What can be solar, wind, fuel, and nuclear? The right guess would be energy or power engineering. Thermonuclear power engineering does not exist as yet, though thermonuke offers an inexhaustible and fairly safe source of energy, which, however, has found the only practical application so far, a hydrogen bomb…
Afamous physicist L. A. Artsimovich, whose contribution to the progress in thermonuclear research is invaluable, believed that physicists had never faced a problem as hard as controlled thermonuclear fusion for the whole history of physics. Yet, solving this problem would mean freeing people from energy-hungry forever.
Thermonuclear (as well as nuclear) power engineering is based on the Einstein’s mass defect*. When light nuclei fuse into heavier ones, this “lost” mass becomes converted to energy carried by particles emitted during the fusion: neutrons, protons, alpha particles, etc. It is the thermonuclear reactions that are responsible for the existence of stars, including the Sun, a class G-2 yellow dwarf next to the Earth. People are very much tempted to “stick” to this source of energy in the Universe.
Nuclear reactions in stars were first mentioned in 1928. The very idea belonged to R. Atkinson, F. Houtermans, and G. Bethe who later became a Nobel Prize winner. G. A. Gamow, a famous Russian-born physicist, also contributed to the knowledge of thermonuclear reactions. He received a fantastic offer from N. I. Bukharin, a member of the Soviet government, in 1932 after his presentation at the Presidium of the USSR Academy of Sciences. G. A. Gamow recalled: “I was offered to have the total electric power of the Moscow region at my command for a few minutes once a week at the night time to obtain a nuclear reaction by letting the energy through a thick copper conductor saturated with small bubbles of a lithium-hydrogen mixture. I refused, and I’m glad I did because it would have led nowhere at that time.”
Systematic research of controlled thermonuclear fusion (CTF) started in the 1950s simultaneously in three countries: UK, USA, and USSR. One can easily guess that the prime purpose was not a peaceful one. Naturally, all research was classified, until N. S. Khrushchev and academician I. V. Kurchatov visited the UK in 1956 where I. V. Kurchatov delivered a lecture on experimental thermonuclear research in the USSR.
The three countries disclosed some of their secrets at the 1958 conference on the peaceful use of atomic energy, and it turned out that their approaches to the development of controlled thermonuclear fusion almost coincided.
Plasma traps and thermonuclear fuel
What is needed to initiate controlled thermonuclear fusion? Self-sustained thermonuclear reactions are possible only in plasma, which is an ionized gas consisting of positively charged nuclei and electrons. This is a typical state of the greater part of matter in the Universe, including stars.
The main problem is that CTF requires high-temperature plasma to be restrained from spreading for some time and, simultaneously, to be insulated from the reactor walls. The task is really challenging. The plasma of the Sun and stars is securely held by gravity forces. Therefore, thermonuclear reactions do occur there though the highest temperature on the Sun is “as low” as 16 million degrees. Sustaining the reaction in the terrestrial conditions, however, requires heating plasma to as hot as 100–200 million degrees.
Although numerous, the systems for confinement of high-temperature plasma ever proposed by physicists are of two main groups. Systems of the first group employ the so-called inertial confinement (used to develop hydrogen weapons) in which a specially prepared target containing high-density thermonuclear fuel is rapidly compressed in some way, for instance, by X rays, ion or laser beams. Those of the other group imply thermal insulation of plasma using magnetic field.
In 1950, A. D. Sakharov and I. E. Tamm, Soviet physicists, put forward the idea of a closed magnetic thermonuclear reactor. A year later, L. Spitzer from the US suggested a stellarator. Actually, Sakharov meant both systems. His reasoning was as follows: putting plasma into a toroid (a hollow bagel) and generating a toroidal magnetic field provides thermal insulation across the field but causes plasma expulsion toward the low field. To help the problem, a helical magnetic field can be generated in plasma itself using a system of external conductors. This device is actually a stellarator.Deuterium, the heavy hydrogen isotope with its nucleus containing a neutron in addition to a proton, occurs in common water, though in a fairly low concentration of 1 atom per 6000 atoms of normal hydrogen. Nuclear fusion of deuterium, extracted from one liter of water and then ionized and heated as hot as to leap over the Coulomb repulsion barrier, would release the amount of energy equivalent to that from 300 liters of gasoline!
Yet, Sakharov abandoned the idea as too sophisticated. Another idea was to transmit current over plasma along a toroid and generate the toroidal magnetic field simultaneously. This produces a helical magnetic field in plasma as well. It was the very idea implemented in the USSR in systems called tokamaks. Spitzer’s stellarator was based on the more sophisticated magnetic winding.
When justifying the first thermonuclear reactor, Sakharov suggested using the D-D reaction (D stands for deuterium, a stable heavy hydrogen isotope).
