Accelerating science: Linde’s cryogenic expertise

It’s not just cutting edge – it’s on the cusp of breakthrough discoveries. How Linde’s cryogenic solutions are helping the European Spallation Source hit its target.

Today, it’s a 750,000 m² muddy construction site amidst the green fields on the edge of Lund, in southern Sweden. But it is here, that in a few years’ time, scientists will conduct cutting edge research using some of the most advanced technology and instrumentation. Indeed, the construction site of today will become a site for the boundless discoveries of tomorrow – all with the help of Linde Kryotechnik AG in Switzerland. 

The site in question is that of the European Spallation Source (ESS), a multi-disciplinary research facility based on the world’s most powerful neutron source. A neutron source is a milestone visualisation tool, much like the Hubble Space Telescope, for observing the things in our universe that the naked eye cannot. But instead of outer space, a neutron source looks at ’inner space’. It allows scientists to better understand the very make-up of different materials at an atomic level: think of it as the most powerful microscope you could imagine.

“There are many possibilities to research a material, using x-ray or electrons for example, but there are some things that you can actually only find out with neutrons. And that requires one of these large facilities,” explains Philipp Arnold, Section Leader, Cryogenics at ESS. 

And large, it most certainly is: one of the largest science and technology infrastructure projects being built today in fact. The ESS is a multi-million euro, pan-European project with 15 European member and observer nations and over 100 partnering labs contributing. When it opens its doors to researchers in 2023, it will also be opening up new opportunities across the spectrum of scientific discovery. Everything from materials and life sciences to energy, environmental technology, cultural heritage and fundamental physics.

Coldbox on its way to Lund
The largest single technical component of the ESS makes its way to Lund (Photo: Wolfgang Hees / ESS)
Visualisation ESS in Lund, Sweden
How the completed ESS in Lund, Sweden will look (Photo: Visualisation: ESS)

Step on the (proton) accelerator

Today however, cranes dominate the skyline, trucks traverse the terrain and high-vis clad workers in hard hats scurry from one scaffolding platform to the next. But beneath the building bedlam, one can see the complex taking shape. The focal point of activity is a large circular expanse at the opening of a tunnel that stretches back for nearly two thirds of a kilometre: the most powerful linear proton accelerator ever built.

At one end of the tunnel, a proton beam is generated by an ion source. The protons enter the accelerator in short pulses where they are accelerated, first in a warm section and then a cold section with superconducting cavities. The protons then leave the tunnel and smash against a four-tonne, helium-cooled, rotating tungsten target wheel at 96% of the speed of light. It is here that the spallation process takes place: neutrons are scattered from the tungsten nucleus. The neutrons are then cooled by moderators before being guided to the 22 state-of-the-art instruments in the adjoining experiment halls. By analysing the scatter patterns of the neutrons against a sample material, scientists can get precise information about its structure and dynamics.

The challenge of chilling to -271°C!

As the section head and work package manager for cryogenics at ESS, Philipp Arnold’s job is to create just the right cold conditions. “My work package is to take care of three cryogenic plants and all cryogenic distribution,” he explains. “That includes the Accelerator Cryogenic Plant (ACCP) which must cool the superconducting cavities down to 2K (-271°C) using helium. Cryogenics is a challenging technology.” 

Luckily, Linde Kryotechnik AG in Switzerland is up to the challenge. As the world’s leading manufacturer of cryogenic equipment, its expertise in planning, designing and constructing such plants is second to none. Having been awarded the project in 2014, Linde immediately got to work on the 150 page ESS specification to develop the refrigeration plant. The solution consists of three recycle gas compressors operating at ambient temperature and one coldbox which includes Linde’s proprietary expansion turbines as well as cold compressors.

“This was a big delivery with a very detailed specification. In cryogenics, for this size of plant, you can only have tailored solutions. It’s a complicated cycle with many interdependencies. Also, we’re a green field facility and we need extensive acceptance testing,” explains Arnold.

Big in more ways than one. The coldbox is the largest single technical component in the entire ESS, and on 18th August 2017, it was delivered to the site – after what was quite a logistics undertaking. Too big for sea transport from any ports near Lund, the giant refrigerator, measuring 14 meters long and 4.7 metres high with a diameter of 3.5 metres and a weight of 50 tonnes had to travel by road from Basel to Gent for shipping to Gothenburg since the ports around Lund couldn’t handle this size of cargo. An advance team had to clear some “road furniture” such as signs and other obstacles to make it possible. In the end the transporter even had to navigate an underpass with a guideline clearance of as little as three centimetres! Arnold remembers it well: “I was here when it arrived and can verify that it took a day to get it from the entrance point to the hall where it’s housed and another day to position it precisely!”

Reliability:  The potential costs of downtime 

Once the facility is fully up and running and the user program begins in 2023, scientists from all over the world will be applying for their “beam time”: a, for instance, 48-hour window in which to run their potentially Nobel prize-winning experiments. If downtime occurs within that window due to a cryogenic hitch, ESS is not only running the risk of impeding scientific progression, but also of leaving users dissatisfied, as Arnold explains: 

“This can’t happen very often, otherwise you could risk losing users. And many of these users will have been planning these experiments for a long time before. This is why we put so much on reliability.”

All of the scientific endeavours depend on one proton beam, which depends on one cryogenic plant, which depends on a number of components – each one essential for a reliable operation of the facility. And at the end of the day, unplanned downtime is simply not an option at ESS. Thankfully, Linde’s experience and expertise mean its solutions meet these requirements. Linde Kryotechnik AG is also currently developing a cooling plant for the target moderators planned for delivery in 2018.

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A Linde engineer surveys the inside of the “giant fridge” for the accelerator cryoplant