The extremely intense light pulses generated by free-electron lasers (FELs) are versatile tools in research. In the x-ray range, they can be used to analyze the details of atomic structures of a wide variety of materials, and to follow fundamental ultrafast processes with great precision. Up to this point, FELs such as the the European X-Ray Free-Electron Laser (XFEL) in Germany have been based on conventional electron accelerators, which makes them long and expensive. Now, an international team led by Synchrotron SOLEIL, France, and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, has demonstrated seeded FEL lasing in the ultraviolet regime based on laser-plasma acceleration. In the future, the advancement could allow researchers to build more compact systems, which would considerably expand the possible applications of FELs. A German-French research team built a free-electron laser at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) driven by particles from a plasma accelerator — and generated well-controllable laser flashes for the first time with the still young technology. In the foreground, the beam line framed by a light-blue magnet arrangement, the undulator; in the background, the metallic beam chamber for the high-power laser DRACO at HZDR. Courtesy of HZDR. The principle behind the operation of x-ray FELs involves an accelerator — helped by strong radio-frequency waves — bringing electrons close to the speed of light. Then the particles, bundled into bunches, fly through the undulator — a magnet arrangement with periodically alternating fields that forces the electron bunches into slalom paths. This causes the bunches to reorganize into many smaller groups of electrons, known as micro-bunches, which together emit extremely powerful, laser-like light pulses. These can then be used to decipher previously unknown properties of materials or to track extremely fast processes, such as chemical reactions that take place in quadrillionths of a second. However, these systems demonstrate drawbacks. “They are several hundred meters or even a few kilometers long,” said Ulrich Schramm, director of the HZDR Institute of Radiation Physics. “That’s why we’re working on an alternative technology to make such facilities smaller and more cost-effective, and then they could be closer to users at universities and industry in the future.” Laser-plasma acceleration is the basis for the alternative technology. Here, using a high-power laser, short, ultrastrong light flashes are fired into a plasma. In the plasma, the light pulse then generates a strong wave of alternating electric fields, similar to the wake of a ship, according to HZDR physicist Arie Irman. This wave can rapidly accelerate electrons to higher speed over a very short distance. In principle, this could shrink an accelerator that is now a hundred meters long to a length of much less than 1 m. The first controllable laser flash from a free-electron laser driven by a plasma accelerator: FEL-beam at the right side of 'seeded' light. Courtesy of Nature Photonics (2022). DOI: 10.1038/s41566-022-01104-w. In principle, electrons have long been accelerated using this technique. Only recently has it been possible to send such fast particle bunches from a plasma accelerator through an undulator and then convert them into laser-light flashes. “A plasma accelerator installed in Dresden, driven by the high-power laser DRACO at HZDR, delivered fast electron bunches of high beam quality,” SOLEIL physicist Marie-Emmanuelle Couprie said. “Behind it, we then built an undulator along with the associated accelerator beam line, which had previously been optimized for electron beam-transport methods, generation of undulator radiation, seed generation, and shaping including overlap issue and methodologies over several years in the French plasma accelerator laboratory Laboratoire d'Optique Appliquée in Palaiseau jointly with PhLAM in Lille.” To generate FEL laser flashes in the ultraviolet regime, the researchers solved several essential problems. “We had to produce particle bunches that contain copious amounts of electrons,” Irman said. “At the same time, it was important that these electrons possess as equal energies as possible.” To prevent the electron bunches from diverging too quickly, a refined trick was used: the so-called plasma lens. In addition, the team deployed a method called “seeding.” In the method, synchronously to the electron bunches, the team shot external laser-light pulses into the undulator, which is crucial to accelerate the FEL process and allowed for an improved beam quality of the FEL laser flashes. “For 15 years, people in the advanced accelerator physics community have been dreaming about realizing a free-electron laser like this,” Schramm said. Before a plasma-drive FEL can be put to practical use, there are still challenges to overcome. For example, while the setup in Dresden was able to generate UV pulses, research requires high-intensity x-ray flashes — for which the electrons would have to be accelerated to much higher energies. According to Schramm, this has already been demonstrated in principle with plasma acceleration, though the quality of the electron bunches to date is still too poor and too unstable for an x-ray FEL. With a new generation of high-power lasers, Schramm said the team is hopeful it can overcome the barrier. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01104-w).
