Controlled Nuclear Fusion. INSTITUTION Atomic Energy Commission, Washington, D.C

Home , Beta (plasma physics), Deuterium fusion, Kink instability, Magnetic mirror, Model C stellarator, Pinch (plasma physics)

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ED 107 520 SE 019 209 AUTHOR Glasstone, Samuel TITLE Controlled Nuclear Fusion. INSTITUTION Atomic Energy Commission, Washington, D.C. Office of Information Services. PUB DATE 74 NOTE 95p. 'AVAILABLE FROM USAEC Technical Information Center, P.0. Box 62, Oak Ridge, TN 37830

EDRS PRICE MF-$0.76 HC-$4.43 PLUS POSTAGE DESCRIPTORS *Energy; *Nuclear Physics; Physics; Radioisotopes; *Scientific Research IDENTIFIERS AEC; Atomic Energy Comthission; NuclearReactors ABSTRACT This publication isone of a series of information booklets for the general public publishedby The United States Atomic Energy 'Commission. Among the topics discussedare: Importance of Fusion Energy; Conditions for NuclearFusion; Thermonuclear Reactions in Plasmas; Plasma Confinement byMagnetic Fields; Experiments With Plasmas; -High-Temperature Plasma Studies;Ntciear Fusion Reactors With Magnetic Confinement; InertialConfinement; and Nuclear Fusion Research Programs. A reading list andfree-loan film list are included. Scho-AS and public librariesmay obtain the booklets without charge. (BT) US DEPARTMENT OFHEALTH. EDUCATION & WELFARE NATIONAL INSTITUTE OF EDUCATION THIS DOCUMENT HAS(TEEN REPRO OUCED EXACTLY ASRECEIVED FROM THE PERSON OR ORGANQATION ORIGIN ATING IT POIN Tf 0FVIEW OR OPINIONS STATED DO NOT NECESSARILY SENT OFFICIAL NATIONAL REPRE INSTITUTE OF EDUCATION POSITION OR POLICY

,:t, This is madeavailable by ,, ERDA , Research 7 iUnited States Energy ,:3and DevelopmentAdministration . Center 't, _i" Technical Information c.o. Oak Ridge,TN37830 Nuclear energyis playing a vital role in the life of every man, woman, and child inthe United States today. In the years ahead it will affect increasingly all the peoples of the earth. Itisessentialthatall Americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them. The United States' Atomic Energy Com- mission provides this booklet to help you achieve such understanding. I

TIlE AUTHOR

Samuel Glasstone Ph.D. (1922), D.Sc. (1926)- ,University of London, inPhysical Chemistryholds a preeminent position as a lucid expositor of scientific subject matter. He has written 34 books, sometimes with the cooperation of other scientists. In 1959 the American Society of Mechanical Engineers awarded him the Worcester Reed Warner Medal in recognition of his "outstanding contribution to permanent engineering literature in ... writings on atomic energy "(in 1968 he received the Arthur Holly Compton Award from the American Nuclear Society for "his distinguished contribu- tions to nuclear science and engineering education". Dr. Glasstone's best known book in the nuclear field is Sourcebook on Atomic Energy, first published in 1950 and revised in 1958 and 1967, it is still a best seller. He has also written for scientists and engineers about reactor theory. nuclear engineering, nuclear weapons, and controlled thermonuclear research, as wellas various aspects of physical chemistry. 5 IMPORTANCE OF FUSION ENERGY 5 Need for Energy Sources 6 Nuclear Energy 7 Nuclear Fusion Energy 8 CONDITIONS FOR NUCLEAR FUSION 8 Realization of Fusion 9 Requirements for Fusion Reactions 10 Deuterium and Tritium Fusion 13 ,High-EnergyRequirement 14 THERMONUCLEAR REACTIONS IN PLASMAS 14 Thermonuclear Reactions 15 Plasmas. Fourth State of Matter 16 Critical Ignition Temperature 18 PLASMA CONFINEMENTBY MAGNETIC FIELDS 18 Need for Confinement 19 Magnetic Confinement and Plasma Density 21 The Lawson Criterion 22 The Significance of Beta 23 Motion of Plasma Particles in a Magnetic Field 25 Particle Drift in Magnetic Fields 27 Open-ended and Closed Confinement 29 Plasma Diffusion 3i- , Plasma Instabilities 33 Factors Affecting Confinement Times 36EXPERIMENTS WITH PLASMAS 36 Plasma Formation 37 Plasma Heating 39 Plasma Diagnostic Techniques 40 HIGH-TEMPERATURE PLASMA STUDIES 40 Understanding Plasma Behavior 42 Low-Beta Pinch 45 High-Beta Pinch 49Stellarator Systems 53 Tokamak Systems 56-Magnetic-Mirror Systems 63 Internal-Ring Systems 3 65 NUCLEAR FUSION REACTORS WITH MAGNETIC CONFINEMENT 65 Toroidal Confinement System 68 Environmental Aspects 69 Direct Conversion Reactor 71 INERTIAL CONFINEMENT 71 Heating by Laser Beams 74 Laser-Fusion Reactor 76 NUCLEAR FUSION RESEARCH PROGRAMS 76Support by U. S. Government and Industry 77 Historical Outline 80 United States Programs 80 Foreign Programs 82 CONCLUSION 1 85 READING LIST 6 87 MOTION PICTURES Vic /ear 111.,10n.Oh: pro(c,.,1)j 1011(11 .,101 and HI )tyr, ;oh:raj(tlicir energy. VIO 1111; 4.1Creit)pc'd to produce electrical /mil 0' OilCarib.It .sale. aila lb111ci, eVI.Nb In ablIlldallcc ordinar1 water. The cloiterium 01 114 °coin) proud(' citotis.,r1-7 elk:rift lu Nan4 f C/ 'L 111110' Pc'/itli'tI1iC/If a! _tor 111('111(c In rate uj con.s1011010,1 1car,. 110/(c. 10 ',mild ti111,(1011 rcac tor by dhow the(ar I 9 Y

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- IMPORTANCE OF FUSION ENERGY

Need for EnergySources The availability of energy (or power)for the operation of machinery has been a decisive factor inthe improvment of man's standard of thing. With the steadyrise in population andinthe use of mechanical devices toincreasethe productivity of labor, the world's powerrequirements are growing rapidly. In the past, the chief sourcesofenergy were the fossil fuelscoal, oil, and natural gas-and, to a smaller extent, water power. Theincreasing demand for energy has spurred worldwide exploration forfuels, especially for oil, and so far the newly discovered reserveshave kept pace with consumption. Although this appears to be a satisfactorysituation, there are limitations. First, thetime must come, possibly by the end of the present century, when the reservesbegin to dwindle and a shortage of fossil fuelsbecomes a definite prospect.And, second,the distribution of oil and coal reservesis such that many industrialized countries must import their fuel supplies atconsiderable cost. Furthermore, the combustion of fossil fuels, especiallycoal, causes problems of atmospheric pollution by producingoxides of sulfur and nitrogen and fine particles of ash. A newand clean source of energy with a basicfuel that is cheap, abundant, and available to all, would thus be a greatboon to mankind. Scientists hope that nuclear fusion willprovide such an energy source. In order tounderstand this processitis necessaryto know something about thebackground of nuclear energy.

Photo I.Large loop prominences on the sun, caused by a locally intense magnetic field. The U. S. programin controlled '.lsionis devoted to research on fusion reactions similar to those fromwhich the sun derives its energy.

5 Nuclear Energy

Soon after radioactivitywas discovered at the end of the 19th century. physicistsbeganto conjecture about the energy. which they called 'atomic energy,"that was appar- ently stored within the atom. Later.when the nuclear theory 01 the atom was developed in 1911.scientists realized that it was the nucleus. the centralpart of the atom, that was the source of this energy.* Hence. it shouldbe called "nuclear energy" rather than atomicenergy. Until 1939. however,no one knew how this energy could bereleased in a useful manner. In the course of his studies in1905 on the theory of retativity. Albert Einstein showedthat mass and energywere, in asense. equivalent. Consequently.energy should be liberated in any process associatedwith a net decrease in mass. By considering the measuredmasses of atomic nuclei, it became apparent that therewere two general ways whereby nuclear energy could be martsavailable by using reactions accompanied by a decrease inmass. One is by the splitting (fission) of the heaviest nucleiinto nuclei of intermediate mass. and the other is by the combination(fusion) of some of the lightest nuclei. Actually, there are many othernuclear processes that are accompanied bya liberation of energy. But only with nuclear fission and fusionis there the possibility of producing more energy than is consumedduring the reaction. In other words. only for these reactionsis there 4 prospCct that the process. once started,can be self-sustaining like a fire.

The discovery of nuclear fissionin 1939 revealed anew and highly concentrated source ofenergy. Some 6 years later, this energy was first used in the atomic bomb, andsince that time nuclear reactors have been developed inwhich fission energy is liberated as heat and convertedinto electric power.t The

*Ior a description of atomic theory,see Inner Space. The Structure of the Atom, another booklet in this series. +See uelear Reactors and Nuclear Power Plants,other booklets in this series.

6 ,mpact of nuclear fission energyis already being felt in many countries. In 1973, about =I%of the electricity used inthe United States was generated innuclear reactor power plants and this proportion is expected toincrease to more than 20% by 1980. But fissionis not the complete solution tothe energy problem. Itis true that the world resourcesof the basic materials, namelyuranium and thorium minerals, are fairly abundant. There are,however, many countries that either do not possess theseminerals or do not have the means for producing nuclear fuelsfrom them. Furthermore, special precautions must be takenin the operation of fission power plants to prevent undesirableenvironmental effects.

Nuclear Fusion Energy fusion of Itis such considerationsthat make nuclear exceptional interest as apossible source of power.The fuel is "hell% y hydrogen" or a form (isotope)of hydrogen. called that is present in all water:for every 6500 or so deuterium. there is one atoms of ordinary(light) hydrogen in water, atom of deuterium.In other words, thereis1 pound of pounds of water. Butthe deuteriumin some 30.000 enonnous volumes of oceanand other surface waters on of earth contain more than10 million million(10' 3)tons deu terium! Calculations show thatthe energy that could,in theory. be present in a produced by the fusionof the deuterium nuclei from gallon (S pounds) of wateris equal to that obtainable The large amounts the combustion of300 gallons of gasoline. virtually of deuterium available onearth thus represent a inexhaustible potential sourceof energy. water is not The cost of obtainingthe deuterium fuel from to extract a large. At the present.it costs only a few cents of water. The quantity of deuteriumequal to that in a gallon to require lithiumin firstfusion systems areexpected to deuterium,but lithium is not anexpensive addition will be seen material and amplesupplies are available, as

7 shortly. In fact, it isanticipated that the fuelcosts for fusion powerwillbe negligible.Furthermore,the safety and environmental aspects offusion power are expectedto be favorable (see page 68). Herethen is apparentlythe ideal source of energy. Unfottunately,this is not the whole For one thing, story. as in a nuclear fission system,*theprice of the fuel would representonly a small proportionof the cost of the electric power produced. For another,there are difficult problems to be solvedbefore fusionpower can be a reality. The purpose of this booklet is to indicate thenature of these problems and to show howsolutions are being sought.

CONDITIONS FOR NUCLEARFUSION

Realization of Fusion

Before describing theconditions that must bemet if fusion energy is to be released in a practical manner (i.e., ina fusion "reactor"), we will reviewthe evidence whichshows that nuclear fusion is indeed possible. In the first place,fusion is the energy source of the sun and other stars. The sun's fuelis not deuterium but ordinaryhydrogen; in a series ofnuclear reactions, four hydrogennuclei are fused togetherto form a helium nucleus. Thereare good reasons, however, forstating that the sun'sprocesses take place much too slowlyto be useful on earth. in thesun, for example, the abilityto generate energy at a highrate depends on theenormous quantity of hydrogenpresent. Laboratory experiments haveshown that nuclear fusion can be achieved with deuterium.Deuterons (i.e., deuterium nuclei) can be acceleratedto high velocity (and kinetic energy) in a charged-particleaccelerator, such asa cyclotron or similar machine.* If thesedeuterons strike a solidtarget containing deuterium, fusionreactions occur. But, of the collisions between the accelerateddeuterons and those in the

*Sec Accelerators, another bookletin this series.

8 target, only a very small proportion lead tofusion. In the mat majority of collisions, the impingingdeuteron is merely deflected (scattered) and at the same time loses some ofits energyit then becomes essentially incapable of fusing with another deuteron. In effect, most of the energyof the acceleraied deuterons is lost as heat in the target. Much more energyis consequently spent in accelerating the 'deuterons than is produced by the small number of fusionreactions that occur. Although the acceleration procedure is not abasis for the practical liberation of energy, it does show thatfusion between two deuterium nuclei is possible. Finally, nuclear fusion is the source of the large amount of energy releasedin the so-called hydrogen (or H-) bomb. Weapons of this type contain deuterium as one of their components together with a nuclear fissionbomb that serves as a trigger. The latter supplies the energyrequired to make fusion reactions occur.

Requirements for Fusion Reactions In order to see how controlled nuclear fusionmight be realized. let us examine the essential requirements.First, the two light nuclei must come close enough topermit interaction. Since each nucleus carries a positiveelectrical charge, the two nuclei repel each other more and more asthey come closer together. Consequently, for the nuclei to interactthey must hate enough initial energy to overcomethe force of electrostatic repulsion that tends to keep them apart.The maemtudc of the repelling force increases with theelectrical charges onthetwo nuclei. To keepthis force small, therefore.theinteractingnuclei should have the lowest possible charge (or atomic number*). The element with the smallest atomic number ishydrogen, since its nuclei (and those of its isotopes) carry but asingle charge. The obvious choice for fusion reactions onearth is

* I he atomic number of .in isotope is the number of protons inthe nucleus.

9 thus sonic form of hydrogen. The factthat it happens also to be cheap and abundant is, ofcourse, an advantage. Three isotopes of hydrogenare known. The lightest, with amass number* of one, is ordinary hydrogen,H; the nucleus of an atom of this isotope is calleda proton. Ordinary hydrogen is the isotope that undergoes fusionin the sun. The next isotope is deuterium,mass number two, represented by 'II or, more commonly,by the symbol D; its nucleus, the deuteron, is also indicatedby D, although D+ is used when it isnecessary to distinguish between the neutral atom and the positively chargednucleus. It occurs inall natural water, as mentioned earlier,and can be extracted Without too much difficulty. Finally,there is tritium, mass number three, represented by 311or T; the nucleus is called a triton. This isotope is radioactiveand is very rare in nature, butit can be made by the interactionof neutrons with lithium nuclei.

