the power of the reactors could be doubled (after the revision of a basic law)

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Nuclear fusion is one of tomorrow’s most promising sources of energy, especially in the context of the climate crisis. Physicists from the EPFL (Ecole Polytechnique Fédérale de Lausanne) have recently revised one of the fundamental laws of nuclear fusion, called the “Greenwald border”, in the context of extensive European cooperation. For three decades, this law has been the basis of plasma and fusion research, even regulating the design of mega-projects such as 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 combination of two atomic nuclei into one, releasing large amounts of energy. It is this process that is at work in 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. Tokamak is an experimental machine designed to harness fusion energy. In the confinement 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 – “stellar energy” – can be used as a large-scale, CO2-free energy source to produce electricity. Its primary goal is to create a high temperature plasma that provides the ideal environment for fusion and energy production. The results of the ITER scientific program will be crucial in paving the way for tomorrow’s electricity-producing fusion power plants.

As part of the continuous improvement of these reactors, EPLF physicists are revealing that it is possible to use a larger amount of hydrogen in full safety, thus obtaining more energy than is possible. This revision of the Greenwald boundary will be carried out in practice for testing in the ITER reactor when in operation. The new equation that updates this limit has been published in the journal Physical review letters.

A new frontier for tokamaks, future producers of clean energy

Researchers have been working for more than 50 years to achieve a viable controlled fusion. Unlike nuclear fission, which produces energy by breaking very large atomic nuclei, nuclear fusion could generate much more energy by connecting very small nuclei. In addition, 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 which occurs in the Sun, by using hydrogen atoms. On Earth, however, the pressure that prevails in the heart of a star is not reproducible. This pressure is necessary to convert hydrogen into the plasma medium, where hydrogen atoms can fuse 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 an annular vacuum chamber, under the influence of extreme temperature and pressure, hydrogen gas becomes 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 limit and control the plasma.

Simplified section of the reactor with the annular vacuum chamber. © US ITER

Several fusion energy projects are now at an advanced stage. Nevertheless, ITER is basically not designed to produce electricity, but to test the production limits and define the exact conditions for performing such fusion reactions. However, ITER-based tokamaks, called DEMO reactors, are being designed and able to operate in 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 containment. “. This is why one of the limits of plasma production in a tokamak is the amount of hydrogen that can be injected into it. The higher the density, the harder it is to actually keep the plasma obtained stable.

More precisely, the more fuel 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 interference. Paolo Ricci explains in simple terms: “ We completely lose the containment and the plasma goes everywhere. In the 1980s, we were trying to find some kind of law that would allow us to predict the maximum density of hydrogen that we could inject into a tokamak. “. It was discovered in 1988 by physicist Martin Greenwald, and establishes a relationship between the density of the fuel, the smaller radius of the tokamak (radius of the inner circle of the ring) and the current circulating in the plasma inside the tokamak. So far, the experiments have performed with these machines confirmed this “Greenwald boundary”, which is the core of the ITER design strategy.

Plasma history

Researchers have long suspected that the Greenwald border could be improved. To test their hypothesis, in collaboration with teams from other tokamaks, the Swiss Plasma Center designed and conducted a revolutionary experiment that made it possible to use highly sophisticated technology with the aim of precisely controlling the amount 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 PhD student in Paolo Ricci’s team, began analyzing the physical processes that limit the density of tokamaks, in order to establish a basic law that allows the correlation between 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 effect of a fusion reaction increases. In other words, tokamaks like ITER can efficiently use almost twice as much fuel to produce plasma without fear of interference. Paolo Ricci says: This result is important because it shows that the density that can be achieved in a tokamak increases with the force required to operate it. DEMO will operate at significantly higher power than current tokamaks and ITER, which means that higher 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

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