Cooling the giant neutron microscope
- 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 cold boxes, each being a one of a kind.
Two mobile cranes with long telescope arms are already waiting as the heavy low-bed trailer arrives at the giant building site in Lund. Its valuable cargo, a coldbox, has the shape of a white zeppelin, measuring 10 metres in length and 3.5 metres in diameter. While this shell may look unimposing, it actually contains sophisticated technical equipment, a one-of-a-kind coldbox that will cool parts of the world’s most powerful neutron source down to temperatures around 16 kelvins, or -257 degrees Celsius. In September 2018, after a long journey through Europe, the 40-ton coldbox reached its destiny, the site of the European Spallation Source (ESS).
ESS is a pan-European science project and one of the world’s largest research infrastructures in construction today. Thousands of scientists from a broad range of disciplines will run experiments investigating actual and upcoming matters on an atomic or molecular scale in research fields such as material sciences, energy industry, medicine, and environmental research. Linde Kryotechnik, the leading provider of cryogenic systems for world-scale research facilities such as CERN and Fermilab, has provided both big cryogenic systems for the complex cooling system at ESS. “In total, we have three different helium cold boxes,” says Philipp Arnold, Section Leader Cryogenics at ESS. “Two for the operation of the ESS machine, one for the testing of equipment and providing liquid helium to the neutron instruments.” The cooling systems for operation, including compressors, oil removal system (ORS), the gas management panel, and more, have been designed and delivered by Linde Kryotechnik. Because every coldbox has its own function, they are built to their individual purpose. One cryoplant is needed for the acceleration of protons, one for moderating neutrons. Accordingly, they are called ACCP (Accelerator Cryogenic Plant) and TMCP (Target Moderator Cryogenic Plant).
It starts with protons
Reliable cooling systems are essential for the 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 electromagnetic fields. Contrarily, neutrons, as their name implies, are electrically neutral. Electromagnetic 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 the 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 percent 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.
A giant passes the chokepoint
Now, one year later, the second vessel is being lifted by two cranes in front of the gate at the side of the oblong klystron gallery that runs along the accelerator tunnel. Since it is too big to be driven in on the back of a trailer, it has to be offloaded and placed on small rolls. The crane operators are deepening the 40-ton cylinder slowly and carefully laying it down. Success is a matter of centimetres, but the experienced workers manage to place the vessel safely on the rolls. Supported by a forklift truck, they pull the coldbox inside the hall, passing a jungle of pipelines, machines, measuring apparatuses, and more. Now, however, the task is a matter of millimetres. The workers need a whole day to maneuver it to its place, next to its big brother, the ACCP.
Although they look similar at first sight, these are completely different machines. The ACCP cools down the accelerator to boost protons that 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 ongoing 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.
Meanwhile, work is going on all over the giant ESS site. “Until now, we have completed 52 percent of the construction project,” Julia Öberg says. 2019 will be an intensive year with the 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.