Deuterium and Tritium Fusion

Fusion processes with deuterons andtritons take place fast enough to make them reasonablypossible sources for the release of energy at ausefulrate. These isotopes are, therefore, the most practical fusionfuels. Because of the low cost and availability of deuterium, it wouldbe preferable to use this isotope alone; the fusionprocess would then involve only deuterons. Two such reactions,occurring with roughly equal probabilities, are known; theyare

D D -+ 3 He + n + 3.2 MeV and D+ D-+T+11+4.0MeV,

*The mass number of an isotope is the total numberof protons and neutrons in the nucleus,

10 where n represents a neutron. The energy releaseis expressed (MeV)Y. In the first of these in units of million electron volts i two reactions, the products are ahelium-3 nucleus and a neutron, and in the second they are a tritonand a hydrogen nucleus (proton). The triton formed in this manner canthen react fairly rapidly with another deuteron, thatis,

D 1- T--> 'I-lei-11+ 17.6 MeV, leading to the formation of a helium-4 (ordinaryhelium) nucleus and a neutron, plus the large energy releaseof 17.6 MeV. The fusion of deuterium alone would be thepreferable reaction for the release of energy, but we shall seethat the conditions required to make the process practical are very severe. It is possible that theseconditions could be alleviated by adding asmall proportion of tritiumina so-called "catalyzed" DD reaction. The general feeling at present, however,isthatcontrolled fusion willfirst be realized through the reaction between deuterium and tritiumnuclei, in accordance with the D-T reaction given above.Since the Ennui]] would not be available from natural sources,it would be made artificially by the interaction of neutronswith lithium nuclei. The neutrons released in the D-Treaction would be used for this purpose. The process is consequently referred to as tritium "breeding". The raw materials for the production of energy by theDT reactionwould thus be deuterium and lithium. Ample reserves of the latter are available on land at amoderate cost. Infact, the known and reasonably inferred reserves in the United States will last for hundreds of years and probable reserves would last for a few thousand years.Even larger amounts are present in the oceans, but, before thelithium

*An electron volt (eV) is the energy acquired by a unit (electronic)charge in accelerating through a potential of I volt. The MeV (million electronvolt) unit is equivalent to 1.60 x IV erg or 3.8 x IV* caloric.

.;, 0 11 supplies on land are exhausted,the conditions for fusion of deuterium alone should be realized. A pictorial representation of thethree fusion processes of immediate interestis given in Figure 1, whichshows the rearrangements among the constituentneutrons (n) and protons (p). A deuteron consists ofone proton and one neutron, and a triton ofone proton and two neutrons. The energy released appears as kineticenergy and is divided between the two products ininverse proportion to their masses. The amount of energy carriedby each of the products is indicated. In thereaction between deuterium and tritium nuclei mostabout80% of the fusionenergy is associated with the neutron, whichis not electrically charged.

D 3He

0.8 MeV 2.4 MeV

D H + 0 1.0 MeV 3.0 MeV

T 4He (9* 40k 0 3.5 MeV 14.1 MeV Figure 1. Fusion reactions withdeuterium (D) and tritium (T) nuclei. The numbers indicatehow the fusion energy is divided between the products in eachcase. The neutrons (n) are not electrically charged, but the otherproducts carry electrical charges.

12 5 1' In addition to the three fusion reactionsjust described, which involve only isotopes of hydrogen, thereis also some interest in the nuclear fusion reaction betweendeuterium and helium-3, namely,

D+3He-->4He+H+18.3MeV.

The helium-3 required for this process would begenerated by the first of the DD reactions. TheD-3 He reaction will not be discussed but itwill be mentioned later in a special context (page 69). Both products, 4 He and H, carryelectrical charges.

High-Energy Requirement So far, we have established that thenuclear reactions of primary interest for the controlled releaseof fusion energy involve either deuterium alone, deuterium andtritium, or deuterium and helium-3. The forces of repulsion between these light nuclei are the minimum possible, andthe resulting reactions could be expected to take place at areasonable rate. The next point to consider ishow to give sufficient energyto the nucleito permit them to overcome the repelling forces. One obvious way is to make useof an accelerator and a solid target, as described earlier. But we saw that this procedure wastes too much energy tobe of practical value. It has been used extensively, however,in the laboratory to study the probabilities(nuclear cross sections) of the DD and DT reactions. Much of what is nowknown about the conditions under which these reactionswill occur was determined in this manner. Another way of supplying energy to thenuclei is to raise the temperature of a gas consistingof the isotope (deuterium) or isotopes (deuterium and tritium) that areto undergo fusion. The kinetic energy of an atom(or nucleus)is

13 proportional to its absolute temperature*; hence, it is onlya matter of attaining a sufficientlyhigh temperature to permit fusion reactions to occur. This is precisely what happensin the sun. At first sight, itmight appear that the situationis somewhat similar to that in whichparticles are accelerated. However, if the nucleiare confined in some manner,so that they cannot escape, theconsequences are very different. Although many ofthenuclearcollisionsinahigh- temperature system result in scatteringinstead of fusion, the effect isa redistribution of energy rather thana loss of energy. Temperature and averageenergy are unchanged. In a confined space, the nucleimoving in random directions would collide repeatedly untilfusion reactions take place.

THERMONUCLEAR REACTIONS INPLASMAS

Thermonuclear Reactions

Fusion reactions broughtabout by means of hightemperatures are referred toasthermonuclear reactions.Strictly speaking, however, the adjectivethermonuclear implies temperature equilibrium, in which theenergies (and velocities) of the nuclei or other particleshave a range (or distribution)of values determined by theirrandom motion. The equilibrium energy distribution can be calculatedtheoretically, and it is found that, although most ofthe nuclei have energies inthe vicinity of the most probablevalue, there are alwayssome with lower and others withhigher energies. It is therelatively small proportion of nuclei withthese high energiesmuch higher than the averagethatis responsible for thegreat majority of thermonuclear fusionreactions. The thermonuclear approach,through the use ofvery high temperatures, appears to be themost promising for con-

*Temperatures on the absolute (or Kelvin)scale are obtained by adding 273to the temperature on the Celsius (centigrade)scale (e.g., 25°C = 25 + 273 = 298°K).

14 trolled fusion. The actual temperature required depends on the particular fusion reaction that is being employed. From calculations based on measured cross sections, and other considerations to be described shortly, it has been determined that a system for the release of energy by nuclear fusion of deuterium and tritium would have to operate at a temperature of about 100,000,000°K. The fusion of deuterium alone would require even higher temperatures. Itis evident, therefore, that exceptionally high temperatures, ..such higher than the 15,000,000°K of the sun's interior, must be reached before useful thermonuclear fusion reactions can occur on eart1 . Such temperatures have been achieved in a number of experiments; however, to release practical amounts of energy using thermonuclear fusion reactions, itis necessary to confine the high temperature gases for a longer period of time than has been attained to date.

Plasmas: Fourth State of Matter At these very high temperatures, essentially all thehydro- gen atoms will be stripped of theirelectrons. Such a gas, consisting entirely for almost entirely) of positively charged nuclei (ions) and free negative electrons, is said to behighly ionized. A highly ionized gas is commonly called a plasma.It should be remembered that, although a plasma contains free positive ions and free negative electrons, the numbers of positive and negative electrical charges balance exactly, and the plasma as a whole is neutral. Because of the presenceof the electrically charged particles, plasmas have a numberof interesting properties, some of these can be used to advantage incontrolledfusion research,but others create unique problems. The unusual characteristics of plasmas have led to the revival in recent years of the expression, the"fourth state of matter", firstused by William Crookes in1879, to describe them.

15 Critical Ignition Temperature

In a reactor that wouldproduce useful energy by fusion, energy would haveto be supplied initiallyin order to establish the proper conditions(temperature and ion density) for the thermonuclearreactions to take place ata significant rate. Once these conditionsare reached in the plasma, they must be maintained long enoughto generate recoverable energy by fusion atleast equal to the amountsupplied initially to bring thereactor to its operating condition. If this requirement is met, thesystem would be describedas "self sustaining". Anyexcess energy beyond that needed fora self-sustaining process would beavailable for use as apower source for external purposes. Thus,a fusion system must be at least self-sustaining for itto have any practical value. One of the conditions fora self-sustaining fusion reactor is the temperature, as will now be shown;another is related to the ion (tensity anti tne confinmenttime (see page 21). At the very hightemperatures necessary for nuclear fusion reactions, a plasma losesa considerable amount of its energy in the form of radiation.This loss takes placeso rapidly that the energy would not beavailable for heating the reacting nuclei. The radiationenergy is not lost entirely, because it can be absorbed and used tosome extent. But it escapes from the region where it is neededto maintain the fusion ruction temperature. Clearly,a system cannot be self-sustaining unless the rate at whichenergy is produced by fusion excceds the rate at which it is lost fromthe plasma as radiation. This requirement determines theminimum operating temperature for a nuclear fusion reactor. Calculations have been madeof therate of energy production in a plasma by fusion,on the one hand, arid of the energy loss by radiation,on the other hand, over a range of temperatures. At lowertemperatures, the radiation loss rateislarger than the rate ofenergy generation and a self-sustainingreaction would not bepossible.As the temperature is increased, both ratesincrease, but the energy

16 r.%1 Figure 2. Evaluation of thecriticalignition temperature that is the minimum for a thermonuclear fusion reac- t iontobeself-. sustaining. Below this temperaturethe rate of energy loss as radia- CRITICAL tion exceeds the rate IGNITION TEMPERATURE atwhichenergyis produced bythe fusion reactions. TEMPERATURE

production increases fasterthan does the radiation loss (Figure 2). Therefore, above a certain temperature,called the critical ignition temperature, more energy isproduced by fusion than islost. At this temperature, a sell sustaining fusion reactor istheoretically possible, although a higher operating temperature would be needed in a systemfor producing significant amounts of fusion energy. Forpractical purposes.the minimum operating temperaturefor the deuteriumtritiumreaction is believedtobeabout 100,000,000 °K and that for the fusion of deuterium aloneis about 500,000,000°.* The energylosscalculationsusedinderivingcritical ignition temperatures are based on the suppositionthat the

innuclear fusion studies, temperatures arc expressedin terms of the kilo-electron volt or keV (1 keV = 1000 electron volts). Thus, 1keV is equivalent to a temperature of 1.16 x IV degrees K or veryroughly 10,000,000°. Hence, the minimum useful thermonuclear temperatures arc thought tobe about 10 keV for the DT reaction and 50 keV for the DD reactions.

17 energy consists solely ofbremsstrahlung,which is radiation resulting from deceleratinginteractions between rapidly moving charged particles inthe plasma, chiefly electrons interacting with ions. Moreover,it is assumed that the only nuclei in the systemare those of hydrogen isotopes.The presence of nuclei of highercharge, that is, higher atomic number, will increase therate of energy loss as bremsstrahlung. Hence, impurities, andeven the helium nuclei formed in the fusion reactions, willtend to raise the ignitiontempera- ture. Inaddition,thereisapossibility that,atvery high temperatures, another kind ofradiation loss may become important. This is thesynchrotron radiationthat is emitted by high-energy chargedparticles (electrons inparticular) moving in a magnetic field;as will be seen shortly, such fields play an important role inplans for achieving controlled fusion. However, whereasthe fraction of the fusionenergy lost as bremsstrahlung is thesame for all plasma densities, the loss as synchrotron radiationdecreases at higher densities. Consequently, the synchrotronradiation loss can probably be minimized by operatingat the highest possible plasma density.

PLASMA CONFINEMENTBY MAGNETIC FIELDS

Need for Confinement

Suppose, for the moment,that we know how to produce a deuteriumtritium plasmaatatemperature of some 100,000,0000 or more. How is such a plasma confined?The difficulty does not lie in thevery high temperature because, at the low gas densities ina fusion reactor, the totalenergy content .of theplasma would be insufficientto cause any significant damage toa containing vessel if the plasmawere to come in contact with it.

Cj 4 18 Is The problem arises from the loss of energy by the nuclei when they strike the walls of the container. At a temperature of 100,000.000°. the nuclei (and electrons) in a plasma are moving randomly in all directions at average speeds of several thousand miles per second. Consequently, witiOi a small fraction of a second, all the particles will have hit the walls of the containing vessel, as a result, they lose essentially all their kinetic energy. In other words. the plasma would be rapidly cooled. Evenifthehigh-temperature plasma could be generated instantaneously, it would not last long enough to allow a significant number of fusion reactions to take place. A method must therefore be found to prevent the plasma particles from striking the walls of the containing vessel. In this connection the electrical charges carried by the nuclei and electrons can be used to advantage. It is difficult for charged particles to cross the lines of force of a magnetic field. A plasma can therefore be confined by a magnetic field of suitable form. (See Photo 2.) The concept of magnetic confinementis often described as the use of a "magnetic bottle". Many different magnetic field arrangements have been proposed for the confinement of the high-temperature plasmas requiredfor fusion reactions. Some of the more promising will be discussed later. Another possibletype of confinement, called inertial confinement, which does not involve magnetic fields, has been proposed (see page 71). For the moment, however, we will restrict ourselves to magnetic confinement.

Magnetic Confinement and Plasma Density A plasma, like a normal gas, exerts a pressure that comes from the motion of the particles present; this pressure is proportional to the absolute temperature and to the particle density (i.e., the number of particles per cubic centimeter). The maximum plasma pressure that can be confined by a magnetic field depends on the strength of the latter. Because there is a practical limit to the magnetic field strength,. there / i / 19 Photo 2. Light emitted by plasma confined ina "magnetic bottle".

is a corresponding limit to the particlepressure 8)f the plasma that can be confined. Since the temperaturesare extremely high. the other factor that determines the particlepressure, namely. the plasma particle densitymust inevitably be low so that it does not exceed the limit on the plasma pressure. In deciding upon the plasma density. another factormust be considered in addition to the magnetic field strength. The rate at which energy would be generated per unit volume of a fusion reactor increases with the particle density. Forenergy to be released at a useful rate, therefore:. the density must not be too low. On the other hand, it must not beso high that the nuclear fusion energy cannot be removed (and used)as fast as itis released. Both consideration of magnetic field strength and heat removal suggestthat the plasma in a controlled fusion *stem will probably have a density of 1014 to 1016 particles per cubic centimeter. Thismay be compared with 3 X 1019 particles (molecules) in agas at normal room temperature and pressu.:. At the temperatures that must be attained in a deuterium .tritium fusion reactor, adensity,of 10" particles per cubic centimeter would

20 k.1 inch. It represent a pressure ofabout 200 pounds per square magnetic isthis pressure that mustbe contained by the bottle.

