This X-ray laser is getting faster and brighter

“Watch your head; watch your feet,” Eric Fauve says as he enters a cryoplant sitting in a landscape of dry grass, eucalyptus trees, and oaks in Northern California. “We have rattlesnakes, deer, mountain lions—we’re pretty remote out here.”

Fauve, who directs the cryoplant at SLAC National Accelerator Laboratory, points to a series of heat exchangers and compressors that cool helium to a liquid at 4 K. That liquid helium travels through a series of pumps that gradually drop the pressure, expanding the liquid and cooling it to 2 K. Its destination is 30 m underground, where the helium will help cool parts of a 3 km long apparatus that passes under Interstate 280, a nearby freeway connecting San Francisco and San Jose.

This facility is SLAC’s Linac Coherent Light Source (LCLS). That supercooled helium helps a new part of the light source called the LCLS-II fire a record-breaking 1 million pulses of extremely bright X-rays per second. With that kind of firepower available on a regular basis early next year, chemists and materials scientists can effectively slow down time and zoom in on the ultrafast, ultrasmall, and extremely weird world of atoms and electrons.

“In a femtosecond flash, you can see atoms stand still, single atomic bonds breaking,” says Leora Dresselhaus-Marais, a materials scientist with joint appointments at Stanford University and SLAC.

This type of superbright, superfast light source, called an X-ray free-electron laser (XFEL), has been around for a decade, but it is becoming more and more powerful, says Sakura Pascarelli. She is the scientific director of another such source, located outside Hamburg, Germany: the European XFEL. Pascarelli is excited about SLAC’s current upgrades and expects the LCLS and other such light sources to become even more powerful in the coming years.

With these machines’ improved ability to observe the behavior of atoms and electrons, Pascarelli expects the lasers to finally illuminate the mechanisms of photosynthesis, watch the movement of electrons in real time as chemical bonds form and break, and help scientists create new classes of materials that will only form under extreme conditions in which, she says, “chemistry as we know it doesn’t happen anymore.”

The power of X-rays

X-ray free-electron lasers are just one source of X-rays used in spectroscopy and imaging. For decades, X-ray spectroscopy has helped scientists identify and map the atoms within crystals and molecules. The technique’s strength comes from the unique properties of this part of the electromagnetic spectrum, says Linda Young, an atomic physicist at the Argonne National Laboratory and the University of Chicago. X-ray wavelengths are extremely short, which means that bombarding a sample with these photons can spot individual atoms. Elements emit light at characteristic X-ray wavelengths, allowing chemists to identify the atoms they spot. High-energy, or “hard,” X-rays can also penetrate hard materials and even extremely dense, high-energy plasmas that other wavelengths of light cannot.

Until SLAC started up the LCLS in 2009, the brightest X-ray sources were synchrotrons. These facilities accelerate electrons along circular paths and use magnets to wiggle them, inducing the electrons to emit X-rays. Synchrotrons provide what physicists deem relatively long pulses of light, lasting tens of picoseconds, Young says. This pulse length limits scientists’ ability to unblur fast-moving events, restricting researchers to the capture of static images of slower processes in molecules and materials.

Still, that’s no knock on synchrotrons, which are the workhorses of X-ray science, according to Pascarelli. “Synchrotrons are extremely useful for a large community,” she says. For example, at these facilities, biochemists have snapped images of complex protein structures such as the SARS-CoV-2 spike protein, and materials scientists have examined the nanostructures of battery materials.

But X-ray free-electron lasers are pushing the limits of X-ray imaging. When SLAC switched on the LCLS, it was immediately the brightest X-ray source in the world. Its success was followed by other machines, including the European XFEL and ones constructed in Japan, Switzerland, and Korea. China is also building one.

Like synchrotron sources, these lasers use undulating magnets to wiggle high-energy electrons and induce them to emit light. But in a free-electron laser, the electrons collect in compact bunches that move at nearly the speed of light. As they wiggle, they remain coupled to the photons they emit for relatively long distances. This interaction causes the light to become highly coherent, a property of all laser light. Once they’ve done their job, the electrons are separated from the laser beam with magnets. The result is a beam of very short, very bright pulses of X-rays with tunable wavelengths.

Since these light sources have come on line in the last decade, chemists have used their powerful pulses to make molecular movies of the formation of chemical bonds between carbon and oxygen, determine the structure of small molecules that form crystals too small to map with other methods, watch photosynthetic enzymes go to work, and simulate the formation of diamonds in planetary interiors.

The upgrades

The LCLS currently produces 120 X-ray pulses per second at energies from about 200 eV to about 11 keV. (X-ray beams are typically described in terms of energy rather than wavelength—the higher the energy, the shorter the wavelength.) With the upgrades that go on line early next year, the facility will produce two beams, one with up to 1 million pulses per second and energies reaching about 5 keV, and one at the original pulse rate that can max out around 25 keV. The upgrades will also allow scientists to better tune the energies of X-rays produced.

These upgrades are mainly focused on the equipment that manipulates the electron beam. LCLS gets its electron beam from a huge particle accelerator that began operating in Menlo Park, California, in 1962. In the original LCLS, the electron beam from the accelerator gets fired through a vacuum inside a copper tube at room temperature. Current pumped through the copper generates undulating electromagnetic fields that inject the electrons with more and more energy as they travel. But copper can conduct only so much electricity, so these copper cavities must be turned on and off to prevent damage, limiting the X-ray laser’s rate to 120 pulses per second.

