Ice-Cold Electron Beams For Ultra-Compact X-ray Lasers

Ice-cold electron beams simulated in research at the University of Strathclyde could pave the way to reducing X-ray free-electron lasers (X-FELs) to a fraction of their current size.

X-FELs convert the kinetic energy of an electron beam into powerful photon pulses, down to hard X-ray wavelengths, and are often called ‘engines of discovery.’ X-FELs are used to create extreme matter conditions for hot-dense matter research, to study properties of materials for next-generation microchips, to resolve the structure of complex biomolecules for new medications, and many other applications.

At the heart of FELs are electron beams swinging on a path inside a device, known as an undulator, with an alternating magnetic field. As a result of the wiggling motion, the electron beam emits bursts of photons and a positive feedback effect structures the electron beam into micro-bunches at the radiation wavelength. Consequently, the radiation power grows exponentially along the undulator and becomes highly coherent.

This self-organizing effect can occur only if the electron beam is of high quality at relativistic energies. However, to meet the stringent electron beam quality requirements, state-of-the-art X-FELs are kilometre-scale finely tuned machines, costing as much as a billion-pounds. Consequently, only a handful of X-FEL facilities exist worldwide, with none in the UK so far.

The Strathclyde research shows, with high-fidelity start-to-end simulations, that a Plasma Wakefield Accelerator (PWFA), equipped with a ‘Trojan horse’ advanced electron injection method called plasma photocathode, can produce electron beams 100,000 times brighter than state-of-the-art. This is because of the low momentum spread distribution, producing extremely cold electron beams.

The PWFA also has an accelerating electric field, with a capacity of tens to hundreds of gigavolt-per-meters, which enables the realisation of the accelerator on a centimetre scale, compared with the kilometre scales of traditional accelerators.

The study explores how to extract, transport, isolate and inject the ultra-high brightness, ice-cold electron beams from plasma photocathode PWFA into an undulator without charge and quality loss; they remain cold and do not ‘melt.’ Focused into an undulator, the ultrahigh quality electron beam produces powerful coherent, laser-like photon pulses, with pulse durations in the attosecond regime (1×10−18 of a second). In addition to the extreme quality of the electron and resulting photon pulses, the entire system may have a spatial footprint of only few tens of metres, in contrast to state-of-the-art, kilometre-size X-FEL machines.

The scientists working on the study believe that the three milestones achieved in the study could be a gateway to next-generation ultra-compact X-FELs. The study has been published in the journal Nature Communications and forms part of the nationwide UK X-FEL project.

Fahim Habib, a Research Associate in Strathclyde’s Department of Physics and leading post-doctoral researcher of the study, said: “The prospects of ultra-compact plasma-X-FELs based on this scheme are mind-boggling. Our results are important first milestones, but much more work is ahead of us towards experimental realization of the approach.”

Dr Brian McNeil, a Reader in the Department of Physics, said: “Brightness is the key performance parameter for free-electron-lasing. If electron beams as bright and short as shown in our computational study can be realized from plasmas, this could have an enormous impact on photon science.”

Professor Hidding works on a parallel project, funded by the European Research Council, named NeXource: Next-generation Plasma-based Electron Beam Sources for High-brightness Photon Science.

Dr McNeil was awarded the international Free-Electron-Laser Prize in 2022, in recognition of his outstanding contribution to the field.