The Lawson Criterion In addition to thespecifications of temperatureamd plasma another particledensity,afusionreactor must satisfy condition. This is known asthe Lawson criterionbecause it in l'ogland in 1957. It was first pointedout by J. D. Lawson system, is based on the requirementthat, in a self-sustaining produce the reacting nuclei mustbe confined long enough for the sufficient recoverable energyby fusion to compensate The criterion is amount suppliedinitially to heat the plasma. (in expressed as the product nr,where n is the plasma density secon::3) for particles per cubic centimeter)and T is time (in confined by the which the plasma ofthat density can be magnetic field. For apractical fusion reactor, /ITmust exceed 1016 for the about 1014 for theDT reaction and about DD reactions. particle density Since the Lawson criterionis a product of conditions over and confinement time,there is a range of density is at the which it can be satisfied.For example, if the cubic centimeter, lower practical limitof 1014 particles per particlesina theconfinementtimefor the charged deuteriumtritium system mustexceed1 second. On the other hand, in the vicinityof the upper limit ofabout 1016 would particles per cubic centimeter,the confinement time hundredth part) of have only to be longerthan 0.01 (i.e., one a second. production of The three essentialrequirements for the the next useful energy by nuclearfusion are summarized on require is higher page. For deuteriumalone, the temperature for a mixture of and the confinementtime is longer than deuterium and tritium.It is clear, therefore,that a deuterium devise than one fusion reactor wouldbe more difficult to for the based on deuteriumand tritium. This is the reason

21 ESSENTIAL REQUIREMENTS FOR FUSIONREACTORS Minimum Minimum temperature Particle density Reaction confinement time (°K) (per cm') (seconds) DT DD 100,000,000 10 "-10' 500,000,000 10 " 1-0.01 -10' 100 -1

statement madeearlier that practical controlledfusion may be reached firstby way of theDT reaction. hand, the engineering On the other problems of a DDreactor could easier since itwould not require prove the breeding oftritium. The Significanceof Beta A magnetic field that confinesa plasma effectively pressure on the plasma. exerts a The ratio of theplasma particle pressure to thepressure due to the magnetic field iscalled beta (Greek, 0);thus,

betaplasma particlepressure magnetic field pressure' The value of beta for a confinedplasma can when the plasma range from 1, pressure is equal to themagneticpressure, down to almostzero, when the plasma For magnetic ressure is very low. confinement to bepossible, beta exceed unity, for must not otherwise the plasmawould immediately escape from the magneticfield. The plasmas connection with being studied in controlled fusionfallinto two broad categories: High beta, in which beta isgenerally 0.5 and low beta with or more, beta equal to about0.2 or less. If beta wereexactly unity, that is to say, if theplasma and magnetic pressureswere equal, the magnetic enclose the plasma field lines would without penetratingit; in other words, there would beno magnetic field lines inside the plasma.In practice, however,beta is invariably less than unity andthe lines of force ofthe magnetic field penetrate the plasmato

22 6 .'") i,,,d some extent. The smallerthe value of beta, the greater is the penetration of the plasma by the fieldlines. Nevertheless, the magnetic field can still confine theplasma as will be seen in the next section.

Motion of Plasma Particles in aMagnetic Field The presence of a magnetic fieldin a plasma causes the electrically charged particles to movein a particular manner. For example, in the absence of amagnetic field, an assembly (or plasma) of charged particlesin a cylindrical vessel will move in straight lines inrandom directions and will quickly strike the walls (Figure 3a).Suppose, next, that a uniform (homogeneous) magnetic field isapplied. The particles will now be compelled tofollow helical (i.e., corkscrew orspiral) paths,as depicted inFigure 3b, encircling the lines of magnetic force. Positivelycharged particles spiral in one direction and negatively chargedparticles in the opposite

Figure 3. Effect of uniform magneticfield on charged particles. In (a), no field is present.In (b), a homogeneous magnetic field is applied.

(a)

MAGNETIC (b) FIELD

23 direction. Asa result, the particles are not free tomove across the magnetic field lines: access to thewalls of the vessel is thus restricted.in a sense, each line of force particle is "tied"to a alone which ittravels in a helical constant radius. path of The radius of curvature of the path ofa charged particle alone a field linedepends on three factors: (I) thestrength of the magnetic field. (2) the mass ofthe particle, and(3) the component of the particlevelocity in the direction alleles to the lines at right of force. The firsttwo factors can be explained quite easily. but the third willrequire a littlemore consideration. Other things being the same. theradius of the spiral's curvature is inversely proportional to thefield strength and directly proportional to the mass of theparticle. Hence, for two particles with the same mass (and electriccharge). the greater the magneticfield strength the smaller thespiral's radius. Furthermore.in a plasma confined by a riven magnetic (and penetrated) field. the electronswill move in much tighter spirals thando the much heavier We will now ions (atomic nuclei). consider the effectof the right -angle nent of the particle compo- velocity. Supposer in Figure 4 represents

Figure 4.The velocity of a particlemoving in a giren direction. indicated byv,can be treated 'Ira thematlathy as consisting oftwo MAGNETIC components: One.vii, FIELD parallelto the mag- DIRECTION. netic field lines.and the other,vi, perpen- diculartothe field lines.

24 this the magnitude and directionof he velocity of a particle; one, can be divided(mathematically) into two components, at ro, parallel to themagnetic field lines, and theother, right angles. The radiusof the spiral path is thenproportional to the component The other component, 1,11,determines how fast the particletravels in the directionof the field during the course of itsspiraling around the field lines. Two extreme cases ofparticle motion in a magneticfield are of interest.First, suppose the particleis moving at right angles to the line of force; 1,is then the same asrl,andII is zero. The path ofthe particle would be acircle and not a spiral (or helix). since thereis no motion alone thefield lines. The other extreme occurswhen the velocity r isdirected along the lines of force; then ris equal to 1,11, and pi is zero. The radius of the spiral is now zero,and the particle travels in Between these two a direct manneralong the line of force. extremes, there is aninfinite number of possibilities.Thus, even in a uniformplasma contained in, a uniformmagnetic field, there is a widevariation in the radius of curvatureof the spiral paths and in the rateof progression of theparticles in the direction of themagnetic field lines becauseof the many differentvelocities and their components. motion, a As a consequence ofthese variations in its spiral charged particle willoccasionally collide withothers, both of positive and negative. As aresult, the center of curvature the path can shift from onefield line to another. Inthis way, lines of it is possible for a chargedparticle to move across the force and there is a possibilitythat it will eventually escape of the confining action ofthe magnetic field. The process gradual escape of particles bymotion across the fieldlines is referred to as plasmadifflision; more will be said aboutthis phenomenon later.

Particle Drift in Magnetic Fields There are other situations inwhich charged particles can move across thelines of force of a magneticfield; an uf 25 DRIFT

UNIFORM FIELD

WEAKER FIELD 4,////wisw.49www.o://////..-.:-,/r,ST NGER FIELD %. (a) (b) Figure S. Motion of a positively charged particlein (a) a homogeneous. (uniform) magnetic field and (b)an inhomogeneous (nonuniform) magnetic field directed upwardper- pendicular to the plane of the paper. In (b),the field is stronger in the shaded region than above it. In thiscase, the positively chargedparticles driftto the right. Negatively charged particles would spiral around the lineof force (and drift) in the oppositedirection.

important one arises when the magnetic fieldis not uniform (i.e., inhomogeneous) in strength. Theparticles no longer follow aspiral path of constant radius because,as seen earlier. the radius varies inversely with the fieldstrength. The general behavior can be illustrated byconsidering a simple example. Let the magnetic field lines run perpendicularto the plane of the page so that a line of forceappears as a point. If the field is uniform, the spiral path of a chargedparticle is then represented by a circle, as in Figure 5a. Supposenow that the magnetic field is nonuniform and varies in such-a manner that itis stroneer below a horizontal line and weakerabove the line (Figure 5b). The radius of curvature of theparticle's path is then smaller below the line than it is above,as shown. The net resultis that the charged particle drifts to theright, across thefieldlines, and may reachthe walls of the containing vessel. The particle drift in a nonuniform field is ina direction perpendicular both to the magnetic field (out ofthe page) and to the direction in which the field strengthvaries (stop to

26 coril. t2f bottom). Since particles with differentelectric charges spiral inoppositedirections,thepositivelychargedparticles (nuclei) drift in one direction whilethe negative particles (electrons) drift in the oppositedirection. Thus, in the case represented in Figure 5b, the nuclei move tothe right and the electrons to the left. The separationof positive and negative charges generates a local electricalfield, and now the particles are subjected toboth electrical and magnetic forces.The combined action causes a motion of ionsand electrons, that is,the plasma as a whole, in thedirection of decreasing magnetic field. This drift can result inthe plasma moving to the walls of the containing vessel.Specially shaped magnetic fields are commonly used to eliminate orreduce plasma drift in nonuniform fields. As seen above, the tendencyfor positively and negatively charged particles to drift in oppositedirections results in the development of an electric field thatthen causes motion of the plasma as a whole. The samesituation is produced in either uniform or nonuniform magneticfields if an electric held is deliberately applied in adirection perpendicular to the magnetic field lines. The chargedparticles travel in paths similar to those shown in Figure 5b,except that both positive and negative particles now movein the same direction. Consequently, the plasma as a wholetends to drift across the magnetic field with no tendencyfor charge separation to occur.

Open-ended and Closed Confinement

Magneticconfinement systems fall into two general categories, depending upon whether thevessel, chamber, or tube containingtheplasmaisopen ended or closed.In an open-ended arrangement,like a cylinder, the magnetic lines of force run parallel to the length (axis)of the cylinder, as in Figure 3b. Escape of plasma to the wallsis hindered as a result of the difficulty experiencedby the charged particles in crossing the field lines. Escapethrough the open ends

27 I-712 -c--1"-TT. MAGNETIC MIRRORS

DISTANCE ALONG CYLINDER Figure 6. Ina magnetic mirror system, the the ends of magnetic field at a cylinder containing a plasmais stronger than in the central region.Electrically charged particles the mirror regions approaching (stronger magneticfield) incertain directions are turned back reflected) and soare prevented from escapingat the ends of the cylinder.

along the lines of force can be almost entirely preventedby having the magnetic field much stronger at theends than at the center of the cylinder (Figure 6). Suchan arrangement is called a magnetic mirror because, undersuitable conditions, the charged particles moving toward the end,where the field strength is highest, will be reflected back towardthe region of lower field strength. In fact, the particlesmay be reflected back and forth, from one mirror to the other,many times before they ultimately escape.* The magneticmirrors thus serve as partial "stoppers" for the open-endedcontaining vessel. Closed magnetic confinementsystems are generally in the form of a torus, that is, a doughnut-shaped h011owchamber.

*The Van Allen radiationbelt of charged particles surrounding its existence to the mirror effect the earth owes of earth's magnetic field; thisis stronger near the poles, where reflectionoccurs, than near the equator.

28 tit)c,r) The magnetic field lines are closedand charged particles cannot escape by traveline,along the field lines, but the plasma can drift to the walls. especiallyif the magnetic field is nonuniform. Suitable step,must then be taken to decrease this effect. The magnets. fields for confiningplasmas are generated by electric currents. Two cases are of interestin connection with the confinement of plasmas. For purposesof illustration, we will ;.onsider atoroidal containment chamber, bearingin mind that the conclusions reached are equallyapplicable to a plasma in an open-ended vessel. In Figure 7a, the electric current ispassed through rings surrounding the torus; these may be separate orthey may be connected in series to form a coil (orsolenoid). The current flows in the same direction in all therings, or the turns, of a coil. The magnetic fieldthus produced is described as a toroidal field. itis also referred to as a longitudinalfield or, especially in a straight tube, as an axialfield. In Figure 7b, the current flows thrOugh the plasma orin a metal ring- suspended within the torus; the resultingmagnetic field is called a potoida/ field or sometimes ana:inithal field. These two types of fieldstoroidaland poloidal play important roles in the confinement of plasmas.

Plasma Diffusion Suppose a plasma could be confined in acylindrical tube by a uniform magnetic field in thedirection parallel to the tube axis, as in Figure 3b. In the absenceof collisions, the electrically charged particles wouldsimply spiral along the field lines without crossing them. As notedearlier, however, because of the variations in thecharacteristics of the spiral motions, collisions inevitably occur. If twoions collide, fusion may result, but collisions ofelectrons with ions may cause the plasma todiffuse across the lines of force and escape from confinement.

29 The simplest type of plasma diffusion,resulting from particle collisions, is calledclassical diffusion. Loss of from a confining plasma magnetic field as a result ofthis diffusion cannot be avoided, but calculations show that theeffect is not serious. Moreover, by increasingthe magnetic field strength, the spiral paths of the charged particlesaround the field lines become tighter (i.e., they havea smaller radius of curvature) and collisions are less frequent. Itis expected, therefore, thatin an operatingnuclear fusion reactor, in which a strong magnetic field is used to confinethe plasma, losses due to classical diffusionwill not be important.

ELECTRIC ELECTRIC CURRENT CURRENT ELECTRIC CURRENT

MAGNETIC FIELD

TOROIDAL POLOIDAL MAGNETIC FIELD MAGNETIC FIELD la)

Figure 7. An electric current is always associatedwith a magnetic field perpendicular to the direction ofcurrent flow. A toroidal magnetic field is shown in (a)and a poloidal magnetic field is shown in (b). Similar magneticfields can be produced in a linear (cylindrical) tubein an analogous manner: the field corresponding to (a) is then calledan axial magnetic field.

