In the southern Swedish city of Lund, a pan-European consortium is building one of today’s largest research facilities, the European Spallation Source (ESS). Based on the world’s most powerful neutron source, the facility will serve as a massive laboratory for scientists working on research projects covering areas such as materials sciences, energy, health and the environment. This spallation source would not be possible without sophisticated and extremely capable cooling systems. Linde Kryotechnik supplied the most important cryogenic parts, including two coldboxes, each being a one of a kind.
It starts with protons
Reliable cooling systems are essential for experimental infrastructure of particle accelerators. The reason lies within the acceleration process. “In order to get neutron beams for science, we first have to create a powerful beam of electrically charged particles” says Julia Öberg, Press Officer at ESS, “this is, in our case, protons.” Particles with an electrical charge can be accelerated using electro-magnetic fields. Contrarily, neutrons, as their name implies, are electrically neutral. Electro-magnetic fields do not have any effects on them. “So, the main purpose of the ESS accelerator is to accelerate a powerful beam of protons and to shoot them against a rotating Target wheel containing bricks made of tungsten, which is a neutron rich heavy metal,” Öberg says. “When the protons hit the target, the tungsten releases neutrons in a process called spallation”. Those neutron beams are extremely useful for scientists, since they allow investigation of materials providing different information than experiments based on electron or synchrotron radiation.
Accelerating the protons to the speed needed in the spallation process – approximately 96 per cent of the speed of light – requires radio-frequency cavities. ESS uses superconducting cavities to lower the resistive losses. Since the cavities only become superconducting at extremely low temperatures, they have to be cooled with superfluid helium kept at temperatures of 2 kelvins, which is minus 271 degrees Celsius. This is exactly what the first coldbox designed by Linde Kryotechnik does: The Accelerator Cryogenic Plant (ACCP) was delivered in early August 2017.
Since September 2018, the second Coldbox TMCP has been standing next to its big sister ACCP. Although they look similar at first sight, these are completely different machines. The ACCP cools down the accelerator to boost protons which hit the target wheel. Those crashes force the tungsten in the target to release neutrons. “To make these neutrons usable for science, they are slowed down by passing ESS’ innovative hydrogen moderators. These moderators are cooled by the second coldbox, the Target Moderator Cryogenic Plant (TMCP),“ Öberg explains.
Close partnering and exchange of expertise
Even for a highly specialised technology company like the Linde Group, designing a cooling system for such a complex project is not a daily routine. “You cannot simply order a coldbox like this,” says Philipp Arnold. Thus, Linde Kryotechnik was already involved at an early stage during the conception phase. Lars Blum, Head of Sales & Business Development at Linde Kryotechnik, describes the collaboration with engineers and scientists from ESS as “a close partnership with an on-going exchange of information and expertise.”
This close partnership is especially necessary when it comes to challenging details and special requests. “One difficulty was to keep efficiency high at several power levels and stages of expansion,” Blum explains. An additional challenge is caused by the distance between the TMCP and the target. Since they are 300 metres away from each other, the gas molecules take minutes to travel forth and back from the target to the coldbox. In order to react quickly to changes of pressure, flow rate and temperature at the target station, Linde’s and ESS’ engineers have developed a special control concept to cope with the lag of the system response.
Half of the construction work is done
The TMCP has reached its final position. But Linde’s work has not ended. “It will take several months, maybe a year, to install the cooling system completely,” Arnold says. A complex snarl of pipelines and valves has to be attached properly. Therefore, a small group of Linde engineers will stay at the site during installation, commissioning and testing.
Finally, after a day of work and maneuvering, the TMCP reaches its position next to its brother, the Accelerator Cryogenic Plant (ACCP). It takes several months to a year to install the cooling system completely. (Credit: Ole Øystein Bakke/ESS)
Meanwhile, work is going on all over the giant ESS site. “Until now, we have completed 52 per cent of the construction project,” Julia Öberg says. 2019 will be an intensive year with installation and commissioning of technical equipment in various parts of the facility. The user program for scientists is planned to start in 2023. Then, the optimal environment for multi-disciplinary research and scientific breakthroughs will be in place – cooled by Linde Kryotechnik.