IGNITOR

in the Context of

Controlled Thermonuclear Fusion


Ignitor is a compact high magnetic field device intended for producing an ignited plasma.

At present it is the only experiment designed to  achieve ignition in a fusion reacting plasma.

A fundamental step towards demonstrating that a fusion reactor can produce net power.

Copyright © Luisa Bonolis 2009-2015 All Rights Reserved

INTRODUCTION

The inconveniences connected with the use of the energy sources available at present (oil, coal, renewable sources, fission nuclear energy) are well known. Exhaustion of natural resources, inadequacy to meet long term energy needs, environmental impact, safety problems, possible major accidents, are all drawbacks which are connected, depending on the cases, with their use. The last one being particularly important for nuclear fission reactions.

   However, a different kind of energy source, almost completely free from the above-mentioned drawbacks, is conceivable, even if not available at present, consisting in the exploitation of the energy released in fusion reactions. In fact, as in the case of fission reactions, in which heavy elements are split apart, in fusion reactions, in which light elements are fused, the total mass of the reaction products is less than the mass of the reacting elements.

The difference in mass ∆m is then converted into energy according to the relativistic formula

E=∆m×c2 (c = speed of light). Fusion reactions take place in a “plasma”, i.e. in an ionized gas made up of positive ions and electrons, which is also referred to as the “fourth state of matter”.

   In the visible UNIVERSE about 99% of the matter is in the state of plasma. Plasmas are, for instance, the nebulae, the interstellar gas, the inner part of stars, the heliosphere, the X-ray stars, the quasars, and the “galaxy clusters”, the main objects made up of high temperature plasmas.

   On the EARTH instead, plasmas are rare. Natural plasmas are Van Allen belts, aurora borealis, lightnings. Man-made plasmas are discharge tubes, plasma torches, plasma TV devices and nuclear fusion devices.

   The fusion reactions taking place in the stars have characteristic times of the order of billion years and the power delivered per unit mass is very little. The huge total power delivered is due to their enormous mass. To produce fusion energy on the earth reactions with much shorter characteristic times are to be taken into consideration. Poiché comunque bisogna poter portare i nuclei interagenti ad energie elevatissime, in un primo tempo la cosa sembrò impossibile.

At first, due to the huge amount of energy which must be given to ions, the task seemed impossible. However, as will be seen below, it was the progress made in the work on nuclear fission that recaptured the attention on the possibility of obtaining energy from fusion processes. The research which gave rise to the exploitation of nuclear energy began around 1939 and concerned the fission process. Subsequently, a sequence of discoveries led, in 1942 in Chicago, to the first nuclear pile under the supervision of Enrico Fermi.

   The Chicago pile was part of a gigantic U.S. activity called Manhattan Project whose main aim was the construction of nuclear weapons.

   As is known, in July 1945 an A bomb exploded at Alamogordo, New Mexico. On August 6, 1945, an atomic bomb was dropped on the Japanese city of Hiroshima and on August 9 another bomb was dropped on Nagasaki.

   In 1952, under the leadership of Edward Teller, the first thermonuclear explosion, the H bomb, took place at the Eniwetok Atoll. In this device the energy produced in a nuclear explosion is used to trigger fusion reactions. In 1953 the Russians, too, realized a thermonuclear explosion.

   The first fusion experiments were then carried out for military purposes and were as a consequence classified.

   Energy was produced in explosions, i.e. in uncontrolled processes. However, this gave rise to a research aimed at producing controlled thermonuclear fusion. This research was declassified during the second Conference on Atoms for Peace which was held in Geneva in 1958. After this event, well before the fall of the “iron curtain”, an international collaboration concerning the achievement of nuclear fusion energy was initiated.


Controlled thermonuclear fusion − The scientific feasibility of a reactor

The fusion reactions which are realistically considered for practical applications involve hydrogen isotopes (deuterium and tritium) and helium3. Among these the one having the possibility to be triggered at temperatures attainable in a fusion reactor is that in which deuterium and tritium nuclei fuse producing neutrons with an energy of about 14 MeV and α-particles with an energy of about 3.5 MeV.

   To solve the problem of the controlled thermonuclear fusion is then convenient to produce at first a deuterium and tritium plasma (DT plasma for short), having a high particle density n (≥1013 cm−3) 15

   In a reactor the neutrons escaping from the plasma would be captured in a lithium-containing blanket surrounding the plasma in which they would breed tritium by reactions with lithium. This regeneration is necessary because tritium, being radioactive with a relatively short half-life (about 12 years) does not exist in nature in significant quantities. The heat produced by neutrons would be extracted and used to produce electricity with a conventional thermal cycle.

