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Fusion: Energy of the Future

JET

Inside view of a tokamak chamber: JET-EFDA, the flagship of fusion research. Courtesy of JET-EFDA, Culham, Abingdon, United Kingdom

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What is fusion? Nuclear fusion provides the energy that powers the Sun and the stars. One possible fusion reaction occurs when different forms (or isotopes) of hydrogen collide and fuse to form nuclei of helium. These collisions also release high energy neutrons. Similar fusion reactions have been made to happen here on Earth by using the hydrogen isotopes deuterium and tritium. But to generate enough energy to produce electricity efficiently using fusion, reactions need to take place at a rate that requires temperatures of around 100 million degrees Celsius. This temperature is around ten times hotter than the interior of the Sun. At this temperature matter forms a plasma, the fourth state of matter, which consists of ionized atoms and free electrons (electrically charged particles).

Why so hot? The forces between hydrogen nuclei normally cause them to repel each other. Inside the Sun the strong powerful effect of gravity pulls the atoms together. But in situations where the force of gravity is much weaker, for example on Earth, complex techniques are needed to confine the hydrogen isotopes and to produce the high temperatures required to drive fusion reactions. Therefore, developing fusion as a new source of energy is a formidable scientific and technological challenge

Why using deuterium and tritium as fusion fuel? In a commercial fusion power station the fuel will consist of a 50-50 mixture of deuterium and tritium (D-T), since this mixture fuses at the lowest temperature and its energy yield is the largest compared with other fusion reactions. Deuterium can easily be extracted from seawater, where 1 in 6700 hydrogen atoms is deuterium. Tritium can be produced from lithium, which is widely distributed in the Earth's crust. Thus, the primary fuels for D-T fusion reactors are so abundant in nature that, practically speaking, D-T fusion is an inexhaustible source of energy for global energy requirements. For comparison, if the deuterium in 50 cups of seawater were used in a D-T fusion reactor, the energy produced would be equal to that gained from the burning of 2 tonnes of coal. In addition, the primary fuels (deuterium, lithium) and the direct end product (helium) of fusion are neither toxic nor radioactive, and they do not produce atmospheric pollution nor do they contribute to the greenhouse effect.

Fusion reaction

The fusion fuels deuterium and helium (the heavy forms of hydrogen) fuse into helium, releasing a high energy neutron.

How can the plasma be heated to 100 million degrees? This is a challenging task. Possible heating methods include: (1) compressing the fuel -- like air in a piston, (2) forcing an internal electric current through it -- like a toaster, (3) bombarding the fuel with high energy neutral particles, and (4) supplying power from microwaves or lasers..

How can the hot plasma be confined? Three methods for plasma confinement exist: (1) gravitational (as occurs in stars), (2) inertial and (3) magnetic. The most successful method for containing plasmas thus far is a magnetic bottle, which is toroidal -- or doughnut -- shaped and in which the plasma forms a continuous circuit. The most highly developed magnetic bottle is the tokamak, which was invented by Russian scientists. The tokamak uses strong externally applied magnetic fields to confine the plasma and maintain separation of the plasma from the walls of the containing vessel, which could not withstand the 100 million degree temperature of the plasma.

Who carries out nuclear fusion research? There are several major (national and international) and many smaller fusion programmes carried out worldwide. Because of the complexity and cost of developing fusion as an energy source, a high degree of international collaboration is required, as is being realized, for example, at JET-EFDA (Joint European Torus -- European Fusion Development Agreement), Culham, Abingdon, United Kingdom. Private industry and governmental agencies are also co-operating in the fusion endeavour.

What does the future of fusion research hold? ITER, the International Thermonuclear Experimental Reactor conducted under the auspices of the IAEA, is the next generation large device. It will be by far the largest fusion device ever built: 30 meters in diameter and 30 meters in height. Engineering Design Activities began in 1992 and will be completed in mid-2001 with the production of the ITER Final Design Report. Negotiations will then result in the drawing up of an agreement for the construction and exploitation of ITER, preparation for licensing, and a decision on the location of the site.

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