Recently, the Moscow Institute of Physics and Technology hosted a Russian presentation of the ITER project, within which it is planned to create a thermonuclear reactor operating on the tokamak principle. A group of scientists from Russia spoke about the international project and the participation of Russian physicists in the creation of this object. Lenta.ru attended the ITER presentation and spoke with one of the project participants.

ITER (ITER, International Thermonuclear Experimental Reactor) is a thermonuclear reactor project that allows the demonstration and research of thermonuclear technologies for their further use for peaceful and commercial purposes. The creators of the project believe that controlled thermonuclear fusion can become the energy of the future and serve as an alternative to modern gas, oil and coal. Researchers note the safety, environmental friendliness and accessibility of ITER technology compared to conventional energy. The complexity of the project is comparable to the Large Hadron Collider; The reactor installation includes more than ten million structural elements.

About ITER

Tokamak toroidal magnets require 80 thousand kilometers of superconducting filaments; their total weight reaches 400 tons. The reactor itself will weigh about 23 thousand tons. For comparison, the weight of the Eiffel Tower in Paris is only 7.3 thousand tons. The volume of plasma in the tokamak will reach 840 cubic meters, while, for example, in the largest reactor of this type operating in the UK - JET - the volume is equal to one hundred cubic meters.

The height of the tokamak will be 73 meters, of which 60 meters will be above the ground and 13 meters below it. For comparison, the height of the Spasskaya Tower of the Moscow Kremlin is 71 meters. The main reactor platform will cover an area of ​​42 hectares, which is comparable to the area of ​​60 football fields. The temperature in the tokamak plasma will reach 150 million degrees Celsius, which is ten times higher than the temperature at the center of the Sun.

In the construction of ITER in the second half of 2010, it is planned to involve up to five thousand people simultaneously - this will include both workers and engineers, as well as administrative personnel. Many of ITER's components will be transported from the port near the Mediterranean Sea along a specially constructed road approximately 104 kilometers long. In particular, the heaviest fragment of the installation will be transported along it, the mass of which will be more than 900 tons, and the length will be about ten meters. More than 2.5 million cubic meters of earth will be removed from the construction site of the ITER installation.

The total cost of design and construction works is estimated at 13 billion euros. These funds are allocated by seven main project participants representing the interests of 35 countries. For comparison, the total costs of building and maintaining the Large Hadron Collider are almost half as much, and building and maintaining the International Space Station costs almost one and a half times more.

Tokamak

Today in the world there are two promising projects of thermonuclear reactors: tokamak ( That roidal ka measure with ma rotten To atushki) and stellarator. In both installations, the plasma is contained by a magnetic field, but in a tokamak it is in the form of a toroidal cord through which an electric current is passed, while in a stellarator the magnetic field is induced by external coils. In thermonuclear reactors, reactions of synthesis of heavy elements from light ones (helium from hydrogen isotopes - deuterium and tritium) occur, in contrast to conventional reactors, where the processes of decay of heavy nuclei into lighter ones are initiated.

Photo: National Research Center “Kurchatov Institute” / nrcki.ru

The electric current in the tokamak is also used to initially heat the plasma to a temperature of about 30 million degrees Celsius; further heating is carried out by special devices.

The theoretical design of a tokamak was proposed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm, and the first installation was built in the USSR in 1954. However, scientists were unable to maintain the plasma in a steady state for a long time, and by the mid-1960s the world was convinced that controlled thermonuclear fusion based on a tokamak was impossible.

But just three years later, at the T-3 installation at the Kurchatov Institute of Atomic Energy, under the leadership of Lev Artsimovich, it was possible to heat the plasma to a temperature of more than five million degrees Celsius and hold it for a short time; Scientists from Great Britain who were present at the experiment recorded a temperature of about ten million degrees on their equipment. After this, a real tokamak boom began in the world, so that about 300 installations were built in the world, the largest of which are located in Europe, Japan, the USA and Russia.

Image: Rfassbind/ wikipedia.org

ITER Management

What is the basis for confidence that ITER will be operational in 5-10 years? On what practical and theoretical developments?

On the Russian side, we are fulfilling the stated work schedule and are not going to violate it. Unfortunately, we see some delays in the work being carried out by others, mainly in Europe; There is a partial delay in America and there is a tendency that the project will be somewhat delayed. Detained but not stopped. There is confidence that it will work. The concept of the project itself is completely theoretical and practically calculated and reliable, so I think it will work. Whether it will fully give the declared results... we'll wait and see.

Is the project more of a research project?

Certainly. The stated result is not the obtained result. If it is received in full, I will be extremely happy.

What new technologies have appeared, are appearing or will appear in the ITER project?

The ITER project is not just a super-complex, but also a super-stressful project. Stressful in terms of energy load, operating conditions of certain elements, including our systems. Therefore, new technologies simply must be born in this project.

Is there an example?

Space. For example, our diamond detectors. We discussed the possibility of using our diamond detectors on space trucks, which are nuclear vehicles that transport certain objects such as satellites or stations from orbit to orbit. There is such a project for a space truck. Since this is a device with a nuclear reactor on board, complex operating conditions require analysis and control, so our detectors could easily do this. At the moment, the topic of creating such diagnostics is not yet funded. If it is created, it can be applied, and then there will be no need to invest money in it at the development stage, but only at the development and implementation stage.

What is the share of modern Russian developments of the 2000s and 1990s in comparison with Soviet and Western developments?

The share of Russian scientific contribution to ITER compared to the global one is very large. I don't know it exactly, but it is very significant. It is clearly no less than the Russian percentage of financial participation in the project, because in many other teams there are a large number of Russians who went abroad to work in other institutes. In Japan and America, everywhere, we communicate and work with them very well, some of them represent Europe, some represent America. In addition, there are also scientific schools there. Therefore, about whether we are developing more or more what we did before... One of the greats said that “we stand on the shoulders of titans,” therefore the base that was developed in Soviet times is undeniably great and without it we are nothing we couldn't. But even at the moment we are not standing still, we are moving.

What exactly does your group do at ITER?

I have a sector in the department. The department is developing several diagnostics; our sector is specifically developing a vertical neutron chamber, ITER neutron diagnostics and solves a wide range of problems from design to manufacturing, as well as carrying out related research work related to the development, in particular, of diamond detectors. The diamond detector is a unique device, originally created in our laboratory. Previously used in many thermonuclear installations, it is now used quite widely by many laboratories from America to Japan; they, let's say, followed us, but we continue to remain on top. We are now making diamond detectors and are going to reach the level of industrial production (small-scale production).

What industries can these detectors be used in?

In this case, these are thermonuclear research; in the future, we assume that they will be in demand in nuclear energy.

What exactly do detectors do, what do they measure?

Neutrons. There is no more valuable product than the neutron. You and I also consist of neutrons.

What characteristics of neutrons do they measure?

Spectral. Firstly, the immediate task that is solved at ITER is the measurement of neutron energy spectra. In addition, they monitor the number and energy of neutrons. The second, additional task concerns nuclear energy: we have parallel developments that can also measure thermal neutrons, which are the basis of nuclear reactors. This is a secondary task for us, but it is also being developed, that is, we can work here and at the same time make developments that can be quite successfully applied in nuclear energy.

What methods do you use in your research: theoretical, practical, computer modeling?

Everyone: from complex mathematics (methods of mathematical physics) and mathematical modeling to experiments. All the different types of calculations that we carry out are confirmed and verified by experiments, because we directly have an experimental laboratory with several operating neutron generators, on which we test the systems that we ourselves develop.

Do you have a working reactor in your laboratory?

Not a reactor, but a neutron generator. A neutron generator is, in fact, a mini-model of the thermonuclear reactions in question. Everything is the same there, only the process there is slightly different. It works on the principle of an accelerator - it is a beam of certain ions that hits a target. That is, in the case of plasma, we have a hot object in which each atom has high energy, and in our case, a specially accelerated ion hits a target saturated with similar ions. Accordingly, a reaction occurs. Let's just say this is one way you can do the same fusion reaction; the only thing that has been proven is that this method does not have high efficiency, that is, you will not get a positive energy output, but you get the reaction itself - we directly observe this reaction and the particles and everything that goes into it.

The second half of the 20th century was a period of rapid development of nuclear physics. It became clear that nuclear reactions could be used to produce enormous energy from tiny amounts of fuel. Only nine years passed from the explosion of the first nuclear bomb to the first nuclear power plant, and when a hydrogen bomb was tested in 1952, there were predictions that thermonuclear power plants would come into operation in the 1960s. Alas, these hopes were not justified.

Thermonuclear reactions Of all the thermonuclear reactions, only four are of interest in the near future: deuterium + deuterium (products - tritium and proton, released energy 4.0 MeV), deuterium + deuterium (helium-3 and neutron, 3.3 MeV), deuterium + tritium (helium-4 and neutron, 17.6 MeV) and deuterium + helium-3 (helium-4 and proton, 18.2 MeV). The first and second reactions occur in parallel with equal probability. The resulting tritium and helium-3 “burn” in the third and fourth reactions

The main source of energy for humanity today is the combustion of coal, oil and gas. But their supplies are limited, and combustion products pollute the environment. A coal power plant produces more radioactive emissions than a nuclear power plant of the same power! So why haven't we switched to nuclear energy sources yet? There are many reasons for this, but the main one recently has been radiophobia. Despite the fact that a coal-fired power plant, even during normal operation, harms the health of many more people than emergency emissions at a nuclear power plant, it does so quietly and unnoticed by the public. Accidents at nuclear power plants immediately become the main news in the media, causing general panic (often completely unfounded). However, this does not mean that nuclear energy does not have objective problems. Radioactive waste causes a lot of trouble: technologies for working with it are still extremely expensive, and the ideal situation when all of it will be completely recycled and used is still far away.

