Armaflex Cryogenic Systems in the nuclear fusion reactor Wendelstein 7-X

All systems go for the German stella rator

The sun shows us how to do it: how to gain an almost inexhaustible supply of energy without risks and harmful residues. With the commissioning of the nuclear fusion device Wendelstein 7-X research has moved another step closer to the fusion of atomic nuclei. The Max Planck Institute for Plasma Physics in Greifswald used the innovative cryogenic system made by Armacell to insulate the relief pipe.

Mankind has long dreamed of harnessing the sun’s energy for use on Earth. Stars like our sun generate energy through the fusion of atomic nuclei. Nuclear fusion promises infinite generation of electricity with minimal fuel input and without the risk of catastrophic incidents. Unlike nuclear fission, nuclear fusion produces only very small quantities of radioactivity. From the point of view of physics, nuclear fusion is possible. The scientific and technical feasibility of generating energy in this way has been the subject of research for over 50 years. In nuclear fusion, hydrogen atoms are fused into helium atoms. At a pressure of around 2 bar hydrogen gas is heated to temperatures of 100 to 150 million degrees Celsius, the electrons are separated from the atomic nuclei and the gas becomes an electrically conductive plasma. Currently, the most promising concepts for reactors are tokamaks and stellarators.

The world’s largest stellarator is now being built at the Max Planck Institute for Plasma Physics in Greifswald (Germany):

Wendelstein 7-X allows the magnetic confinement of fusion plasma in continuous operation. The new building was begun in 1997 and opened in April 2000. At the end of 2003, the first main components – a non-planar superconducting magnetic coil and the first section of the plasma vessel were delivered. In May 2014, preparations for the operation of the device began.

 

Cryogenics for superconducting magnetic coils


Stellarators create the magnetic field needed to confine the plasma via energized coils arranged outside the plasma vessel. In the case of Wendelstein 7-X these coils are superconducting, i.e. once supplied the electric current can flow indefinitely without electrical resistance and thus permanently maintain the magnetic field. To this end the coils are cooled with liquid helium. The superconducting coils and the steel structures which support them have to be thermally insulated both against their environment and against the hot plasma. Following the principle of a thermos flask (although in this case the cold material is inside) they are cooled in a cryostat: the coils are located in a vacuum chamber, which is formed by the plasma vessel on the one side and the outer vessel of the device on the other side. Cryogenic shields  surround the coils and – themselves cooled – keep residual heat radiation away from them. Access to the plasma through this vacuum vessel and between the superconducting coils – for heating, cooling pipes or diagnostics – is enabled by 254 approximately 1.8 m long, thermally insulated ports. The stellarator is supplied by the helium cryosystem, water cooling devices, vacuum pumps and systems for providing electrical energy. 

Despite thermal insulation, 5 kW heat output have to be dissipated during the experiments to cool the magnets and their enclosure (around 425 tonnes of material) to superconductivity temperature and then keep them cool. This permanent cooling is needed due to the residual thermal conductivity of the insulation materials used. At temperatures close to absolute zero, cooling can no longer be achieved by a standard refrigeration system, but requires liquid helium which boils at 4.22 K (−268.93 °C). This cooling system must be exceedingly gas-tight in order to prevent helium diffusing into the insulating vacuum of the stellarator and the insulation deteriorating.
Relief pipe insulated with Armaflex Cryogenic Systems

To insulate the relief pipe for the helium the project management installed Armacell’s  innovative system for extremely low temperatures. Armaflex Cryogenic Systems are specially developed for applications at temperatures ranging from -180 to +125 °C. The multi-layer systems ensure exceptional thermal insulation, reduce the risk of corrosion under insulation (CUI) and are much easier to install than rigid foams. The core of the insulation system is Armaflex LTD, a specially developed polymer, which prevents thermal tension. Armaflex Cryogenic Systems maintain their flexibility even at extremely low temperatures. This flexibility ensures that vibrations and impact are absorbed and the risk of cracking as a result of extreme temperature cycles is significantly reduced. A key advantage of the cryogenic foams is that they need neither additional expansion joints nor vapour barriers.

 

Special insulation construction

On the relief pipe of the Wendelstein 7-X stellarator a special insulation construction was used. To better withstand possible vibrations of the pipework, a layer of Armaflex LTD anti-abrasive foil was applied both directly to the pipework and to the first Armaflex LTD insulation layer. It gives the insulation greater surface strength and acts as an additional vapour barrier. Then a further layer of Armaflex LTD and a third insulation layer of NH/Armaflex, the halogen-free elastomeric insulation material made by Armacell, were applied.

During the installation the greatest care was taken to ensure precise and neat workmanship: each layer was applied step-by-step and then inspected by a project engineer. For the elbows with various radii the shapes needed were plotted, printed on paper and cut out to serve as templates for the sheet material. The employees of the Max Planck Institute who carried out the work were trained by Armacell in the use of the products, especially in adhesion, beforehand.