   α-particles instead, being electrically charged, can be confined in the plasma so giving it part of their energy. If the amount of energy produced is large enough corresponding to a considerable amount of fusion reactions, it is then possible to overcome energy losses. The situation in which the fusion power equals the one which must be injected into the plasma in order to heat and confine it is called breakeven. Of course this condition must be exceeded because what matters in practice is the net energy production. The situation to be attained is that in which the power given to the plasma by α-particles (about 1/5 of the total reaction power) is sufficient to counterbalance the energy losses. In these ignition conditions the plasma is self sustaining.

In order to reach this situation it is necessary, as is logical and as said before, that density, temperature and confinement time, or some combination of these quantities, be large enough. On this regard the Lawson criterion exists which, based on an energy balance and taking plasma losses into account, fixes for the product (confinement parameter) a value not less than about 1014cm-3s, with a plasma temperature greater than about 10 keV.

   It is common to refer to the ignition condition as the condition of scientific feasibility of controlled thermonuclear fusion. It is important to specify the meaning of this terminology because quite often it is used in the wrong way. Only after attaining this important goal could one start designing a prototype reactor and, subsequently, a commercial one.

   In all existing fusion devices fusion reactions take place with production of energy; but, scientific feasibility as also breakeven, has not been obtained yet.

   In any fusion device the plasma must be kept far from the walls of the containing vessel. This confinement, taking place spontaneously in the stars, in which big masses of plasma are held together by immense gravitational forces, can be obtained in the laboratory by means of two different systems.

a) Inertial confinement In this system a small DT pellet is compressed to high density (greater than about one thousand times the density of a liquid) by means of laser or charged particles beams. The compression time is very short, so that the fuel, constrained by its own inertia, burns before dispersing. Lawson criterion is satisfied with plasma densities greater than 1024 cm-3 and with confinement times less the 10-10 s.

b)  Magnetic confinementHere the plasma, being a mixture of charged particles, is slowed down in the diffusion towards the container walls by the action of suitable magnetic fields. In this case one can have, for instance, a confinement time of 0.5 s with a peak density of about 1015 cm-3 .

   A major problem in thermonuclear research is that of instabilities. In the case of magnetic confinement plasma instabilities can be subdivided into two different types: macroscopic and microscopic. When dealing with macroscopic instabilities the plasma  is considered as a globally neutral and highly conductive fluid whose behavior under the effect of electromagnetic forces is studied. These instabilities consist in global observable movements of the plasma, altering its configuration and bringing it, in a very short time, in contact with the vessel walls.

   Microscopic instabilities, instead, are due to small deviations from thermodynamic equilibrium. Their effect is that of producing small and intermittent losses of plasma across the magnetic lines of force. These instabilities are not observable, but can be revealed by perturbations in the signals of diagnostic apparatuses.


TOKAMAKS − LARGE AND COMPACT

Among the various devices exploiting the plasma magnetic confinement the one that up to now has been considered as the most promising and which has then been mainly studied is the tokamak, a simple description of which is given below.

In the following figure a schematic representation of the machine is given.



The plasma contained in a suitable vacuum chamber has a toroidal configuration which can have a circular cross section with major radius R0 and minor radius a. However, in order to improve some characteristics, quite often an elongated configuration is adopted with minor radii a and b.

   The central transformer induces the current Ip which heats the plasma and originates the poloidal magnetic field Bp which, of course, is different from zero only when the current in the primary changes. The toroidal magnet produces the toroidal magnetic field BT. The poloidal and toroidal fields combine into the helical field pattern required for equilibrium and stability. In the figure the coils used to center the toroidal plasma in the discharge chamber are also shown.

   In principle, to reach ignition conditions, two different approaches can be conceived, corresponding to two different ways to obtain high values of the confinement parameter.

   One way is that of obtaining long confinement times with low plasma densities. This requires the construction of large machines (intending with “large” machines having a major radius Rnot less than 2 m). This of course implies very high costs and construction times. Furthermore, in this case, the uncertainties concerning the physical plasma behavior and the possibility of attaining ignition are considerable.

   The alternative way is that of realizing short confinement times with high densities. In this case the smaller dimensions of the devices allow the application of high magnetic fields; hence the possibility of reaching higher densities of the current circulating in the plasma and, as a consequence, high plasma densities. Thermonuclear plasma are so produced in which heating is mainly produced by α-particles. In large machines instead the use of external heating devices (neutral beams, radio frequencies, etc.) are necessary, so that the plasma cannot be considered to be thermonuclear.

   It can also be seen that high magnetic fields and compact dimensions make it possible to bring the plasma current to considerably high values without running into instabilities.