Of all the thermonuclear reactions, only four are of interest in the near future: deuterium + deuterium (products - tritium and proton, released energy 4.0 MeV), deuterium + deuterium (helium-3 and neutron, 3.3 MeV), deuterium + tritium (helium -4 and neutron, 17.6 MeV) and deuterium + helium-3 (helium-4 and proton, 18.2 MeV). The first and second reactions occur in parallel with equal probability. The resulting tritium and helium-3 “burn” in the third and fourth reactions.

From fission to fusion

A potential solution to these problems is the transition from fission reactors to fusion reactors. While a typical fission reactor contains tens of tons of radioactive fuel, which is converted into tens of tons of radioactive waste containing a wide variety of radioactive isotopes, a fusion reactor uses only hundreds of grams, maximum kilograms, of one radioactive isotope of hydrogen, tritium. In addition to the fact that the reaction requires an insignificant amount of this least dangerous radioactive isotope, its production is also planned to be carried out directly at the power plant in order to minimize the risks associated with transportation. The synthesis products are stable (non-radioactive) and non-toxic hydrogen and helium. In addition, unlike a fission reaction, a thermonuclear reaction immediately stops when the installation is destroyed, without creating the danger of a thermal explosion. So why has not a single operational thermonuclear power plant been built yet? The reason is that the listed advantages inevitably entail disadvantages: creating the conditions for synthesis turned out to be much more difficult than initially expected.

Lawson criterion

For a thermonuclear reaction to be energetically favorable, it is necessary to ensure a sufficiently high temperature of the thermonuclear fuel, a sufficiently high density and sufficiently low energy losses. The latter are numerically characterized by the so-called “retention time”, which is equal to the ratio of the thermal energy stored in the plasma to the energy loss power (many people mistakenly believe that the “retention time” is the time during which hot plasma is maintained in the installation, but this is not so) . At a temperature of a mixture of deuterium and tritium equal to 10 keV (approximately 110,000,000 degrees), we need to obtain the product of the number of fuel particles in 1 cm 3 (i.e., plasma concentration) and the retention time (in seconds) of at least 10 14. It does not matter whether we have a plasma with a concentration of 1014 cm -3 and a retention time of 1 s, or a plasma with a concentration of 10 23 and a retention time of 1 ns. This criterion is called the Lawson criterion.
In addition to the Lawson criterion, which is responsible for obtaining an energetically favorable reaction, there is also a plasma ignition criterion, which for the deuterium-tritium reaction is approximately three times greater than the Lawson criterion. “Ignition” means that the fraction of thermonuclear energy that remains in the plasma will be enough to maintain the required temperature, and additional heating of the plasma will no longer be required.

Z-pinch

The first device in which it was planned to obtain a controlled thermonuclear reaction was the so-called Z-pinch. In the simplest case, this installation consists of only two electrodes located in a deuterium (hydrogen-2) environment or a mixture of deuterium and tritium, and a battery of high-voltage pulse capacitors. At first glance, it seems that it makes it possible to obtain compressed plasma heated to enormous temperatures: exactly what is needed for a thermonuclear reaction! However, in life, everything turned out, alas, to be far from so rosy. The plasma rope turned out to be unstable: the slightest bend leads to a strengthening of the magnetic field on one side and a weakening on the other; the resulting forces further increase the bending of the rope - and all the plasma “falls out” onto the side wall of the chamber. The rope is not only unstable to bending, the slightest thinning of it leads to an increase in the magnetic field in this part, which compresses the plasma even more, squeezing it into the remaining volume of the rope until the rope is finally “squeezed out.” The compressed part has a high electrical resistance, so the current is interrupted, the magnetic field disappears, and all the plasma dissipates.

The principle of operation of the Z-pinch is simple: an electric current generates an annular magnetic field, which interacts with the same current and compresses it. As a result, the density and temperature of the plasma through which the current flows increases.

It was possible to stabilize the plasma bundle by applying a powerful external magnetic field to it, parallel to the current, and placing it in a thick conductive casing (as the plasma moves, the magnetic field also moves, which induces an electric current in the casing, tending to return the plasma to its place). The plasma stopped bending and pinching, but it was still far from a thermonuclear reaction on any serious scale: the plasma touches the electrodes and gives off its heat to them.

Modern work in the field of Z-pinch fusion suggests another principle for creating fusion plasma: a current flows through a tungsten plasma tube, which creates powerful X-rays that compress and heat the capsule with fusion fuel located inside the plasma tube, just as it does in a thermonuclear bomb. However, these works are purely research in nature (the mechanisms of operation of nuclear weapons are studied), and the energy release in this process is still millions of times less than consumption.

The smaller the ratio of the large radius of the tokamak torus (the distance from the center of the entire torus to the center of the cross-section of its pipe) to the small one (the cross-section radius of the pipe), the greater the plasma pressure can be under the same magnetic field. By reducing this ratio, scientists moved from a circular cross-section of the plasma and vacuum chamber to a D-shaped one (in this case, the role of the small radius is played by half the height of the cross-section). All modern tokamaks have exactly this cross-sectional shape. The limiting case was the so-called “spherical tokamak”. In such tokamaks, the vacuum chamber and plasma are almost spherical in shape, with the exception of a narrow channel connecting the poles of the sphere. The conductors of magnetic coils pass through the channel. The first spherical tokamak, START, appeared only in 1991, so this is a fairly young direction, but it has already shown the possibility of obtaining the same plasma pressure with a three times lower magnetic field.

Cork chamber, stellarator, tokamak

Another option for creating the conditions necessary for the reaction is the so-called open magnetic traps. The most famous of them is the “cork cell”: a pipe with a longitudinal magnetic field that strengthens at its ends and weakens in the middle. The field increased at the ends creates a “magnetic plug” (hence the Russian name), or “magnetic mirror” (English - mirror machine), which keeps the plasma from leaving the installation through the ends. However, such retention is incomplete; some charged particles moving along certain trajectories are able to pass through these jams. And as a result of collisions, any particle will sooner or later fall on such a trajectory. In addition, the plasma in the mirror chamber also turned out to be unstable: if in some place a small section of the plasma moves away from the axis of the installation, forces arise that eject the plasma onto the chamber wall. Although the basic idea of ​​the mirror cell was significantly improved (which made it possible to reduce both the instability of the plasma and the permeability of the mirrors), in practice it was not even possible to approach the parameters necessary for energetically favorable synthesis.

Is it possible to make sure that the plasma does not escape through the “plugs”? It would seem that the obvious solution is to roll the plasma into a ring. However, then the magnetic field inside the ring is stronger than outside, and the plasma again tends to go to the chamber wall. The way out of this difficult situation also seemed quite obvious: instead of a ring, make a “figure eight”, then in one section the particle will move away from the axis of the installation, and in another it will return back. This is how scientists came up with the idea of ​​the first stellarator. But such a “figure eight” cannot be made in one plane, so we had to use the third dimension, bending the magnetic field in the second direction, which also led to a gradual movement of the particles from the axis to the chamber wall.

The situation changed dramatically with the creation of tokamak-type installations. The results obtained at the T-3 tokamak in the second half of the 1960s were so stunning for that time that Western scientists came to the USSR with their measuring equipment to verify the plasma parameters themselves. The reality even exceeded their expectations.

These fantastically intertwined tubes are not an art project, but a stellarator chamber bent into a complex three-dimensional curve.

In the hands of inertia

In addition to magnetic confinement, there is a fundamentally different approach to thermonuclear fusion - inertial confinement. If in the first case we try to keep the plasma at a very low concentration for a long time (the concentration of molecules in the air around you is hundreds of thousands of times higher), then in the second case we compress the plasma to a huge density, an order of magnitude higher than the density of the heaviest metals, in the expectation that the reaction will have time to pass in that short time before the plasma has time to scatter to the sides.

Originally, in the 1960s, the plan was to use a small ball of frozen fusion fuel, uniformly irradiated from all sides by multiple laser beams. The surface of the ball should have instantly evaporated and, expanding evenly in all directions, compressed and heated the remaining part of the fuel. However, in practice, the irradiation turned out to be insufficiently uniform. In addition, part of the radiation energy was transferred to the inner layers, causing them to heat up, which made compression more difficult. As a result, the ball compressed unevenly and weakly.

There are a number of modern stellarator configurations, all of which are close to a torus. One of the most common configurations involves the use of coils similar to the poloidal field coils of tokamaks, and four to six conductors twisted around a vacuum chamber with multidirectional current. The complex magnetic field created in this way allows the plasma to be reliably contained without requiring a ring electric current to flow through it. In addition, stellarators can also use toroidal field coils, like tokamaks. And there may be no helical conductors, but then the “toroidal” field coils are installed along a complex three-dimensional curve. Recent developments in the field of stellarators involve the use of magnetic coils and a vacuum chamber of a very complex shape (a very “crumpled” torus), calculated on a computer.

