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Nuclear fusion is one of the best energy sources of tomorrow, especially in the context of the climate crisis. Physicists from the EPFL (Ecole Polytechnique Fédérale de Lausanne), in a large collaboration in Europe, recently revised one of the basic laws of nuclear fusion, called the “Greenwald limit”. For three decades, this law has been the basis of plasma and fusion research, even overseeing the design of megaprojects such as ITER (International Thermonuclear Experimental Reactor). The group of physicists found it possible to double the amount of hydrogen injected into a thermonuclear reactor to produce double the amount of energy. This discovery thus changes 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 fusion of two atomic nuclei into one, releasing a large amount of energy. This is the process that works within the Day. The heat thus comes from the mixing 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 power 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. As with conventional power plants, a fusion power plant uses this heat to generate steam and then, through turbines and generators, electricity.
ITER aims to show that fusion – “the energy of stars” – can be used as a large, CO2 -free energy source to generate electricity. Its main purpose is to create a high temperature plasma that provides an ideal environment for fusion to occur and generate energy. The results of the ITER scientific program could be decisive in opening the way to power-generating fusion power plants of tomorrow.
As part of the ongoing development of these reactors, EPLF physicists have revealed that it is possible to use, in complete safety, a greater amount of hydrogen, and thus obtain more energy than is possible. This Greenwald limit change will be implemented for ITER reactor tests if applicable. The new equation, which updates this limit, was published in the journal Physical Review of Letters.
A new limit for tokamaks, future producers of clean energy
Scientists have been working for more than 50 years to obtain a controlled fusion. Unlike nuclear fission, which produces energy by breaking up very large atomic nuclei, nuclear fusion can generate more energy, by fusing very small nuclei. In addition, the fusion process creates less (almost no) radioactive waste than fission, and neutron-rich hydrogen for fuel is easily available.
As already mentioned, the nuclear reaction here is similar to what happened during the Sun, using hydrogen atoms. However, on Earth, the pressure that overwhelms a star’s heart cannot be changed. This pressure is needed to convert hydrogen into plasma-the medium in which 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, i.e. about 150 million degrees Celsius.
As a result, in the heart of a tokamak, which is formed in 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 mixing of atomic nuclei is absorbed in the form of heat by the walls of the vacuum chamber. Very strong magnetic fields are used to inhibit and control the plasma.
Many fusion energy projects are now in the advanced stage. However, ITER is not designed, primarily, to generate electricity, but to test production limits and define the exact conditions for performing such fusion reactions. However, ITER -based tokamaks, called DEMO reactors, are designed and could be operational by 2050 to generate electricity.
Paolo Ricci, from the Swiss Plasma Center (EPFL), explained in a press release: “ To produce a plasma for fusion, three elements must be considered: high temperature, high hydrogen density and good confinement. “. This is why one of the limitations of plasma production in a tokamak is the amount of hydrogen that can be injected into it. In fact, the higher the density, the harder it is to keep the solid obtained. of plasma.
More precisely, the more fuel injected at the same temperature, the more specific parts of the plasma will cool, and the more difficult the current will flow later, thus causing disturbances. Paolo Ricci explains in simple terms: “ The fluid is completely gone and the plasma goes everywhere. In the 1980s, we tried to find some kind of law that allowed us to predict the maximum hydrogen density we could inject into the tokamak. “. It was discovered in 1988 by the physicist Martin Greenwald, and established 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 circulating inside in plasma.tokamak.Currently, experiments performed with these machines confirm this “Greenwald limit”, which is at the heart of ITER’s construction strategy.
Scientists have long doubted that the Greenwald limit could be improved. To test their hypothesis, in collaboration with teams from other tokamaks, the Swiss Plasma Center designed and conducted a revolutionary experiment, enabling the use of highly sophisticated technology with the aim of accurately controlling the amount of fuel injected. in tokamak. Major experiments have been performed on the world’s largest tokamak, the Joint European Torus (JET) in the UK, the ASDEX Upgrade in Germany (Max Planck Institute) and the EPFL’s TCV tokamak.
At the same time, Maurizio Giacomin, a doctoral student on Paolo Ricci’s team, began to analyze the physical processes that limit the density of tokamaks, to establish a basic law that allows correlation of fuel density and tokamak size. Part of this work involved the use and advanced plasma simulation performed using a computer model.
The key is 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 by a tokamak increases the power required to operate it. The DEMO will operate at much higher power than current tokamaks and ITER, meaning that greater fuel density can be added without a production limit, contrary to what is intended by Greenwald’s law. And that’s very good news “.