thermonuclear fusion, the reaction of fusion of light atomic nuclei into heavier nuclei, occurring at superhigh temperatures and accompanied by the release of huge amounts of energy. Nuclear fusion This is a reaction that is the reverse of atomic fission: in the latter, energy is released due to the splitting of heavy nuclei into lighter ones. see also NUCLEAR FISSION; NUCLEAR POWER.

According to modern astrophysical concepts, the main source of energy for the Sun and other stars is thermonuclear fusion occurring in their depths. In terrestrial conditions, it is carried out during an explosion hydrogen bomb. Thermonuclear fusion is accompanied by a colossal energy release per unit mass of reacting substances (about 10 million times greater than in chemical reactions). Therefore, it is of great interest to master this process and, on its basis, create a cheap and environmentally friendly source of energy. However, despite the fact that large scientific and technical teams in many developed countries are engaged in research on controlled thermonuclear fusion (CTF), there are still many complex problems to be solved before industrial production fusion energy will become a reality.

Modern nuclear power plants using the fission process only partially satisfy the world's electricity needs. The fuel for them is the natural radioactive elements uranium and thorium, the prevalence and reserves of which in nature are very limited; therefore, for many countries there is a problem of their import. The main component of thermonuclear fuel is the hydrogen isotope deuterium, which is contained in sea water. Its reserves are publicly available and very large (the oceans cover ~ 71% of the Earth's surface area, and deuterium accounts for about 0.016% of the total number of hydrogen atoms that make up water). In addition to the availability of fuel, fusion power sources have the following important advantages over nuclear power plants: 1) the UTS reactor contains much less radioactive materials than a nuclear fission reactor, and therefore the consequences of an accidental release of radioactive products are less dangerous; 2) thermonuclear reactions produce less long-lived radioactive waste; 3) TCB allows direct electricity generation.

Artsimovich L.A. Controlled thermonuclear reactions. M., 1963
Thermal and nuclear power plants(book 1, section 6; book 3, section 8). M., 1989

Find "NUCLEAR FUSION" on

NUCLEAR FUSION
thermonuclear fusion, the reaction of fusion of light atomic nuclei into heavier nuclei, occurring at superhigh temperatures and accompanied by the release of huge amounts of energy. Nuclear fusion is a reaction that is the reverse of atomic fission: in the latter, energy is released due to the splitting of heavy nuclei into lighter ones. see also
NUCLEUS FISSION ;
NUCLEAR POWER . According to modern astrophysical concepts, the main source of energy for the Sun and other stars is thermonuclear fusion occurring in their depths. Under terrestrial conditions, it is carried out during the explosion of a hydrogen bomb. Thermonuclear fusion is accompanied by a colossal energy release per unit mass of reacting substances (about 10 million times greater than in chemical reactions). Therefore, it is of great interest to master this process and, on its basis, create a cheap and environmentally friendly source of energy. However, despite the fact that large scientific and technical teams in many developed countries are engaged in research on controlled thermonuclear fusion (CTF), there are still many complex problems to be solved before the industrial production of thermonuclear energy becomes a reality. Modern nuclear power plants using the fission process only partially satisfy the world's electricity needs. The fuel for them is the natural radioactive elements uranium and thorium, the prevalence and reserves of which in nature are very limited; therefore, for many countries there is a problem of their import. The main component of thermonuclear fuel is the hydrogen isotope deuterium, which is found in sea water. Its reserves are publicly available and very large (the oceans cover 71% of the Earth's surface area, and deuterium accounts for about 0.016% of the total number of hydrogen atoms that make up water). In addition to the availability of fuel, thermonuclear energy sources have the following important advantages over nuclear power plants: 1) the UTS reactor contains much less radioactive materials than a nuclear fission reactor, and therefore the consequences of an accidental release of radioactive products are less dangerous; 2) thermonuclear reactions produce less long-lived radioactive waste; 3) TCB allows direct electricity generation.
PHYSICAL FOUNDATIONS OF NUCLEAR FUSION
The successful implementation of the fusion reaction depends on the properties of the atomic nuclei used and the possibility of obtaining a dense high-temperature plasma, which is necessary to initiate the reaction.
Nuclear forces and reactions. The energy release during nuclear fusion is due to extremely intense attractive forces operating inside the nucleus; these forces hold together the protons and neutrons that make up the nucleus. They are very intense at NUCLEAR fusion distances of 10-13 cm and weaken extremely rapidly with increasing distance. In addition to these forces, positively charged protons create electrostatic repulsive forces. The radius of action of electrostatic forces is much greater than that of nuclear forces, so they begin to dominate when the nuclei are further apart. Under normal conditions, the kinetic energy of the nuclei of light atoms is too small to overcome the electrostatic repulsion, they could approach each other and enter into a nuclear reaction. However, the repulsion can be overcome by "brute" force, for example, by colliding nuclei with a high relative speed. J. Cockcroft and E. Walton used this principle in their experiments conducted in 1932 at the Cavendish Laboratory (Cambridge, Great Britain). Irradiating a lithium target with protons accelerated in an electric field, they observed the interaction of protons with lithium nuclei Li. Since then, a large number of such reactions have been studied. Reactions involving the lightest nuclei - proton (p), deuteron (d) and triton (t), corresponding to the hydrogen isotopes protium 1H, deuterium 2H and tritium 3H - as well as the "light" helium isotope 3He and two lithium isotopes 6Li and 7Li are presented in the table below. Here n is a neutron, g is a gamma quantum. The energy released in each reaction is given in millions of electron volts (MeV). With a kinetic energy of 1 MeV, the speed of a proton is 14,500 km/s.
see also ATOMIC NUCLEI STRUCTURE.

Fusion REACTIONS

As G. Gamov showed, the probability of a reaction between two approaching light nuclei is proportional to

, where e is the base of natural logarithms, Z1 and Z2 are the numbers of protons in interacting nuclei, W is the energy of their relative approach, and K is a constant factor. The energy required to carry out a reaction depends on the number of protons in each nucleus. If it is more than three, then this energy is too high and the reaction is practically impossible. Thus, as Z1 and Z2 increase, the probability of a reaction decreases. The probability that two nuclei will interact is characterized by a "reaction cross section" measured in barns (1 b = 10-24 cm2). The reaction cross section is the area of the effective cross section of the nucleus, into which another nucleus must "get" in order for their interaction to occur. The cross section for the reaction of deuterium with tritium reaches its maximum value (NUCLEAR fusion5b) when the interacting particles have a relative approach energy of the order of 200 keV. At an energy of 20 keV, the cross section becomes less than 0.1 b. Out of a million accelerated particles hitting the target, no more than one enters into nuclear interaction. The rest dissipate their energy on the electrons of the target atoms and slow down to speeds at which the reaction becomes impossible. Consequently, the method of bombarding a solid target with accelerated nuclei (as was the case in the Cockcroft-Walton experiment) is unsuitable for CTS, since the energy obtained in this case is much less than the energy expended.

