Looking Back at Half a Century of Fusion Research

P. STOTT

Association Euratom-CEA, Centre de , 13108 Saint Paul lez , .

This article gives a short overview of the origins of and of its development as a potential source of terrestrial .

1 Introduction

A hundred years ago, at the dawn of the twentieth century, physicists did not understand the source of the ‘s energy. Although classical physics had made major advances during the nineteenth century and many people thought that there was little of the physical sciences left to be discovered, they could not explain how the Sun could continue to radiate energy, apparently indefinitely. The law of energy conservation required that there must be an internal energy source equal to that radiated from the Sun‘s surface but the only substantial sources of energy known at that time were wood or coal. The mass of the Sun and the rate at which it radiated energy were known and it was easy to show that if the Sun had started off as a solid lump of coal it would have burnt out in a few thousand years. It was clear that this was much too shortœœthe Sun had to be older than the Earth and, although there was much controversy about the age of the Earth, it was clear that it had to be older than a few thousand years. The realization that the source of energy in the Sun and is due to nuclear fusion followed three main steps in the development of science. The first came in 1905 with Albert Einstein's prediction that mass can be converted into energy. The step came about ten years later with Francis Aston's precision mass spectrographic measurements, which showed that the mass of a atom was slightly lighter than the mass of four hydrogen atoms. These two key results led and others, around 1920, to propose that the source of the Sun‘s energy might be explained in terms of four hydrogen atoms combining to form a single helium atom. The third step came with the development of in the late 1920s (according to classical physics, the Sun was not hot enough for nuclear fusion), which provided a complete understanding of the chain of fusion reactions in the Sun and stars. After fusion was identified as the energy source of the Sun and the stars it was natural to ask whether the process of turning mass into energy could be demonstrated on Earth and, if so, could it be put to use for man's benefit. The novelist H G W ells had predicted the use of nuclear energy in a novel published in 1914, but , the famous physicist and discoverer of the structure of the atom, thought otherwiseœœhe told the British Association for the Advancement of Science in 1933 "We cannot control atomic energy to an extent that would be of any use commercially, and I believe we are not ever likely to do so". The prospects for nuclear energy improved in 1939 when Otto Hahn

1 and Fritz Strassmann demonstrated that bombarding a nucleus with caused it to split with the release of a large amount of energy. This was the birth of fission, but the quest for fusion energy proved to be much more difficultœœin order to fuse, two nuclei have to overcome strong repulsive electrostatic forces. Although the prospect of fusion energy was contemplated by many groups worldwide, the first tangible steps came in the in May 1946 when George Thomson (the son of J J Thomson, the discoverer of the ) and Moses Blackman at Imperial College, London University filed a patent for a fusion reactor. Programmes of fusion research started at Liverpool, Oxford and London Universities in the UKœœlater the work was made secret and relocated to the Atomic Energy Research Establishments at Harwell and Aldermaston. The general approach was to try to heat gas to a high so that the colliding nuclei had sufficient energy to fuse together. By using a to confine the hot it was thought that it should be possible to allow adequate time for the fusion reactions to occur. Fusion research was taken up in the 1950s in the and the under secret programmes and subsequently, after being declassified in 1958, in many other technically advanced countries. The most promising fusion reaction was between the two rare of hydrogen, deuterium and . Deuterium is present naturally in water and is therefore readily available. Tritium is not available naturally and has to be produced in situ in the reactor. This can be done by using the neutrons produced by the fusion reaction to interact with the light metal in a layer surrounding the reaction chamber in a breeding cycle. Thus the basic for nuclear fusion are lithium and waterœœboth are readily and widely available. Most of the energy is released as heat that can be extracted and used to make steam and drive turbines as in any conventional power plant. A schematic diagram of the proposed arrangement is shown in figure 1.

