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 23, 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. (courtesy Princeton magnetic field. Plasma Physics Laboratory) Downloaded by guest on September 23, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) l'ost: Fusion Power 1933

10 HIGH SHEAR X 10 DEEP MAGNETIC WELL

10 0,1 x10 I l PLASMA PROBE SIGNAL c 5 10 15 20 TIME (msecl (2 msec/div )

-= III I l; - - vLOW SHEAR

NO MAGNETIC

WELL t 0.1 X10 0 5 10 15 TIME (msec)

Stabilization improvement in a field of the "spher- fluctuations and conIfinement inl toroidal (olitaiiment confined conductor ator" type, achieved by introducing magnetic shear and magnetic well. (courtesy Princeton Plasma Physics Laboratory)

netic mirrors (the mirrors are the strong field regions at the nagnetic axis) slpiral as they go around the tube- -sometimes ends). The closed bottle is a primitive forerunner of the stel- being nearer to the outside wall, sometimes nearer to the larator idea that I will discuss later. inside wall. This effect is called rotational transform and it was Like many simple and neat solutions to difficult I)roblems, first used in the stellarator. Its effect is to average out the these two are virtually worthless as originally proposed. Nev- "downhill magnetic gradient effect". The second effect of ertheless, they embody the basic confinement principles of sheared fields is that it can also inhibit plasma instabilities later magnetic bottles that have proved to be effective. other than A1HD that could cause losses across the field. Why did these first systems fail? They failed because they Shear, in effect, "shorts out" the localized unstable accumula- were subject to the simlplest and most catastrophic of all tion of charges by taking advantage of the high electrical plasma instabilities-the MIHD or hydromagnetic instabil- conductivity of plasma along field lines. ity, in which the plasma moves as a whole across the confining Strong shear is one basic principle, the magnetic well the field, in the downhill direction on any average magnetic gra- other. Magnetic wells are made by shaping the magnetic field dient that happens to be present. In the simple toroid the within and around a plasma so that the plasma sits at a point field weakens away from the central axis of symmetry at of relative minimum in the field. If this trick can be accom- every point on every magnetic line; confinement is not pos- plished, any direction in which the plasma tries to move is sible and the plasma slides to the outer wall in microseconds. "uphill" on the magnetic gradient. In deep magnetic wells, In the simple open system a similar catastrophe can happen; .MHD instabilities are impossible. here, it occurs sideways between the mirrors. Magnetic well fields can be made either by modifying the The first major success of fusion research, about a decade field coils or by a combination of currents flowing in conduc- ago, was the suppression of AMHD instabilities. This success tors outside the plasma, and currents induced in the plasma was achieved in both open and closed systems by reshaping itself, or in conductors floated inside the plasma. Perhaps the the confining fields. From this work emerged two basic prin- best-known application of the magnetic-well idea is to the ciples of plasma stabilization now used in one form or another mirror machine. The first experimental test was carried out by in almost every modern fusion experiment. These two )rin- loffe and his coworkers in MIoscow in 1961. Fig. 3 shows the ciples are magnetic shear and the magnetic well. general shape of the field lines and the confined plasma in a Fig. 2 shows the idea of magnetic shear. In a sheared field modern version of the magnetic-well type of mirror machine the lines are all helices (except for the one down the axis). But as made by a "baseball coil"-one shaped like the seam on a what's important is that the helical pitch angle varies between baseball. successive layers of field lines (if you can think of layers of Suppressing MHI) modes does not, however, end insta- such things). This means that the lines form a kind of multi- bility. When these modes are gone, some less violent but still layered basket-weave pattern. worrisome modes may remain. Even though these modes can- The helical nature of the field lines in a closed toroidal system not expel the plasma bodily, they can cause it to diffuse exces- with magnetic shear has two important consequences: first, sively rapidly across the field lines of a torus, or out from the all field lines (except the one in the middle of the tube-the ends of a mirror machine. Whereas MHD modes dump plasma Downloaded by guest on September 23, 2021 1934 N. A. S. Syrmposium: Energy for the Future Proc. Nat. Acad. Sci. USA 68 (1971)

