American scientists have introduced a new type of spherical tokamak

National Spherical Torus Experiment (NSTX-U) (Photo: PPPL Office)

The Laboratory of Plasma Physics at Princeton University is now working on a new fusion reactor design. Scientists call their reactor a “star in a bank”. Moreover, the design of the "banks" is very important. From its form and a number of other parameters depends on the efficiency of the reactor.

Usually the central element of a tokamak looks like a donut. The new design of the National Spherical Torus Experiment (NSTX-U) reactor looks more like an apple. To date, the most advanced spherical tokamaks in the world are NSTX-U in the USA and MAST in the UK. According to the developers, the spherical tokamak allows nuclear fusion reaction to be carried out at a much lower energy cost than in the case of the traditional tokamak.

The fact is that the size of the hole in the center of a tokamak, where plasma is formed and held in spherical fusion reactors, can be two times smaller than a similar hole in a conventional reactor. In a spherical reactor, a high-pressure plasma is formed in a relatively weak magnetic field, which requires much less energy to create than is necessary in a standard analog.
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Tokamak (toroidal chamber with magnetic coils) is an installation for magnetic plasma confinement in order to achieve the conditions necessary for the flow of controlled thermonuclear fusion. The plasma in the tokamak is not held by the chamber walls , which are not able to withstand the temperature required for thermonuclear reactions, but by a specially created combined magnetic field — by the toroidal external and poloidal field of the current flowing through the plasma cord. Compared to other installations using a magnetic field to confine a plasma, the use of electric current is the main feature of a tokamak. The current in the plasma ensures that the plasma is heated and the plasma cord is kept in equilibrium in the vacuum chamber. This tokamak, in particular, differs from the stellarator , which is one of the alternative containment schemes in which both toroidal and poloidal fields are created using external magnetic coils.


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Scientists have published information about their project in the authoritative scientific publication Nuclear Fusion . So far, experts say, it’s too early to talk about creating a full-sized reactor that can be used for commercial purposes. These are only real fusion reactor models. But in such a model, the first energy can already be obtained.

In NSTX-U, materials that are considered suitable as structural elements of a fusion reactor will be tested. It is possible that a self-sustaining thermonuclear reaction can be obtained in the reactor. In the NSTX-U tokamak, tritium will be produced, which is also a fuel. Initially, a certain amount of the initial fuel (deuterium ( ² H) and tritium ( ³ H)) will need to be loaded into the reactor. As scientists hope, the reactor will be able to synthesize one atom of the same hydrogen isotope for each tritium atom consumed.

The special design of the tokamak will ensure the appearance of a special magnetic field. This, in turn, will help to avoid the negative effects of exposure to too high temperatures on the reactor. The various elements of a tokamak can be extracted independently for maintenance or repair. Technical work will be performed using robots, remotely.

Superconducting rings will replace copper in a new type of tokamak. Such a replacement will have both positive and negative effects. A positive point is the reduction of energy costs. Negative - additional protection of superconducting rings against temperature and radiation will be required. Because of this, you have to slightly increase the size of the installation. Thanks to the new high-temperature conductors, the current density in them can be raised. This means that the section of the magnets for the spherical tokamak will be reduced. In the central column it will be possible to place a blanket, which is responsible for the “reproduction” of tritium. American experts say that now the tritium reproduction rate will be increased to 1.04-1.1.


Normal donut tokamak (illustration: Wikimedia)

“NSTX-U and MAST-U will expand current horizons in physics, give new information about high-temperature plasma and bring the time of commercial installations closer,” says project leader Stewart Prager. He also argues that in the MAST reactor, the production of high-temperature high-density plasma will take place without problems and negative points for the installation itself. These two tokamaks are prototypes of the full-scale fusion nuclear fusion reactor, which is planned to be built in 15 years.

“The main reason for the search for a new tokamak design is the hope of conducting a nuclear fusion reaction with less resource expenditure than with standard tokamak,” says one of the project participants. It may well be that spherical tokamaks are the forerunners of full-fledged installations that will give humanity an inexhaustible source of energy. Perhaps the time when the “star in the bank” will start giving energy to humanity is near.