Figure 1œœSchematic of a fusion energy power plant. The deuterium and tritium fuel burns at a very high temperature in the central reaction chamber. The energy is released as charged particles, neutrons and radiation, and is absorbed in a lithium blanket surrounding the reaction chamber. The neutrons convert the lithium into tritium fuel. A conventional steam generating plant is used to convert the nuclear energy to electricity. The waste product from the is helium.

2 However, the problems of heating and confining the hot fusion fuel with magnetic fields turned out to be much more difficult than at first envisaged and the quest for fusion energy proved to be one of the most difficult challenges faced by scientists. It inspired theoretical and experimental study of the physics of plasmas and the development of diagnostic techniques to make accurate measurement. After many years, the scientific feasibility of via the magnetic confinement route has been demonstrated and the next generation of inertial confinement experiments is expected to reach a similar position. Developing the technology and translating these scientific achievements into power plants that are economically viable will be a major step that will require much further time and effort. Some have hoped that they could find easy ways to the rewards offered by fusion energy. This line of thinking led to many blind alleys and even to several false claims of success; the most widely publicized being the so-called ”‘ discoveries in 1989 and the more recent claims for ”‘ in 2002.

2 M agnetic Confinement Fusion

Many different magnetic configurations were proposed and explored in the early years of fusion research. The results were presented at the United Nations Atoms for Peace Conference in Geneva in 1958œœsome actual experiments were even taken to Geneva and exhibited working. A full discussion of all the lines that have been explored (and mostly abandoned) is beyond the scope of this paper, but it is interesting to review very briefly some of the main concepts. A broad classification divides the magnetic configurations into closed and open-ended systems and then into steady-state or rapidly-pulsed devices. The earliest magnetic confinement devices in the UK were toroidal pinches that attempted to confine with a strong, purely poloidal magnetic field produced by a toroidal plasma current. The so-called effect had been discovered in in a hollow conductor that had been permanently squashed during a storm. With a sufficiently strong current, the magnetic field compresses (or pinchesœœhence the name) the plasma, pulling it away from the walls. But the toroidal pinch proved to be seriously unstableœœthe plasma thrashed about like a snake or constricted itself like a string of sausages (figure 2). External coils adding a weak magnetic field in the toroidal direction improved stability and (some years later) further improvement was found when this toroidal field was made to reverse direction outside the plasmaœœa configuration now known as the reverse field pinch (RFP). The potential to work at high β (the ratio of plasma to magnetic field) would be a possible advantage of the RFP but good energy confinement has proved elusive.

3

Figure 2œœPhotograph of an unstable toroidal pinch discharge with current 1.3 kA in a xenon plasma. The glass tube had a major radius of about 0.3 m.

Bigger and ever more powerful pinch experiments were built at Harwell through the early 1950s, culminating in the ZETA machine that started to operate in 1957. ZETA (figure 3) was a bold venture and a remarkable feat of engineering for its day. The aluminium of three meters major diameter and one meter bore was intended to contain a plasma current of nine hundred thousand amps. Already in the first few weeks of operation in deuterium plasmas large numbers of neutrons were recorded, up to a million per pulse. This caused great excitement, but the important question was, were these neutrons thermonuclear? The Russian Academician had visited Harwell and warned already of the possibility that beams of might be accelerated to high and produce neutrons that could be misinterpreted as coming from the hot plasma. The distinction, though subtle, was very important and would seriously affect the way the results would extrapolate to a fusion reactor. The uncertainty could have been resolved easily if it had been possible to measure the plasma temperature accurately, but plasma diagnostic techniques were still in their infancy.

Figure 3œœThe ZETA experiment at Harwell soon after its first operation in 1957. The core consists of two 4 m diameter rings surrounding the 1 m bore torus.

