Chapter 4

Fusion Science and Technology Chapter 4 Fusion Science and Technology

Great progress has been made over the past for lack of funds; science had a higher funding 35 years of fusion research. Nevertheless, many priority. In addition, fusion technologies that re- scientific and technological issues have yet to be quire a source of fusion power to be tested and resolved before fusion reactors can be designed developed have had to await a device that could and built. Fundamental questions in plasma sci- supply the power. Until recently, the fusion sci- ence remain, especially involving the behavior ence database has not been sufficient to permit of plasmas that actually produce fusion power. such a device to be designed with confidence. Other plasma science questions involve the be- This chapter discusses the various confinement havior and operation of the various confinement concepts under study, the systems required in a concepts that might be used to hold fusion fusion reactor, and the issues that must be re- plasmas. solved before such systems can be built. It then To date, engineering issues have not been stud- outlines the research plan required to resolve ied as extensively as plasma science issues. For these issues and estimates the amount of time and many years, engineering studies were deferred money that such a research plan will take.

CONFINEMENT CONCEPTS’

Most of the fusion program’s research has fo- Table 4-1 lists the principal confinement schemes cused on different magnetic confinement con- presently under investigation in the United States cepts that can be used to create, confine, and and classifies them according to their level of de- understand the behavior of plasmas. In all of velopment. The concepts are described in the fol- these concepts, magnetic fields are used to con- lowing section. fine the plasma; the concepts differ in the shape At this stage of the research program, it is not of the fields and the manner in which they are known which confinement concept can best generated. These differences have implications form the basis of a fusion reactor. The tokamak for the requirements, complexity, and cost of the is much more developed than the others, and engineering systems that surround the plasma. tokamaks are expected to demonstrate the basic scientific requirements for fusion within a few

1 Th is chapter d Iscusses on Iy magnetic confinement approaches. years. However, several alternate concepts are Other approaches to fusion are discussed In app. B. The concepts under investigation in order to gain a better un- mentioned In this section are described in greater detail in pp. 156- derstanding of the confinement process and to 204 ot’ Physics Through the 19905: P/asmas and F/uids, by the Na- tional Research Council (Wa~hington, DC: National Academy Press, explore possibilities for improving reactor per- 1 986). formance.

Table 4-1 .—Classification of Confinement Concepts

Well-developed Moderately developed Developing knowledge base knowledge base knowledge base Conventional tokamak Advanced tokamak Spheromak Tandem mirror Field-reversed configuration Stellarator Dense Z-pinch Reversed-field pinch SOURCE Adapted from Argonne National Laboratory, Fusion Power Program, Technical Planning Activity Final Report, commissioned by the U S Department of Energy, Off Ice of Fusion Energy, AN UFPP-87-1, January 1987, p 15

57 58 ● Starpower: The U.S. and the International Quest for Fusion Energy

The major scientific questions to be answered Figure 4-1 .—Tokamak Magnetic Fields for each confinement approach are whether and with what confidence the conditions necessary for a sustained, power-producing fusion reaction can be simultaneously satisfied in a commercial- scale reactor. Much of the experimental and theoretical work in confinement studies involves the identification and testing of scaling relation- ships that predict the performance of future de- vices from the results of previous experiments. ideally, such scaling models should be derivable from the basic laws of physics. However, the be- havior of plasmas confined in magnetic fields is so complicated that a general theory has not yet been found. With some simplifying assumptions, limited theoretical models have been developed, but they are not broad enough to extrapolate the behavior of a concept to an unexplored range. Without a sound theoretical base, the risk of tak- ing too large a step is great. A series of interme- diate-scale experiments is needed to bridge the gap between concept development and a full- scale reactor. Even with the tokamak—the most studied con- finement concept–scaling properties are not fully understood. Although tokamaks have attained by far the best experimental performance of any confinement concept, no proven theoretical ex- planation of how that performance scales with parameters such as size, magnetic field, and plasma current has yet been derived. Without a complete theoretical basis, “empirical” scaling relationships deduced from past observations must be used. Such empirical relationships may well prove sufficient for designing a machine ca- pable of forming reactor-scale plasmas before a fundamental theoretical understanding of toka- mak behavior is reached.

SOURCE: Princeton Plasma Physics Laboratory Information Bulletin NT-1: Fu- “Closed” Concepts sion Power, 1984, p. 4. In “closed” magnetic confinement configura- field. This field is generally created by external tions, the plasma is contained by magnetic lines magnet coils, called toroidal field coils, through of force that do not lead out of the device. Closed which the plasma torus passes. A magnetic field configurations all have the basic shape of a dough- perpendicular to the toroidal field, encircling the nut or inner tube, which is called a “torus. ” A torus the short way, is called a “poloidal” field. magnetic field can encircle a torus in two differ- This field is generated by electrical currents in- ent directions (figure 4-1 ). A field running the long duced to flow within the plasma itself. Together, way around the torus, in the direction that the toroidal and poloidal magnetic fields form the to- tread runs around a tire, is called a “toroidal” tal magnetic field that confines the plasma. Ch. 4.—Fusion Science and Technology ● 59

Conventional Tokamak shaped plasmas according to this principle, Other variants on tokamak design would permit more In a tokamak, the principal confining magnetic compact fusion cores to be constructed, which field is toroidal, and it is generated by large ex- could lead to less expensive reactors; these im- ternal magnets encircling the plasma. This field provements are under study. alone, however, is not sufficient to confine the plasma. A secondary poloidal field, generated by Still other improvements would permit toka- plasma currents, is also required. The combina- maks to run continuously. The technique typi- tion of poloidal and toroidal fields produces a to- cally used today to drive the plasma current in tal field that twists around the torus and is able a tokamak can be run only in pulses. Technol- to confine the plasma (figure 4-1). ogies for driving continuous, or steady-state, plasma currents are being investigated at a num- The tokamak concept was developed in the So- ber of different experimental facilities. viet Union, and, since the Iate 1960s, it has been the primary confinement concept in all four of the world’s major fusion research programs. It Stellarator has also served as the principal workhorse for The stellarator is a toroidal device in which developing plasma technology. The scientific both the toroidal and poloidal confining fields are progress of the tokamak is far ahead of any other generated by external magnets and do not de- concept. Major world tokamaks are listed in ta- pend on electric currents within the plasma. The ble 4-2. external magnets are consequently more com- plicated than those of a tokamak (figure 4-2). Advanced Tokamak However, the absence of plasma current in a stel- Various features now under investigation may Iarator enables steady-state operation to be substantially improve tokamak performance. Mod- achieved more directly without the need for cur- ifying the shape of the plasma cross-section can rent drive. increase the maximum plasma pressure that can be confined with a given magnetic field. The The stellarator concept was invented in the Doublet II I-D (D III-D) tokamak at GA Technol- United States. After the discovery of the tokamak ogies and the Princeton Beta Experiment Modifi- in the late 1960s, however, the United States con- cation (PBX-M) tokamak at Princeton Plasma verted its stellarators into tokamaks. The stella- Physics Laboratory are being used to investigate rator concept was kept alive primarily by research

Table 4-2.—Major World Tokamaksa

Device Location Status JET ...... European Community (United Kingdom) Operating D III-D ...... United States (GA Technologies) Operating Alcator C-Mod ...... i ...... United States (MIT) Under construction T-14 ...... U.S.S.R. (Kurchatov) Under construction TFTR ...... United States (PPPL) Operating JT-60 ...... Japan (Naka-machi) Operating T-15 ...... U.S.S.R. (Kurchatov) Under construction ASDEX-Upgrade ...... Federal Republic of Germany (Garching) Under construction Tore Supra...... France (Cadarache) Under construction Frascati Tokamak Upgrade ...... Italy (Frascati) Under construction PBX-M ...... United States (PPPL) Under construction TEXTOR ...... Federal Republic of Germany (Julich) Operating aListed in decreaing order of plasma current, one of the many parameters that determines tokamak capability. NO single factor by itself measures capability well; current is used here only to give a rough distinction between those devices at the top of the list and those at the bottom. Ranking by size, magnetic field, or other parameter would rearrange the list somewhat. NOTE: This table includes only the largest tokamaks. The World Survey of Activities in Corrtrolled Fusion Research, 1986 Edition (published in Nuclear Fusion, Special Supplement 1966) lists a total of 77 existing and proposed tokamaks at 54 sites in 26 countries. SOURCE Office of Technology Assessment, 1987

60 ● Starpower: The U.S. and the International Quest for Fusion Energy

Figure 4-2. —Magnet Coils for the Advanced Toroidal Facility, A Stellarator

SOURCE Oak Ridge National Laboratory

in the Soviet Union, Europe, and Japan, and, due the new Japanese device would be the largest to good results, the United States has recently re- operational non-tokamak fusion experiment, vived its stellarator effort. Stellarators today per- form as well as comparably sized tokamaks. Reversed-Field Pinch Major world stellarator facilities that are oper- In a reversed-field pinch, the toroidal magnetic ating or under construction are listed in table 4-3. field is generated primarily by external magnets Not shown on the table is the Large Helical Sys- and the poloidal field primarily by plasma cur- tem proposed to be built in Japan at a cost sev- rents. The toroidal and poloidal fields are com- eral times that of the largest stellarator machine parable in strength, and the toroidal field reverses now u nder construction; if built and operated, direction near the outside of the plasma, giving

Table 4-3.—Major World Stellaratorsa

Device Location Status ATF ...... United States (ORNL) Under construction Wendelstein VII-AS ...... Federal Republic of Germany (Garching) Under construction URAGAN-2M ...... U.S.S.R. (Kharkov) Under construction Heliotron-E...... Japan (Kyoto University) Operating URAGAN-3 ...... U.S.S.R. (Kharkov) Operating CHS ...... Japan (Nagoya University) Under construction L-2 ...... U.S.S.R. (Lebedev) Operating H-1 ...... Australia (Canberra) Under construction aListed in order of decreasing stored magnetic energy, a parameter which in turn depends both on magnetic field strength and plasma

SOURCE Office of Technology Assessment, 1987; from data provided by Oak Ridge National Laboratory,

Ch. 4.—Fusion Science and Technology “ 61

Figure 4-3.— Reversed. Field Pinch without the complex and costly external heating Toroidal field systems required by tokamaks. windings Los AIamos National Laboratory in New Mex- ico is the center of U.S. reversed-field pinch re- search. The Confinement Physics Research Fa- cility (CPRF) to be built there will hold the largest reversed-field pinch device in the United States. A variant of the reversed-field pinch, the Ohmi- cally Heated Toroidal Experiment, or OHTE, was built at GA Technologies in San Diego, Califor- nia. Reversed-field pinch research is also con- ducted in both Europe and Japan. Table 4-4 lists the major world reversed-field pinches.

Spheromak The spheromak is one of a class of less devel- oped confinement concepts called “compact toroids,” which do not have toroidal field coils SOURCE: Adapted from National Research Council, Physics Through the 1990s: Plasmas and FluIds (Washington, DC: National Academy Press, 1988). linking the plasma loop and therefore avoid the engineering problem of constructing rings locked within rings. Conceptually, if the torodial field the concept its name (see figure 4-3). In a toka- coils and inner walls of a reversed-field pinch mak, the toroidal field dominates and points in were removed and the central hole were shrunk the same direction throughout the plasma. to nothing, the resultant plasma would be that The reversed-field pinch generates more of its of the spheromak. Its overall shape is spherical; magnetic field from plasma currents and less from although the internal magnetic field has both external magnets, permitting its external magnets toroidal and poloidal components, the device has to be smaller than those of a comparably per- no central hole or external field coil linking the forming tokamak. The nature of the magnetic plasma (figure 4-4). The plasma chamber lies en- fields in a reversed-field pinch may also permit tirely within the external magnets. If the sphero- steady-state plasma currents to be driven in a mak can progress to reactor scale, its small size much simpler manner than is applicable in a toka- and simplicity may lead to considerable engineer- mak. Moreover, a reversed-field pinch plasma ing advantages. However, the present state of may be able to heat itself to reactor temperatures knowledge of spheromak physics is rudimentary.