30 t...!..0 Theoretical studies indicate, however, thatclassical diffu- sionismodified (and increased) in a system with closed magnetic field lines (e.g., in a torus). The curvaturein the lines of force causes an increase in thediffusion rat,In addition, the schemes used to minimize plasmadrift arising from nonuniformity in the confining magneticfield (page 25 et seq.)can also affect the diffusion rat,These schemes generallyinvolve a combination of toroidal andpoloidal magnetic fields that lead to local variationsin the net field strength. In some circumstances, such variations canincrease the plasma diffusion rate by permitting thecharged particles to move across the field lines. By takingthese arguments into consideration, calculations have been made of whatis called neoclassicaldiffitsion.Althoughthediffusionratesare substantially higher than for classical diffusion,they are still not considered to be high enough to preventthe operation of a useful fusion reactor. Another ty pe of plasma diffusion at onetime presented a serious threat to magnetic confinement forcontrolled fusion. Itis actually a form of plasma instabilityand it will be described shortly.

Plasma Instabilities A plasma in a magnetic field has a tendencyto be unstable; as a result, it can break upand escape from confinement by the field. Plasma instabilities are duebasically to the presence of electrically charged particles; theelectric and magnetic fields produced by their motion cause the particles toact in a collective(or cooperative) manner. An exampleof such collective actionisthe drift of a plasma in a nonuniform magnetic field, described above. Similar collectiveeffects give riseto plasma instabilities. Theseinstabilities fall into two broad categories called grosshi dromagnetic instabilitiesand more localizedmieroinstabilities. Suppose a small displacement of a plasma occurs in a magnetic field, if the system reacts in such a way as torestore

1.-'1. V. 31 the original condition, then itis stable. in the case ofa hydronmnetic (or gross) instability,however, the plasma does not recover, but the displacementincreases rapidly in magnitude. The whole plasmamay then break up and escape even from a strong magnetic field. Microinstabilities, as thename implies, are on a small scale compared with the dimensions agile plasma.As a rule, these instabilities do not lead to completeloss of confinement, but rather to an increase in the rateat which the plasma diffuses out of the magnetic field. Nlicroinstabilitiesapparently result from the interaction of electricallycharged particles with electromagnetic waves, similarto radio waves, in the plasma. They are, therefore, sometimescalled ware-particle instabilities, Under certain conditions, theenergies of the charged particles can be repeatedly addedto the waves so that they grow in amplitude. As a result, a high-frequencyturbulence can develop in the plasma that makes itsescape From the confining field easier. One of the most seriousconsequences of turbulence. especiallyintoroidal(closed) systems.iscalled Bohni diffucion.* Little attentionwas paid to Bohm diffusion until the earlyI960s when its importance in controlledfusion studies became very apparent. Therate of Bohm diffusion is so high that, unless it could be controlled, the realizationof a practicalfusion reactor with magneticconfinement was considered to be almost impossible. The ability to understand andovercome plasma instabilities is vitalto the success of the controlled Fusionresearch program. Consequently, the highly complex problemsin- volved have been the subject ofextensive exiwrimentaLand. theoretical studies. As a result of thestep-by-step interplay between theory and experimentsover a period of years, considerable progress has been made.

*So called because it was first mentionedin a report, published in 1949, by D. Bohm and others of observations madeduring World War II on a weakly ionized arc plasma in a magnetic field.

32 L C Magnetic field configurations have been devised inwhich hydromagnetic (gross) instabilities are no longer a serious problem. In *stems using toroidal magnetic fields,microin- stabilitieshavenot beensignificant,exceptfor Bohm diffusion in some cases. Fortunately, there are operating conditions for which this phenomenon is largely suppressed and the observed diffusion rates are in fair agreementwith the classical or neoclassical requirements. Severaldifferentkinds of microinstabilitieshave been identified in straight, open-ended (mirror) systems, butthey have been controlled. Other types havebeen predicted theoretically but not yet detected; should they occur, means have been proposed for dealine with them.Methods are also known for suppressing hydromagneticinstabilities in magnetic mirror confinement and thesewill be described in due course. In mirror systems, lossof plasma as a result of instabilities is now a much less serious problemthan escape out of the ends of the confining tube(page 63). Studiesof high-temperature plasmas duringthepast 20 years or so have been full of surprises, mostof them unpleasant. However, thereis now a general feeling of optimism among scientiststhatplasmainstabilitiesare reasonably well understood.It appears that they can be controlled, at least to the extent thatthey will not interfere withtherealization of a fusion reactor. Indeed, some instabilities are now being utilized to heatplasmas.

Factors Affecting Confinement Times The timefor which a plasma can be confined by a particular magnetic field arrangement is one ofthe factors in the Lawson criterion. It is desirable for theconfinement time to be as long as possible and this meanskeeping the various types of plasma diffusion to aminimum. We are assuming, of course, that hydromagneticinstabilities have been controlled. Diffusion rates and hence confinement times canbe changed

33 by varying such quantitiesas magnetic field strength, plasma temperature, and the dimensions ofthe containing chamber. When Bohm diffusion does occur, it increases withtem- perature, thus leading to shorterconfinement times at higher temperatures. It now appears, however,that Bohm diffusion can be avoided if the charged-particlemean free path (i.e., the average distancea particle travels between successive collisions) is long in comparisonwith the dimensions of the containing vessel. Hightemperatures and low densities leadto an increase in mean free path and thusto a decrease in Bohm diffusion.

Under conditions that resultin mean free paths thatare long in comparison with theplasma dimensions, neoclassical diffusion predominates and,fortunately, the confinement timeincreases markedly as thetemperature israised

102

102 103 104 .105 ELECTRON TEMPERATURE ( °K)

Figure 8. Plasma confinementtime as a Auction of electron temperature; the confinement timeincreases with plasma temperature. The broken line shows thevariation expected for classical (or neoclassical)diffusion.

34 Figure 9. The major radius of a torus isR and the minor radius is r: the aspect ratio is Rjr.

(Figure 8). Increase in magnetic field strengthincreases the plasma confinement time for all typesof diffusion and the relativeincreaseisgreater forclassical and neoclassical diffusion than for Bohm diffusion.These facts explain why lossesbydiffusionacrossthemagneticfieldlines are expected to be of minor significancein a fusion reactor in which a strong magnetic field would beused to confine a plasma at a very high temperature. The geometry of a torus is determinedby its major radius, R. and its ininor mints, r (Figure9); the ratio of the major radius to the minor radius (i.e.,R/r) is called the aspect ratio of the torus. For a given aspectratio, the confinement time with respect to diffusion increases as the squareof the minor radius: that is 'ay, if the minor radius isincreased by a factor of 2. the confinement time isincreased (2)2 = 4 fold. This is a useful method for increasingconfinement times. For neoclassical diffusion, the confinementtime can also be increased by decreasing the aspect ratioof the torus. In a fusion reactor, the toroidal containmentchamber may thus resemble a large doughnut with amoderately small central hole. Experiments are under way todetermine if some new type of plasma instability(or diffusion loss) occurs in a reaction chamber of this shape.

35 In a magnetic mirrorsystem, high temperatures andstrong magnetic fields are helpful in increasingthe confinement time for diffusion across the fieldlines, but no advantage isto be gained by increasing the radiusof the cylindrical containing vessel. Confinement times thendepend on classical diffusion* andon theextentto which microinstabilitiescan be controlled, assuming thathydromagnetic instabilitiesare essentiallyabsent. Experimentalmeasurements have con- firmed the theoretical predictionthat microinstabilitiescan be decreased by reducingthe " .ance between the magnetic mirrors that serve as partialstoppers of the open-ended tube,

EXPERIMENTS WITH PLASMAS

Plasma Formation

Once a thermonuclear fusionreactor begins operation, the temperatrre should be high enoughto ionize the injected deuteriumtritium or deuteriumgas and to convert it into a plasma. In starting up, !.owever,and also in controlled fusion research,itis necessary to generatea plasma by special methods, The simplestway of forming a plasma is topass a high - voltage electrical .discharge through agas; this is done, for example, in fluorescentand neon lamps. High-frequency alternating electric fields, suchas arc employed in radio communications, are particularly usefulfor producing partial ionization (or " breakdown") ofthe gas, The weak plasmaso formed is an electricalconductor, and further ionizationcan be readilyachieved by means ofa direct current or a low-frequency alternatingcurrent. If a high voltage is to be applied to the gas in anyevent, as is sometimes thecase, the breakdown stage may not benecessary. The discharge method isused when the plasmacan be formed directly in the containingchamber. In some experi-

*Neoclassical diffusion occurs only ina toroidal system (see page 31).

36 If 4 mental systems employed incontrolled fusion research, plasma or high-energy particles areproduced outside and various devices are used forinjection into the chamber. Among such devices are ion sourcesand plasma guns. In general, a plasma is produced by anelectrical discharge, and fieldor by a itisthenaccelerated by anelectrical combination of electric and magneticfields into the experimental vessel.

Plasma Heating Several methods have been proposedfor attaining the very high temperatures required fornuclear fusion reactions. For temperatures up to a few milliondegrees K, a deuterium tritium or deuterium plasma canbe heated by passing an electric current through it. Theprocedure is called ohmic heating (or resistance heating)because it depends on the resistance (in ohms) of themedium carrying the current. The principle is the same as in an ordinaryelectric tiglit bulb or an electric heater. To heat a plasmain this manner, the current is generally induced fromoutside, to avoid the need for inserting electrodes into the gas.As the temperature increases, the resistanceof the plasma decreases andeventually becomes too low for resistanceheating to be useful. A number of techniques havebeen developed for heating plasmas to higher temperatures:One method is based on compression of the plasma by amagnetic field. It is well known that a gas can be heatedif it is compressed with a pump or in any otherconvenient manner. Similarly, aplasma is heated ifitis compressed by suddenlyincreasing the strength of the confining magneticfield. As seen earlier, a magnetic field exerts a pressure on aplasma of charged particles. By increasing the strengthof the field, the confined plasma can be compressedand heated.11 two or more successive stages of magneticcompression are employed or if the plasma is preheatedprior to compression, veryhigh temperatures can be attained.

37 Another procedure is based on the injectionof high-energy neutral atoms into a magnetic field. Neutralatoms cannot be accelerated to high velocities (or highenergies) directly and so they are produced inan indirect manner. First, ions* with energies deuterium in the kilo-electronvolt (I kilo-electron volt = 1000 electron volts) rangeare generated inan ion source, and the resulting high-energy D+ ionspass into a chamber containing neutral deuteriumgas. Here a charge exchange occurs; the high-energy ions transfertheir charge to low-energy neutral deuterium atoms (W),that is,

D4 (high energy)D° (lowenergy)

D° (highenergy) + D+ (lowenergy).

The result is theformation of a stream of high-energyneutral deuterium atoms thatenter the magnetic region already field region. If this contains somecharged particles,such as a weak plasma, theywill collide withthe neutral ionize them. Hence, atoms and a high-energy plasma willbe formed and trapped within themagnetic field. If a strong turbulenceoccurs in a plasma, the tend to become plasma will unstable andescape from the magnetic There are indications field. that turbulencenot severe enough cause significant instability to can be induced ina plasma by applying an electricfield for a very short is time. After the field removed, theturbulence dies outand the energy transferred to the is plasma, whichmay thus be heatedto high temperatures. A process called magnetic pumping hasbeen proposed for heating plasmas.A toroidal chamber is surroundedat one or more regions by magnetic coils whereby thelocal magnetic

*Although the earliest fusion reactors willprobably use both deuterium tritium, researcheson controlled nuclear fusion and are performed with hydrogen, deuterium, or heliumin order to avoid tritium, handling radioactive (andexpensive)

38 field strengthis continuously increased anddecreased in rapid succession. When the fieldis increased, the plasma is compressed and heated, but, duringthe subsequent decrease infield strength, the plasma expandsand is cooled. If the frequency of the alternations is chosencorrectly, the heating during the compression phaseexceeds the cooling in the expansion phase. The net resultis that the plasma is heated. An approach somewhat related tomagnetic pumping is the ion cyclotron methodof plasma heating. By adjustingthe that it is alternatingfrequencyof the local magnetic field so slightly lower than the frequencyat which the ions spiral about the confining magneticfieldlines, a wave motion develops in the plasma. Thedamping of these ion cyclotron waves results in theconversion of their energy intoheat. In electron cyclotron heating,electron cyclotron waves are produced in a similar manner.The plasma is then heated by the damping of these waves.Plasma heating is also being studied with waves at other(lower) frequencies. In addition, the generationand heating of plasmas by means of laser lightbeams, intense beams ofhigh-energy electrons, microwave radiation,and shock waves are receiving increasing attention. A differentapplication of laser heating in controlled fusion is described on page71.

Plasma Diagnostic Techniques In studying the behaviorof magnetically confinedand heated plasmas, it is necessary todetermine such properties electron as temperature, pressure,electron and ion densities, current andionenergies,magneticfielddistribution, strength. and extent ofthermonuclear reaction. Theexperi- mental procedures used toobtain such information are referred to as "diagnostictechniques". The nature of plasmas required to is such that considerableskill and ingenuity are of the apply these techniquesand to interpret the results measurements made. Someplasma properties that arebeing in the studied and the methodsemployed are summarized

39 accompanying table. (See Photo 3.) Sinceneutrons are produced in fusion reactions, the numberand energy of these particles liberated provide some indicationof the extent of such reactions. Detailed explanationof these procedures beyond the scope of is this booklet, but thelisting indicates the complexity of plasma research and the varietyof equipment and techniques inuse.

PLASMA DIAGNOSTICTECHNIQUES Quantity measured Diagnostic method

Plasma current Current shunts; current transformers (Rogovsky coils) Plasma pressure Magnetic probes; piezoelectric distribution crystals Electron density Microwave interferometer; Ion density Thomson scattering Optical spectroscopy (Starkeffect); Langmuir probes Plasma density Laser interferometer Electron energy Microwave emission; X-rayemission; and temperature Langmuir probes; Thomsonscattering; optical spectroscopy Ion energy and Optical spectroscopy (Dopplereffect); temperature mass spectroscopy Electric and Magnetic probes; Ilallcutrent probes; optical magnetic fields spectroscopy (Zeeman effect); Neutron emission thallium ion beams Boron counters; scintillationdetectors; silver or indium foil activation

HIGH - TEMPERATURE PLASMASTUDIES

Understanding PlasmaBehavior With the factsalready presented as a background, itis possible to outline some of the more importantexperimental researches on plasmas, as related to controlledfusion, now being conducted. Because the behaviorof magnetically confined plasmas at high temperatures isso complex, several different lines of investigation arc beingfollowed. Basically, the approaches differ in the arrangement(or geometry) of

40 I ' a

Photo 3. Ruby laser interference patterns(interPrograms) are used to determineparticle densities in a plasma.The upper photograph showsthe pattern obtained in the absence of a plasma. the /alto' oneis the interferogram obtainedwith corresponds to a a plasma. Eachcircle in the central region specific particle density.

