Inside world’s ultimate X-ray machine before it becomes more powerful

The klystron gallery, a concrete corridor lined with evenly spaced metal cylinders, is long enough to extend beyond my field of vision. But standing inside it, I know that beneath my feet lies something even more impressive.

Beneath the Klystron Gallery is a giant 3.2-kilometer-long metal tube: the Linac Coherent Light Source II (LCLS-II). This machine, located at the SLAC National Accelerator Laboratory in California, produces X-ray pulses more powerful than those produced by any other facility in the world, and I'm visiting it because it recently broke one of its own records. However, its most powerful components will soon be taken offline for upgrades. Once it is turned on again, perhaps as early as 2027, its X-rays will have more than double the energy.

“It will be like going from a flicker to a light bulb,” says James Cryan in SLAK.

Describing LCLS-II as a simple flicker is a gross understatement. In 2024, it produced the most powerful X-ray pulse ever recorded. It lasted only 440 billionths of a billionth of a second, but transmitted almost a terawatt of power, far exceeding the average annual output of a nuclear power plant. Moreover, in 2025, LCLS-II generated 93,000 X-ray pulses in one second, a record for an X-ray laser.

Cryan says this latest recording offers researchers unprecedented insight into the behavior of particles inside molecules after they absorb energy. It's like turning a black-and-white film of their behavior into a sharper, more color-saturated one. With this achievement and upcoming upgrades, LCLS-II has the chance to radically improve our understanding of the subatomic behavior of light-sensitive systems, whether they are photosynthetic plantsor candidates for the best solar cells.

LCLS-II achieves all this by accelerating electrons until they reach the speed of light—the ultimate cosmic speed limit. The cylindrical devices I saw, namely the klystrons that give the Klystron Gallery its name, are responsible for producing the microwaves that achieve this acceleration. Once they reach sufficient speed, the electrons pass through rows of thousands of magnets, whose poles are carefully positioned so that the accelerating electrons wobble. This in turn produces X-ray pulses. Similar to medical X-rays, these pulses can then be used to produce images of the interior of materials.

On the day of my visit, I visit one of several experimental rooms where X-rays complete their journey by colliding with molecules. I look at some cameras where the molecule and the x-ray meet. They resemble something out of a futuristic submarine: thick metal cylinders with round glass windows that are carefully bolted together to keep out stray air molecules that might interfere with the experiment.

The day before my visit, Cryan and his colleagues conducted an experiment examining the movement of protons within molecules. Imaging techniques other than X-rays have difficulty determining exactly how protons move, but the precise details of the process are important for solar cell development, he says.

What will happen to such research when LCLS-II completes its “high energy” upgrade and becomes LCLS-II-HE? The ability to study the behavior of particles and charges inside molecules will increase significantly, Cryan says. However, getting there will not be an easy task.

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John Shmerge SLAC says the more energetic the electron beam becomes, the more the team has to worry about even a few particles going astray. He says he once saw an imperfectly controlled beam burn a hole through an instrument at another site, so there's little room for error. SLAC Yuantao Ding says all the new parts the team will install during the upgrade are designed to handle the new, higher power of the facility, but it's critical to gradually increase the power and make sure everything works as designed. “We'll turn on the beam and watch closely what happens,” he says.

He and his colleagues will spend much of 2026 putting in a major engineering effort to get all the pieces in place, which will then prepare them for this incremental process over the next year or two. If all goes according to plan, researchers around the world will be able to use LCLS-II-HE by 2030. Conversations between researchers who use X-rays, like Cryan, and those who operate them, like Shmerge and Ding, will also play a big role. “Ultimately, this is a great tool, and people will learn how to use it correctly,” Schmerge says. “We will continually adjust it.”