Table 4.4.—Major World Reversed-Field Pinchesa

Device Location Status CPRF ...... United States (LANL) Under construction RFX ...... Italy (Padua) Under construction OHTE ...... United States (GA Technologies) Operating HBTX 1-B ...... United Kingdom (Culham) Operating ZT-40M . . . ! ...... United States (LANL) Operating MST ...... United States (University of Wisconsin) Under construction ETA BETA 11 ...... , Italy (Padua) Operating Repute I...... Japan (Tokyo University) Operating TPE-1RM(15) ...... Japan (Tsukuba University) Operating STP-3M ...... Japan (Nagoya University) Operating aListed in order of decreasing plasma current, a rough measure of reversed-field pinch performance. SOURCE: Office of Technology Assessment, 1987; from data supplied by the Los Alamos National Laboratory.

62 “ Starpower: The U.S. and the International Quest for Fusion Energy

Figure 4-4.—Spheromak spheromak research effort takes place at the Uni- versity of Maryland. Spheromaks also are being studied in Japan and the United Kingdom. Ma- jor world spheromak devices are listed in table 4-5.

Field= Reversed Configuration The field-reversed configuration (FRC) is another form of compact toroid. Despite the similar name, it does not resemble the reversed-field pinch. It is unusual among closed magnetic confinement concepts in providing confinement with only poloidal fields; the FRC has no toroidal field. The plasma is greatly elongated in the poloidal direc- tion and from the outside has a cylindrical shape (figure 4-5). Like the spheromak, the FRC does not have ex- ternal magnets penetrating a hole in its center; all the magnets are located outside the cylindri- cal plasma. The FRC also has the particular vir- tue of providing extremely high plasma pressure for a given amount of magnetic field strength. if its confining field is increased in strength, the FRC plasma will be compressed and heated. Such heating may be sufficient to reach reactor con- ditions, eliminating the need for external heat- ing. Existing FRC plasmas are stable, but whether stability can be achieved in reactor-sized FRC plasmas is uncertain. A new facility, LSX, is under construction at Spectra Technologies in Bellevue, Washington, to investigate the stability of larger plasmas. U.S. FRC research started at the Naval Research Spheromak research at Los Alamos National Laboratory in Washington, D. C., in the late 1960s. Laboratory was terminated in 1987 due to fiscal Increased effort in the United States in the late constraints, and another major U.S. device at 1970s, centered at Los Alamos, was undertaken Princeton Plasma Physics Laboratory is to be ter- largely in response to experimental results ob- minated in fiscal year 1988. The remaining U.S. tained earlier in the decade from the Soviet Union

Table 4-5.—Major World Spheromaksa

Device Location Status Ch. 4.—Fusion Science and Technology “ 63

Figure 4-5.— Field-Reversed Configuration Magnetic Mirrors Fusion plasmas can be confined in an open- ended tube by strengthening, and thereby com- pressing, the magnetic fields near the ends. Strengthening the magnetic field near the ends “reflects” plasma particles back into the center much as narrowing the ends of a sausage helps keep in the meat. However, the ends of a sim- p/e magnetic mirror (figure 4-6a) are not other- wise sealed. Just as the meat eventually forces its way out of an unsealed sausage when squeezed, a simple magnetic mirror cannot confine a plasma well enough to generate fusion power. In addi- tion, simple mirrors are usually unstable, with the plasma as a whole tending to slip out sideways. A variation of the simple mirror is the mini- mum-B mirror (figure 4-6 b), one version of which SOURCE: National Research Council, Physics Through the 1990s: Plasmas and Fluids (Washington, DC National Academy Press, 1988), uses a coil shaped like the seam on a baseball to create a magnetic field that is lowest in strength at the center and increases in strength towards the outside. Particles leaving the center tend to and the Federal Republic of Germany. Soviet re- be reflected back by the increasing magnetic field search has continued, but German and British re- at the outside, just as particles leaving the sim- search programs have stopped. Meanwhile, a ple mirror tend to be reflected back at the ends. program in Japan has begun. Major field-reversed This configuration is stable, and there is no ten- configuration experiments around the world are dency for the plasma as a whole to escape. How- listed in table 4-6. ever, despite these improvements, the minimum- B mirror cannot confine a plasma well enough "Open” Concepts to generate net fusion power. plasmas in open magnetic confinement devices The tandem mirror (figure 4-6c) improves the are confined by magnetic fields that do not close simple magnetic mirror by utilizing additional back on themselves within the device but rather mirrors to improve the plugging at each end. extend well outside the device. Since plasma par- These plugs, called end cells, are themselves mag- ticles can easily travel along magnetic field lines, netic mirrors. Rather than trapping the main some additional mechanism is required to reduce plasma, the end cells hold particles that gener- the rate at which plasma escapes out the ends ate an electric field. This electric field, in turn, of an open confinement device. keeps the plasma in the central cell from escap-

able 4.6.—Major World Field-Reversed Configurationse

Device Location Status LSX ...... United States (Spectra Technologies) Under construction FRX-C ...... United States (LANL) Operating BN, TOR ...... U.S.S.R. (Kurchatov) Operating TRX-2 ...... United States (Spectra Technologies) Operating OCT, PIACE ...... Japan (Osaka University) Operating NUCTE ...... Japan (Nihon University) Operating aListed approximately by decreasing order of size; similarly sized devices at the same institution are listed together. SOURCE Office of Technology Assessment, 1987; from information supplied by the Los Alamos National Laboratory.

64 ● Starpower: The U.S. and the International Quest for Fusion Energy

Figure 4-6.—Magnetic Mirrors

(a) Simple mirror (b) Minimum-B mirror

Plasma /

SOURCE: Lawrence Livermore National Laboratory, “Evolution of the Tandem Mirror,” Energy and Technology Review, November 1986 ing. While particle losses from the end cells are Dense Z= Pinch high, these losses can be compensated by inject- In this concept, a fiber of frozen deuterium- ing new particles. tritium fuel is suddenly vaporized and turned into The tandem mirror concept was developed plasma by passing a strong electric current through simultaneously in the United States and the So- it. This current heats the plasma while simultane- viet Union in the late 1970s. The Mirror Fusion ously generating a strong magnetic field encircling Test Facility B (MFTF-B), located at Lawrence the plasma column (figure 4-7), “pinching” it long Livermore National Laboratory in California, is enough for fusion reactions to occur. Many de- the largest mirror device in the world and the vices investigated in the earliest days of fusion largest non-tokamak magnetic confinement fu- research in the 1950s operated in a similar man- sion experiment. Budget cuts, however, forced ner, but they were abandoned because their MFTF-B to be moth balled before it could be used plasmas had severe instabilities and were una- experimentally. The Tandem Mirror Experiment ble to approach the confinement times needed Upgrade (TMX-U) at Livermore, a smaller version to generate fusion power. of MFTF-B, was terminated as well, and the TARA device at the Massachusetts Institute of Technol- The dense z-pinch differs from the 1950s pinches ogy will be shut down in 1988. At that point, in several important aspects that, as calcuIations Phaedrus at the University of Wisconsin will be and experiments have shown, improve stability. the only operational U.S. mirror machine. Mir- Crucial to the modern experiments are precisely ror research is still conducted in the Soviet Union controlled, highly capable power supplies that and Japan. Table 4-7 presents a list of major world would have been impossible to build with 195os tandem mirror facilities. technology, and the use of solid, rather than gase- —

Ch. 4.—Fusion Science and Technology ● 65

Table 4-7.—Major World Tandem Mirrorsa

Device Location Status MFTF-B ...... United States (LLNL) Moth balled TMX-U ...... United States (LLNL) Mothballed Gamma-10 . . . .Japan (Tsukuba University) Operating TARA ...... United States (MIT) To be terminated, fiscal year 1988 Phaedrus . . . . . United States (University of Wisconsin) Operating Ambal M . . . . . USSR (Novosibirsk) Under construction aListed in decreasing order of size. SOURCE: Office of Technology Assessment, 1987; from data supplied by the Lawrence Livermore National Laboratory.

Figure 4-7. —Dense Z-Pinch ● Many fusion concepts are under study be- cause the frontrunner tokamak, while likely to be scientifically feasible, may yet be found weak in some critical area or less economi- cally attractive than alternatives. Features being studied in alternate concepts include eased conditions for steady-state operation, reduced external magnet complexity and cost, and improved use of the magnetic field. Searching for optimum reactor configura- tions and developing further understanding of the fusion process mandate that the range of concepts under investigation not be prematurely narrowed. The tokamak concept is by far the most developed, and it has attained plasma con- ditions closest to those needed in a fusion reactor. At present and for the next several years, studies of reactor-like plasmas will be done with tokamaks because no other con- SOURCE: Office of Technology Assessment, 1987 cept has yet proven that it can reach reactor conditions. A number of other confinement OUS, fuel to initiate the discharge. However, it is concepts have features that might make much too early to tell whether this concept can them preferable to the tokamak if they are be developed successfully. If the concept can be capable of progressing to an equivalent stage developed, the device has the potential to be far of performance. It remains to be seen which smaller and far less expensive than devices based of these concepts will attain that perform- on other concepts. External magnets are not ance level, what their development will cost, needed since the plasma current supplies the en- and to what degree the tokamak concept it- tire confining field. Dense z-pinch research is tak- self will further improve. ing place in the United States at two facilities: the ● Different confinement studies complement Naval Research Laboratory and Los Alamos Na- each other. Knowledge obtained through re- tional Laboratory. search on a specific concept often can be generalized. Throughout the history of fusion research, plasma science issues originally in- Conclusions Concerning vestigated because of their relevance to a Confinement Approaches particular concept have become important to studies of other concepts as well. A number of general conclusions can be drawn ● A great deal of progress in understanding from studies of the confinement concepts that fusion plasmas and confinement concepts have evolved over the past 10 to 15 years: has been made to date. Many concepts 66 ● Starpower: The U.S. and the International Quest for Fusion Energy

studied earlier, such as the simple magnetic require study at greater levels of capability mirror, are no longer studied today because before their potential as reactor candidates they cannot compare attractively to improved can be assessed. Moreover, since this has or alternate concepts. At the same time, as largely been an empirical program, advanced in the case of the dense z-pinch, problems studies will require larger and increasingly once considered intractable may be solved more expensive facilities. Fiscal constraints with additional scientific understanding and will almost certainly require that not all of more advanced technology. the concepts be “promoted” to subsequent ● Research on all confinement concepts has stages of development. Criteria such as de- benefited from international cooperation. velopment cost, characteristics of the end Studies undertaken by different groups in product, and likelihood of success must be different countries enhance each other sig- developed for selecting which concepts are nificantly. Furthermore, advances by one to be pursued further. program have frequently stimulated addi- ● progress in fusion science depends on prog- tional progress in other programs. interna- ress in fusion technology. Time after time, tional cooperation in fusion research is dis- the exploration of new ranges of plasma be- cussed further in chapter 7. havior has been made possible by the devel- ● Not all confinement concepts can be devel- opment of new heating, fueling, and plasma oped to reactor scale. Promising concepts shaping technologies.