4 News of the existence of the top-secret ZETA machine and of its neutrons quickly leaked out into the press. Pressure grew for an official statement from Harwell and delays in doing so merely heightened the speculation. It was decided to publish the results from ZETA alongside papers from other British and American fusion experiments in the scientific journal at the end of January 1958. The carefully worded ZETA paper made no claims at all about the possible thermonuclear origin of the neutrons but Harwell‘s director was less cautious and at a press conference he was drawn into saying that he was "ninety percent certain" that the neutrons were thermonuclear. The press reported this with great enthusiasm about stories of cheap electricity from . The matter was soon resolved by a series of elegant measurements on the neutrons that showed they were not thermonuclear. ZETA went on to make many valuable contributions to the understanding of fusion plasmas, but plans to build an even larger version were abandoned.

Figure 4œœA simplified plan of the C- at Princeton USA, showing the location of the main components. Two half-tori with helical windings were joined by straight sections into a ”racetrack‘ configuration. After 1968 the machine was converted into the fully toroidal ST .

At in New Jersey, the astrophysicist invented a toroidal plasma confinement device that he called the stellarator, with a magnetic field produced entirely by external coils. The original idea for the stellarator had been to confine plasma in a toroidal magnetic field but it was soon realized that a purely toroidal magnetic field cannot confine plasma and that it was necessary to add a twist to the field. In the first stellarator experiments this was done simply by physically twisting the whole torus into the shape of a ”figure 8‘. Later the same effect was produced using a second set of twisted coilsœœthe helical winding (figure 4). The coils in modern have evolved in various sophisticated ways but share the same basic principle of providing a twisted toroidal magnetic field. Koji Uo‘s group in Kyoto arrived at a similar configuration from a different starting point. The first Heliotron devices had toroidal fields produced by circular conductors, arranged in pairs with opposite but unequal currents, similar to a ”picket fence‘ or ”‘. At first the rings were outside the vessel, later they were submerged in the plasma as in internal-ring devices. Already in 1961, Uo had proposed a helical version of this configuration, which he called a helical heliotron, and which led by 1999, via a series of experiments in Kyoto and Nagoya, to the superconducting (LHD) at the NIFS laboratory in Toki (figure 5). 5

Figure 5œœOverview of the superconducting Large helical Device (LHD) at the National Institute for Fusion Science, Toki, .

The third toroidal confinement scheme was the tokamak that had been invented in at the . The name is an acronym of the Russian words toroidalnaya kamera for toroidal chamber and magnitnaya katushka for magnetic coil. The tokamak had evolved, like the stellarator, out of the realization that plasma cannot be confined in a purely toroidal magnetic fieldœœthe solution in this case was to twist the field with a poloidal field generated by a current in the plasma. It is also possible to think of the tokamak as a pinch with very strong stabilizing toroidal field. The tokamak (figure 6) became the leading contender for a magnetically confined fusion reactor.

Figure 6œœSchematic of the tokamak magnetic confinement scheme. Current is induced in the plasma by a transformer effect. The toroidal magnetic field component due to the external coils and the poloidal field component due to the plasma current combine to produce a helical magnetic field with a gentle twist.