Thermonuclear fuels. Reactions involving p, which play the main role in the processes of nuclear fusion in the Sun and other homogeneous stars, are of no practical interest under terrestrial conditions, since they have a too small cross section. For the implementation of thermonuclear fusion on earth, a more suitable type of fuel, as mentioned above, is deuterium. But the most probable reaction is realized in an equal component mixture of deuterium and tritium (DT-mixture). Unfortunately, tritium is radioactive and, due to the short half-life (T1 / 2 NUCLEAR fusion 12.3 years), practically does not occur in nature. It is obtained artificially in fission reactors, and also as a by-product in reactions with deuterium. However, the absence of tritium in nature is not an obstacle to the use of DT - fusion reactions, because tritium can be produced by irradiating the 6Li isotope with neutrons formed during fusion: n + 6Li (r) 4He + t. If the thermonuclear chamber is surrounded by a layer of 6Li (natural lithium contains 7%), then it is possible to carry out complete reproduction of the consumable tritium. And although in practice some of the neutrons are inevitably lost, their loss can be easily replenished by introducing such an element as beryllium into the shell, the nucleus of which, when one fast neutron hits it, emits two.
The principle of operation of a thermonuclear reactor. The fusion reaction of light nuclei, the purpose of which is to obtain useful energy, is called controlled thermonuclear fusion. It is carried out at temperatures of the order of hundreds of millions of kelvins. This process has only been implemented in laboratories so far.
Time and temperature conditions. Obtaining useful thermonuclear energy is possible only if two conditions are met. First, the mixture intended for synthesis must be heated to a temperature at which the kinetic energy of the nuclei ensures a high probability of their fusion upon collision. Secondly, the reacting mixture must be very well thermally insulated (i.e., the high temperature must be maintained long enough for the required number of reactions to occur and the energy released due to this exceeds the energy spent on heating the fuel). In quantitative form, this condition is expressed as follows. To heat a thermonuclear mixture, one cubic centimeter of its volume must be supplied with energy P1 = knT, where k is a numerical coefficient, n is the density of the mixture (the number of nuclei in 1 cm3), T is the required temperature. To maintain the reaction, the energy imparted to the thermonuclear mixture must be conserved for a time t. In order for a reactor to be energetically profitable, it is necessary that during this time more thermonuclear energy be released in it than was spent on heating. The released energy (also per 1 cm3) is expressed as follows:

where f(T) is a coefficient depending on the temperature of the mixture and its composition, R is the energy released in one elementary act of synthesis. Then the energy profitability condition P2 > P1 will take the form

The last inequality, known as the Lawson criterion, is a quantitative expression of the requirements for the perfection of thermal insulation. The right side - the "Lawson number" - depends only on the temperature and composition of the mixture, and the larger it is, the more stringent the requirements for thermal insulation, i.e. the more difficult it is to create a reactor. In the range of acceptable temperatures, the Lawson number for pure deuterium is 1016 s/cm3, and for an equal-component DT mixture it is 2×1014 s/cm3. Thus, the DT mixture is the preferred fusion fuel. In accordance with the Lawson criterion, which determines the energetically favorable value of the product of the density and the retention time, in a thermonuclear reactor, as large as possible n or t should be used. Therefore, studies of CTS diverged in two different directions: in the first, researchers tried to keep relatively rarefied plasma with the help of a magnetic field for a sufficiently long time; in the second - with the help of lasers for a short time to create a plasma with a very high density. Much has been devoted to the first approach. more works than the second.
Magnetic confinement of plasma. During the fusion reaction, the density of the hot reactant must remain at a level that would provide a sufficiently high yield of useful energy per unit volume at a pressure that the plasma chamber can withstand. For example, for a mixture of deuterium - tritium at a temperature of 108 K, the yield is determined by the expression

Assuming P to be 100 W/cm3 (roughly equivalent to the energy released by fuel cells in nuclear fission reactors), the density n should be approx. 1015 cores/cm3, and the corresponding pressure nT is about 3 MPa. The retention time in this case, according to the Lawson criterion, should be at least 0.1 s. For deuterium-deuterium plasma at a temperature of 109 K

In this case, at P = 100 W/cm3, n " 3x1015 cores/cm3 and a pressure of approximately 100 MPa, the required retention time will be more than 1 s. Note that these densities are only 0.0001 of the density of atmospheric air, so the reactor chamber must The above estimates of confinement time, temperature and density are typical minimum parameters necessary for the operation of a fusion reactor, and they are more easily achieved in the case of a deuterium-tritium mixture. , then it should be borne in mind that, due to completely different conditions, in the first case they proceed very quickly, and in the second - extremely slowly compared to the processes in a thermonuclear reactor.
Plasma. When a gas is heated strongly, its atoms partially or completely lose electrons, resulting in the formation of positively charged particles called ions and free electrons. At temperatures above a million degrees, a gas consisting of light elements is completely ionized, i.e. each atom loses all of its electrons. A gas in an ionized state is called a plasma (the term was introduced by I. Langmuir). The properties of a plasma differ significantly from those of a neutral gas. Since there are free electrons in the plasma, the plasma conducts electric current very well, and its conductivity is proportional to T3/2. Plasma can be heated by passing an electric current through it. The conductivity of a hydrogen plasma at 108 K is the same as that of copper at room temperature. The thermal conductivity of the plasma is also very high. To keep the plasma, for example, at a temperature of 108 K, it must be reliably thermally insulated. In principle, the plasma can be isolated from the walls of the chamber by placing it in a strong magnetic field. This is provided by the forces that arise during the interaction of currents with a magnetic field in the plasma. Under the action of a magnetic field, ions and electrons move in spirals along its lines of force. The transition from one line of force to another is possible in collisions of particles and in the imposition of a transverse electric field. In the absence of electric fields, high-temperature rarefied plasma, in which collisions rarely occur, will only slowly diffuse across magnetic field lines. If the lines of force of the magnetic field are closed, giving them the shape of a loop, then the plasma particles will move along these lines, being held in the region of the loop. In addition to such a closed magnetic configuration, plasma confinement has also been proposed open systems(with field lines extending outward from the ends of the chamber), in which particles remain inside the chamber due to magnetic "plugs" that restrict the movement of particles. Magnetic mirrors are created at the ends of the chamber, where a narrowing beam of field lines is formed as a result of a gradual increase in the field strength. In practice, magnetic confinement of a sufficiently high density plasma turned out to be far from simple: magnetohydrodynamic and kinetic instabilities often arise in it. Magnetohydrodynamic instabilities are associated with bends and breaks in magnetic field lines. In this case, the plasma can begin to move across the magnetic field in the form of bunches, leave the containment zone in a few millionths of a second and give off heat to the chamber walls. Such instabilities can be suppressed by giving the magnetic field a certain configuration. Kinetic instabilities are very diverse and have been studied in less detail. Among them are those that disrupt orderly processes, such as the flow of a constant electric current or a stream of particles through a plasma. Other kinetic instabilities cause a higher plasma transverse diffusion rate in a magnetic field than that predicted by collision theory for a quiet plasma.
Systems with a closed magnetic configuration. If a strong force is applied to an ionized conducting gas electric field, then a discharge current will appear in it, simultaneously with which a magnetic field surrounding it will appear. The interaction of the magnetic field with the current will lead to the appearance of compressive forces acting on the charged particles of the gas. If the current flows along the axis of the conducting plasma filament, then the resulting radial forces, like rubber bands, compress the filament, moving the plasma boundary away from the walls of the chamber containing it. This phenomenon, theoretically predicted by W. Bennett in 1934 and experimentally demonstrated for the first time by A. Ware in 1951, is called the pinch effect. The pinch method is applied to plasma confinement; its remarkable feature is that the gas is heated to high temperatures by the electric current itself (ohmic heating). The fundamental simplicity of the method led to its use in the very first attempts to contain a hot plasma, and the study of a simple pinch effect, despite the fact that it was subsequently supplanted by more advanced methods, made it possible to better understand the problems that experimenters face today. In addition to plasma diffusion in the radial direction, there is also a longitudinal drift and its exit through the ends of the plasma column. Losses through the ends can be eliminated if the chamber with plasma is shaped like a donut (torus). In this case, a toroidal pinch is obtained. For the simple pinch described above, the magnetohydrodynamic instabilities inherent in it are a serious problem. If a small bend occurs at the plasma column, then the density of magnetic field lines with inside bending increases (Fig. 1). The magnetic lines of force, which behave like bundles resisting compression, will rapidly "bulge" so that the bend will increase until the entire structure of the plasma filament is destroyed. As a result, the plasma will come into contact with the walls of the chamber and cool down. To exclude this disastrous phenomenon, before the passage of the main axial current, a longitudinal magnetic field is created in the chamber, which, together with the circular field applied later, “straightens” the incipient bending of the plasma column (Fig. 2). The principle of stabilization of a plasma column by an axial field is the basis for two promising projects of thermonuclear reactors - a tokamak and a pinch with a reversed magnetic field.