The problem of unevenness was solved by significantly changing the design of the target. Now the ball is placed inside a special small metal chamber (it is called “holraum”, from the German hohlraum - cavity) with holes through which laser beams enter inside. In addition, crystals are used that convert IR laser radiation into ultraviolet. This UV radiation is absorbed by a thin layer of hohlraum material, which is heated to enormous temperatures and emits soft X-rays. In turn, X-ray radiation is absorbed by a thin layer on the surface of the fuel capsule (ball with fuel). This also made it possible to solve the problem of premature heating of the internal layers.

However, the power of the lasers turned out to be insufficient for a noticeable portion of the fuel to react. In addition, the efficiency of the lasers was very low, only about 1%. For fusion to be energetically beneficial at such a low laser efficiency, almost all of the compressed fuel had to react. When trying to replace lasers with beams of light or heavy ions, which can be generated with much greater efficiency, scientists also encountered a lot of problems: light ions repel each other, which prevents them from focusing, and are slowed down when colliding with residual gas in the chamber, and accelerators It was not possible to create heavy ions with the required parameters.

Magnetic prospects

Most of the hope in the field of fusion energy now lies in tokamaks. Especially after they opened a mode with improved retention. A tokamak is both a Z-pinch rolled into a ring (a ring electric current flows through the plasma, creating a magnetic field necessary to contain it), and a sequence of mirror cells assembled into a ring and creating a “corrugated” toroidal magnetic field. In addition, a field perpendicular to the torus plane, created by several individual coils, is superimposed on the toroidal field of the coils and the plasma current field. This additional field, called poloidal, strengthens the magnetic field of the plasma current (also poloidal) on the outside of the torus and weakens it on the inside. Thus, the total magnetic field on all sides of the plasma rope turns out to be the same, and its position remains stable. By changing this additional field, it is possible to move the plasma bundle inside the vacuum chamber within certain limits.

A fundamentally different approach to synthesis is proposed by the concept of muon catalysis. A muon is an unstable elementary particle that has the same charge as an electron, but 207 times more mass. A muon can replace an electron in a hydrogen atom, and the size of the atom decreases by a factor of 207. This allows one hydrogen nucleus to move closer to another without expending energy. But to produce one muon, about 10 GeV of energy is spent, which means it is necessary to perform several thousand fusion reactions per muon to obtain energy benefits. Due to the possibility of a muon “sticking” to the helium formed in the reaction, more than several hundred reactions have not yet been achieved. The photo shows the assembly of the Wendelstein z-x stellarator at the Max Planck Institute for Plasma Physics.

An important problem of tokamaks for a long time was the need to create a ring current in the plasma. To do this, a magnetic circuit was passed through the central hole of the tokamak torus, the magnetic flux in which was continuously changed. The change in magnetic flux generates a vortex electric field, which ionizes the gas in the vacuum chamber and maintains current in the resulting plasma. However, the current in the plasma must be maintained continuously, which means that the magnetic flux must continuously change in one direction. This, of course, is impossible, so the current in tokamaks could only be maintained for a limited time (from a fraction of a second to several seconds). Fortunately, the so-called bootstrap current was discovered, which occurs in a plasma without an external vortex field. In addition, methods have been developed to heat the plasma, simultaneously inducing the necessary ring current in it. Together, this provided the potential for maintaining hot plasma for as long as desired. In practice, the record currently belongs to the Tore Supra tokamak, where the plasma continuously “burned” for more than six minutes.

The second type of plasma confinement installation, which has great promise, is stellarators. Over the past decades, the design of stellarators has changed dramatically. Almost nothing remained of the original “eight”, and these installations became much closer to tokamaks. Although the confinement time of stellarators is shorter than that of tokamaks (due to the less efficient H-mode), and the cost of their construction is higher, the behavior of the plasma in them is calmer, which means a longer life of the first inner wall of the vacuum chamber. For the commercial development of thermonuclear fusion, this factor is of great importance.

Selecting a reaction

At first glance, it is most logical to use pure deuterium as a thermonuclear fuel: it is relatively cheap and safe. However, deuterium reacts with deuterium a hundred times less readily than with tritium. This means that to operate a reactor on a mixture of deuterium and tritium, a temperature of 10 keV is sufficient, and to operate on pure deuterium, a temperature of more than 50 keV is required. And the higher the temperature, the higher the energy loss. Therefore, at least for the first time, thermonuclear energy is planned to be built on deuterium-tritium fuel. Tritium will be produced in the reactor itself due to irradiation with the fast lithium neutrons produced in it.
"Wrong" neutrons. In the cult film “9 Days of One Year,” the main character, while working at a thermonuclear installation, received a serious dose of neutron radiation. However, it later turned out that these neutrons were not produced as a result of a fusion reaction. This is not the director’s invention, but a real effect observed in Z-pinches. At the moment of interruption of the electric current, the inductance of the plasma leads to the generation of a huge voltage - millions of volts. Individual hydrogen ions, accelerated in this field, are capable of literally knocking neutrons out of the electrodes. At first, this phenomenon was indeed taken as a sure sign of a thermonuclear reaction, but subsequent analysis of the neutron energy spectrum showed that they had a different origin.
Improved retention mode. The H-mode of a tokamak is a mode of its operation when, with a high power of additional heating, plasma energy losses sharply decrease. The accidental discovery of the enhanced confinement mode in 1982 is as significant as the invention of the tokamak itself. There is no generally accepted theory of this phenomenon yet, but this does not prevent it from being used in practice. All modern tokamaks operate in this mode, as it reduces losses by more than half. Subsequently, a similar regime was discovered in stellarators, indicating that this is a general property of toroidal systems, but confinement is only improved by about 30% in them.
Plasma heating. There are three main methods of heating plasma to thermonuclear temperatures. Ohmic heating is the heating of plasma due to the flow of electric current through it. This method is most effective in the first stages, since as the temperature increases, the electrical resistance of the plasma decreases. Electromagnetic heating uses electromagnetic waves with a frequency that matches the frequency of rotation around the magnetic field lines of electrons or ions. By injecting fast neutral atoms, a stream of negative ions is created, which are then neutralized, turning into neutral atoms that can pass through the magnetic field to the center of the plasma to transfer their energy there.
Are these reactors? Tritium is radioactive, and powerful neutron irradiation from the D-T reaction creates induced radioactivity in the reactor design elements. We have to use robots, which complicates the work. At the same time, the behavior of a plasma of ordinary hydrogen or deuterium is very close to the behavior of a plasma from a mixture of deuterium and tritium. This led to the fact that throughout history, only two thermonuclear installations fully operated on a mixture of deuterium and tritium: the TFTR and JET tokamaks. At other installations, even deuterium is not always used. So the name “thermonuclear” in the definition of a facility does not mean at all that thermonuclear reactions have ever actually occurred in it (and in those that do occur, pure deuterium is almost always used).
Hybrid reactor. The D-T reaction produces 14 MeV neutrons, which can even fission depleted uranium. The fission of one uranium nucleus is accompanied by the release of approximately 200 MeV of energy, which is more than ten times the energy released during fusion. So existing tokamaks could become energetically beneficial if they were surrounded by a uranium shell. Compared to fission reactors, such hybrid reactors would have the advantage of preventing an uncontrolled chain reaction from developing in them. In addition, extremely intense neutron fluxes should convert long-lived uranium fission products into short-lived ones, which significantly reduces the problem of waste disposal.

Inertial hopes

Inertial fusion is also not standing still. Over the decades of development of laser technology, prospects have emerged to increase the efficiency of lasers by approximately ten times. And in practice, their power has been increased hundreds and thousands of times. Work is also underway on heavy ion accelerators with parameters suitable for thermonuclear use. In addition, the concept of “fast ignition” has been a critical factor in the progress of inertial fusion. It involves the use of two pulses: one compresses the thermonuclear fuel, and the other heats up a small part of it. It is assumed that the reaction that begins in a small part of the fuel will subsequently spread further and cover the entire fuel. This approach makes it possible to significantly reduce energy costs, and therefore make the reaction profitable with a smaller fraction of reacted fuel.

Tokamak problems

Despite the progress of installations of other types, tokamaks at the moment still remain out of competition: if two tokamaks (TFTR and JET) back in the 1990s actually produced a release of thermonuclear energy approximately equal to the energy consumption for heating the plasma (even though such a mode lasted only about a second), then nothing similar could be achieved with other types of installations. Even a simple increase in the size of tokamaks will lead to the feasibility of energetically favorable fusion in them. The international reactor ITER is currently being built in France, which will have to demonstrate this in practice.

However, tokamaks also have problems. ITER costs billions of dollars, which is unacceptable for future commercial reactors. No reactor has operated continuously for even a few hours, let alone for weeks and months, which again is necessary for industrial applications. There is no certainty yet that the materials of the inner wall of the vacuum chamber will be able to withstand prolonged exposure to plasma.