I rv^^uNEN C. WEN A:~~~A H FCA IN

.~~~~ ....~ ~~~~~~~~~~~~~~~~~~~

FIG. 8. Illustration of deep magnetic well created by hot electron plasma produced by microwave heating. (Courtesy Oak Ridge National Laboratory.) a in microseconds, these weaker modes dissipate it in milli- seconds. In all but very-high-density pulsed systems, milli- QUARTZ. WINDOW second confinement is too short. In a steady-state reactor, BEAM VALVYEE SPLUTTER /VA~~~~~~~~JLI ii. tenths of seconds up to a second or so would be required. The modes I am now discussing are describable as plasma LASER i =_M- 'I .i turbulences, involving laser-like wave-amplification effects X VACUUM SCENE PLASMA P Imp that lead to unstable plasma oscillations. They are the last BEAM hurdle before scientific feasibility-and major progress has recently been made in overcoming them. The bad effects of turbulence modes were probably first \iv_-BEFEFRNCEen BEAMW ~~~~~~~~~. noticed in closed systems. There, the experiments strongly suggested a fundamental limitation on confinement, proposed b by Bohm about 30 years ago to explain some peculiar effects HOLOGR-,IM - in gas discharges in magnetic fields. According to this dismal law (dismal for fusion, that is), magnetic confinement should FIG. 6. Laser interferometric picture of a stable high-density get progressively worse with increasing plasma temperature, plasma column in the Los Alamos Scylla IV experiment. (Cour- tesy Los Alamos Laboratory.) rather than getting better, as in classical diffusion. Not only that, but confinement was predicted to vary only as the first power of the magnetic field, rather than improving as the square, as in classical diffusion. Thus, high magnetic fields would be of little help. For years experimental evidence in toroidal systems seemed to support the Bohm law. Fig. 4 experimental points - shows the accuracy with which this law was found to hold in the Princeton Model C stellarator-at that time the largest

theory: and most powerful toroidal experiment in the world. last (Bohm)ditfusion _ Faced with the fact of Bohm diffusion, the fusion commun- \slow(classical)diffusion ity went back to the drawing board. Probably the first ray of light came when magnetic wells were tested in toroidal geom- etry. Special experiments were set up in which ring conductors inside the plasma chamber were used to shape the field. electrons, Though no one thinks of these suspended ring experiments as no/cc leading to reactors, they provide critical tests of stabilization principles in closed confinement systems. A clear-cut example of plasma stabilization in such a torus is provided by the Princeton spherator experiment, where both shear and mag- netic-well depth can be varied. Fig. 5 shows the stabilization achieved. The upper scope traces and density curve show sta- ~~~~~~~~~~wal~~~@ _ I ble confinement; the lower ones show turbulence and loss of confinement. plasma radius, cm There is another important aspect of fusion plasma confine- FIG. 7. Demonstration of classical diffusion in a high-density ment not mentioned so far. It has to do with the "beta value" plasma column. (courtesy Culham Laboratory, U.K.) of a fusion plasma. Beta measures the energy density of the Downloaded by guest on September 23, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Post: Fusion Power 1935

cm2/sec K)9 T DM IHD

'E 6 1 106 -ID U BETTER Bo L-4 CONFINEMENT

z 2 w w 103 C TDC 31

FIG. 10. Scale of diffusion constants in toroidal system, show- ing reactor requirements as being less than theoretical (classical) value.

