If it works, fusion power offers vast amounts of clean energy with a near limitless fuel source and virtually zero carbon emissions. That’s if it works. In February last year a new chapter of fusion energy research commenced with the formal opening of Wendelstein 7-X. This is an experimental €1 billion (A$1.4bn) fusion reactor built in Greifswald, Germany, to test a reactor design called a stellarator.It is planned that by around 2021 it will be able to operate for up to 30 minutes duration. Which would be a record for a fusion reactor. This is an important step en-route to demonstrating an essential feature of a future fusion power plant: continuous operation.
But the W-7X isn’t the only fusion game in town. In southern France a $US20 billion (A$26.7bn) experimental fusion reactor ITER is being built. This machine uses a different design called a tokamak. However, even though the W-7X and ITER employ different designs. The two projects complement each other, and innovations in one are likely to translate to an eventual working nuclear fusion power plant.
Twists and turns
Fusion energy seeks to replicate the reaction that powers our Sun. The resulting fused atom ends up slightly lighter than the original two atoms. The difference in mass is then converted to energy according to Einstein’s formula E=mc². The difficulty comes in encouraging the two atoms to fuse, requiring them to be heated to millions of degrees Celsius. What makes the W-7X particularly interesting is its stellarator design. It comprises a vacuum chamber embedded in a magnetic bottle created by a system of 70 superconducting magnet coils. In these experiments a strong toroidal (or ring) magnetic field creates a magnetic bottle to confine the plasma.
However, in order for the plasma to have good confinement in the doughnut-shaped chamber, the magnetic field needs to have a twist. In a tokamak, such as in the ITER reactor, a large current flows in the plasma to generate the required twisted path. However, the large current can drive “kink” instabilities. In a stellarator, the twist in the magnetic field is obtained by twisting the entire machine itself. This removes the large toroidal current, and makes the plasma intrinsically more stable. The cost comes in the engineering complexity of the field coils and reduced confinement thus the plasma is less easily contained within the magnetic bubble.
While the W7-X and ITER use different approaches, most of the underlying technology is identical. They are both toroidal superconducting machines, and both use external heating systems such as radio frequency and neutral beam injection to heat the plasma, and much of the plasma diagnostic technology is in common. In a power plant, heavy isotopes of hydrogen (deuterium and tritium) fuse to form helium along with an energetic neutron. The neutron is has a neutral electric charge, and shoots off into the “blanket” surrounding the plasma. The helium generates electricity.
A common feature across fusion power is the need to develop materials that can withstand y the fusion reaction. The first wall of a fusion reactor has to withstand a massive bombardment from high energy particles throughout its lifetime. At this stage, it’s too early to tell whether the tokamak design used by ITER or the stellarator used by W-7X will be better suited for a commercial fusion power plant. But the commencement of research operation of W-7X will not only help decide which technology might be best to pursue. Without a doubt will contribute valuable knowledge to any future fusion experiments, and perhaps one day a true energy revolution.