the power of the reactors could be doubled (following the revision of a fundamental law)

fusion nucleaire nouvelle limite greenwald puissance double tokamak couv

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Nuclear fusion is one of the most promising energy sources of tomorrow, especially in the context of the climate crisis. Physicists from EPFL (Ecole Polytechnique Fédérale de Lausanne), within a vast European collaboration, have recently revised one of the fundamental laws of nuclear fusion, called the “Greenwald limit”. For three decades, this law has been the basis of plasma and fusion research, even governing the design of megaprojects like the ITER (International Thermonuclear Experimental Reactor). The team of physicists found that it was possible to double the amount of hydrogen injected into a thermonuclear reactor to produce twice as much energy. This discovery thus redraws the limits of fusion, when some experts believe that the first reactors for industrial use will only be profitable from 2040-2050.

Nuclear fusion involves the combining of two atomic nuclei into one, releasing large amounts of energy. It is this process that is at work within the Sun. The heat thus comes from the fusion of hydrogen nuclei into helium atoms, which are heavier.

In France, in the Bouches-du-Rhône department, 35 countries are involved in the construction of the largest tokamak ever designed, as part of the ITER project. The tokamak is an experimental machine designed to harness the energy of fusion. In the enclosure of a tokamak, the energy generated by the fusion of atomic nuclei is absorbed, in the form of heat, by the walls of the vacuum chamber. Like conventional power plants, a fusion power plant will use this heat to produce steam and then, through turbines and generators, electricity.

ITER aims to demonstrate that fusion — “the energy of the stars” — can be used as a large-scale, CO2-free source of energy to produce electricity. Its primary objective is the creation of a high temperature plasma providing the ideal environment for fusion to take place and produce energy. The results of the ITER scientific program will be decisive in opening the way to the power-generating fusion power plants of tomorrow.

As part of the continuous improvement of these reactors, EPLF physicists reveal that it is possible to use, in complete safety, a greater quantity of hydrogen, and thus obtain more energy than is possible. thought before. This revision of the Greenwald limit will be put into practice for tests in the ITER reactor when it is in operation. The new equation, updating this limit, is published in the journal Physical Review Letters.

A new limit for tokamaks, future producers of clean energy

Scientists have been working for more than 50 years on obtaining a viable controlled fusion. Unlike nuclear fission, which produces energy by breaking very large atomic nuclei, nuclear fusion could generate much more energy, by joining together very small nuclei. Additionally, the fusion process creates far less (almost no) radioactive waste than fission, and neutron-rich hydrogen for fuel is relatively easy to obtain.

As mentioned earlier, the nuclear reaction here is identical to that occurring within the Sun, using hydrogen atoms. However, on Earth, the pressure prevailing in the heart of a star is not reproducible. This pressure is needed to convert hydrogen into plasma — the medium in which hydrogen atoms can fuse together and generate energy. It is therefore necessary to bring the gases to a temperature 10 times higher than that of the Sun, that is to say about 150 million degrees Celsius.

As a result, in the heart of a tokamak, formed by a ring-shaped vacuum chamber, under the influence of extreme temperature and pressure, hydrogen gas turns into plasma. In the enclosure, the energy generated by the fusion of atomic nuclei is absorbed in the form of heat by the walls of the vacuum chamber. Very strong magnetic fields are used to confine and control the plasma.

Simplified section of the reactor with the ring-shaped vacuum chamber. © US ITER

Several fusion energy projects are now at an advanced stage. Nevertheless, ITER is not designed, basically, to produce electricity, but to test the production limits and define the exact conditions for carrying out such fusion reactions. However, ITER-based tokamaks, called DEMO reactors, are being designed and could operate by 2050 to generate electricity.

Paolo Ricci, from the Swiss Plasma Center (EPFL), explains in a press release: “ In order to produce a plasma for fusion, three elements must be taken into account: a high temperature, a high density of hydrogen and a good confinement. “. This is why one of the limits of plasma production in a tokamak is the amount of hydrogen that can be injected into it. Indeed, the higher the density, the more difficult it is to keep the plasma obtained stable.

More precisely, the more fuel is injected at the same temperature, the more certain parts of the plasma cool down, and the more difficult it is for the current to flow in the latter, thereby causing disturbances. Paolo Ricci explains in simple terms: “ We completely lose containment and the plasma goes anywhere. In the 1980s, we tried to find a kind of law allowing us to predict the maximum density of hydrogen that we can inject into a tokamak. “. It was discovered in 1988 by physicist Martin Greenwald, and establishes a correlation between the density of the fuel, the minor radius of the tokamak (the radius of the inner circle of the ring) and the current which circulates in the plasma inside. tokamak. So far, the experiments carried out with these machines have confirmed this “Greenwald limit”, which is at the heart of the ITER construction strategy.

Plasma history

Scientists have long suspected that the Greenwald limit could be improved. In order to test their hypothesis, in collaboration with teams from other tokamaks, the Swiss Plasma Center designed and conducted a revolutionary experiment, making it possible to use very sophisticated technology with the aim of precisely controlling the quantity of fuel injected into a tokamak. Massive experiments have been performed in the largest tokamaks in the world, the Joint European Torus (JET) in the UK, the ASDEX Upgrade in Germany (Max Planck Institute) and the TCV tokamak at EPFL.

At the same time, Maurizio Giacomin, a doctoral student in Paolo Ricci’s team, began to analyze the physical processes that limit density in tokamaks, in order to establish a fundamental law allowing the correlation of fuel density and tokamak size. Part of this work involved using an advanced plasma simulation performed using a computer model.

The key was the discovery that a plasma can support greater fuel density as the power output of a fusion reaction increases. In other words, tokamaks like ITER can effectively use almost twice the amount of fuel to produce plasmas, without fear of disturbances. Paolo Ricci says: This result is important because it shows that the density that can be achieved in a tokamak increases with the power required to operate it. DEMO will operate at significantly higher power than current tokamaks and ITER, which means that greater fuel density can be added without limiting production, contrary to what Greenwald’s law intended. And that’s very good news “.

Source: Physical Review Letters