Thermonuclear reactor using open traps. Press approach
The RAS press service has previously reported about this event in the news on 09.08. and 08/15/2016. Details of this landmark scientific achievement are now being published.
SIBERIAN PHYSICISTS HEATED PLASMAUP TO 10 MILLION DEGREES
INfusion plant
Scientists from the Institute of Nuclear Physics named after. G.I. Budker of the Siberian Branch of the Russian Academy of Sciences, in experiments on a gas-dynamic trap, achieved stable heating of the plasma to 10 million degrees. This is a very significant result for the prospects of controlled thermonuclear fusion. The plasma retention time is still milliseconds.
Scientists began to consider options for creating a thermonuclear reactor based on an open trap.
Scientists intend to achieve acceptable fusion energy yields for systems approximately 100 meters long. This is very compact systems. A thermonuclear reactor based on an open trap, an alternative to TOKAMAK, can be created within the next 20-30 years.
Academic scientists from the Siberian Branch of the Russian Academy of Sciences managed to create hot plasma using electron cyclotron heating, which made it possible to abandon plasma guns and, thus, conduct experiments under more controlled conditions.
With plasma parameters already achieved, such a system, in particular, can be used for research in the field of materials science, since it produces high neutron fluxes.
Deputy Director of the BINP SB RAS A. Ivanov noted that research has already been carried out on the interaction of plasma with the walls of the reactor, record values of energy density per unit area have been obtained. “We now know how tungsten plates erode,” he said.
Scientists believe that developed at the Institute of Applied Physics Russian Academy of Sciences for the BINP SB RAS radiation sources - gyrotrons will be promising for heating, which will make it possible to achieve higher plasma parameters.
Earlier, the BINP SB RAS announced plans to create a prototype thermonuclear reactor. It is assumed that the BINP SB RAS will develop a technical design and a feasibility study for the installation, after which the stage of negotiations with potential partners from other countries will begin.
As reported, the development of a prototype thermonuclear reactor based on a gas-dynamic “multi-mirror” trap is being carried out within the framework of a grant from the Russian Science Foundation. The duration of the program is 2014-2018, the amount of project funding from the Russian Science Foundation is 650 million rubles.
Previously, scientists from the Institute of Nuclear Physics SB RAS obtained a record temperature of 4.5 million degrees (400 electron volts) in a gas dynamic trap (GDT), which used to contain hot plasma in a magnetic field; in 2014, this temperature was increased to 9 million degrees.
HEATING THERMONUCLEAR PLASMAUP TO 10 MILLION DEGREES
A series of successful experiments on electron cyclotron resonance (ECR) plasma heating was carried out at the GDL installation (Fig. 1). The purpose of the experiment was to test the scenario of combined plasma heating by neutral beams (NB) with a power of 5 MW and ECR heating with a power of up to 0.7 MW, to study the physical mechanisms of the magnetohydrodynamic instability of the plasma observed during such heating and to search for ways to suppress it.
The ECR heating system at the GDL installation consists of two pulsed gyrotrons with a frequency of 54.5 GHz and a power measured at the plasma input of 300 and 400 kW. Each of the gyrotrons is powered from specially designed high-voltage power supplies that form a rectangular high-voltage pulse with an amplitude of 70 kV (with stability no worse than 0.5%), a current of up to 25 A, and a duration of up to 3 ms. Gyrotron radiation is supplied through separate closed quasi-optical lines and introduced into a vacuum chamber in the vicinity of two magnetic mirrors as shown in Fig. 3.
To create optimal conditions for ECR heating, an increase in the magnetic field in individual coils located around the absorption region is required. The additional current required to implement effective absorption at the opposite ends of the trap was obtained by reducing the magnetic field in the main body of the trap (from 0.35 to 0.27 T in the center of the installation). Such a perturbation of the magnetic configuration led to a significant deterioration in plasma confinement; in particular, without ECR heating, the electron temperature decreased from 250 eV to 150 eV.
Two ECR heating scenarios were optimized in this magnetic configuration. The first scenario was optimized to increase the lifetime of hot ions resulting from the capture of heating neutral beams by plasma. This regime was characterized by the absorption of gyrotron radiation over almost the entire plasma cross-section, which led to an increase in the electron temperature throughout the entire volume of the plasma.
Since the lifetime of hot ions is proportional to the electron temperature to the power of 3/2, during ECR heating the energy content of the plasma and the flux of D-D fusion neutrons resulting from collisions between hot ions increased significantly (Fig. 4). It was possible to obtain a stable discharge in this mode with an ECR heating power not exceeding 400 kW. The electron temperature at the GDL axis reached 200 eV.
The second scenario was optimized to obtain the maximum electron temperature. In this mode, the main part of the microwave power captured by the plasma was absorbed in a narrow paraxial region. Therefore, when the gyrotrons were turned on, a discharge with a central temperature of up to 1 keV was formed within a few hundred microseconds (Fig. 5). Despite the fact that the radial temperature profile was strongly peaked, the energy balance showed that plasma confinement in the paraxial zone occurs in the gas-dynamic regime, radial transport and classical longitudinal (Spitzer) electron thermal conductivity are strongly suppressed. Measurements using the Thomson scattering method showed that energy is redistributed between thermal electrons, that is, we are talking specifically about the electron temperature, and not about the energy stored in the “tail” of energetic electrons. During these experiments, a record electron temperature for open systems in a quasi-stationary (-1 ms) discharge was achieved at the GDT facility, and for the first time the plasma parameters approached values comparable to toroidal systems.
This circumstance allowed us to conclude that there are good prospects for thermonuclear applications based on open traps. For comparison, in Fig. Figure 6 shows a graph reflecting the progress of the increase in electron temperature in experiments at the GDT facility over the 25 years of the facility’s existence.