Open magnetic configurations. In systems with an open configuration, the problem of plasma confinement in the longitudinal direction is solved by creating a magnetic field, the lines of force of which near the ends of the chamber have the form of a converging beam. Charged particles move along helical lines along the field line and are reflected from regions of higher intensity (where the density of field lines is greater). Such configurations (Fig. 3) are called magnetic mirror traps or magnetic mirrors. The magnetic field is created by two parallel coils in which strong currents flow in the same direction. In the space between the coils, the lines of force form a "barrel" in which the contained plasma is located. However, it has been experimentally established that such systems are unlikely to be able to contain the plasma of the density required for reactor operation. At present, there is not much hope for this method of retention.
see also MAGNETIC HYDRODYNAMICS.

inertial hold. Theoretical calculations show that thermonuclear fusion is possible without the use of magnetic traps. To do this, a specially prepared target (a ball of deuterium with a radius of about 1 mm) is rapidly compressed to such high densities that the thermonuclear reaction has time to complete before the fuel target evaporates. Compression and heating to thermonuclear temperatures can be performed by super-powerful laser pulses, uniformly and simultaneously irradiating the fuel ball from all sides (Fig. 4). With instantaneous evaporation of its surface layers, the ejected particles acquire very high velocities, and the ball is under the action of large compressive forces. They are similar to the reactive forces driving a rocket, with the only difference being that here these forces are directed inward, towards the center of the target. This method can create pressures of the order of 1011 MPa and densities 10,000 times higher than the density of water. At this density, almost all of the thermonuclear energy will be released in the form of a small explosion during the NUCLEAR Fusion time of 10-12 s. Occurring microexplosions, each of which is equivalent to 1-2 kg of TNT, will not cause damage to the reactor, and the implementation of a sequence of such microexplosions at short intervals would make it possible to realize an almost continuous production of useful energy. For inertial containment, the arrangement of a fuel target is very important. A target in the form of concentric spheres made of heavy and light materials will make it possible to achieve the most efficient evaporation of particles and, consequently, the greatest compression.

Calculations show that at the energy laser radiation of the order of a megajoule (106 J) and a laser efficiency of at least 10%, the thermonuclear energy produced must exceed the energy expended for pumping the laser. Thermonuclear laser facilities are available in research laboratories in Russia, the USA, Western Europe and Japan. The possibility of using a heavy ion beam instead of a laser beam or a combination of such a beam with a light beam is currently being studied. Thanks to modern technology this method of initiating the reaction has an advantage over the laser method, since it allows you to get more useful energy. The disadvantage is the difficulty in focusing the beam on the target.
INSTALLATIONS WITH MAGNETIC RETENTION
Magnetic plasma confinement methods are being studied in Russia, the USA, Japan, and a number of European countries. The main attention is paid to toroidal-type devices, such as tokamak and pinch with reversed magnetic field, which appeared as a result of the development of simpler pinches with a stabilizing longitudinal magnetic field. To contain the plasma using a toroidal magnetic field Bj, it is necessary to create conditions under which the plasma would not be displaced towards the walls of the torus. This is achieved by "twisting" the magnetic field lines (the so-called "rotational transformation"). This twisting is done in two ways. In the first method, a current is passed through the plasma, leading to the configuration of the already considered stable pinch. The magnetic field of the current Bq Ј -Bq together with Bj creates a total field with the necessary twist. If Bj Bq, then the resulting configuration is known as a tokamak (an abbreviation of the expression "TOROIDAL CAMERA WITH MAGNETIC COILS"). The tokamak (Fig. 5) was developed under the guidance of L. A. Artsimovich at the Institute atomic energy them. I. V. Kurchatov in Moscow. At Bj NUCLEAR fusion Bq, a pinch configuration with a reversed magnetic field is obtained.

In the second method, special helical windings around the toroidal plasma chamber are used to ensure the equilibrium of the confined plasma. The currents in these windings create a complex magnetic field, which leads to twisting of the lines of force of the total field inside the torus. Such an installation, called a stellarator, was developed at Princeton University (USA) by L. Spitzer and his co-workers.
Tokamak. An important parameter on which the confinement of the toroidal plasma depends is the "stability margin" q, equal to rBj/RBq, where r and R are the small and large radii of the toroidal plasma, respectively. At small q, helical instability can develop, which is analogous to the instability of a straight pinch bend. Scientists in Moscow have experimentally shown that for q > 1 (ie Bj Bq) the possibility of helical instability is greatly reduced. This makes it possible to effectively use the heat released by the current to heat the plasma. As a result of many years of research, the characteristics of tokamaks have improved significantly, in particular, by increasing the field uniformity and efficient cleaning of the vacuum chamber. The encouraging results obtained in Russia stimulated the creation of tokamaks in many laboratories around the world, and their configuration became the subject of intensive research. The ohmic heating of the plasma in the tokamak is not sufficient to carry out the thermonuclear fusion reaction. This is due to the fact that when the plasma is heated, its electrical resistance, and as a result, the heat generation during the passage of current is sharply reduced. It is impossible to increase the current in the tokamak above a certain limit, since the plasma column can lose stability and be transferred to the chamber walls. Therefore, various additional methods are used to heat the plasma. The most effective of them are the injection of beams of high-energy neutral atoms and microwave irradiation. In the first case, the ions accelerated to energies of 50-200 keV are neutralized (to avoid their "reflection" back by the magnetic field when introduced into the chamber) and injected into the plasma. Here they are again ionized and in the process of collisions they give up their energy to the plasma. In the second case, microwave radiation is used, the frequency of which is equal to the ion cyclotron frequency (the rotation frequency of ions in a magnetic field). At this frequency, the dense plasma behaves as an absolutely black body, i.e. completely absorbs the incident energy. On the JET tokamak of the countries of the European Union, a plasma with an ion temperature of 280 million Kelvin and a confinement time of 0.85 s was obtained by injection of neutral particles. A thermonuclear power reaching 2 MW was obtained on deuterium-tritium plasma. The duration of the reaction is limited by the appearance of impurities due to the sputtering of the chamber walls: impurities penetrate into the plasma and, being ionized, significantly increase energy losses due to radiation. Currently, work on the JET program is focused on research on the possibility of controlling impurities and their removal, the so-called. "magnetic diverter". Large tokamaks were also created in the USA - TFTR, in Russia - T15 and in Japan - JT60. The research carried out at these and other facilities laid the foundation for the next stage of work in the field of controlled thermonuclear fusion: in 2010, it is planned to launch a large reactor for technical tests. This is expected to be teamwork USA, Russia, European Union countries and Japan.
Reversed field pinch (FOP). The POP configuration differs from the tokamak in that it has Bq Bj, but the direction of the toroidal field outside the plasma is opposite to its direction inside the plasma column. J. Taylor showed that such a system is in a state with a minimum energy and, despite q Stellarator. In a stellarator, a closed toroidal magnetic field is superimposed by a field created by a special helical winding wound around the camera body. The total magnetic field prevents the plasma from drifting away from the center and suppresses certain types magnetohydrodynamic instabilities. The plasma itself can be created and heated by any of the methods used in a tokamak. The main advantage of the stellarator is that the method of confinement used in it is not related to the presence of current in the plasma (as in tokamaks or in devices based on the pinch effect), and therefore the stellarator can operate in a stationary mode. In addition, the helical winding can have a "divertor" effect, i.e. purify the plasma from impurities and remove reaction products. Plasma confinement in stellarators is being comprehensively studied at facilities in the European Union, Russia, Japan, and the United States. On the stellarator "Wendelstein VII" in Germany, it was possible to maintain a non-current-carrying plasma with a temperature of more than 5x106 kelvins, heating it by injection of a high-energy atomic beam. The latest theoretical and experimental studies showed that in most of the described installations, and especially in closed toroidal systems, the plasma confinement time can be increased by increasing its radial dimensions and confining magnetic field. For example, for a tokamak, it is calculated that the Lawson criterion will be fulfilled (and even with some margin) at a magnetic field strength of 100 kG and a small toroidal chamber radius of approx. 2 m. These are the installation parameters for 1000 MW of electricity. When creating such large installations with magnetic plasma confinement, completely new technological problems arise. To create a magnetic field of the order of 50 kG in a volume of several cubic meters using water-cooled copper coils, a source of electricity with a capacity of several hundred megawatts is required. Therefore, it is obvious that the windings of the coils must be made of superconducting materials, such as alloys of niobium with titanium or with tin. The resistance of these materials electric current in the superconducting state is zero, and, consequently, to maintain the magnetic field will be spent minimal amount electricity.
reactor technology. The device of a thermonuclear power plant is schematically shown in fig. 6. There is a deuterium-tritium plasma in the reactor chamber, and it is surrounded by a lithium-beryllium "blanket" where neutrons are absorbed and tritium is reproduced. The generated heat is removed from the blanket through a heat exchanger to a conventional steam turbine. The windings of the superconducting magnet are protected by radiation and thermal shields and are cooled with liquid helium. However, many problems associated with the stability of the plasma and its purification from impurities, radiation damage to the inner wall of the chamber, fuel supply, removal of heat and reaction products, and thermal power control have not yet been resolved.
see also
NUCLEAR POWER ;
HEAT EXCHANGER .

