What is the mechanism behind a tokamak, the doughnut-shaped artificial sun?

The project seems too dizzying to exist, and yet we visited it. In our new documentary, in the south of France, in the heart of the mountainous landscape of Saint-Paul-Les-Durance, we find ITER, an “artificial sun”, a nickname for a nuclear fusion reactor. This potential energy source would be immense, profitable and carbon-free. Currently under construction and the result of collaboration between 35 countries, the ITER reactor will have to reach 150 million degrees Celsius. It takes an astonishing donut shape: it's a tokamak. But how does a tokamak work?

Our extraordinary behind-the-scenes documentary at ITER can be discovered on YouTube:

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The little story of the “tokamak”

Against all expectations, “tokamak” is an acronym, taken from the Russian expression: toroidalnaya camera s magnitnymi katouchkami (to-ka-ma-k). Which means “toroidal chamber with magnetic coils”. The word comes from Russian, because the very concept of the tokamak was invented in Russia, by two physicists, Igor Tamm and Andreï Sakharov, during the 1950s. The expression was invented by Oleg Lavrentiev, who also contributed to research on nuclear fusion.

The tokamak principle, studied extensively throughout the world, is today considered the most promising for developing a nuclear fusion reactor.

How does fusion start in a tokamak?

To begin to understand how a tokamak works, we must break down its meaning — “toroidal chamber with magnetic coils” — into two:

  • Toroidal chamber: a “tor” is a ring. We speak of a “toroidal chamber” because it is a (hollow) vacuum chamber in the shape of a ring, which also earns it the nickname “donut”, since seen from above, it takes exactly the shape of one. . The nuclear fusion reaction takes place within the vacuum chamber.
  • Magnetic coils: in a tokamak, the fusion reaction is achieved and maintained thanks to an extremely powerful magnetic field, coming from the outside and the center. This is achieved using magnetic coils, a type of giga magnet.
The ITER tokamak, seen from above. Donut shaped. // Source: ITER
The ITER tokamak, seen from above. Donut shaped. // Source: ITER

The nuclear fusion reaction within a tokamak takes place in several stages. Take the example of ITER:

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  • We inject a few grams — in the form of gas — of a mixture of two isotopes of hydrogen: deuterium and tritium. These are atoms with so-called “light” nuclei.
A mixture of two isotopes is injected: deuterium and tritium. // Source: Nino Barbey for NumeramaA mixture of two isotopes is injected: deuterium and tritium. // Source: Nino Barbey for Numerama
A mixture of two isotopes is injected: deuterium and tritium. // Source: Nino Barbey for Numerama
  • When the tokamak starts up, these isotopes are heated and put under extreme pressure by the combination of several systems (an intense current and external heating systems, using microwaves or particle beams).
  • These conditions create a new state of matter – a dense and hot plasma – which has a particularity: the nuclei of atoms can meet. This is where nuclear fusion comes in: this encounter generates heavier atomic nuclei, helium. A physical process similar to that at the heart of stars. And like in the stars, this releases a colossal amount of energy.
The plasma circulates within the vacuum chamber of the tokamak. // Source: ITERThe plasma circulates within the vacuum chamber of the tokamak. // Source: ITER
The plasma circulates within the vacuum chamber of the tokamak. // Source: ITER

At ITER, this plasma will have to rise to 150 million degrees, a temperature ten times that of our Sun. “ It's not just the temperature that matters », Explains physicist Alain Boutilet, scientific leader of the project, in our documentary. “ It is the density, multiplied by the temperature, multiplied by the energy confinement time: the quality of isolation from the environment. The sun has a monstrous isolating quality. It has energy confinement times of several hundred thousand million years. » The Sun therefore has such a high density that it does not need a temperature so high that so that the reactions take place. Whereas, for us, on Earth, a temperature that is too low will not generate enough reactions: we need a very, very high temperature.

Due to its extreme heat, the plasma must never touch the walls. This is why a tokamak works based on magnetic confinement: the powerful magnets, around and in the center, keep the plasma in suspension, they make it constantly swirl within the vacuum chamber, which is why it is in the shape of a ring (like a particle accelerator).

How do we recover the energy from this artificial sun?

To withstand such temperatures, not only is the plasma sculpted within the vacuum chamber, but the whole thing is also enclosed in a cryostat, a structure maintained at -200°C; in addition to a heat shield in its center. In this case, how can we hope to use the energy emitted by fusion reactions?

These are neutrons. The latter, released during the metamorphosis of deuterium-tritium into helium, have one advantage: they are not charged. The magnets can't hold them with the rest. They therefore escape by bombarding the wall. No less than 80% of the energy produced goes away with these neutrons.

The fusion reaction bombards the walls with neutrons, the energy leaves with them, this is how we can recover it, via the walls. // Source: Nino Barbey for NumeramaThe fusion reaction bombards the walls with neutrons, the energy leaves with them, this is how we can recover it, via the walls. // Source: Nino Barbey for Numerama
The fusion reaction bombards the walls with neutrons, the energy leaves with them, this is how we can recover it, via the walls. // Source: Nino Barbey for Numerama

By bombarding the wall, heat accumulates there. This heat can be recovered to supply steam to turbines, which themselves are supposed to produce electricity. The ITER project intends to multiply the starting energy by 10: within the tokamak, the fusion reaction will need to be self-sustaining, so that the energy cost-benefit ratio is profitable.

The first ignition should take place during the 2030s. If ITER is the largest artificial sun – and the largest tokamak – in the world, it is not the first of its kind. Others are currently in operation and provide a better understanding of how to control and improve such a machine.

The ITER tokamak in figures

The figures from the ITER tokamak allow us to understand the dizziness of such a project:

  • The plasma temperature will be 150 million degrees;
  • The cryostat is maintained at -200 degrees;
  • Including the cryostat, the machine is 60 meters high by 30 meters wide;
  • These are 840 cubic meters of plasma at 150 million degrees which must be sculpted continuously by the magnets;
  • The tokamak weighs nearly 30,000 tonnes;
  • The ITER complex as a whole covers 42 hectares;
  • Construction began in 2010 and is expected to be completed around 2030.

France of the future, episode 2

ITER, the artificial sun that humanity needs? » is the second episode of our documentary series The France of the future, with the support of the National Center for Cinema and Animated Images. An episode by Marcus Dupont-Besnard and Louise Audry, with the collaboration of Nino Barbey and the voice of Benoît Allemane.


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