41 the magnetic field for confining thehot plasma. The of this many-pronged purpose attack is to study theproperties of plasmas undera wide variety of confinement Only by understanding conditions. these properties willit be possible to develop a reactor for producing usefulpower from controlled nucleat fusion. At pt ?,sent, four general classes ofmagnetic confinementof high-temperature plasmasare receiving major attention scientists in many by countries. Theyare (a) pinch systems, (b) the stellarator, (c) thetokamak, and (d) magnetic the required mirrors. If conditions of plasmatemperature, density, and confinement timecan be realized inany one of these systems, scaling up toa practical fusion reactor to be possible. In would seem addition, studiesare being made with devices that do not directly form thebasis of reactors butare intended for investigatingplasma behavior, related to stability. especially as

Low-Beta Pinch In the phenomenon known as the pincheffect, an electric current flowing through aplasma producesa poloidal magnetic field, as in Figure 7b, which confinesthe plasma. In a toroidal chamber, the current is induced in theplasma from outside in the general manner shown in Figure 10.The torus containing the plasmapasses through an iron yoke the core of that forms a transformer. The primarycircuit is wound around the yoke wherea; the plasma actsas the secondary circuit. If a rapidly increasing (or varying)current is passed through the primary,a corresponding current is the plasma, thus induced in generating the magneticfield that both confines and compresses (i.e., pinches)the plasma. An early stage of pinch formation is represented inFigure 11. When the pinch effect was firstproposed for plasma confinement, therewere hopes that the plasma heated. Part of the would also be heating would beresistance heating,

42 i' 4

TORUS PRIMARY COIL CURRENT INDUCED IN SECONDARY IRON YOKE OF TRANSFORMER

Figure 10. A varying current passed throughthe primary coil induces a corresponding current in theplasma (contained in the torus) acting as. the secondary ofthe transformer. In some cases, two identicalyokes with primary coils may be used (see Figures 13 and 15). caused by the flow of electric current inthe plasma, and part would result from compression by the strongpoloidal field. Although such heating did occur, it soonbecame apparent that the pinched plasma was highlyunstable and did not persist for more than a few millionthsof a second (Photo 4). This time was too short for theproduction of a significant

Figure 11. Representation of the pincheffect in a toroidal tube. The darker ring is the plasma inwhich an electric currentisinduced; thecircles surrounding the plasma indicate the poloidal magnetic field producedby the current.

43 Photo 4. The bright horizontal strip in thecenter of the upper picture is a portion of a pinched plasmawhile it is stable. A few millionths of a second later itbreaks up, owing to the development ofinstabilities; asseen in the lower picture.

amount of fusion energy, even if the temperaturehad been high enough. Theoretical studiesindicated that it mightbe possible to overcome hydromagnetic instabilities of the pinchedplasma by trapping within it a toroidal magnetic field.The basic thought was that the field lines runningaround the torus (sec Figure 7a) would act as a sort of stiffener,so that it would be more difficult for the plasma to break up. Anotherstabilizing device is the inclusion ofa metal shell either withinor outside the toroidal chamber. These proceduresundoubtedly increased the plasmastability to some extentbut did not eliminate instabilitiesentirely. Furthermore,it was found that the stabilizing magnetic field includedin the plasma opposed the pinch action and thus limited thecompression.

44 The pinch effect just describedis called a 2 pinch, whereZ stands for the longitudinaldirection around the torus. Itis also referred to as alow-beta pinch because theplasma particle pressure is small incomparison with the magnetic field pressure, (i.e., beta issmall). At one time, studiesof low-beta Z pinches played animportant role in controlled activityhas fusionresearch. But inrecentyears,the diminished and is now largelyconcerned with the 'effort to understand and overcome instabilitiesin low-beta plasmas.

High-Beta Pinch high-beta pinch, a wide Inthesimplest form of the containing a plasma at single-turncoil surrounds a tube ordinarytemperature (Figure 12a).By using a bank of is suddenly capacitors(see Photo 5),a powerful current switched into the coil; thisgenerates within the tube a to sharply increasing magneticfield in the direction parallel the axis. The situation issimilar to that in Figure 7a, except that a single coil around astraight tube is used insteadof a series of coils around a torus. The surface of the plasmaforms a cylindrical sheaththat is driven rapidly inward bythe fast-rising magneticfield as indicated in Figure I 2a. Theplasma is consequently heated followed by by a shock originatingfrom the moving sheath compression (or pinching) whenthe magnetic field increases more slowly. Asthe field reaches itsmaximum strength, held there is a relatively quiescentphase in which the plasma is in a cigar-like shape as depictedin Figure 12b. However,in a short time, the plasma is lostby escape from the ends as shown by the arrow. The current that generatesthe magnetic field runsaround the containing tube inwhat is called the theta(Greek, 0) direction. The effect observedis thus referred to as atheta patch. Since the plasma isstrongly compressed, its pressureis high and so also is the betavalue. This is why thefast magnetic-compression phenomenonjust described is termed

45 SINGLE-TURN COIL --0- CURRENT

ESCAPE OF PLASMA PLASMA SHEATH MOVING INWARD COMPRESSED PLASMA (a) (b) Figure 12. Developmentof a theta pinch: (a) phase, and (b) shock-heating quiescent compressionphase. the high-beta theta pinch. As will beseen shortly, high-beta Z pinches are alsopossible. Experiments on linear (open-ended) theta pincheshave led to the production ofdeuterium plasmas with the vicinity of temperatures in 50,000,000°K anddensities up to 5 X 1016 about particles per cubiccentimeter. Nuclear reactions have been fusion observed under theseconditions. How- ever,because of the rapid escape of theplasma, the confinement timeshave beenvery low, generally millionths of a few a second, so that themaximum Lawsonnr product has been roughly2.5 x 101 1, compared with at least 10'4 required for adeuteriumtritium system. The confinement time of the plasmain a linear theta pinch is limited by the escape of plasma from theends. One possible way whereby this escape can beprevented is to bend the tube into a circle, i.e., a torus,so that the two ends close

Photo 5. Some thermonuclearresearchrequireslarge amounts of electricity in short pulses. Thephoto shows the capacitor bank forScyllac, a toroidal the Los Alamos theta-pinch device,at Scientific Laboratory.The full toroidal Scyllac experiment will have 3240primary bank capacitors, 15,000,000 joules of stored energy, 60,000volts maximum bank voltage, and 130,000,000amperes maximum current. 46 r

on themselves. Although there will be no ends from which the plasma can leak away, losses to the walls will be possible and methods are being developed to minimize them. Experi- ments are under way to test the feasibility of establishing a fairly stable toroidal theta pinch (see page 77). It is expected that ultimately the magnetic fields will be programmed so as to provide distinct shock- heating and compression phases. The linear theta pinch is remarkably free from aross plasma instabilities that would drive the plasma as a whole to the walls of the tube. Thisisindicatedclearly by the laser interferencepatternsin Photo 6.taken at various times during the quiescent phase. The number of interference rings produced by the plasma decreases, indicating a decrease in density due to losses, but there is no appreciable change in location or circular cross section of the plasma. Furthermore, other observations show that,itleast at temperatures of about I.000.000°K, the loss of plasma by diffusion across the magnetic fields lines is approximately classical and is very much less than would result from Bohm diffusion. A similar situationis expected to exist atthe higher temperatures required for a nuclear fusion reactor. Although the linear, high-beta theta pinch appears to have little or no hydromagnetic instability, theory suggests that

2.4 ilsec 3.6 Photo 6.Laser intcriCrograms of a theta pinch at successive times (inmillionths of a second) after formation. The decrease in the number of intofrifence rings indicates the

48 such instability mayoccur in a toroidal theta pinch. Various methods are being considered whereby thistype of instability can be controlled when atoroidal chamber is used to eliminate escape of plasma from the ends. In view of the success of the'high-beta theta pinch, there has been a revival of interest in the Z pinch,using the same fastmagnetic-compressionconcept toobtainhigh-beta plasmas. Experiments were first made bypassing an electric current through a straight tube in sucha manner that a rapidly increasing poloidal magnetic fieldwas produced. The results have been so promising thata toroidal, high-beta Z pinch system has been built. fly dromagneticinstabilities are expected, just as in the theta pinch, but it ishoped that they can be overcome.

Stellara tor Systems Itwould seem, atfirstthought, that a simple way of confining a plasma in a torus would be bymeans of a toroidal field obtained in the manner shown in Figure7a. Apart from losses by diffusion across' die fieldlines, the electrically chargedparticles might be expectedtospiral endlessly around the lines of the magnetic fiekl.Unfortunately, this is

4.9 6.1 gradual disappearance of the plasma but itsluLation remains unchanged,

49 not the case. Because the innercircumference of the torus is shorter than the outer circumference,the coils carrying the electric current must be closer on theinside than on the outside. Consequently, tht, magneticfield is strongest near the inner circumference and itbecomes progressively weaker fieldis inthe outward direction.In other words, the of non:in:torn] (inhomogeneous) overthe minor cross section the torus. In view of the argumentsdeveloped earlier,itis evident that the plasma as awhole will drift toward the outer wall, that is, in the directionof the weaker magnetic field. Confinement is thus not possible. of any In a ring-shaped tube or,in fact, in a closed tube shape, such as an oval or racetrack, that lies in one plane, each line of force closes uponitself as it is followed around the tube. In all such tubes,the magnetic field is inhomoge- shown, neous. and plasmadrift must take place. It has been field is distorted or twistedin ,however, that if the magnetic close upon sucha way thatthe lines of force do not themselves after making. onecomplete circuit, the plasma drift will be ereatly reduced or eveneliminated. This is the basic principle of the stellaratorsystem. In the earlier models ofthe stellarator, the required result of a figure was achieved bytwisting: the tube into the shape eight, somewhat like a pretzel, sothat it was no longer in one plane. Later.it was realized that the sameresult could be of achievedina closedplanar tube by using two sets magnetic field coils. One set.called theconfining field coils (Figure 13).is of the simple type forproducing a toroidal field (compare Figure 7a). Theother, indicated as the heliad (stalnig) windings,with currentflowinginopposite directions in alternate turns.provides the required twist. these minium also pros ide adegree of stabilization against hydromunetic instability. For conveniencein adding various pieces of equipment.stellarators have generally been constructed in the form of a race track,but this is not essential. In fact. circular (toroidal)chambers may be less subject to diffusion losses.

50 In the operation of a stellarator, formation and heating of the plasma sire generally accomplished in three stages. First, a radio-frequency discharge is used to preionize the deuterium .(or other) gas in the tube. The resulting weakly ionized plasmais confined by the magnetic fields, and a toroidal currentisthen induced from outside by means of a transformer with an iron y oke (see Figure 10). The ionization of the plasma is thereby increased, and its temperature is raisedby resistanceheating to about 1,000,000°K. For reasons thatwill be apparent shortly (page 56), this approaches the maximum possiblefor resistanceheating. Ilence, subsequent increase of temperature must be attained m other ways, such as ion cyclotron heating ormagnetic pumping.

RESISTANCE HEATING TRANSFORMER '- HELICAL (STABILIZING) WINDINGS

CONTAINING TUBE Figure 13. Repie.sentation of a racetrack (planar) stellarator. (Thc c,mjining field cools and the helical windings go around the entire tube. but parts are vim tied in the diagram for sunpliciti.) The field coilsproduce the toroidal magnetic Jiehl. and the helical windings,with current passing in uppo.site directions in adjacent turns. provide the twist for stabili=ing the plasma.

51 In stellarators and similardevices in which the magnetic field lines are twisted in the manner indicated above,there is an upper limit to the toroidalcurrent that can be passed through the plasma. Thislimiting current is knownas the Kruskal-Shafranor limit.Ate' M. 1). Kruskal ofthe United States and V. D. Shafrano%of the U.S.S.R. whopredicted it independently. If this limit is exceeded, the plasmadevelops a hydromagnetic instability called thekink instability(see Figure l4). Suppose thata small kink (actuallya helical distortion) develops in the plasma, as indicated in Figure14. The lines of force of the encircling magneticfield are closer together on the inside (bottom) than on the outside(top) of the kink. The field strength is thus greateron the inside, and, as a result of the difference inlick! strength, the kinkis distended even more. This continues until thepinched plasma is so badly distorted that it touches the wallsof the vessel, as in Photo 4. or breaksup entirely. There have been limn},severe losses arising from Bohm diffusion in much of the stellarator work. Atone time it was thought that this behavior might becharacteristic of all stellarator systems. butsuch is not thecase. By increasing the plasma temperature and by taking care in theconstruction of the toroidal magnetic field coils to avoidnonuniform regions, confinement times in stellarators have approachedthe values

Figure 14.The kink instability in a plasma, (The plasmais actually in the form of a spiral whichis shown here ina two-dimensional representation.)

52 expected from neoclassical diffusion, Since thereis essentially no compression of the plasma, stellarators are low-beta devices.

