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Fusion Power Proc. Nat. Acad. Sci. USA Vol. 68, No. 8, pp. 1931-1937 August 1971 Fusion Power R. F. POST Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550 Contributed to Symposium on Energy for the Future, April 26, 1971 Last November the late Theos Thompson, then a commis- density would be quite low-about 10-5 of the density of the sioner in the Atomic Energy Commission, gave a talk here in atmosphere-practically a vacuum. Even so, the fusion power Washington to the Plasma Physics Division of the American released per unit volume would be about 10 MW or more per Physical Society. The title of his talk was "Fusion Power- cubic meter of reacting fuel. Higher fuel densities are possible, the Uncertain Certainty". Characteristically, in picking that but since fusion power increases as the square of the fuel title Tommy Thompson had put his finger on the key feature density, reactors working at much higher densities must of the topic he was to discuss. I feel that I could do no better in necessarily operate in a pulsed mode-that is, with an inter- adopting his phrase as a theme for my talk. mittent combustion cycle, like an internal combustion engine. Fusion power does not exist today. What exists is a well- In any fusion reactor, however, the total amount of fuel pres- grounded worldwide research effort specifically aimed toward ent at any one time would only be a few tens of milligrams. achieving it, plus recent evidence that says this research Confinement of the hot fuel gas, that is isolating it from phys- effort is on the right track. There is growing acceptance of the ical contact with the reactor chamber walls, has always been proposition that power from fusion not only will be achieved, the central problem. At first, this problem seemed insoluble but that fusion offers an almost ideal answer to our future even in principle. Then it was realized that the high tempera- energy needs. Fusion will be and needs to be achieved-that is ture necessary for fusion itself could help solve the problem. the certainty. But there is also a feeling on the part of many I am referring to the fact that at fusion temperatures matter that not only is fusion a highly desirable goal, but that it is not exists only in the plasma state - ions and electrons, and that necessarily a distant one-if we move now. Fusion power: is these can be controlled by magnetic fields. Since the earliest it a distant dream or a near-term possibility? That is the days of fusion research this has been the main approach: to use uncertainty. intense magnetic fields to control the plasma, in other words, to Controlled fusion research began about 20 years ago, in attempt to confine it within a magnetic bottle. secrecy and essentially simultaneously in the U.S., the As I implied in using the words "attempt to confine", the United Kingdom, and the USSR. Secrecy was ended by inter- stumbling block for fusion research in the past has been the national agreement in 1958 and since that date an unusually failure of magnetic bottles to live up to expectations. This high degree of international cooperation has existed in this single circumstance dominated the research picture for many field. The present worldwide fusion effort is about $120,000,000 years. This is the circumstance that has now changed so equivalent-about 50% in the Soviet Union, 25% in the U.S. radically. (that is, about $30,000,000 /year), and the rest in the U.K., To anticipate things I will later discuss, let me say that the Western Europe, and Japan. present situation is that the origins of the past failures of mag- Before getting to details I'll review some basic principles. netic confinement, namely plasma instabilities, are well under- Fusion research seeks ways to extract useful power from stood, and that powerful and effective means for dealing with nuclear reactions among light elements. The primary fuel for these instabilities have been devised and proved out in the lab- fusion is deuterium. Deuterium exists in sufficient quantity to oratory. In fact, the recent optimism in the fusion community satisfy any conceivable energy demands for thousands of mil- stems from the circumstance that the stabilization principles lions of years. The cost of obtaining it by isotope separation involved are being found to remain effective in plasmas whose from water is so low that, as a fuel, deuterium would cost less properties are not far below those needed in a fusion reactor. than 1% of the present cost of coal, on a per-unit-of-energy Furthermore, new experiments are coming up that should push basis. these limits even farther. Nuclear fusion is nuclear combustion, the process that heats The optimists