SCIENTIFIC PROGRESS AND REACTOR DESIGN Different Dimensions of Progress understanding of fusion technology and future societal needs improves. To form the basis of a viable fusion reactor, a confinement concept must meet two objectives. No matter how it is evaluated, reactor poten- First, it must satisfy scientific performance require- tial is a requirement that, along with scientific per- ments—temperature, density, and confinement formance, must be satisfied by at least one con- time—necessary for a plasma to produce fusion finement concept before fusion power can be power. progress towards those requirements is realized. Figure 4-8 shows two different paths by easy to measure. which a concept can develop toward commer- cial use. Along the “performance-driven” path, Second, a confinement concept must demon- a concept first demonstrates the ability to attain strate “reactor potential. ” Unlike scientific per- plasma parameters near those required to pro- formance, reactor potential is difficult to meas- duce fusion power; subsequently, innovations or ure. A viable reactor must be built, operated, and successive refinements show that the concept’s maintained reliably, and it must be economically, scientific capabilities can be used in a viable re- environmentally, and socially acceptable. While actor design. Alternatively, along the “concept- fusion’s acceptability in these respects depends improvement-driven” path, features that are at- on factors external to fusion technology, it also tractive in a fusion reactor—e.g., compact size, depends on the choice of confinement concept. ease of maintenance, simple construction, and reliable operation—are apparent before the sci- Each concept may have different advantages, entific performance necessary to produce fusion and, in the absence of quantitative measures, the power is demonstrated. process of i dentifying the concepts t hat offer t he most attractive reactors depends in large part on The actual development of any given confine- the innovation and technological optimism of the ment concept will fall somewhere between these reactor designer. Also, attributes of an attractive extremes. Development of the tokamak appears reactor can be identified today, but the relative to be closer to the performance-driven curve. Its importance of these attributes may change as our scientific performance, along with its use in de- Ch. 4.—Fusion Science and Technology ● 67

Figure 4-8.—Alternate Paths for Concept Development

SOURCE Adapted from Argonne National Laboratory, Technical Planning Activity: Final Report, commissioned by the U.S. Department of Energy, Office of Energy Research, AN L/FPP-87-1, January 1987, figure 1.5, p. 56. veloping plasma technology and diagnostics, has Input and output power are measured at some been the primary motivation for study to date; instant after the plasma has reached its operat- improvements to the basic tokamak concept are ing density and temperature. In experimental currently focused on improving reactor poten- plasmas that do not contain tritium and therefore tial. Other concepts are more concept-improve- do not produce significant amounts of fusion ment-driven in that their features might make power, an “equivalent Q“ is measured. It is de- them preferable to the tokamak if they can reach fined as the Q that would be produced by the reactor scale. However, the ability of other con- plasma if it were fueled equally by both deu- cepts to attain the necessary plasma conditions terium and tritium (D-T) and if it had attained the is much less certain because their experimental same plasma parameters. z databases are less developed. The numerator of the Q ratio includes all the fusion power produced by the plasma, even Scientific Progress though most of the output power (80 percent) Energy Gain “’Equivalent Q“ is either calculated from the measured plasma An important measure of scientific progress density and temperature or derived from actual measurements of towards attaining reactor-relevant conditions is deuterium-deuterium (D-D) fusion reactions. Since the ratio be- energy gain, denoted as “Q. ” Energy gain is the tween the D-T reaction rate and the D-D reaction rate under the ratio of the fusion power output that a device gen- same conditions IS believed known, a measurement of D-D re- actions can be used to infer what the D-T fusion yield would be erates to the input power injected into the plasma. under the same conditions. 68 “ Starpower: The U.S. and the International Quest for Fusion Energy

immediately escapes from the plasma via ener- crates output power without auxiliary input power getic neutrons. The denominator of the ratio– from external sources. (Power to drive currents the power used to heat the plasma—greatly un- in the plasma and to cool the magnets to their derestimates the amount of power actually con- operating temperature will be required even for sumed. Losses incurred in generating heating ignited plasmas, but, as stated above, this power power and delivering it to the plasma are not in- is not included in Q.) cluded, nor is the power needed for the confin- Successfully reaching ignition—or at least suc- ing magnets, vacuum system, and other support cessfully generating a plasma that produces many systems. In present-generation experiments, the times more power than is input into it—will be power excluded from the definition of Q is as a major milestone in determining fusion’s tech- much as 35 times greater than the power ac- nological feasibility. The energy and the reaction counted for in this ratio.3 products generated in a plasma producing apprecia- Q excludes most of the power drawn by a fu- ble amounts of fusion power will significantly sion experiment because it is a scientific meas- affect the plasma’s behavior. Understanding these ure that is not intended to gauge engineering effects may be crucial to utilizing self-sustaining progress. Present experiments, needing only to fusion reactions in reactors, and these effects can- operate for short pulses, have not been designed not be studied under breakeven conditions alone. to minimize consumed power; to lessen construc- tion cost, they use magnets that are far less effi- Breakeven cient than those likely to be used in future re- The breakeven curve in figure 4-9 shows the actors. Similarly, the inefficiencies in generating conditions under which a plasma generates as the externally applied plasma heating power are much power through fusion reactions as is in- not included because auxiliary heat is not re- jected into it to maintain the reactions. Although quired once a plasma generates enough fusion reaching breakeven will be a major accomplish- power to become self-sustaining. Even so, some ment, it will not have the technical significance external power will be required in any steady- of reaching ignition. Due to the way that energy state plasma device except the stellarator to main- gain is defined, the breakeven threshold in some tain electrical currents within the plasma. respects is arbitrary, and it depends significantly Figure 4-9 shows the plasma temperatures and on the manner in which the plasma is heated. confinement parameters needed to obtain Qs of Crossing the threshold does not cause a signifi- at least 1, a condition known as “breakeven. ” cant change in plasma behavior and in no way The plasma temperatures and confinement pa- indicates that the experiment is able to power it- rameters that have been attained experimentally self. Problems that are not fully evident under by various confinement configurations are also breakeven conditions may yet be encountered shown. No device has yet reached breakeven, on the way to ignition. although tokamak experiments have clearly come The breakeven curve in figure 4-9 is calculated the closest. for plasmas that are uniformly heated. If the plasma is heated in such a way that a small frac- Ignition tion of the plasma particles become much hot- The most significant region in figure 4-9 is ig- ter than the rest, this fraction will produce a dis- nition in the top right corner. An ignited D-T proportionate amount of fusion power and the plasma not only generates net fusion power but breakeven requirements can be substantially lo- also retains enough heat to continue producing wered. For this reason, plasmas heated with neu- fusion reactions without external heat. The Q of tral beams can reach breakeven under conditions an ignited plasma is infinite, since the plasma gen- that would not be sufficient without the use of neutral beams.

Jsee specifications for the Tokamak Fusion Test Reactor at the Neutral beams are extremely hot jets of neu- Princeton Plasma Physics Laboratory, footnote 4 below. tral atoms that can penetrate the confining mag-

Ch. 4.—Fusion Science and Technology ● 69

Figure 4-9.— Plasma Parameters Achieved by Various Confinement Concepts

10

1

0.1

Confinement parameter (particle –see cm-3) KEY: S-1: Spheromak-1; Princeton Plasma Physics Laboratory, Princeton, NJ. TMX-U: Tandem Mirror Experiment Upgrade; Lawrence Livermore National Laboratory, Livermore, CA. ZT-40M: Toroidal Z-pinch, -40, Modified; Los Alamos National Laboratory, Los Alamos, NM. FRX-C: Field-Reversed Experiment C; Los Alamos National Laboratory, Los Alamos, NM. OHTE: Ohmically Heated Toroidal Experiment; GA Technologies, Inc., San Diego, CA. Gamma-IO: University of Tsukuba, Ibaraki, Japan. WVII-A: Wendelstein VII-A; Institute for Plasma Physics, Garching, Federal Republic of Germany. HEL-E: Heliotron-E; Kyoto University, Kyoto, Japan. D Ill: Doublet Ill; GA Technologies, Inc., San Diego, CA. JET: Joint European Torus; JET Joint Undertaking, Abingdon, United Kingdom. TFTR: Tokamak Fusion Test Reactor; Princeton Plasma Physics Laboratory, Princeton, NJ. ALC-C: Alcator C; Massachusetts Institute of Technology, Cambridge, MA. SOURCE Office of Technology Assessment, 1987.

netic fields to enter the plasma. Beam atoms col- significantly hotter than the original plasma par- lide with particles inside the plasma and become ticles. (Once the beams are turned off, the in- electrically charged, thereby becoming trapped jected particles cool down to the temperature of by the magnetic field. Through collisions, much the remaining plasma.) Since the fusion reaction of the energy carried by the beams is transferred rate increases very rapidly with temperature, the to the “target” plasma, heating it up. In the proc- hotter particles from the neutral beam have a ess, the beam particles themselves cool down. much higher probability of generating fusion re- actions than other particles in the plasma. in this However, it will take many collisions for the manner, a beam-heated plasma can achieve beam particles to cool down to the temperature breakeven with plasma parameters up to a fac- of the target plasma. As long as the beams are tor of 10 lower than those needed for plasmas on, the most recently injected beam particles are heated by other mechanisms. However, since the 70 ● Starpower: The U.S. and the International Quest for Fusion Energy beams themselves require so much power to of the confinement concepts to date. Tokamak operate, it is not expected that beam-heated experiments have clearly made the most progress plasmas will be used in reactors. Therefore, the in terms of coming the closest to the ignition lower breakeven threshold for beam-heated region. plasmas may not translate into lower require- ments for a practical reactor. TFTR, in particular, has reached the highest temperature and confinement parameters of any The Tokamak Fusion Test Reactor (TFTR) at magnetic fusion experiment. In 1986, TFTR at- Princeton Plasma Physics Laboratory was de- tained ion temperatures of 20 kiloelectron volts signed to take advantage of beam heating. It is (kev) or more than 200 million degrees C, well expected that breakeven-equivalent (breakeven over the temperature needed for breakeven or conditions in a plasma not containing tritium) will ignition. However, these high-temperature results be obtained sometime between fall 1987 and were obtained in a relatively low-density plasma spring 1988. Experiments to realize true break- having a confinement parameter of 1013 second- even using tritium are scheduled for the end of particles per cubic centimeter, which is about half 1990. These achievements will be important be- of the confinement parameter needed to reach cause, for the first time, a significant amount of breakeven at that temperature. The equivalent heat from fusion power will be produced in a Q actually attained by the plasma was 0.23. Use magnetic fusion device. Moreover, successful D-T of neutral beam heating under these conditions operation of TFTR will provide important tritium- reduces the breakeven threshold by almost a fac- handling experience necessary for future reactor tor of four; a plasma heated to 20 keV without operation. the use of neutral beams would need a confine- Nevertheless, TFTR–not being an engineering ment parameter 7.5 times higher than was at- facility–does not address most of the technologi- tained to reach equivalent breakeven. cal issues that must be resolved before a fusion In a separate experiment at a lower tempera- reactor can be built. Moreover, it will not reach ture of 1.5 keV, TFTR reached a confinement pa- ignition, and the advantage it derives from using rameter of 1.5 X 1014 second-particles per cubic neutral beams will probably not translate into a centimeter. Had this confinement been attained workable reactor. It does not incorporate ad- at a temperature of 20 keV, TFTR would have vanced physics aspects that have been identified been well above equivalent breakeven, coming since its design in the 1970s. TFTR will not—and close to meeting the equivalent ignition condi- never was intended to—have the capability to tion. However, in practice, TFTR will not be able generate electricity from the fusion power it will to attain temperature and confinement values this produce. Even on attaining breakeven, the TFTR high simultaneously. Temperature can be raised experiment as a whole—as opposed to the TFTR at the expense of confinement, and vice versa, plasma alone—will produce less than 3 percent 4 but the product of the two–which determines of the power it will consume. equivalent Q—is difficult to increase. With addi- tional neutral beam power and other improve- State of the Art ments, TFTR may well be able to raise its equiva- Temperature and Confinement.–Figure 4-9 lent Q from 0.23 to 1 and reach equivalent shows results that have been attained by each breakeven. However, it is extremely unlikely that equivalent Qs much greater than 1 are attaina- 4TFTR is being upgraded to deliver up to 27 megawatts of neu- ble in TFTR. tral beam power to the plasma. To reach breakeven, where the fusion power generated equals the external power injected into Beta.–The beta parameter, also called the the plasma, 27 megawatts of fusion power would have to be gen- erated in the plasma. If reaching breakeven were to require TFTR “magnetic field utilization factor, ” measures the to draw near the maximum amount of power available from its elec- efficiency with which the energy of the magnetic trical supply, it could consume close to 1,000 megawatts of elec- field is used to confine the energy of the plasma. tricity. This amount IS 37 times greater than the fusion power to be produced at breakeven. Beta is defined as the ratio of the plasma pres- Ch. 4.—Fusion Science and Technology c 71

