Термоядерный синтез и его огромный потенциал для будущего мировой энергетики

Онгена Джозеф

ЭНЕРГЕТИКА

1.12. ТЕРМОЯДЕРНЫЙ СИНТЕЗ И ЕГО ОГРОМНЫЙ ПОТЕНЦИАЛ ДЛЯ БУДУЩЕГО МИРОВОЙ ЭНЕРГЕТИКИ

Онгена Джозеф, заместитель директора лаборатории физики плазмы Королевской военной академии Бельгии, партнер трехстороннего кластера Euregio (TEC), Брюссель, Бельгия. E-mail: [email protected]

Аннотация. Мировая энергетическая проблема ставит масштабную задачу «декарбонизации» нынешней энергетической системы, извлекающей ~85 % первичной энергии из ископаемых источников. В настоящее время предлагаются только два варианта сокращения выбросов CO2: возобновляемая энергия (солнце, ветер, вода, ...) или атомная энергетика. Их вклады в мировой энергетический баланс: ~2 % ветра и солнца, непостоянных по своей природе, ~6 % гидроэнергетики и ~5 % атомная энергетика, должны быть значительно увеличены за относительно короткое время для достижения целей, поставленных политиками. Сегодня в ряде стран, и в частности в ЕС, наблюдается растущая тенденция к исключению атомной энергетики из общего баланса производства энергии. Поэтому, при существующих в настоящее время энергетических технологиях, вряд ли «декарбонизация» энергетики является возможным путем, поскольку не позволит избежать трудностей в энергоснабжении. Следовательно, дополнительные экологически безопасные варианты производства энергии будут приветствоваться. Термоядерный синтез является таким кандидатом, причем очень важным из-за его неотъемлемых свойств: практической неисчерпаемости, отсутствия производства парниковых газов или долгоживущих отходов и безопасности. В статье изложены принципы магнитного удержания термоядерной плазмы и описаны два основных варианта установок для магнитного удержания - токамак и стелларатор. Рассмотрено современное состояние магнитного термоядерного синтеза и кратко представлены следующие шаги в освоении энергии синтеза, ITER и DEMO.

Ключевые слова: мировая энергетика, термоядерный синтез, магнитное удержание, плазмы токамак, стелларатор, ИТЭР, ДЕМО.

1.12. NUCLEAR FUSION AND ITS LARGE POTENTIAL FOR THE FUTURE WORLD ENERGY SUPPLY1

Ongena Josef, Dr, research director at the Plasma Physics Laboratory, Royal Military Academy, Brussels, Belgium. Formerly the Task Force Leader at JET, and Scientific Assistant to the JET director

Abstract. The energy problem in the world is outlined to show the enormous task of «decarbonizing» the current energy system, with ~85 % of the primary energy from fossil sources. Currently only two options offer a solution to reduce CO2 emissions: renewable energy (sun, wind, hydro,...) or nuclear fission. Their contributions, ~2 % for intermittent sun and wind, ~6 % for hydro power and ~5 % for fission, must be enormously increased in a relatively short time to meet the targets set by policy makers. In several countries, and in particular in the EU, there is a growing tendency to rule out nuclear fission for energy production. It is therefore questionable if «decarboniztion» is feasible with the currently available techniques, without avoiding difficulties in the energy supply. Additional environmentally friendly options to produce energy will therefore be very welcome. Fusion is such a candidate and a very important one, because of its inherent properties: nearly inexhaustible, no production of greenhouse gases or long term waste and safe. The principles of magnetic confinement are outlined and the two main options for magnetic confinement, tokamak and stellarator, explained. The status of magnetic fusion is summarized and the next steps in fusion research, ITER and DEMO, briefly presented.

Key words: global energy, thermonuclear fusion, magnetic confinement, plasma, tokamak, stellarator, ITER, DEMO.

Introduction

Climate and energy are getting an ever increasing public of glaciers and ice at the poles are becoming regular news items.

attention in recent years. Worrying phenomena like extreme The human being could well be one of the species suffering from

droughts and rainfall, tremendous tropical storms, melting the destruction of the habitat in the near or far future.

1 This paper is dedicated to Prof. Yu.N. Dnestrovskii of the NRC Kurchatov Institute, in recognition of his lifelong outstanding commitment to fusion research, in particular to the study of transport in fusion plasmas. PACS numbers: 88.00.00; 52.55 Fa; 52.55 Hc; 25.70.Jj.