A sharp and significant increase in electron temperature when ECR heating is turned on leads to the development of flute-type MHD plasma instability. To suppress this instability in a standard GDL discharge (without ECR heating), the “vortex confinement” method is used. It consists in the fact that a constant electric potential is applied to the periphery of the plasma, causing it to rotate in crossed electric and magnetic fields. To effectively suppress transverse losses during the development of flute instability, the applied radial potential must be comparable to the electron temperature. With a strong increase in plasma temperature during ECR heating, this condition may be violated. To solve this problem, a method of stepwise increasing the radial potential was used, which tracks the increase in temperature when ECR heating is turned on. As a result, it was possible to realize relatively stable ECR plasma heating with a power of 700 kW for a time comparable to the total duration of the discharge in the installation.
The demonstration of a discharge with a record high electron temperature became possible due to the development of optimal scenarios for EC heating of the plasma by an extraordinary wave at the first harmonic in the main volume of the trap. This result provides a reliable basis for creating nuclear fusion reactors based on open traps, which have the simplest axisymmetric magnetic field configuration from an engineering point of view. The immediate application of such reactors may be a powerful source of neutrons from the fusion reaction of deuterium and tritium nuclei, which is necessary for solving a number of problems in thermonuclear materials science, as well as controlling subcritical nuclear reactors, including devices for destroying radioactive waste. Further development of this approach will make it possible to consider the creation, based on open traps, of a “pure” thermonuclear reactor using low-neutron or neutron-free fusion reactions.
Experiments on the GOL-3 installation to improve longitudinal retention in an open trap
The parameters of the plasma in the facility obtained as a result of many years of work and the new ideas that have emerged make it possible to evaluate the prospects of this scheme for confining high-temperature plasma much more optimistically than it was before the start of work on GOL-3 (Fig. 2). The main conclusion is that the main processes occur against the background of a fairly high level of plasma turbulence. A new type of instability has been discovered in the end cells of a multimirror trap, leading to more efficient exchange between groups of transient and trapped particles under conditions of low plasma density near the ends.
August 9, 2016 at 10.40 A press approach with key participants of the 11th international conference on open magnetic systems for plasma confinement will take place at the Institute of Nuclear Physics SB RAS (11 Akademika Lavrentiev Avenue, Novosibirsk). They will talk about the latest results from leading scientific centers engaged in research in this area. For example, scientists at the Institute of Nuclear Physics SB RAS have developed a promising method for generating plasma using high-power microwave radiation in a large-scale open-type magnetic trap (GDT). This method allowed successful experiments to improve plasma confinement with parameters in the thermonuclear range. In addition, at the installation of the Institute of Nuclear Physics SB RAS, the dispersion of splashes of liquid tungsten in thermonuclear reactors of the future was studied.
Participants of the press approach:
1. Alexander Alexandrovich IVANOV, Doctor of Physical and Mathematical Sciences, Deputy Director of the Institute of Nuclear Physics SB RAS for scientific work.
2. Alexander Gennadievich SHALASHOV, Doctor of Physical and Mathematical Sciences, Head of the Sector of Microwave Methods of Plasma Heating at the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod).
3.Yosuke NAKASHIMA , Professor, Center for Plasma Research, University of Tsukuba, Japan. (Prof. Nakashima Yousuke, Plasma Research Center, University of Tsukuba, Japan)
4. Taehyup Oh, Professor, National Institute of Thermonuclear Research, Daejeon, Korea. (Prof. Lho Taihyeop, National Fusion Research Institute, Daejeong, Korea).
The conference is held every two years, alternately at the sites of scientific centers in Russia (Novosibirsk, BINP SB RAS), Japan and Korea. The main areas that will be presented are the physics of plasma confinement in open traps, heating systems for open traps, plasma diagnostics, interaction of plasma with a surface.
There are several options on the basis of which in the future it will be possible to build a thermonuclear reactor - tokamak, stellarator, open traps, reversed field configuration and others. Nowadays, tokamaks are the most developed area, but alternative systems also have a number of advantages: they are technically simpler and can be more economically attractive as a reactor. Perhaps in the future the tokamak will be replaced or will begin to coexist with other types of traps. BINP SB RAS is working on an alternative direction - open traps for plasma confinement.
Previously, it was believed that this type of installation could be considered more as a tool for studying the fundamental properties of plasma, as well as as stands for supporting experiments for the first experimental thermonuclear reactor ITER.
However, recent results - heating plasma to a temperature of 10 million degrees in an open GDL trap (BINP SB RAS, Russia) and demonstrating a quasi-stationary state of plasma at the S-2 installation (Tri Alpha Energy, USA) - have shown that in alternative systems it is possible to achieve much more higher plasma parameters than previously thought.
The largest open traps operate in Russia, Japan, China, South Korea and the USA.
Contacts for accreditation:
Alla Skovorodina,
public relations specialist, BINP SB RAS,
r.t.+7 383 329-47-55, m.t.+7 913 9354687, e-mail:
Brief information on types of fusion reactor models
Tokamak(short for “toroidal magnetic chamber”), a closed magnetic trap shaped like a torus and designed to create and contain high-temperature plasma. Tokamak was designed and created to solve the problem of controlled thermonuclear fusion and create a thermonuclear reactor.