Soon it became, however, clear that at least the first thermonuclear reactors should operate on the basis of fusion with participation of the nuclei of stable heavy (D, deuterium) and radioactive superheavy (T, tritium) hydrogen isotopes. The point is that the D-D reaction with release of excess energy is possible at about one billion degrees, whereas the D-T reaction proceeds in a much “colder” plasma of 100–200 million degrees.Tritium, the superheavy hydrogen isotope is never found in nature because its half-life is as short as 12 years. Therefore, it has to be produced somehow.
If a thermonuclear reactor is coated with a lithium “blanket”, each neutron emanating from plasma and interacting with lithium will spend a great part of its energy on lithium heating. Moreover, the reaction additionally yields one and a half atoms of tritium.
Thus reproduction of tritium is secured once the reactor is set into operation
Some alternative ideas were developed almost concurrently. The most popular one implied the use of the so-called open magnetic trap suggested in 1953 independently by G. I. Budker from the USSR and R. Post from the USA. Six years later, it was proved valid in an experiment performed by S. N. Rodionov, a scientist from the newly founded Institute of Nuclear Physics of the Siberian Branch of the USSR Academy of Sciences in Novosibirsk.
From pessimism to hope
Let us turn back to the tokamaks. One key problem thermonuke faced in its early years was plasma diffusion across magnetic field. A thermonuclear reactor could have a reasonable size in the case of classical diffusion. Yet, back in 1948, D.Bohm, a US scientist, discovered high turbulent diffusion across magnetic field when analyzing experiments with arc discharge in rarefied gases in high magnetic fields. Bohm diffusion leaves almost no hope for successful implementation of thermonuke in devices of a technologically feasible size. Nobody knew then what the real situation was.
Launching the first tokamak of a rather moderate size in 1955 was also the first failure, as plasma cooled rapidly because of impurities. In 1958, Spitzer drew rather pessimistic conclusions from experiments on a stellarator as he attributed the plasma loss from the trap to Bohm diffusion, which made the research pointless. Three years later, however, B. B. Kadomtsev from the Institute of Atomic Energy, a future academician, analyzed the results of open trap experiments and showed that no Bohm diffusion existed. After that A. A. Galeev and R. Z. Sagdeev (also future academicians) from the Institute of Nuclear Physics reconciled the two viewpoints by creating a neoclassical theory of plasma diffusion for closed magnetic configurations, which remains the basis for thermonuclear studies.
After N. A. Yavlinsky, the author of the first toroidal system, had perished, Academician L. A. Artsimovich took over to lead the tokamak research. The progress was quite appreciable: The electron temperature reached 150 eV (1.75 million degrees) in 1963 and increased fourfold to as high as 600 eV in only three years! Nevertheless, the scientific community doubted those results because the achievements in other fields were much less encouraging. To dispel the doubts, Artsimovich bet with US physicists from Princeton and invited UK scientists to measure the electron temperature by the most reliable method of Thomson scattering.
So, a joint presentation of UK and USSR physicists at the IAEA International Conference on plasma physics and controlled thermonuclear reactions held in Novosibirsk in 1968 demonstrated that the electron temperature in the T-3 tokamak reached 1000 eV for the first time in the world. The US scientists lost the bet.
In 1975, the USSR launched the T-10 setup which was the largest tokamak at that time. The ion and electron temperatures in it reached 1 keV and 3 keV, respectively. Naive people would wonder why should electrons be heated, as thermonuclear fusion involves positively charged nuclei. Actually the nuclei cannot keep hot for a long time being cooled by cold electrons, which reduces the lifetime of plasma and impedes self-sustained thermonuclear burn.
By the end of the 1970s, an ion temperature of 7 keV was obtained in the PLT tokamak (USA), and slightly later, the electron temperature reached 10 keV in the T-10 tokamak. Those were the absolute world records. The path forward was open.
Greater and more powerful
Thus, the outcome of the first thirty years of thermonuclear research was that, first, the idea of Bohm diffusion was disproved and, second, plasma confining was found possible in principle.
The greatest success was achieved with tokamak-type closed traps. This prompted many developed countries to abandon their experiments with alternative systems, which now appears a serious mistake. Only Germany, USSR, and Japan kept stellarators, and the USSR and Japan continued with open traps, whereas the USA actually canceled any research other than tokamaks.
Studies of open traps went on at the Institute of Nuclear Physics in Novosibirsk. These were scientists from the Institute who developed all advanced approaches to open magnetic systems which found application in thermonuclear research worldwide.The name “tokamak” was coined by I. N. Golovin, a follower of academician Kurchatov. Originally it was “tokamag”, an abbreviation from the Russian for “toroidal magnetic chamber” but N. A. Yavlinsky, the author of the first toroidal system, suggested to substitute “mak” for “mag” for euphony. This is the version borrowed by all languages
After the success with tokamaks, the respective designing in various countries tended to achieve an ever bigger size of the systems, because the bigger the setup the longer the lifetime of plasma and the easier the approach to self-sustained thermonuclear fusion. The objective of the next stage was to develop systems with a “non-negative balance”, i.e., with the value of the Q parameter (ratio of the energy released in thermonuclear reactions to the energy spent on maintaining plasma) of the order of unity.