O 0! 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 TIME IN MILLISECONDS classically, not by the Bohm law. Fig. 7 shows the evidence: FIG. 9. History of plasma decay curves in 2X experiment at theory and experiment. Lawrence Radiation Laboratory, Livermore. Shown are im- One important additional aspect of high-j plasmas is that provements caused by magnetic-well fields and other stabiliza- because they can dig their own magnetic well they can actu- tion measures. Theoretical decay rate is uppermost curve. ally help stabilize themselves. This idea is also implicit in the Astron experiment at our laboratory, where the well is to be plasma, as compared to the energy density of the confining dug with high-energy particles. field. When jB is small, the plasma hardly perturbs the mag- A fine experimental example of both stable high-f confine- netic field by its presence; when it is large, it may markedly ment and self-dug magnetic wells is shown in Fig. 8 (from Oak weaken it. # = 1 is the limit; there, the plasma has pushed out Ridge). The upper half shows contour lines of magnetic inten- the field entirely, and magnetic fields can hold no more. Betais sity, as modified by mirror-trapped high-energy electrons. The not too important in academic studies of confinement. It is lower half shows the magnetic field contours without plasma. very important in a reactor, since it measures how efficiently The plasma electrons indeed have dug a well for themselves the magnetic field is being used. Since fusion power varies as and are quite happy to stay there. j2, high # is desirable from an economic standpoint. Thus, magnetic confinement, at first a wobbly and poorly Experiments have now been performed that exhibit both understood business, has been refined to the point where it can high ,3 and classical diffusion in a tubular plasma. These exper- be made to work. Is it good enough for fusion reactors? iments are called "theta pinches", since they work by magneti- First, let's recognize that, with one exception, all the experi- cally squeezing the plasma, with currents flowing in coils ments mentioned so far have been preparatory ones-that is, around it (in the theta direction). A striking example of a sta- they were experiments conducted on non-fusion (low-density ble, high-# plasma column is shown in Fig. 6. The device is the or low-temperature) test plasmas. The exception was the theta Los Alamos Scylla IV and the picture shows a laser interfer- pinch, where the plasma was both hot and very dense. Beyond ometer picture of the compressed plasma. In a similar experi- that experiment what else is known about confining real ment in the U.K., it was shown that such columns diffuse fusion plasmas? Two more experiments, plus follow-ups, have

FIG. 11. Artist's conception of a 1000-MW direct converter for use with a mirror reactor. Downloaded by guest on September 23, 2021 1936 N. A. S. Symposium: Energy for the Future Proc. Nat. Acad. Sci. USA 68.'(1971)

1C930 1940 1950 1960 1970 1980 1990 2000

------|--- RESEARCH------

i--PP N--' - - U-A PPLICATION------

FIG. 12. An optimist's fusion power timetable.

encouraged fusion researchers to feel that magnetic confine- Alamos Scyllac theta pinch facility, just recently commis- ment really should work for fusion reactors. The first of these sioned; the new 2X II experiment at LRL, just coming into is the Tokomak T-3 experiment in Moscow. A tokomak is a operation; and the new LRL Baseball II, also due on immi- toroid in which magnetic shear and magnetic-well effects are nently. It uses neutral beams to create the plasma and has the produced by a combination of the usual external fields with largest high-field superconductor magnet yet built-a base- fields from a heavy, induced electrical current circulating in ball magnet of course. the plasma itself. In T-3, plasma-ion temperatures of about 10 There are also many other new-generation experiments in million degrees were achieved, at close to reactor densities. preparation in other laboratories around the world-includ- The confinement time was about 10 msec, a world's record for ing, we understand, some new and larger tokamaks in the that density and temperature. But what excited the fusion USSR. Will these new generation experiments be successful- community most was the fact that the measured confinement all of them? any of them?-We don't know for sure. time was within about a factor of 3 of the calculated classical The recent encouraging results on fusion confinement are lifetime in a torus, and was far above the Bohm value. The heady stuff, and you can't blame those of us in fusion for Soviet results have since been confirmed at Princeton and beginning to think about reactor problems. One immediate should soon be extended even further in new experiments. question is: how good does confinement have to be for a reac- The second experiment to come within shooting distance of tor, and how close might we be to it? It is here that the toroi- classical confinement, in this case a mirror experiment, is the dal systems really have the edge. The point is, in a MHD-sta- 2X experiment at our laboratory. In this experiment, the ble torus, particles can escape only by diffusion across the mag- plasma was hotter (nearly 100 million degrees); the density netic field. Since diffusion times vary as the square of the was the same. Here the classical time was not as long. Fig. 9 dimensions, you can always buy back confinement time, even shows a historical sequence of plasma confinement curves in in a somewhat nonclassical torus, just by increasing the size a 2X, showing the improvement in confinement caused by the bit. Fig. 10 illustrates this point. As you see, even a failure to progressive elimination of instabilities over the years, and the achieve classical confinement by a factor of 10 or more would approach to classical confinement (top curve). apparently not be a worry. Of course we cannot assume before I have already mentioned the third fusion-plasma-confine- the fact that the present favorable trends will continue all the ment result: the Scylla IV theta pinch at Los Alamos. These way to a reactor-but there are certainly strong reasons for three experiments and other recent experiments represent the hope. visible basis for present optimism in fusion. Less visible, but Suppose we assume that we are indeed on the track in at no less important, is a massive accumulation of prior experi- least one of the approaches to fusion. What, then, are the prob- ments and theory. Plasma theory is a very difficult business, lems of engineering a fusion reactor and what might it cost? but there are more and more cases where theory and experi- The answer will, of course, depend on which system and on ment agree closely, so that prediction based on theory is other details, but some things have turned up through studies. becoming much more of a science and less of an art. First there do not appear to be major engineering problems- Earlier, I mentioned new-generation experiments, based on with some important exceptions-that have not already been the successes of the old ones, that are about to come on line in faced in the fusion program, or in the space program, or in the near future. They are: the Princeton ST Tokomak, now the fission-reactor program. By that I mean the problems of operative but to be modified-the Soviet T-3 results were con- generating intense fields, of obtaining high vacua, of heat firmed and extended with this experiment; the new ORMAC transfer, and of other nuclear-related technology. Some have tokomak experiment at Oak Ridge, on line soon; the new Los not been faced-for example, those to do with the way the Downloaded by guest on September 23, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Post: Fusion Power 1937