Open traps- a type of magnetic trap for confining thermonuclear plasma in a certain volume of space, limited in the direction along the magnetic field. Unlike closed traps (tokamaks, stellarators), which have the shape of a toroid, open traps are characterized by linear geometry, with magnetic field lines intersecting the end surfaces of the plasma. Open traps have a number of potential advantages over closed traps. They are simpler in engineering terms, they use the energy of the magnetic field confining the plasma more efficiently, the problem of removing heavy impurities and thermonuclear reaction products from the plasma is easier to solve, and many types of open traps can operate in a stationary mode. However, the possibility of realizing these advantages in a fusion reactor based on open traps requires experimental evidence.
Based on materials from D. D. Ryutov, Open traps, UFN 1988, vol. 154, p. 565.
Fusion trap
The Institute of Nuclear Physics, like all institutes of the Siberian Branch of the Russian Academy of Sciences, is relatively young: in 2008 it will turn only 50 years old - the same as the average age of its employees. It is gratifying to see that recently many graduate and undergraduate students have appeared at the BINP who plan to continue their scientific research within its walls. It is known that today’s youth are drawn to where it is interesting, where there are prospects for growth. And at the INP there are undoubtedly such prospects. It should also be emphasized that carrying out the most complex modern experiments requires the efforts of not one person, but a powerful team of like-minded people. That is why the influx of fresh forces is so important for the institute...
Plasma is a mysterious matter,
possessing the property of self-organization
Plasma is a fully or partially ionized gas in which the total negative charge of the particles is equal to the total positive charge. And therefore, in general, it is an electrically neutral medium, or, as physicists say, it has the property of quasineutrality. This state of matter is considered the fourth (after solid, liquid and gaseous) aggregate state and is a normal form of existence at temperatures of the order of 10,000 degrees Celsius and above.
Research into this unusual state of matter in nature has been going on for more than a century. Since the second half of the 20th century, the “general direction” has been the implementation of a self-sustaining controlled thermonuclear fusion reaction (CTF). High-temperature plasma clots are very widespread in the Universe: just mention the Sun and stars. But on Earth there is very little of it. Cosmic particles and the solar wind ionize the upper layer of the Earth's atmospheric shell (ionosphere), and the resulting plasma is retained by the Earth's magnetic field. In other words, it is a kind of earthly magnetic trap. During periods of increased solar activity, the flow of charged particles from the solar wind deforms the planet’s magnetosphere. Due to the development of hydromagnetic instabilities, plasma penetrates into the upper atmosphere in the region of the poles - and atmospheric gases, interacting with charged plasma particles, are excited and emitted. This is responsible for the phenomenon of the aurora, which can only be observed at the poles.
Along with the “general direction” in the study of plasma physics, there are other, no less important, applied ones. This has led to the emergence of numerous new technologies: plasma cutting, welding and metal surface treatment. Plasma can be used as a working fluid in spaceship engines and fluorescent lamps for lighting. The use of plasma technologies has caused a real revolution in microelectronics. Not only has processor performance significantly increased and memory capacity has increased, but the amount of chemicals used in production has also been significantly reduced - thus, the level of environmental damage has been minimized.
Dense high-temperature plasma exists only in stars; on Earth it can only be obtained in laboratory conditions. This unusual state of matter amazes the imagination with a large number of degrees of freedom and, at the same time, the ability to self-organize and respond to external influences. For example, plasma can be held in a magnetic field, causing it to take on different shapes. However, it strives to accept the state that is most energetically favorable for it, which often leads to the development of various instabilities, and, like a living organism, to break free from the rigid “cage” of a magnetic trap if the configuration of this trap does not suit it. That is why the task of physicists is to create such conditions so that the plasma is stable, “lives” in a trap for a long time and calmly, and heats up to thermonuclear temperatures of about 10 million degrees Celsius.
Today, two unique large plasma traps are successfully operating at the BINP, which were the result of the practical application of original ideas and principles born within the walls of the institute. These are open-type traps, significantly different from popular closed magnetic systems. They amaze with their mysterious grandeur and at the same time ease of operation. Over the entire history of work at the facilities, scientists have been able to obtain important results on heating and confining dense hot plasma, as well as make a number of discoveries related to the fundamental properties of this fourth state of matter. Every year presented something new and unusual in one or another conditions for life in traps when changing the configuration of the magnetic field, when creating electric fields, when adding various impurities, as well as when injecting powerful beams into the plasma and “probing” plasma with various diagnostics. And the plasma, “reacting” to such actions, albeit reluctantly, shared its deepest secrets with the researchers...
Gas dynamic trap (GDT)
The GDL installation, created at the Novosibirsk Institute of Nuclear Physics in 1986, belongs to the class of open traps and serves to contain plasma in a magnetic field.
The configuration of the magnetic field in a classical open axially symmetric trap is an elongated region of a uniform magnetic field with maxima at the edges, which are achieved using ring coils of a strong magnetic field. The areas under these coils (those areas of space occupied by the magnetic field in which it reaches its maximum value) are usually called “magnetic plugs”, and a trap arranged according to this principle is called a “mirror cell”. In the simplest case, the magnetic field in the mirror cell is created only by magnetic mirrors.
Charged plasma particles (negative electrons and positive ions) move along the magnetic field lines between the magnetic mirrors, being reflected from them and thus performing oscillatory movements. Particles with kinetic energy sufficient to overcome the potential barrier of the plug leave the trap in one flight.