ing that have been obtained by certain other con- finement concepts (albeit to date at much poorer temperatures and confinement parameters). How- ever, physical phenomena in the plasma prevent beta values from being increased indefinitely. These phenomena, which differ from concept to concept, are not completely understood; gain- ing additional understanding in this area is a high priority. Low beta values can also be compensated by raising the magnetic field strength. Whereas rais- ing beta primarily involves plasma physics issues, the issues involved in raising the magnetic field strength are primarily engineering-related: stronger magnetic fields are more difficuIt and expensive to generate and place greater stress on the magnet Photo credit: Princeton Plasma Physics Laboratory structures. At some field strength, the advantages The PBX tokamak at Princeton Plasma Physics of stronger magnetic fields will be outweighed Laboratory. by the additional expense of the magnets. Scaling.–Understanding how tokamak per- sure to the magnetic field pressures Record toka- formance can be expected to improve is crucial mak values for beta of 5 percent, in the PBX ex- to evaluating the tokamak’s potential for future periment at Princeton Plasma Physics Laboratory, reactors as well as to designing next-generation and 6 percent, in the D II I-D experiment at GA tokamak experiments. As mentioned earlier, the Technologies, have been attained. These results complete theoretical mechanism determining are especially important in that they generally tokamak scaling has yet to be understood. Ob- validate theoretical models that predict how fur- servationally, plasma confinement has been found ther improvements in beta can be obtained. to improve with increased plasma size. Empiri- In a fusion reactor, the fusion power output per cal data also show that tokamak confinement im- unit volume of the plasma would be proportional proves when plasma density is increased, but that to beta squared times the magnetic field strength this behavior holds only for ohmically heated to the fourth power. Since tokamaks have rela- plasmas. Non-ohmically heated plasmas follow tively low betas compared to many of the other what has come to be known as “L (Low) -mode” confinement concepts currently studied, improv- scaling, in which confinement degrades as in- ing the beta of tokamaks can be useful. Betas creasing amounts of external power are injected. greater than 8 percent are indicated in some sys- A few years ago, experiments on the German tem studies as being necessary for economical Axisymmetric Divertor Experiment (ASDEX) dis- performance,6 and values considerably exceed- covered a mode of tokamak behavior described by a more favorable scaling, labeled “H (High)- ~plasma pressure IS equal to plasma temperature times density and is proportional to the plasma energy per unit volume; mag- mode. ” In this mode, performance even with netic field pressure, which is proportional to the square of the mag- auxiliary heating behaved more like the original, netic field strength, is a measure of the energy stored in the mag- netic field per unit volume. ohmically heated plasmas. However, H-mode Typical reactor studies indicate that the plasma in an operating scaling could be achieved only with a particuIar fusion reactor WIII have a pressure several times that of the earth’s combination of device hardware and operating atmosphere at sea level, The plasma density, however, will only conditions. Subsequently, additional work at be about 1/1 00,000 the density of’ the atmosphere at sea level That so tew par-t lcles can exert such a h lgh pressure IS a measure ot thel r other tokamaks has broadened the range of con- extreme tern peratu re about 10,000 electron volts, or more than ditions under which this more favorable behavior 100 million degrees C. 6For ~xam Pie, J, Sheffield, et a[., Cost Assewnent of.? ~eflerlc can be found. The challenge to tokamak re- Magnet/c Fus/orr Rex-tor, oak Rlcige National Laboratory, searchers is to obtain H-mode scaling in config- (3RNLKM-931 1, March 1986, p. 5. u rations and operating regimes that are also con-

72 . Starpower: The U.S. and the International Quest for Fusion Energy

many systems besides those that heat and con- fine the plasma. Fusion’s overall engineering fea- sibility will depend on supporting the fusion re- action, converting the power released into a more usable form of energy, and ensuring operation in a safe and environmentally acceptable man- ner. Developing and building these associated systems and integrating them into a functional whole will require a technological development effort at least as impressive as the scientific challenge of creating and understanding fusion plasmas. The following section describes the systems in a fusion reactor. Since the tokamak confinement Photo credit: Commission of the European Communities concept and the D-T reaction are the most ex- The ASDEX tokamak at the Max-Planck Institute for tensively studied, a tokamak-based reactor fueled Plasma Physics, Garching, Federal Republic of Germany. with D-T is used as an example. However, most of the systems described here would be found, in some form, in reactors based on other con- ducive to attaining reactor-like temperatures and cepts as well. densities. The overall fusion generating station (figure 4- Reactor Design 10) consists of a fusion power core, containing the systems that support and recover energy from Just as an automobile is much more than spark the fusion reaction, and the balance of plant that plugs and cylinders, a fusion reactor will contain converts this energy to electricity using equip-

Figure 4-10.—Systems in a Fusion Electric Generating Station

Rejected heat Fusion power core inside

Balance of plant -

Electric power out to grid

In

Ch. 4.—Fusion Science and Technology “ 73

ment similar to that found in present electricity station. It consists of the plasma chamber, the sur- generating stations. Features that might convert rounding blanket and first wall systems that re- fusion power to electricity more directly in ad- cover the fusion energy and breed tritium fuel, vanced fusion reactors are described in a subse- the magnet coils generating the necessary mag- quent section. netic fields, shields for the magnets, and the fuel- ing, heating, and impurity control systems. Be- fore an acceptable design for a fusion power core Fusion Power Core can be developed, the behavior of fusion plasmas The fusion power core, shown schematically must be understood under all conditions that in figure 4-11, is the heart of a fusion generating might be encountered. Furthermore, significant

Figure 4-11.—Systems in the Fusion Power Core

Electromagnetic radiation

Neutrons

SOURCE. 74 . Starpower: The U.S. and the International Quest for Fusion Energy advances must be made in plasrna technologies, tions. These systems use heat provided by the fu- which confine and maintain the plasma, and nu- sion core to produce steam that drives turbines c/ear technologies, which recover heat from the and generates electricity. The steam is cooled by plasma, breed fuel, and ensure safe operation. passing through the turbines, and the remaining heat in the steam is exhausted through cooling Balance of Plant towers or similar mechanisms. Balance of plant generally describes the systems More advanced systems that convert plasma of a fusion generating station outside of the fu- energy directly into electricity also may be pos- sion power core. In the example shown in fig- sible. Fusion reactors incorporating such systems ure 4-11, the balance-of-plant resembles systems could be made more efficient than those using found in other types of electric generating sta- steam generators and turbines.

FUSION POWER CORE SYSTEMS

The Fusion Plasma sult, ohmic heating is not sufficient to reach ig- nition in many configurations. At the center of a fusion reactor, literally and figuratively, is the fusion plasma. A number of Neutral Beam Heating. –Energetic charged or supporting technology systems create and main- neutral particles can be used to heat fusion tain the plasma conditions required for fusion re- plasmas. However, the same magnetic fields that actions to occur. These technologies confine the prevent the plasma from escaping also prevent plasma, heat and fuel it, remove wastes and im- charged particles on the outside from easily get- purities, and, in some cases, drive electric cur- ting in. Therefore, beams of energetic neutral (un- rents within the plasma. They also recover heat, charged) particles that can cross the field lines breed fuel, and provide shielding. are usually preferred for heating the plasma. Further development of many of these plasma Radiofrequency Heating. -Electromagnetic ra- technologies is required before they will be ca- diation at specific frequencies can heat a plasma pable of producing a reactor-scale plasma. Fur- like a microwave oven heats food. Radiofrequency thermore, each of these supporting systems af- or microwave power beamed into a plasma at fects plasma behavior, and the interactions are the proper frequency is absorbed by particles in incompletely understood. progress in both plasma the plasma. These particles transfer energy to the technology and plasma science is therefore rest of the plasma through collisions. needed before reactor-scale fusion p lasmas can Compression Heating. –Increasing the confin- be created. ing magnetic fields can heat a plasma by com- pressing it. This technique has been used in toka- Heating mak devices and is one reason for studying the field-reversed configuration confinement ap-

Description.–Some heat loss from I a plasma proach. As stated earlier, there is hope that com- is inevitable (see box 4-A), but, with good con- pression may be sufficient to heat an FRC plasma finement, the losses can be made up by external to ignition. heating and/or by fusion self-heating. Different Fusion Self-Heating.–The products of a D-T fu- mechanisms for heating the plasma, illustrated sion reaction are a helium nucleus—an alpha in figure 4-12, are listed below. particle–and a neutron. The neutron, carrying Ohmic Heating. -Like an electric heater, a most of the reaction energy, is electrically un- plasma will heat up when an electrical current charged and escapes from the plasma without re- is passed through it. However, the hotter a acting further. The alpha particle, carrying the rest plasma gets, the better it conducts electricity and of the energy from the fusion reaction, is charged therefore the harder it is to heat further. As a re- and remains trapped within the confining mag-

Ch. 4.—Fusion Science and Technology ● 75

Photo credit: GA Technologies Inc View inside vacuum vessel of the D II I-D tokamak at GA Technologies, San Diego, California. The plasma is contained within this vessel.

netic fields. Hundreds of times hotter than the gigahertz (billions of cycles per second), are un- surrounding plasma, the alpha particle heats der study. Each frequency range involves differ- other plasma particles through collisions. ent technologies for generation and transmission. Status.– Recent system studies show that radio- Issues.—Additional research and development frequency (RF) heating offers significant advan- (R&D) in heating technologies is essential to meet tages over neutral beam heating. Consequently, the needs of future experiments and reactors. Key the U.S. neutral beam research program has been technical issues in RF heating are the develop- reduced while the RF heating program has grown. ment of sufficiently powerful sources of radiofre- Various types of RF heating, using different fre- quency power (tens of megawatts), particularly quencies of radiation from tens of megahertz (mil- at higher frequencies, and the development of lions of cycles per second) to over a hundred launchers or antennas to transmit this power into

the plasma, particularly at lower frequencies. Fueling Resolution of these issues will require technologi- –Any fusion reactor that operates cal development as well as improved understand- Description. in pulses exceeding a few seconds in length must ing of the interaction between radio waves and plasmas. be fueled to replace particles that escape the plasma and, to a lesser extent, those that are con- Since no ignited plasma has yet been produced, sumed by fusion reactions. Firing pellets of fro- the effects of fusion self-heating on plasma con- zen deuterium and tritium into the plasma cur- finement and other plasma properties are not rently appears to be the best approach for fueling. experimentally known. Confinement could de- Both pneumatic (compressed gas) and centrifugal grade, just as it does with other forms of auxiliary (sling) injectors have been used (figure 4-13). heating. Although self-heating can be simulated Neutral beam fueling has been used in experi- in some ways in non-ignited plasmas, its effects ments, but fueling reactors in this way would take can be fully studied only upon reaching high excessive amounts of power. energy gain or ignition. The ignition milestone, therefore, is crucial to the fusion program, and Status.–Pellets up to 4 millimeters in diameter understanding the behavior of ignited plasmas is have been fired into experimental plasmas at one of the program’s highest scientific priorities. speeds of up to 2 kilometers per second and at

Ch. 4.—Fusion Science and Technology . 77

Figure 4-12.—Plasma Heating Mechanisms

Radiofrequency heating

hydrogen atoms Magnetic deflector Hydrogen //ion Neutralizer source Neutral injection Accelerator heating

