iter.org. Both the mission and physics of ITER can be reduced to a single letter: Q. To understand the Q of ITER is to understand its most essential operating parameter as well as the raison d’être of the ITER Project.
What then is the meaning of Q?
Quantitatively, Q is the out-versus-in power amplification ratio of the fusion reaction: the ratio of the amount of thermal power produced by hydrogen fusion compared to the amount of thermal power injected to superheat the plasma and initiate the reaction. ITER is designed to produce plasmas having Q ≥ 10: meaning that injecting 50 megawatts of heating power into the plasma will produce a fusion output of at least 500 megawatts.
Qualitatively, the Q of ITER signifies the achievement of a ”burning plasma”—a state of matter that has never been produced on Earth and that will usher in a new era of fusion research. In a burning plasma, the energy of the helium nuclei produced when hydrogen isotopes fuse (see box below) becomes large enough—because of the large number of reactions—to exceed the plasma heating that is injected from external sources. This is an essential condition for one day generating electricity from fusion power, and enabling scientists from 35 countries to study burning plasmas is the primary scientific motivation of the ITER Project.
Essentially, all other major aspects of tokamak plasma physics have been demonstrated and studied in smaller machines. In engineering terms, the construction of a tokamak capable of creating and sustaining a burning plasma for periods ranging from hundreds to thousands of seconds requires the development of ”reactor-like” tokamak systems across virtually the entire range of fusion technologies. The study of burning plasmas in ITER is intended to demonstrate the feasibility of building commercial fusion power plants for electricity generation.
Plasma energy breakeven, or Q=1, has never been achieved in a fusion device: the current record
is held by the European tokamak JET (UK), which succeeded in generating a Q of 0.67. ITER’s Q value of ≥10 makes it a first-of-kind machine.
How did ITER’s designers choose the specific Q value? Accounting for the size of ITER’s vacuum vessel (830 cubic metres) and the strength of the confining magnetic field (5.3 Tesla), the ITER plasma can carry a current of up to 15 megaamperes. Under these conditions, an input thermal power of 50 megawatts is needed to bring the hydrogen plasma in the vessel to about 150 million degrees Celsius. This temperature in turn translates to a high enough velocity, among a sufficient population of hydrogen nuclei, to induce fusion at a rate that will produce at least 500 megawatts of thermal power output. Read the full article here (link).
Written by: Laben Coblentz, Head of Communication