The differences between a gas-dynamic trap (GDT) and a conventional mirror cell described above are the large extent of the homogeneous field section in the center of the trap and a very large “mirror ratio” (the ratio R = B 1 /B 2 of the magnetic field values in the mirror and in the center of the trap). In this configuration, the mean free path of the ions is small compared to the length of the section of a uniform magnetic field, so the outflow of plasma from the installation occurs according to the laws of gas dynamics, similar to the outflow of gas into a vacuum from a vessel with a small hole, which explains the name of the installation. By making the “holes” in the magnetic mirrors very small and the volume occupied by the plasma large, it is possible to obtain a plasma confinement time sufficient to carry out a controlled thermonuclear reaction. True, the length of such a mirror reactor will be several kilometers. However, the use of various devices, so-called ambipolar plugs, which reduce the plasma flow into the plugs, will reduce the length of the trap to reasonable limits. Therefore, the reactor prospects of such a trap remain attractive. The most promising thermonuclear application of the plasma confinement scheme is the creation, based on the GDT, of a simple and reliable source of fast neutrons with an energy of 14 MeV, which are born in the fusion reaction of deuterium and tritium nuclei. In fact, this is the same thermonuclear reactor (only with low efficiency), consuming energy and producing neutrons. Such a neutron generator can be used to conduct materials science tests of the first wall of a future industrial thermonuclear reactor or to feed a fission reactor with low-energy neutrons, which makes modern nuclear power safe. The project of a neutron source based on a gas-dynamic trap has been developed for many years at the Institute of Nuclear Physics. In order to practically test the predictions of the theory and accumulate a database for creating a neutron source, an experimental model of a gas-dynamic trap - a GDL installation - was created at the Institute of Nuclear Physics SB RAS.
Currently, the international scientific community, dealing with the solution to the problem of CTS, has begun construction of the largest tokamak-type plasma trap called ITER. In the coming decades, ITER should demonstrate the possibility of operating a self-sustaining controlled thermonuclear power plant based on the fusion reaction of deuterium and tritium.However, it is obvious that for the further development of thermonuclear energy of the future and the construction of such stations that will operate for decades and even centuries, today it is necessary to select reliable materials that can withstand strong neutron fluxes throughout their entire service life. To test such materials, a powerful neutron source is required. The BINP has been developing a project for such a source based on GDL for many years.
All the physical principles underlying a compact and relatively inexpensive neutron source based on an open gas-dynamic trap are currently being studied in a real experiment on the accumulation, confinement and heating of plasma in a GDT installation. Already today, direct measurements of the emitted neutron flux are being carried out in experiments with deuterium injection. The deuterium-deuterium fusion reaction under the given experimental parameters produces, in general, a small flux compared to the deuterium-tritium reaction. But for checking model calculations, which are planned to be used in the future for calculations of the source reactor, they are quite sufficient. This December, the installation turns 22 years old: the first plasma was obtained at the end of 1985. Those who built and launched it are still working in the laboratory today.
But the team has also been replenished with new, young and energetic employees: some of them are the same age as the GDL installation itself
The main part of the installation is an axisymmetric mirror cell 7 m long, with a field of 0.3 T in the center and up to 10 T in the plugs, designed to contain two-component plasma.
One of the components - warm “target” plasma - has an electron and ion temperature of up to 100 eV (this is approximately 1,200,000 degrees Celsius) and a density of ~ 5 10 19 particles per cubic meter. This component is characterized by the gas-dynamic confinement mode described above. The other component is fast ions with an average energy of ~ 10,000 eV and a density of up to 2 10 19 particles per cubic meter. They are formed as a result of ionization in the target plasma of powerful beams of atoms, obliquely injected into the trap using special devices - neutral atom injectors. This fast component is characterized by the same confinement mode as in a classical mirror cell: fast ions move in magnetic orbits along magnetic field lines and are reflected from a region of strong magnetic field. In this case, fast ions are slowed down when interacting with particles of the target plasma (mainly electrons) and heat it to 100 eV and higher. With oblique injection and small angular scattering of particles, the density of fast ions turns out to be strongly peaked (large) near the reflection region, and this circumstance is most attractive for the implementation of a neutron source. The fact is that the neutron flux in the fusion reaction is proportional to the square of the density of deuterium and tritium ions. And therefore, with such a density pick, it will be concentrated only in the stopping area, where the “test zone” will be located. The rest of the installation space will experience a much lower neutron load, which will eliminate the need for expensive neutron protection of all generator components.
An important problem on the way to creating a reactor and neutron source based on an axially symmetric mirror cell is plasma stabilization across the magnetic field. In the GDT scheme, this is achieved thanks to special additional sections with a magnetic field profile favorable for stability, which are located behind the magnetic plugs and ensure reliable plasma stabilization.
Another important problem of controlled thermonuclear fusion (CTF) based on open traps is thermal insulation of the plasma from the end wall. The fact is that, unlike closed systems such as a tokamak or stellarator, plasma flows out of an open trap and enters the plasma receivers. In this case, cold electrons emitted under the action of the flow from the surface of the plasma receiver can penetrate back into the trap and greatly cool the plasma. In experiments studying longitudinal confinement at a GDL installation, it was possible to demonstrate that the expanding magnetic field behind the plug in front of the plasma receiver in the end tanks - expanders - prevents the penetration of cold electrons into the trap and provides effective thermal insulation from the end wall.
Within the framework of the GDL experimental program, ongoing work is being carried out related to increasing the stability, target temperature and density of fast plasma particles; with the study of its behavior under various operating conditions of the installation, etc. The study of fundamental properties is also being conducted. It is worth emphasizing that the range of scientific interests and research related to plasma is very wide.
The GDL installation is equipped with the most modern diagnostic tools. Most of them were developed in our laboratory and, among other things, are supplied on a contract basis to other plasma laboratories, including foreign ones.