The concept of a tokamak with a strong field can make the project less expensive. By increasing the field by two to three times, it is planned to obtain the required plasma parameters in a relatively small installation. This concept, in particular, is the basis for the Ignitor reactor, which, together with Italian colleagues, is now beginning to be built at TRINIT (Trinity Institute for Innovation and Thermonuclear Research) near Moscow. If the engineers’ calculations come true, then at a cost many times lower than ITER, it will be possible to ignite plasma in this reactor.

Forward to the stars!

The products of a thermonuclear reaction fly away in different directions at speeds of thousands of kilometers per second. This makes it possible to create ultra-efficient rocket engines. Their specific impulse will be higher than that of the best electric jet engines, and their energy consumption may even be negative (theoretically, it is possible to generate, rather than consume, energy). Moreover, there is every reason to believe that making a thermonuclear rocket engine will be even easier than a ground-based reactor: there is no problem with creating a vacuum, with thermal insulation of superconducting magnets, there are no restrictions on dimensions, etc. In addition, the generation of electricity by the engine is desirable, but It’s not at all necessary, it’s enough that he doesn’t consume too much of it.

Electrostatic confinement

The concept of electrostatic ion confinement is most easily understood through a setup called a fusor. It is based on a spherical mesh electrode, to which a negative potential is applied. Ions accelerated in a separate accelerator or by the field of the central electrode itself fall inside it and are held there by an electrostatic field: if an ion tends to fly out, the electrode field turns it back. Unfortunately, the probability of an ion colliding with a network is many orders of magnitude higher than the probability of entering into a fusion reaction, which makes an energetically favorable reaction impossible. Such installations have found application only as neutron sources.
In an effort to make a sensational discovery, many scientists strive to see synthesis wherever possible. There have been numerous reports in the press regarding various options for so-called “cold fusion.” Synthesis was discovered in metals “impregnated” with deuterium when an electric current flows through them, during the electrolysis of deuterium-saturated liquids, during the formation of cavitation bubbles in them, as well as in other cases. However, most of these experiments have not had satisfactory reproducibility in other laboratories, and their results can almost always be explained without the use of synthesis.
Continuing the “glorious tradition” that began with the “philosopher’s stone” and then turned into a “perpetual motion machine”, many modern scammers are offering to buy from them a “cold fusion generator”, “cavitation reactor” and other “fuel-free generators”: about the philosophical Everyone has already forgotten the stone, they don’t believe in perpetual motion, but nuclear fusion now sounds quite convincing. But, alas, in reality such energy sources do not exist yet (and when they can be created, it will be in all news releases). So be aware: if you are offered to buy a device that generates energy through cold nuclear fusion, then they are simply trying to “cheat” you!

According to preliminary estimates, even with the current level of technology, it is possible to create a thermonuclear rocket engine for flight to the planets of the Solar System (with appropriate funding). Mastering the technology of such engines will increase the speed of manned flights tenfold and will make it possible to have large reserve fuel reserves on board, which will make flying to Mars no more difficult than working on the ISS now. Speeds of 10% of the speed of light will potentially become available for automatic stations, which means it will be possible to send research probes to nearby stars and obtain scientific data during the lifetime of their creators.

The concept of a thermonuclear rocket engine based on inertial fusion is currently considered the most developed. The difference between an engine and a reactor lies in the magnetic field, which directs the charged reaction products in one direction. The second option involves using an open trap, in which one of the plugs is deliberately weakened. The plasma flowing from it will create a reactive force.

Thermonuclear future

Mastering thermonuclear fusion turned out to be many orders of magnitude more difficult than it seemed at first. And although many problems have already been solved, the remaining ones will be enough for the next few decades of hard work of thousands of scientists and engineers. But the prospects that the transformations of hydrogen and helium isotopes open up for us are so great, and the path taken is already so significant that it makes no sense to stop halfway. No matter what numerous skeptics say, the future undoubtedly lies in synthesis.

Fusion reactor E.P. Velikhov, S.V. Putvinsky

E.P. Velikhov, S.V. Putvinsky.
Report dated October 22, 1999, carried out within the framework of the Energy Center of the World Federation of Scientists

annotation

This article provides a brief overview of the current state of fusion research and outlines the prospects for fusion power in the 21st century energy system. The review is intended for a wide range of readers familiar with the basics of physics and engineering.

According to modern physical concepts, there are only a few fundamental sources of energy that, in principle, can be mastered and used by humanity. Nuclear fusion reactions are one such source of energy and... In fusion reactions, energy is produced due to the work of nuclear forces performed during the fusion of nuclei of light elements and the formation of heavier nuclei. These reactions are widespread in nature - it is believed that the energy of stars, including the Sun, is produced as a result of a chain of nuclear fusion reactions that convert four nuclei of a hydrogen atom into a helium nucleus. We can say that the Sun is a large natural thermonuclear reactor that supplies energy to the Earth's ecological system.

Currently, more than 85% of the energy produced by humans is obtained by burning organic fuels - coal, oil and natural gas. This cheap source of energy, mastered by man about 200 - 300 years ago, led to the rapid development of human society, its well-being and, as a result, to the growth of the Earth's population. It is assumed that due to population growth and more uniform energy consumption across regions, energy production will increase by about three times by 2050 compared to the current level and reach 10 21 J per year. There is no doubt that in the foreseeable future the previous source of energy - organic fuels - will have to be replaced by other types of energy production. This will happen both due to the depletion of natural resources and due to environmental pollution, which, according to experts, should occur much earlier than cheap natural resources are developed (the current method of energy production uses the atmosphere as a garbage dump, throwing out 17 million tons daily carbon dioxide and other gases accompanying the combustion of fuels). The transition from fossil fuels to large-scale alternative energy is expected in the middle of the 21st century. It is assumed that the future energy system will use a variety of energy sources, including renewable energy sources, more widely than the current energy system, such as solar energy, wind energy, hydroelectric power, growing and burning biomass and nuclear energy. The share of each energy source in the total energy production will be determined by the structure of energy consumption and the economic efficiency of each of these energy sources.

In today's industrial society, more than half of the energy is used in a constant consumption mode, independent of the time of day and season. Superimposed on this constant base power are daily and seasonal variations. Thus, the energy system must consist of base energy, which supplies energy to society at a constant or quasi-permanent level, and energy resources, which are used as needed. It is expected that renewable energy sources such as solar energy, biomass combustion, etc. will be used mainly in the variable component of energy consumption and. The main and only candidate for base energy is nuclear energy. Currently, only nuclear fission reactions, which are used in modern nuclear power plants, have been mastered to produce energy. Controlled thermonuclear fusion is, so far, only a potential candidate for basic energy.

What advantages does thermonuclear fusion have over nuclear fission reactions, which allow us to hope for the large-scale development of thermonuclear energy? The main and fundamental difference is the absence of long-lived radioactive waste, which is typical for nuclear fission reactors. And although during the operation of a thermonuclear reactor the first wall is activated by neutrons, the choice of suitable low-activation structural materials opens up the fundamental possibility of creating a thermonuclear reactor in which the induced activity of the first wall will decrease to a completely safe level thirty years after the reactor is shut down. This means that an exhausted reactor will need to be mothballed for only 30 years, after which the materials can be recycled and used in a new synthesis reactor. This situation is fundamentally different from fission reactors, which produce radioactive waste that requires reprocessing and storage for tens of thousands of years. In addition to low radioactivity, thermonuclear energy has huge, practically inexhaustible reserves of fuel and other necessary materials, sufficient to produce energy for many hundreds, if not thousands of years.

It was these advantages that prompted the major nuclear countries to begin large-scale research on controlled thermonuclear fusion in the mid-50s. By this time, the first successful tests of hydrogen bombs had already been carried out in the Soviet Union and the United States, which confirmed the fundamental possibility of using energy and nuclear fusion in terrestrial conditions. From the very beginning, it became clear that controlled thermonuclear fusion had no military application. The research was declassified in 1956 and has since been carried out within the framework of broad international cooperation. The hydrogen bomb was created in just a few years, and at that time it seemed that the goal was close, and that the first large experimental facilities, built in the late 50s, would produce thermonuclear plasma. However, it took more than 40 years of research to create conditions under which the release of thermonuclear power is comparable to the heating power of the reacting mixture. In 1997, the largest thermonuclear installation, the European TOKAMAK (JET), received 16 MW of thermonuclear power and came close to this threshold.

What was the reason for this delay? It turned out that in order to achieve the goal, physicists and engineers had to solve a lot of problems that they had no idea about at the beginning of the journey. During these 40 years, the science of plasma physics was created, which made it possible to understand and describe the complex physical processes occurring in the reacting mixture. Engineers needed to solve equally complex problems, including learning how to create deep vacuums in large volumes, selecting and testing suitable construction materials, developing large superconducting magnets, powerful lasers and X-ray sources, developing pulsed power systems capable of creating powerful beams of particles, develop methods for high-frequency heating of the mixture and much more.

§4 is devoted to a review of research in the field of magnetic controlled fusion, which includes systems with magnetic confinement and pulsed systems. Most of this review is devoted to the most advanced systems for magnetic plasma confinement, TOKAMAK-type installations.