reactor plasma is to be heated (which has not yet been tested point where I would like to venture some predictions. These full-scale), and with radiation damage at the reactor chamber predictions will be those of an optimist-but I hope not those wall. But even these problems are not immutable: in some of a fool. I also hope I have made you aware of the fact that no approaches, plasma heating should not be a major problem, decisive experiment has yet been performed which proves con- and in others, radiation-damage effects should be minimal. clusively that fusion is feasible, however encouraged we may The point is that fusion is not one thing. It represents a broad be. new approach to power generation, with new flexibilities and Fig. 12 tries to trace the course of development of fusion new potentialities. power from the time deuterium was first discovered up to the An example of new possibilities is direct conversion. Some present and into a possible future. The width of the bands fusion reactions, the best example being the 2H-,He (deu- measures the effort or urgency level (within that category) as a terium-helium-3) reaction, release their energy as kinetic function of time. As you see, I believe plasma technology is energy carried entirely by charged reaction products. Since already in good shape. Confinement physics is soon to be over electricity is the flow of charges, it should be possible to con- the hump, and engineering should grow. At the bottom, I show vert particle energy directly to electrical power. In principle, my idea of the overlapping sequences: research and develop- the efficiency of this conversion could be 90% or possibly ment and practical application. By my timetable, the first more. At the Lawrence Radiation Laboratory, we are in- applications of fusion power could come in the 1980s, and eco- vestigating an idea for direct conversion that uses the end nomic and environmental aspects should be in full swing by 1990. This all presumes that the critical next 5 years of con- leakage out of the mirror machine. In recent small-scale tests finement physics goes as shown. If that is to happen, fusion we have confirmed many of the principles involved and research will have to have an expanding budget, and to reach have achieved measured conversion efficiencies of over 80%. practical power on this timetable, engineering development Just for fun, Fig. 11 shows an artist's concept of what a 1000 must clearly grow. MW direct converter attached to a mirror reactor might look As Tommy Thompson implied, fusion power is indeed an like. One of the nice features of the direct converter is that uncertain certainty-but I believe we have the clear and pres- kilowatt converted might be substantially lower than that of ent opportunity to bring that desirable certainty much closer. steam turbine-alternator conversion systems. There is really no reason to wait for our grandchildren to The question of cost brings me to the end of my talk, the finish the job. Downloaded by guest on September 23, 2021