Prospects for thermonuclear research. Experiments carried out on installations of the tokamak type have shown that this system is very promising as a possible basis for the UTS reactor. The best results to date have been obtained on tokamaks, and there is hope that with a corresponding increase in the scale of installations, they will be able to implement an industrial controlled fusion. However, the tokamak is not economical enough. To eliminate this shortcoming, it is necessary that it does not work in a pulsed mode, as it is now, but in a continuous mode. However, the physical aspects of this problem are still poorly understood. It is also necessary to develop technical means, which would improve the plasma parameters and eliminate its instabilities. Considering all this, one should not forget about other possible, although less developed options for a thermonuclear reactor, for example, a stellarator or a reversed field pinch. The state of research in this area has reached the point where there are conceptual reactor designs for most high temperature plasma magnetic confinement systems and for some inertial confinement systems. An example of the industrial development of a tokamak is the Aries project (USA). The next generation of tokamaks must decide technical problems associated with industrial reactors UTS. It is obvious that considerable difficulties will arise before their creators, but it is also certain that as people become aware of the problems related to environment, sources of raw materials and energy, the production of electricity by the new methods discussed above will take its rightful place. see also

Since nuclear forces of attraction act between atomic nuclei at small distances, when two nuclei approach each other, their fusion is possible, i.e., the synthesis of a heavier nucleus. All atomic nuclei have a positive electric charge and, therefore, repel each other at large distances. In order for the nuclei to approach and enter into a nuclear fusion reaction, they must have sufficient kinetic energy to overcome the mutual electrical repulsion, which is greater, the greater the charge of the nucleus. Therefore, the synthesis of light nuclei with a small electric charge is most easily carried out. In the laboratory, fusion reactions can be observed by firing at a target with fast nuclei accelerated in a special accelerator (see Charged Particle Accelerators). In nature, fusion reactions occur in very hot matter, for example, in the interiors of stars, including in the center of the Sun, where the temperature is 14 million degrees and the energy of the thermal motion of some of the fastest particles is sufficient to overcome the electrical repulsion. Nuclear fusion occurring in a heated substance is called thermonuclear.

Thermonuclear reactions taking place in the interiors of stars play a very important role in the evolution of the Universe. They are the source of nuclei chemical elements, which are synthesized from hydrogen in stars. They are the energy source of the stars. The main source of solar energy is the reactions of the so-called proton-proton cycle, as a result of which a helium nucleus is born from 4 protons. The energy released during fusion is carried away by the resulting nuclei, quanta electromagnetic radiation, neutrons and neutrinos. By observing the flow of neutrinos coming from the Sun, it is possible to establish which nuclear fusion reactions and with what intensity take place in its center.

A unique feature of thermonuclear reactions as an energy source is a very large energy release per unit mass of reacting substances - 10 million times more than in chemical reactions. The entry into the synthesis of 1 g of hydrogen isotopes is equivalent to the combustion of 10 tons of gasoline. Therefore, scientists have long sought to master this gigantic source of energy. In principle, we already know how to obtain the energy of thermonuclear fusion on Earth today. It is possible to heat matter to stellar temperatures using energy atomic explosion. This is how the hydrogen bomb works - the most terrible weapon of our time, in which the explosion of a nuclear fuse leads to instantaneous heating of a mixture of deuterium and tritium and a subsequent thermonuclear explosion.

But scientists are not striving for such an uncontrolled synthesis, capable of destroying all life on Earth. They are looking for ways to implement controlled thermonuclear fusion. What conditions must be met for this to happen? First of all, of course, it is necessary to heat the thermonuclear fuel to a temperature where fusion reactions can occur with a noticeable probability. But this is not enough. It is necessary that during fusion more energy be released than is spent on heating the substance, or, even better, that fast particles born during synthesis themselves maintain the required temperature of the fuel. For this, it is necessary that the substance entering into synthesis be reliably thermally isolated from the environment and, naturally, cold on Earth, i.e., that the cooling time, or, as they say, the energy retention time, be sufficiently long.

Temperature and holding time requirements depend on the fuel used. The easiest way to carry out the synthesis is between the heavy isotopes of hydrogen - deuterium (D) and tritium (T). In this case, as a result of the reaction, a helium nucleus (He 4) and a neutron are obtained. Deuterium is found on Earth in huge quantities in sea water (one atom of deuterium per 6000 hydrogen atoms). Tritium is absent in nature. Today it is obtained artificially by irradiating lithium with neutrons in nuclear reactors. The absence of tritium is not, however, an obstacle to using D-T fusion reactions, since the neutron formed during the reaction can be used to reproduce tritium by irradiating lithium, whose reserves on Earth are quite large.

For implementation D-T reactions the most favorable temperatures are about 100 million degrees. The requirement for the energy retention time depends on the density of the reacting substance, which at such a temperature will inevitably be in the form of a plasma, i.e., an ionized gas. Since the intensity of thermonuclear reactions is the higher, the higher the plasma density, the requirements for the energy confinement time are inversely proportional to the density. If the density is expressed as the number of ions in 1 cm 3, then for the D-T reaction at the optimum temperature, the condition for obtaining useful energy can be written as: the product of the density n and the energy retention time τ must be greater than 10 14 cm −3 s, i.e. i.e., a plasma with a density of 10 14 ions in 1 cm 3 should noticeably cool down no faster than in 1 s.

Since the thermal velocity of hydrogen ions at the required temperature is 10 8 cm/s, the ions travel 1000 km in 1 s. Therefore, special devices are needed to prevent the plasma from reaching the walls that insulate it. Plasma is a gas consisting of a mixture of ions and electrons. Charged particles moving across the magnetic field are affected by a force that bends their trajectory and makes them move in circles with radii proportional to the momentum of the particles and inversely proportional to the magnetic field. Thus, the magnetic field can prevent charged particles from escaping in a direction perpendicular to the lines of force. This is the basis for the idea of magnetic thermal insulation of plasma. The magnetic field, however, does not prevent the motion of particles along the lines of force: in general case particles move in spirals, winding around the lines of force.