Tokamak Systems

Sinceabout1969, when results of considerable consequence for controlledfusion were reported from the U.S.S.R., there has been widespread interest in the tokamak concept. The tokamak is a low-beta toroidal system with features of both the stabilized Z pinch and the stellarator. The plasma is contained in a metal-walled torus and toroidal and poloidalfieldsare applied, the former by coils surrounding the torus and the latter by inducing a current that flows around the torus (Figure 15). This is similar to the stabilized low-beta pinch system (page 42), the difference lies in the relative strengths of the two magnetic fields. In the tokamak, thetoroidalfieldis much stronger thanthe poloidal field (about 10 to 1), whereas in the Z pinch the reverse is true (less than 1 toI ). The tokamak is similar to the stellarator in that the magnetic field is twisted so that the lines do notclose on themselves after a single circuit. However, inthe tokamak the twistis produced by the combination of toroidal and poloidal fields and can be varied, whereas in the stellarator it is achieved by means of the fixed helical windings. Byvaryingthetoroidal and poloidal magnetic fields independently, a region of exceptiolial plasma stability was discovered in the tokamak...A low-beta plasma, with a density approaching 5 X 1013particles per cubic centimeter, was confined for about one-fiftieth (0.02) of a second. The Lawson 11T product was thus almost 1012, this was achieved in1972 andisthe largestvalue attainedsofarina high-temperature plasma. The ion temperature, which determines the fusion rate, was about 5,000,000°, although the electron temperature was higher. There is no evidence of 13olim diffusion and the confinement times correspond to FIELD COILS (FOR TOROIDAL FIELD)

PRIMARY COIL

INDUCED PLASMA CURRENT RESULTANT (SPIRAL) (PRODUCES POLOIDAL FIELD) MAGNETIC FIELD

Figure 15.Representation of a common type of tokamak. The current in the field coilsgenerates a toroidal magnetic field (not shomt): the plasmacurrent induced by current in the primary coils producesa poluidal magnetic field (not shown) These two fields combineto form a spiral magnetic field, as shown, which stabili:esthe plasma.(In some tokamaks, the plasmcurrentis induced by current in primary coils running around themajor circumference of the torus. i.e., perpendicular to the toroidal field coils.)

neoclassical (o classical)diffusion.Since1969, various experimental tokamak devices have been builtin several countries, including the United States. (See Photo7.) The ion and electron temperatures mentionedabove result from resistance heating by the toroidal current induced, in the plasma. In this type of heating, theenergy is first absorbed by theelectrons andisthen shared with the ions by collisions: this explains why the electrontemperature is greater than the ion temperature. The temperaturesattained in tokamak experiments have been higherthan expected from calculated values of the plasmaresistance. This anoma- lous (larger) resistanceis attributed to minor turbulence, perhaps due to microinstabilitie, in theplasma.

54 As in the stellarator, thetoroidal current, which heatsthe plasma and also generates thepoloidal field in the tokamak, must be kept below theKruskal-Shafranov limit in order to avoid hydromagnetic (kink)instability. To achieve maximum resistance heating, the limitshould be as large as possible. According to theory, this canbe achieved by increasing the toroidal magnetic field strengthand decreasing the aspect ratio of the torus (see Figure9). The smallest practical aspect ratiois about 3, but with such afat torus, it would be field; hence, a com- difficultto attain a strongtoroidal promise would be necessary. Although the tokamak concept appearsto be a promising approach tocontrolled nuclear fusion,thereare many problems still to be solved. Theproblem of increasing the

MIL

Photo 7. The Model ST(Symmetric Tokamak) atPrinceton Plasma Physics Laboratory.This device ivas made by recon- structing the Model C stellarator.

55 Lawson nr product at least a hundredfoldis obvious, and there are two others that should beconsidered. First, the plasma temperature that can be reachedby resistance heating is probably 10,000,000° at the very most,partly because of the decrease in the plasma resistanceat high temperatures and partly because of the Kruskal-Shafranovlimit on the current strength. Furtherheating will thus be attain the temperatures necessary to required for a nuclearfusion reactor. Several methods of heating are beingstudied; these include plasma turbulence, magnetic compression,and the injection of high-energyneutral atoms. A second major problem is associatedwith the very low beta values in a tokamak device. At lowbeta, the fusion energy generated per unitvolume of a reactor low. This would also be means that the smaller thebeta value, the would be a reactor larger designed to producea certain amount of power. Furthermore, when beta is low, lossesfrom synchrotronradiation(page 18) becomelarge.Withexisting tokamaks, long confinement times are associatedwith low- beta plasmas, but possibilities are beingstudied for increasing beta withoutsignificant decrease of confinement.Beta should be increased by decreasing theaspect ratio of the torus, but this will mean some sacrifice inthe magnitude of the resistanceheating current. An optimum tokamakdesign can result only from a balance among severalfactors.

Magnetic Mirror Systems -Inits simplest form, the magnetic mirrorarrangement consists of a number of coils woundaround a straight tube with the coils being closer at theends than in the (Figure 16a). If the middle same current is passedthrough each coil, the magnetic field generated is strongerat the ends, leading to the result shownearlier in Figure 6. The lines of forceare closest where thefieldis strongest, as indicatedin Fig- ure lob. The strongermagnetic fields at the slow down and, ends serve to under certain conditions,turn back the

56 FIELD COILS

(a)

(b)

Figure 16.Representation of a simple magneticmirror system: (a) showsthe windings of the field coils.and ( b) indicates the form of the lines offorce. charged particles that mightotherwise escape. As seenearlier, the stronger magnetic fields atthe ends can thus act as mirrors for the reflectionof the charged particles in aplasma. and As a charged particle spiralsalong a magnetic field line enters the mirror regionwhere the field is stronger. aforce begins to act on the particlethat tends to push itback toward the central region.The rate of motion of theparticle parallel to the field lines, or via (seeFigure 4),_is thus reduced, and the separations betweensuccessive turns of the spiral in the path become smaller and"smaller. If the magnetic field mirror region is strongenough, it will first bring theparticle motion along the fieldlines to a stop and then reverseit. In of other words, the Particle isreflected back into the region weaker magnetic field. Such aparticle will spiral back and is forth, after successive reflectionsat the two ends, so that it trapped between the magneticmirrors. Not allthe charged particles in aplasma will undergo reflection in the mirrors. Forexample, a particle with allits velocity parallel to the fieldlines in the region betweenthe

57 mirrors, i.e., with r = rii in Figure 4, willmove straight along thericld lines, without spiraling,and escape through mirror. For the a particle to be reflectedand trapped between themirror fields,it must have an appreciablevelocity component ri perpendicular to the lines of forcein the central region. Particles with smaller values of(a will escape. The greater the strength of the magneticfield in the mirror regions relative to that in between, thesmaller the value of for which reflection !a is possible. Thus,the stronger the mirror fields, for a given strength in the centralregion, the greater the proportion of charged particles thatcan be trapped. Another way of viewing the situation is to thinkof the particles as movingin a magnetic valley (centralregion) between two peaks (mirror regions),as depicted in Figure 6. If a particle has sufficient velocity inthe right directionas it approaches a peak, itwill, in asense, go right over and escape. But if the velocity is sufficient onlyto take it part of the way up the peak, it will fall backto the valley. Particles in this category are trapped. The direction ofapproach to the peaks is clearly important: if itis head-on,escape will be easier than if the particle, with thesame velocity, is moving at an angle. Moreover, the higher the peaks(the mirror fields), with respect to the valley, themore difficult it will be for particles, even those approachingalmost head-on, to escape. Resistance heating in a magnetic mirror systemdoes not seem to be practical: consequently, other plasmaheating techniques must be used. An initial plasmamay be created by electron cyclotronheating or by theuse of microwave radiation. Subsequentheating bymeans of high-energy neutral atoms appears to be apromising approach.In addition, heating has been achieved bymagnetic compression. This can be done by changing themagnetic fields inone or both of two ways. First, by increasing the strengthof the field in the central region, the plasma iscompressed toward the axis of the tube and is thus heated. Themagnetic field in the mirror region should be increased at thesame time to

58 4 t.....i. method, retain the same trapping,characteristics. The second is to bring the which may be combinedwith thefirst. The plasma is magnetic mirror fieldsrapidly closer together. then squeezed into asmaller space and isheated.

In the earlier magneticmirror experimentshydromatmetic has now been solved. instabilitywas a problem.butit stable if Theoretical studies indicatedthat a plasma would be it could be confined by amagnetic field that had aminimum strength in the center andincreased outward in alldirections. Such an :immanent iscalled a magnetic. well or aminimum- employed /3 configuration. since13 is the s mbol commonly field. In 1961 the to represent thestrength of a mag»etic Russian physicist M. S.toile showed that such aconfigura- tion could be achievedinamirror system by placing a "Joffe Bars". number of conductors.generally referred to as parallel to the central axisof the tube containing theplasma adjacent and passing currents inopposite directions through conductors (Figure 17). Sincethe magnetic well concept was virtually introduced. hydromagneticinstabilityhas been eliminated in magnetic mirrordevices. (See Photo 8.)

An ingenious schemefor producing in a simple manner a mirror field that includes amagnetic well is known asthe "baseball"-tor "tennis-ball"-) scamcoil, so named because its winding is shaped like the seamof a*baseball or tennis ball, as of the magnetic seen in Figure 18.The general configuration coil field generated by passing anelectric current though the angles to is also shown. The mirrorfields, which are at right close each other. occur wherethe turns in the coil come which together. The central regionis a magnetic well from Plasma the field strength increasesoutward in all directions. confinement systems havebeen constructed withmagnetic fields produced bybaseball-seam coils. The plasmas were the suppression of the foundto behave as expectedin is hydromaenetic instability.The baseball-seam coil system nuclear fusion reactor. one possibilityof confinement in a (See Photo 9.)

59 COILS FOR MIRRORFIELOS7

IOFFE BARS Figure 17. Representation of four lolfe barsin a magnetic mirror system:currents flow in opposite adjacent bars. The directions in magnetic field., producedby the currents in the mirror field coils and in the lojii! barscombine to form mininnon-B (or a magnetic-well) field inwhich the strength increases from the center outward in alldirections.

Photo 8. Part of the 2X 11 magneticmirror experiment for confining and heating plasmas at theLawrence Livermore Laboratory in California.It includes themagnetic well concept to achieve plasmastability.

60 As a result of painstaking researches, both theoretical and experimental, soeral plasmamicroinstabilities have been identified in mnirroi systems, and methods haNe been developed for controlling them. The procedures include a1oidanee of impuritiesinthe hydrogen- isotopeplasmas, suitable variationsinthe strength of the magnetic field, and a relati ely short distame between the mirrors. As a result of these and other measures. 13ohm diffusion does not occur, and theloss of plasma acrossthe magnetic fieldlines approaches that corresponding to classical diffusion. Plasma temperatures as high as 200,000,000° have been repoited by the injection of energetic neutral atoms into a mirror system. The maximum value of the Lawson product attained, although notinthe same experiment, is about

2 X I OI I.This moderate value is mainly the consequence of the low-beta characteristics of the plasmas employed in most

Figure 18. Magni tic -itcll field prod:tied b,i current in as coil shaped like the scam of a baseball (or tennis ball).Two Jan-shapid mirror fiLlds arc formed at right angles to each other in the regions where parts of the coil come close together. c' being conducted to magnetic mirrorexperiments Research is confined discover a means toincreasethe beta of the promising. plasmas, and the earlyobservations appear to be suppressed, magnetic Evenifallinstabilities could be loss of plasma by mirror systems wouldstill suffer from the thermonuclear plasma, itis escape throughthe mirror. In a inevitable that a fractionof the particles willhave substantial field lines. Such velocity componentsparallel to the magnetic Because of the particles will not betrapped by the mirrors. temperature resulting loss of energy,itis possible that the be higher in required for self - sustainingnuclear fusion may confinement. mirror systems than inother types of magnetic of decreasing Consideration is beinggiven to the possibility mirrors by applyingradio- escape ofplasma through the frequency fields.

Internal-Ring Systems which one or Internal-ring devices aretoroidal systems in suspended either mechani- more conducting(metal) rings arc torus (figure 19). cally or by magneticlevitation within the of the toroidal The ring c:rcumferenceis parallel to that generally contain one orfour rings; chamber. These systems devices. Current maybe thelatter arecalled octopole making them serve as induced in the rinesfrom outside by devices have been secondaries of atransformer. Single-ring that once current constructed withsuperconducting rings, so low temperature is induced, it persists aslong as the required

of the photograph isthe Baseball Pbot9. In the upper part which is the largest magnetof 11 superconducting magnet, Underneath is the this type ever builtfor fusion research. is now sealed atthe vacuum chamberin which the magnet Lawrence LivermoreLaboratory.

63 is maintained (see page 67). Alternatively,as in the D. C. octopole, direct-current leads are connecteddirectly to the rings. (See Photo 10.) Passage of current through the ringsgenerates poloidal fieldsinthe usual manlier. In one-rine devices,windings around the torus also provide a toroidalmagnetic field. Such a field may or may not be included withoctopole arrangements. Plasma is injected into or generatedwithin the torus and it tends to form a sleeve around the ring(or rings). The internal-ring systems are not intendedas prototypes for nuclear fusion reactors. Their main.purpose is to study plasma stability and instabilities. It has beenfound that when the poloidal magnetic field is producedby current flowing a metal ring rather than in the plasma, it isa relatively simple matter to achieve stability. The diffusionrates are thenvery close to thoseexpected from classical considerations. By varying the experimental conditions, therequirements for stability can be determined. Furthermore,by disturbing the magnetic fields invarious ways, the factors that leadto instabilities can beunderstood.

Figure 19. Sections through a torus with(a) one suspended conducting coil and (b) four suspended coils(octopole).

(a) (b)

64 .v

Tr V`, .-.4"17*

.014i1M,

deriee at Gulf Photo 10. Interior ofthe D. C octopole supported me- General Atomic.Four conducting rings are caically Iriti the torus.

