Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and laser technology company, TAU Systems, built a compact x-ray free-electron laser (XFEL) using laser plasma accelerators to generate and sustain high-quality electron beams. XFELs are powerful light sources and are typically large research instruments that see use in medicine, biology, physics, materials, and other fields of basic and applied science. The research work undertaken by Berkeley Lab and Tau Systems sought to develop smaller and cheaper XFELs to be implemented in research facilities on a broader scale than currently possible. Specifically, the work advanced the use of laser plasma accelerators (LPAs) to generate high-quality electron beams and make XFELs more compact. In a conventional linear accelerator, researchers are limited to electron beam acceleration gains of about 50 MV per meter. With plasmas, 100 gigavolts (GeV) per meter are possible, which is more than 1000× stronger, thereby shrinking the accelerator distance Researchers Sam Barber and Stephen Milton align the electron beamline made up of a series of steering magnets, quadrupoles, and high-resolution diagnostics. Credit of Lawrence Berkeley National Laboratory. In addition to reaching high energies, XFELs require high-quality electron beams —another area in which the high fields of the plasma can prove helpful, if controlled correctly. If these conditions are met, and when the electron beam traverses special magnetic devices called undulator magnets, the wiggling beam begins to radiate. If conditions are just right, the radiation amplifies exponentially as it moves along the undulator, creating especially bright x-rays. 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 research team produced high-quality electron beams demonstrating exponential growth of the FEL radiation with stability and reliability over hours of operation. 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 Carl Schroeder, a senior scientist at Berkeley Lab. “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, offers a path toward upgrading existing XFEL facilities to extend their reach. Conventional FELs typically reach the 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 steps for the LPA field. “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 Berkeley Lab’s Accelerator Technology & Applied Physics Division.