Keeping Cool for Frontier Science

For decades, physicists have explored the subatomic world to understand the fundamental building blocks of the universe. And yet, despite thousands of experiments and observations, only five percent of the universe’s particles have been accounted for. To fill in the blanks, the international particle physics community continuously strives towards developing ever more complex and fascinating precision machines.

From the start, the advancement of knowledge in the field of particle physics has been inextricably linked with the evolution of particle accelerator technologies. “They act as giant telescopes, able to peer at unimaginably small energy levels”, explains John Urbin, Business Unit Manager, Linde Cryogenics. The Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, represents an international hotspot in the endeavor to continuously push the limits of detection. Several of Fermilab’s current projects are aimed at testing and further developing critical system components for future particle accelerator assemblies. Among these core components are superconducting radio-frequency (SRF) cryomodules.

“Cryomodules are vessels that contain superconducting cavities in a particle accelerator”, says Urbin. Inside, liquid helium cools the cavities to -271°C, only slightly warmer than the coldest possible temperature. Once the cavities reach these super-cool temperatures, they gain the ability to boost the particles that are racing through the accelerator at nearly the speed of light. Because future particle accelerators will require as many as 2000 cryomodules, the design teams must consider two very important factors: manufacturability and cost. And achieving a perfect balance between these two factors requires many design iterations and tests. “For this, Fermilab enables beam-based experiments in prototypic conditions that cannot be reproduced anywhere else”, says Matthias Kocher, Linde’s site manager at Fermilab.
Several stand-alone superconducting radio-frequency (SRF) cryomodule test stands are currently being completed inside a new building on Fermilab’s campus. The so-called Cryomodule Test Facility (CMTF) also features a new Linde Kryotechnik coldbox that has been integrated into the existing cryogenic infrastructure. The setup is also designed to be capable of supporting simultaneous operations within the new facility. Due to the necessary power and the wide scope of tests in different conditions, Fermilab requested a very flexible refrigeration plant that is able to deliver adjustable temperatures between 1.8K and 4.5K.

This refrigeration system sets a new standard”, says Philipp Treite, Process Engineer of Linde Kryotechnik. “It is the first superfluid helium refrigerator plant realized within the last decade that provides a highest degree of operator-friendliness, flexibility and compactness”, he adds. In order to generate the required temperatures, Linde Kryotechnik implemented a so-called mixed cycle. It consists of warm vacuum compressors and a series of three cold compressors.
The turbo-machines pump off helium vapor at sub-atmospheric pressures in a very cost-efficient way. “In large-sized units such as this one, they are indispensable”, says Treite. The essential advantage lies in a drastic size reduction of the additional downstream equipment. “But operating three cold compressors in a row is like dancing samba on a high wire”, jokes Lars Blum, Head of Sales & Business Development, Linde Kryotechnik AG. “Their operational tolerance is very narrow.” So the engineers at Linde Kryotechnik developed a cutting edge and operator-friendly control system that enables very easy handling. Operators can now follow the state of operation within the compressor map and are always up-to-date. In addition, an integrated safety feature functions in the background and supports the operating system by automatically avoiding critical states. .
In the future, the new cryogenic facility will help Fermilab to develop many advances in particle accelerator engineering. And Linde’s commitment to the project didn’t stop with the plant’s acceptance. Urbin: “As we go forward, the people who worked on the project’s realization will also continue to contribute to its continued success and long lifetime.

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