NUCLEAR FUSIONREACTORS WITH MAGNETIC CONFINEMENT Toroidal ConfinementSystem thatthe isafeeling mom; plasmaphysicists There of gen- scientific possibility ofnuclear fusion as a means in the early erating useful powerwill probably be established the 1980s. Consequently. somethought is being given to take. The following Cormthatafusionreactor might description indicates thegeneral outlines of asteady-state, which the energy is toroidal magneticconfinement reactor in Sys- produced by the deuteriumtritium nuclear reaction.* of them will tems of other typeshave been studied and two be described in latersections.

that operates continuouslyrather than in pulses. *"1 eatly .tate react a is one

65 We mentionedon page l2 that some 80% of theenergy of the DT reactionis carried by the neutrons.Since these particles are notelectrically charged, they are notconfined by a magneticfield. Consequently, when DT fusionhas occurred, thehigh-energy neutrons will escape fromthe plasma. The heliumnuclei produced in the fusionreaction will remain inthe chamber and their energy willionize and heat the incomingdeuteriumtritium helium gas gas. Eventually, the will be dischargedas a completely material that innocuous may, however, be usedin the operationof the fusion reactor(see below). The toroidal reaction chamberwill be surrounded "blanket" of molten by a lithium, either inelemental form salt (Figure 20) for or as a breeding tritium. Theblanket shell would probably be madeof a refractory metal, suchas niobium, vanadium, or molybdenum, although stainlesssteel is also possibility. The a energetic neutronsenter the blanket interact with the and lithium nuclei toproduce tritium; removed as gas and this is returned to the reactionchamber. At the same time, the neutrons deposit theirenergy in the lithium, which will thus be heated totemperatures up to about 1100°C (2000°F).The lithium blanket will be surroundedby

-- TOROIDAL REACTOR TRITIUM REMOVAL ELECTRICITY ELECTRICITY

POTASSIUM VAPOR STEAM STEAM TURBINE GENERATOR TURBINE WASTE HEAT LIQUID POTASSIUM PLASMA WATER \....MAGNETIC LITHIUM FIELD COILS BLANKET THERMAL AND NEUTRON SHIELD

Figure 20. Section through a torusshowing the basicprici- ples of a possible.fusion reactor.

66 )-`) a thermal insulator,which can also absorb stray neutrons, and finally by the magnetic confinementcoils. The heat deposited in the blanketby the neutrons can be transferred to potassium by passing liquidpotassium through pipes immersed in thelithium. The potassium will boil and the hot vapor will be used to drive aturbine-generator for the production of electricity. The potassium vaporexhaust from the turbinewill be of sufficiently high temperatureto produce steam from water by wayof a water potassium vapor heat exchanger.The steam will be employed in a conventional turbine to generate moreelectricity. Another possibility would be tocirculatea gas, such as helium, through the lithium blanket. The hot gaswould then be used to operate a gas turbine.Instead of circulating the liquid potassium or helium gas through pipeswithin the lithium blanket, the transfer of heat may occuroutside the blanket (see page 66). Maintenance of the strong magneticfields would normally require considerable amountsof electric power. A possible way to decrease the powerrequirements would be to use superconducting magnets.* Whencertain metals and alloys are cooled to extremelylow temperatures, usually less than 10°K (i.e.. 263°C below the freezingpoint of water), they become superconductors. Once aflow of current has been started in a superconductor, itwill continue indefinitely in a is closedcircuit,even after the sourceof the current removed. Magnets constructed withsuperconducting coils have already been used incontrolled fusion (and other) research, although their dimensionsand field strengths are small compared with those for a reactor.It is of interest that liquid helium is commonly used tomaintain the temperature required for superconductivity. Theuse of superconductingmagnets would permit an overall decrease in power requirements,but energy would he needed to operate the refrigerationmachinery for keeping

*See Cryogenics, another booklet in this %cries.

67 the magnets at a low temperature. A fusion reactorsystem, as outlined above, would thushave an extremely hot regionin the interior andan extremely cold regionon the exterior. The problems of thermal insulation would thus be difficultto handle,

Environmental Aspects

By taking advantage of thehigh temperature in thelithium blanket to introducea potassium "topping" cycle preceding the usual steam -water cycle in the operation of theturbine- generators, more than 50f,`4 of theenergy deposited in the lithium by the neutrons could be converted intoelectricity. Thisconversion efficiencymay be compared with the maximum of about 40% forthe most efficient fossil-fuelor nuclear fission power plants.A nuclear fusion reactorwould thus release less heat to theenvironment. During normal operation,tritiumis the only radioactive material that could escape, either into the air or water,from a fusion reactor. Special precautionswould be taken to keep leakage to the lowestpracticable levels. Inany event, the system would be designedto prevent loss of lithiumand potassium. Since most ofthe tritium would bepresent in the lithium, the escape of tritium would be minimizedat the same time. After a period of operation, the niobium (or other)vessel containing the lithium willhave to be replaced becauseof damage that the high-energyneutrons caused. As a result of neutron capture, the discardedniobium will have become radioactive and means will haveto be found for disposing of it safely. After a few years, however, the activitywill have decayed sufficiently for itto be refabricated bymeans of remote-handling techniques. Ifvanadium were used instead of niobium to contain thelithipm, the problems wouldbe greatly decreased since theradioactivity caused byneutrons is quite small.

68 Yyi The most serious hazard from a fusion reactorplant would probably arise from a lire because both lithiumand potassium burn readilyinair. The amounts of deuterium and tritium present in the reaction chamber at anytime ale so small that the energyeleased in an accident would not be important. There t.s «vain no clanger of a thermonuclear explo.ston. The total energy liberated if all componentsof the system were consumed in d firewould be a boat the same as from a large tank of fuel oil. Use of the molten salt,lithium beryllium fluoride (LiBel4 ), in plaLe of elementallithium, as has been suggested, would greatly reduce the fireha/n(1. The main danger associated with a fire would bethe release of tritium, but normal operations wouldbe direLted at keeping the amount of tritium as small as possible.

Direct Conversion Reactor We have seen that, because of the loss of chargedparticles through the ends of magnetic mirror confinement systems, the operating temperatures will have tobe higher than for closedsystems.Advantage may betaken of the high temperature to comeit fusion energydirectly into electricity. At the same time, the energy of escapingparticles would be recovered.IntheI) D reactions,that would occurif deuterium alone were used in a fusion system,almost (7`4 of the energy released is Larded by thecharged particles. In the reaction between deuterium andhelium-3 the amount is 100'; since charged parades are the solereaction products (page 13),althoughinpractical systems there would be neutrons from D D reactions. II plasma temperatures exceeding 1000million degrees K could be attained in a mirror system,both the 1),1) and D 31ie reactions could take place.The latter would be conversion of fusion especiallyfavorablefor the direct energy into electricity. Ilelium-3is extremely rare in nature and thereis no simple way in which itcan be made, as

69 tritium can be. However, helium-3is a product of one of the I)- D reactions and this wouldprovide the helium-3 for the D--3 lle reaction. On the assumption that theconditions for the D'lie (or even the D- D) reaction could be realized ina magnetic mirror system, theenergy of the charged particles could be converted intoelectricity in the followingmanner (Fig- ure 21).The chargedparticles.consisting of positively charged nuclei and negatively chargedelectrons, emerging from the ends of the cylindricalreaction chamber would first be expanded into a broad beamby a magnetic field. The charged particles are guided anddirected outward by the gradually weakening field. Since theelectrons are lighter than the ions, they are more easilydeflected and they would be collectedat an electrode, calledthe "charge separator", which is grounded. The ions would continueon and pass through a series of "decelerating electrodes" threeare shown in Figure 21

ION DECELERATION AND COLLECTION ELECTRODES ELECTRON COLLECTOR x. .------..---- ESCAPING = - DECREASING I PLASMA IMAGNETIC FIELD

,...--- -.. EXPANDER-SEPARATOR **, SECTION vocracq ADJUSTERS

+ D. C. OUTPUT

Figure 21. Outline ofa scheme for the (lirect conversion into electricity of the energy carriedby the charged particles escaping from a magnetic mirror fusionreactor.

70 their positive charges where they wouldbe collected and most easily deposited. The nucleiwith the lowest energies are electrode: those of decelerated and arecollected at the first subsequent electrodes. higher energy wouldbe collected at the deceleratingelectrodes will steadily The voltages at be appropriate multipliersand dividers will decrease and In required to bring themto the samefinal output voltage. will be gen,.!ratt.d:the the system justdescribed direct current pole and the charge separator wouldconstitute the negative decelerating, electrodesthe positive pole. have environmental The direct conversionprocess would it should be possible and other advantages.In the first place, 90% of the charged-particle to convertinto electricity up to much less wasteheat energy producedby fusion. Thus, environment. incidentally, would have to bereleased to the in a decrease in this high conversionefficiency would result criterion. which isbased on a 33%efficiency. the Lawson irr in one of the Furthermore. most ofthe tritium (brim(' and be consumed bythe DT reaction DD reactions would time. little would be presentin the system at any very energies. would be Finally. fewer neutrons.and with lower and tritium nuclei. produced than in thefusion of deuterium structural Materials Hence. damage andradioactivityin considerably less. Anothermaterial, other than would also be neutrons, since the lithium. could beused to absorb the required. There wouldnot breeding of tritiumwould not be materials, but the lire only be a largerchoice of structural hazard could bedecreased.

INERTIAL CONFINEMENT

Heating by Laser Beams inertial confinementis that a pellet of a The basic idea of rapidly to deuteriumtritium mixturewould be heated very Such a system couldproduce thermonuclear temperatures.

71 energy from plasmaswith densities for which much greater thanthose magnetic fieldscould be used When an for confinement. incident pulse ofhigh-intensity laser absorbed by the energy is pellet, a plasmais formed from surface material. the outer The resulting"blow-off" ing pellet causes the remain- mass to implode (i.e.,to move very rapidly The convergingshock inward). pressure compressesand heats the material toa state in which the 1.)---T reactionoccurs rapidly. Thereafter, thetremendous energy explode pressures generated bythe fusion the pellet. Mostof theenergy from the interaction ofdeuterium and tritium nuclei is carriedby the neutrons produced,and this would venient be removed ina con- manner, as will bedescribed shortly, generate electricity, and used to A new deuterium-- tritium pellet would then be introducedand the process repeated andso on. Laser-fusion reactions would occurso rapidly that inertial forces wouldprovide adequate confinement for thereacting nuclei, For suchconfinement to be amount of possible, theconsiderable energy required toheat the solidto temperatures in the vicinityof I 00,000,0000K would have to be (and absorbed)in the extremely delivered short period ofa fraction of a nanosecond, thatis, in less than a billionth part (10-9)of a second. The onlypresently known ways in which thismight be done is bymeans of pulsed laser of high-energy beams* or focusedbeams electrons. ,Themajor activitiesrelated to inertial confinementfor nuclear fusion arepresently concerned with theuse of lasers. Laser beams can be readilycontrolled and very small areas, focused onto thereby permittingefficient irradiation pellets that mightbe I millimeter of (i.e., about 5inch) or less in diameter.Lasers can deliver large amountsof energy in *See Lasers :mother booklet in thisseries,

Photo H. Ashort-pulse energy of up neodymium glasslaser havingan to 200 joules isshown at the Livermore Laboratory. Lawrence

72 bi I a ,II . , . . . . ,...... ea, k.. 04 ' -1, AP' 4 4 7"..r."'" , r very short times, although notyet in the quantities that would be required for nuclearfusion. Laboratory experiments haveestablished the fact that nuclear fusion reactionscan indeed be induced by theaction of lasers on soliddeuterium. However, theamount of energy released is only a small fraction of that in the laserbeam. More powerful lasers with shorter pulses are requiredand effortsarebeing made to achievethis objective. One possibility being studied forincreasing the power isto focus several individual laser beams onto the solid pellet ofreacting material. Calculations have shown thatif the laser pulse is properly shaped in time, the pellet ismore highly compressed. At the very high densities that can be reachedin this manner, the laser energy required toattain fusion conditionsshould be greatly decreased.

Laser-Fusion Reactor The major immediate problemin connection with laser fusion is concerned with thedevelopment of lasers capable of repeatedly delivering hugeamounts of energy invery short and properly shaped pulses.A solution does notseem to lie outside the realm of technologicalpossibility. Since the situationappears promising, preliminary designs havebeen proposed for nuclearfusion power reactors utilizing inertial confinementwith laser heating. The main structure of the reactor would bea large chamber capable of withstandingthe repeatedexplosionsoccurringwhen deuterium tritiumpelletsareheatedbyalaser beam (Figure 22). A pellet wouldbe injected into the vesseland when itreaches the centera powerful pulsed laser beam would be focused onto it fora period of about a nanosecond or less. After the lapse ofa few seconds, another pellet would be injected and heated, zinc(so on.

74 I he neutrons produced in the I) Treaction. carrying some SO' , of thefusion CI Ian, wouldbe taken up blithium. The basic scientific principles from now onwould be the same as for a fusion reactor with toroidal magneticconfinement wage o6). although the designof the *stem would be different. Hie lithium might be introducedinto the reaction chamber as a film of liquid covering theinterior surface or as a swirl of droplets (orboth). The energy of theneutrons would be deposit in the lithium and at the sametime tritium would be generated forsubsequent use in the fusion reaction. [he heated lithium would be drawn offfrom the bottom of the reaction chamber. passed through aheat exchanger, and then returned at a lower temperature.The heat removed in the heat exchanger would be used toproduce steamor possibly a hot gas) for operation of aturbine-generator in the usual manner.

7-PELLET INJECTOR LITHIUM FILM LITHIUM ELECTRICITY

PULSED WASTE LASER HEAT BEAM

REACTOR WATER VESSEL TRITIUM REMOVAL LITHIUM

Figure 22. Basic principles of a conceptualspherical (or approximate!) spherkal) reactor utilizinglaser Palm? of deuterium tritium pellets.

H.j 75 NUCLEAR FUSION RESEARCHPROGRANIS

Support by U. S. Governmentand Industry The study of controlled fusion in the United Stateswas begun in1951and 1952 atthe Los Alamos Scientific Laboratoryin New Mexico, andatwhat are now the Lawrence Livermore Laboratoryand the Lawrence Berkeley Laboratory. whichare both in California, and the Princeton Plasma Physics Laboratoryin New Jersey. A substantial program was started at the Oak Ridge National Laboratoryin Tennessee in 1955, although there had been an interestin controlled fusion in earlieryears. Except lot a smallamount of research and development. the Berkeley Laboratoryis no longer a major contributor in this area, but at theother four laboratories the 1/ S. Atomic EnergyCommission (AEC) supports research aimed at the ultimate development ofa reactor for obtaining usefulenergy from controlled fusion. In addition, the AEC has sponsoredboththeoretical and experimental research on high-temperature plasmas atseveral universities. The NationalScience Foundation, the National Aeronautics and Space Administration,and the Department of Defense also finance plasma physics research,which is somewhat related to controlledfusion. Electricalutilities have providedsome support for controlled fusion studies although to a much smallerextent than the AEC. In 1957, the Texas AtomicEnergy Research Foundation. a consortiumof privateutility companies, started a project in conjunctionwith what is now Gulf General Atomic in La Jolla,California, Some 10years later, the financial supportwas transferred to the Universityof Texas. Work at Gulf GeneralAtomic is being continuedwith help from the AEC. In recent years, several utilities havebeen contributing to the cost ofplasma research at a-.numberof universities. Private industryis also supporting laserfusion research and a company named KMS Fusion, Inc., hasbeen formed exclusively for thispurpose.