Linear magnetic configurations were also explored. In the linear Z-pinch, a plasma current along the axis between two end electrodes produced an azimuthal magnetic field that compressed the plasma away from the walls. Unfortunately, as in the toroidal pinch, the plasma rapidly became unstable, broke up and hit the wall. Variants of the Z-pinch were studied, including the configuration known as the plasma focus that flourished during the 1960s. It has long been abandoned as a serious candidate for a fusion reactor but it continues to provide a source of dense plasma for academic studies. The linear Z-pinch survives today in the form of ultra fast, very high current devices (sometimes using metal filaments to initiate the plasma) where it is hoped that the plasma compression and heating can take place on a faster time scale than the instabilities. The potential of these schemes as a route to magnetic confinement is doubtful but these ultra-fast 6 pinches produce copious bursts of X-rays and are being studied as drivers for inertial confinement fusion. A fast-rising azimuthal current in a single turn external conductor wrapped around the plasma tube produced an axial magnetic field that compressed and heated the plasma in the theta pinch. The SCYLLA theta pinch, developed at Los Alamos in 1958, was the first magnetic confinement system to produce really hot fusion plasmas with neutrons from thermonuclear reactions and, for a time, the theta pinch moved into the lead. The pulse duration of a theta pinch was very short; typically about one microsecond, but even on this short time scale plasma was lost by instabilities and end losses. Attempts to stopper the ends with magnetic fields, material plugs and by joining the two ends to make a toroidal theta pinch all failed. Both the Z-pinch and the theta pinch were inherently pulsed devices and, even if all the problems of end losses and instabilities were to be solved, a fusion reactor based on these configurations would have been a doubtful proposition due to the large re-circulating energy. The third linear magnetic confinement scheme had the potential to run steady state. This was the machine where a solenoid coil produced a steady-state axial magnetic field that increased in strength at the ends. These regions of higher fieldœœthe magnetic mirrorsœœserved to trap the bulk of the plasma in the central lower field region of the solenoid, though ions and with a large parallel component of velocity could escape through the mirrors. Mirror machines looked promising but had difficulties in reaching the higher densities needed for a reactor. Instabilities and collective effects caused losses that could not be overcome in spite of adding complicated end cells to the basic mirror configuration. Development of mirror machines in the USA culminated in the massive MFTF-B project at Livermore, but this was stopped in 1986. Mirror programmes were prominent in the former USSR but were wound down due to lack of research funding and the only significant mirror machine that remains in operation is GAMMA10 in Tsukuba, Japan.

3

The invention of the in 1958 had a double impact on fusionœœthe intense light source allowed new methods of plasma diagnostics and it opened up an alternative route to fusion energy known as inertial confinement. Laser-based diagnostics held the key to resolving the long-standing problem of determining the temperature of fusion plasmas. The first measurements of incoherent scattering from laboratory plasmas using ruby lasers were published in 1963 and the technique of soon became the established method for measuring electron and densities. An early application was to the understanding of shock waves, for which good time and space resolution were important. A well-known success of Thomson scattering was the measurement of the electron temperature in the tokamak T-3 device. Collective scattering was more elusiveœœalthough proof of principle experiments were convincing, the development of a routine laser-based diagnostic for temperature in fusion plasmas proved more difficult. Lasers also

7 became important in developing interferometry and polarimetry, topics well known to the LAPD series of conferences, and all of these laser diagnostics found applications in plasmas outside fusion.

4 Inertial Confinement Fusion

The potential to focus intense bursts of laser energy onto small targets came just at a time when magnetic confinement fusion had reached something of an impasse, so laser enthusiasts saw an alternative approach to fusion. The idea was to rapidly compress and heat small fuel pellets or capsules in a series of mini explosionsœœcalled inertial confinement because the fusion fuel was to be confined only by its own inertia (figure 7). The close overlap with nuclear weapons had the consequence that many of the details remained a closely guarded secret, even more secretive than the early days of magnetic confinement. In 1972, John Nuckolls and his collaborators at the Lawrence Livermore National Laboratory in California outlined the principles in the international science journal Nature. Optimists predicted that inertial confinement fusion might be achieved within a few years, but soon it became clear (as with magnetic confinement) that it was not going to be so easy.

Figure 7œœThe four stages of a fusion reaction in an inertial confinement capsule: (a) laser heating of the outer layer, (b) ablation of the outer layer compresses the capsule, (c) the core reaches the density and temperature for ignition, (d) the fusion reaction spreads rapidly through the compressed fuel.

One of the inherent problems with inertial confinement was that the compressed capsule distorted and became unstable before reaching the conditions for fusion. The British physicist Lord Rayleigh had observed instability in ordinary fluids many years earlier and a similar effect (now known as the Rayleigh-Taylor instability) caused the compressed capsule to distort unless its surface was heated very uniformly. A uniformity of better than one percent was called for, requiring many separate laser beams. An ingenious way of obtaining a higher degree of uniformity was proposed around 1975, although the details remained secret for many years. The capsule is supported inside a small metal cylinder called a hohlraum (the German word for a cavity) typically a few centimetres across and made of a heavy metal such as gold (figure 8). The laser beams are focused through holes onto

