P/2410 USA Peaceful Uses of Fusion By Edward Teller * INTRODUCTORY REMARKS ticles involved in that explosion will necessarily move It is a great pleasure to discuss with you the topic with high velocity. of the day and I should like to begin by expressing The second and more important difference is the the same sentiments with which the previous speaker following: The fission chain reaction is regulated finished his paper. It is wonderful that over a large by assembling the correct amounts of active materials and important area of research we can now all talk and absorbers. If the correct amount is exceeded, and work together freely. I hope that this spirit the energy release rises in a rather slow manner due of cooperation will endure, that it will be generally to the action of delayed neutrons. In contrast, the exercised throughout the world in this field and that main regulating feature in a fusion reaction is the it will be extended also to other fields. temperature. When a certain ignition temperature has been exceeded, the reaction itself raises the It is remarkable how closely parallel the develop- temperature and this leads to an exceedingly rapid ments in the different countries are and this, of course, acceleration of further energy release. Slow energy is due to the fact that we all live in the same world release is possible only at low densities ; but this raises and obey the same laws of nature. I was particularly further problems. impressed by the wealth of detail given by the previous speaker. I should like to tell you a few generalities and, perhaps, a few little details which might help you HISTORY or might amuse you. Additional details can, of course, be found in other papers submitted to this The problem of controlled fusion is difficult but not conference. necessarily insoluble. Some of us discussed it in a rather detailed manner during the war in Los Alamos. FUSION AND FISSION This group included Enrico Fermi, John von Neumann, James Tuck and Luis Alvarez. The problem of how to release fusion energy in an explosion process was solved several years ago. Some elementary general facts were recognized at How to release this energy in a slow and controlled that time : That deuterium gas could react t above manner has proved to be a much harder question. an ignition point of approximately 35 kilovolts; that This situation is in sharp contrast with the history of deuterium-tritium mixtures could react î at a con- energy release from fission. One year of intensive siderably lower temperature of a few kilovolts; that the gas should be introduced at an exceedingly low work had sufficed to produce the first nuclear reactor 14 15 3 in the early winter of 1942, while several more years density of approximately 10 to 10 particles/cm were needed to perform the first successful nuclear in order to make the reaction rates sufficiently slow detonation. and in order to avoid excessively high pressures; that at these high temperatures the gas will be Among the reasons for this difference I should like completely ionized (such an ionized gas is called a to mention two which are simple and important. plasma) ; that with the presence of magnetic fields, the Fission energy is released by the mechanism of a ions in this plasma follow spiral paths and that by chain reaction. The chain-carrying neutrons can be appropriate arrangement of the magnetic fields the slowed easily. This facilitates a controlled reaction losses to the walls can be reduced; that the pressure not only because the time that the process takes of the plasma on the field leads to a thermal expansion becomes longer but also because slow neutrons are of the plasma which tends to stabilize the reaction; more easily controlled by specific absorbers and other that equilibrium with radiation is not established and means. On the other hand, release of fusion energy that the energy emitted as bremsstrahlung should be is accomplished by a heat explosion or, to use the treated as a loss ; that for this reason atoms other than technical term, a thermonuclear reaction. The par- t The reactions areD + D->T + H Mev and * University of California Radiation Laboratory, Livermore, D + D -> He3 + n + 3.25 Mev. California. t The reaction is D + T -* He3 + n + 18 Mev. 27 28 SESSION 4 P/2410 E. TELLER hydrogen isotopes must be eliminated as completely Laboratory, at the Los Alamos Scientific Laboratory as possible; and that even the equilibrium between and at the Naval Research Laboratory. electron and positive ion energies will not be complete at the highest temperatures. Very particularly, it was also noticed that in a simple closed field along a torus, A COMPARISON OF THE APPROACHES the particles will not continue to spiral indefinitely around the same magnetic line of force but that they Each of these proposals has its peculiar advan- will drift in a direction perpendicular to both the mag- tages and its peculiar difficulties. The pinch effect netic field and the field gradient. This leads to smaller is based on an earlier idea of Willard Bennett \ who but nevertheless prohibitive wall losses. For the time derived the relations according to which a violent gas being, no experimental work was undertaken for the discharge will contract into a narrow filament under specific purpose of carrying the theoretical considera- the influence of its own magnetic field. The great tions into practice. virtue of this scheme lies in the fact that starting from a rather moderate plasma density one can push In 1952 and 1953, under the stimulus of a successful the system to very much higher densities and pressures man-made release of thermonuclear energy, such an and at the same time expose the containing system experimental approach was started. To a very great to much more moderate pressures. One price that extent this was due to the initiative and confidence one has to pay is that the high densities are obtained of the chairman of the Atomic Energy Commission, only for short periods of time. In other words, we Lewis L. Strauss. It was recognized that the prob- are dealing with a pulsed machine. In order to avoid lems of heating and containing the plasma would be instability and breakup of the pinch, the original extremely difficult. For this reason, several parallel simplicity of the concept had to be compromised by approaches were undertaken which, between them, introducing magnetic fields parallel to the direction amounted to an attack on plasma research on a broad of the plasma current. The detailed theory of the front. pinch contraction, as well as the theory of pinch stabilization, was worked out by M. Rosenbluth at Los Alamos. THE PRESENT APPROACHES Rather early in the game it was noticed that the pinch effect in deuterium gives rise to copious neutron Two of these approaches, the pinch and the stella- emission. The first public announcement of this fact rator, were based on the idea of containing the plasma was made by I. V. Kurchatov.2 Unfortunately, these in an annular region. The main difference between neutrons do not appear to be of thermonuclear origin. these two approaches is that in the pinch an axial Considerable progress has been made by S. A. Colgate current is established with magnetic lines of force in explaining the mechanism by which these neutrons encircling the current, while in the stellarator the roles are produced. A general difficulty in the pinch effect of current and magnetic lines are reversed with the seems to arise from the circumstance that during the principal currents established outside the reacting formation of the high-current filament most of the region. In the United States, the work on the pinch original energy output is lost to the walls. It seems effect was started by James Tuck in Los Alamos. to be of the greatest importance to understand and Later, similar work was undertaken in the University prevent these losses. of California Radiation Laboratory by W. R. Baker The Astron is based on the following observation: and S. A. Colgate and still later at other sites. This In contrast to the instability of the pinch current, it is approach has also been very strongly emphasized by expected that a current of relativistic electrons will be workers in the United Kingdom and in the USSR. stable. If this proves to be correct then one can con- Nicholas С Christofilos proposed an ingenious steady- fine a plasma in the field which encircles this rela- state arrangement, the Astron, which bears some tivistic current for a longer period than in an unstable similarity to the pinch effect. The stellarator was pinch. This arrangement is under development at proposed by Lyman Spitzer at the very beginning of Livermore. the thermonuclear program and work on it was The thorough theoretical studies of the stellarator, carried out in Princeton. carried out by L. Spitzer, M. Kruskal, E. Frieman Taking an entirely different approach, the advan- and others, led to the recognition that the simplest tages of a confinement geometry based on externally arrangements of magnetic lines are unstable in this generated, cylindrically symmetric magnetic fields case also. In order to prevent these instabilities, were recognized by Richard Post and Herbert York. additional magnetic fields are being introduced in a Starting with these concepts, Post evolved an approach direction perpendicular to the original fields. Thus which has come to be called the " Mirror Machine ", the present pinch and stellarator designs are no longer which uses various applications of the magnetic as sharply different as they used to be.