The team of scientists, engineers and technicians conducting research at the GDT facility is small, but incredibly capable. The high level of qualifications of all its members helps them achieve high results. In addition, the scientific workforce is constantly being replenished with “young blood” - graduates of Novosibirsk State University and Novosibirsk State Technical University. Students of various courses, undergoing practical training in the laboratory, from the first days take an active part in experiments, thereby making a direct contribution to the creation of new knowledge. After the first coursework, they remain for practical training in the laboratory, successfully defend their diplomas, enter graduate school and prepare their candidate's dissertations. We will not hide that this makes us, scientific leaders, extremely happy.
Another trap - "GOL-3" - and a slightly different angle on thermonuclear fusion
Humanity is experiencing a shortage of electricity, and in the near future this problem will become a priority: reserves of fuel - oil and gas - used in the main modern power plants, alas, are being depleted. That is why thermonuclear reactors should become the basis of the electric power industry of the future.
Thermonuclear reactions are reactions of fusion of light nuclei, such as the hydrogen isotopes deuterium and tritium, releasing large amounts of energy. To carry out these reactions, high temperatures are required - more than 10 million degrees Celsius. It is known that any substance at a temperature of more than 10 thousand degrees Celsius becomes plasma. Contact with a solid body leads to instantaneous cooling and explosive destruction of the surface of the solid body, so the plasma must be isolated from the structure: for this purpose it is placed in a magnetic field.
It is extremely difficult to heat a substance to enormous temperatures and hold it in a magnetic field for a long time - and therefore many experts consider controlled thermonuclear fusion (CTF) to be the most difficult task ever faced by humanity.
The GOL-3 installation at the Institute of Nuclear Physics SB RAS is designed to heat and contain thermonuclear plasma in a multiple-mirror magnetic field. The installation consists of three main parts: the U-2 accelerator, a 12-meter solenoid (a unit for creating a strong magnetic field) and an output unit.
The electron beam used in the installation is created by the most powerful accelerator in the world (in its class) U-2. In it, electrons are pulled by an electric field from an explosive emission cathode and accelerated by a voltage of about 1 million volts. At a current of 50,000 Amps, the system power reaches 50 GW. (But the entire Novosibirsk consumes 20 times less energy during the daytime.) With a beam duration of about 8 microseconds, it contains up to 200,000 J of energy (which is equivalent to the explosion of a hand grenade).
In the main solenoid, when a beam passes through a deuterium plasma with a density of n = 10 20 -10 22 particles per cubic meter, due to the development of two-stream instability, a large level of microturbulence arises and the beam loses up to 40% of its energy, transferring it to plasma electrons. The heating rate is very high: in 3-4 microseconds, plasma electrons are heated to a temperature of about 2,000-4,000 eV (23-46 million degrees Celsius: 1 eV = 11,600 degrees Celsius) - this is a world record for open traps (for comparison: on at the 2XIIB installation in the USA, the temperature did not exceed 300 eV versus 2,000-4,000 eV at GOL-3).
The magnetic field in the main solenoid is multi-mirror (55 mirror cells), i.e. the maxima (5 T) and minima (3 T) of the field alternate, and the distance between the maxima (22 cm) is of the order of the ion path length. What does this lead to: if an ion leaves a single mirror cell and flies along the magnetic field, then in a neighboring mirror cell it will collide with another particle, as a result it can be captured by a neighboring mirror cell, and then it will “forget” where it was flying. Thus, the expansion of plasma from the trap is significantly slowed down. But the hot plasma retention time on GOL-3 is up to 1 millisecond, which can be considered an undoubted achievement of scientists.
Multiple mirrors lead to inhomogeneity in the transfer of energy from the beam to the plasma electrons: where the magnetic field is stronger, the heating of the electrons is stronger. When heated by a beam, a high level of turbulence contributes to a strong (more than a thousand times) suppression of electronic thermal conductivity, so temperature inhomogeneities are not equalized, and consequently, large differences in plasma pressure occur: for this reason, the plasma begins to move as a whole. From areas of high pressure to pressure minima on both sides, two counter plasma flows begin to move, which collide and warm up to a temperature of 1-2 keV (it is slightly higher than in the center of the Sun). This rapid heating mechanism was discovered on GOL-3 four years ago during experiments. From the theory it followed that it should be accompanied by sharp jumps in plasma density, which were soon discovered by Thomson scattering of a laser beam.
After passing the main solenoid, the beam enters the output node, which is capable of receiving a powerful beam of electrons, as well as a plasma flow, without being destroyed. To do this, the magnetic field in the output node must be divergent, which reduces the energy density in the beam by 50 times, and the beam receiver must be graphite. The peculiarity of graphite, firstly, is that it does not have a liquid phase, it evaporates immediately; secondly, it has a low density (2 g/cm 3 ), due to which the electron range in it is higher than in metals, and therefore, the energy is released in a larger volume and does not exceed the threshold of explosive destruction of graphite, and therefore the erosion of graphite is small - about 1 micron per shot. The presence of a powerful plasma flow at the output of the installation makes it possible to conduct experiments on the irradiation of materials for thermonuclear reactors of the future: these reactors will be subjected to such a high level of thermal loads, which is still unrealistic to achieve in other plasma installations today.
Another important task that can be solved using the output node is ensuring the safety of beam transportation through the main solenoid. The complexity of the problem lies in the fact that the beam current in the solenoid (30 kA) is greater than the stability threshold (for the GOL-3 camera - 12 kA), so the beam is unstable and can be thrown onto the wall or intra-chamber structures, which will lead to their destruction. For this purpose, before beam injection, a discharge (lightning) must be struck in the output node, and then the main solenoid will be filled with relatively cold (several eV) preliminary plasma, in which, upon injection of the electron beam, a counter current is induced, and it completely compensates for the beam current, which in general will ensure stability of the system (the total current will not exceed 3 kA).