SOURCE. Oak Ridge National Laboratory

repetition rates ofs to 40 pellets per second. U.S. development of pellet fueling technology, cen- tered at Oak Ridge National Laboratory, is well ahead of fueling technology development else- where in the world. By building state-of-the-art pellet injectors for use on foreign experiments, the United States is able in return to gain access to foreign experimental facilities. Issues.—Reactor-scale plasmas will be denser, hotter, and perhaps bigger than the plasmas made o date in fusion experiments; moreover, reactor pIasmas will contain energetic alpha particles. All these factors will make it much more difficult for pellets to penetrate reactor plasmas than plasmas made in present-day facilities. Penetration to the center of the plasma, most desirable from a theo- Photo credit: Princeton Plasma Physics Laboratory retical point of view, probably will be extremely Princeton Large Torus at PPPL, showing waveguides difficult in reactor plasmas. Experiments are now for the RF heating system. underway to understand how deeply a pellet

78 ● Starpower: The U.S. and the International Quest for Fusion Energy

Figure 4-13.—Centrifugal Pellet Injection

SOURCE: Oak Ridge National Laboratory must penetrate. Tokamaks, for example, appear difficult to produce, but they may disturb the to have a mechanism, not yet understood, that plasma too much; additional work needs to be transports fuel to the center of the plasma. Fuel done to determine how fuel pellets affect plasma might be brought into the center more effectively behavior, If larger pellets cannot be used, higher in some of the alternate confinement concepts injection speed will be required, which is tech- with turbulent plasmas, such as the reversed-field nologically much more difficult. Improving pres- pinch or the spheromak. ent techniques is unlikely to increase injection speeds by more than about a factor of 2. New If deeper penetration is required than can now techniques capable of producing much higher in- be attained, either larger pellets or higher injec- jection speeds are being investigated, but the tion speeds will be needed. Larger pellets are not pellets themselves may not survive injection at Ch. 4.—Fusion Science and Technology ● 79 these speeds due to fundamental limitations in “pushing” directly on electrons in the plasma or their mechanical properties.7 by selectively heating particles traveling in one direction. Experiments have confirmed the the- Current Drive ory of radiofrequency current drive and have suc- ceeded in sustaining tokamak current pulses for Description.–Several confinement concepts, several seconds. including the tokamak, require generation of an electric current inside the plasma. In most present Some other confinement concepts, such as the experiments, this current is generated by a trans- reversed-field pinch or the spheromak, can gen- former. In a transformer, varying the electric cur- erate plasma currents with small, periodic varia- rent in one coil of wire generates a magnetic field tions in the external magnetic fields. Such current- that changes with time. This field passes through drive technologies do not involve complex ex- a nearby second coil of wire—or in this case the ternal systems. conducting plasma—and generates an electric Issues.–The principal issues involving steady- current in that coil or plasma. Varying the mag- state current drive are cost and efficiency, espe- netic field is essential; a constant magnetic field cially under reactor conditions. I n particular, the cannot generate current. radiofrequency technique becomes less efficient in tokamak experiments, a coil located in the as the plasma density increases. This inefficiency “doughnut hole” in the center of the plasma could pose problems because reactors will prob- chamber serves as one coil of the transformer. ably operate at higher densities to maximize gen- Passing a steadily increasing current through this erated power. At this time, it is not known whether coil creates an increasing magnetic field, which the efficiency of continuous current drive can generates current in the plasma. When the cur- be increased to the point where it could replace rent in the first coil levels off at its maximum the pulsed transformers now used in tokamaks. value, its magnetic field becomes constant, and However, radiofrequency current drive might the current in the plasma peaks and then starts also be used to augment the puIsed transformer to decay. If the fusion plasma requires a plasma by starting up the plasma current in a period of current, its pulse length is limited by the maxi- low-density operation. Once the plasma current mum magnetic field of the first coil and the length was started, the density could be raised and the of time taken for the plasma current to decay.8 transformer used to sustain the current. The ra- diofrequency current drive together with the Status.–Techniques are now being studied for transformer would be able to generate longer last- generating continuous plasma currents, rather ing pulses than the transformer alone. than pulsed ones, because steady-state reactors are preferable to ones that operate i n puIses. g In- Reaction Product and Impurity Control jecting radiofrequency power or neutral beams into the plasma might be able to generate such Description .–Alpha particles, which build up steady-state currents in tokamaks. The injected as reaction products in steady-state or very long- power or beams generate currents either by pulse fusion reactors, will have to be removed so that they do not lessen the output power by 7Argon ne National Laboratory, Fusion Power Program, Techni- diluting the fuel and increasing energy loss by ra- ca/ Planning Activity: Find/ Report, commissioned by the U.S. De- diation, Devices that collect ions at the plasma partment of Energy, Office of Fusion Energy, AN L/FPP-87-l, janu- ary 1987, p. 189. edge can be used to remove alpha particles from 81n tokamaks, this decay time can be thousands of seconds. For the plasma. Alpha particles, when combined with confinement concepts with plasmas that are more resistive to elec- electrons that are also collected at the plasma tric currents, the decay time is shorter, on the order of hundreds of seconds. Operating for periods of time longer than the decay edge, form helium gas that can be harmIessly re- time requires non-transformer current drive mechanisms. leased. Unburned fuel ions also will be collected; 9 Components in pulsed systems undergo periodic stresses not these will be converted to deuterium and tritium experienced in steady-state systems. Pulsed reactors also require some form of energy storage to ellminate variations in their elec- gas, which will have to be separated from the trical output. helium and reinfected into the plasma. 80 ● Starpower: The U.S. and the International Quest for Fusion Energy

The same devices that collect ions at the plasma issues such as the conditioning and cleaning of edge help prevent impurities from entering the surfaces in contact with the plasma, the erosion plasma. Even small amounts of impurities can of these surfaces and redeposition of their mate- cool the plasma by greatly accelerating the rate rials elsewhere in the plasma chamber, the ef- at which energy is radiated away. fects of high heat loads, the development of cool- ing systems, and the degree and effects of tritium permeation. Status.–Two types of devices are being con- sidered for these tasks: pumped limiters and Burn Control diverter-s. A limiter is a block of heat-resistant ma- terial that, when placed inside the reaction cham- Description. —When a fusion reactor plasma ber, defines the plasma boundary by intercept- is ignited, it provides its own heat and no longer ing particles at the plasma edge. A variant, the depends on external heating. Two opposing ten- pumped limiter, combines a limiter with a vacuum dencies make it difficult to determine how sta- pump to remove the material collected by the ble, or self-regulating, an ignited plasma will be. limiter. A divertor generates a particular magnetic An ignited plasma may be inherently unstable field configuration in which ions diffusing out of due to the strong temperature dependence of the the fusion plasma, as well as those knocked out fusion reaction rate. If, for whatever reason, a hot of the vessel walls and drifting towards the spot forms in the plasma, the fusion reaction rate plasma, are diverted away and collected by ex- there will go up. As a result, more fusion power ternal plates. will be generated in that area, heating it further and compounding the original problem. Both limiters and diverters are in direct con- tact with the plasma edge. Although temperatures If the plasma particles mix with sufficient speed, at the edge are far below the 100-million-degree hot spots that form will not persist long enough C temperatures found in the plasma center, these to grow. If, on the other hand, the mixing is slow, components will nevertheless get very hot. All this thermal instability might make it very diffi- the energy injected into or produced by the cult to maintain a steady reaction. Formation and plasma that is not carried away by neutrons or growth of hot spots could cause output power radiated away as electromagnetic energy is even- levels to fluctuate considerably, and in the worst tually deposited on the limiter or divertor plates case these hot spots could grow until much of by electrons and ions. Therefore, these devices the fuel present in the reaction chamber was con- must withstand high heat loads under energetic sumed. The amount of fuel would not be large— ion and neutral particle bombardment while be- at most a few seconds’ worth-making this proc- ing exposed to intense neutron radiation. In a fu- ess more of an operational problem than a safety sion environment, they will become radioactive one. The reactor would have to be designed so due to neutron-induced reactions and, to a much that it could not be damaged, and its contents lesser extent, to permeation with tritium. Their not be released, by the maximum amount of reliability must be high since they will be located energy that could be produced in this way. deep within the reactor, inside the vacuum ves- Countering this possible instability is a self- sel, where maintenance will be difficult. regulating mechanism that limits the maximum issues.–A key issue for reaction product and attainable value of the beta parameter. Any in- impurity control will be the choice between stability that heated the plasma would increase pumped limiters and diverters. The devices not the plasma beta, which is proportional to tem- only have different efficiencies but have differ- perature. However, since the power generated ent effects on plasma confinement. Limiters are by a fusion reactor is proportional to beta squared, simpler, but diverters may have operational ad- a reactor would probably already be operating vantages. 10 More R&D is necessary to investigate

IOThe H.mode of tokamak operation, in which confinement prop- ters. This mode, now thought to depend on processes occurring erties are significantly improved, is seen in tokamaks with diver- at the plasma edge, may be difficult to reproduce with Iim iters. Ch. 4.—Fusion Science and Technology ● 81

at the highest value of beta consistent with good ated by fusion reactions and transfer it to other performance. Further increases in beta would de- parts of the facility to generate electricity. De- grade plasma confinement and increase energy pending on plant design and materials selection, loss. These increased losses would cool the these cooling systems also might be needed to plasma back down, counteracting the initial in- remove afterheat from the radioactive decay of stabiIity. materials in the blanket after a plant shutdown. In addition, the tritium fuel required by the re- These beta-limiting processes could maintain actor is produced, or “bred,” in the blanket. Fur- a steady reactor power level. The plasma would thermore, the blanket must support itself and any tend to operate just under the limiting beta value, other structures that are mounted on it. and, by adjusting the magnetic field or the plasma density, the power level corresponding to the The safety of the plant will be greatly influenced limiting beta value could be controlled. by the blanket breeder, coolant, and other sub- systems. Since the blanket will perform multiple Status and Issues.– It is impossible, without functions, its development will require an in- creating and studying an ignited plasma, to de- tegrated R&D program. This program must have termine how a plasma will behave in the face of two primary aspects: it must develop the capa- the two opposing tendencies described above. bility to predict the behavior of blanket compo- Neither the causes of beta-limiting processes nor nents and systems under actual reactor usage, their effects on the plasma are fully understood and it must develop technologies that can pro- in general. More research is necessary before the duce fuel and recover energy in the blanket while ability of these processes to stabilize a fusion maintaining attractive economic, safety, and envi- plasma can be determined. ronmental features. The processes that control the reaction rate and Intense irradiation by neutrons produced in the burn stability of an ignited plasma are probably fusion plasma will make blanket components the most device-dependent and least understood radioactive, with the level of induced radioactivity of any aspect of burning plasma behavior. 11 Even depending on the materials with which these for tokamaks, the properties that determine sta- components are made. Tritium bred within the bility have been studied only under conditions blanket will add to the blanket’s total radioactive well short of ignition; still less is known about the inventory. As the largest repository of radioactive properties of other confinement concepts. If these materials in a fusion plant, the blanket will be the issues are indeed concept-specific, only limited 12 focus of environmental and safety concerns. information from a burning plasma experiment using one confinement concept can be used to The discussion below focuses on blankets that predict the behavior of another. would be used in fusion reactors that generate electricity. Reactors used for other purposes, The Fusion Blanket and First Wall some of which are discussed in appendix A, would have different blanket designs. The region immediately surrounding the fusion plasma in a reactor is called the blanket; the part Description of the blanket immediately facing the plasma is called the first wall. In some designs, the first wall Energy Conversion.–The first wall is heated is a separate structure; most often, however, the by radiation from the plasma as well as by energy first wall refers to the front portion of the blan- carried by particles leaking out of the plasma. ket that may contain special cooling channels. Energetic neutrons produced in the plasma pene- trate the blanket, where they slow down and con- The blanket serves several functions. Cooling vert their kinetic energy into heat. coolant cir- systems in the blanket remove the heat gener- cuIating within the blanket and first wall transfers this heat to other areas of the plant, where it is

