Wendelstein 7-X Reports Good Results After Graphite Wall Fusion Tests
The Max Planck Institute in Greifswald, Germany has announced that a series of experiments with the nuclear fusion reactor Wendelstein 7-X, in which the vessel wall is covered with graphite tiles, has been positive. Record values have been achieved for this specific type of nuclear fusion reactor.
The results of the recent series of experiments confirm the theoretically predicted properties of the plasmas produced in the Wendelstein 7-X stellarator, according to the researchers at the German Institute. The design of the magnetic field, which is supposed to contain the hot plasma, performed well. The convoluted interior of the vacuum vessel is able to withstand higher temperatures and longer plasma discharges due to the use of graphite tiles.
Professor Sunn Pedersen is very optimistic because the values measured are very good for the size of Wendelstein 7-X. In addition, the results were obtained under realistic conditions, ie at high temperatures of the plasma ions. The duration of the energy confinement, during which the hot plasma is held in check with magnets, as it were, so that it cannot touch the wall, was also good, according to him, at 200 ms. This, he says, is an indication that the Wendelstein 7-X stellarator concept and its optimizations are working.
Plasma’s were produced that stayed within the magnetic field of the reactor for 6 seconds; we are currently working to increase this to 26 seconds. When the first hydrogen plasma was created in the reactor in early 2016, the plasma did not last longer than a second. New plasma experiments will follow in July, with another major expansion in August. The graphite tiles are replaced by carbon-reinforced components that are water-cooled. That should enable discharges of 30 minutes. Once that is possible, it can be definitively verified whether Wendelstein 7-X satisfies the properties expected based on the optimizations.
Wendelstein 7-X is a so-called stellarator, while the experimental nuclear fusion reactor in France, called ITER, is a so-called tokamak. In a stellarator, the reactor vessel is a kind of convoluted wok, which is turned around its axis, while in a tokamak the reactor vessel has a symmetrical, flowing donut shape. Tokamaks are the most commonly used and are relatively easy to convert due to their simple shape. A stellarator is much more difficult to make, but has the advantage that much less power is needed to hold the plasma in place. In addition, there is more wall surface to be able to dissipate the enormous heat from the plasma.
Nuclear fusion is about fusing hydrogen atoms together. The temperature of the plasma can rise to 150 million degrees Celsius in a nuclear fusion. Plasma is the fourth state of matter next to solid, liquid and gas; it concerns a phase in which the atoms have lost their electrons, or are ionized. The magnets protect the reactor wall against the hot plasma, but the outlet of the reactor, a divertor, does come into direct contact with the plasma. As a result, it must be able to withstand a bombardment of charged particles and the enormous heat. The ultimate aim is to use the heat dissipated by the exhaust for power generation.