The first milestones in the assembly of the Wendelstein 7-X fusion device at the Greifswald branch of Max Planck Institute of Plasma Physics (IPP), Germany, have been reached with the completion of the first of five modules of the large-scale experiment: one-fifth of the inner core of the device is now ready. Construction of the complex device will take about another six years. Wendelstein 7-X will then be – next to the Large Helical Device in Japan – the world’s largest fusion device of the stellarator type. With discharges lasting up to 30 minutes it is to demonstrate the stellarator’s essential property – continuous operation at reactor relevant plasma parameters.

The components

Industrial production of the essential components for Wendelstein 7-X is almost complete. Manufacturing of the core of the device – 50 complex shaped, helium-cooled superconducting magnet coils about 3.5 metres high – has been finished. They were produced by a German-Italian consortium headed by Babcock Noell GmbH in Würzburg and ASG Superconductors S.p.A. in Genoa. In order to vary the magnetic field, a second set of 20 planar, likewise superconducting coils, are superposed on the stellarator coils. The manufacturer, Tesla in the UK, has delivered all 20 planar coils. More than half of the 70 coils have been successfully tested under cryogenic and high voltage conditions, demonstrating superconductivity at full current and the ability to withstand the voltage required in case of quench. A massive ring-shaped support structure, already half completed by the Spanish company ENSA, will hold the coils in their exact positions.

The entire coil configuration will be enclosed by a cryostat 16 metres in diameter. Two of its five sections have already been finished by MAN DWE in Deggendorf, Germany. A refrigeration plant will later provide cold helium to cool the magnets and supports to the temperature of a few Kelvin needed to achieve superconductivity. Inside the coils the plasma vessel has a peculiar shape matched to the twisting of the plasma contour, designed to provide optimised confinement and stability. Its 20 sections were likewise produced by MAN DWE. More than 250 openings are engineered in the vessel to allow the plasma to be observed and heated and for cooling tubes to penetrate for cooling the plasma facing components. An equal number of ports produced and supplied by the Romabau Gerinox company in Switzerland connect these openings with the outer wall of the cryostat.

The assembly

The assembly of Wendelstein 7-X is organized in six stages making use of the five-fold symmetry of the magnetic field structure. Five almost identical modules are pre-assembled before being joined into a torus in the experiment hall. In the first stage the coils are threaded onto the vacuum vessel and joined to the central support ring, while installing the mechanical support elements between the coils. This work is done separately for each half module. For this stage two assembly rigs Ia and Ib are used, allowing parallel assembly of two half modules simultaneously. The half modules are then joined together to form a full magnet module in assembly rig II. On this rig the preparations for the installation of the superconducting bus and of the cryo-pipes for the liquid helium supply also take place.

In September 2008, the first of five modules moved on from assembly rig II to rig IIIa inside the torus hall (see picture), where the superconducting bus and cryo-pipes are being installed. From now on work can progress on three magnet modules in parallel. Subsequent assembly stages are the completion of the five single magnet modules, the final alignment of these modules on the machine base, the successive connection of the modules and the set-up of the periphery, including electrical connections and cooling system.

A major task concerning the first magnet module during recent months was the manufacturing and installation of the support brackets and clamps of the superconducting bus, which is provided by Forschungszentrum Jülich, and the design and manufacturing of the cryopipes. Because of limited space inside the cryostat and the calculated movement of the coils and structures for the different magnetic field load cases, a very complex collision analysis has been required. Assembly trials for the holders of the superconducting bus have turned out to be very time consuming. Countermeasures, to avoid delays during these working steps, require an increase in design, manufacturing and assembly capacities.

Parallel to the advancing assembly in Greifswald, the first half shells of the cryostat vessel have been delivered. In a first step to prepare the cryostat for the final assembly, experts from MAN DWE are applying the thermal insulation to the cryostat which is required to reduce thermal radiation to the cold structures. After enclosing the magnet modules with the lower and upper half shells of the cryostat vessel the port installation can start.

Isabella Milch