The scope of this review allows us to discuss only the most significant aspects of research on controlled thermonuclear fusion. The reader interested in a more in-depth study of various aspects of this problem may be advised to consult the review literature. There is an extensive literature devoted to controlled thermonuclear fusion. In particular, mention should be made of both now classic books written by the founders of controlled thermonuclear research, as well as very recent publications, such as, for example, which outline the current state of thermonuclear research.

Although there are quite a lot of nuclear fusion reactions leading to the release of energy, for practical purposes of using nuclear energy, only the reactions listed in Table 1 are of interest. Here and below we use the standard designation for hydrogen isotopes: p - proton with atomic mass 1, D - deuteron, with atomic mass 2 and T - tritium, isotope with mass 3. All nuclei participating in these reactions with the exception of tritium are stable. Tritium is a radioactive isotope of hydrogen with a half-life of 12.3 years. As a result of β-decay, it turns into He 3, emitting a low-energy electron. Unlike nuclear fission reactions, fusion reactions do not produce long-lived radioactive fragments of heavy nuclei, which makes it possible in principle to create a “clean” reactor, not burdened with the problem of long-term storage of radioactive waste.

Table 1.
Nuclear reactions of interest for controlled fusion

All reactions shown in Table 1, except the last one, occur with the release of energy and in the form of kinetic energy and reaction products, q, which is indicated in brackets in units of millions of electron volts (MeV),
(1 eV = 1.6 ·10 –19 J = 11600 °K). The last two reactions play a special role in controlled fusion - they will be used to produce tritium, which does not exist in nature.

Nuclear fusion reactions 1-5 have a relatively high reaction rate, which is usually characterized by the reaction cross section, σ. The reaction cross sections from Table 1 are shown in Fig. 1 as a function of energy and colliding particles in the center of mass system.

Due to the presence of Coulomb repulsion between nuclei, the cross sections for reactions at low energy and particles are negligible, and therefore, at ordinary temperatures, a mixture of hydrogen isotopes and other light atoms practically does not react. In order for any of these reactions to have a noticeable cross section, the colliding particles need to have high kinetic energy. Then the particles will be able to overcome the Coulomb barrier, approach at a distance on the order of nuclear ones, and react. For example, the maximum cross section for the reaction of deuterium with tritium is achieved at a particle energy of about 80 KeV, and in order for a DT mixture to have a high reaction rate, its temperature must be on the scale of one hundred million degrees, T = 10 8 ° K.

The simplest way to produce energy and nuclear fusion that immediately comes to mind is to use an ion accelerator and bombard, say, tritium ions accelerated to an energy of 100 KeV, a solid or gas target containing deuterium ions. However, the injected ions slow down too quickly when colliding with the cold electrons of the target, and do not have time to produce enough energy to cover the energy costs of their acceleration, despite the huge difference in the initial (about 100 KeV) and energy produced in the reaction ( about 10 MeV). In other words, with this “method” of energy production and the energy reproduction coefficient and,
Q fus = P synthesis / P costs will be less than 1.

In order to increase Q fus, the target electrons can be heated. Then fast ions will decelerate more slowly and Q fus will increase. However, a positive yield is achieved only at a very high target temperature - on the order of several KeV. At this temperature, the injection of fast ions is no longer important; there is a sufficient amount of energetic thermal ions in the mixture, which themselves enter into reactions. In other words, thermonuclear reactions or thermonuclear fusion occur in the mixture.

The rate of thermonuclear reactions can be calculated by integrating the reaction cross section shown in Fig. 1 over the equilibrium Maxwellian particle distribution function. As a result, it is possible to obtain the reaction rate K(T), which determines the number of reactions occurring per unit volume, n 1 n 2 K(T), and, consequently, the volumetric density of energy release in the reacting mixture,

P fus = q n 1 n 2 K(T) (1)

In the last formula n 1 n 2- volumetric concentrations of reacting components, T- temperature of reacting particles and q- energy yield of the reaction given in Table 1.

At a high temperature characteristic of a reacting mixture, the mixture is in a plasma state, i.e. consists of free electrons and positively charged ions that interact with each other through collective electromagnetic fields. Electromagnetic fields, self-consistent with the motion of plasma particles, determine the dynamics of the plasma and, in particular, maintain its quasineutrality. With very high accuracy, the charge densities of ions and electrons in plasma are equal, n e = Zn z, where Z is the charge of the ion (for hydrogen isotopes Z = 1). The ion and electron components exchange energy due to Coulomb collisions and at plasma parameters typical for thermonuclear applications, their temperatures are approximately equal.

For the high temperature of the mixture you have to pay with additional energy costs. First, we need to take into account the bremsstrahlung emitted by electrons when colliding with ions:

The power of bremsstrahlung, as well as the power of thermonuclear reactions in the mixture, is proportional to the square of the plasma density and, therefore, the ratio P fus /P b depends only on the plasma temperature. Bremsstrahlung, in contrast to the power of thermonuclear reactions, weakly depends on the plasma temperature, which leads to the presence of a lower limit on the plasma temperature at which the power of thermonuclear reactions is equal to the power of bremsstrahlung losses, P fus /P b = 1. At temperatures below the threshold bremsstrahlung power losses exceed the thermonuclear release of energy and, and therefore in a cold mixture a positive energy release is impossible. The mixture of deuterium and tritium has the lowest limiting temperature, but even in this case the temperature of the mixture must exceed 3 KeV (3.5 10 7 °K). The threshold temperatures for the DD and DHe 3 reactions are approximately an order of magnitude higher than for the DT reaction. For the reaction of a proton with boron, bremsstrahlung radiation at any temperature exceeds the reaction yield, and, therefore, to use this reaction, special traps are needed in which the electron temperature is lower than the ion temperature, or the plasma density is so high that the radiation is absorbed by the working mixture.

In addition to the high temperature of the mixture, for a positive reaction to occur, the hot mixture must exist long enough for the reactions to occur. In any thermonuclear system with finite dimensions, there are additional channels of energy loss from the plasma in addition to bremsstrahlung (for example, due to thermal conductivity, line radiation of impurities, etc.), the power of which should not exceed the thermonuclear energy release. In the general case, additional energy losses can be characterized by the energy lifetime of the plasma t E, defined in such a way that the ratio 3nT / t E gives the power loss per unit plasma volume. Obviously, for a positive yield it is necessary that the thermonuclear power exceed the power of additional losses, P fus > 3nT / t E , which gives a condition for the minimum product of density and plasma lifetime, nt E . For example, for a DT reaction it is necessary that

nt E > 5 10 19 s/m 3 (3)

This condition is usually called the Lawson criterion (strictly speaking, in the original work, the Lawson criterion was derived for a specific thermonuclear reactor design and, unlike (3), includes the efficiency of converting thermal energy into electrical energy). In the form in which it is written above, the criterion is practically independent of the thermonuclear system and is a generalized necessary condition for a positive output. The Lawson criterion for other reactions is one or two orders of magnitude higher than for the DT reaction, and the threshold temperature is also higher. The proximity of the device to achieving a positive output is usually depicted on the T - nt E plane, which is shown in Fig. 2.

nt E

Fig.2. Region with a positive yield of a nuclear reaction on the T-nt E plane.
The achievements of various experimental installations for confining thermonuclear plasma are shown.

It can be seen that DT reactions are more easily feasible - they require a significantly lower plasma temperature than DD reactions and impose less stringent conditions on its retention. The modern thermonuclear program is aimed at implementing DT-controlled fusion.

Thus, controlled thermonuclear reactions are, in principle, possible, and the main task of thermonuclear research is the development of a practical device that could compete economically with other sources of energy and.

All devices invented over 50 years can be divided into two large classes: 1) stationary or quasi-stationary systems based on magnetic confinement of hot plasma; 2) pulse systems. In the first case, the plasma density is low and the Lawson criterion is achieved due to good energy retention in the system, i.e. long energy plasma lifetime. Therefore, systems with magnetic confinement have a characteristic plasma size of the order of several meters and a relatively low plasma density, n ~ 10 20 m -3 (this is approximately 10 5 times lower than the atomic density at normal pressure and room temperature).

In pulsed systems, the Lawson criterion is achieved by compressing fusion targets with laser or x-ray radiation and creating a very high-density mixture. The lifetime in pulsed systems is short and is determined by the free expansion of the target. The main physical challenge in this direction of controlled fusion is to reduce the total energy and explosion to a level that will make it possible to make a practical fusion reactor.

Both types of systems have already come close to creating experimental machines with a positive energy output and Q fus > 1, in which the main elements of future thermonuclear reactors will be tested. However, before moving on to a discussion of fusion devices, we will consider the fuel cycle of a future fusion reactor, which is largely independent of the specific design of the system.

1) TOKAMAK T-15 has so far only operated in the mode with ohmic plasma heating and, therefore, the plasma parameters obtained with this installation are quite low. In the future, it is planned to introduce 10 MW of neutral injection and 10 MW of electron cyclotron heating.

2) The given Q fus was recalculated from the parameters of the DD plasma obtained in the setup to the DT plasma.

And although the experimental program on these TOKAMAKs has not yet been completed, this generation of machines has practically completed the tasks assigned to it. TOKAMAKs JET and TFTR for the first time received high thermonuclear power of DT reactions in plasma, 11 MW in TFTR and 16 MW in JET. Figure 6 shows the time dependences of thermonuclear power in DT experiments.

