The world’s most powerful x-ray laser, the Linac Coherent Light Source II (LCLS-II) at SLAC National Accelerator Laboratory, officially produced its first x rays on 12 September. A $1.1 billion upgrade more than a decade in the making, the x-ray free-electron laser (XFEL) is now capable of firing 1 million x-ray pulses per second—8000 times as many as the first-generation LCLS, which began operation in 2009. Each pulse is 10 000 times as bright as those of the original LCLS. The new machine reestablishes the US facility as the top performer among the world’s five XFELs, surpassing the six-year-old European XFEL in Schenefeld, Germany.
“This upgrade to the most powerful x-ray laser in existence keeps the United States at the forefront of x-ray science, providing a window into how our world works at the atomic level,” Energy Secretary Jennifer Granholm said in a statement. The LCLS-II “absolutely” becomes the world-leading XFEL, agrees European XFEL managing director Robert Feidenhans’l. Europe’s machine will continue to edge out the LCLS-II on average power until the SLAC machine fully ramps up. Scientists are temporarily limiting the beam rate to nearly 1 kHz to gain experience and confidence in their ability to safely operate the facility.
LCLS-II experiments will begin in November, says LCLS director Mike Dunne. The facility is already oversubscribed by a factor of four to five.
The LCLS-II will be capable of “entirely new classes of experiments that weren’t possible with the first generation of XFELs,” says Robert Schoenlein, LCLS deputy director for science. Its few-femtoseconds-long pulses will greatly improve the fidelity of the “molecular movies” that the LCLS had been producing to show subatomic interactions occurring during chemical reactions. “If you want to understand the light-harvesting processes or charge-transfer processes that drive the chemistry for the storage and transformation of energy, we need new insights into the valence electrons,” he says.
Quynh Le Nguyen, an LCLS associate scientist, says that the LCLS-II’s vastly increased x-ray intensity will yield more data and a stronger signal for researchers who are trying to engineer materials with new properties. Improved characterization could be coupled with artificial intelligence to design new materials for a range of applications, including electronics, energy storage, and aerospace. The laser x rays could also be used to further modify properties of the newly designed materials, she says.
The LCLS-II can better illuminate the structures of biological macromolecules such as proteins and can observe biological processes such as protein folding and photosynthesis, Schoenlein says. “You can take a billion snapshots per day and map out the entire ensemble of biological structures while in their near-physiological environments.”
The LCLS-II can vary its x-ray energy from 250 eV to 5000 eV. That tuning allows scientists to capture the behavior of various elements, each of which has characteristic energy levels. When studying a complex molecule, experimenters will be able to observe what’s happening to specific elements—an advantage that x-ray sources have over ultrafast optical lasers, where such information is jumbled together. And the LCLS-II’s new instrumentation will enable the measurement of electron and atomic information at the same time, Nguyen says. “We can look at more complicated systems than we could before.”
The upgraded XFEL features a superconducting accelerator composed of 37 cryogenic modules built by Fermilab and the Thomas Jefferson National Accelerator Facility. New undulators were developed by Lawrence Berkeley and Argonne National Laboratories. The project also included the construction of two new cryoplants to cool the required 4 tons of helium to 4 K (see Physics Today, September 2023, page 18).
The new accelerator will work in parallel with the LCLS’s existing nonsuperconducting copper version, which despite its lower pulse rate and intensity produces photon beam energies of up to 25 keV. Experimenters who work with hard x rays (5–20 keV) will continue to use the LCLS until another upgrade, expected to be completed in 2027 or 2028, that will extend the LCLS-II’s capabilities to the hard x-ray regime.
“Hard x rays give you access to the atomic structure, at the angstrom scale,” Schoenlein says. “The science requires both, because the electrons don’t act independently from the atomic structure.”
The combination of both LCLS generations will allow researchers to capture detailed higher-resolution snapshots of rapid processes, probe delicate samples that are beyond the reach of other light sources, and gather more data in less time. The LCLS-II also will greatly increase the number of experiments that can be accommodated at the SLAC facility.
Because it outputs waves continuously, the LCLS complements the European XFEL, which produces x rays in bursts. The German facility’s strength, Feidenhans’l says, is its high pulse power and high photon energy, which are well suited to crystallography and materials science experiments that collect sufficient data from a single pulse.
