Nuclear fusion: Advances in magnetic prison

Le tokamak KSTAR en Corée du Sud, vu de l

Unlike nuclear fission, it breaks a heavier nucleus to form two lighter, fusions, a process that heals the hearts of the stars, combining the two light nuclei to form a heavier one. Repeating the conditions that lead to fusion is a challenge because these reactions take place under extreme pressures and temperatures, reaching hundreds of millions of degrees Celsius. Under such conditions, matter is controlled by a magnetic field in the form of a plasma, an ionized gas under the influence of heat, in which electrons and atomic nuclei rotate freely.

However, there are irregularities in the magnetic field that control the plasma, which are small enough to move the molten material, and then the fusion reaction disappears. A study published in the journal Physical examination letters Explains a promising method of compensating for these abuses.

Magnetic imprisonment, to control and maintain reactions

The method used by the researchers to induce the fusion reaction is to control the particles in the plasma by a magnetic field, particularly the use of Tocamax (Russian acronym for “Droid room with magnetic coils”). They are ring-shaped structures called tori, around which coils are installed to create a magnetic field.

Construction of the magnetic field-generating foilial coils at the “International Thermal-Nuclear Testing Reactor” (IDER) in Togolese, Saint-Paul-Les-Tourans, in the south of France. Credit: CLEMENT MAHOUDEAU / AFP

The idea is to keep the charged particles of plasma in closed paths, not too close to the center of the ring or too close to the outer walls. The use of the magnetic field, combined with the heat supply through the outer walls, makes it possible to heat the plasma to temperatures favorable for nuclear fusion and to keep it unrestricted. At the moment, research is focusing on the possibility of nuclear fusion as an industrial activity. Especially in terms of the magnetic configuration: this is thanks to the coils around the ring.

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The magnetic field is modified and deformed

But the path of the particles is practically not completely symmetrical. First, the number of coils around the toggle is limited: the resulting magnetic field, instead of being uniform and axial, is then modulated, and its modulation depends on the spacing between the coils. Catarach CEO scientists are working on this issue, including Xavier Corbett Science and the future On the issue of research around nuclear fusion.

In addition, for a reason Incomplete alignment of the coils forming the magnetic field, which has irregular deformations, which makes the plasma unstable. These alignment errors are very small, in the order of ten thousandths of the value of the magnetic field, but they are sufficient to produce a significant impact. In fact, slight alignment distortion causes variation in the magnetic field and creates instability in the plasma.

Finally, instability is generated in the plasma itself independently of magnetic field errors. To compensate for this intrinsic plasma instability, additional magnets are placed inside and outside the toxomas. “Their effect is most important when placed near molten plasma. “ Xavier Corbett, a researcher at the Institute of Research on Fusion Magnetic (IRFM), explains.

But these magnets add other effects: as they stabilize the plasma, they slow down. This breaking can then lead to the appearance of new instabilities, but also the release of particles from the plasma, especially alpha particles (helium nuclei). These are necessary to maintain plasma temperature, so their loss can lead to a drop in reactivity. Researchers at the Plasma Physics Laboratory (PPPL) in Princeton, the US Department of Energy (DOE), worked on this problem.

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Semi-symmetrical, to eliminate distortions without side effects

To avoid these side effects of additional magnets, the Princeton team of researchers found that semi-symmetrical, a type of magnetic disturbance that alters the helical path of plasma particles. This pathway, although it does not strictly speak of “symmetry”, also controls plasma.

The researchers’ technique is to create additional barriers that compensate for the original errors. “One way to preserve their rotation while ensuring stability is to change the shape of the magnetic field so that the particles act as if there is no magnetic field distortion.”, declares in a Communication PPPL physicist Jong-Qi Park, principal author of the study.

To do this, the researchers first performed a flow measurement within the tocopherols. Thanks to this measurement, they found out what the magnetic field errors were and then calculated the field to be used to compensate for them. “We need to create an almost symmetrical 3D magnetic field within the plasma to mislead the particles and act as if they are not affected by the field.”, Continue to Park.

Interruptions in the first test phase

They then used coils already placed in the tocos to create “semi-symmetric” magnetic confusions there: they did not create symmetry around an axis, but made it possible to control the plasma. Tests were carried out at DIII-D Tokamax in San Diego and KSTAR in South Korea, during which they observed whether there was a braking effect similar to other magnetic disturbances.

The results were exciting: no breaking effect or particle loss! Process “Provides a reliable way to fully enhance magnetic field errors in fusion combustion plasmas”, According to the article. “This work may be the basis for the development of future control strategies for these sectors.”, Explained Boss-Solden, co-author and physicist of the Tokamak DIII-T publication.

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It remains to be seen whether these disturbances perform their primary function, which is to observe the instability of the plasma. Finally, “Last Step”, Park said, this concept should be applied to ITER, “This will allow you to fix the error fields in this Tokamak.”

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