New undulator (in the middle) at FLASH. (Credit: DESY)
Following a comprehensive 14-month upgrade as part of the FLASH2020+ project, DESY’s free-electron laser FLASH is restarting – now offering new, unique capabilities for research on its FLASH1 branch. Once scientific user operations resume, tailored, extremely stable and spectrally narrow X-ray pulses will be available for experiments. Combined with FLASH’s high repetition rate, this opens up entirely new opportunities for precise studies in biology, chemistry and materials science, with exceptionally high temporal resolution in the femtosecond range.
FLASH, which underwent a major overhaul from June 2024 to August 2025, is now almost unrecognisable. The 300-metre-long facility sparkles and shines – in recent weeks, technicians, engineers and scientists have been putting the final touches to everything: checking connections and supply lines, testing motors and magnets, adjusting mirrors and preparing for the restart. As a result of the upgrade, the free-electron laser (FEL) has become a new machine with unique properties, enabling revolutionary experiments in the X-ray regime. It is now being gradually and cautiously recommissioned. Thanks to the FLASH2020+ project, FLASH is set to remain an X-ray laser pioneer and trailblazer for innovative technologies.
A central feature is the special configuration of undulators. Undulators are periodic magnetic structures which force the electron beam to oscillate as it passes through. The electrons can generate intense, focused FEL pulses or else can be manipulated in a targeted way via interaction with strong lasers. The interplay with the laser allows certain properties to be transferred to the electron beam — a process known as ‘seeding’. An electron beam thus prepared offers many benefits for the generation of X-rays: The pulses are spectrally narrower (i.e. more monochromatic), more temporally precise and far more stable from shot to shot — not only spatially coherent, as at FLASH2 and in future at PETRA IV, but now also temporally. This will soon enable researchers not only to analyse samples with the highest temporal precision but also to study processes in which timing is critical.
“Our electron bunches are also forced to do gymnastics because of these more complex undulators,” says DESY scientist Rolf Treusch, FLASH group leader for scientific user operations. “They no longer simply oscillate side to side, but, if desired, can follow adjustable spiral trajectories which may even set the light itself rotating. The light bursts produced with seeding become significantly more stable in intensity and reproducibility from shot to shot. This allows for more precise, reproducible experiments which is especially important for time-resolved studies of ultrafast processes — a major advantage for forthcoming research in biology or biochemistry.”
In order to construct the new FLASH1 beamline, the rear half of the FLASH tunnel was completely cleared out – providing the perfect opportunity to modernise the infrastructure, repaint all the walls, install new lighting and lay new cable trays. "This is thanks to the dedicated efforts of everyone involved in preparation and installation. With the new FLASH1, we have laid the foundation for FLASH’s scientific future and will soon be tackling exciting new research questions," says FLASH project manager Lucas Schaper. Beam diagnostics have also been considerably enhanced, allowing even better external control and prompt correction of the generated X-ray pulses from the accelerator control room.
The improved light pulse properties have also necessitated updates at the end of FLASH1 near the experimental stations. For example, new mirror chambers have been installed to guide the generated light to its destination with minimal loss. "The polished surfaces of these X-ray mirrors are manufactured with sub-nanometre precision," says Schaper. "Producing them externally took two years. We must take all these special requirements into account here at DESY and develop the necessary concepts and solutions."
The conversion of FLASH was only possible thanks to the close collaboration of multiple groups at DESY, ensuring processes were handled as efficiently and economically as possible. This ‘DESY spirit’ has already proven its worth, for example in the construction of the European XFEL, and will make future flagship projects such as PETRA IV possible. We have created new structures here for the FLASH upgrade in which science, technology development, workshops and administration work together optimally and deadlines are adhered to, even with extremely complex schedules,” says Britta Redlich, Director of Photon Science. “This gives me great confidence in our ambitious plans for the future. With PETRA IV, we are planning to build the world's leading synchrotron radiation source, and FLASH, with its unique parameters, is already opening up opportunities for many new types of experiments.”
Within the first phase of FLASH2020+, from 2021 to 2022, the first section of FLASH — from the electron source to the end of the accelerator — was upgraded to improve the quality and energy of electron beams and thereby generate X-rays of shorter wavelengths. The now completed upgrade enables FLASH1 and FLASH2 undulator sections to be operated in parallel and independently, allowing more research groups to access X-ray laser pulses optimally tailored for their experiments and substantially increasing the beamtime available at FLASH.
When FLASH returns to full operation, not only will the number of potential users rise sharply but so too will the diversity and scope of possible experiments. FLASH will continue to be a research facility of unique global standing. Much still remains to be done, however, before scientific work with seeded FLASH can fully commence: "The commissioning of such a complex beamline as FLASH1 will take several months, as all components must interact optimally to produce customised photon pulses for experiments," says Lucas Schaper. "Parallel operation of external seeding and self-amplified spontaneous emission (SASE) — each with its own very different requirements — is a world first and a particular challenge." The first experiments at FLASH2 are planned for November; the comprehensively renewed FLASH1 is expected to provide its first seeded X-ray light for experiments in spring or summer 2026.
"Our teams have done an amazing job upgrading the machine, and after just a few days since the restart they already managed to guide the electron bunches through the entire machine,” says Wim Leemans, Director of the Accelerator Division. “This is an outstanding achievement after such a massive conversion, during which the accelerator also had to be warmed up from its operating temperature of -271 degrees Celsius to room temperature. With this major enhancement, we will soon offer unrivaled capabilities to our users, opening the door to exciting new scientific opportunities. The transformation of FLASH marks the completion of the first chapter in DESY’s strategic trilogy for our core photon facilities, to be followed by PETRA IV and European XFEL upgrades. Together, these upgrades will secure our capabilities for the future.”
(Partly from DESY news)