among us-and I'm one of them-look on the sun and the stars. To achieve controlled fusion on earth we these new experiments as being the likely immediate prede- must carry out the following steps: first, heat a small quantity cessors of what might be called "scientific feasibility demon- (how small I will mention later) of fusion fuel above its ignition strations" for fusion, i.e., experiments in which the principles point-about 100 million degrees kinetic temperature; second, of magnetic confinement would be proved at plasma densities, maintain this fuel in a heated condition long enough for the temperatures, and confinement times comparable to those release of fusion energy to exceed the heat input; and third, projected for fusion reactors. convert the energy released to useful form, namely electricity. Before I launch into more details let me digress again and The requirement for high temperatures to achieve fusion is say a word about optimism and pessimism inside and outside unavoidable; the choice of operating fuel density is open. For the fusion community. Optimism has kept us going through example, in a large reactor operating in steady state the fuel some very black times-it has also in the past led to pre- 1931 Downloaded by guest on September 25, 2021 1932 N. A. S. Symposium: Energy for the Future Proc. Nat. Acad. Sci. USA 68 (1971) PLASMA OPEN SYSTEM - SIMPLE MAGNETIC MIRROR PLASMA a COIL FIG. 3. Field lines in a magnetic-well type of mirror field as produced by a "baseball" coil. around for at least 30 years. It represents both the starting point and the reference standard for magnetic confinement. FIELD LINES To convert the basic idea into a useful plasma confinement system is another story. This kind of magnetic confinement is CLOSED SYSTEM - SIMPLE TORUS good for stopping transverse flow, but worthless in inhibiting flow along field lines. There are three ways to resolve this di- FIG. 1. Simple examples of open (mirror) and closed (toroidal) lemma. (a) Make the tube so long that its open ends don't mat- magnetic containment configurations. ter. This means a kilometer or so for a high-density pulsed mature rejoicing as soon as a little light showed through the reactor or hundreds of kilometers for a steady-state reactor: clouds. On the other hand, pessimism has little it can say not a very attractive possibility. (b) Restrain the flow of about what are now solid scientific accomplishments in fusion plasma along the field lines; this is the mirror machine idea. research, nor even about the ultimate importance and value (c) (this one is the most popular choice for many reasons) of achieving fusion power. Where the confrontation occurs, Return the field lines on themselves within a doughnut-shaped and where it will remain until fusion power is a reality, is at chamber, so that plasma particles can move freely along the the question of the significance of new results as a valid guide lines without eseaping. The last two ways gave rise during the to the future and on the timetable for translating these earliest days of fusion research to two simple, neat, and intui- results into practical hardware. tively obvious forms: the so-called open and closed magnetic Charged particles immersed in a magnetic field must execute bottles shown in Fig. 1. The open system is called a mirror helical-coil, spring-like orbits in that field. If the field is suf- machine, since it works by trapping particles between mag- ficiently intense, the diameter of the helices will be small com- lw I I I I I I pared to that of the reactor chamber. When this is true, parti- \ . , , cles should be able to reach the chamber walls only in the @-ALKALI PLASMA transverse direction (across the field lines) by step-by-step dif- fusion arising from collisions between the particles-the so- called "classical" rate of diffusion of plasma across a magnetic [.,,RESISTIVE MICROWAVE in a tube this classi- 0 w HEATING field. At fusion temperatures and straight EE' 10 cal rate is calculated to be orders of magnitude slower than LAJ + ELECTRON CYCLOTRON This " that needed to satisfy fusion requirements. result, namely - +.'-RESONANCE HEATING that a strong magnetic field in a straight tube should effec- I-P- tively prevent the flow of particles across that field, has been 2c OHMIC HliEATING: LAJ AFTERGILOW LSo BOHM DIFFUSION,-, L- I Tt Te OHMIC / \ HEATING A ION CYCLOTRO>K -( DATA NORMIALIZED TO RESONANCE HETING 12.3 kG, 5.U.Oc RADIUS) n0 l I..... I fll QI 1.0 10 100 kTe (ELECTRON TEMPERATURE) (eV) FIG. 4. Stellarator model C confinement results compared FIG. 2. Illustration of the magnetic field lines in a sheared with the prediction of Bohm diffusion.
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