One of the most serious problems of CTS is plasma stability, i.e., creating conditions under which plasma could not leave the trap across the magnetic field due to the development of various plasma instabilities. For open traps, the most dangerous is groove instability. Its essence is that the plasma pushes apart the magnetic lines of force and slips out between them. In the GOL-3 plasma, this instability is suppressed due to the shift of magnetic field lines at different plasma radii, which arises due to the complex configuration of currents in the plasma. The beam current flows in the center of the plasma, and there is also a high level of turbulence. The reverse current flows through the plasma, but due to turbulence at the center, its resistance increases - and the reverse current flows along the surface of the plasma cord. The straight-line current creates a circular magnetic field around itself, which, together with the longitudinal field of the solenoid, gives a spiral magnetic field. At different radii, the current is different (and flows in different directions) - therefore, the pitch and direction of the spiral are also different. That is why when a plasma groove pushes magnetic field lines apart at one radius, it encounters field lines at a different angle and cannot move them apart - this is how the groove instability is suppressed.
Diagnosing hot plasma is also a difficult task, i.e. determining its temperature, composition, density, magnetic field strength and much more. You can't insert a thermometer there - it can explode - and the plasma will cool down. It is necessary to use various special methods, which are divided into passive and active. Using passive diagnostics, you can study what the plasma emits. With the help of active ones, inject, for example, laser light or beams of atoms into the plasma and see what comes out of it.
Among the passive diagnostics, the GOL-3 installation operates photon detectors and spectrometers in the visible, ultraviolet, x-ray and gamma regions, neutron detectors, a charge exchange neutral detector, diamagnetic probes and Rogowski belts. Active ones include several laser systems, an atomic beam injector and an injector of solid-state grains.
Although tokamaks are now closest to reactor parameters (they have a higher temperature and confinement time), thanks to GOL-3, multimirror traps are also being considered as a variant of a fusion reactor. The plasma density in GOL-3 is almost a hundred times higher than in tokamaks on average; moreover, unlike tokamaks, there are no restrictions on plasma pressure in this installation. If the pressure is comparable to the pressure of the magnetic field (5 T creates a pressure of ~100 atmospheres), then the trap will go into the “wall” confinement mode - the magnetic field pushed out of the plasma (since the plasma is diamagnetic) will concentrate and increase near the walls of the chamber and will still be able to hold plasma. At present, there is not a single reason that would fundamentally limit the growth of the main thermonuclear parameters (n, T and confinement time) in multimirror traps.
The main task facing the team of the GOL-3 installation today is the development of the concept of a multiple-mirror thermonuclear reactor, as well as experimental verification of the main provisions of this concept.
Not by bread alone... But by bread too
Plasma research cannot be carried out without diagnostics, and therefore the developments of the BINP are readily purchased. The institute enters into contracts for the supply of certain diagnostic tools, and researchers are engaged in the development and assembly of these tools in their own workshops. These are mainly diagnostic injectors, but there are also some optical devices, interferometers, etc. The matter does not stand still: BINP also knows how to earn money.
Literature
1. A. Burdakov, A. Azhannikov, V. Astrelin, A. Beklemishev, V. Burmasov at all. Plasma heating and confinement in GOL-3 multimirror trap // Transactions of Fusion Science and Technology. - 2007. - Vol. 51. - No. 2T. - Pp. 106-111.
2. A. V. Arzhannikov, V. T. Astrelin, A. V. Burdakov, I. A. Ivanov, V. S. Koidan, S. A. Kuznetsov, K. I. Mekler, S. V. Polosatkin, V. V. Postupaev, A. F. Rovenskikh, S. L. Sinitsky, Yu. S. Sulyaev, A. A. Shoshin. Study of the mechanism of rapid heating of ions in the multimirror trap GOL-3 // Plasma Physics. - 2005. - T. 31. - No. 6. - P. 506-520.
There is probably no field of human activity as full of disappointments and rejected heroes as attempts to create thermonuclear energy. Hundreds of reactor concepts, dozens of teams that consistently became the favorites of the public and state budgets, and finally there seemed to be a winner in the form of tokamaks. And here again - the achievements of Novosibirsk scientists are reviving interest around the world in a concept that was cruelly trampled upon in the 80s. And now more details.
An open GDL trap that yielded impressive results
Among the variety of proposals on how to extract energy from thermonuclear fusion, they are most oriented towards the stationary confinement of a relatively loose thermonuclear plasma. For example, the ITER project and more widely - tokamak toroidal traps and stellarators - come from here. They are toroidal because this is the simplest form of a closed vessel made of magnetic fields (due to the hedgehog combing theorem, a spherical vessel cannot be made). However, at the dawn of research in the field of controlled thermonuclear fusion, the favorites were not traps with complex three-dimensional geometry, but attempts to contain plasma in so-called open traps. These are usually also cylindrical magnetic vessels in which the plasma is well retained in the radial direction and flows out from both ends. The idea of the inventors here is simple - if the heating of a new plasma by a thermonuclear reaction proceeds faster than the consumption of heat leaking from the ends - then God bless him, with the openness of our vessel, energy will be generated, but the leakage will still occur into the vacuum vessel and the fuel will be walk in the reactor until it burns out.
The idea for an open trap is a magnetic cylinder with plugs/mirrors at the ends and expanders behind them.
In addition, in all open traps, one or another method is used to stop the plasma from escaping through the ends - and the simplest here is to sharply increase the magnetic field at the ends (install magnetic “plugs” in Russian terminology or “mirrors” in Western terminology), while incoming charged particles will, in fact, spring back from the mirror plugs and only a small part of the plasma will pass through them and enter special expanders.
And a slightly less schematic image of the heroine of today - a vacuum chamber is added in which plasma flies, and all sorts of equipment.
