Following nine years of construction work and more than a million assembly hours, the main assembly of the Wendelstein 7-X was completed in April 2014. The operational preparations have been under way ever since. Each technical system was tested in turn, the vacuum in the vessels, the cooling system, the superconducting coils and the magnetic field they produce, the control system, as well as the heating devices and measuring instruments. On 10th December, the day had arrived: the operating team in the control room started up the magnetic field and initiated the computer-operated experiment control system. It fed around one milligram of helium gas into the evacuated plasma vessel, switched on the microwave heating for a short 1,3 megawatt pulse – and the first plasma could be observed by the installed cameras and measuring devices. “We’re starting with a plasma produced from the noble gas helium. We’re not changing over to the actual investigation object, a hydrogen plasma, until next year,” explains project leader Professor Thomas Klinger: “This is because it’s easier to achieve the plasma state with helium. In addition, we can clean the surface of the plasma vessel with helium plasmas.”
The first plasma in the machine had a duration of one tenth of a second and achieved a temperature of around one million degrees. “We’re very satisfied”, concludes Dr. Hans-Stephan Bosch, whose division is responsible for the operation of the Wendelstein 7-X, at the end of the first day of experimentation. “Everything went according to plan.” The next task will be to extend the duration of the plasma discharges and to investigate the best method of producing and heating helium plasmas using microwaves. After a break for New Year, confinement studies will continue in January, which will prepare the way for producing the first plasma from hydrogen.
Background
The objective of fusion research is to develop a power source that is friendly to the climate and, similarly to the sun, harvests energy from the fusion of atomic nuclei. As the fusion fire only ignites at temperatures of more than 100 million degrees, the fuel – a thin hydrogen plasma – must not come into contact with cold vessel walls. Confined by magnetic fields, it floats virtually free from contact within the interior of a vacuum chamber. For the magnetic cage, two different designs have prevailed – the tokamak and the stellarator. Both types of system are being investigated at the IPP. In Garching, the Tokamak ASDEX Upgrade is in operation and, as of today, the Wendelstein 7-X stellarator is operating in Greifswald.
At present, only a tokamak is thought to be capable of producing an energy-supplying plasma and this is the international test reactor ITER, which is currently being constructed in Cadarache in the frame of a worldwide collaboration. Wendelstein 7-X, the world’s largest stellarator-type fusion device, will not produce energy. Nevertheless, it should demonstrate that stellarators are also suitable as a power plant. Wendelstein 7-X is to put the quality of the plasma equilibrium and confinement on a par with that of a tokamak for the very first time. And with discharges lasting 30 minutes, the stellarator should demonstrate its fundamental advantage – the ability to operate continuously. In contrast, tokamaks can only operate in pulses without auxiliary equipment.
The assembly of Wendelstein 7-X began in April 2005: a ring of 50 superconducting coils, some 3.5 metres high, is the key part of the device. Their special shapes are the result of refined optimisation calculations carried out by the “Stellarator Theory Department”, which spent more than ten years searching for a magnetic cage that is particularly heat insulating. The coils are threaded onto a ring-shaped steel plasma vessel and encased by a steel shell. In the vacuum created inside the shell, the coils are cooled down to superconduction temperature close to absolute zero using liquid helium. Once switched on, they consume hardly any energy. The magnetic cage that they create, keeps the 30 cubic metres of ultra-thin plasma – the object of the investigation – suspended inside the plasma vessel.
The investment costs for Wendelstein 7-X amount to 370 million euros and are being met by the federal and state governments, and also by the EU. The components were manufactured by companies throughout Europe. Orders in excess of 70 million euros were placed with companies in the region. Numerous research facilities at home and abroad were involved in the construction of the device. Within the framework of the Helmholtz Association of German Research Centres, the Karlsruhe Institute of Technology was responsible for the microwave plasma heating; the Jülich Research Centre built measuring instruments and produced the elaborate connections for the superconducting magnetic coils. Installation was carried out by specialists from the Polish Academy of Science in Krakow. The American fusion research institutes at Princeton, Oak Ridge and Los Alamos contributed equipment for the Wendelstein 7-X that included auxiliary coils and measuring instruments.
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