How it works

To generate the X-ray flashes, bunches of electrons are first accelerated to high energies and then directed through special arrangements of magnets. Electrons are first brought to high energies in a superconducting accelerator. They then fly on a slalom course through a special arrangement of magnets (called an "undulator"), in which they emit laserlike flashes of radiation.

Principle of a free-electron laser: the SASE process relies on energized electrons traveling through alternating magnetic fields.

1. Bringing electrons to high energies

The first part of the facility is a 1.7 kilometre-long particle accelerator that brings bunches of electrons to high energies at nearly the speed of light.

The electrons are accelerated in special cavities, the so-called resonators. In these resonators, an oscillating microwave transfers its energy to the electrons.

Niobium cavities being prepared in France. (Image credit: DESY)

The resonators are made of the metal niobium and are superconducting: When they are cooled to a temperature of -271 degrees Celsius, they lose their electrical resistance. Electrical current then flows through the resonators with no losses whatsoever, and nearly the entire electrical power is transferred to the particles. Moreover, the superconducting resonators deliver a very fine and even electron beam. A particle beam of such extremely high quality is the absolute prerequisite to operate an X-ray laser.

The electron bunches are generated by knocking the particles out of a piece of metal using a conventional laser. The specifications the electron source has to meet are very challenging, as even the smallest irregularities at the beginning would amplify in the course of the acceleration process and result in an electron beam of insufficient quality.

The electron bunch gains energy as it moves through the cavity, which is chilled to -271°C and infused with radiofrequency.

2. Inducing the electrons to emit light

The accelerated electrons then race through so-called undulators, periodic arrangements of magnets that force the electrons onto a tight slalom course. In the process, each individual electron emits X-ray radiation that amplifies more and more.

This amplification process is induced by the interaction of the X-ray radiation with the electrons: Because the radiation is faster than the electrons speeding along their slalom path, the radiation overtakes the electrons flying ahead and interacts with them along the way, accelerating some of them and slowing others down. As a result, the electrons gradually organize themselves into a multitude of thin disks. The key property of this process is the fact that all of the electrons in a given disk emit their light “in sync.” This produces extremely short and intense X-ray flashes with the properties of laser light.

The undulators generate laser-like X-ray light from accelerated electrons through the SASE process. (Image credit: Option Z / European XFEL)

This is the SASE principle of self-amplified spontaneous emission. Since the structure of thin disks takes some time to fully form in the undulators, free-electron lasers require very long undulators. At the European XFEL, they will be more than 100 metres long.

As one electron accelerator can drive several undulators at the same time, it is possible to generate radiation with different properties for different experiment stations. In its initial configuration, the European XFEL will provide three undulators with six experiment stations. Eventually, this will be expanded to five undulators with ten instruments, and perhaps even more.

3. Using the X-ray flashes for research

The X-ray flashes of the European XFEL will enable a large variety of very different experiments at several different experiment stations.

The experiments are similar in their basic setup: Depending on the experimental requirements, the X-ray flashes can be widened, focused, filtered, or weakened using optical elements such as mirrors, gratings, slits, or crystals. The samples are provided in the experiment station, where they interact with the X-ray flashes. The results of these interactions are measured using special detectors. The data is recorded and processed for analysis. The researchers can follow the progress of the experiments from a neighbouring control room.