Scientists at Germany’s Max Planck Institute for Plasma Physics in Greifswald on 4 February generated the first hydrogen plasma at the Wendelstein 7-X stellarator, the world’s largest and most modern stellarator type fusion device.

Researchers injected a tiny amount of hydrogen into a special device and heated it until it became a plasma. Wendelstein 7-X had been working with helium plasma in preparation for the first hydrogen plasma since the beginning of December 2015.

Helium is easier to heat and also has the advantage of "cleaning" any minute dirt particles left behind during the construction of the device. Wendelstein 7-X device produced more than 300 plasmas using helium. These served primarily to clean the plasma vessel, allowing the plasma temperature to increase. The first plasma in the machine had a duration of one-tenth of a second and achieved a temperature of around one million degrees Celsius. Eventually, a plasma temperature of six million degrees Celsius was achieved.

American physicist Lyman Spitzer invented the stellarator in 1950. It differs from a tokamak fusion reactor such as the Joint European Torus in the UK or the Iter device under construction in France. While a tokamak is based on a uniform toroid shape, a stellarator twists that shape in a figure eight. This avoids the problems tokamaks face when magnetic coils confining the plasma are necessarily less dense on the outside of the toroidal ring. Thomas Klinger, who heads the project, says the device should be able to keep plasma in place for much longer. "The stellarator is much calmer," he said. "It’s far harder to build but easier to operate."

The Max Planck Institute said investment costs for the Wendelstein 7-X totalled €370m ($405m) and were provided by the German federal and state governments and the European Union. The components were manufactured by companies throughout Europe. Orders in excess of €70m were placed with companies in the region. Wendelstein 7-X, is not designed to produce any energy but to test the extreme conditions such devices would face, said John Jelonnek, a physicist at the Karlsruhe Institute of Technology, Germany. Jelonnek’s team is responsible for a key component of the device, the massive microwave ovens that will turn hydrogen into plasma, eventually reaching 100 million degrees Celsius.

Hans-Stephan Bosch, head of the division responsible for operation of Wendelstein 7-X, said: "With a temperature of 80m degrees and a lifetime of a quarter of a second, the device’s first hydrogen plasma has completely lived up to our expectations." The initial experimentation phase will last until mid-March, after which the plasma vessel will be opened in order to install carbon tiles to protect the vessel walls and a "divertor" for removing impurities.

"These facilities will enable us to attain higher heating powers, higher temperatures, and longer discharges lasting up to 10 seconds," explained Klinger. Successive extensions are planned until, in about four years, discharges lasting 30 minutes can be produced and it can be checked at the full heating power of 20MWt whether Wendelstein 7-X will achieve its optimisation targets.

Photo: The first plasma at Wendelstein 7-X in December (Credit: IPP)