According to Lawson’s criterion, energy released in thermonuclear reactions exceeds that spent on maintaining high-temperature plasma under certain conditions defined by plasma density and lifetime; the ion temperature is another important factor. Therefore, the triple product of the three parameters is the most reasonable indicator of contiguity to the conditions of self-sustained thermonuclear burn. Its value is getting almost twice as high every two years, which corresponds to the growth rate of computer memory.
What are the most relevant recent achievements? A record-beating ion temperature of 400 million degrees was obtained in the JET tokamak in Europe. A power of more than 16 MW was released in the same setup in neutrons produced in the deuterium-tritium reaction.
TORE SUPRA, the only big superconducting tokamak operated today allowed confining plasma for 4.5 minutes. This experiment proved the possibility of stationary sustaining toroidal current in a tokamak.
The problem of the Q ratio has been solved in three world largest tokamaks (TFTR, JET, and JT-60U). These systems are the most suitable to approach self-sustained thermonuclear burn.
As for alternative systems, the Japanese superconducting stellarator LHC produced plasma with its parameters only slightly inferior to those achieved in the biggest tokamaks. Moreover, the system sustained plasma for 3.5 hours!
Open magnetic traps are going to have their application as well. They can be effective in future in the fields where tokamaks are helpless: where the energy of thermonuclear reactions is released in charged particles rather than in neutrons. It is clear, however, that the first thermonuclear reactors will be based on the deuterium-tritium reaction which provides a high-density thermonuclear neutron flux. Therefore, the nearest goal is to study the properties of materials for reactors in terms of radiation resistance.
Testing the existing and newly developed materials requires a reliable, efficient, and powerful source of thermonuclear neutrons. The easiest way to solve the problem is to use a device based on a gas-dynamic trap designed at the Institute of Nuclear Physics in Novosibirsk which has already been appreciated by the worldwide thermonuclear community.
An electron temperature of the order of 2 keV recently reached in a multi-mirror open magnetic trap at the same institute far exceeds that obtained before in similar systems. About the same ion temperature was achieved with a plasma density almost two orders of magnitude higher than that commonly used in tokamaks.
The last step
Today’s attitude to thermonuclear power engineering is controversial. Some scientists find it absolutely pointless to study thermonuke being discouraged by over 50 years of failure and by the vagueness of success. Yet, others take the situation differently. Academician V. L. Ginzburg gave the CTF problem top priority in the science of the 21st century: “This is a problem of great importance, and yet unresolved; therefore, I would leave it out of the list [of priorities] only after the first thermonuclear reactor has been set into operation.”According to Lawson’s criterion, the energy released in thermonuclear reactions exceeds that spent on maintaining high-temperature plasma at n τ > 2•1020m-3s, where n is the density (number of particles per cubic meter) and τ is the lifetime (characteristic cooling time in seconds) of plasma. The contiguity to the conditions of self-sustained thermonuclear burn is indicated by the triple product nTi τ (Ti is plasma ion temperature). Its value has been getting ever higher lately.
The scientific community has recently started a project on a tokamak-type international thermonuclear experimental reactor called ITER. The countries involved in the ITER project are the European Community, Russia, USA, Japan, China, and Korea. In addition, India and Brazil expressed their wish to join the project.
The project duration is expected to be 20 years: 8 years for construction, 5 years for hydrogen-plasma-related activities, and 7 years for tritium-related activities. The construction of an experimental fusion power plane DEMO will begin only after this campaign aiming at proving the possibility for a self-sustained thermonuclear reaction has been completed.Plasma physics in the frame of thermonuclear research has offered a number of novel technologies which found numerous applications in microelectronics, TV broadcasting, metallurgy, refractory production, space research, ecology, and many other fields of human activity
It is difficult to guess which principles DEMO will be based on: tokamak, stellarator, open trap, or some other system of plasma confinement. One thing can be predicted for sure: the ITER project will succeed and thermonuclear fusion will contribute notably to world power engineering in the second half of this century.P.S. An important event occurred while the manuscript was being edited: The ministers of foreign affairs of the ITER-member countries agreed to start the construction of the first experimental thermonuclear reactor and signed the agreement in Cadarache (France) on June 28, 2005, after long negotiations.
It sounds like things are moving…
The achieved success gives a hope that this long-awaited time will really come and the first man-made thermonuclear “stars” will light up on our planet and will lighten substantially the energy burden of the mankind. Basic science has to take the last step to attain to the goal.
*mass defect is the difference between the sum of masses of interacting bodies before (in the free state) and after (in the bound state) interaction (ed.)