Wendelstein 7-X upgrade moves to divertor stage

The installation of water-cooled inner cladding of the plasma vessel will make the facility suitable for higher heating power and longer plasma pulses. Production of the new cladding's centrepiece, the divertor, was taken over by the institute's Garching branch.

The divertor is the most heavily loaded component of the plasma vessel. In 10 double strips on the inner wall of the plasma vessel, the divertor tiles follow the curved contour of the plasma edge. They protect those wall areas to which particles from the edge of the plasma are magnetically directed. A pump behind a gap in the middle of each double strip removes the impinging plasma and impurity particles. In this way, the divertor can be used to control the purity and density of the plasma.

At the end of 2018, the experiments on Wendelstein 7-X - at the Max Planck Institute for Plasma Physics in Greifswald - were temporarily terminated after two successful work phases. Upgrading of the plasma vessel has been ongoing since then.

"First of all, most of the old components had to be taken out. Installation of the new ones can now begin," said Hans-Stephan Bosch, whose division is responsible for technical operation of the device. Whereas most of the wall protection components were previously operated uncooled, large sections of the wall will be water-cooled starting with the next round of experiments. "This will then enable Wendelstein 7-X to generate plasma pulses lasting up to 30 minutes," Bosch said.

In the high-performance experiments planned, the new water-cooled divertor plates, which replace the previous uncooled ones, are designed to withstand a load of up to 10 megawatts per square metre. Without water cooling, the heat-resistant divertor tiles (made of carbon-fibre-reinforced carbon) would not be able to withstand this load for the intended 30-minute plasma pulses. They are therefore welded onto water-cooled plates made of a copper-chromium-zirconium alloy. The coolant, supplied by small steel tubes, ensures that the heat energy is removed.

Plasma operation is expected to resume at the end of 2021. It is planned to begin with low water cooling, low heating power and short plasma pulses in order to allow testing of all installations in operation after the long break in experiments. With full cooling, longer pulses with plasma energies of up to one gigajoule should be possible - a target that will be slowly approached. Instead of the previous hundred-second pulses with heating powers of two megawatts and plasma energies of 200 megajoules, the cooled high-performance divertor should later allow pulses lasting up to 30 minutes at full heating power.

Wendelstein is a stellarator fusion reactor - different to a tokamak fusion reactor such as the Joint European Torus in the UK or the Iter device under construction in France. A tokamak is based on a uniform toroid shape, whereas a stellarator twists that shape in a figure-8. This gets round the problems tokamaks face when magnetic coils confining the plasma are necessarily less dense on the outside of the toroidal ring.

The Wendelstein 7-X will not be used to produce energy but should demonstrate whether stellarators are suitable as a power plant. It should show that stellarators have the ability to operate continuously. In contrast, tokomaks can only operate in pulses without auxiliary equipment.