8 the interior surfaces of this cavity rather than directly onto the capsule. The intense laser energy evaporates the inner surface of the cavity producing dense metal plasma. The laser energy is converted into X-rays, which bounce about inside the hohlraum, being absorbed and reemitted many times rather like light in a room where the walls are completely covered by mirrors. The bouncing X-rays strike the capsule many times and from all directions smoothing out any irregularities in the original laser beams. X-rays can penetrate deeper into the plasma surrounding the heated capsule and couple their energy more effectively than longer wavelength light. This approach is known as indirect drive in contrast to the direct drive arrangement where the laser beams are focused directly onto the capsule.

Figure 8œœ An experimental cavity, or hohlraum, of the type used for indirect drive experiments. Intense laser light is shone into the cavity through the holes and interacts with the wall creating intense x-rays. These X-rays heat the capsule, causing ablation, compression and heating.

It is important to avoid heating the capsule core too strongly during the early part of the compression phase because much more energy is then needed to compress the hot plasma. Ideally the core of the capsule should just reach the ignition temperature at the time of maximum compression. Once the core of the capsule starts to burn, the energy produced by the fusion reactions will heat up the rest of the capsule and the fusion reaction will spread outwards. A more sophisticated scheme for direct drive, known as fast ignition where an ultra-fast laser is fired to heat and ignite the core after it has been compressed, may reduce the overall energy requirements for the driver. A proof of principle demonstration of this concept was made recently with the Gekko XII facility in Osaka. Estimates of the amount of laser energy required to compress and heat a fusion capsule have increased several fold since the early days and present calculations show that at least one MJ will be required. This requires very advanced types of laser and the most suitable ones presently available use neodymium glass. The laser and its associated equipment fill a building larger than an aircraft hanger. These lasers consist of many parallel beams that are fired simultaneously and focused uniformly onto the small fusion capsule. Typical of these types of laser was the facility at the Lawrence Livermore National Laboratory in the USA that came into operation in 9 1984 and had ten beams. It was capable of producing up to 100 kJ in a burst of light lasting for a billionth (10-9) of a second. For that brief instant its output was equivalent to 200 times the combined output of all the electricity generating plants in the United States. Large lasers of similar type have also been developed at the Lebedev Institute in , at Osaka University in Japan, at Limeil in France and at other laboratories around the world. New lasers with state of the art technology and energies more than ten times that of existing facilities are under construction in the National Ignition Facility at Livermore and the Laser Megajoule project at Bordeaux in France. The big neodymium lasers presently at the forefront of inertial confinement fusion research are very inefficientœœtypically less than one percent of electrical energy is converted into ultra violet light and there is a further loss in generating X-rays in the hohlraum. A commercial inertial confinement plant will require a much higher driver efficiency otherwise all its output will go into the driver. Using light-emitting diodes instead of flash tubes to pump a solid state laser would improve the efficiency and permit the rapid firing rate needed for a power plantœœbut diode systems are extremely expensive at the moment and costs would have to fall dramatically. Other types of laser might also be developed and research is looking also at alternative drivers using beams of energetic ions and intense bursts of X-rays.

5 From T-3 to ITER

The early 1960s were depressing times for magnetic confinement fusion research. Theoreticians proposed new and more threatening instabilities and experimentalists retreated from plans to build bigger successors to experiments like ZETA (figure 3) and C-stellarator (figure 4) to concentrate on attempts to understand the physics of hot plasmas in magnetic fields. But in the second half of the 1960s the experimental results began to improve and the success of the Russian tokamak T-3 brought optimism to the IAEA Fusion Conference at in 1968. The only cause to doubt the tokamak results was that the electron temperature had been measured rather indirectly. To put the issue beyond all doubt, a team of British scientists was invited to go to Moscow with the newly developed Thomson scattering diagnostic technique. W hen these results were published, even the most sceptical fusion researchers were convinced. The leading American fusion laboratory at Princeton moved quickly, converting its biggest stellarator into a tokamak. This was followed by the construction of many new tokamak experiments not only in the Soviet Union but in the United States, Europe and Japan. Thereafter set the pace for magnetic fusion research. The increasing emphasis on the tokamak after 1968 was rewarded by steady progress, and the encouraging performance of the newly built machines added more fuel to the tokamak fire. In the aftermath of acute oil shortages resulting from conflict in the Middle East, fusion budgets increased in the 1970s. Although the short-term effects of the oil crisis were soon overcome, they accentuated long-term concerns about energy reserves and industrial nations readily supported fusion research, if only as an insurance premium against the risk of future energy shortages. In this climate it was not too difficult to decide upon the construction of three very large tokamaksœœJET in

