Simulations: plasma-accelerator X-ray laser for the water window reaches multi-gigawatt pulses

A European team from Heinrich Heine University Düsseldorf (HHU), the ELI Beamlines centre in Dolní Břežany (Czechia) and the University of Strathclyde has reported start-to-end simulations of a compact X-ray free-electron laser for the water window. The paper was published in Nature Communications on 5 November 2026 and is available on nature.com nature.comnature.comnature.comOpen source ↗. The design combines a metre-scale plasma wakefield accelerator with a metre-scale undulator. The results are computational: the authors show them in model simulations.

Why the water window matters

The water window covers X-ray wavelengths of roughly 2.3 to 4.4 nanometres. Biological systems consist largely of water, while biological molecules contain atoms such as carbon and nitrogen. In this range water is transparent and the other atoms absorb strongly, which suits imaging of samples in their natural aqueous environment nature.comnature.comnature.comOpen source ↗.

Until now, radiation of sufficient quality and intensity in this range has come only from free-electron lasers in large accelerator facilities. Bernhard Hidding of HHU, the corresponding author, said the research community has relied on kilometre-long systems built on conventional electron accelerators. The team instead proposes a compact accelerator based on laser-plasma technology.

How the scheme works

In the simulated design, a plasma photocathode releases an ultra-low-emittance electron beam inside a metre-scale plasma wakefield accelerator nature.comnature.comnature.comOpen source ↗. By tuning the beam charge, space-charge forces balance the beam fields against the wakefields. According to the authors, this reduces beam energy spread and improves energy stability, both critical for extracting the beam, transporting it and focusing it into the undulator.

The resulting ultra-bright beams produce wavelength-tunable, coherent, femtosecond-scale photon pulses at strong gain. The authors report reliable generation of multi-gigawatt-class X-ray laser pulses across the water window. Tunability comes from the witness beam charge, and the output is robust against variations in plasma wakefield strength and plasma photocathode laser jitter nature.comnature.comnature.comOpen source ↗.

In a laser-plasma accelerator, an intense laser pulse drives a plasma wave in a gas, which can accelerate electrons to almost light speed within a few centimetres. HHU's Arcturus laser is used for research and development toward X-ray flashes of this kind. The authors state that the findings could help broaden access to coherent, high-power soft X-ray radiation.