I I Argonne Nat I. na I La t)orato ry, Technic,;/ P/arming ActI\It)’: F;- na/ Report, op. cit., p. 155. I J plant safety characterlst ICS are d ISCU JSed tu rther I n ch. J 82 . Starpower: The U.S.. and the International Quest for Fusion Energy

used to generate electricity. The coolant also pre- vents blanket and first wall components from overheating during reactor operation; depend- ing on plant design, coolant also may be needed to prevent overheating after plant shutdown, whether scheduled or emergency. Depending on the level of radioactivity within the blanket and coolant, secondary heat exchangers like those now used in nuclear fission plants may be re- quired to isolate the coolant. Fusion neutrons slow down by colliding with the nuclei of blanket materials, transferring energy to the blanket in the process. Additional heat is also generated in reactions that occur when the neutrons are captured by materials in the blan- ket. Depending on their energy, the neutrons travel up to several centimeters between colli- sions. Collisions change the neutrons’ directions, and a blanket thickness of from one-half meter to one meter is enough to capture most of the neutron energy. Tritium Breeding.–Through reactions with fu- sion neutrons, the nuclei in the blanket can be changed into other nuclei that are either stable or radioactive. In particular, if a fusion neutron is captured by a lithium nucleus, it will induce a reaction that produces tritium (see box 4-B).13 Therefore, the presence of lithium in the blan- ket is necessary for tritium breeding. The number of tritium nuclei produced in the fusion blanket per tritium nucleus consumed in the fusion plasma, called the breeding ratio, must be at least 1 for the reactor to be self-sufficient in tritium supply. Accounting for losses and im- reactive and may pose safety problems i n fusion perfections in the blanket, as well as uncertain- reactors. Liquid lithium’s reactivity can be les- ties in the data used to calculate tritium breed- sened by alloying it with molten lead, but the ing rates, this ratio probably should be in the addition of lead substantially increases the pro- range of 1.1 to 1.2. duction of undesired radioactive materials in the blanket. Lithium can be contained in the blanket in ei- ther solid or liquid form. Lithium metal has a low Liquid metal coolants can be avoided by sep- melting point (186 o C) and excellent heat trans- arating the function of cooling the blanket from fer properties, making it attractive as a coolant that of breeding tritium. A non-lithium-containing in addition to its use in breeding tritium. How- fluid can be used as the coolant, and solid lith- ever, in its pure form, liquid lithium can be highly ium-containing compounds can be used to pro- duce tritium. However, solid lithium compounds 1 JLithium is a reactive metal that does not occur in its pure form contain other elements, such as oxygen and alu- in nature. However, chemical compounds containing lithium are found in many minerals and in the waters of many mineral springs. minum, that would capture some of the fusion Fuel resources for fusion are discussed in ch. 5. neutrons and lower the breeding ratio. To com- Ch. 4.—Fusion Science and Technology ● 83

pensate for the lost neutrons, substances called Other fusion neutrons will penetrate the blanket neutron rnultipliers can be added to the blanket. to make reactor structures outside the blanket

Neutron multipliers convert one very fast fusion radioactive. Since the degree of radioactivity gen- neutron to two or more slower neutrons by erated within the reactor structure strongly de- means of a nuclear reaction. pends on the reactor’s composition, develop- ment and use of /ow-activation materials that do Recovering the tritium from solid breeder ma- not generate long-lived radioactive products terials is more difficult than from liquid breeders. under neutron bombardment will greatly lessen When tritium is bred in a liquid coolant, the cool- induced radioactivity, ant carries the tritium directly outside the blan- ket where it can be extracted. Tritium produced Due to radiation damage, blanket and first wall in solid lithium-containing compounds, on the components in a fusion reactor wil I require peri- other hand, must first diffuse out of those com- odic replacement. After their removal, the old pounds before it can be collected and flushed components will constitute a source of radio- out of the blanket by a circulating stream of active waste. ’ 5 helium gas. Impact on Fusion Reactor Design .–Although Blanket Structure.—The heat loads, neutron the blanket and first wall components themselves fluxes, and radiation levels found in the blanket may not represent a large fraction of the cost of place stringent requirements on the materials a fusion reactor, blanket design has a substan- with which the blanket is made. Conditions are tial influence on total reactor cost. The blanket most severe at the first wall, which is bombarded thickness (along with that of the shield, described by neutron and electromagnetic radiation from below) determines the size and cost of the mag- the plasma, by neutral particles, and by plasma nets, which are substantially more expensive than electrons and ions that escape confinement. First the blanket. The blanket coolant temperature de- wall issues are similar to many of the issues asso- termines the overall efficiency with which the ciated with limiters and diverters (discussed pre- plant converts fusion power into electricity, di- viously in the section on “The Fusion Plasma, ” rectly affecting the cost of electricity. The selec- under the heading “Reaction Products and im- tion of materials in the blanket determines the purity Control”), which undergo even higher heat amount of long-term radioactive waste and the and particle fluxes than the first wall. amount of heat produced by radioactive decay in the blanket after plant shutdown; both the Neutron irradiation introduces two major prob- waste and the heat affect the reactor’s environ- lems in the blanket materials. First, irradiation can mental and safety aspects. Finally, the ability of lead to brittleness, swelling, and deformation of the blanket materials to withstand heat loads and the reactor structural materials. During the serv- neutron irradiation levels determines the amount ice lives of first wall and blanket components, of fusion power that can be generated in a plant each atom in those components will be displaced of a given physical size, which has a significant several hundred times by collisions with fusion effect on reactor size and cost and on its behavior neutrons. The amount of radiation damage that during accidents. the blanket materials can withstand determines component lifetimes and also places an upper Status and Issues limit on reactor power for a blanket of a given size. A wide variety of designs have been proposed for the blanket and first wall. However, since the Second, neutron irradiation makes the blanket radioactive. Not all the neutrons penetrating the 1 sAccord ! ng to Argonne National Laboratory, Technical P/.lnn/nL~ blanket will be captured in lithium to breed Acti\ity: F/na/ Repor?, op. cit., p. 283, the amount ot long-l{~ed tritium, Some will be absorbed by other blanket (more than 5 years) radioactive waste depends primarily on the materials, making those materials radioactive. amounts of copper, molybcien u m, nitrogen, niobium, and nickel used In the blanket. Radioacti\lty levels, radioactive wastes from I J Beryl I I u m ~ ncj lead are common Iy used I n rea~to r stud I es Js fusion reactors, and Iow-activation materials are discussed tu rther neutron mu Itlpliers, In ch. 5. 84 . Starpower: The U.S. and the International Quest for Fusion Energy fusion research program has concentrated to date for high radiation areas inside the reactor. Since primarily on plasma science issues, relatively lit- a typical effect of radiation damage on insulators tle experimental work has been done on blan- is to increase electrical conductivity, developing ket design or fusion nuclear technologies in gen- materials that will remain insulators under high eral. As the program moves from establishing radiation fluxes is a challenging task. scientific feasibility to demonstrating engineering Special Materials.–Other materials require- feasibility, engineering issues will become much ments for a fusion reactor may include special more important. materials to coat plasma-facing surfaces to mini- Tritium Self-Sufficiency .-Engineering designs mize their effect on the plasma, coatings or clad- must be developed to produce tritium at a rate dings used to form barriers to contain tritium, and equal to the rate of consumption in the plasma advanced superconducting magnet materials (de- plus an additional margin, The extra tritium, 10 scribed in the section on magnets, below). Many to 20 percent of the amount consumed in the of the specific requirements for these materials plasma, is needed to compensate for losses due have not yet been determined. to radioactive decay and to provide the initial in- Compatibility.–Certain combinations of ma- ventory to start up new reactors. Improvements terials, each suitable for a particular task, may in in calculating neutron flow through the reactor combination prove unacceptable in a reactor de- structure and in collecting additional basic nu- sign. For example, liquid lithium reacts violently clear data such as reaction rates are necessary with water, so a liquid lithium-cooled blanket de- to develop adequate engineering designs. Exper- sign would probably prohibit use of water as an imental verification of the calculation methods additional coolant. and data is also required to demonstrate tritium self-sufficiency. Tritium Permeation and Recovery.–Once pro- duced inside the blanket, tritium must be recov- Structural Materials.–Structural materials in ered and removed. However, tritium will perme- the first wall and deeper in the blanket must be ate many materials that are continuously exposed developed that can withstand neutron-induced to it. Its interactions with blanket materials un- effects such as swelling, brittleness, and defor- der the conditions inside a fusion reactor will mation. Stainless steel alloys already have been have to be understood. In particular, tritium may identified that appear to show adequate perform- be difficult to collect from solid breeder materials. ance under neutron fluxes at the low end of those expected in a reactor. However, these materials Liquid Metal Flow.—Liquid metal coolants in produce more radioactive products than may be a fusion reactor will be subject to strong magnetic desirable for commercial reactors. Developing fields created both by the plasma and by exter- low-activation materials that also have acceptable nal magnets. Liquid metals are conductors of physical properties under irradiation remains a electricity; when electrical conductors move significant challenge. The task will require further through magnetic fields, voltages and currents are basic research in materials science as well as generated.16 The currents induced in the cool- progress in materials technology. ant, in turn, are subject to forces from the mag- netic field that oppose the motion of the cool- Non-Structural Blanket Materials.–The tritium- ant, increasing coolant pressure and adding to breeding properties of various lithium-containing the power required for pumping. materials must be studied and compared. The choice between solid and liquid breeder mate- rials, in particular, will greatly affect overall blan- The Shield ket design. in addition to lithium, other materi- Description and Status als may be required in the blanket such as neutron multipliers and moderators (which slow Since many neutrons will penetrate all the way down neutrons to make them more easily ab- through the blanket during fusion reactor oper- sorbed in the blanket). Insulators, or materials that do not conduct electricity, also maybe required IGThis process IS the basis of electrical generators. Ch. 4.—Fusion Science and Technology ● 85 ation, a shield may be required between the blan- The most important choice concerning design ket and the magnet coils.17 The shield may be of the magnets is whether they will be made of composed of materials such as steel and water, superconducting materials or of conventional will probably contain a circulating coolant, and conductors such as copper. Copper is an excel- would have a thickness from tens of centimeters lent conductor of electricity but nevertheless has to over a meter. The shield would provide extra sufficient resistance to electric currents that a protection to the magnets and could reflect es- great deal of power is wasted as heat when the caping neutrons back into the blanket to improve magnet is running. This heat must be removed the efficiency of tritium breeding. Additional by cooling systems. Superconducting coils lose shielding wouId probably surround the entire re- all resistance to electricity when cooled suffi- actor core, perhaps in the form of thick walls for ciently; below a temperature called the critical the enclosing building. temperature, their magnetic fields can be sus- tained without any additional power. However, Most existing fusion experiments have not been power is required to establish the fields initially, designed to use tritium and are incapable of gen- and a small amount of refrigeration power is re- erating significant amounts of fusion power. Con- quired to keep superconducting magnets at their sequently, shielding has generally not been an operating temperature. No heat is generated in- important issue for the research program. It has, side a superconducting magnet, but heat that however, been a factor in the design of devices leaks in from the outside must be removed. such as TFTR and the Joint European Torus that are intended to use tritium. As future machines Although recent discoveries could revolution- are designed that will generate appreciable amounts ize the field (see “issues” section below), all su- of fusion power, shielding will become more im- perconducting materials that have so far been portant. used in large magnets have critical temperatures within about 20° K of absolute zero. The only Issues substance that does not freeze solid at these tem- peratures is helium, and the only way to cool su- The intensity of the incident neutron radiation; perconducting magnets to these temperatures is the size, shape, and effectiveness of the shield; to circulate liquid helium through them. Use of and the permissible levels of neutron irradiation liquid helium makes superconducting magnets penetrating the shield must all be determined to more complicated and expensive to build than evaluate shielding requirements. As improved copper magnets; superconducting magnets also magnet materials are developed that are less sen- require thicker shields. However, superconduct- sitive to neutron radiation, shielding requirements ing magnets require much less electricity to run, for plant components will lessen. However, pro- substantially lowering their operating costs. tection of plant personnel alone will require sub- stantial shielding. Conceptual design studies typically have shown The Magnets that the operational savings from using supercon- ducting magnets in commercial fusion reactors Description would more than compensate for their higher ini- The external confining magnetic fields in a fu- tial cost. However, there may be exceptions, sion reactor are generated by large electric cur- especially for confinement concepts with higher rents flowing through magnet coils surrounding beta values that are able to confine fusion plasmas the plasma. These magnets must withstand tremen- at lower magnetic field strengths. At lower field dous mechanical forces, strengths, copper magnets, which do not require as much shielding as superconducting magnets, can be made to fit more closely around the .. —-—-—. . plasma chamber. The resultant reduction in size I Plf th magnets are superconducting, the shield will be requl red. e of the magnet/shield combination might reduce If they are made of copper, the shield may not be necessary. How- ever, without a shield, the coils would require periodic replace- its cost enough to outweigh the operational in- ment. See the following section on magnets. efficiencies of copper magnets. 86 . Starpower: The U.S. and the International Quest for Fusion Energy