Fig.6. Dependence of thermonuclear power on time in record deuterium-tritium discharges at the JET and TFTR tokamaks.

This generation of TOKAMAKs reached the threshold value Q fus = 1 and received nt E only several times lower than that required for a full-scale TOKAMAK reactor. TOKAMAKs have learned to maintain a stationary plasma current using RF fields and neutral beams. The physics of plasma heating by fast particles, including thermonuclear alpha particles, was studied, the operation of the divertor was studied, and modes of its operation with low thermal loads were developed. The results of these studies made it possible to create the physical foundations necessary for the next step - the first TOKAMAK reactor, which will operate in combustion mode.

What physical restrictions on plasma parameters are there in TOKAMAKs?

Maximum plasma pressure in TOKAMAK or maximum value β is determined by the stability of the plasma and is approximately described by Troyon's relation,

Where β expressed in %, Ip– current flowing in the plasma and β N is a dimensionless constant called the Troyon coefficient. The parameters in (5) have the dimensions MA, T, m. Maximum values ​​of the Troyon coefficient β N= 3÷5, achieved in experiments, are in good agreement with theoretical predictions based on calculations of plasma stability. Fig.7 shows the limit values β , obtained in various TOKAMAKs.

Fig.7. Comparison of limit values β achieved in Troyon scaling experiments.

If the limit value is exceeded β , large-scale helical disturbances develop in the TOKAMAK plasma, the plasma quickly cools and dies on the wall. This phenomenon is called plasma stall.

As can be seen from Fig. 7, TOKAMAK is characterized by rather low values β at the level of several percent. There is a fundamental possibility to increase the value β by reducing the plasma aspect ratio to extremely low values ​​of R/ a= 1.3÷1.5. Theory predicts that in such machines β can reach several tens of percent. The first ultra-low aspect ratio TOKAMAK, START, built several years ago in England, has already received values β = 30%. On the other hand, these systems are technically more demanding and require special technical solutions for the toroidal coil, divertor and neutron protection. Currently, several larger experimental TOKAMAKs than START are being built with a low aspect ratio and plasma current above 1 MA. It is expected that over the next 5 years, experiments will provide enough data to understand whether the expected improvement in plasma parameters will be achieved and whether it will be able to compensate for the technical difficulties expected in this direction.

Long-term studies of plasma confinement in TOKAMAKs have shown that the processes of energy and particle transfer across the magnetic field are determined by complex turbulent processes in the plasma. And although plasma instabilities responsible for anomalous plasma losses have already been identified, the theoretical understanding of nonlinear processes is not yet sufficient to describe the plasma lifetime based on first principles. Therefore, to extrapolate plasma lifetimes obtained in modern installations to the scale of the TOKAMAK reactor, empirical laws—scalings—are currently used. One of these scalings (ITER-97(y)), obtained using statistical processing of an experimental database from various TOKAMAKs, predicts that the lifetime increases with plasma size, R, plasma current I p, and elongation of the plasma cross section k = b/ A= 4 and decreases with increasing plasma heating power, P:

t E ~ R 2 k 0.9 I р 0.9 / P 0.66

The dependence of the energy lifetime on other plasma parameters is rather weak. Figure 8 shows that the lifetime measured in almost all experimental TOKAMAKs is well described by this scaling.

Fig.8. Dependence of the experimentally observed energy lifetime on the one predicted by ITER-97(y) scaling.
The average statistical deviation of experimental points from scaling is 15%.
Different labels correspond to different TOKAMAKs and the projected TOKAMAK reactor ITER.

This scaling predicts that a TOKAMAK in which self-sustaining thermonuclear combustion will occur should have a large radius of 7-8 m and a plasma current of 20 MA. In such a TOKAMAK, the energy lifetime will exceed 5 seconds, and the power of thermonuclear reactions will be at the level of 1-1.5 GW.

In 1998, the engineering design of the TOKAMAK reactor ITER was completed. The work was carried out jointly by four parties: Europe, Russia, the USA and Japan with the aim of creating the first experimental TOKAMAK reactor designed to achieve thermonuclear combustion of a mixture of deuterium and tritium. The main physical and engineering parameters of the installation are given in Table 3, and its cross-section is shown in Fig. 9.

Fig.9. General view of the designed TOKAMAK reactor ITER.

ITER will already have all the main features of the TOKAMAK reactor. It will have a fully superconducting magnetic system, a cooled blanket and protection from neutron radiation, and a remote maintenance system for the installation. It is assumed that neutron fluxes with a power density of 1 MW/m 2 and a total fluence of 0.3 MW × yr/m 2 will be obtained on the first wall, which will allow nuclear technology tests of materials and blanket modules capable of reproducing tritium.

Table 3.
Basic parameters of the first experimental thermonuclear TOKAMAK reactor, ITER.

ITER is planned to be built in 2010-2011. The experimental program, which will continue on this experimental reactor for about twenty years, will make it possible to obtain plasma-physical and nuclear-technological data necessary for the construction in 2030-2035 of the first demonstration reactor - TOKAMAK, which has already will produce electricity. The main task of ITER will be to demonstrate the practicality of the TOKAMAK reactor for generating electricity and.

Along with TOKAMAK, which is currently the most advanced system for implementing controlled thermonuclear fusion, there are other magnetic traps that successfully compete with TOKAMAK.

In addition to TOKAMAKs and STELLARATORs, experiments, although on a smaller scale, continue on some other systems with closed magnetic configurations. Among them, field-reversed pinches, SPHEROMAKs and compact tori should be noted. Field-reversed pinches have a relatively low toroidal magnetic field. In SPHEROMAK or compact tori there is no toroidal magnetic system at all. Accordingly, all these systems promise the ability to create plasma with a high parameter value β and, therefore, may in the future be attractive for the creation of compact fusion reactors or reactors using alternative reactions, such as DHe 3 or rB, in which a low field is required to reduce magnetic bremsstrahlung. The current plasma parameters achieved in these traps are still significantly lower than those obtained in TOKAMAKS and STELLARATORS.

A study of the interaction of laser radiation with matter showed that laser radiation is well absorbed by the evaporating substance of the target shell up to the required power densities of 2÷4 · 10 14 W/cm 2 . The absorption coefficient can reach 40÷80% and increases with decreasing radiation wavelength. As mentioned above, a large thermonuclear yield can be achieved if the bulk of the fuel remains cold during compression. To do this, it is necessary that the compression be adiabatic, i.e. It is necessary to avoid preheating the target, which can occur due to the generation of energetic electrons, shock waves, or hard X-rays by laser radiation. Numerous studies have shown that these unwanted effects can be reduced by profiling the radiation pulse, optimizing the tablets, and reducing the radiation wavelength. Figure 16, borrowed from the work, shows the boundaries of the region on the plane power density - wavelength lasers suitable for target compression.

Fig. 16. The region on the parameter plane in which lasers are capable of compressing thermonuclear targets (shaded).

The first laser installation (NIF) with laser parameters sufficient to ignite targets will be built in the USA in 2002. The installation will make it possible to study the physics of compression of targets that will have a thermonuclear output at the level of 1-20 MJ and, accordingly, will allow obtaining high values Q>1.

Although lasers make it possible to carry out laboratory research on the compression and ignition of targets, their disadvantage is their low efficiency, which, at best, so far reaches 1-2%. At such low efficiencies, the thermonuclear yield of the target must exceed 10 3, which is a very difficult task. In addition, glass lasers have low pulse repeatability. In order for lasers to serve as a reactor driver for a fusion power plant, their cost must be reduced by approximately two orders of magnitude. Therefore, in parallel with the development of laser technology, researchers turned to the development of more efficient drivers - ion beams.

Ion beams

Currently, two types of ion beams are being considered: beams of light ions, type Li, with an energy of several tens of MeV, and beams of heavy ions, type Pb, with an energy of up to 10 GeV. If we talk about reactor applications, then in both cases it is necessary to supply an energy of several MJ to a target with a radius of several millimeters in a time of about 10 ns. It is necessary not only to focus the beam, but also to be able to conduct it in the reactor chamber at a distance of about several meters from the accelerator output to the target, which is not at all an easy task for particle beams.

Beams of light ions with energies of several tens of MeV can be created with relatively high efficiency. using a pulse voltage applied to the diode. Modern pulsed technology makes it possible to obtain the powers required to compress targets, and therefore light ion beams are the cheapest candidate for a driver. Experiments with light ions have been carried out for many years at the PBFA-11 facility at Sandywood National Laboratory in the USA. The setup makes it possible to create short (15 ns) pulses of 30 MeV Li ions with a peak current of 3.5 MA and a total energy of about 1 MJ. A casing made of large-Z material with a target inside was placed in the center of a spherically symmetric diode, allowing for the production of a large number of radially directed ion beams. The ion energy was absorbed in the hohlraum casing and the porous filler between the target and the casing and was converted into soft X-ray radiation, compressing the target.

It was expected to obtain a power density of more than 5 × 10 13 W/cm 2 necessary for compressing and igniting targets. However, the achieved power densities were approximately an order of magnitude lower than expected. A reactor using light ions as a driver requires colossal flows of fast particles with a high particle density near the target. Focusing such beams onto millimeter targets is a task of enormous complexity. In addition, light ions will be noticeably inhibited in the residual gas in the combustion chamber.