IPCC reports claim an increase in the temperature of a few degrees in the coming decades due to the continuous release of CO2 and other greenhouse gases in the atmosphere. The heads of State at the G7 conference in May 2015 prepared for the first time bold statements, declaring their intention to decarbonize the world energy production.

However, such decisions, if not prepared correctly, could have unwanted negative effects. Technology development times and realistic potentials of the various options need to be taken into account to avoid disruptions in the energy supply.

This paper is intended to document the energy problem, to discuss possible solutions for the future and to highlight the role of fusion energy to contribute to a «decarbonized» energy system.

The world energy problem

With about 7.6 billion people on earth in 2017 and a daily primary energy consumption per capita (world average) of about 60kWh, the total amount of energy currently consumed is about

60 kWh/person/day x 7.5 billion people x 365 days s s 19 TWyr.

An estimate of what might be needed in the next 50 years can be found with the following two assumptions:

(i) the primary power consumption per capita in 2070 remains the same or increases with about 1/4 to about 75kWh/day (note that this is about 1/2 of what is already used in Europe and one fourth of what is currently used in the USA), and

(ii) the world population in 2070 will stabilise at a number between 8.5 and 13 billion people («low» and «high» variant prediction by the UN [1]).

Thus, in 50 years we expect the world to consume yearly an amount of primary energy equal to

(60-75) kWh/person/day x 8.5 x 3 billion people x 365 days = = 21.3 - 40.6 TWyr.

This is between 1.1 and 2.1 times the current energy consumption.

If we believe these numbers, this would be equivalent to an increase of minimum ~2.5 TWyr and maximum ~20 TWyr in 50 years time or an increase of minimum 50 GW and maximum about 400 GW installed power capacity per year running 24h a day. Thus at least the equivalent of at least one additional large conventional power plant (with a continuous power production of 1 GW) should be built every week (or in the worst case every day), somewhere in the world - a staggering prospect.

Is this possible? In fact, it is already happening in just one country in the world - for electricity production - in China: on average about 110 GW of additional electric generation capacity was built over the period 2011-2015, but unfortunately about 1/2 of this number or about 55 GW is using fossil fuels [2]. This is more than one new fossil power plant of 1GW per week (burning mainly coal and gas)... in just one country in the world.

But existing systems will also need to be replaced, first of all because of aging, but most importantly to convert them into much more environmentally friendly systems. Thus we have to nearly double the effort.

To further realize the task that lays in front of us, Table I shows the current contribution of various energy sources to the primary energy production in the world [2]. We immediately note the overwhelming dominance of fossil sources,

Ongena J.

contributing to ~85 % to the world primary energy production. The rest, ~15 %, is thus the contribution from non-fossil options: hydro, wind and solar and nuclear fission. If nuclear fission is dismissed then the candidate replacement options contribute now for a mere ~10 %. Thus we need to crank up the contribution of the non-fossil primary energy options at least 5 times if we want to arrive at a fully decarbonized energy production.

Table I

Contribution of various energy sources to the primary energy production in the world

Primary energy source Contribution to primary energy production in the world (2014), %

Oil 35.4

Coal 28.7

Gas 22.7

Fission 6.8

Hydro-electricity 4.4

Solar, wind, wood, waste. 2.0

Renewable energy will have to play an important role in the future. But we need to realize that although renewable energy resources in the world are large and inexhaustible, they have, unfortunately, serious drawbacks. One of them is their low energy density, another one is their intermittency [3]. At the moment the focus for renewables is mainly to produce electricity. Thus backup electrical capacity will be needed for those days where sun and wind cannot supply the electricity needed. This backup capacity cannot - by definition - be renewable, and thus is only fossil or nuclear based. But if the electrical system has to be decarbonized and nuclear fission, for whatever reasons, is not accepted (as is the case in several EU countries), then we have a problem. Any additional electricity source, free from the production of CO2, like fusion, is then very much needed. An additional solution to this problem could consist in building colossal electricity storage systems, but this is non-existent at the scale needed (hundreds of GWh of energy in a few hours in the case of e.g. wind in Germany) and a massive R&D programme is urgently needed to develop such large systems at an affordable cost, in as far as technically possible.

