History of magnetic fusion

A short history of magnetic fusion

1 - The "Prehistoric Period" (1920 - 1938)

The early beginnings of research on fusion energy can be traced back to the nineteen twenties. At this time, the physicist Aston had measured the "mass defect" in helium, which suggested the possibility of recovering a large amount of energy by making a helium nucleus from lighter elements. After this discovery, the English astronomer Eddington suggested that the energy of the stars was "sub-atomic", and dreamt that " Man will learn one day to free [this energy] and use it to his own ends". Experiments were set up in the United States as early as 1938 to attempt to confine a hot plasma with magnetic fields.

2 - The time of the pioneers (1946 - 1958)

Shortly after the Second World War, a wave of international interest in for thermonuclear fusion broke out. This was notable in the United Kingdom, where Thomson and Blackman, of the University of London, filed a patent for a fusion reactor in 1946. Although the scale of this device was overly optimistic, the device already featured a vacuum chamber in a torus shape and current generation by radio-frequency waves, two important aspects found on today’s tokamaks!

Figure 1 : 1946 : the " fusion reactor", patent filed by Thomson and Blackman

Figure 2 : 1946 : the magnetic confinement devices tested by Thoneman
(tori made of glass and metal), in the Clarendon laboratory (Oxford, United Kingdom)

In the 1950s, during the Cold War, fusion was stamped top-secret. The Americans, Russians and the British intensified their research and were joined by the French, the Germans and the Japanese in the late 1955s.

3 - The first international collaboration (1958 - 1968)

The year 1958 marked an important turning point in the history of controlled fusion, with the unveiling of secret research: at the "Atoms for Peace" conference in Geneva, the different countries revealed the magnetic configurations that they had been working on; toroidal pulses, stellarators, mirror machines, Z and theta-pinches. The foundations of magnetic confinement had been laid, as expressed by the Russian physicist Artsimovitch, at the end of the conference: "we are witnessing here the emergence of an insight into the scientific foundations on which the methods to solve the problems of fusion reactions will probably be based ". The physicists also started to realise that mastering fusion would not be an easy task, due to plasma instability, losses in magnetic configurations and so on. The physicist E. Teller was to say: "I think that [controlled fusion] may be achieved, but I don’t think that it will come to any concrete importance during this [the XXth] century".

Figure 3 : Magnetic confinement devices studied by
au CEA during the 1960's in Fontenay-aux-Roses (France)

To be in a position to take up the huge scientific and technological challenges presented by the mastery of fusion energy, collaboration was set up on an international scale. At the European level, associations were set up between the European Atomic Agency EURATOM and the research organisations of the member countries, so as to coordinate their efforts. The association of EURATOM-CEA, set up in 1959, was the first of these. These structures predated the current international organisation of research (EFDA, ITER project), now more important than ever, given the huge resources demanded.

4 - The era of the tokamaks (from 1968 to today)

A sensation occurred in 1968, when the Russian scientists in the Kurchatov Institute announced that they had obtained performance well beyond the others using a very specific magnetic configuration: the tokamak. Confirmed in 1969 by a British team, which, right in the middle of the Cold War, went to Moscow to measure the temperature in the T3 tokamak, this milestone result opened the era of the tokamaks in other copuntries. They were to rapidly replace the other magnetic configurations in the research for controlled fusion. Today, only the stellarators are still considered as a possible alternative to the tokamaks, although their current performance is significantly lower than the latter.

Figure 4 : Diagram of the Kurchatov Institute’s T1 tokamak in Moscow

Figure 5 : Overhead view of the TFR -Tokamak in Fontenay-aux-Roses- (CEA-France)

The French TFR Tokamak led the world from 1973 to 1976 reaching temperatures of 2 keV. Crucial results on confinement and plasma heating have been achieved on this installation.

The construction of the big modern tokamaks (JET , JT60, TFTR) were launched in the middle of the 1970s, both due to very encouraging scientific results and a significant increase in the budgets attributed to research on controlled fusion. France, having helped Europe enter the tokamak era with the TFR machine, prepared the technology and the physics of continuous operation of fusion reactors right from the 1980s, with the building of a large supra-conducting toroidal coil tokamak, TORE SUPRA, which went into operation in 1988.

Figure 6 : Artist’s drawing of the European tokamak JET

5 - The current state of affairs: 30 years of considerable progress

In the past 30, considerable advances have been made towards the achievement of controlled thermonuclear fusion: the energy balance of the plasma, measured by the triple product nTt of the density, temperature, and confinement time of the energy, has been increased by 1000! This huge leap forward is comparable to (and even slightly greater than) the growth in the performance of micro-processors (Moore's Law). At the end of the 1990s, in the tokamaks JET and JT60-U, deuterium plasmas were obtained, in which the energy balance was close to equilibrium, that is to say where the fusion power released by using a balanced mixture of deuterium and tritium was roughly equal to the power injected into the plasma to heat it. In parallel to this progress in performance, the duration of pulses in the large tokamaks was extended up to two minutes (TORE SUPRA), thus opening the way for continuous operation of a future reactor. Another major achievement is the production of 17 MW of fusion power, from plasmas of a deuterium-tritium mixture obtained in JET, in 1997. These major breakthroughs are the result of progress achieved on tokamaks over a period of thirty years, as much from the point of view of technological know-how as from the comprehension of physical phenomena.

Figure 7 : The triple product (density, temperature and energy confinement time, see also Lawson's criterion) plotted against the centre plasma temrerature. The yellow arrow shows the progress accomplished in the course of the recent decade 1989-1998. The open symbols correspond to experiments with deuterium plasmas, and the solid symbols to plasmas of a mixture of deuterium and tritium.

6 - The future: the ITER project (decision to build is espected in 2003)

The implementation of controlled fusion for energy production requires conditions beyond those attained in the current tokamaks: a factor of around 10 must be attained on the triple product nTt, and the duration of pulses must be lengthened to demonstrate the possibility of continuous operation of the installation. Thus, maintaining pulses over long periods (more than 1000 seconds), where the plasma is mainly heated by particles from fusion reactions, constitutes a crucial goal for the next stage. These new challenges will be taken up by the international ITER, project, the next step in research before construction of an industrial prototype of a controlled thermonuclear fusion reactor.

Figure 8 : artist’s drawing of the ITER project