Whereas the magnets in future fusion reactors worked very well and have exceeded their origi- will operate for long pulses, if not continuously, nal design specifications. 20 magnets in present-day fusion experimental fa- cilities generally operate only for several seconds Issues at a time. For pulses this short, the cost of elec- Recent discovery of new superconducting ma- tricity is less of a factor in determining magnet terials with critical temperatures far above those design, making the simpler construction of cop- of previously known materials, and possibly with per magnets preferable in most cases. A notable the capability to reach very high magnetic field exception is the MFTF-B device at Lawrence Liv- strengths, will have a profound impact on a great ermore National Laboratory, which was built with many fields, including fusion. Materials have been superconducting magnets because copper coils identified with critical temperatures higher than would have been prohibitively expensive to oper- the 77 Kelvin (–196° C) threshold that would ate even for 30-second puIses. permit use of liquid nitrogen as a refrigerant. Liq- uid nitrogen is cheaper and easier to handle than Status liquid helium, and its use could reduce the cost The first fusion device built with superconduct- and complexity of superconducting magnets. ing magnets was the Soviet T-7 tokamak, com- However, the discovery of these “high-temper- pleted 7 years before any Western fusion device ature’ using superconducting magnets. The Soviets are superconductors does not necessarily mean that they can soon be utilized in fusion ap- now building T-1 5, a much larger superconduct- plications. Little is known about the physical ing tokamak. Difficulties with the T-1 5 magnets processes underlying superconductivity in these have been among the reasons that the project’s materials, and they present great engineering completion has been delayed for several years; challenges. They are difficult to fabricate into however, these difficulties apparently have been magnet coils, and they may not be able to with- resolved. The Tore Supra tokamak being built in stand the forces exerted in fusion magnets. Al- France will also use superconducting magnets though their current-carrying capability is improv- and will probably exceed the parameters of T- 15.18 In the United States, MFTF-B was completed ing, they may not be able to carry high enough currents under high magnetic fields to be useful in 1986; its superconducting magnets have been in large-scale magnets. Moreover, their response successfuIly tested at their operating conditions. to neutron irradiation is not known. If these ma- Overall, DOE considers U.S. magnet develop- terials are highly susceptible to radiation damage, ment to be comparable to that in Europe and Ja- their use in fusion magnets could be difficult. pan and ahead of that in the Soviet Union.19 Conversely, if they proved more resistant to ra- Generally, magnet development has been asso- diation effects than previous superconducting ciated with individual fusion confinement exper- materials, thinner shields and correspondingly iments rather than with facilities dedicated specifi- smaller magnets could be used. cally to magnet development. A major exception Further research is required to see whether the is the Large Coil Task, an international program new superconducting materials can be used in to build and test superconducting magnets. Mag- practical applications. The great economic advan- nets developed through the Large Coil Task have tage that they would have in numerous applica- tions ensures that much of this research will be 161 nformation on Soviet tokamak development IS from an oral undertaken independently of the fusion program. presentation on “Assessment of Soviet Magnetic Fusion Research” However, fusion does have particular require- by Ronald C. Davidson, Director of the Plasma Fusion Center, Mas- sachusetts Institute of Technology, to the Magnetic Fusion Advi- ments for large, high-field magnets that may not sory Committee, Princeton, Nj, May 19, 1987. otherwise be investigated. l~lnternational comparisons are from a presentation by R.J. Dowl- ing, Director of the Division of Development and Technology, Of- fice of Fusion Energy, U.S. Department of Energy, to the Energy Research Advisory Board, Sub-Panel on Magnetic Fusion, Wash- ington, DC, May 29, 1986. zOThe Large Coi I Task is discussed further in ch. 7.

Ch. 4.—Fusion Science and Technology ● 87

- . 3

88 . Starpower: The U.S. and the International Quest for Fusion Energy

tritium in a fusion reactor once it has been produced and separated. The task of extracting tritium from a blanket under reactor conditions while at the same time generating electric power with high efficiency has yet to be done.

Status To acquire experience with tritium handling for fusion applications, DOE has built and is operat- ing the Tritium Systems Test Assembly (TSTA) at Los Alamos National Laboratory. A prototype of the tritium processing and handling facilities needed for a full-scale fusion reactor, TSTA in- cludes plant safety equipment such as a room atmosphere detritiation system. TSTA operators have developed system maintenance procedures that minimize or eliminate tritium release. This system, however, does not duplicate the produc- tion or extraction of tritium from a fusion blanket.

Issues Specific issues involved with tritium process- ing include monitoring, accountability, and safety. Being radioactive, tritium cannot be allowed to Issues for superconducting fusion magnet ma- diffuse out of the reactor structure. if tritium col- terials include further development and investi- lects on inaccessible surfaces within the reactor, gation of the new high-temperature materials. If it cannot be completely recovered, and it will these new materials prove unacceptable for fu- make those surfaces radioactive. Developing trit- sion, improvements in the strength, workability, ium processing systems will require additional re- and maximum current capacity of the previously search in measuring basic tritium properties such known superconductors will be important. Issues as diffusivity, volubility, and oxidation chemistry. for copper magnets include fully exploiting cop- Safety needs include developing and maintain- per’s strength and developing joints (needed to ing the capability to contain and recover tritium assemble and maintain the magnet) that can carry from air and from water coolants (if any) in the large electric currents. event of tritium contamination.

Fuel Processing Remote Maintenance Description Due to their inventory of radioactive tritium Tritium fuel contained in the exhaust from the and the activation of their structural components, plasma, or generated in the reactor blanket, must the interior of all subsequent fusion experiments be extracted, purified, and supplied back to the that burn D-T will become too radioactive for fueling systems for injection into the plasma. Con- hands-on maintenance. Therefore, remote main- siderable experience has been developed in han- tenance is a key issue not only for future power dling tritium, particularly within the nuclear weap- reactors, but also for near-term D-T experiments. ons program, making tritium technology more Nearly all aspects of the research program, from highly developed than many of the other nuclear design of experiments to operation and mainte- technologies required for fusion. However, this nance to decommissioning, will be affected by experience is applicable primarily to handling the the need for remote maintenance.

Ch. 4.–Fusion Science and Technology “ 89

Photo credit Oak Ridge National Laboratory

The Swiss superconducting magnet coil being installed in the International Fusion Superconducting Magnet Test Facility (Large Coil Task) at Oak Ridge National Laboratory.

The fusion program at present is relying on tenance requirements for fusion facilities will in- activities outside the fusion community for gen- clude transporters able to move heavy loads (over eral development of remote maintenance equip- 100 tons) with precision alignment; manipulators ment. Much work in remote manipulation and made of nonmagnetic material that can operate remote maintenance has been done, but some under high vacuum conditions; and rapid and applications are likely to be unique to fusion and precise remote cutting, welding, and leak detec- will require special development. Remote main- tion equipment. The first challenge in this field

90 “ Starpower: The U.S. and the International Quest for Fusion Energy

Photo credit: Los Alamos National Laboratory The Tritium Systems Test Assembly at Los Alamos National Laboratory will be identification of the remote maintenance quently, needs for test facilities and reactors must requirements for near-term facilities and the de- be identified and assessed. velopment of any necessary equipment. Subse-

ADVANCED FUEL AND ENERGY CONVERSION CONCEPTS

Even though the fusion power core systems de- Advanced Fuels scribed in the previous section have not yet been developed, researchers already are designing re- The fusion power core described in the previ- actors using more advanced concepts. Improve- ous section uses D-T fuel because it is by far the ments described below are not mere refinements most reactive of all potential fusion fuels. This of the systems already described; they are qualita- reactivity can be increased still further by align- tively new features that may be much more at- ing the internal spins of the deuterium and tritium tractive. In general, these improvements involve nuclei, a technique known as spin polarization. use of either advanced fuels or advanced meth- If the spins can be aligned initially, the magnetic ods of converting fusion energy into useful forms. field of the fusion reactor will tend to keep them Ch. 4.—Fusion Science and Technology ● 91

in alignment. Therefore, research is ongoing at ● They might permit the use of more efficient Princeton Plasma Physics Laboratory to develop methods to generate electricity from fusion intense sources of spin-polarized fuel. energy. I n advanced fuel fusion reactions, more energy is released i n the form of ener- The principal disadvantage of D-T fuel is that getic charged particles, such as protons or the D-T reaction produces energetic neutrons that alpha particles, than is the case in the D-T cause radiation damage and induce radioactiv- reaction. Therefore, these advanced fuels ity in reactor structures. Moreover, reactors using may be amenable to various techniques that D-T must breed their own tritium, substantially generate electricity directly from the fusion adding to reactor complexity and radioactivity plasma or from plasma-generated radiation levels. For these reasons, the possibility of using without having to first convert the energy other fuels in fusion reactors is being investigated. into heat. (See the following section on “Ad- Fuels other than D-T require higher tempera- vanced Energy Conversion.”) tures and Lawson confinement parameters to reach ignition and higher beta values to perform Table 4-8 presents five fusion fuel cycles, in- economically. Achieving these parameters will re- cluding the “baseline” D-T cycle and four pos- quire stronger magnetic fields, higher plasma cur- sibilities for advanced fuel cycles. Of the advanced 3 rents, and substantial improvements in other cycles, the D- He cycle is currently drawing the plasma technologies beyond those needed to most attention within the fusion community. The reach ignition with D-T fuel—a task that in itself primary reaction produces no neutrons, and neu- has not yet been accomplished. However, re- trons resulting from corollary D-D reactions can actions that use advanced fuels would have a be minimized by using a mixture consisting mostly 21 number of advantages: of 3He or by using spin-polarization. c They wouId require little to no tritium, re- ZI Deuterium, being relatively scarce i n a 3He -rich mixture, would be much more likely to react with a 3He nucleus than with another ducing or eliminating the need for the blan- deuterium nucleus, making D-D reactions relatively rare. However, ket to breed tritium and permitting a much one consequence of this mode ot operation, in addition to mini- wider range of blanket designs. Tritium in- mizing neutron generation, wou Id be the lessening of output power since most of the 3He nuclei would be unable to find D nuclel with ventories would be smaller and the conse- which to react. Increasing the ratio of D to ‘He to more nearly equal quent radioactivity levels would be lower. proportions, theretore, would increase both the output power and ● They would generate fewer and lower energy the neutron generation. Spin polarization suppresses the D-D reaction rate because, un- neutrons, alleviating radiation damage and like the D-T reaction, two deuterium nuclel whose spins are allgned minimizing radioactive wastes. are Jess likely to react with each other,

Table 4-8.—Fusion Fuel Cyclesa 92 ● Starpower: The U.S. and the International Quest for Fusion Energy