The low power density of renewables, as illustrated in Table II, unavoidably implies also considerable land use and/or investment in materials. E.g. in the case of biomass this implies areas of several 1000 km2 even for a relatively low power production of 100MW and CO2 emissions from the production of fertilizers/pesticides, harvesting, drying and transportation have to be taken into account. It leads for some «low carbon» technologies to a serious decrease of their potential to contribute effectively to CO2 reductions, or depending on the case, even to a further increase of CO2 emissions! For an interesting analysis see [4, 5]. For a very interesting discussion on sustainability, energy efficiency and subsidies see [6]; a critical assessment of the consequences of the current German energy policy («Energiewende»), so often praised by press, politicians and various lobbies, is given in [7-10]. These and other arguments should be carefully taken into account in discussing future energy options.

Table II

Power production per m2 of land or sea surface occupied.

Renewable energy is rather diffuse, leading to large, country-sized facilities in order to contribute substantially (from [3])

Nuclear fusion

Fusion is still in development as we all know, but it holds the promise of being a safe, practically inexhaustible and rather clean energy production method. As such it could become one of the best compromises between nature and the energy needs of mankind.

The reaction in the sun transforms H nuclei to 4He nuclei using the so-called p-p reaction chain, a complex set of reactions that starts from 4 protons and ends up in a 4He nucleus. The 4He nucleus contains however two neutrons, so there is a conversion needed from proton to neutron. This is the slowest part in the p-p chain, possible via inverse P decay, a process with a low probability. For this reason the p-p reaction in the sun cannot be used on earth. A clever trick is to use isotopes of H as fuel, which contain already the necessary neutrons for the synthesis of 4He. Thus the nuclides need only to be «rearranged», a process that is much more easy. Measured data for a number of fusion reactions [11] between light nuclei containing neutrons are listed in Table III. The fusion reaction that shows the best combination of a large cross section and large energy gain is the so-called D-T reaction:

D + T ^ 4He (3.5 MeV) + n(14.1 MeV), (1)

where D symbolizes deuterium (the stable isotope of hydrogen with a nucleus consisting of one proton and one neutron) and T is the symbol for tritium (the radioactive hydrogen isotope with a nucleus of 2 neutrons and 1 proton).

The reaction products are an a-particle (4He nucleus) and a neutron. From the conservation of momentum, the lighter neutron carries 80 % of the reaction energy and the a-parti-cle 20 %, as also indicated in Eq.1. The neutron does not feel the presence of the magnetic field and escapes immediately from the reactor volume, while the charged a-particle is confined. The kinetic energy of these escaping fast neutrons will be converted into heat in a blanket around the reactor and then into electricity using conventional technology (steam). About one million times more energy is released from a fusion reaction in comparison with a chemical one (MeV's instead of eV's for the latter). This is the reason why so little fuel can produce so much energy: when burnt in a fusion reactor, the deuterium contained in 1 litre of ordinary water (about 33 mg) will produce as much energy as burning 260 l of gasoline.

Other possible fusion reactions of interest between isotopes of hydrogen and helium are:

D + D ^ 3He (0.82 MeV) + n (2.45 MeV); (2)

D + D ^ T (1.01 MeV) + H (3.02 MeV); (3)

D + 3He ^ 4He (3.6 MeV) + H (14.7 MeV). (4)

Reaction a at 10 keV, barn a , barn max' Center of mass energy, keV, for omax Energy released, MeV

D + T ^ 4He + n 2.72 • 10-2 5.0 64 17.59

D + D ^ T + p 2.81 • 10-4 0.096 1250 4.04

D + T ^ 3He + n 2.78 • 10-4 0.11 1750 3.27

T + T ^ 4He + 2n 7.90 • 10-4 0.16 1000 11.33

D + 3He ^ 4He + p 2.2 • 10-7 0.9 250 18.35

p + 6Li ^ 4He + 3He 6 • 10-10 0.22 1500 4.02

p + nB ^ 3 4He 4.6 • 10-17 1.2 550 8.68

p + p ^ D + e+ + v 3.6 • 10-26 - - 1.44 + 0.27(v)

