Europe's Powerful XFEL Laser Reboots After Record-Long Shutdown for Major Upgrades

European XFEL: Hamburg's X-Ray Laser Chills Out for a Comeback

After an extended break, the European XFEL is being prepared to resume scientific operations. We took one last look inside before the deep freeze.

December 17, 2025

The forecast for Hamburg-Bahrenfeld: deep freeze through April—very deep freeze, in fact, down to 2 Kelvin, or minus 271 degrees Celsius. That's the temperature to which the accelerator section of the European XFEL is now being cooled, paving the way for research to restart in the spring.

The facility has been dormant since summer. "We began warming up the system on June 21," says Winfried Decking, head of the accelerator team at the European XFEL. Since then, crews have installed a new electron injector, performed extensive maintenance that usually gets deferred, and added a second cooling system.

Now, the accelerator is being chilled back down to 2 Kelvin. During this phase, no one is allowed in the tunnel for safety reasons—if the cooling system were to fail, the consequences could be severe, Decking explains. The tunnel is off-limits during normal operation anyway due to radiation hazards. This was our last chance to descend multiple stories beneath Hamburg for a glimpse of the world's most powerful X-ray laser.

The European XFEL—short for X-Ray Free-Electron Laser—generates ultra-bright X-ray flashes from electron beams in femtosecond pulses. To probe matter at the molecular and atomic level, the machine requires a facility of monumental scale.

Stretching beneath Hamburg and the neighboring town of Schenefeld, the XFEL is nearly 3.5 kilometers long, with a branching tunnel network totaling almost 6 kilometers. Scientists from disciplines as diverse as physics, astrophysics, chemistry, and materials science work alongside biologists, medical researchers, and pharmacologists. Twelve nations collaborated on the €1.5 billion project, which has been operational since 2017. This marks its first extended maintenance shutdown; typically, the XFEL pauses for just three to four weeks at a time.

The journey begins in Hamburg at the German Electron Synchrotron (DESY) campus, which recently celebrated its 66th anniversary. Here, the injector—nicknamed the "gun"—and the 1.7-kilometer linear accelerator (the section now being cooled) are housed. Inside the gun, a copper disk the size of a coin is struck by a laser, dislodging roughly a billion electrons. These electrons, now a tightly packed bundle, embark on their high-speed voyage toward Schenefeld.

There's a catch, though: electrons carry a negative charge, so they repel one another. "Otherwise, we wouldn't gain much," Decking notes. "The trick is to generate the electrons and then accelerate them to near-light speed almost instantly." This happens within a cavity just a few centimeters wide. The result? A two-centimeter-wide electron beam hurtling forward at 99.99999996% the speed of light.

Next, the beam—initially two centimeters across—is compressed to just 20 micrometers. Quadrupole magnets, acting like focusing lenses, squeeze the beam into shape.

From there, the electrons enter the linear accelerator, a series of yellow, 80-centimeter-long cavity resonators. Inside, niobium structures channel the electron packets. Niobium becomes superconducting at 2 Kelvin, hence the helium cooling system. Over the next 1.7 kilometers, these helium-cooled resonators boost the electrons' energy to 17.5 giga-electronvolts.

"In a warm accelerator, we could handle about 100 electron pulses per second," explains XFEL Director Thomas Feurer. "But in a cold one, we're now achieving around 27,000 pulses per second."