The transition to heavy ions and high particle energies makes it possible to significantly mitigate these problems and, in particular, to reduce the particle current densities and, thus, alleviate the problem of particle focusing. However, to obtain the required 10 GeV particles, huge accelerators with particle accumulators and other complex accelerating equipment are required. Let us assume that the total beam energy is 3 MJ, the pulse time is 10 ns, and the area on which the beam should be focused is a circle with a radius of 3 mm. Comparative parameters of hypothetical drivers for target compression are given in Table 6.

Table 6.
Comparative characteristics of drivers on light and heavy ions.

*) – in the target area

Beams of heavy ions, as well as light ions, require the use of a hohlraum, in which the energy of the ions is converted into X-ray radiation, which uniformly irradiates the target itself. The design of the hohlraum for a heavy ion beam differs only slightly from the hohlraum for laser radiation. The difference is that the beams do not require holes through which the laser beams penetrate into the hohlraum. Therefore, in the case of beams, special particle absorbers are used, which convert their energy into X-ray radiation. One possible option is shown in Fig. 14b. It turns out that the conversion efficiency decreases with increasing energy and ions and increasing the size of the region on which the beam is focused. Therefore, increasing the energy and particles above 10 GeV is impractical.

Currently, both in Europe and in the USA, it has been decided to focus the main efforts on the development of drivers based on heavy ion beams. It is expected that these drivers will be developed by 2010-2020 and, if successful, will replace lasers in next-generation NIF installations. So far, the accelerators required for inertial fusion do not exist. The main difficulty in their creation is associated with the need to increase particle flux densities to a level at which the spatial charge density of ions already significantly affects the dynamics and focusing of particles. In order to reduce the effect of space charge, it is proposed to create a large number of parallel beams, which will be connected in the reactor chamber and directed towards the target. The typical size of a linear accelerator is several kilometers.

How is it supposed to conduct ion beams over a distance of several meters in the reactor chamber and focus them on an area several millimeters in size? One possible scheme is self-focusing of beams, which can occur in a low-pressure gas. The beam will cause ionization of the gas and a compensating counter electric current flowing through the plasma. The azimuthal magnetic field, which is created by the resulting current (the difference between the beam current and the reverse plasma current), will lead to radial compression of the beam and its focusing. Numerical modeling shows that, in principle, such a scheme is possible if the gas pressure is maintained in the desired range of 1-100 Torr.

And although heavy ion beams offer the prospect of creating an effective driver for a fusion reactor, they face enormous technical challenges that still need to be overcome before the goal is achieved. For thermonuclear applications, an accelerator is needed that will create a beam of 10 GeV ions with a peak current of several tens of spacecraft and an average power of about 15 MW. The volume of the magnetic system of such an accelerator is comparable to the volume of the magnetic system of the TOKAMAK reactor and, therefore, one can expect that their costs will be of the same order.

Pulse reactor chamber

Unlike a magnetic fusion reactor, where high vacuum and plasma purity are required, such requirements are not imposed on the chamber of a pulsed reactor. The main technological difficulties in creating pulsed reactors lie in the field of driver technology, the creation of precision targets and systems that make it possible to feed and control the position of the target in the chamber. The pulse reactor chamber itself has a relatively simple design. Most projects involve the use of a liquid wall created by an open coolant. For example, the HYLIFE-11 reactor design uses molten salt Li 2 BeF 4, a liquid curtain from which surrounds the area where the targets arrive. The liquid wall will absorb neutron radiation and wash away the remains of the targets. It also dampens the pressure of micro-explosions and evenly transfers it to the main wall of the chamber. The characteristic outer diameter of the chamber is about 8 m, its height is about 20 m.

The total flow rate of the coolant liquid is estimated to be about 50 m 3 /s, which is quite achievable. It is assumed that in addition to the main, stationary flow, a pulsed liquid shutter will be made in the chamber, which will open synchronized with the supply of the target with a frequency of about 5 Hz to transmit a beam of heavy ions.

The required target feeding accuracy is fractions of millimeters. Obviously, passively delivering a target over a distance of several meters with such precision in a chamber in which turbulent gas flows caused by explosions of previous targets will occur is a practically impossible task. Therefore, the reactor will require a control system that allows tracking the position of the target and dynamically focusing the beam. In principle, such a task is feasible, but it can significantly complicate reactor control.

The thermonuclear reactor is not working yet and will not work soon. But scientists already know exactly how it works.

Theory

Helium-3, one of the isotopes of helium, can be used as fuel for a thermonuclear reactor. It is rare on Earth, but is very abundant on the Moon. This is the plot of the Duncan Jones film of the same name. If you are reading this article, then you will definitely like the film.

A nuclear fusion reaction is when two small atomic nuclei fuse into one large one. This is the opposite reaction. For example, you can smash two hydrogen nuclei together to make helium.

With such a reaction, a huge amount of energy is released due to the difference in mass: the mass of particles before the reaction is greater than the mass of the resulting large nucleus. This mass is converted into energy thanks to.

But in order for the fusion of two nuclei to occur, it is necessary to overcome their force of electrostatic repulsion and strongly press them against each other. And at small distances, on the order of the size of the nuclei, much greater nuclear forces act, due to which the nuclei are attracted to each other and combine into one large nucleus.

Therefore, the thermonuclear fusion reaction can only take place at very high temperatures, so that the speed of the nuclei is such that when they collide, they have enough energy to get close enough to each other for nuclear forces to work and a reaction to occur. That's where the "thermo" comes from in the name.

Practice

Where there is energy, there are weapons. During the Cold War, the USSR and the USA developed thermonuclear (or hydrogen) bombs. This is the most destructive weapon created by humanity, in theory it can destroy the Earth.

Temperature is the main obstacle to using thermonuclear energy in practice. There are no materials that can hold this temperature without melting.

But there is a way out, you can hold the plasma thanks to the strong energy. In special tokamaks, plasma can be held in a donut shape by huge, powerful magnets.

A fusion power plant is safe, environmentally friendly and very economical. It can solve all the energy problems of humanity. All that's left to do is learn how to build thermonuclear power plants.

International Experimental Fusion Reactor

Building a fusion reactor is very difficult and very expensive. To solve such a grandiose task, scientists from several countries combined their efforts: Russia, the USA, EU countries, Japan, India, China, the Republic of Korea and Canada.

An experimental tokamak is currently being built in France, it will cost approximately 15 billion dollars, according to plans it will be completed by 2019 and experiments will be carried out on it until 2037. If they are successful, then perhaps we will still have time to live in the happy era of thermonuclear energy.

So concentrate harder and start looking forward to the results of the experiments, this is not a second iPad for you to wait for - the future of humanity is at stake.

Without exaggeration, the international experimental thermonuclear reactor ITER can be called the most significant research project of our time. In terms of the scale of construction, it will easily outshine the Large Hadron Collider, and if successful, it will mark a much bigger step for all of humanity than a flight to the Moon. Indeed, potentially controlled thermonuclear fusion is an almost inexhaustible source of unprecedentedly cheap and clean energy.

This summer there were several good reasons to brush up on the technical details of the ITER project. Firstly, a grandiose undertaking, the official start of which is considered to be the meeting between Mikhail Gorbachev and Ronald Reagan back in 1985, is taking on material embodiment before our eyes. Designing a new generation reactor with the participation of Russia, the USA, Japan, China, India, South Korea and the European Union took more than 20 years. Today, ITER is no longer kilograms of technical documentation, but 42 hectares (1 km by 420 m) of a perfectly flat surface of one of the world's largest man-made platforms, located in the French city of Cadarache, 60 km north of Marseille. As well as the foundation of the future 360,000-ton reactor, consisting of 150,000 cubic meters of concrete, 16,000 tons of reinforcement and 493 columns with rubber-metal anti-seismic coating. And, of course, thousands of sophisticated scientific instruments and research facilities scattered across universities around the world.

March 2007. First photo of the future ITER platform from the air.

Production of key reactor components is well underway. In the spring, France reported the production of 70 frames for D-shaped toroidal field coils, and in June, winding of the first coils of superconducting cables, received from Russia from the Institute of Cable Industry in Podolsk, began.

The second good reason to remember ITER right now is political. The new generation reactor is a test not only for scientists, but also for diplomats. This is such an expensive and technically complex project that no country in the world can undertake it alone. The ability of states to reach agreement among themselves both in the scientific and financial spheres determines whether the matter will be completed.

March 2009. 42 hectares of leveled site are awaiting the start of construction of a scientific complex.

The ITER Council was scheduled for June 18 in St. Petersburg, but the US State Department, as part of sanctions, banned American scientists from visiting Russia. Taking into account the fact that the very idea of ​​a tokamak (a toroidal chamber with magnetic coils, which is the basis of ITER) belongs to the Soviet physicist Oleg Lavrentiev, the project participants treated this decision as a curiosity and simply moved the meeting to Cadarache on the same date. These events once again reminded the whole world that Russia (along with South Korea) is most responsible for fulfilling its obligations to the ITER project.

February 2011. More than 500 holes were drilled in the seismic isolation shaft, all underground cavities were filled with concrete.

