Fusion Stellarator Wendelstein 7-x Fires Up for Real

IEEE Spectrum By Alexander Hellemans 3 Feb 2016

outside view of the stellarator.

Hydrogen plasma navigates the tortuous turns of the stellarator.

Today the German Chancellor Angela Merkel, at a ceremony at the Max Planck Institute for Plasma physics in Greifswald in Germany, pressed a button that caused a two-megawatt pulse of microwave radiation to heat hydrogen gas to 80 million degrees for a quarter of a second.

No, she was not setting off some new kind of hydrogen bomb. She was inaguriating the fusion reactor Wendelstein 7-X, the world’s largest stellarator, by generating its first hydrogen plasma.

Completed in April 2014 the toroidal reactor with its complicated magnetic field is viewed by many as a serious competitor to tokamak-style fusion reactors, such as ITER. One of the advantages of a stellarator is that nuclear fusion reactions can take place continuously, while a tokamak operates in a pulsed mode, making it much less efficient as an energy source.

However, whether plasma would survive in an untested toroidal field created by a set of coils of unprecedented complexity was an open question. The coils were the result of a decade of an enormous computational effort, only made possible by the power of modern supercomputers. On 10 December 2015 researchers at Greifswald loaded the nuclear reactor, also known as W-X with a helium plasma, and to their relief, the plasma behaved exactly as expected. “This was really the beginning, and the machine works nicely. The confinement time was very large, we knew we were on the right path,” says Hans-Stephan Bosch, who directs the division responsible for the operation of the stellarator.

Hydrogen plasma navigates the tortuous turns of the stellarator.

Hydrogen plasma navigates the tortuous turns of the stellarator.

Hydrogen, of course, is the real stuff. “The behavior of hydrogen as a plasma is somewhat different, and with hydrogen we will be in the real regime of operation,” says Bosch. Experiments with hydrogen plasma will continue until March when protective carbon tiles and a divertor for the elimination of impurities will be mounted inside the reactor vessel. The microwave plasma heating power will then be increased to 20 megawatts, allowing plasmas to last as long as 30 minutes.

In a first stage, the stellarator will only confine hydrogen, without any fusion reactions, says Bosch. “In a later phase of W-X, starting in 2019, we will use deuterium and we will get fusion reactions, but not enough to get more energy out than we are putting in,” he says. There are no plans to add tritium to the hydrogen plasma, which would be required to achieve break even, he adds.

Scientists expect that the fusion experiments with deuterium would amply demonstrate that a stellarator would be suitable for power production. But whether the concept of a stellarator will overtake ITER remains questionable. There are currently no plans for a stellarator that would demonstrate net fusion energy production, says Bosch.