   Also, it is of paramount importance the fact experimentally verified that, in a high density plasma, the impurity degree, i.e. the percentage of heavy elements, is quite low. As to heavy elements radiation losses are mainly due, a high impurity degree would hinder the attainment of ignition conditions.

   Obviously, the need to operate with high magnetic fields and currents implies technological problems which may be important if one has to deal with a reactor rather than with an experimental device. On the other hand, the demonstration of the scientific feasibility of the nuclear fusion and the study of the behavior of an ignited plasma, must be considered as a priority and then put before the design of the reactor. And in this strategy the advantages offered by compact machines are evident.


THE CHOICE OF THE FUEL

Let us now go back to the problem of the choice of the fuel. As has been said, the DT reaction is easier to be triggered with respect to other reactions involving light elements (deuterium-helium3, proton-lithium6, proton-boron11, etc.). However, the DT fuel cycle involves two negative aspects. On one hand, tritium is radioactive, and on the other, high energy (about 14 MeV) neutrons are produced which, escaping from the plasma, induce activation and damage to the structural materials so giving rise to safety and environmental problems.

   It is therefore reasonable to use the DT fuel only in a first experimental phase in a machine aimed at obtaining plasma ignition. The next logical step is to look for a better alternative. Now, among the various possible reactions, the one which appears as the most convenient is the deuterium-helium3 (D3He) reaction which does not involve tritium and does not produce neutrons (apart from the fewer and less energetic ones produced in secondary and side reactions), but only α-particles with an energy of 3.6 MeV and protons with an energy of 14.7 MeV. The positively charged  α-particles and protons could also be used directly to obtain energy (direct conversion) so avoiding the need for a thermal cycle with the resulting yield decrease. Also, in this kind of reactor, an external blanket, with all its technological difficulties, is not necessary any more.

   The ignition of a D3He plasma could be made easier using a DT plasma as a “match”; in the sense that one would start with an ignited DT plasma and, while temperature rises, one would supply 3He, so allowing a transition from an ignited DT to an ignited  D3He plasma.

   One difficulty which would arise when using the D3He reaction is the scarcity of 3He on the earth. It would then be necessary to produce it from other elements. For instance it could be obtained from the decay of the tritium contained in the nuclear warheads; and this would be sufficient for an experimental program. For a long-term reactoristic program  an interesting possibility is offered by the extraction of 3He from lunar soil. On this regard studies have been carried out for many years in America, in particular by NASA. In the recent years also China and India are undertaking this kind of research.


RESEARCH WITH TOKAMAKS

The device tokamak was conceived in 1951 by Sakharov and Tamm. During the 4th International Conference on the Plasma and Controlled Thermonuclear Fusion Physics, held in Novosibirsk in 1968, Artsimovich of the Moscow Kurchatov Institute and director of the Russian fusion research program reported the results obtained with their T-3 tokamak arousing great interest. Since then many tokamaks were constructed all around the world obtaining interesting results.

   In the following two Tables some of the most “important” tokamaks are shown, considering as important not only the ones which gave significant results, but also those which, independently of their scientific value, involved considerable effort and high costs. 

   In Table 1 some large and medium size tokamaks are reported; while Table 2 shows compact high magnetic field tokamaks. In both Tables the main characteristics are given.


Table 1 − LARGE AND MEDIUM SIZE TOKAMAKS

** Various values for b are foreseen


Table 2 − COMPACT HIGH MAGNETIC FIELD TOKAMAKS



After having realized JET (Joint European Torus) the European community initiated the study of a second large tokamak called NET (Next European Torus) which was soon abandoned to start designing INTOR (INternational TORus), set apart in its turn to undertake the project ITER (International Thermonuclear Experimental Reactor) whose realization was decided by Reagan and Gorbachev in 1985.

   A first version of this huge tokamak (major radius about 6 meters) with a declared cost of 10 billion dollars, was advertised by the proponents as capable of achieving plasma ignition. But, as studies went on, it was demonstrated that instabilities would have prevented it from reaching this goal. This project was then abandoned and the design was undertaken, and are still being carried out at present, of a new device called ITER-FEAT, a reduced version of the original ITER, in which the ignition requirement was dropped.

Let us now come to more recent times. In June 2005 the decision was announced to construct ITER in France, in the nuclear site of Cadarache. This ITER, with a major radius of 6.2 meters, a plasma current of 15 MA and a toroidal magnetic field of 5.3 T, with an auxiliary heating of 50 MW, would produce, according to the designers, a power of 500 MW. If Q is the ratio of the produced power to that introduced into the plasma, one has Q = 10 (Q should be ∞ for at ignition).