p + 12C ^ 13N + Y 1.9 • 10-26 1.0 • 10-4 400 1.94

Renewable category Renewable source Power output, w/m2

Solar heating 53

Concentrating solar power (deserts) 15

Solar photovoltaics 5-20

Sun based Solar chimney 0.1

Ocean thermal 5

Wind 2-3

Waves (Pelamis farm) 30

Tidal power 6

Gravitationbased Tide pool 3

Hydro-electricity 11

Biogas 0.02

Rape seed oil 0.13

Agriculturebased Bio-ethanol (sugar cane) 1.2

Energy crops 0.5

Wood 0.1-0.2

Earthbased Geothermal Heat 0.017

Table III

Measured data for a number of fusion reactions between light nuclei containing neutrons, together with their cross-sections o at the centre-of-mass energy of 10 keV, the maximum cross-section omax, (in barn), the location of the maximum Emax and the energy released per reaction (in MeV). In the line for the p-p reaction 1.44 MeV is the energy in the positron

and deuteron, 0.27 MeV is the average energy in the neutrino

They require higher temperatures and are thus more difficult to achieve and have a lower power density than the D-T reaction, but show even more benign environmental features. The D-D reaction would eliminate the need for tritium and produce neutrons with lower energies which are therefore easier to absorb and shield. A reactor based on the D-3He reaction would proceed with very low neutron production (some neutrons would be produced in competing but much less occurring D-D reactions) and lead to much less induced radioactivity in the reactor structures. However, the prospects for these 'advanced' fuels are still too speculative and only the D-T reaction has immediate future prospects.

Advantages of fusion power

A close look at the D-T fusion reaction, the reaction to be used in first fusion reactors, shows immediately the nice prospects of fusion.

1. The reactants are D and T. D can be obtained from seawater with conventional techniques in a cheap way (1/6000 of all hydrogen on earth consists of D); T is the radioactive isotope of hydrogen. It decays to 3He by the emission of an electron, with the rather short half-life of 12.3 years:

T ^ 3He + e- + 18.7 keV. (5)

Aside from an estimated 10kg of T produced by cosmic rays in the upper atmosphere, it is thus essentially nonexistent in nature and will have to be artificially made. The neutrons produced in the fusion reactions will be used to breed it by bombarding a blanket around the burn chamber containing a lithium compound, according to:

6Li + n ^ 4He (2.05 MeV) + T (2.73 MeV); (6)

7Li + n ^ 4 He + T + n - 2.47 MeV. (7)

Thus the main inputs to a fusion reactor are D and Li, two products that abundant and free from any radioactivity.

2. Nearly inexhaustible source of energy. Very little fuel consumption for a huge amount of energy. To supply an «average» EU citizen with electricity during 80 years (assuming the use of steam turbines, and taking into account the efficiency of the conventional Carnot cycle of ~35-40 %) only about 15 g of a mixture of D and T is needed. Taking into account the reserves of D in seawater and the terrestrial reserves of Li, one finds easily that fusion is a source for several thousand years, if based on the D-T reaction. If later the D-D reaction could be used, then we would have a source of energy for millions of years.

3. Strongly reduced energy dependency. The fact that D and Li are abundant and cheap reduces to a large extent the dependence on foreign countries for fuel imports. This is an important element in the discussion on world peace. It also avoids the enormous concentration of money in the oil rich regions of the world, with all the very negative consequences we see happening in the last decades.

4. No long term storage of nuclear waste. There is radioactivity from two sources:

(i) tritium is radioactive, but is fuel for the reaction;

(ii) the 14.1 MeV neutron will induce activity in the structural elements of the reactor.

But this can be minimized by making a good choice for the structural elements in the reactor, in the sense that one would look for materials with a short half life (~50-100 years) after irradiation with the 14.1 MeV neutrons Due to the specific spectrum of the neutrons, a dedicated test facility will

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Ongena J.

have to be built that can test the proposed material solutions. Such a facility is IFMIF (International Fusion Materials Irradiation Facility), and a prototype device is now successfully built in Rokkasho, Japan [12].

5. Safety. Fusion reactions take place at extremely high temperatures (~150 million degrees, see below) and the fusion process in itself does not rely on a neutron multiplication reaction. An uncontrolled burn (nuclear runaway) of the fusion fuel is therefore excluded on physical grounds, as one can close at any moment the supply of D and T. In addition, the amount of fuel (D and T) available at each instant is sufficient for operation during only a few seconds, in sharp contrast with a fission reactor where fuel for several years of operation is stored in the reactor core. Third, residual heating is not sufficient to cause melting of the reactor structure. Even in case of a total loss of active cooling, no safety problems are expected [13].

