The Sun on Earth

The Max Planck Institute for Plasma Physic’s (IPP) Wendelstein 7-X fusion experiment (W7-X) will see its technical acceptance inauguration on 20th May 2014. The experimental facility is considered a major milestone in fusion energy research. Linde Kryotechnik AG engineered, designed, installed and commissioned a high-tech turnkey cryogenic helium plant needed to run the fusion experiment.
Nuclear fusion – it is nature’s way of making energy. The sun and all the stars in the universe shine bright because of it. For decades, scientists around the world have worked on replicating this ultimate form of energy production here on earth. “One gram of fusion fuel can deliver the same amount of energy as eleven tons of coal”, explains Michael Nagel, Cryogenic Department Head at IPP. With the W7-X experimental facility, fusion experts around the world have come another step closer to harnessing the almost infinite stellar energy source that is fueled by hydrogen isotopes that are available in abundance. “There is enough fusion fuel to power all of our energy needs for hundreds of years”, says Nagel.

But although the resources are available in abundance, the technical challenges remain at the boundary of what is currently possible. “The problem is forcing atomic hydrogen nuclei that are both positively charged to join and release energy”, explains the IPP scientist. The reaction will only function if the atoms are in a state of superheated plasma at around 100 million degrees Celsius. To achieve and sustain these temperatures, the plasma must be contained and channeled with high precision, as any outside influence would immediately cool it down and halt the fusion reaction. The answer comes in form of a magnetic cage with an almost otherworldly shape. At first sight, this so-called Stellarator looks like a large metal donut that has been randomly squashed by a giant hand in some places. In reality its bizarre geometry is the result of a decade of supercomputer calculations and state-of-the-art metal working techniques.
“A Stellarator is far more difficult to build than other fusion devices”, says Nagel. “But the advantage is that the plasma won’t attempt any wild jailbreaks as it does in other designs – it is a lot easier to control.” W7-X is the largest Stellarator fusion device in the world. The entire machine construction has a mass of 725 tons. Its oddly shaped plasma vessel is circled by 50 superconducting niobium-titanium magnet coils about 3.5 metres high that look equally strange. They are cooled to superconduction temperature close to absolute zero with supercritical helium by forced flow. To allow the magnetic field to be varied, a second set of 20 planar superconducting coils has been superposed on the core structure.

“W7-X’s core components are helium cooled by a Linde Kryotechnik AG refrigeration system”, says Uwe Nüsslein, Project Manager at Linde Kryotechnik AG. “It is the most complex refrigeration plant we have ever built.” Linde Kryotechnik AG’s contribution started with a technical feasibility study in 1997. “Groundbreaking developments do not happen from one day to the next”, says Nüsslein. But the result is without comparison: “The flexibility of the cryogenic plant says it all”, Michael Nagel adds.
The refrigerating system for Wendelstein 7-X has a capacity of roughly 7 kW at 4.5 K. The lowest bath temperatures in the system are at individual 3.3 and 3.8 K levels and are generated by separated cold compressor stages to minimize installation space compared to a single bath. Four supercritical helium pumps supply the lowest guaranteed temperature of forced helium flows at 3.4 K. Although these capacity specifications seem to be modest compared to other systems already implemented, they are very unique. The system owns a multitude of breakthroughs aggregated into a single cryogenic refrigerator with very interesting particularities: “We like to compare the cold compressor and helium pump starting sequence to a remote-controlled Formula One car on ice”, says Nüsslein. “Starting these high-tech machines requires so much sensitivity due to the helium property and aerodynamics that even the slightest driver’s mistake is unforgivable.” The system also possesses high efficiency across a wide capacity range, considerable load variation between different operation and sub modes and a complex distribution system for six different W7-X consumers including the magnetic coils, coil housings and support structures, cryo-vacuum pumps, current leads, cryostat heat radiation shield and the shield of the cryo-vacuum pumps. Last but not least, the system is highly automated: the steady state operation in each operating mode as well as the transitional steps between these modes are automatically controlled and do not need any further attendance. “A 10’000-liter Dewar allows us to temporarily store the cold and enables fast and economic transitions between operating modes up to the demanded peak load. In other words, we don’t let any energy escape”, says Nüsslein. Also, each W7-X consumer can be individually connected or disconnected during cold operation from the self-adjusting helium refrigerator with its 15 main machines. Over 100 control valves and around 500 analogue signals from instruments are processed by three programmable logic controllers.

“Throughout the entire time we always felt that we were receiving excellent support in all areas from Linde Kryotechnik. Our expectations were fulfilled and surpassed. Even in phases that IPP needed adjustments to the scope”, says Nagel. For Linde Kryotechnik, the experiences gained in this project were invaluable and many have already been of great use in other areas of the company’s portfolio. Nüsslein: “We can now proudly say that Linde Kryotechnik has mastered cryogenic leading-edge technology for an energy source that could well transform the world! To have worked on this unique project is extremely special to me as an engineer” says Nüsslein.