   At present seven partners are participating in the project: EU (contributing with 45% of the cost), Japan, China, Russia, South Korea, India, USA.

   The cost of ITER was initially estimated at 5 billion euros for its construction and the same amount for its operation for 20 years. But, due to variations in the design and to inflation, the present estimated cost is considerably higher.

   At present no definite design exists for ITER.

   Let us know briefly describe in a historical sequence the line of compact, high magnetic field tokamaks. As can be seen in Table 2, the Alcator A (ALto CAmpo TORus) was first realized at MIT according to a design by professor Bruno Coppi.

The tokamak FT (Frascati Torus) represents the natural successive step. Due to its importance, it is worthwhile to briefly report its history.

   In summer 1968, during the IAEA Conference in Novosibirsk, Coppi and professor Bruno Brunelli, then director of the Laboratori Gas Ionizzati Euratom-CNEN, had the opportunity of discussing about starting in Frascati a research line on magnetically confined plasmas; a subject for which no competence existed at Frascati laboratories at the time. Talks went on after Novosibirsk and led to a seminar held by Coppi at the University of Rome in 1969 concerning the most promising experimental lines aiming at producing plasmas capable of burning for fusion reactions.

   Later on, Coppi agreed to a proposal by Brunelli and professor Carlo Salvetti, then vice-president of CNEN, to study a toroidal plasma device to be constructed in Frascati. And, as is known, this study led to the design of FT.

   Unfortunately, in the summer 1970, Brunelli gave up the direction of the Laboratori Gas Ionizzati; laboratories he had started and run for thirteen years.

   However, in the summer 1971 Coppi accepted the invitation by Salvetti to spend about two months at Frascati Laboratories. At the end of this period Coppi had completed the definition of the essential parameters of FT, whose construction was completed in 1978.

   From 1981 to 1983 FT held the record for the confinement parameter (about 4x1013 cm−3 s) with a temperature slightly higher than 1 keV.

   In the late 1980s the project FTU (FT Upgrade) was undertaken, aiming at studying the plasma behavior with microwave systems. FTU was completed in 1990 and is still operating.


I G N I T O R

The compact high magnetic field machine called Ignitor is a device designed to achieve ignition in a DT plasma. The account given below is deliberately quite short because, given its great scientific and practical importance, Ignitor is described in detail in a separate document [Ignitor in the context of the problem of controlled thermonuclear fusion] also reporting the historical and logical road which has led to its design.

   Ignitor design has been realized under the scientific and technical leadership of Bruno Coppi. It is conceived to produce high density plasmas. At present it is the only magnetic field confinement device meant to reach ignition conditions in a DT plasma, employing now available technologies and based on present day knowledge of plasma physics.

   A success of this experiment would then be equivalent to the demonstration of the scientific feasibility of controlled thermonuclear fusion.

   The main parameters of Ignitor are: R0 = 1,32 m,   a = 0,47 m,   b =  0,86 m,   Ip = 11 MA,   BT  =  13 T.

   Full size prototypes of its main components have been constructed.

   Obviously enough, costs and construction times of Ignitor are by far lower than those foreseeable for large machines.

   In April 2010 an agreement Italy-Russia was signed in Milan, according to which Ignitor would be constructed in collaboration with the Kurchatov Institute at the Trinity site (Troitsk, near Moscow). The site will be accessible to scientists from different countries.

A detailed information of the Ignitor Project can be found in a dedicated website: http://ignitorproject.com/index.html


IGNITOR

Diameter: 7 m

Height: 8 m

Weight: 770 Ton


               

 

Controlled Thermonuclear Fusion

Research and History Issues

edited by
Luisa Bonolis and Franca Magistrelli


Producing an ignited plasma will be a truly notable achievement for mankind and will capture the public’s imagination. Resembling a burning star, the ignited plasma will demonstrate a capability with immense potential to improve human well-being. Ignition is analogous to the first airplane flight or the first vacuum-tube computer. As in those cases, the initial model need not resemble the one that is later commercialized; much of what would be learned in a tokamak ignition experiment would be applicable both to more advanced tokamak approaches and to other confinement concepts.



The U.S. Program of Fusion Energy, Report of the Fusion Review Panel

The President's Committee of Advisors on Science and Technology (PCAST) July 1995 , p. 22


[http://science.energy.gov/~/media/fes/fesac/pdf/1990-99/1995_jul.pdf]

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Italian

Ignitor in the context of the problem of controlled thermonuclear fusion

                       (extended version)

Ignitor in the context of the problem of controlled thermonuclear fusion

                       (extended version)