The LCLS-II upgrades have made it possible to produce an additional beam. A section of the copper line has been replaced with superconducting cavities made of 3 mm thick sheets of nitrogen-doped niobium shaped into ribbed tubes that look like doughnuts on a stick. The liquid helium pumped from Fauve’s cryoplant cools these cavities down to 2 K, the temperature at which the material becomes superconducting. As a result, the material can carry much higher amounts of energy without getting damaged, and the laser can stay on constantly.

Because the superconducting electronics can operate continuously, they can pump a tremendous amount of energy into the speeding electrons, which enables LCLS-II’s faster pulse rate. The original copper tube line will remain in operation, so the system will provide two X-ray beams at once; that will allow scientists to gather more data in a shorter time and to perform new types of experiments.

These enhancements come on top of ones completed in 2020 that gave users the ability to tune the X-ray beam’s energies for experiments and added two new sets of movable, undulating magnets—one each for hard and soft X-rays.

Some scientists have already benefited from the undulator upgrades. SLAC quantum physicist Taran Driver used the ability to tune the X-ray beam energy to zero in on individual atoms of interest in a sample. For example, he says, “I can tune the beam to a photon energy where the probability of interacting with N or with O is greater.” Earlier this year, Driver used the upgraded light source to follow the motions of electrons in nitrogen oxide after exciting the molecule with the X-ray beam (Science 2022, DOI: 10.1126/science.abj2096). At these scales, he says, researchers can directly observe “spooky quantum effects” in electrons. He and other scientists are interested in how quantum mechanics influence charge transfer in light-driven processes—and they want to know more about how quantum effects might explain, for instance, why photosynthesis is so efficient.

Driver is excited about next combining the beam’s tunability with LCLS-II’s high pulse rate. His group has designed a spectrometer that can simultaneously detect the electron and the ion generated when a molecule is bombarded by X-rays. At the slower pulse rates of the original LCLS, it wasn’t statistically possible to determine whether both particles came from the same molecule. At the higher rates, Driver says, the researchers will be able to make this determination, which will allow them to study single molecular events in ways not possible before.

The faster pulse rates will also be a boon for researchers studying the behavior of materials under extreme conditions. What chemists call standard temperature and pressure “is not what the universe would consider standard,” says SLAC physical chemist Benjamin Ofori-Okai. With LCLS-II, he’ll study the properties of materials under the extreme conditions found at the centers of planets.

At one of the experimental stations at the LCLS, a powerful optical laser can be used to push materials to pressures and temperatures that mimic planetary cores, create never-before-seen allotropes of carbon, and generate extreme states of matter that otherwise don’t exist on Earth’s surface. Materials scientists can then use the new higher-energy, much faster X-ray beams to penetrate these extreme states of matter and image them at high speeds. Dresselhaus-Marais and Ofori-Okai say this kind of research will help in the development of materials for nuclear fusion reactors and lead to basic insights into geology and astronomy.

But the new beam will also aid the study of less extreme systems. SLAC scientist Kristjan Kunnus looks forward to having easier ways to study molecules in solution. Kunnus works with the ChemRIXS instrument. These experiments must be done in a vacuum—easy enough with a solid sample, but tricky with a liquid one such as a molecule in solution. The ChemRIXS cycles a liquid sample through a micrometers-thick channel inside a vacuum chamber so that the solution can be illuminated with a soft X-ray beam. To get enough data on the molecules using the current rate of X-ray pulses, chemists have to provide highly concentrated solutions.

“You have to put more molecules in to get more emissions,” Argonne’s Young says. “But that might not be the system you want to study.” Many interesting reactions happen in relatively dilute systems. According to Kunnus, the faster pulse rates of LCLS-II will make it possible to study these more realistic systems.

Despite all the exciting opportunities, LCLS-II’s high-energy X-rays and fast pulses will also present some unique experimental hurdles for scientists to overcome. For example, in some experiments, researchers will use supersensitive sensors to detect single electrons. When LCLS-II’s rapid X-ray pulses cause samples to emit millions of electrons per second, “all these super-, supersensitive detectors are going to be destroyed,” Driver says. The faster series of X-ray blasts would not only physically damage existing detectors but overload their circuits with data that the electronics can’t unload quickly enough.

Driver says his collaborators have been developing event-driven cameras with pixels that turn on only when they’re hit by an electron from the sample they’re imaging. And they’re designing circuits that will toss out uninteresting data on the spot, before they’re passed on.

Once they get up to speed with how to use these new X-ray sources, scientists are confident they will provide unique insights into critical scientific questions, particularly in the field of renewable energy. Unlocking the secrets of photosynthesis is one of the big selling points for X-ray free-electron lasers. And that knowledge could lead to the development of synthetic systems that harvest energy from sunlight. Photosynthesis is “very complicated, and we still don’t know how to reproduce it efficiently,” Pascarelli says. “But if we could, we can use the sun to solve a lot of our problems.”

For scientists to study photosynthesis more thoroughly, these X-ray lasers must combine speedy pulse rates with beam energies that move further into the hard X-ray spectrum. Beams with higher energies will allow scientists to pinpoint the movements of particular electrons on heavier elements, including iron. That’s not possible today. But Pascarelli says more planned upgrades at SLAC and other facilities over the next decade will push the wavelength of high-pulse-rate sources further into the hard X-ray range, where metals really sing.

Pascarelli worked in the synchrotron world for 30 years before joining the European XFEL. Knowing what these X-ray lasers can do, she expects that these powerful light sources will become as routine as synchrotrons are today.