A New Milestone For Laser Technology: Seeded Freeelectron Lasers

The extremely intense light pulses generated by free electron lasers (FELs) are very versatile research tools. In particular, in the X-ray range, they can be used for detailed analysis of the atomic structure of various materials and for tracking fundamental ultrafast processes with great accuracy.

Until now, FELs such as the European XFEL in Germany have relied on traditional electron accelerators, making them long and expensive. An international team led by Synchrotron SOLEIL, France and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, has now made progress in finding a cost-effective alternative solution: they have succeeded in implanting a FEL laser in permanent UV mode. Accelerate new laser and plasma technologies.

In the future, this may allow researchers to create more compact systems, which will significantly expand the possibilities of FEL applications. The research collaboration presents their findings in the journal Nature Photonics .

Electronless X-ray lasers are the most powerful and sophisticated research machines in the world. Principle: Using powerful radio frequency waves, the accelerator brings electrons to speeds close to the speed of light. The particles then fly in groups through a "wave fan," a magnetic array of periodically changing fields that cause the electron beams to follow a serpentine path.

This rearranges the clusters into much smaller clusters of electrons – microclusters – which together emit extremely powerful laser pulses of light. It can then be used to understand previously unknown properties of matter, or to discover extremely fast processes, such as chemical reactions that take place in a quarter of a second.

However, the multibillion-dollar European XFEL and other similar infrastructures have a drawback: “They are hundreds of meters or even kilometers long,” says the professor. Ulrich Schramm, director of the HZDR Institute for Radiation Physics. “Therefore, we are working on alternative technologies to make these facilities smaller and more accessible so that they can be closer to university and industry users in the future.” It is based on a new acceleration technology under development – laser-plasma acceleration.

Physicist HZDR Dr. Ari Erman explains. "In a plasma, a pulse of light creates a powerful wave of electric field, like a ship." These waves can accelerate electrons to high speeds over very short distances. In principle, this could reduce the accelerator, which was now a hundred meters long, down to a meter.

Successful teamwork

In principle, with the help of this technique, electrons have been accelerated for a long time. Recently, although still in its infancy, it has become possible to send bunches of these fast particles through waves from plasma accelerators and then convert them into flashes of laser light. In order to create a highly steerable FEL laser beam using plasma acceleration for the first time, HZDR teamed up with experts from the French synchrotron SOLEIL.

“The plasma accelerator installed in Dresden and powered by the DRACO high-energy laser produced high-quality, fast electron beams,” says SOLEIL physicist Dr. Doctor. Marie Emmanuelle Cooper.

“After that, we then created a wave with the corresponding beam line of the accelerator, improved several years ago at the French Plasma Accelerator Laboratory with the problems and methods of combining the methods of electron beam transmission, wave emission generation, seed generation and formation. ''L'L' Lab d' Optic Appliquée in Palaiseau.

To create an ultraviolet (UV) FEL laser pulse, the researchers had to solve several fundamental problems. “We had to create clusters of particles with a lot of electrons,” Ehrman explained. "At the same time, it was important that these electrons have as much energy as possible."

To prevent the electron beams from scattering too quickly, a sophisticated technique was used: the so-called plasma lens. In addition, the team created a method called "seed": in combination with electron beams, they emit external pulses of laser light in the form of waves, which is necessary to speed up the FEL process and improve the quality of the laser beam. . blinking. . .

Breakthrough in laser technology

With this setup, the team finally achieved their goal: as expected, the FEL plasma screen produced very short flashes of the UV laser. “For 15 years, people involved in advanced accelerator physics have dreamed of creating such a free electron laser,” says Ulrich Schramm. "You can imagine how happy we are to have achieved this now in Dresden."

Ari Ehrman's dream has also come true: “The free electron plasma laser has long been considered one of the most important moments in our field. Through our experiments, we have made tremendous progress.”

Before a plasma-based FEL can be used in practice, several problems must be solved. For example, while the Dresden facility could generate ultraviolet pulses, the research required high-intensity X-ray flashes, which required electrons to be accelerated to much higher energies.

“In principle, this has already been demonstrated with plasma acceleration, but so far the quality of the electron beam is too low and too unstable for an X-ray FEL,” says Schramm. “But with a new generation of high-energy lasers, we hope to solve this problem.” If the effort is successful, the free electron laser may enter the institute's vaults in the future and thus become available to many more research groups than it is today.

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