The United States has regained the global lead in x-ray lasers. The Linac Coherent Light Source II (LCLS-II), a so-called x-ray free electron laser (XFEL) at SLAC National Accelerator Laboratory in California, has produced its first flashes of x-rays, lab officials announced today. Supporting studies of everything from the structures of biomolecules to the conditions in the cores of planets, the upgraded machine will produce data 8000 times faster than the original LCLS and in some ways surpass its main rival, the European XFEL near Hamburg, Germany.
“There isn’t anything you can do at the European XFEL that you can’t do at LCLS-II once it is up and running,” says Saša Bajt, an x-ray optics scientist at the German Electron Synchrotron Laboratory (DESY). In fact, some work will be easier at the new machine, she says. However, both machines serve large communities of users, so the LCLS-II won’t in any way render its rival obsolete. “There is, of course, friendly competition,” Bajt says, “and that is healthy.”
SLAC researchers began planning the $1.1 billion upgrade almost as soon as the original LCLS came on in 2009. The Department of Energy, which owns SLAC, approved construction in 2016. “The last few years have been agonizing and enticing and exciting all in equal measure,” says Mike Dunne, a physicist at SLAC and director of the LCLS. “But we’re finally there.”
In a free electron laser, a linear accelerator, or linac, shoots a beam of high-energy electrons into a string of complex magnets called undulators. The undulators make the electrons wiggle sideways, causing them to emit photons. The photons travel along with the electrons, which are moving at near–light-speed, and herd them into clusters known as microbunches. The microbunches then radiate in concert, greatly amplifying the radiation. The result is a laser burst of x-rays billions of times brighter and far shorter than the pulses from circular particle accelerators called synchrotrons, another major kind of x-ray source.
The first free-electron lasers were built in the 1970s, but only in 2009 did the LCLS push the concept into the regime of “hard” x-rays—those with wavelengths short enough to resolve atomic-scale detail. SLAC scientists used a 1-kilometer length of the lab’s namesake 3-kilometer linac—which for 50 years fed experiments in particle physics—to shoot electrons through a single string of undulators.
XFELS have opened new horizons in x-ray research. Their intense flashes can determine the structure of a crystallized protein in a single shot, avoiding damage caused by repeated x-ray exposures. The flashes can also be used as a strobe light to make “movies” of molecules responding to, say, a blast of ordinary laser light. “You can watch molecular motions on their natural timescales and length scales,” says Linda Young, an atomic physicist at Argonne National Laboratory.
In 2017, the LCLS and other machines in Japan, Switzerland, and South Korea were surpassed by the European XFEL. The earlier machines employed conventional linacs made of copper accelerating cavities. In contrast, the €1.2 billion European facility deployed a linac made of superconducting metal. It must be cooled to near–absolute zero with liquid helium, but it can produce electron pulses far faster. The European XFEL produces 27,000 x-ray flashes per second—compared with the original LCLS’s 120—and feeds multiple experiments simultaneously. “The European XFEL came on and blew everyone out of the water,” Dunne says.
To keep up, SLAC researchers have replaced a 1-kilometer stretch of their old linac with a new superconducting linac. They’ve also replaced the original string of undulators with two new ones and rebuilt the instruments they feed. With those changes and additions, the LCLS-II should eventually produce 1 million x-ray pulses per second, seizing the lead in repetition rate.
But the LCLS-II won’t just be faster than its rival. The European XFEL generates its flashes in bursts, separated by 0.1 seconds, within which flashes are separated by just 0.25 microseconds. That irregular rhythm and tight spacing makes designing timing experiments harder and pushes the limit of detectors. “They had to invent a lot of fancy detectors to actually be able to run at high rep rate,” Young says. In comparison, the LCLS-II will produce a continuous stream of pulses separated by a more manageable 1 microsecond. “That is a little easier to work with and also easier on detectors,” Bajt says.
Still, the European XFEL will continue to produce higher energy, shorter wavelength x-rays than the LCLS-II. In fact, SLAC’s new superconducting linac will run at a lower energy and produce longer wavelength x-rays than the old copper linac. So until a proposed $710 million upgrade to the brand-new linac is completed in 2030, the lab will continue to run both accelerators, Dunne says.
The world of x-ray lasers should soon become even more competitive. China is building the Shanghai High Repetition Rate XFEL and Extreme Light Facility, which will deploy a superconducting linac. Also known as SHINE, the machine aims to match the LCLS-II in repetition rate and rival the European XFEL in energy. It is scheduled to be completed in 2027.
