research towards obtaining useful power by controlled
thermonuclear reactions, or nuclear fusion as it is
known, began about fifty years ago at the end of the
Second World War. Before the war, thermonuclear reactions
had been looked for in gas discharges and exploding wires
loaded with deuterium, but none had been found.
Rutherford, remarking in 1933 that anyone who looks
for a source of power in the transformation of the atom
is talking moonshine, had identified the central
problem as seen in pre-fission days. Although in
individual collisions between nuclei there could be a
large release of energy, it seemed impossible to design
an overall system capable of producing more power than it
consumed. By the end of the war, however, ideas from
astrophysics, plasma physics and nuclear physics had
developed to the extent that
some adventurous enthusiasts felt able to propose fusion power as a serious possibility. Although many remained sceptical, the idea took root.
Certainly there were speculative discussions among scientists at Los Alamos, where at the end of the war much of the relevant physics was being studied in connection with the possibility of a fusion bomb, the Super. This activity did not lead to a coherent theoretical or experi-
mental programme ; the team at Los Alamos dis-persed into other fields. Some ideas did, nevertheless, make their way to Liverpool University, where there was expertise in high-power arcs and sparks. In 1949 Reynolds and Craggs searched for neutrons in sparks in deuterium at atmospheric pressure, but none was found. Meanwhile major initiatives, not linked to the Los Alamos work, were launched in England in 1946 by G.P. (Sir George) Thomson, Professor of Physics at Imperial College, who had worked with his father (J.J.) on ring discharges in gas during the twenties and thirties, and by Peter Thonemann, a young Australian who arrived in Oxford from Sydney in 1946, already well read in the appropriate literature and with ideas of what he wanted to do. Both soon got down to work and by 1947 were investigating toroidal systems, with the aim of confinement by the pinch effect. Thomson had already worked out a complete (but rather impractical !) installation, described in a secret patent dated 1946 ; Thonemann with more limited means at his disposal set off with a sequence of experiments to look at the problems one at a time. Both were soon joined by additional workers, and in a few years Harwell had assumed financial responsibility for the work, by now secret because of its potential as a powerful neutron source for making plutonium for weapons. Thonemanns team moved to Harwell, Thomsons to the industrial AEI Laboratories at Aldermaston court.
Meanwhile work had started up quite independently elsewhere ; in the U.S.A. Lyman Spitzer a prominent astrophysicist at Princeton had proposed and built his figure-of-eight Stellarator as well as studying the requirements for a net power producing system. At Los Alamos,
James Tuck (who had been at Oxford and
worked with Thonemann) started the Perhapstron, an inductively driven toroidal system. By 1952 the American programme was well established, with other ideas including mirrors emerging elsewhere. By this time the Russian programme was also developing. Sakharov and Tamm had suggested magnetic insulation in 1950, and an experimental programme was well under way soon after. Knowledge of this burst upon the world in Kurchatovs sensational Harwell lecture in 1956.
Details of what happened in these early years are recorded elsewhere ; here I can only give my personal reminiscence of an exciting and stimulating if chaotic era. The air was thick with unanswered questions, and with ingenious, crazy, and here and there brilliant ideas. It was in 1954 that I was drawn in to the interdisciplinary confusion group hurriedly set up in the wake of rumours that the Americans had made a spectacular breakthrough something to do with shock waves ; I was instructed to study the subject. In the end, I took up Rutherfords point of emphasising that it was necessary to ensure that one could get out more energy than is put in. This led to the new criterion, a sort of razor to clear the air of wilder schemes.
By 1958 the subject had gathered enormous momentum, it was no longer secret and made a major impact at the 1958 Atoms for Peace conference in Geneva, where an impressive exhibition, including working models, was assembled. Despite the collapse of over-inflated hopes when the ZETA neutrons were found not to be thermonuclear, hopes ran high and many alternative concepts were presented.
During the following decade a more sober realisation of the intractability of the containment problem was gained, and hopes flagged. These were revived, however, by the success of the Soviet Tokamak, and the whole subject received a boost during the 1973 Energy Crisis ; large scale tokamak and mirror machines were planned and built in the years that followed. The range of possibilities was now augmented by the inertial confinement concept, which emerged from the American weapons programme. A succession of miniature hydrogen bombs, millimetres in diameter, would be ignited by bathing them in an intense flash of laser light, from enormous laser installations. Further study showed that irradiation by intense bursts of light or heavy ions might be more attractive than lasers, provided that the very challenging demands presented to the accelerator community could be met. Inertial confinement also poses extreme and unusual demands on the designers of the reactor system. This approach, absent from the Geneva exhibition of 35 years ago, is a prominent feature of the present one.
Towards the end of the 1970, some of the urgency of the programme as seen at the beginning of the decade had subsided, but the need for a steady development programme was appreciated. This would require very large installations to explore conditions closer to those expected to occur in a net power producing reactor. During the following decade, large Tokamak experiments were built in the U.S.A., Japan and, most notably, in Europe where JET, the Joint European Torus, first came into operation in 1983. Much has been learnt on these machines, particularly with regard to heating by radio frequency and by injected beams, and with regard to finding an optimum configuration for the magnetic field. The production of over 1 MW of fusion power was demonstrated in a short experiment in 1991.
The next stage requires an experiment so large that it can only be attempted as a world wide cooperative venture. A design study, involving full participation of the European Community, America, Japan and Russia has already started on a large Tokamak ITER, the International Thermonuclear Experimental Reactor intended to demonstrate all the features, physical and technical, required for a commercial reactor.
Inertial confinement studies by enthusiastic groups continue in Europe, the U.S.A. and Japan, but very large extrapolations from present technology are still required for the driver and reactor systems before the technical feasibility of a reactor can be established.
How far are we from the final goal ? As seen at present, even though a demonstration of physical and technical feasibility in ITER might be achieved, it may be that a reliable operating reactor would be so complex that it would be judged uneconomic by present criteria.
Neither the final outcome of fusion research, nor the kind of world in which fusion power might make a major contribution to mankinds energy supply can be foreseen. Certainly, if we can imagine a sustainable, stable, technological world where present political and environmental crises are under reasonable control, the provision of our power by fusion energy would seem to be an essential ingredient. It is an option that we must surely keep open in the years to come.
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