Magnetic Confinement of the hot plasma

At the high temperatures necessary for nuclear fusion, the fuel is nearly fully ionised. Magnetic fields can thus be used to confine the fuel, since charged particles will follow a helical path around the field lines owing to the Lorentz force. With an appropriate geometry for the magnetic field, it is thus possible to keep the hot particles away from the walls of the container.

In a tokamak, a set of coils placed around the doughnut-shaped plasma chamber produces the main toroidal magnetic field. The conducting plasma ring itself serves as the sole secondary winding of an enormous transformer. A current pulse in the primary winding induces a large current in the secondary, i.e. in the plasma ring itself. This induced plasma current is accompanied by a poloidal magnetic field. The combination of this poloidal field with the main toroidal field results in a helical magnetic field (Fig. 1). The magnetic structure thus generated consists of an infinite set of nested toroidal magnetic surfaces (Fig. 2), each with a slightly different twist, reducing further the leakage of particles and heat from the plasma. On each of these surfaces, the plasma pressure is constant.

Induced Plasma Current Poloidal (by Transformer) Magnetic Field J___

Magnetic Field

Poloidal Magnetic Field

Fig. 1. The combination of the poloidal field generated by the plasma current and the main toroidal field generated by external toroidal field coils in the tokamak configuration results in a helical magnetic field, needed for the stability of the plasma

Fig. 2. The structure of the magnetic fields in a tokamak: a set of nested toroidal magnetic surfaces, each with a different twist.

The innermost surfaces have the largest twist. For didactical purposes the twist of the mangnetic surfaces is greatly exaggerated

The tokamak is an intrinsically pulsed device, since the transformer that induces the plasma current needs a steadily increasing current to provide the necessary change in magnetic flux that induces the current in the plasma ring. One could argue that non-inductive current drive (e.g. by electromagnetic waves, injection of fast particles unidirectionally on axis etc) could solve this problem. The reality is however that the current drive efficiency of all known systems is rather low. They thus consume large amounts of energy to produce a given plasma current. For a future fusion reactor, present knowledge learns that the so-called «re-circulating power» will be increased to such an extent that it becomes questionable if non inductive current drive will ever be useful.

Continuous operation of a fusion device can be obtained if the need for inducing a plasma current could be avoided. The stellarator is such a solution, and it relies on currents external to the plasma for the helical magnetic configuration. In its basic configuration, extra helical coils around the toroidal plasma provide the necessary additional twist to the toroidal magnetic field generated by the main field coils (Fig. 3). Initial stellarator configurations lacked good confinement properties. Modern stellarators have optimised confinement properties, and are equipped with a complex set of coils, that are determined numerically (Fig. 4). Several devices of the stellarator type are in operation or in construction at this moment all over the world. The largest stellarator is Wendelstein 7-X, in Greifswald, Germany (Fig. 5), and first promising results have been obtained recently [14].

Helical winding

Resulting helical magnetic field

Fig. 3. In early stellarators extra helical coils provided the necessary additional twist to the toroidal magnetic field generated by the main field coils

Fig. 4. Modern stellarators are equipped with

a complex set of coils, resulting in optimised confinement properties

The temperatures required for fusion can be reached with two main types of heating systems. The first type consists of accelerating positive H, D or He ions to high energies (in current tokamaks up to ~150 keV) and injecting them into the plasma. A complication is caused by the confining magnetic field. If the fast particles are charged, they will be deflected by the magnetic field and not be able to enter the hot plasma. Therefore the accelerated ions must be neutralized before they can penetrate into the hot plasma. To this end, the highly energetic ions are sent through a cloud of neutral gas. The accelerated positive ions «steal» electrons from atoms in this neutral gas cloud, undergo a charge-exchange reaction, thus become energetic neutral particles and will pass the magnetic field unhindered. Once in the hot plasma, they are almost immediately ionised again and deposit their energy via collisions to the rest of the plasma particles. Powers of up to several million watts per neutral injector can be delivered in this way. However, as the cross-section for the charge-exchange neutralization reaction for positive ions decreases at higher energies, use will have to be made from accelerated negative ions at energies above ~200 keV, as is the case for the ITER Neutral Beam Injectors. Therefore negative ions have to be generated before acceleration and then again neutralized. This requires a totally different technology [15] and a new laboratory is set up in Europe for the development of this technique [16-18].

