A milestone for laser technology

The extremely intense light pulses generated by free electron lasers (FELs) are versatile tools in research. Especially in X-rays, they can be used to analyze the details of the atomic structures of a wide range of materials and to follow fundamental ultrafast processes with great precision. Until now, FELs such as the European XFEL in Germany have been based on conventional electron accelerators, making them time-consuming and expensive. An international team led by the Synchrotron SOLEIL, France, and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, has now achieved a breakthrough towards an affordable alternative solution: they were able to demonstrate the FEL laser seeded in the cosmic ultraviolet based on still young technology – laser-plasma acceleration. In the future, this may allow building more compact systems, which will greatly expand the possible applications of FELs. The research collaboration presents its results in the journal Nature photonics.

Free-electron X-ray lasers are among the most powerful and complex research machines in the world. The principle: using strong radio frequency waves, an accelerator brings electrons closer to the speed of light. Then the particles, grouped in bundles, fly through the “undulator” – an arrangement of magnets with periodically alternating fields that force the electron bunches into slalom courses. This causes the clusters to reorganize into many smaller groups of electrons – microbundles, which together emit extremely powerful, laser-like pulses of light. 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.

But the European XFEL and other similar billion-dollar infrastructures have a disadvantage: “They are hundreds of meters or even a few kilometers long,” said Professor Ulrich Schramm, director of the Department of Physics. HZDR radiation. “That’s why we’re working on alternative technology to make these facilities smaller and more cost-effective, so they can be closer to users in universities and industry in the future.” The basis is a new accelerator technology that is still under development – ​​laser-plasma acceleration.

“Using a powerful laser, we send short, ultra-powerful flashes of light into a plasma, which is an ionized gas of negatively charged electrons and positively charged ions,” explained the HZDR physicist. Dr. Ari Irman. “In the plasma, the light pulse then generates a strong wave of alternating electric fields, similar to the wake of a ship.” This wave can rapidly accelerate electrons to a higher speed over a very short distance. In principle, it can reduce an accelerator that is currently around 100 meters long to well under one metre.

Successful teamwork

In principle, electrons have long been accelerated using this technique. But only recently, albeit still at an early stage, has it been possible to send such fast packets of particles from a plasma accelerator through an undulator and then turn them into flashes of laser light. To generate well-controllable FEL laser light driven by plasma acceleration for the first time, HZDR teamed up with experts from the French synchrotron SOLEIL.

“A plasma accelerator installed in Dresden, powered by the high-power DRACO laser, delivered fast electron beams of high beam quality,” described SOLEIL physicist Dr. Marie-Emmanuelle Couprie. “Behind it, we then built an undulator with the associated accelerator beamline, previously optimized for electron beam transport methods, undulator radiation generation, generation and commissioning. seed form, including the overlapping problem and multi-year methods in the French plasma accelerator. laboratory Laboratory of Applied Optics in Palaiseau in collaboration with PhLAM in Lille.”

To generate FEL laser flashes in the ultraviolet (UV) regime, researchers had to solve several key problems. “We had to produce bundles of particles that contained large amounts of electrons,” Irman explained. “At the same time, it was important that these electrons have as equal energies as possible.”

To prevent the electron packets from diverging too quickly, a sophisticated trick was used: the so-called plasma lens. In addition, the team implemented a method called “seeding”: synchronously with the electron beams, they sent external laser light pulses into the undulator, which is crucial for accelerating the FEL process and has made it possible to improve the quality of the beam from the FEL laser flashes. .

Breakthrough for laser technology

With this setup, the team was finally able to achieve its goal: the hoped-for demonstration of ultrashort UV laser flashes generated by the plasma FEL. “For 15 years, members of the advanced accelerator physics community have dreamed of realizing a free-electron laser like this,” enthused Ulrich Schramm. “You can imagine how happy we are to have achieved this in Dresden.” For Arie Irman, too, a dream has come true: “A plasma-driven free electron laser has always been considered one of the most important milestones in our field. Thanks to our experience, we have now made enormous progress.

Before a plasma-based FEL can be put into practice, there are still several challenges to overcome. For example, while the Dresden facility was able to generate UV pulses, the research requires high-intensity X-ray flashes – for which electrons had to be accelerated to much higher energies.

“This has already been demonstrated in principle with plasma acceleration, but so far the quality of the electron beams is still too poor and too unstable for an X-ray FEL,” said Schramm. “But with a new generation of powerful lasers, we hope to solve this problem.” If the effort is successful, free-electron lasers may in the future fit into the institute’s basement – and may therefore be available to many more research teams than today.

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