X-rays with wavelengths between approx. 2.3 and 4.4 nanometres, i.e. in the so-called water window, are particularly well suited to examining biological samples in their natural, aqueous environment. To date, however, it has been technically difficult to access this type of radiation at an appropriate quality and intensity, as only so-called free-electron lasers in large-scale accelerator systems are capable of generating it. A European research team from Heinrich Heine University Düsseldorf (HHU), the ELI Beamlines research facility and the University of Strathclyde have now demonstrated in model simulations that laser-plasma-based particle accelerators can offer easy access to this radiation range, as they report in the scientific journal Nature Communications.
Biological systems largely comprise water. By contrast, biological molecules contain e.g. carbon and nitrogen. When it comes to depicting such molecules at high resolution, electromagnetic radiation – for which water is transparent but which is strongly absorbed by other atoms – is helpful. And this applies precisely to soft x-ray radiation in the "water window".
Professor Dr Bernhard Hidding, leader of the strong-field laser plasma physics group at HHU and corresponding author of the study: "To date, the research community has been reliant on kilometre-long free-electron laser systems – which are based on conventional electron accelerators – in order to obtain sufficiently bright radiation for biological examinations. Together with the ELI Beamlines facility in Dolní Břežany in Czechia and the University of Strathclyde in Scotland, we are taking a different approach: We want to use a compact electron accelerator based on laser-plasma technology, key aspects of which were developed in Düsseldorf."
In a laser-plasma accelerator, a high-intensity laser pulse generates a plasma wave in a gas. Its extremely high field intensity can accelerate electrons to almost light speed in the space of just a few centimetres.
Lily Berman, doctoral researcher in the working group of Professor Hidding at the University of Strathclyde and lead author of the study: "Our simulations show that the novel, ultra-bright electron pulses in 'magnetic undulators' can produce x-ray flashes, in which the maximum amount of electron energy is converted into x-ray energy, within just a few metres. The process should also be very robust and adjustable on a flexible basis."
Dr Fahim Habib, Principal Investigator at the ELI (Extreme Light Infrastructure) research facility: "Whether a radiation source like that can work as a free-electron laser depends primarily on the quality of the electron beams. Our study shows that a novel, hybrid laser-plasma accelerator could create the necessary prerequisites. We will test this approach experimentally in the near future."
Professor Hidding on the further perspectives: "As such a system is very compact and can be built at a fraction of the cost of conventional particle accelerators, it could also be sited at universities and research facilities, or used for commercial purposes. Water-window x-ray imaging and spectroscopy can thus be expanded from just a few major national laboratories into the wider research landscape."
The study was conducted within the framework of a strategic partnership between HHU and ELI Beamlines, and funded by the German Research Foundation, the Federal Ministry of Research, Technology and Space, and via European research funding programmes such as the European Plasma Research Accelerator with eXcellence (EuPRAXIA).
Background: Undulators and free-electron lasers
An undulator is a structure comprising a series of dipole magnets with alternating polarity. When an electron passes through these magnets, it is forced into a sinusoidal path. The continuous acceleration that occurs causes the particles to emit synchrotron radiation – electromagnetic radiation, the wavelength of which can be precisely controlled via the shape and amplitude of the sinusoidal line.
Building on the undulator technology, this method exploits the fact that the already emitted radiation interacts with subsequent electrons, bunching them into small packages. During the next pass through the undulator, these packages emit their synchrotron radiation in phase – all light waves oscillate at exactly the same rate. Overall, the radiation in the free-electron laser is amplified by an avalanche mechanism – in the same way as in a conventional laser. The system produces ultra-short, extremely bright light pulses that are tuneable to a wide range of wavelengths.
Original publication
L. Berman, D. Campbell, E. Hartmann, T. Heinemann, T. Wilson, B. Hidding, A. F. Habib. Strong-gain water-window X-ray laser driven by plasma photocathode wakefield acceleration. Nature Communications (2026)
DOI: 10.1038/s41467-026-78880-4