Physicists have come up with various tricks to prevent particles from escaping along the lines of force. It is possible, for example, to make "magnetic plugs" - areas with a stronger magnetic field that reflect part of the particles, but it is best to fold the lines of force into a ring, use a toroidal magnetic field. But even one toroidal field, it turns out, is not enough.

The toroidal field is inhomogeneous in space - its intensity decreases along the radius, and in the inhomogeneous field there is a slow movement of charged particles - the so-called drift - across the magnetic field. This drift can be eliminated by passing a current through the plasma along the bypass of the torus. The magnetic field of the current, adding to the toroidal external field, will make the total field helical.

Moving in spirals along the lines of force, the charged particles will move from the upper half-plane of the torus to the lower and back. At the same time, they will always drift in one direction, for example, up. But, being in the upper half-plane and drifting upwards, the particles leave the middle plane of the torus, and being in the lower half-plane and also drifting upwards, the particles return to it. Thus, the drifts in the upper and lower halves of the torus are mutually compensated and do not lead to particle losses. This is exactly how the magnetic system of Tokamak-type installations is arranged, on which the best results in plasma heating and thermal insulation are obtained.

In addition to the thermal insulation of the plasma, it is also necessary to ensure its heating. In a Tokamak, the current flowing through the plasma column can be used for this purpose. In other devices where the holding is carried out without current, as well as in the Tokamak itself, other heating methods are used to heat up to very high temperatures, for example, using high-frequency electromagnetic waves, injection (introduction) into the plasma of beams of fast particles, light beams generated by high-power lasers, etc. The greater the power of the heating device, the faster the plasma can be heated to the required temperature. Development in last years very powerful lasers and sources of beams of relativistic charged particles made it possible to heat small volumes of matter to thermonuclear temperatures in a very short time, so short that the substance has time to heat up and enter into fusion reactions before being scattered due to thermal motion. Under such conditions, additional thermal insulation was unnecessary. The only thing that keeps particles from flying apart is their own inertia. Thermonuclear devices based on this principle are called inertial confinement devices. This new direction of research, which is called inertial thermonuclear fusion, is being intensively developed at the present time.

As a child, I loved to read the magazine "Science and Life", in the village there was a file starting from the 60s. There they often talked about thermonuclear fusion in a joyful way - it's almost there, and it will be! Many countries, in order to be in time for the distribution of free energy, built Tokamaks (and set up a total of 300 of them around the world).

The years have gone by... It's 2013 and humanity still gets most of its energy from burning coal, like it did in the 19th century. Why did it happen, what prevents the creation of a thermonuclear reactor, and what can we expect in the future - under the cut.

Theory

The nucleus of an atom, as we remember, consists in the first approximation of protons and neutrons (= nucleons). In order to tear off all neutrons and protons from an atom, a certain energy must be expended - the binding energy of the nucleus. This energy is different for different isotopes, and naturally, in nuclear reactions, the energy balance must be maintained. If we plot the binding energy for all isotopes (per 1 nucleon), we get the following:

From here we see that we can get energy either by separating heavy atoms (like 235 U), or by connecting light ones.

The most realistic and interesting in practical terms are the following synthesis reactions:

1) 2 D+ 3 T -> 4 He (3.5 MeV) + n (14.1 MeV)
2) 2 D+ 2 D -> 3 T (1.01 MeV) + p (3.02 MeV) 50%
2 D+ 2 D -> 3 He (0.82 MeV) + n (2.45 MeV) 50%
3) 2 D+ 3 He -> 4 He (3.6 MeV) + p (14.7 MeV)
4) p+ 11 B -> 3 4 He + 8.7 MeV

These reactions use Deuterium (D) - it can be obtained directly from sea water, Tritium (T) - a radioactive isotope of hydrogen, now it is obtained as a waste product in conventional nuclear reactors, it can be specially produced from lithium. Helium-3 - like on the Moon, as we all already know. Boron-11 - natural boron consists of 80% boron-11. p (Protium, hydrogen atom) - ordinary hydrogen.

For comparison, the fission of 235 U releases ~202.5 MeV of energy, i.e. much more than with a fusion reaction based on 1 atom (but based on a kilogram of fuel - of course, thermonuclear fuel gives more energy).

According to reactions 1 and 2, a lot of very high-energy neutrons are obtained, which make the entire design of the reactor radioactive. But reactions 3 and 4 - "without neutron" (aneutronic) - do not give induced radiation. Unfortunately, side reactions still remain, for example, from reaction 3 - deuterium will react with itself, and there will still be a small neutron radiation.

Reaction 4 is interesting because as a result we get 3 alpha particles, from which, theoretically, energy can be directly removed (because they actually represent moving charges = current).

In general, there are enough interesting reactions. The only question is how easy it is to implement them in reality?

On the complexity of the reaction Mankind has mastered the fission of 235 U relatively easily: there is no difficulty here - since neutrons do not have a charge, they can literally "crawl" through the nucleus even at a very low speed. In most fission reactors, just such thermal neutrons are used - in which the speed of movement is comparable to the speed of the thermal movement of atoms.

But during the fusion reaction - we have 2 nuclei that have a charge, and they repel each other. In order to bring them closer to the distance necessary for the reaction, it is necessary that they move at a sufficient speed. This speed can either be achieved in an accelerator (when all atoms move at the same optimal speed as a result), or by heating (when atoms fly randomly in random directions and at a random speed).

Here is a graph showing the reaction rate (cross section) versus the speed (=energy) of the colliding atoms:

Here is the same, but constructed from the plasma temperature, taking into account the fact that the atoms there fly at a random speed:

We immediately see that the D + T reaction is the “easiest” (it needs a miserable 100 million degrees), D + D is about 100 times slower at the same temperatures, D + 3 He goes faster than the competing D + D only at temperatures of the order 1 billion degrees.

Thus, only the D + T reaction is at least remotely accessible to a person, with all its shortcomings (radioactivity of tritium, difficulties in obtaining it, radiation induced by neutrons).

But as you understand, taking and heating something up to one hundred million degrees and leaving it to react will not work - any heated objects emit light, and thus quickly cool down. Plasma heated to hundreds of millions of degrees - shines in the X-ray range, and what is most sad - it is transparent to him. Those. Plasma with such a temperature fatally cools down quickly, and in order to maintain the temperature, it is necessary to constantly pump in gigantic energy to maintain the temperature.

However, due to the fact that there is very little gas in a thermonuclear reactor (for example, in ITER - only half a gram), everything turns out not so bad: to heat 0.5 g of hydrogen to 100 million degrees, you need to spend about the same amount of energy as to heat 186 liters of water per 100 degrees.

The project ended on September 30, 2012. It turned out that there were inaccuracies in the computer model. According to the new estimate, the pulse power achieved at NIF is 1.8 megajoules - 33-50% of what is required to release as much energy as was expended.

Sandy Z-machine The idea is this: take a large pile of high-voltage capacitors, and abruptly discharge them through thin tungsten wires in the center of the machine. The wires instantly evaporate, and a huge current of 27 million amperes continues to flow through them for 95 nanoseconds. Plasma heated to millions and billions (!) degrees - radiates x-rays, and compresses a capsule with a deuterium-tritium mixture in the center (the energy of the X-ray pulse is 2.7 megajoules).

It is planned to upgrade the system using a Russian power plant (Linear Transformer Driver - LTD). In 2013, the first tests are expected, in which the energy received will be compared with the energy spent (Q=1). Perhaps this direction in the future will have a chance to compare and surpass tokamaks.

Dense Plasma Focus-DPF- "collapses" the plasma running along the electrodes to obtain gigantic temperatures. In March 2012, a temperature of 1.8 billion degrees was reached at an installation operating according to this principle.

Levitated Dipole- "inverted" tokamak, in the center of the vacuum chamber hangs a toroidal superconducting magnet which holds the plasma. In such a scheme, the plasma promises to be stable on its own. But the project does not currently have funding, it seems that the synthesis reaction was not carried out directly at the facility.

