Cold, colder, European XFEL

After six months of maintenance and upgrades to the infrastructure, European XFELs accelerator is now being cooled again. A bath of liquid helium is being used to cool down the cavities in the 1.7-kilometre-long accelerator to -271 °C, just two degrees above absolute zero. At this temperature, the metal niobium is superconducting, enabling efficient energy transfer to the free electrons. In the summer of 2025, the accelerator was “defrosted”, i.e. returned to room temperature, for the first time in nine years of operation in order to allow for work that readies the facility to remain a leader in its field. 

Thomas Feuer, Chair of the European XFEL management board, says that the upgrades and preparatory work are an important step towards achieving the company's Strategy 2030+ goals. "These upgrades enable us to remain at the forefront of free-electron laser research," says Feurer. "The preparatory work on the infrastructure paves the way for future important milestones." One of European XFEL's goals is to deliver even brighter X-ray flashes in faster succession. During the installation and maintenance period, an entirely new electron source called "GUN5" was installed in the injector building on the DESY campus, where the European XFEL originates. It benefits from a refined shape, integrated field probes, enhanced cooling, improved mechanics for swapping cathodes, and a double input window. “The designing of the new electron source started years ago, followed by extensive testing”, says Wim Leemans, director of the accelerator division at DESY. “We are now in the commissioning phase and already seeing excellent performance.” The new electron source will increase the number of electron bunches, and therefore the number of light flashes generated, by 30 per cent to up to 36,000 flashes per second.

Hundreds of individual tasks were completed as planned during the long installation and maintenance period, including the replacement of pumps, filters and control units in the accelerator's cooling system, as well as the renewal of helium valves, which made turning off the accelerator necessary in the first place. “Completing all of this work on time is indicative of considerate planning as well as the continuing excellent partnership between European XFEL and DESY”, says Leemans.

Over the past half year, scientists, engineers and workers carried out maintenance tasks and installed several upgrades to equipment across the nearly 5.8 kilometres of underground tunnels . These tasks included replacing components such as cables and mirrors. The latter had been worn out by constant exposure to intense X-ray flashes. The replacement mirrors received a different coating to withstand even stronger photon pulses. Significant additions to the beamlines were also installed. "We've added a lot of new features," says Thomas Tschentscher, one of the scientific directors at European XFEL. "For instance, we installed additional devices that will enable us to generate attosecond pulses.". An attosecond is a quintillionth of a second (10⁻¹⁸ s). Light pulses this short will enable scientists to observe, for instance, chemical bonds forming in chemical reactions.  Another upgrade Tschentscher mentions is the installation of a photon chicane. “With this new X-ray delay line, we can move two light pulses relative to each other, adjusting the distance between them,” he explains.

The undulators, which force the electrons to emit the desired X-ray beams for scientific experiments, were a key focus during the maintenance phase. A planned upgrade involves installing superconducting undulators, that will enable unprecedented X-ray pulses: ultra-intense, short and at wavelengths below an angstrom. This will allow exploring matter deeper and at even smaller scales. "We have used this period to prepare for any anticipated future changes", says Sara Casalbuoni, who leads the undulator group at European XFEL. Tasks that would have taken too long during the short regular maintenance periods each summer and winter were carried out over the past several months. "We installed most of the electron beam line components as well all infrastructure such as power and water supplies for the superconducting undulators in one of the tunnels that transport the electron and photon beams," says Casalbuoni, "and we’re now prepared to install the first new undulators in 2027.

Thomas Feurer expects that European XFEL will be ready to welcome scientific users from around the globe back to Schenefeld in April 2026 as scheduled: "Warming up the accelerator worked flawlessly, and I’m sure the cooling process, which will be carried out extremely carefully over the course of multiple weeks, will go just as smoothly", says Feurer. During the cooldown process, the accelerator will shrink by more than a metre in length. It might not be warmed up again for many years to come.