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The site for the International Thermonuclear Experimental Reactor (ITER) has finally been chosen: southern France. Both the European Union and Japan were bidding to host ITER, and the selection of one of them opens the way to the scientific demonstration of controlled fusion energy production and removes perhaps the last major impediment to a project under consideration for nearly 20 years.

This result is good news for the two bidders, for the rest of the ITER consortium (the United States, Russia, China, and South Korea), and for the citizens of the world, since it enables us to take the next step toward developing a sustainable energy source – nuclear fusion, the process that powers the sun – that produces zero climate-changing emissions.

Nuclear reactions that release energy by combining light nuclei like hydrogen’s to form heavier nuclei such as helium’s are called fusion. They are, in a sense, the opposite of the fission reactions that generate power in present-day nuclear plants. Fission breaks up the nuclei of heavy elements such as uranium. Fusion has the potential to provide practically inexhaustible energy with greatly reduced radioactive waste.

The fuel in a fusion reaction must be subjected to tremendous heat, which turns it into an electrically conducting gas called a plasma. The plasma state must be maintained long enough for the reactions to occur. In stars like our sun, gravity confines the plasma in a wonderfully stable and long-lived configuration. A human-scale fusion reactor must use a much stronger confining force: a magnetic field. ITER will use a donut-shaped magnetic containment device called a tokamak.

But confining a plasma tightly enough to enable useful energy release is far more difficult than early researchers had hoped. Many important optimizations have been developed, but one unavoidable measure is to make the plasma large. Existing large tokamaks typically have a plasma radius of three meters and have demonstrated substantial energy releases. But keeping their fuel in a plasma state has required additional heating.

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