The first experiment with a “mirror” or “open” trap, the Q-cucumber, was carried out in 1955 at the American Lawrence Livermore National Laboratory. For many years, this laboratory has become a leader in the development of the concept of CTS based on open traps (OT).
The world's first experiment - an open trap with magnetic mirrors Q-cucumber
Compared to closed competitors, the advantages of OL include the much simpler geometry of the reactor and its magnetic system, and therefore its low cost. Thus, after the fall of the first favorite of the CTS - Z-pinch reactors, open traps received maximum priority and funding in the early 60s, as they promised a quick solution for little money.
Early 60s, Table Top Trap
However, it was not by chance that the same Z-pinch retired. His funeral was associated with a manifestation of the nature of plasma - instabilities that destroyed plasma formations when attempting to compress the plasma with a magnetic field. And it was precisely this feature, poorly studied 50 years ago, that immediately began to irritate experimenters with open traps. Flute instabilities force us to complicate the magnetic system, introducing, in addition to simple round solenoids, “Ioffe sticks”, “baseball traps” and “yin-yang coils” and reduce the ratio of magnetic field pressure to plasma pressure (parameter β).
“Baseball” superconducting magnet trap Baseball II, mid-70s
In addition, plasma leakage occurs differently for particles with different energies, which leads to plasma nonequilibrium (i.e., a non-Maxwellian spectrum of particle velocities), which causes a number of other unpleasant instabilities. These instabilities, in turn, “rocking” the plasma accelerate its exit through the end mirror cells. At the end of the 60s, simple versions of open traps reached the limit on the temperature and density of the confined plasma, and these figures were many orders of magnitude less than those needed for a thermonuclear reaction. The problem was mainly the rapid longitudinal cooling of the electrons, which then caused the ions to lose energy. New ideas were needed.
The most successful ambipolar trap TMX-U
Physicists are proposing new solutions related primarily to improving longitudinal plasma confinement: ambipolar confinement, corrugated traps and gas-dynamic traps.
- Ambipolar confinement is based on the fact that electrons “flow” from an open trap 28 times faster than deuterium and tritium ions, and a potential difference arises at the ends of the trap - positive from the ions inside and negative from the outside. If the fields with dense plasma are amplified at the ends of the installation, then the ambipolar potential in the dense plasma will keep the internal less dense contents from scattering.
- Corrugated traps create a “ribbed” magnetic field at the end, in which the expansion of heavy ions is slowed down due to “friction” against the trap field locked in the “cavities”.
- Finally, gas-dynamic traps create with a magnetic field an analogue of a vessel with a small hole, from which plasma flows at a lower speed than in the case of “mirror plugs”.
The diagram of the magnetic system and plasma heating of Gamma-10 clearly illustrates how far OL solutions had come from simple solutions by the 80s.
At the same time, in 1975, at the 2X-IIB trap, American researchers were the first in the world to achieve a symbolic ion temperature of 10 keV - optimal for the thermonuclear burning of deuterium and tritium. It should be noted that in the 60s and 70s they were marked by the pursuit of the desired temperature in any way, because... temperature determines whether the reactor will work at all, while two other parameters - density and the rate of energy leakage from the plasma (or more commonly called “holding time”) can be compensated for by increasing the size of the reactor. However, despite the symbolic achievement, 2X-IIB was very far from being called a reactor - the theoretical power output would have been 0.1% of that spent on plasma confinement and heating. A serious problem remained the low temperature of electrons - about 90 eV against the background of 10 keV ions, associated with the fact that one way or another the electrons were cooled against the walls of the vacuum chamber in which the trap was located.
Elements of the now defunct ambipolar trap AMBAL-M
The beginning of the 80s marked the peak of development of this branch of CTS. The peak of development is the American MFTF project worth $372 million (or $820 million in today’s prices, which brings the project closer in cost to a machine such as the Wendelstein 7-X or the K-STAR tokamak).
Superconducting magnetic modules MFTF…
And the body of its 400 ton end superconducting magnet
It was an ambipolar trap with superconducting magnets, incl. masterpiece terminal “yin-yang”, numerous systems and heating of plasma diagnostics, a record in all respects. It was planned to achieve Q=0.5, i.e. The energy output of a thermonuclear reaction is only two times less than the cost of maintaining the operation of the reactor. What results has this program achieved? It was closed by a political decision in a state close to readiness for launch.
End "Yin-Yang" MFTF during installation in a 10-meter vacuum chamber of the installation. Its length was supposed to reach 60 meters.
Despite the fact that this decision, shocking from all sides, is very difficult to explain, I will try.
By 1986, when MFTF was ready to launch, another favorite star lit up in the horizon of TCB concepts. A simple and cheap alternative to the “bronze” open traps, which by this time had become too complex and expensive against the backdrop of the original concept of the early 60s. All these superconducting magnets of puzzling configurations, fast neutral injectors, powerful radio frequency plasma heating systems, puzzling instability suppression circuits - it seemed that Such complex installations will never become the prototype of a thermonuclear power plant.
JET in original limiter configuration and copper coils.
So tokamaks. In the early 80s, these machines reached plasma parameters sufficient to burn a thermonuclear reaction. In 1984, the European JET tokamak was launched, which should show Q=1, and it uses simple copper magnets, its cost is only 180 million dollars. In the USSR and France, superconducting tokamaks are being designed, which waste almost no energy on the operation of the magnetic system. At the same time, physicists working on open traps for years have been unable to make progress in increasing plasma stability and electron temperature, and promises for MFTF achievements are becoming increasingly vague. The next decades, by the way, will show that the bet on tokamaks turned out to be relatively justified - it was these traps that reached the level of power and Q that were of interest to power engineers.
