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European Spallation Source ESS, Lund, Sweden
Cooling the Giant Microscope
The refrigeration systems were handed over in 2020. The construction phase of the overall project is scheduled to be completed in 2025. First research activities are planned for 2027.
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.
Cryogenics at the ESS; Pictures of the construction site in 2019
A giant passes the chokepoint
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 specialized 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 cold box 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.
Please find the latest information on the European Spallation Source (ESS) currently under construction in Lund at the ESS homepage.
Header image © Roger Eriksson / ESS
Picture of Unloading: © Ole Øystein Bakke / ESS