Nuclear fusion
March 17, 2010
At first glance it looks as though all the building components have fallen from the sky and landed in an untidy heap. Virtually no component at the Wendelstein 7-X research reactor is straight or symmetrical.
Among the objects are vast metal rings - two meters in diameter - which to the untrained eye look like they have become gnarled in a bad accident. But in actual fact they have been purposely bent and shaped down to the millimeter.
Lutz Wegener is the technical supervisor on the construction of "Wendelstein 7-X", and as such is responsible for 800 or so steps leading up to the planned completion of the country's largest research reactor in 2014.
And, as he explained to Deutsche Welle, when that time comes, the metal rings will play a decisive role in the fusion process.
"Every magnetic coil produces a magnetic field and all the magnetic coils together create a three dimensional magnetic tube," he said. "That is what keeps the hot fusion plasma in place."
Plasma needs optimal conditions
The fuel of fusion is plasma, formed when an extremely thin hydrogen gas mixture is exposed to high pressure and extreme heat, its neutrons morph and become electrically-charged particles. It is the same process which gives the sun its energy.
According to Professor Robert Wolf, who is in charge of optimization at the Greifswald reactor, the principle of plasma creation is quite simple: as matter is heated, it will change from a solid to a liquid, when you heat it further, the liquid will become a gas, and if you heat it even more, it turns into plasma.
The process involves combining two hydrogen isotopes – deuterium and tritium – which leads to the creation of helium gas and frees up a free neutron. Unlike regular neutrons, free neutrons carry a charge, and it's that energy that can be turned into electricity. And the best thing about it is that the raw materials are available in unlimited supply. Deuterium can easily be harvested from water and tritium is easily produced from lithium.
The nuclear fusion of just one gram of both substances generates the same amount of energy as 11 tons of coal, and it does it without dangerous carbon-dioxide emissions, long-term harmful radioactive waste or the risk of explosion.
Complex technology
Seems simple enough, but there are plenty of potential pitfalls that go along with recreating the sun in a reactor, the least of which is the unimaginable 100 million degrees Celsius temperature required.
The greatest worry for the operators, however, is the plasma coming into contact with the outer wall of the reactor during fusion, thereby grinding the process to a halt. In order to prevent that from happening, giant magnetic coils - 70 in total - have to create a stable magnetic cage, and for that, they require some extremely strong magnets.
At full capacity, 100 tons of magnetic force will be focused on a steel skeleton no larger than a human hand. The magnets are cooled down to negative 269 degrees using liquid helium, making them superconductive and allowing the necessary electricity to flow unhindered. This is the only way to produce enough electricity in a short enough time to get the plasma to burn at 100 million degrees.
This type of fusion reactor is known as a stellarator. It solves issues faced by tokamak fusion reactors, first built in the 1950s. Tokamak reactors have been the preferred choice of physicists up until now because their simpler geometry made them easier to build.
But this simplicity has its downside, says Lutz Wegener. Tokamak reactors can so far only burn plasma for 10 to 30 seconds at a time. Even the most advanced tokamak set up in the world, the ITER in Cadarache, France, will only be able to run for a couple of minutes at time. Certainly not long enough to make it a viable energy source for the future, which is where the Greifswalder stellerator might come in.
As Professor Robert Wolf told Deutsche Welle "the greatest drawback in today's fusion research is that we have never developed a fusion power plant."
But even if the 430 million euro project doesn't deliver any energy, it ought to prove whether or not permanent nuclear fusion is actually possible.
Author: Richard Fuchs (tkw)
Editor: Mark Mattox