Farnsworth–Hirsch fusor The idea is simple - we place two spherical grids in a vacuum chamber filled with deuterium, or a deuterium-tritium mixture, apply a potential of 50-200 thousand volts between them. In an electric field, atoms begin to fly around the center of the chamber, sometimes colliding with each other.

There is a neutron yield, but it is rather small. Big losses energy into bremsstrahlung X-rays, the inner grid quickly heats up and evaporates from collisions with atoms and electrons. Although the design is interesting from an academic point of view (any student can assemble it), the efficiency of neutron generation is much lower than linear accelerators.

Polywell is a good reminder that not all fusion work is public. The work was funded by the US Navy, and was classified until negative results were obtained.

The idea is a development of the Farnsworth–Hirsch fusor. The central negative electrode, which had the most problems, we replace with a cloud of electrons held by a magnetic field in the center of the chamber. All test models had regular, not superconducting, magnets. The reaction produced single neutrons. In general, no revolution. Perhaps the increase in size and superconducting magnets would have changed something.

Muonic catalysis- a radically different idea. We take a negatively charged muon and replace it with an electron in an atom. Since the muon is 207 times heavier than an electron, 2 atoms in a hydrogen molecule will be much closer friend to a friend, and a fusion reaction will occur. The only problem is that if helium is formed as a result of the reaction (chance ~ 1%), and the muon flies away with it, it will no longer be able to participate in reactions (because helium does not form chemical compound with hydrogen).

The problem here is that the generation of the muon on this moment requires more energy than can be obtained in a chain of reactions, and thus energy cannot be obtained here yet.

"Cold" thermonuclear fusion(this does not include "cold" muon catalysis) - has long been a pasture of pseudoscientists. There are no scientifically confirmed and independently repeatable positive results. And sensations at the level of the yellow press were more than once before Andrea Rossi's E-Cat.

According to modern astrophysical concepts, the main source of energy for the Sun and other stars is thermonuclear fusion occurring in their depths. Under terrestrial conditions, it is carried out during the explosion of a hydrogen bomb. Thermonuclear fusion is accompanied by a colossal energy release per unit mass of reacting substances (about 10 million times greater than in chemical reactions). Therefore, it is of great interest to master this process and, on its basis, create a cheap and environmentally friendly source of energy. However, despite the fact that large scientific and technical teams in many developed countries are engaged in research on controlled thermonuclear fusion (CTF), there are still many complex problems to be solved before the industrial production of thermonuclear energy becomes a reality.

Modern nuclear power plants using the fission process only partially satisfy the world's electricity needs. The fuel for them is the natural radioactive elements uranium and thorium, the prevalence and reserves of which in nature are very limited; therefore, for many countries there is a problem of their import. The main component of thermonuclear fuel is the hydrogen isotope deuterium, which is found in sea water. Its reserves are publicly available and very large (the world ocean covers ~ 71% of the Earth's surface area, and deuterium accounts for about 0.016% of the total number of hydrogen atoms that make up water). In addition to the availability of fuel, thermonuclear energy sources have the following important advantages over nuclear power plants: 1) the UTS reactor contains much less radioactive materials than a nuclear fission reactor, and therefore the consequences of an accidental release of radioactive products are less dangerous; 2) thermonuclear reactions produce less long-lived radioactive waste; 3) TCB allows direct electricity generation.

PHYSICAL FOUNDATIONS OF NUCLEAR FUSION

The successful implementation of the fusion reaction depends on the properties of the atomic nuclei used and the possibility of obtaining a dense high-temperature plasma, which is necessary to initiate the reaction.

Nuclear forces and reactions.

The energy release during nuclear fusion is due to extremely intense attractive forces operating inside the nucleus; these forces hold together the protons and neutrons that make up the nucleus. They are very intense at distances of ~10–13 cm and weaken extremely rapidly with increasing distance. In addition to these forces, positively charged protons create electrostatic repulsive forces. The radius of action of electrostatic forces is much greater than that of nuclear forces, so they begin to dominate when the nuclei are further apart.

As G. Gamov showed, the probability of a reaction between two approaching light nuclei is proportional to , where e – base of natural logarithms, Z 1 And Z 2 are the numbers of protons in interacting nuclei, W is the energy of their relative approach, and K is a constant multiplier. The energy required to carry out a reaction depends on the number of protons in each nucleus. If it is more than three, then this energy is too high and the reaction is practically impossible. Thus, with increasing Z 1 and Z 2 the probability of a reaction decreases.

The probability that two nuclei will interact is characterized by a “reaction cross section” measured in barns (1 b = 10–24 cm 2). The reaction cross section is the area of the effective cross section of the nucleus, into which another nucleus must “get” in order for their interaction to occur. The cross section for the reaction of deuterium with tritium reaches its maximum value (~5 b) when the interacting particles have a relative approach energy of about 200 keV. At an energy of 20 keV, the cross section becomes less than 0.1 b.

Out of a million accelerated particles hitting the target, no more than one enters into a nuclear interaction. The rest dissipate their energy on the electrons of the target atoms and slow down to speeds at which the reaction becomes impossible. Consequently, the method of bombarding a solid target with accelerated nuclei (as was the case in the Cockcroft-Walton experiment) is unsuitable for CTS, since the energy obtained in this case is much less than the energy spent.

Thermonuclear fuels.

Reactions involving p, which play the main role in the processes of nuclear fusion in the Sun and other homogeneous stars, are of no practical interest under terrestrial conditions, since they have a too small cross section. For the implementation of thermonuclear fusion on earth, a more suitable type of fuel, as mentioned above, is deuterium.

But the most probable reaction is realized in an equal component mixture of deuterium and tritium (DT-mixture). Unfortunately, tritium is radioactive and, due to its short half-life (T 1/2 ~ 12.3 years), is practically never found in nature. It is obtained artificially in fission reactors, and also as a by-product in reactions with deuterium. However, the absence of tritium in nature is not an obstacle to the use of DT - fusion reactions, since tritium can be produced by irradiating the 6 Li isotope with neutrons produced during fusion: n+ 6 Li ® 4 He + t.

If the thermonuclear chamber is surrounded by a layer of 6 Li (natural lithium contains 7%), then it is possible to carry out complete reproduction of the consumable tritium. And although in practice some of the neutrons are inevitably lost, their loss can be easily replenished by introducing such an element as beryllium into the shell, the nucleus of which, when one fast neutron hits it, emits two.

The principle of operation of a thermonuclear reactor.

The fusion reaction of light nuclei, the purpose of which is to obtain useful energy, is called controlled thermonuclear fusion. It is carried out at temperatures of the order of hundreds of millions of kelvins. This process has only been implemented in laboratories so far.

Time and temperature conditions.

Obtaining useful thermonuclear energy is possible only if two conditions are met. First, the mixture intended for synthesis must be heated to a temperature at which the kinetic energy of the nuclei ensures a high probability of their fusion upon collision. Secondly, the reacting mixture must be very well thermally insulated (i.e., the high temperature must be maintained long enough for the required number of reactions to occur and the energy released due to this exceeds the energy spent on heating the fuel).

In quantitative form, this condition is expressed as follows. To heat a thermonuclear mixture, one cubic centimeter of its volume must be supplied with energy P 1 = knt, Where k- numerical coefficient, n- the density of the mixture (the number of nuclei in 1 cm 3), T- required temperature. To maintain the reaction, the energy imparted to the thermonuclear mixture must be conserved for a time t. In order for a reactor to be energetically profitable, it is necessary that during this time more thermonuclear energy be released in it than was spent on heating. The released energy (also per 1 cm 3) is expressed as follows:

Where f(T) is a coefficient depending on the temperature of the mixture and its composition, R is the energy released in one elementary act of synthesis. Then the condition of energy profitability P 2 > P 1 will take the form

The last inequality, known as the Lawson criterion, is a quantitative expression of the requirements for the perfection of thermal insulation. The right side - "Lawson's number" - depends only on the temperature and composition of the mixture, and the larger it is, the more stringent the requirements for thermal insulation, i.e. the more difficult it is to create a reactor. In the region of acceptable temperatures, the Lawson number for pure deuterium is 10 16 s/cm 3 , and for an equal-component DT mixture it is 2×10 14 s/cm 3 . Thus, the DT mixture is the preferred fusion fuel.

