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In a step toward making ultra-bright X-ray sources more widely available, an international collaboration led by the University of Michigan—with experiments at the U.K.'s Central Laser Facility—has mapped key aspects of electron pulses that can go on to generate laser-like X-ray pulses.

These X-ray pulses have the potential to advance chemistry, biology, material science and physics by enabling researchers to measure the way molecules behave in great detail. The technique may also be useful in clinical medicine for imaging soft tissues and organs.

Because the pulses are so short, quadrillionths of a second (femtoseconds) long, they can take snapshots of chemical reactions, revealing the choreography of atoms and molecules, including larger biomolecules such as proteins. These studies are valuable for both basic research, down to quantum mechanics, and applications of chemistry such as drug discovery.

"We hope that laser-plasma accelerators will be able to shrink XFELs to the size of a tabletop and dramatically increase access to XFEL sources, but one obstacle is the beam quality. This new diagnostic indicates that the beams we produce have much better quality than previously thought," said Alec Thomas, a professor of nuclear engineering and radiological sciences at U-M and corresponding author of the published in Â鶹ÒùÔºical Review X.

Electron pulses used to generate intense X-rays are conventionally produced in accelerators that are hundreds of meters long, available at only one laboratory in the U.S. and five more scattered around the world, according to Thomas. But a way of accelerating electrons with powerful laser pulses could make the technique more accessible, using lower-cost, commercially available parts and requiring a smaller laboratory footprint.

The new approach runs a femtosecond-scale laser pulse through a cloud of gas. The light rips electrons off the atoms in the gas, and some of these electrons are pulled along in the wake of the laser pulse, a phenomenon known as laser wakefield acceleration. The characteristics of this electron beam determine the qualities of the X-ray pulse it can produce. For instance, to generate the laser-like X-ray pulses that are good for imaging soft tissues, the electrons need to be clumped together in bunches within the pulse.

The international team has demonstrated a method for mapping out the electrons in the pulse, where they're headed and how fast they're moving. In particular, they can divide the beam into slices and figure out the energy distributions within those slices.

"The resolution of our method, in time, is approximately one femtosecond, which is better than the diagnostics available at state-of-the-art conventional radio-frequency accelerators," said Yong Ma, an assistant research scientist in nuclear engineering and radiological sciences.

The team worked out how to achieve this resolution through an experiment on the Gemini laser in Didcot, U.K. The wave pattern in the laser light used to accelerate the electrons already imprints on the electron beam, creating a predictable wave pattern. However, the momentum of each electron creates deviations from the expected pattern, and the team was able to read those deviations to reconstruct qualities of the electron beam.

They measured the beam by deflecting it onto a screen, separating the electrons according to energy and measuring the angle at which each electron struck. This gave the momentum of each electron while also pointing back to its original location in the beam. The team then built a machine learning algorithm that could take that data and reconstruct the details of the original pulse.

This information can be used to tune the qualities of electron beams in future compact X-ray facilities. To continue exploring how to measure electron beams produced by laser pulses, the team has an upcoming experiment planned at Europe's Extreme Light Infrastructure (ELI) Beamlines in Czechia, which partners with the U.S. NSF. They also intend to use the new technique on ZEUS, the highest-power laser in the U.S., located at U-M.