Plot of electron beam characteristics
In a step toward making ultrabright 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. The anticipated impact on the future of science from these compact X-ray free-electron lasers (XFELs) helped draw funding for this study from the U.S. National Science Foundation and Department of Energy, as well as British funding agencies.
"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, U-M professor of nuclear engineering and radiological sciences and corresponding author of the study in Physical Review X. Thomas is also a professor of electrical and computer engineering and physics.
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, U-M 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 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 and funded by the NSF.
The team also included researchers from the Central Laser Facility, U.K.; Queens University, Belfast, U.K.; Cockroft Institute, U.K.; Lancaster University, U.K.; Diamond Light Source, U.K.; University College London, U.K.; Imperial College London, U.K.; University of York, U.K.; University of Strathclyde, U.K.; Helmholtz Center Dresden-Rossendorf, Germany; Technical University of Dresden, Germany; Helmholtz Institute Jena, Germany; GSI Helmholtz Centre for Heavy Ion Research, Germany; Institute of Physics of the ASCR, Czechia; Lund University, Sweden; Superior Technical Institute, Portugal; Ergodic LLC, U.S.; Lawrence Livermore National Laboratory, U.S.; and University of California, Los Angeles, U.S.
U.S. support came from NSF Grant No. 1804463 and Department of Energy Grants No. DE- NA0002372, No. DE-SC0022109 and No. DE-SC0016804. U.K. funding came from the Science and Technology Facilities Council, Engineering and Physical Sciences Research Council and Royal Society. Additional funding sources include EuPRAXIA, the European Commission's LASERLAB-EUROPE, EuCARD-2, the Swedish Research Council, the Natural Science and Engineering Research Council of Canada and the Portuguese Foundation for Science and Technology.
Study: Single-shot reconstruction of electron beam longitudinal phase space in a laser wakefield accelerator (DOI: 10.1103/sxqf-l6mp)
Journal
Physical Review X