Scientists burn

The phrase “fusion reactor” makes many people wary. The associative chain is clear: a thermonuclear bomb is more terrible than just a nuclear one, which means that a thermonuclear reactor is more dangerous than Chernobyl.

In fact, nuclear fusion, on which the operating principle of the tokamak is based, is much safer and more efficient than nuclear fission used in modern nuclear power plants. Fusion is used by nature itself: the Sun is nothing more than a natural thermonuclear reactor.

The ASDEX tokamak, built in 1991 at Germany's Max Planck Institute, is used to test various reactor front wall materials, particularly tungsten and beryllium. The plasma volume in ASDEX is 13 m 3, almost 65 times less than in ITER.

The reaction involves nuclei of deuterium and tritium - isotopes of hydrogen. The deuterium nucleus consists of a proton and a neutron, and the tritium nucleus consists of a proton and two neutrons. Under normal conditions, equally charged nuclei repel each other, but at very high temperatures they can collide.

Upon collision, the strong interaction comes into play, which is responsible for combining protons and neutrons into nuclei. The nucleus of a new chemical element—helium—emerges. In this case, one free neutron is formed and a large amount of energy is released. The strong interaction energy in the helium nucleus is less than in the nuclei of the parent elements. Due to this, the resulting nucleus even loses mass (according to the theory of relativity, energy and mass are equivalent). Recalling the famous equation E = mc 2, where c is the speed of light, one can imagine the colossal energy potential nuclear fusion contains.

August 2011. The pouring of a monolithic reinforced concrete seismic isolating slab began.

To overcome the force of mutual repulsion, the initial nuclei must move very quickly, so temperature plays a key role in nuclear fusion. At the center of the Sun, the process occurs at a temperature of 15 million degrees Celsius, but it is facilitated by the colossal density of matter due to the action of gravity. The colossal mass of the star makes it an effective thermonuclear reactor.

It is not possible to create such a density on Earth. All we can do is increase the temperature. For hydrogen isotopes to release the energy of their nuclei to earthlings, a temperature of 150 million degrees is required, that is, ten times higher than on the Sun.

No solid material in the Universe can come into direct contact with such a temperature. So just building a stove to cook helium won’t work. The same toroidal chamber with magnetic coils, or tokamak, helps solve the problem. The idea of ​​​​creating a tokamak dawned on the bright minds of scientists from different countries in the early 1950s, while the primacy is clearly attributed to the Soviet physicist Oleg Lavrentyev and his eminent colleagues Andrei Sakharov and Igor Tamm.

A vacuum chamber in the shape of a torus (a hollow donut) is surrounded by superconducting electromagnets, which create a toroidal magnetic field in it. It is this field that holds the plasma, hot up to ten times the sun, at a certain distance from the walls of the chamber. Together with the central electromagnet (inductor), the tokamak is a transformer. By changing the current in the inductor, they generate a current flow in the plasma - the movement of particles necessary for synthesis.

February 2012. 493 1.7-meter columns with seismic isolating pads made of rubber-metal sandwich were installed.

The Tokamak can rightfully be considered a model of technological elegance. The electric current flowing in the plasma creates a poloidal magnetic field that encircles the plasma cord and maintains its shape. Plasma exists under strictly defined conditions, and at the slightest change, the reaction immediately stops. Unlike a nuclear power plant reactor, a tokamak cannot “go wild” and increase the temperature uncontrollably.

In the unlikely event of destruction of the tokamak, there is no radioactive contamination. Unlike a nuclear power plant, a thermonuclear reactor does not produce radioactive waste, and the only product of the fusion reaction - helium - is not a greenhouse gas and is useful in the household. Finally, the tokamak uses fuel very sparingly: during synthesis, only a few hundred grams of substance are contained in the vacuum chamber, and the estimated annual supply of fuel for an industrial power plant is only 250 kg.

April 2014. Construction of the cryostat building was completed, the walls of the 1.5-meter thick tokamak foundation were poured.

Why do we need ITER?

Tokamaks of the classical design described above were built in the USA and Europe, Russia and Kazakhstan, Japan and China. With their help, it was possible to prove the fundamental possibility of creating high-temperature plasma. However, building an industrial reactor capable of delivering more energy than it consumes is a task of a fundamentally different scale.

In a classic tokamak, the current flow in the plasma is created by changing the current in the inductor, and this process cannot be endless. Thus, the lifetime of the plasma is limited, and the reactor can only operate in pulsed mode. Ignition of plasma requires colossal energy - it’s no joke to heat anything to a temperature of 150,000,000 °C. This means that it is necessary to achieve a plasma lifetime that will produce energy that pays for ignition.

The fusion reactor is an elegant technical concept with minimal negative side effects. The flow of current in the plasma spontaneously forms a poloidal magnetic field that maintains the shape of the plasma filament, and the resulting high-energy neutrons combine with lithium to produce precious tritium.

For example, in 2009, during an experiment on the Chinese tokamak EAST (part of the ITER project), it was possible to maintain plasma at a temperature of 10 7 K for 400 seconds and 10 8 K for 60 seconds.

To hold the plasma longer, additional heaters of several types are needed. All of them will be tested at ITER. The first method - injection of neutral deuterium atoms - assumes that the atoms will enter the plasma pre-accelerated to a kinetic energy of 1 MeV using an additional accelerator.

This process is initially contradictory: only charged particles can be accelerated (they are affected by an electromagnetic field), and only neutral ones can be introduced into the plasma (otherwise they will affect the flow of current inside the plasma cord). Therefore, an electron is first removed from deuterium atoms, and positively charged ions enter the accelerator. The particles then enter the neutralizer, where they are reduced to neutral atoms by interacting with the ionized gas and introduced into the plasma. The ITER megavoltage injector is currently being developed in Padua, Italy.

The second heating method has something in common with heating food in the microwave. It involves exposing the plasma to electromagnetic radiation with a frequency corresponding to the speed of particle movement (cyclotron frequency). For positive ions this frequency is 40−50 MHz, and for electrons it is 170 GHz. To create powerful radiation of such a high frequency, a device called a gyrotron is used. Nine of the 24 ITER gyrotrons are manufactured at the Gycom facility in Nizhny Novgorod.

The classical concept of a tokamak assumes that the shape of the plasma filament is supported by a poloidal magnetic field, which is itself formed when current flows in the plasma. This approach is not applicable for long-term plasma confinement. The ITER tokamak has special poloidal field coils, the purpose of which is to keep the hot plasma away from the walls of the reactor. These coils are among the most massive and complex structural elements.

In order to be able to actively control the shape of the plasma, promptly eliminating vibrations at the edges of the cord, the developers provided small, low-power electromagnetic circuits located directly in the vacuum chamber, under the casing.

Fuel infrastructure for thermonuclear fusion is a separate interesting topic. Deuterium is found in almost any water, and its reserves can be considered unlimited. But the world's reserves of tritium amount to tens of kilograms. 1 kg of tritium costs about $30 million. For the first launches of ITER, 3 kg of tritium will be needed. By comparison, about 2 kg of tritium per year is needed to maintain the nuclear capabilities of the United States Army.

However, in the future, the reactor will provide itself with tritium. The main fusion reaction produces high-energy neutrons that are capable of converting lithium nuclei into tritium. The development and testing of the first lithium reactor wall is one of ITER's most important goals. The first tests will use beryllium-copper cladding, the purpose of which is to protect the reactor mechanisms from heat. According to calculations, even if we transfer the entire energy sector of the planet to tokamaks, the world's lithium reserves will be enough for a thousand years of operation.

Preparing the 104-kilometer ITER Path cost France 110 million euros and four years of work. The road from the port of Fos-sur-Mer to Cadarache was widened and strengthened so that the heaviest and largest parts of the tokamak could be delivered to the site. In the photo: a transporter with a test load weighing 800 tons.

From the world via tokamak

Precision control of a fusion reactor requires precise diagnostic tools. One of the key tasks of ITER is to select the most suitable of the five dozen instruments that are currently being tested, and to begin the development of new ones.

At least nine diagnostic devices will be developed in Russia. Three are at the Moscow Kurchatov Institute, including a neutron beam analyzer. The accelerator sends a focused stream of neutrons through the plasma, which undergoes spectral changes and is captured by the receiving system. Spectrometry with a frequency of 250 measurements per second shows the temperature and density of the plasma, the strength of the electric field and the speed of particle rotation - parameters necessary to control the reactor for long-term plasma containment.

The Ioffe Research Institute is preparing three instruments, including a neutral particle analyzer that captures atoms from the tokamak and helps monitor the concentration of deuterium and tritium in the reactor. The remaining devices will be made at Trinity, where diamond detectors for the ITER vertical neutron chamber are currently being manufactured. All of the above institutes use their own tokamaks for testing. And in the thermal chamber of the Efremov NIIEFA, fragments of the first wall and the diverter target of the future ITER reactor are being tested.

Unfortunately, the fact that many of the components of a future mega-reactor already exist in the metal does not necessarily mean that the reactor will be built. Over the past decade, the estimated cost of the project has grown from 5 to 16 billion euros, and the planned first launch has been postponed from 2010 to 2020. The fate of ITER depends entirely on the realities of our present, primarily economic and political. Meanwhile, every scientist involved in the project sincerely believes that its success can change our future beyond recognition.