In accordance with the Lawson criterion, which determines the energetically favorable value of the product of density and confinement time, in a thermonuclear reactor, as large as possible should be used. n or t. Therefore, studies of CTS diverged in two different directions: in the first, researchers tried to keep relatively rarefied plasma with the help of a magnetic field for a sufficiently long time; in the second, with the help of lasers for a short time to create a plasma with a very high density. Much more work has been devoted to the first approach than to the second.

Magnetic confinement of plasma.

During the fusion reaction, the density of the hot reactant must remain at a level that would provide a sufficiently high yield of useful energy per unit volume at a pressure that the plasma chamber can withstand. For example, for a mixture of deuterium - tritium at a temperature of 10 8 K, the yield is determined by the expression

If accept P equal to 100 W / cm 3 (which approximately corresponds to the energy released by fuel elements in nuclear fission reactors), then the density n should be approx. 10 15 cores / cm 3, and the corresponding pressure nt- about 3 MPa. The retention time in this case, according to the Lawson criterion, should be at least 0.1 s. For deuterium-deuterium plasma at a temperature of 10 9 K

In this case, when P\u003d 100 W / cm 3, n» 3×10 15 cores/cm 3 and a pressure of approximately 100 MPa, the required holding time will be more than 1 s. Note that these densities are only 0.0001 of atmospheric air, so the reactor chamber must be evacuated to a high vacuum.

The above estimates of retention time, temperature, and density are typical minimum parameters required for the operation of a fusion reactor, and are more easily achieved in the case of a deuterium-tritium mixture. As for the thermonuclear reactions that occur during the explosion of a hydrogen bomb and in the interiors of stars, it should be borne in mind that, due to completely different conditions, in the first case they proceed very quickly, and in the second - extremely slowly compared to the processes in a thermonuclear reactor.

Plasma.

When a gas is heated strongly, its atoms partially or completely lose electrons, resulting in the formation of positively charged particles called ions and free electrons. At temperatures above a million degrees, a gas consisting of light elements is completely ionized, i.e. each atom loses all of its electrons. A gas in an ionized state is called a plasma (the term was introduced by I. Langmuir). The properties of a plasma differ significantly from those of a neutral gas. Since there are free electrons in the plasma, the plasma conducts electric current very well, and its conductivity is proportional to T 3/2. Plasma can be heated by passing an electric current through it. The conductivity of a hydrogen plasma at 10 8 K is the same as that of copper at room temperature. The thermal conductivity of the plasma is also very high.

To keep the plasma, for example, at a temperature of 10 8 K, it must be reliably thermally insulated. In principle, the plasma can be isolated from the walls of the chamber by placing it in a strong magnetic field. This is provided by the forces that arise during the interaction of currents with a magnetic field in the plasma.

Under the action of a magnetic field, ions and electrons move in spirals along its lines of force. The transition from one line of force to another is possible when particles collide and when a transverse electric field is applied. In the absence of electric fields, high-temperature rarefied plasma, in which collisions rarely occur, will only slowly diffuse across magnetic field lines. If the lines of force of the magnetic field are closed, giving them the shape of a loop, then the plasma particles will move along these lines, being held in the region of the loop. In addition to such a closed magnetic configuration, open systems (with field lines extending outward from the ends of the chamber) were also proposed for confining the plasma, in which particles remain inside the chamber due to magnetic “plugs” that restrict the movement of particles. Magnetic mirrors are created at the ends of the chamber, where a narrowing beam of field lines is formed as a result of a gradual increase in the field strength.

In practice, magnetic confinement of a sufficiently high density plasma turned out to be far from simple: magnetohydrodynamic and kinetic instabilities often arise in it.

Magnetohydrodynamic instabilities are associated with bends and breaks in magnetic field lines. In this case, the plasma can begin to move across the magnetic field in the form of bunches, leave the containment zone in a few millionths of a second and give off heat to the chamber walls. Such instabilities can be suppressed by giving the magnetic field a certain configuration.

Kinetic instabilities are very diverse and have been studied in less detail. Among them are those that disrupt orderly processes, such as the flow of a constant electric current or a stream of particles through a plasma. Other kinetic instabilities cause a higher plasma transverse diffusion rate in a magnetic field than that predicted by collision theory for a quiet plasma.

Systems with a closed magnetic configuration.

If a strong electric field is applied to an ionized conducting gas, then a discharge current will appear in it, simultaneously with which a magnetic field surrounding it will appear. The interaction of the magnetic field with the current will lead to the appearance of compressive forces acting on the charged particles of the gas. If the current flows along the axis of the conducting plasma filament, then the resulting radial forces, like rubber bands, compress the filament, moving the plasma boundary away from the walls of the chamber containing it. This phenomenon, theoretically predicted by W. Bennett in 1934 and experimentally demonstrated for the first time by A. Ware in 1951, is called the pinch effect. The pinch method is applied to plasma confinement; its notable feature is that the gas is heated to high temperatures by the electric current itself (ohmic heating). The fundamental simplicity of the method led to its use in the very first attempts to contain a hot plasma, and the study of a simple pinch effect, despite the fact that it was subsequently supplanted by more advanced methods, made it possible to better understand the problems that experimenters face today.

In addition to plasma diffusion in the radial direction, there is also a longitudinal drift and its exit through the ends of the plasma column. Losses through the ends can be eliminated if the chamber with plasma is shaped like a donut (torus). In this case, a toroidal pinch is obtained.

For the simple pinch described above, the magnetohydrodynamic instabilities inherent in it are a serious problem. If a small bend occurs near the plasma column, then the density of magnetic field lines on the inner side of the bend increases (Fig. 1). The magnetic lines of force, which behave like strands resisting compression, will quickly begin to "bulge", so that the bend will increase until the entire structure of the plasma filament is destroyed. As a result, the plasma will come into contact with the walls of the chamber and cool down. To exclude this disastrous phenomenon, before the passage of the main axial current, a longitudinal magnetic field is created in the chamber, which, together with the circular field applied later, “straightens” the incipient bending of the plasma column (Fig. 2). The principle of stabilization of a plasma column by an axial field is the basis for two promising projects of thermonuclear reactors - a tokamak and a pinch with a reversed magnetic field.

Open magnetic configurations.

inertial hold.

Theoretical calculations show that thermonuclear fusion is possible without the use of magnetic traps. To do this, a specially prepared target (a ball of deuterium with a radius of about 1 mm) is rapidly compressed to such high densities that the thermonuclear reaction has time to complete before the fuel target evaporates. Compression and heating to thermonuclear temperatures can be performed by super-powerful laser pulses, uniformly and simultaneously irradiating the fuel ball from all sides (Fig. 4). With instantaneous evaporation of its surface layers, the ejected particles acquire very high velocities, and the ball is under the action of large compressive forces. They are similar to the reactive forces driving a rocket, with the only difference being that here these forces are directed inward, towards the center of the target. This method can create pressures of the order of 10 11 MPa and densities 10,000 times higher than the density of water. At this density, almost all thermonuclear energy will be released in the form of a small explosion in ~10–12 s. Occurring microexplosions, each of which is equivalent to 1–2 kg of TNT, will not cause damage to the reactor, and the implementation of a sequence of such microexplosions at short intervals would make it possible to realize an almost continuous production of useful energy. For inertial containment, the arrangement of a fuel target is very important. A target in the form of concentric spheres made of heavy and light materials will make it possible to achieve the most efficient evaporation of particles and, consequently, the greatest compression.