'A/ A 76 (:_:. Historical Outline in 1951 and Studies of the low-betaZ pinch were started Laboratory and at the 1952 at the LosAlamos Scientific This work Lawrence Berkeley andLivermore Laboratories. the United States has now beenlargely abandoned in in other countries(e.g.,the althoughinterestcontinues experi- United Kingdom's largeZETA device). Preliminary ments, which led tothe high-beta (shockheated) theta-pinch Livermore in 1954 and a system, wereoriginally made ... following year at the more promisingapproach began the C, The concept Naval Research Laboratoryin Washington, D. Scientific Laboratory was taken upby the Los Alamos successful Scylla linear around 1956 anddeveloped into the fusion on a small theta pinch in whichdefinite controlled theta-pinch scale was firstachieved in1957. A toroidal has been con- device, called Scyllac(for Scylla closed), Photo 12.) Following11., structed atLos Alamos. (Sec Scylla, the same encouraging results withshock heating in principle was used in alinear Z pinch.Subsequently. a the Los toroidal high-beta Z pinch wasplaced in operation at Alamos ScientificLaboratory. mirror confinement was . Theinvestigation of magnetic in 1951 and_ initiated at the LawrenceLivermore Laboratory around 1955. Studies at the Oak RidgeNational Laboratory fusion: have continued at of this aspect ofcontrolled nuclear the work has been both of theselaboratories. At Livermore stability, confinement.and mainly concernedwiththe Ridge, on the other heating of low-betaplasmas. At Oak inthe production ofhigh- hand,the major interestis by magnetic temperature, high-betaplasmas for confinement mirrors. withL. Spitzerat Thestellaratorconcept originated devices based on the Princeton Universityin 1951 and several principle were figure eight and otherforms of the stellarator in the completionin constructed. Theseactivities culminated feet) Model C stellarator 1961 of the large(overall length 40

77 at the Princeton Plasma 0 Physics Laboratory.Plasma confinement in this systemwas limited by Bohm the encouraging diffusion and after results with theRussian tokamak,the Model C stellaratorwas converted into renamed the ST a tokamak system and device. (See Photo7.) Studies of have been terminated stellarators at Princeton, at leastfor the present, but they are stillin progress in countries other thanthe U. S. In addition tothe ST tokamak, other systems basedon the tokamak principlehave been constructed Princeton and elsewhere since 1970 at in the UnitedStates at the following locations: Oak RidgeNational Laboratory, Institute of Technology, Massachusetts University of Texas,and Gulf General Atomic.(See Photo 13.) Studies of plasmastability by using tors developed from internal-ring conduc- the linear "hard-core"system at the Lawrence LivermoreLaboratory in 1958. through a metal rod Current passing running around theaxis of a cylinder produced a poloidal magnetic field. In thefollowing year, the toroidal Levitron with an internal ring, levitatedby means of a 'pulsed alternatingmagnetic field, was constructed atthe same laboratory. Othertoroidal systems have been built with internal rings recently atLivermore,Princeton, the University of Wisconsin,and Gulf General In 1962, the Atomic. study of laserfusion was started scale at the Lawrence on a small Livermore Laboratorywith the support of the AEC.Interest in the work derived fromthe potential of this technologyto military applications. -later, the activity About 5 years was extei.ded to theSandia Labbratory, Albuquerque, whichperforn s weapons-related work forthe AEC, and in 1970to the Los Alamos Lasers with large Scientific Laboratory. outputs, suitable fornuclear fusion,are also being developed bythe Department of Defense at theNaval Research Laboratory.The AEC is supporting all of theU. S. Photo 12.An arc of the toroidal theta-pinch Scyllac device Los Alamos Scientific at Laboratory. (Scyllacconsists of three such sectors.)

78 CO 4.! go. 141.:

I (e

: av4. 41 44

4 .1

to Photo 13.The °MAK is a fusion researchderice based on thetokamak principle at OakRidge National Laboratoryin Tennessee.

Government effortdirected at achieving laser methods. Related controlled fusion by studies, whollyor partly supported by industry, are being conducted at theUnited Aircraft search Laboratory Re- and at the Universityof Rochester, and KNIS Fusion,Inc., was formedin 1970 to exploit approach to laser fusion. an

United States Programs The major programs of controllednuclear fusion in the United States arc summarized in theaccompanying table. In addition, theoretical and experimentalactivities on a smaller scale are being conductedat several universities.

Foreign Programs Studies aimed at the realization of controlledfusion as a source of useful powerwere started in the United and in the U.S.S.R. Kingdom at about thesame time as in the United

v.. 80 CONTROLLED FUSION PROGRAMSIN THE UNITED STATES

Pinch (High-Beta) Systems Los Alamos Scientific Laboratory(Scylla: Scyllac: Toroidal Z-pinch. ZT- ) Tokamak Systems Princeton PlasmaPhysicsLaboratory(Symmetric Tokamak. ST: Adiabatic ToroidalCompressor. ATC: Princeton Large Tokamak. PLT) Oak Ridge National Laboratory(ORMAK) Massachusetts Institute ofTechnology (Alcator) University of Texas (TexasTurbulent Torus, TTT) Gulf General Atomic (DoubletII) Magnetic Mirror Systems Lawrence Livermore Laboratory(2X II, Baseball II) Oak Ridge National Laboratory(ELMO) United Aircraft Research Laboratory internal-Ring Devices Princeton Plasma Physics Laboratory (Floating Multipole, FM-I ) University of Wisconsin (LevitatedOctopole) Gulf General Atomic (D. C.Octopole) Laser Fusion in Pellets Lawrence Livermore Laboratory Los Alamos Scientific Laboratory Sandia Laboratories. Albuquerque Naval Research Laboratory University of Rochester United Aircraft ResearchLaboratory KMS Fusion. Inc.

States. In fact, the pincheffect in a plasma was firstreported it From the United Kingdomin 195 I. although previously physicists. For had been predictedtheoretically by American several years, all nuclearfusion studies were classifiedbecause

81 k:k) some might have weapon-related applications. However. in 1958,the United States. theUnited Kingdom. andthe U.S.S.R. agreed to declassify all work on controlledfusion and this agreement appears to have been honored. Therehave been extensive exchanges of information andpersonal visits of scientists between these three countries. Afterthe publica- tionin 1958 of the previouslyunavvolable reportsou controlled fusion, several otherninnies started theirown research programs. For sonic time. the U.S.S.R. has devoted substantiallymore manpower than any othercountry to studies of controlled fusion. In recent years. the effort in the U.S.S.R.has been more than twice as great as in the United States. whichis roughly the same as in West Germany. Substantialprograms on nuclear fusion are underway in the United Kingdom. Japan. France. Italy. and the Netherlands.with smaller efforts in Australia. Czechoslovakia. Denmark. Israel.Poland. Sweden. and Switzerland. The major activities on controlled nuclear fusionoutside the United States are summarized in the table:there are also several smile(proesams that are not mentioned.

CONCLUSION

The objective of controlled nuclear fusion research isto develop a major economicsource of energy that should be readily available to allnrankind. This developmentis most likelyto occurfirst by way of the reactionbetween deuterium and tritium nuclei. The basic fuelmaterials will then be deuterium and lithium: the tritium will beobtained by the interaction of neutrons with the latter clement.The quantity of deuterium in the oceans and other water bodiesis virtually ine0austible andthe lithium thatcan be extracted from heavy brines and minerals(e.g., pegmatites) should be adequate for several hundredyears. If it should ultimately prove necessary, the lithium inseawater could be extracted

82 CONTROLLED FUSIONPROGRAMS IN OTHER COUNTRIES

Pinch Systems UniNd Kingdom (high-betaand low-beta) U.S.S.R. (high-beta) Germany (higli-beta) Netherlands (high-beta) Stellarator Systems United Kingdom U.S.S.R. West Clerma,w Japan akSystems TokamTokamak

West Germany France Italy Japan Magnetic Mirror Systems United Ki112(10111 U.S.S.R. France Laser Fusion in Pellets U_S.S.R. France Italy Germany Japan Israel

of for an acceptable price.However, before terrestrial sources fusion of deuterium lithium are exhausted,the conditions for nuclei alone (or ofdeuterium and helium3)should be required. achieved. In this event,lithium will no longer be

83 The costto the consumer of electricitygenerated by nuclear fusioncannot be predicted with any certainty,largely because all thetechnological problems However, it is are not yet known, clear that thefuel materials in relation will be verycheap to the amountof energy from them. that can beobtained The bestestimates at electric power present indicatethat -from nuclearfusion will be price with competitive iif power from othermajor sources, nuclear fission. such as coal and Research onmagnetically confined plasmas sincethe early I 950s hasrevealed many problems relatingto the attainment of controlledfusion on a useful scale.Extensive theoretical and experimentalresearches have standing of led to a betterunder- plasmainstabilities andhow they controlled. If.as is not improbable, can be additional instabilities should becomeapparent, plasma for studying physicists have themeans them and for learninghow to deal with The general them consensus is that thereis no fundamental reason why theconditions of plasma temperature. particle density, andconfinement time required forpractical nuclear fusion cannotbe achieved, conceivably earlyin the next decade. Suchan achievement would establishscientific feasibility. Therealization of laser-fusion materials would with solid reacting represent an importantalternative to the confinement andheating of plasmas in a mag,.eticfield. Even if thescientific feasibility of nuclearfusion can be established, therewill still be to be overcome many technologicaldifficulties before fusioncan be usedas an economic source of electricpower. This might 10- to 20 possibly requireanother years. But the rewardsof a successful would be outcome so great as to beworth almost achieve it. The any effort made to solution of theproblems of controlled fusion has been nuclear well describedas one of the greatest and technical scientific challenges of the20th-eentury.

84 OM(SICIWN crogeoc pro al,'moNalgt pa tor $

r Ok

Photo 14. Dr. Dix). Lee Ray.Clutirwn of the AtomicEnergy Commission. and Dr. RobertL. Hirsch. Director ofthe .4 EC's Diriston of controlledThermonuclear Research. discuss recent progros in theORAL-IK experiment at the OakRidge Aratimial Laboratory.

READING LIST

Books and Reports R. W. ProgressinControlledTher»u»nulear Research. Ribs, U. S. Gould,II. P.Furth,R. F. Post, and F. L. Government Printing Office,Washington, D. C., December 1970, S1.00.

1 85 tf Summary of the USAEC Programin ControlledThero- nuclear Research,Division of Controlled Research, U. S. Thermonuclear Atomic EnergyCommission, Washington, D. C., June 1971.73 pp., free. Controlled Thermonuclear Research. HearingsBea lore the Subcommitteeon Research, Development, of the Joint and Radiation Committee on AtomicEnergy, Congress United States. 92nd of the Congress, U. S.Government Printing Office, Washington, D. C.. November1971, Part 2 (Ap- pendices), 397pp., S1.50.

Articles

Controlled FusionResearch and R. F. Post, High-Temperature Plasmas, Annual Review ofNuclear Science, (1970). 20: 509 The Leakage Problem in FusionReactors, F. C. Chen, Scientific American.217: 76 (July1967). The .Prospects of Fusion Power, W.C. Gough and Eastlund. Scientific B. J. American, 224: 50(February 1971). Fusion by Laser,M. J. Lubin and A. P. Fraas,Scientific American, 224: 21(June 1970). The Tokamak Approach in FusionResearch, B. Coppi J. Rem. Scientific and American. 227: 65(July 1972). Controlled Nuclear Fusion Status andOutlook, D. J. Rose, Science, 172: 797(1971). Laser Fusion: A New Approachto Thermonuclear W. D. Mete Power, Science, 177: 1180(1972). Prospects for FusionPower, R. F. Post, 30 (April 1973). Physics Today, 26: Laser-Induced Thermonuclear Fusion,J. Nuckolls, J.Em- mett. and L. Wood,Physics Today. 26: World Survey 46 (August 1973). of Major Facilitiesin Controlled Nuclear Fusion, Fusion, Special Supplement1970.

86 MOTION PICTURES

Available for loan without chargefrom the USAEC Film Library-T1C, P. 0. Box 62, Oak Ridge,Tennessee 37830. To Bottle the Sun, 51/2 minutes,color, 1973. The principles of fusion are explained. Variousresearch projects are discussed as well as the problemsthat must be overcome before fusion reactors are a reality. A Superconducting Magnetfor Fusion Research, 22 minutes, color, 1971. Intense magneticfields are generally agreed to be the most promising meansof confining hydrogen- isotope plasma to producecontrolled fusion on earth. As part of this research a I3-ton superconducting magnethas been builtfor theBaseball-IIneutral beam injection experiment at the LawrenceLivermore Laboratory in California. This film describes the generalconcept of the experiment, the winding and installationsof the magnet system, and initial testing ofthe new fusion research facility.

87 PHOTO CREDITS

Cover courtesy LickObservatory Contents pate from Cosmic Rays, A. W.\Volfendale, Philo- sophical Library, © 1963.Reproduced by permission. Pages 2 & 3 Lick Observatory Photo I Sacramento Peak Observatory,AFCRL Photos 3. 4, 5 & 6 Los Alamos ScientificLaboratory Photo 7 Princeton Plasma PhysicsLaboratory Photos 8 & 9 Lawrence LivermoreLaboratory Photo 10 Gulf General Atomic Photo 11 Lawrence LivermoreLaboratory Photo 12 Los Alamos ScientificLaboratory Photo 13 Oak Ridge NationalLaboratory

* U.S. GOVERNMENTPRINTING OFFICE, The U. S. AtonaL Luergy Commission publishes this series of information booklets forthe general publiL. The booklets arelisted below by subject category. If you would like to have wines of these booklets. please write to the following address for a booklet price list: USAEC Technical Information Center P. 0. Box 62 Oak Ridge. Tennessee 37830

SLImol and puhlilibraries may obtain a Lomplete set of the booklets without charge. These requests must be nude on st.hool or library stationery

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