When the first x-ray free-electron laser (XFEL) opened in 2009 at SLAC National Accelerator Laboratory in California, it provided a new way to look at the atomic-scale world, revealing details about biochemical processes such as photosynthesis and exotic materials such as superconductors. But since then, only four other such billion-dollar facilities have been built worldwide, and getting time on them is difficult.
A group of researchers at Arizona State University (ASU), Tempe, now plans to build a new kind of free-electron laser, dramatically smaller and cheaper than anything that has come before. This month, ASU announced it would embark on the $170 million Compact X-ray Free Electron Laser (CXFEL) project after it received a $91 million grant from the National Science Foundation. The design could put the machines within reach of university laboratories and expand their accessibility.
“It’s an elegant idea,” says Claudio Pellegrini, a physicist at SLAC who first proposed its XFEL in 1992. “Everybody would like to make a smaller system.”
XFELs are excellent probes of the atomic world because short-wavelength x-rays can resolve details that would be invisible to longer wavelength light. Moreover, the short, femtosecond x-ray pulses work like a high-speed camera, helping researchers capture ultrafast processes such as the movement of electrons and atoms.
To reach such supreme spatial and temporal resolution, a standard XFEL requires a kilometer-long linear accelerator. It boosts electrons up to energies of 10 gigaelectronvolts (GeV), or 99.9999995% the speed of light. Then, the electrons pass through “undulators”—a series of magnets arranged in alternating polarity. The electrons emit x-rays as they wiggle through the magnetic fields. Interactions between the light and the electrons cause the electrons to bunch up and radiate in concert like a laser.
The ASU team plans to replace the bulky magnetic undulators with a laser shone directly into the oncoming train of electrons. The laser, like all electromagnetic emissions, has a magnetic field associated with it, says Bill Graves, an ASU physicist and CXFEL’s chief scientist. “When the electrons encounter the laser, they will wiggle just like they do in an undulator.” But where the polarity of undulator fields alternates over a few centimeters, the laser’s field seesaws along with the wavelength of the light—just 1 micrometer.
This ultrahigh-frequency undulator means electrons can be made to wiggle and emit x-rays at much lower energies. They only need to be accelerated to a mere 30 megaelectronvolts, a much easier feat than the 10 GeV needed in a standard XFEL. This vastly reduces the footprint of the XFEL, bringing it down from 1 kilometer to just 10 meters.
With a lower energy electron beam, the team can use crystal diffractors and magnets to finely pattern the electrons into tightly packed bunches. The bunched electrons wiggle more synchronously with one another and as a result, produce more coherent x-ray light. The bunching also results in a shorter pulse of less than a femtosecond.
Such short pulses could potentially reveal the way chlorophyll molecules capture sunlight during photosynthesis, says Petra Fromme, an ASU biochemist and CXFEL team member. “We can look at things that nobody has seen before.”
Sam Teitelbaum, an ASU physicist, plans to use CXFEL as a sensitive probe of electron behavior in materials, which can produce a host of unexplained phenomena, from high-temperature superconductivity to exotic magnetic states. Lessons learned could inspire new superconducting materials or more reliable data storage devices.
Although the new device will have fast, coherent pulses, it won’t pack nearly the same punch as a standard XFEL. Its pulses are much less bright and the individual x-ray photons have longer wavelengths than those from its larger predecessors. This means the CXFEL will miss some of the tiniest details that larger XFELs can see. On the other hand, the lower energy pulses will cause less damage to samples that are typically obliterated by the larger facilities.
“The big machines—they’re like a hammer,” Graves says. “We’re more like a scalpel.”
Pellegrini remains cautious in the face of such an ambitious project. In particular, he says, the team’s plan to shape the electron pulses has not yet been demonstrated. “Before selling it as an XFEL there is a lot of work to do.”
Still, the researchers behind the project are optimistic. They’ve already begun to build CXFEL and expect to be operating it in 5 years. “Anytime that you can see things moving faster, you’re going to get a sense of how dynamic the world is at that time scale,” Teitelbaum says. “There is definitely going to be some problem that’s going to be totally broken open by this fact.”
Source: Science Mag