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The announcement in the last week that the site for the International Thermonuclear Experimental Reactor (ITER) is finally resolved is a source of relief and anticipation to nuclear fusion researchers worldwide. It opens the way to the construction of an experiment that promises the scientific demonstration of controlled fusion energy production. It also removes perhaps the last major impediment to embarking on a project that has been under consideration for nearly 20 years.

Decades of fundamental scientific research and detailed engineering design have bolstered confidence that this project can succeed. But, despite the strong support of the leaders of the major industrial nations, including President Bush, the project had seemed stalled by a diplomatic impasse, as Japan and Europe vied for the prestige and economic benefits associated with hosting so major a technological undertaking. The agreement announces a compromise whereby the non-host (Japan) will be supported in the development of a subsidiary fusion research facility with financial assistance from the ITER host (France), and will also be guaranteed a major role for its industries in the ITER construction project.

This is a good deal for both of the major parties, and for the rest of the ITER consortium. It is a good deal, too, for the citizens of the world, since it enables us to take the next step towards a sustainable energy source that will have zero climate-changing emissions.

The nuclear reactions that release energy by combining light nuclei, like hydrogen, to form heavier nuclei, such as helium, are called fusion. They are, in a sense, the opposite of the nuclear fission reactions that power 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 levels of radioactive waste compared with fission.

To make fusion reactions take place requires the fuel to be heated to tremendously high temperatures (over 100 million degrees), so that it enters an electrically-conducting state beyond that of a gas. This state of matter is called plasma. The plasma must also be maintained long enough for the reactions to occur.

Fusion is the energy source that powers the sun and stars. In these natural fusion reactors, it is gravity that confines the plasma in a wonderfully stable and long-lived configuration. A human-scale fusion reactor must also use a non-material container, but to make the reactor small enough, it must use a much stronger force than gravity: the force of a magnetic field. ITER is to be a magnetic confinement device of the type called a tokamak, which has a toroidal (donut-shaped) configuration and a strong, confining magnetic field. The tokamak configuration has been under study by fusion plasma scientists since the 1960s, and has proven to have the best confinement of all the configurations so far envisioned.

Even so, the achievement of sufficiently good confinement of the plasma to permit useful release of energy has turned out to be far more difficult than the first fusion researchers hoped. Many important optimizations have been discovered and developed. One unavoidable way to obtain sufficient confinement is to make the plasma large. The existing large tokamak experiments typically have plasma radii of three meters. Fueled with the most reactive isotopes of hydrogen, those tokamaks demonstrated substantial release of fusion energy. For example, the world’s largest tokamak, JET (Joint European Torus), obtained up to 16 megawatts of fusion reactions for just under a second. But, to sustain the plasma in these devices required additional heating that was larger.

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