Expansion of the Wendelstein 7-X stellarator fusion device at Germany's Max Planck Institute for Plasma Physics (IPP) in Greifswald is entering a new stage with the final delivery of components for the divertor.

Preparations to install the water-cooled inner cladding components have been completed, with installation work expected to continue into 2021.

Wendelstein 7-X is the world’s largest stellarator fusion device. Its goal is to investigate the suitability of such facilities for power production.

Stellarators differ from a tokamak fusion reactor such as the Joint European Torus (JET) in the UK or Iter under construction in France. While a tokamak is based on a uniform toroidal shape, a stellarator twists that shape in a figure eight. This avoids problems tokamaks face when magnetic coils confining the plasma are necessarily less dense on the outside of the toroidal ring.

Wendelstein 7-X will test an optimised magnetic field for confining the plasma, which will be produced by a system of 50 non-planar and superconducting coils, which is the core piece of the device.

IPP expects that plasma equilibrium and confinement will be of a quality comparable to that of a tokamak of the same size. However, by avoid the disadvantages of a large current flowing in a tokamak plasma , Wendelstein 7-X aims to demonstrate continuous operation.

Upgrading Wendelstein 7-X

The main assembly of Wendelstein 7-X was completed at IPP in Greifswald in 2014, and first plasma was produced in December 2015. At the end of 2018, experiments were temporarily terminated after two successful work phases.

Since then upgrading of the plasma vessel has been underway. “Most of the old components had to be taken out. Installation of the new ones can now begin,” said Dr Hans-Stephan Bosch, whose division is responsible for technical operation of the device.

Whereas most of the wall protection components were previously  uncooled, large sections of the wall will be water-cooled during the next round of experiments: “This will then enable Wendelstein 7-X to generate plasma pulses lasting up to 30 minutes”, he explained.

The new water-cooled divertor plates are designed to withstand a load of up to ten megawatts per square metre. The divertor tiles made of carbon-fibre-reinforced carbon are welded onto water-cooled plates made of a copper-chromium-zirconium alloy. The coolant is supplied by small steel tubes to ensure that the heat energy is removed.

The high-performance components are the result of a long development, manufacturing and testing process. This has been carried out by the Integrated Technical Centre (ITZ) and the Components in the Plasma Vessel working group at IPP, in cooperation with industrial companies.

Preparatory work was extensive. In 2003, a contract was signed with an industrial company for the development and production of divertor elements. After four pre-series and more than 60 prototypes, five years of series production began in 2009.

Completing a divertor element requires a toal of 82 manufacturing steps and 44 tests were necessary.

At IPP’s Garching facility, the divertor elements were then joined together on steel frames to form plates. Pipes were joined using a special welding technique developed at the ITZ.

At Greifswald, everything ready for installation of the high-performance components.

"We have already developed special tools for this purpose – for example to lift and move the 70-kilogram [divertor] plates,” said Wegener.

Plasma operation is expected to resume at Wendelstein 7-X at the end of 2021. It is planned to begin with low water cooling, low heating power and short plasma pulses.

With full cooling, longer pulses with plasma energies of up to one gigajoule should be possible.

Photo: One of the 890 divertor elements and a prototype plate assembled from these elements after a heat test (Credit IPP/Michael Herdlin)