Researchers Make Key Gains in Unlocking the Promise of Compact X-ray Free-Electron Lasers

To exploit the compactness LPAs provide, the plasma-based accelerator also needs to produce high-quality electron beams reliably and stably. In this work, the Berkeley Lab team made a significant step toward this goal by demonstrating strong exponential growth of the FEL radiation with exceptional stability and reliability over hours of operation. The experiments were carried out at the BELLA Center on a system years in the making. Their work is an advance to earlier LPA FEL work by Wang et al. and Labat et al. in terms of LPA FEL physics and light source robustness.

The promise of compact XFELs could open new frontiers in biological research by enabling on-site imaging of complex proteins, in materials science through powerful imaging of nanostructures, and in photolithography for manufacturing the most advanced semiconductor chips.

“LPAs are a high-gradient accelerator technology that has the potential to impact applications where there is a premium on compactness,” said Schroeder, a senior scientist in the BELLA Center. “Development of LPA-based free-electron lasers is an important stepping stone to other applications of this technology, such as linear accelerators for high-energy physics.”

The standalone potential of LPAs to produce extremely bright electron beams also offers a path towards upgrading existing XFEL facilities to extend their reach. Conventional FELs typically reach the so-called saturation regime, where the exponential increase in pulse energy plateaus. Operating an LPA-driven FEL in the saturation regime, and pushing the radiation wavelength into the X-ray regime, are major next steps for the LPA field. “We’re confident that our setup at the BELLA Center gives us an ideal platform to continue to make milestone progress and establish a roadmap towards the development of LPA-driven light sources,” said Barber.

TAU Systems Inc. played a key role in this work. Their team brought expertise on accelerator beam physics and operational concepts to the BELLA Center’s Hundred Terawatt Undulator LPA FEL facility. This proved impactful in successfully coupling the electron beam at the plasma source to and through the undulator. “These FEL results confirm the premise that the LPA has opened a revolutionary paradigm shift in how we view accelerators, what they look like, and what is possible with them,” said Stephen Milton, the lead scientist from TAU Systems Inc. on this project.

“It’s a big result,” Barber said of the recently published work. “The fact that the two to three orders of magnitude FEL gain is so significant proves the LPA is producing the high-quality electron beams required to make FELs work. And the fact that it’s so reliable over tens of successive experimental campaigns speaks to the robustness of the LPA. We’re very happy with the result, and there’s still more on the table.”

“The development of high-quality (low-emittance, high peak-current) LPA electron beams is critical to compact new FELs and upgrades to existing light sources, and also represents a key milestone on the roadmap to high-energy particle colliders where high-brightness beams are a key to high-energy physics colliders experiments at meaningful statistics,” said Cameron Geddes, director of the ATAP Division.

This research was supported by the U.S. Department of Energy (DOE), Office of Science, the Office of Basic Energy Sciences, the Office of High Energy Physics, and through a Cooperative Research and Development Agreement with TAU Systems Inc., and by the Gordon and Betty Moore Foundation.

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