Imitating the Sun on Earth: Fusion Reactors

Last week I visited the Wendelstein 7-X fusion device being built at the Greifswald branch of the Max Planck Institute for Plasma Physics (IPP) and was given a tour of the site in addition to an excellent presentation by former IPP director Professor Friedrich Wagner. This visit took place in the company of a group of young jurist from Germany, Austria and Switzerland who were meeting in Greifswald to discuss the topic of risk and law, or the law of risk. A report of the substantial discussion of the meeting will follow later and will be available from the NCCR Trade Regulation website. (photos)

The idea behind fusion research is to develop a power plant that releases energy by the same mechanism as the Sun does, that is, by fusing light atomic nuclei. What is special about nuclear fusion as a source of energy is that one gramme of fuel – hydrogen isotopes deuterium and tritium (produced from lithium) – generates as much energy as eleven tons of coal (90’000 kilowatt-hours of energy). When these two hydrogen isotope nuclei fuse, helium and neutrons are produced releasing large amounts of energy. In addition, the fuel itself is very abundant on earth and for all intent and purpose one may consider this to be a renewable source of energy. The energy is captured as thermal energy and converted to electrical power using turbine technology. Fusion reactors generate radioactive byproducts however these decay to background levels within one hundred years, and thus do not pose the problem of nuclear waste disposal that fission reactors do.

The sun, like other stars is a naturally occurring nuclear fusion reactor and exists not as a solid, but as a plasma. A plasma consists of electrically charged particles. The plasma of interest in the case of nuclear fusion consists of hydrogen isotope atoms that have been ionized, that is, where the electron that usually keep the atom in its neutral non ionized state has been supplied with enough energy to break loose. On earth, we have all seen plasmas in the form of neon signs or fluorescent light bulbs. Other examples include most flames, polar auroras, welding arcs, lightning and comet tails.

The first commercial commodity fusion power plant is still in the future. The two devices now under construction in Europe, ITER in France, and Wendelstein 7-X in Germany, are research devices meant to demonstrate proof of concept. In addition to these European efforts, there is the Large Helical Device (LHD) in Japan which is the largest supercoducting stellarator in the world. All of this confinement, that is the containment of the plasma is needed because like a coal fire, a fusion fire does not happen on its own, it must be ignited. To ignite a plasma and cause fusion to occur a plasma temperature of 100 million degrees is needed. To produce this kind of plasma temperature, one relies on magnetic a

Of the research fusion reactors being built, ITER is a tokamak and Wendelstein and LHD are stellerators. The difference is one of geometry. A tokamak has a magnetic toroidal confinement (like a doughnut) and the other a Möbius confinement for the plasma. In either case, the torus or the Möbius create a tube closing on itself where the plasma is confined. So, what is this plasma and why does it need confinement?

Last year a film made by the IPP on behalf of the European Fusion Development Agreement with funding from the European Union won the MIDAS Award. It is only nine minutes long and gives an entertaining and informative account on how a fusion power plat will work and what environmental properties are to be expected.