Calculations show that for a laser radiation energy of the order of a megajoule (10 6 J) and a laser efficiency of at least 10%, the thermonuclear energy produced must exceed the energy expended for pumping the laser. Thermonuclear laser facilities are available in research laboratories in Russia, the USA, Western Europe and Japan. The possibility of using a heavy ion beam instead of a laser beam or a combination of such a beam with a light beam is currently being studied. Thanks to modern technology, this method of initiating a reaction has an advantage over laser, since it allows you to get more useful energy. The disadvantage is the difficulty in focusing the beam on the target.

INSTALLATIONS WITH MAGNETIC RETENTION

Magnetic plasma confinement methods are being studied in Russia, the USA, Japan, and a number of European countries. The main attention is paid to toroidal-type devices, such as tokamak and pinch with reversed magnetic field, which appeared as a result of the development of simpler pinches with a stabilizing longitudinal magnetic field.

For confining plasma with a toroidal magnetic field B j it is necessary to create conditions under which the plasma would not be displaced to the walls of the torus. This is achieved by "twisting" the magnetic field lines (the so-called "rotational transformation"). This twisting is done in two ways. In the first method, a current is passed through the plasma, leading to the configuration of the already considered stable pinch. Magnetic field current B q J - B q along with B j creates a total field with the necessary twist. If B j B q , we get a configuration known as a tokamak (an abbreviation of the expression "TOROIDAL CAMERA WITH MAGNETIC COILS"). The tokamak (Fig. 5) was developed under the direction of L.A. Artsimovich at the Institute of Atomic Energy named after V.I. I.V. Kurchatov in Moscow. At B j ~ B q the pinch configuration with reversed magnetic field is obtained.

Tokamak.

An important parameter on which the confinement of a toroidal plasma depends is the “stability margin” q, equal to rB j / R.B. q , where r And R are the small and large radii of the toroidal plasma, respectively. At a small q a helical instability can develop, which is analogous to the instability of the bending of a straight pinch. Scientists in Moscow experimentally showed that when q> 1 (i.e. B j B q) the possibility of helical instability is greatly reduced. This makes it possible to effectively use the heat released by the current to heat the plasma. As a result of many years of research, the characteristics of tokamaks have improved significantly, in particular, by increasing the field uniformity and efficient cleaning of the vacuum chamber.

The encouraging results obtained in Russia stimulated the creation of tokamaks in many laboratories around the world, and their configuration became the subject of intensive research.

The ohmic heating of the plasma in the tokamak is not sufficient to carry out the thermonuclear fusion reaction. This is due to the fact that when the plasma is heated, its electrical resistance greatly decreases, and as a result, the heat release during the passage of current decreases sharply. It is impossible to increase the current in the tokamak above a certain limit, since the plasma column can lose stability and be transferred to the chamber walls. Therefore, various additional methods are used to heat the plasma. The most effective of them are the injection of beams of high-energy neutral atoms and microwave irradiation. In the first case, ions accelerated to energies of 50–200 keV are neutralized (to avoid their “reflection” back by the magnetic field when introduced into the chamber) and injected into the plasma. Here they are again ionized and in the process of collisions they give up their energy to the plasma. In the second case, microwave radiation is used, the frequency of which is equal to the ion cyclotron frequency (the rotation frequency of ions in a magnetic field). At this frequency, the dense plasma behaves like an absolutely black body, i.e. completely absorbs the incident energy. On the JET tokamak of the countries of the European Union, a plasma with an ion temperature of 280 million Kelvin and a confinement time of 0.85 s was obtained by injection of neutral particles. A thermonuclear power reaching 2 MW was obtained on deuterium-tritium plasma. The duration of the reaction is limited by the appearance of impurities due to the sputtering of the chamber walls: impurities penetrate into the plasma and, being ionized, significantly increase energy losses due to radiation. Currently, work on the JET program is focused on research on the possibility of controlling impurities and their removal, the so-called. "magnetic diverter".

Large tokamaks were also created in the USA - TFTR, in Russia - T15 and in Japan - JT60. The research carried out on these and other facilities laid the foundation for the next stage of work in the field of controlled thermonuclear fusion: in 2010, a large reactor is scheduled to be launched for technical testing. It is assumed that this will be a joint work of the United States, Russia, the countries of the European Union and Japan. see also TOKAMAK.

Reversed field pinch (FOP).

The POP configuration differs from the tokamak in that it has B q~ B j , but the direction of the toroidal field outside the plasma is opposite to its direction inside the plasma column. J.Taylor showed that such a system is in a state with a minimum energy and, despite q

The advantage of the POP configuration is that the ratio of the volumetric energy densities of the plasma and the magnetic field (value b) in it is greater than in the tokamak. It is fundamentally important that b be as large as possible, since this will reduce the toroidal field and, consequently, reduce the cost of the coils that create it and the entire supporting structure. Weak side The problem lies in the fact that the thermal insulation of these systems is worse than that of tokamaks, and the problem of maintaining the reversed field has not been solved.

Stellarator.

In a stellarator, a closed toroidal magnetic field is superimposed by a field created by a special helical winding wound around the camera body. The total magnetic field prevents the plasma from drifting away from the center and suppresses certain types of magnetohydrodynamic instabilities. The plasma itself can be created and heated by any of the methods used in a tokamak.

The main advantage of the stellarator is that the method of confinement used in it is not related to the presence of current in the plasma (as in tokamaks or in devices based on the pinch effect), and therefore the stellarator can operate in a stationary mode. In addition, the helical winding can have a "divertor" effect, i.e. purify the plasma from impurities and remove reaction products.

Plasma confinement in stellarators is being comprehensively studied at facilities in the European Union, Russia, Japan, and the United States. On the stellarator "Wendelstein VII" in Germany, it was possible to maintain a non-current-carrying plasma with a temperature of more than 5x10 6 kelvin, heating it by injection of a high-energy atomic beam.

Recent theoretical and experimental studies have shown that in most of the described installations, and especially in closed toroidal systems, the plasma confinement time can be increased by increasing its radial dimensions and confining magnetic field. For example, for a tokamak, it is calculated that the Lawson criterion will be fulfilled (and even with some margin) at a magnetic field strength of ~ 50 ± 100 kG and a small radius of the toroidal chamber of approx. 2 m. These are the installation parameters for 1000 MW of electricity.

When creating such large installations with magnetic plasma confinement, completely new technological problems arise. To create a magnetic field of the order of 50 kG in a volume of several cubic meters using water-cooled copper coils, a source of electricity with a capacity of several hundred megawatts is required. Therefore, it is obvious that the windings of the coils must be made of superconducting materials, such as alloys of niobium with titanium or with tin. The resistance of these materials to electric current in the superconducting state is zero, and, therefore, the minimum amount of electricity will be spent on maintaining the magnetic field.

reactor technology.

Prospects for thermonuclear research.

Experiments carried out on installations of the tokamak type have shown that this system is very promising as a possible basis for the UTS reactor. The best results to date have been obtained on tokamaks, and there is hope that with a corresponding increase in the scale of installations, they will be able to implement an industrial controlled fusion. However, the tokamak is not economical enough. To eliminate this shortcoming, it is necessary that it does not work in a pulsed mode, as it is now, but in a continuous mode. However, the physical aspects of this problem are still poorly understood. It is also necessary to develop technical means that would improve the parameters of the plasma and eliminate its instabilities. Considering all this, one should not forget about other possible, although less developed options for a thermonuclear reactor, for example, a stellarator or a reversed field pinch. The state of research in this area has reached the point where there are conceptual reactor designs for most high temperature plasma magnetic confinement systems and for some inertial confinement systems. An example of the industrial development of a tokamak is the Aries project (USA).