Successful fusion experiments at Germany’s Wendelstein 7-X

28 February 2023

After successful recommissioning in autumn 2022, the Wendelstein 7-X stellarator fusion device at Germany's Max Planck Institute for Plasma Physics (IPP) has achieved some significant breakthroughs. In 2023, an energy turnover of 1 gigajoule was targeted, but researchers have now achieved 1.3 gigajoules. Moreover, a new record for discharge time was achieved, with the hot plasma maintained for eight minutes.

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.

The main assembly of Wendelstein 7-X was completed in 2014, and first plasma was produced in December 2015. At the end of 2018, experiments were temporarily terminated after two successful work phases. Upgrading of the plasma vessel was then started. During the three-year renovation work, Wendelstein 7-X was primarily equipped with water cooling for the wall elements and an upgraded heating system.

The heating system can now couple twice as much power into the plasma as before and nuclear fusion experiments can be operated in new parameter ranges. "We are now exploring our way towards ever higher energy values," explained Professor Dr Thomas Klinger, head of the Stellarator Transport & Dynamics Division at IPP. "In doing so, we have to proceed step by step so as not to overload and damage the facility."

The researchers have now reached a new milestone: for the first time, they were able to achieve an energy turnover of 1.3 gigajoules - 17 times higher than the best value achieved before the conversion (75 megajoules). The energy turnover results from the coupled heating power multiplied by the duration of the discharge.

Infrared images from the Wendelstein 7-X vacuum vessel do not show the plasma itself, but the temperature distribution at the water-cooled divertor baffles. The divertor baffles are used to dissipate the heat from the plasma. A defined line in the centre, the so-called strike line, is clearly visible. This is where the plasma touches the divertor and the temperature is highest. In individual areas, temperatures of up to 600 degrees Celsius were reached (red areas). The divertor tiles can withstand temperatures of up to 1200 degrees Celsius.

Particularly heat-resistant divertor baffle plates are used to dissipate the largest heat flows. They are part of the inner wall, which is now cooled by a system of 6.8 kilometres of water pipes since the completion of the device. No other fusion facility in the world currently has such a comprehensively cooled inner wall. The plasma heating consists of three components: the newly installed ion heating, the heating by neutral particle injection and electron microwave heating.

For the current record, the electron microwave heating system was particularly important because it delivers large amounts of power over periods of several minutes. The energy turnover of 1.3 gigajoule was achieved with an average heating power of 2.7 MW, and the discharge lasted 480 seconds. This is also a new record for Wendelstein 7-X and one of the best values worldwide. Before the upgrade, Wendelstein 7-X achieved maximum plasma times of 100 seconds at much lower heating power. Within a few years, the plan is to increase the energy turnover to 18 gigajoules, with the plasma then being kept stable for half an hour.

Image: Infrared image from the vacuum vessel of Wendelstein 7-X showing the temperature distribution at the water-cooled divertor baffles (courtesy of MPI for Plasma Physics)

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