1 0 Europe, TFTR in the United States and JT-60U in Japan. These three large tokamaks, supported by many smaller experiments, produced the most substantial advances in fusion research. They explored new regimes of improved confinement, both with limiters and with , extended the database for the empirical scaling relations for energy confinement, found ways to reduce the impurity content of their plasmas, observed the onset of heating, brought the fusion triple product, which since 1958 had steadily increased by one or two orders of magnitude per decade, up to the break-even value (figure 9) and produced nearly 20 MW of real fusion power when operating with tritium. Scientific progress in understanding and predicting plasma behaviour flourished and topics like equilibrium and stability, heating, current drive, edge and physics reached new levels of understanding. Much remains to be done however, both in theory and experiment, to quantify energy and particle transport in the presence of micro-turbulence.

JG01.566-2c Figure 9œœ Summary of the progress towards the Ignition D-T Tokamaks 10 10 goals of break-even and ignition. The points indicate JET 1997 TFTR 1994/5 the results actually achieved by individual Small & medium tokamaks Breakeven

1 .sec) ICF JT-60U experiments over many years since the Russian T3 1 JET eV .k 3 experiment in 1968. The temperature is plotted TFTR - m 21 along the horizontal axis and the nTτE product is (10 (atmosphere secs)

0.1 0.1 E E τ

i plotted along the vertical axis. JET, TFTR and Ττ JT60U have come very close to reactor conditions, 0.01 0.01 top right hand corner. The open symbols for JET and TFTR indicate results with DT fuel. All other Fusion Product n Fusion Product P data are for DD plasma. Some inertial confinement 0.001 0.001 T3 fusion data (crosses) are included for comparison. 1968 1 10 100 1000 Central Ion Temperature (million°C)

In the late 1970s, with the construction of the large tokamaks already underway, the next task at hand was to think about the design of the Next Step experiment. Groups in the United States, Soviet Union, European Community and Japan started to design their own versions and they came together to collaborate on a study that was known as INTOR, the International Tokamak Reactor. The INTOR study did valuable work in identifying many of the technical issues for a fusion reactor and started research programmes to solve them. However INTOR fell short on the physics specification for a reactorœœnot surprisingly as JET, TFTR and JT-60U were still only at the construction stage and it was a very large extrapolation to a reactor from existing smaller tokamaks. Optimistic scaling from these small tokamaks led to INTOR being designed around a plasma current of 8 MA, but even before the study was completed it became clear that this was too smallœœat least 20 MA would be needed to reach the conditions for ignition.

