European XFEL: It's getting cold at the Hamburg X-ray laser
The European XFEL is being prepared for scientific operation after a longer break. We were there one last time beforehand.
View into the XFEL tunnel in Hamburg
(Image: Werner Pluta / heisen medien)
The outlook for Hamburg-Bahrenfeld: frosty until April – very frosty, namely down to 2 Kelvin or minus 271 degrees Celsius. This is the temperature at which the accelerator section of the European XFEL is currently being cooled down, so that scientific operation can resume at the facility in the spring.
It has been at a standstill since summer: The facility began to be warmed up on June 21, says Winfried Decking, head of the accelerator team at the European XFEL. Since then, a new electron injector has been installed, many standard maintenance tasks have been carried out that there is otherwise no time for, and a second cooling system has been installed.
Now the accelerator section of the XFEL is being cooled down to 2 Kelvin again. During this time, no one is allowed in the tunnel for safety reasons – in case of damage to the cooling system, Decking explains. The tunnel is closed anyway during operation due to possible radiation exposure. So, it's the last chance to go several stories deep under Hamburg one more time and take a look at the world's most powerful X-ray laser.
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Besuch am European XFEL in Hamburg
European XFEL
)A large facility for glimpses into the very small
The European XFEL – the abbreviation stands for X-Ray Free-Electron Laser – generates X-ray flashes from an electron beam in the femtosecond range. To descend to the smallest, molecular and atomic level, a huge facility is required.
The European XFEL is located underground beneath Hamburg and the neighboring municipality of Schenefeld. It is almost three and a half kilometers long, and its branched tunnel system is almost 6 kilometers long in total. Researchers from a wide variety of disciplines experiment here: physics, astrophysics, chemistry, or material sciences, as well as biology, medicine, or pharmacy. Twelve nations are involved in the 1.5 billion euro project. The facility has been in operation since 2017. Since then, it has been the first extended maintenance period; otherwise, the XFEL is shut down for three to four weeks.
The beginning of the XFEL is in Hamburg on the grounds of the German Electron Synchrotron (DESY), which is celebrating its 66th birthday these days. Here is the injector, also called the gun, and the 1.7-kilometer-long linear accelerator, the part that is now being cooled down. In the gun, a copper piece about the size of a cent is shot with a laser. This releases about a billion electrons from the copper, which then set off as an ordered packet towards Schenefeld.
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Electrons travel at almost the speed of light
However: electrons are negatively charged. This means they repel each other. "You wouldn't have gained much from that," says Decking. "The trick of such a source is to generate electrons and then bring them to the speed of light very quickly." This happens in a cavity a few centimeters in size. From this leaves a bundle of electrons about two centimeters in size traveling at almost the speed of light, to be precise, at 99.99999996 percent of the speed of light.
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Then the packets of about two centimeters are compressed to 20 micrometers. This is done by quadrupole magnets, magnets with four poles, which manipulate the beam size like focusing lenses.
From there, it goes through the linear accelerator, an arrangement of cavity resonators, large yellow, 80-centimeter objects. Inside are structures made of niobium, in which the electron packets are concentrated. Niobium becomes superconducting at two Kelvin, hence the helium cooling system. The helium-cooled resonators accelerate the electron packets over the following 1.7 kilometers up to 17.5 gigaelectronvolts.
"In a warm accelerator, we could accelerate about 100 electron packets per second. In a cold accelerator, we can currently accelerate about 27,000 electron pulses per second," explains XFEL director Thomas Feurer.
