility for next year's run. Researchers will soon have the option of creating plasma for their wakefield experiments with a laser rather than with electrons as is done now. "A laser makes the program more flexible, more accommodating to different experiments," Hogan says. "We can experiment more with the beam itself, making shaped bunches or long bunches."
Next year will also usher in the start of FACET experiments using positrons instead of electrons—a necessity for future colliders such as the International Linear Collider, but a big technical challenge, Hogan says. "The plasma response to positron beams is completely different in ways that make the plasma acceleration harder to control," he explains. "Positrons are the Wild West, the frontier."
Eyes on the prize
But for right now everyone involved is just keeping their eyes on the prize, as Clarke puts it: smaller, cheaper, more efficient accelerators for medicine, for energy technologies, for pharmaceuticals, for basic physics research. "If we can't do it cheaper, we can't do it at all," she says.
Hogan agrees but also shows a bit of the ability to see the bright side that's kept him working long hours since the project was conceived. "It's clear that in today's fiscal environment it's more important than ever that we find techniques to drastically shrink the size and cost of future accelerators," he says, "and the teams working at FACET are excited that we're involved in that challenge."