Successes of open traps and tokamaks by the beginning of the 80s on the “triple parameter” map. JET will reach a point slightly above "TFTR 1983" in 1997.
The decision on MFTF finally undermines the position of this direction. Although experiments at the Novosibirsk Nuclear Physics Institute and at the Japanese Gamma-10 facility continue, the fairly successful programs of their predecessors TMX and 2X-IIB are also being closed in the United States.
End of story? No. Literally before our eyes, in 2015, an amazing quiet revolution is taking place. Researchers from the Institute of Nuclear Physics named after. Budkera in Novosibirsk, who consistently improved the GDL trap (by the way, it should be noted that in the West, ambipolar rather than gas-dynamic traps prevailed) suddenly reach plasma parameters that were predicted as “impossible” by skeptics in the 80s.
Once again GDL. The green cylinders sticking out in different directions are the neutral injectors, which are discussed below.
The three main problems that have buried open traps are MHD stability in an axisymmetric configuration (requiring complex-shaped magnets), nonequilibrium of the ion distribution function (microinstability), and low electron temperature. In 2015, the GDL, with a beta value of 0.6, reached an electron temperature of 1 keV. How did this happen?
The departure from axial (cylindrical) symmetry in the 60s in attempts to overcome groove and other MHD plasma instabilities led, in addition to the complication of magnetic systems, to an increase in heat loss from the plasma in the radial direction. A group of scientists working with GDL used an idea from the 80s to apply a radial electric field that creates a vortex plasma. This approach led to a brilliant victory - with beta 0.6 (let me remind you that this ratio of plasma pressure to magnetic field pressure is a very important parameter in the design of any thermonuclear reactor - since the rate and density of energy release are determined by the plasma pressure, and the cost of the reactor is determined the power of its magnets), compared to the tokamak 0.05-0.1 plasma is stable.
New “diagnostic” measuring instruments allow us to better understand the physics of plasma in GDT
The second problem with micro-instabilities, caused by a lack of low-temperature ions (which are drawn from the ends of the trap by an ambipolar potential) was solved by tilting the neutral beam injectors at an angle. This arrangement creates ion density peaks along the plasma trap, which trap “warm” ions from escaping. A relatively simple solution leads to complete suppression of microinstabilities and to a significant improvement in plasma confinement parameters.
Neutron flux from thermonuclear burning of deuterium in a GDL trap. Black dots are measurements, lines are various calculated values for different levels of micro-instabilities. Red line - micro-instabilities are suppressed.
Finally, the main “gravedigger” is the low electron temperature. Although thermonuclear parameters have been achieved for the ions in the traps, high electron temperature is the key to keeping hot ions from cooling, and therefore high Q values. The reason for the low temperature is the high thermal conductivity “along” and the ambipolar potential sucking “cold” electrons from the expanders behind the ends traps inside the magnetic system. Until 2014, the electron temperature in open traps did not exceed 300 eV, and the psychologically important value of 1 keV was obtained in the GDL. It was obtained through subtle work with the physics of the interaction of electrons in end expanders with neutral gas and plasma absorbers.
This turns the situation upside down. Now simple traps again threaten the primacy of tokamaks that have reached monstrous sizes and complexity (GDML-U, which combines the ideas and achievements of GDT and a method for improving the longitudinal retention of GOL. Although under the influence of new results the image of GDML is changing, it remains the main idea in the field of open traps .
Where do current and future developments stand compared to competitors? Tokamaks, as we know, have reached the value of Q=1, solved many engineering problems, will move on to the construction of nuclear rather than electrical installations, and are confidently moving towards the prototype of a power reactor with Q=10 and a thermonuclear power of up to 700 MW (ITER). Stellarators, which are a couple of steps behind, are moving from studying fundamental physics and solving engineering problems at Q = 0.1, but do not yet risk entering the field of true nuclear installations with thermonuclear combustion of tritium. GDML-U could be similar to the W-7X stellarator in terms of plasma parameters (being, however, a pulsed installation with a discharge duration of several seconds versus the half-hour long-term operation of the W-7X), however, due to its simple geometry, its cost could be several times higher smaller than the German stellarator.
BINP assessment.
There are options for using GDML as a facility for studying the interaction of plasma and materials (however, there are quite a lot of such facilities in the world) and as a thermonuclear neutron source for various purposes.
Extrapolation of the dimensions of the HDML depending on the desired Q and possible applications.
If tomorrow open traps once again become favorites in the race to CTS, one could expect that due to lower capital investments in each stage, by 2050 they will catch up and surpass tokamaks, becoming the heart of the first thermonuclear power plants. Unless the plasma brings new unpleasant surprises...
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Open traps
Open traps are one of the types of installations for magnetic confinement of thermonuclear plasma. Open traps have a number of important advantages over other containment systems: they are attractive from an engineering point of view; they effectively use a plasma-confining magnetic field; they allow work in a stationary mode; they solve the problem of removing thermonuclear reaction products and heavy impurities from plasma in a relatively simple way. However, for a long time it was believed that the prospects of open traps as the basis of a thermonuclear reactor are doubtful due to the too high rate of plasma loss along the magnetic field lines. The situation has changed for the better only during the last decade, when a number of improvements to open traps have been proposed, which have largely eliminated this drawback. The review outlines the physical principles of new types of open traps (ambipolar, centrifugal, multiple-mirror, gas-dynamic, etc.), describes the current state of research on them, and makes forecasts for the future prospects of these systems. The possibilities of using open traps as high-flux neutron generators with an energy of 14 MeV are being considered. Il. 29. Bibliography. references 97 (102 titles).
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