XFELO: New laser system at European XFEL generates super-sharp X-ray light
Hamburg-based researchers have developed a system that generates better X-ray pulses at the European X-Ray Free-Electron Laser. New insights are expected.
One of the two XFELO mirror chambers in a tunnel at the European XFEL in Hamburg
(Image: European XFEL)
The European XFEL in Hamburg is one of the world's most powerful X-ray lasers. A new system called XFELO makes it possible to generate X-ray flashes with photons of very precisely defined energy. This is intended to enable even more precise experiments in the future, the research institution announced.
XFELO stands for: X-Ray Free Electron Laser Oscillator. This generates X-ray light with a uniform wavelength, comparable to a laser. Such coherent radiation has not been possible in the hard X-ray spectrum with this quality until now. The group, consisting of researchers from the European XFEL (X-Ray Free-Electron Laser), the German Electron Synchrotron (Desy), and the University of Hamburg, is presenting the XFELO in the journal Nature.
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The XFEL generates X-ray flashes in the femtosecond range from an electron beam: Bunches of around 100 billion electrons are accelerated to near light speed by a linear accelerator and compressed. The X-ray pulses are generated in undulators. These are 200-meter-long structures made of permanent magnets, arranged alternately polarized one after another.
Electron Bunches on a Slalom Course
The magnets force the electron bunches into a serpentine path. With each change of direction, the electrons emit X-ray flashes with laser properties, up to 27,000 per second.
The X-ray flashes generated in this way are already of high quality. Nevertheless, they have a certain energy blur. The XFELO system changes this: It helps to "extremely reduce the energy blur and thus generate X-ray light with photons of very precisely defined energy," explain the researchers.
(Image: European XFEL)
In XFELO, the X-ray light is sent through a resonator approximately 66 meters long. It consists of two mirrors made of high-purity diamond crystals and a series of undulators in between. As before, the undulators force the electron bunches into a slalom course, where they emit X-ray flashes. This is reflected by one of the diamond mirrors and then hits the next electron bunch, with which it interacts. After several orbits, an X-ray flash with a uniform wavelength is created.
Less Noise
"With each orbit, the noise in the X-ray pulse decreases and the concentrated light becomes sharper," describes Patrick Rauer from Desy. "It becomes more stable, and you start to see this single, clear frequency -- this peak." This peak denotes the individual pulse of X-ray light with a very sharp boundary.
The very sharp X-ray light will enable even more precise experiments in physics, materials science, chemistry, or biology. "Researchers will be able to investigate structures and processes in the future that were previously hardly measurable," says XFEL director Thomas Feurer.
One of the major difficulties in building the system was the precise alignment of the crystals and the synchronization of the X-ray light pulse orbits with the electron bunches. The stability of the 1.7-kilometer-long accelerator in terms of energy, time -- in the femtosecond range -- and position -- to the micrometer -- over several days was crucial for success, says Rauer. "It took years to bring the accelerator into this state, which is unique among high-repetition-rate accelerators."
X-ray Pulses with Very Narrow Bandwidth
The idea of using a resonator at the European XFEL came from Jörg Rossbach, a physicist at the University of Hamburg. However, implementing it proved difficult. Rauer laid the groundwork for this in his doctoral thesis. Together with the accelerator team at Desy and the instrumentation team at the European XFEL, they succeeded in designing the resonator configuration.
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"Our success shows that using a resonator to amplify an X-ray laser is practically feasible," says Harald Sinn, head of the Instrumentation Division at the European XFEL. "Compared to previous methods, it delivers X-ray pulses with a very narrow bandwidth and significantly higher stability and coherence."
Next, the team plans to work on further amplifying the X-ray light, ensuring stability over longer operating times, and making the technology available to other researchers.
(wpl)