A second heating method is based on electromagnetic waves. The waves are coupled to the plasma by antennas or waveguides at the plasma edge. The energy from the waves is most easily absorbed if the frequency used is equal to a «natural» frequency of the particles to be heated. The cyclotron frequency, with which the charged plasma particles gyrate around the magnetic field lines, is such a frequency, and one has the choice between ions and electrons, resulting in Ion Cyclotron Resonance Heating (ICRH) and Electron Cyclotron Resonance Heating (ECRH) systems. Ion cyclotron frequencies are in the MHz range (20 MHz and upwards), while electron cyclotron frequencies are approximately a 1000 times higher (up to 200 GHz), due to the smaller mass of the electrons. Heating powers for high frequency systems range from 100 kW to several tens of MW. A third system exists (Lower Hybrid Heating) that uses frequencies in between those from electrons and ions, and is mostly used to drive part of the plasma current.

Turbulence and Transport in fusion plasmas

The ultimate aim of plasma fusion research is to reach the conditions for a self-sustained burning plasma. However, fusion plasmas have a natural tendency to reduce temperature and density gradients by particle and heat transport, thus counteracting our efforts to make them hot and dense. The first experiments with auxiliary heating systems were fairly disappointing: the confinement time was observed to decrease with increasing additional power (te rc 1/pof); in other words, the larger the injected power, the less efficient it becomes. This is because the transport is governed by turbulence driven by the strong gradients. Above a critical gradient, turbulence levels increase strongly, leading to the decrease of confinement quality with heating power [19-21] and a more detailed understanding of transport processes is needed.

Different approaches exist to describe the turbulence in fusion plasmas: complex calculational approaches used e.g. in the gyrokinetic models [22], theoretical/analytical models like e.g. the Canonical Transport Model [23], and recently also an approach based on general energy characteristics of the system, avoiding the need for a detailed description of the origin and nature of the turbulent heat flux [24]. Various code-systems exist to derive transport coefficients from the experimental data. Important examples are the ASTRA code system [25], pioneered by Prof. Yu.N. Dnestrovskii and D.P. Kostomarov (both authors wrote one of the first transport codes for a tokamak [26]), and TRANSP [27-28] developed at the Plasma Physics Lab of Princeton University, and in use in all major fusion laboratories in the world.

Injecting a given total amount of heating power (sum of ohmic plus auxiliary heating power) the total energy W of the plasma increases at a rate

dW/dt = Ptot - W/te ,

where the last term accounts for losses (by convection, conduction and radiation) characterized by te, the so-called

energy-confinement time. This is the characteristic time during which the plasma maintains its temperature if the heating power is suddenly switched off. This parameter is very important because the product of the number density of the ions n and the energy-confinement time has to satisfy

niTE > 2 • 1020 m-3 • s-1.

In other words, the plasma containing a sufficiently large number of reacting ions must stay hot for a long enough time to allow a sufficiently large number of fusion reactions to take place. This is the basic version of the so-called Lawson criterion [29-30], i.e. without taking into account engineering efficiencies or impurities in the plasma. A more detailed discussion of the Lawson criterion can be found in [31].

Characterising the performance of fusion plasmas

If the Lawson criterion is satisfied, the number of energetic 4He nuclei, produced by the D-T reaction is large enough to sustain the plasma temperature. The plasma is then said to be ignited and the reaction becomes self-sustained. The reactivity of fusion plasmas is quantified by the power amplification factor (the fusion Q-factor) Q = Pfusion/Ptot. Two important landmarks for the value of Q are customary in fusion research. The first, breakeven, is reached when the heating power is equal to the power produced from fusion reactions, corresponding to a Q = 1. The second, ignition, is reached when the additional heating systems can be switched off, corresponding to an infinite value for Q.

