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Nuclear fusion is one of the most promising sources of energy for the future, especially in the context of the climate crisis. Physicists from EPFL (Ecole Polytechnique Fédérale de Lausanne), in the framework of a broad European cooperation, have revised one of the fundamental laws of nuclear fusion, called the “Greenwald Limit”. For three decades, this law has been the basis for plasma and fusion research, even governing the design of giant projects such as the International Thermonuclear Experimental Reactor (ITER). The team of physicists found that it is possible to double the amount of hydrogen injected into a thermonuclear reactor to produce twice the energy. This discovery redraws the boundaries of fusion, when some experts think so first reactors For industrial use it will only be profitable from 2040 to 2050.
The nuclear fusion It involves the merging of two atomic nuclei into one nucleus, which leads to the release of large amounts of energy. This is the process that operates inside the sun. Thus the heat comes from the fusion of hydrogen nuclei into helium atoms, which are heavier.
In France, in the Bouches-du-Rhone department, 35 countries are involved in the construction of the largest tokamak ever built, as part of the ITER project. A tokamak is an experimental machine designed to harness the energy of fusion. In a tokamak container, the energy from the fusion of atomic nuclei, in the form of heat, is absorbed by the walls of the vacuum chamber. Like conventional power plants, a fusion power plant will use this heat to produce steam and then, thanks to turbines and generators, electricity.
ITER aims to demonstrate that fusion – ‘star 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 an ideal environment for fusion to produce energy. The results of the ITER scientific program will be crucial in opening the way for future fusion power plants.
As part of the continuous improvement of these reactors, EPLF physicists have revealed that it is possible to use more hydrogen, quite safely, and thus obtain more energy than was possible, as previously thought. This revision of the Greenwald limit will be put into practice for tests at the ITER reactor when it is operational. The new, updated equation for this limit is published in the journal Physical Review Letters.
A new frontier for tokamaks, future clean energy producers
Scientists have been working for more than 50 years to get controlled, viable fusion. Unlike nuclear fission, which produces energy by cracking very large atomic nuclei, nuclear fusion can generate much more energy, by joining very small nuclei together. Additionally, the fusion process produces less radioactive waste (almost no) than fission, which is rich in hydrogen. neutronsused as fuel, is relatively easy to obtain.
As mentioned earlier, the nuclear reaction here is identical to the reaction that takes place inside the Sun using hydrogen atoms. However, on Earth, the pressure prevailing in the star’s core is not repeatable. This pressure is necessary to turn hydrogen into plasma – the medium in which hydrogen atoms can fuse together and generate energy. Therefore, it is necessary to raise the temperature of gases 10 times higher than the temperature of the sun, that is, about 150 million degrees Celsius.
As a result, in the tokamak core, consisting of a ring-shaped vacuum chamber, under the influence of extreme temperature and pressure, hydrogen gas turns into plasma. In the enclosure, the energy from the fusion of atomic nuclei is absorbed in the form of heat by the walls of the vacuum chamber. Extremely strong magnetic fields are used to confine and control the plasma.
Many fusion energy projects are now up to the level advanced stage. However, ITER was not designed, primarily, to produce electricity, but to test production limits and determine the exact conditions for carrying out these fusion reactions. However, ITER-based tokamaks, called DEMO reactors, are being designed and could be operational by 2050 to generate electricity.
Paolo Ricci, of the Swiss Plasma Center (EPFL), explains in a communication : ” In order to produce fusion plasma, three elements must be considered: high temperature, high hydrogen density and good confinement. This is why one of the limits to plasma production in a tokamak is the amount of hydrogen that can be injected into it. In fact, the higher the density, the more difficult it is to maintain the stability of the plasma.
More precisely, the more fuel is injected at the same temperature, the higher the temperature of certain parts of the plasma, and the more difficult it is for current to flow in the latter, causing turbulence. Paolo Ricci explains in simple terms: We lose containment completely and the plasma goes nowhere. In the 1980s, we tried 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 established a relationship between the density of fuel, the small radius of the tokamak (the radius of the inner circle of the ring) and the current circulating in the plasma inside. Tokamak. So far, experiments with these machines have confirmed this The “Greenwald Limit”, which is at the heart of ITER’s creation strategy.
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, which made it possible to use highly advanced technology with the aim of precisely controlling the amount of fuel injected into the tokamak. Massive trials have been conducted at the world’s largest tokamaks, the Joint European Ring (JET) in the UK, the ASDEX upgrade in Germany (Max Planck Institute) and the TCV tokamak in EPFL.
At the same time, Maurizio Giacomen, a doctoral student on Paolo Ricci’s team, began analyzing the physical processes that limit density in tokamaks, in order to establish a fundamental law that allows linking fuel density and tokamak volume. Part of this work involved the use of advanced plasma simulations using a computer model.
The key was the discovery that plasmas could support a greater fuel density while increasing the energy output of a fusion reaction. In other words, a tokamak like ITER can effectively use nearly twice as much fuel to produce plasma, without fear of disruption. Paulo Ricci says: This result is important because it shows that the achievable density of a tokamak increases with the energy required to operate it. DEMO will operate at much higher power than current tokamaks and ITER, which means greater fuel density can be added without reducing production, contrary to what Greenwald’s law intended. This is very good news “.
source : Physical Review Letters
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