11 INTOR had set the scene for international in fusion reactors and, at the Meeting in 1985, Soviet leader Mikhail Gorbachov proposed to US President that a prototype tokamak reactor should be designed and built as an international collaboration. The United States, the Soviet Union (later the Russian Federation), Japan and the European Community agreed to embark on the design phase of the ITER project. The name ITER (pronounced —eater“) had a dual significanceœœit was an acronym for International Thermonuclear Experimental Reactor and a Latin word meaning ”the way‘. Work on the ITER design concept started in 1988 and was followed in 1992 by agreement to proceed with the detailed design. A dedicated Joint Central Team of scientists and engineers worked full-time supported by many more working part-time in so-called Home Teams. But a lack of accord about where the team should be based and the compromise decision to split the central team between three sites (San Diego in the USA, Munich in Germany and Naka in Japan) warned of future indecision at the political level. Nonetheless the project went ahead with considerable enthusiasm and detailed technical objectives were set. As a next-step experiment, ITER was to test and demonstrate the technologies essential for a fusion reactor, including superconducting magnets and tritium breeding, as well as exploring all the new physics of ignited plasmas. The results from tokamak experiments were now at the stage where it could be predicted reliably that to reach ignition ITER would require a plasma current of about 20 MA with an energy confinement time of about 6 s. ITER would be a tokamak with a divertor and a D-shaped cross-section similar to JET and JT-60U with physical dimensions about three times larger than these machines. The construction was planned to take about ten years at a cost of about six billion dollars. The design had been monitored and checked by independent experts who all agreed that ITER could and should be built. But times had changed by 1998 when it came to reach agreement about the construction phase. The collapse of the Soviet Union meant that a project like ITER could no longer be supported just as a showcase of East-W est collaboration. Now it had to stand on its own merits as a new form of energy. Although there was much talk about environmental issues, the prospects of global warming and of future fuel shortages generated little sense of urgency to carry out the basic research to find alternative forms of energy. In fact government funding for energy research, including fusion, had been allowed to fall in real terms. The situation reached crisis point in the United States where a drastic cut in research funds caused the government to pull out of the ITER collaboration. Japan, Europe and Russia maintained their support for ITER but the Japanese economyœœseemingly impregnable only a few years earlierœœwas in difficulties, Russia was in economic crisis and Europe did not have the determination to take the lead. If anything was to be salvaged it could only be done by accepting a delay until the political climate improved and by reducing the size in order to cut costs. The revised design for ITER (figure 10) published in 2001 has dimensions about 75 percent of those of the previous design with a plasma current of about 15 MA. On the basis of present data this will not be quite big enough to reach ignitionœœthe plasma heating systems will have to be left switched on all the timeœœbut 40 MW of heating will generate about 400 MW of fusion power. ITER will be a realistic test bed for a full size fusion reactor in terms of technology and the physics of 1 2 plasmas where the heating is dominated by alpha particles. It is possible that the reduced size ITER could get even closer to ignition if experiments on improved confinement regimes that are underway on other tokamaks prove successful.

Figure 10œœSchematic of the reduced size version of ITER proposed in 2001.The cut-away shows the interior of the vacuum vessel with the internal and thermal shield. The superconducting toroidal and poloidal magnetic field coils are located outside the vacuum vessel. A man at the bottom right hand side indicates the scale.

The prospects for ITER moved a step closer in 2001 when there was a Canadian offer to host ITER near Toronto on the shore of Lake Ontario. This was followed by offers of sites in Japan at Rokkasho (in the north of Honshu) and in Europe at Cadarache (in the south of France). The United States came back into the project while and joined the existing partners Europe, Japan and Russia. Many details have been agreed, including how the costs will be shared between the partnersœœas ever in such matters the choice of site remains the sticking point, but it is hoped that there will be a decision during 2004. If approved, construction probably will start a few years later and is estimated to take about ten years. The Russian pioneer of fusion, , used to say, —Fusion will be there when society needs it“ and recent history reminds us once again of this.

6 Acknowledgements

This brief account of the history of fusion is drawn largely from two books —Nuclear FusionœœHalf a Century of Magnetic Confinement Fusion Research“ by C M Braams and P E Stott (published in 2002 by Institute of Physics Publishing) and —Nuclear Fusionœœthe Energy of the Universe“ by Garry McCracken and Peter Stott (to be published in 2004 by Elsevier/Academic Press). The first book contains a comprehensive bibliography on magnetic confinement fusion. I would like to acknowledge the implicit contributions of my co-authors to this paper and I am deeply indebted to the late Kees Braams for sharing with me his extensive knowledge of fusion research.

1 3