Status and prospects for fusion research

In the beginning years of fusion research, the energy confinement seemed to follow the so-called Bohm scaling te cx BR2/T. This predicted such low values for te, that any practical realization of a fusion reactor would be excluded. A major breakthrough occurred in 1968 on the T-3 tokamak of the Kurchatov Institute in Moscow. On this magnetic fusion device, a much

higher temperature (10 million C) and much higher values for te were realised than any other device of that time. The energy confinement values obtained on T-3 exceeded the predictions of the Bohm scaling by a factor of more than 30! These spectacular results were announced by Lev Andreevitch Artsimovitch of the Kurchatov Institute at the famous International Atomic Energy Agency (IAEA) Fusion Energy Conference held in Novosi-??? birsk in 1968 [$$] and later independently confirmed by a team

of scientists from the Culham Laboratory (UK), who went with their equipment to Moscow [33]. This resulted in a general redirection of the worldwide fusion programme towards tokam-aks and later also to the plans for the construction of the Joint European Torus (JET, located in Culham, close to Oxford, UK). The most impressive results in fusion research up to now are obtained on this device in October and November 1997. Experiments in 50 %D - 50 %T plasmas resulted in over 16MW of fusion power during about 1 second, with Q values in excess of 0.7 [34]. These are the highest fusion powers and Q values ever reached, thereby effectively resulting in the first demonstration of breakeven in reactor grade D-T fusion plasmas. A quasi steady-state generation of fusion power has also been demonstrated: over 4MW of fusion power were produced for time intervals of more than 5 seconds [35], a duration only limited by the actual technical constraints of JET.

A summary of the different high performance D-T pulses obtained on JET and the Tokamak Fusion Test Reactor (TFTR, Princeton University, USA) is presented in Fig. 6.

The results obtained at JET and other smaller machines provide crucial information for the design of a next large tokamak, aimed at demonstrating the technical feasibility for large scale energy production. This next step is ITER, originally short for International Thermonuclear Experimental Reactor (currently under construction in Cadarache, France)

as a combined effort between Europe, Japan, the Russian Federation, South Korea, India, China and the United States. This device will thus for the first time in history allow mankind to produce huge quantities of energy from nuclear fusion reactions in a controlled way at temperatures over 100 million degrees. ITER is expected to generate fusion powers of the order of 500 MW in pulses of 300-500 seconds. Specifications for ITER (and a few other major tokamaks) are summarised in Table IV. After ITER, the construction of a demonstration reactor is foreseen, currently termed DEMO, which should show not only the technical, but also the economical feasibility of fusion.

JET transient (1997)

JET steady-state . (1997)

>

" « ~ „V.

TFTR , \ steady-state v (1995) I

_L

4.0

5.0

6.0

Fig. 6. An overview of D-T pulses with high performance obtained on JET and TFTR

Machine specifications for ITER and a few other major tokamaks in the world. Data for JET are given for the actual divertor version of the machine

Table IV

T-10 DIII-D JT-60SA JET ITER

Land / Organisation Russian Federation USA Japan GB/EURATOM International

Plasma shape circular Elliptical (D) Elliptical(D) Elliptical (D) Elliptical (D)

Minor radius (m) 0.3 0.67 (hor) 1.18 (hor) 1.25 (hor) 2.0 (hor)

1.74(vert) 2.30(vert) 2.1 (vert) 3.7 (vert)

Major Radius (m) 1.5 1.67 2.96 2.96 6.2

Toroidal Magnetic field (T) 2.5 2.2 2.25 3.5 5.3

Plasma Current (MA) 0.3 3.0 5.5 5.0 15 (17)

Pulse length (s) 1 10 100 60 300-500

Injection of Neutral particle beams (MW) - 20 34 30

Injection of electromagnetic waves (MW) 2.2 8 7 38 73 (130) in total

Conclusions

Some decades are needed to realise the first economical fusion reactor based on the D-T reaction. The most recent results support the attractiveness of the fusion concept for energy production and have led to the definition of ITER, the next large tokamak, now in construction in Cadarache, France. There is no doubt that fusion is a challenging undertaking and that patience

is needed, but it is more than worth the effort given the (rather enormous) difficulties we are facing for the future world energy supply. Fusion is not yet ready for the market, but we are close enough to see the final challenging steps. To make those steps will be the vitally important contribution of the young generation of researchers to the benefit of all people on earth. An enormous challenge, but with an immense reward!

Аннотация научной статьи по электротехнике, электронной технике, информационным технологиям, автор научной работы — Онгена Джозеф

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NUCLEAR FUSION AND ITS LARGE POTENTIAL FOR THE FUTURE WORLD ENERGY SUPPLY

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