However, the D-3He reaction is much more dif- into electricity, with the remainder discharged as ficult to start than the D-T reaction. The minimum waste heat. This efficiency, roughly the same as temperature required to ignite D-3He is several that of fossil fuel and nuclear fission generating times higher than that needed for D-T; the mini- stations, is determined primarily by the process mum confinement parameter is about 10 times of generating electricity from the energy in the higher. Given that the requirements for igniting steam. Efficiency could be raised if advanced, D-T have not yet been experimentally achieved, high temperature materials in the blanket and first attaining conditions sufficient to ignite D-3He is wall of a fusion reactor permitted higher coolant considerably farther off. On top of its technologi- temperatures to be used. 3 cal requirements, H e is scarce. It is an isotope If the intermediate step of heating steam could of helium with one fewer neutron than natural be bypassed, a higher percentage of the energy helium (4He), and it occurs on earth only as the released in fusion reactions couId be converted end-product of tritium decay. The only way to into electricity. Several techniques to integrate collect 3He is to make tritium and wait for it to generation of electricity directly into the fusion decay or to breed 3He as the product of another power core have been conceived. One of these, advanced fuel fusion reaction, the D-D reaction. applicable to D-T reactors as well as to advanced Due to the scarcity of 3He, the D-3He reaction fuel reactors, would convert energy carried off has been considered primarily an academic curi- by escaping charged particles directly to electri- osity until recently. city by collecting the particles on plates. This Today, a resurgence of excitement about 3He technique is most applicable to open confine- comes with the discovery that it is found in sub- ment concepts, in which charged particles can stantial amounts in the uppermost layers of soil be allowed to escape along magnetic field lines. on the moon. Analysis of moon rocks brought Other techniques, which can work with closed back by the Apollo missions shows that 3He, confinement concepts, require plasma temper- which is constantly emitted by the sun and car- atures significantly higher than the 10- to 15- ried by the solar wind, is deposited and retained kiloelectron-volt D-T ignition temperatures. Very in the lunar surface. In principle, a rocket with hot plasmas radiate more energy away in the form the cargo volume of the space shuttle could carry of microwave radiation than cooler plasmas do,23 back enough liquid 3He to generate all the elec- and it appears that this radiation could be cap- tricity now used in the United States in one year. tured at the first wall or in the blanket and con- Of course, the technology to recover 3He from verted directly into electricity. These “direct con- the moon would not be available for decades, version” techniques would be better suited to and the energy and capital investment required advanced fuels, which not only burn at higher to mine, refine, liquefy, and transport the 3He temperatures than D-T but also produce most of have yet to be evaluated.22 their energy in the form of energetic charged par- ticles. Unlike neutrons, which escape from the Advanced Energy Conversion plasma without heating it, charged particles are retained within the plasma. The D-T reaction, in Despite the very high-level technology in the fusion core, a baseline fusion reactor would gen- which only 20 percent of the energy is given to erate electricity in much the same way that pres- charged particles, is less suitable for techniques ent-day fossil fuel and nuclear fission powerplants that recover energy directly from the plasma. do. Heat produced in the reactor would be used Several direct conversion techniques that may to boil water into steam, which would pass through convert well over 35 percent of the fusion energy turbines to drive generators. Through this proc- to electricity have been identified. Until they can ess, about 35 to 40 percent of the energy pro- be tested experimentally under conditions simi- duced in the fusion reaction would be converted lar to those in an advanced fusion reactor, they ———— must be considered speculative. Nevertheless, ~~Use of lunar ‘He is discussed in “Lunar Source ot ‘He for Com- they provide a tantalizing goal. mercial Fusion Power, ” by L.J. Wittenberg, J.F, Santarius, and G.L Kulclnski, Fus/on Technology 10(2): 167, 1986. J ‘See Item 2 In box 4-A, ‘ ‘Plasma Energy Lo\s Mec hanisnl~ ‘‘ Ch. 4.—Fusion Science and Technology ● 93

RESEARCH PROGRESS AND FUTURE DIRECTIONS

In 35 years of fusion research, the technologi- uct provides a rough measure of how well these cal requirements for designing a fusion reactor three requirements have been simultaneously have become clearer, and considerable progress achieved. has been made towards meeting them. Improved The next figure, 4-1 4(b), plots the temperature understanding, based on both experiments and alone and compares it to the minimum temper- increased computational ability, is providing ature below which neither breakeven nor igni- much of the predictive capability needed to de- tion can occur no matter how high the density sign, and eventually to optimize, future plasma and confinement time. The TFTR point shows experiments and fusion reactors. temperatures well into the reactor regime and far Major advances in plasma research have been above that needed for ignition. However, the fact made possible by progress in tokamak plasma that the corresponding TFTR point in figure 4- technologies: 14(a) is below the ignition threshold indicates that high temperature is not sufficient; the product of . By the 1960s, experiments demonstrated the density and confinement time must also be high crucial importance of attaining high vacuum for ignition. and low impurity levels in the plasma to achieve high densities, temperatures, and Figure 4-1 4(c) shows progress in the parame- confinement times. ter beta, the ratio of plasma pressure to magnetic ● In the mid-197os, neutral beam technology field pressure. Note that devices that have achieved was first used to heat plasmas to tempera- high values on one of the three plots often have tures several times higher than those previ- not been the ones that have gotten the highest ously attained. High-performance, high-field values in others. Future devices will have to copper magnets were used to obtain high achieve high values in all areas simultaneously. Lawson confinement parameters in compact tokamak plasmas. The Technical Planning Activity ● The development in the late 1970s of pellet injectors to fuel plasma discharges led to fur- The technological issues to be resolved betore ther advances in plasma density and confine- fusion’s potential as a power producer can be ment. Development of the poloidal divertor assessed have been examined in detail in the at about the same time led to the discovery Technical Planning Activity (TPA), an analysis of the “H-mode,” a mode of tokamak be- commissioned by DOE’s Office of Fusion Energy havior that was not subject to degraded con- and coordinated by Argonne National Labora- finement when auxiliary heating was used. tory.24 Over 50 scientists and engineers from the ● I n the early 1980s, advances in high-power fusion community identified and analyzed the radiofrequency technology gave experimenters tasks and milestones that constitute the research new tools to modify the temperature, cur- needed to reach the goal of the DOE fusion pro- rent, and density distributions within the gram: the establishment of the “scientific and plasma. Much of this new capability has yet technological base required to carry out an assess- to be exploited. ment of the economic and environmental aspects of fusion energy. ”25 According to DOE’s assign- These accomplishments have contributed to ment to TPA, the assessment of fusion wouId be the steady progress in plasma parameters plot- culminated by the construction and operation of ted in figure 4-14. Figure 4-14(a) shows the prod- “one or more integrated fusion faciIities . . . in uct of the temperature, density, and confinement the post 2000 period. ”26 time that has been achieved simultaneously in various experiments over the last 20 years. Since 24 Argon “e N~tl~nc~ I L~bratc)ry, ~6Thf7;cd/ P/JfJfllf7~ Acfll 1~)”” ‘/- nc7/ Report, op. cit. all three of these parameters must be high simul- 251 bid., “Technical Pl~nnlng Acti\ @ Mlsslon Statement, ” p. 348 taneously for the product to be high, this prod- ‘blblci. 94 “ Starpower: The U.S. and the International Quest for Fusion Energy

Figure 4-14.-Progress in Tokamak Parameters

A) B) 20 6 10

5 5

0.5 2

0.2

0.1 1965 1970 1975 1980 1985 1990 1965 1970 1975 1980 1985 1990 1970 1975 1980 1985 1990

Year

The nature of an integrated fusion facility is not properties of burning plasmas, fusion materials, specified by either DOE or TPA. The TPA report and fusion nuclear technology .18 For each of describes it only as “the beginning of the com- these issues, TPA set technical goals and deter- mercialization phase of fusion” that could “per- mined requirements for facilities that could reach haps” take the form of a demonstration power these goals. reactor. 27 The decision to proceed with an in- tegrated facility is scheduled in the TPA report Magnetic Confinement Systems in the year 2005. The key issue in confinement systems is the de- velopment of confinement concepts that would Key Technical Issues and Facilities be suitable for commercial fusion reactors. Prog- DOE defines four “key technical issues” that ress here will require that a series of facilities be must be resolved: magnetic confinement systems, built for whichever concepts are judged worthy of further development. A preliminary experi- zTlbid., p. 26. On p. 11 of the TPA report, table S.2 provides “rep- ment that investigates basic characteristics of a resentative goals” for an integrated fusion facility (IFF) and for a new concept can be done for a few million dol- commercial power reactor. Comparing the two sets of goals shows that the IFF falls well short of commercial performance. h would lars or less. An experiment that looks promising only produce from one-sixth to one-third the heat generated by can be followed up by a larger “proof-of-con- a commercial-scale reactor, and it would have considerably lower availability and shorter lifetime than a commercial plant. The IFF described by these parameters, therefore, could not be considered w .s. @partrnent of Energy, Office of Energy Research, Mag- to be a “demonstration” power reactor. netic Fusion Program P/an, DOE/ER-0214, February 1985, p. 6. Ch. 4.—Fusion Science and Technology ● 95

cept” experiment, costing up to tens of millions follow up on the PBX-M and D II I-D results dis- of dollars; such a device would explore the scal- cussed in the earlier sections “Advanced Toka- ing properties of the concept and determine mak, ” p. 59, and “Beta,” p. 70), steady-state whether it offers the potential for extrapolation tokamak operation, and high-magnetic-field ap- to a reactor. proaches to tokamak confinement.31

If the results are promising, a “proof-of-prin- Properties of Burning Plasmas ciple” experiment would then be required to pro- vide confidence that the concept could be scaled Some of the most critical scientific issues yet up to reactor-level conditions. The JET, TFTR, and to be resolved in the fusion program involve the JT-60 tokamaks are in this category. They are not behavior of ignited, or burning, plasmas. These themselves reactor-level devices, but they will en- issues include the effects of self-heating on con- able decisions to be made about whether to pro- finement and the effects of energetic alpha parti- ceed to the final stage of concept development: cles on plasma stability, burn control, and fueling. demonstration of reactor-level plasma conditions, Effects such as these, which existing experiments including fueling, burn control, ash removal, and cannot yet address, can profoundly influence fu- other functions necessary for reactor operation. sion’s feasibility. TFTR may be able to provide No reactor-level devices have yet been built for some information about the effects of alpha par- any concept, including the tokamak. ticle generation if it attains near-breakeven con- ditions in D-T operation. However, TFTR does Cost is very difficult to estimate for a future not have the capability to reach ignition and proof-of-principle or reactor-level device. The therefore will not be able to resolve burning TPA report estimates costs ranging from 100 mii- plasma issues definitively. The European JET de- Iion to several hundred million dollars or more.29 vice should also have the capability to reach and The costs of the existing JET, TFTR, and JT-60 de- perhaps exceed breakeven, but it too will not be vices range between $600 million and $950 mil- able to resolve many of the burning plasma lion dollars,30 but these devices perform many issues. functions in addition to proof-of-principle for the tokamak concept. There is no reason to think that According to TPA, two different tasks are re- proof-of-principle devices for other confinement quired to study burning plasma issues fully. One concepts would be as expensive. A reactor-scale is a short-pulse ignition demonstration to create device for a particular concept would have more a self-sustaining fusion reaction. The second is stringent technical requirements than its proof- a long-burn demonstration to maintain a self-sus- of-principle device and presumably would be taining fusion reaction long enough to study ef- more expensive. However, the cost of a reactor- fects such as the evolution of the plasma under Ievel device depends on whether it would serve steady-state burn and the buildup of reaction other functions such as the long-pulse burn, nu- products. These two tasks could be done in sep- clear technology demonstration, or system in- arate facilities or in the same facility. tegration functions discussed below. For fiscal year 1988, DOE has requested funds In addition to generic device requirements for to start building a Compact Ignition Tokamak selected alternate confinement concepts, TPA (CIT) to study short-pulse ignition issues (figure also set a requirement for additional tokamaks 4-15). This device, to be located immediately ad- to investigate features such as shaped plasmas (to jacent to TFTR at Princeton Plasma Physics Lab- oratory, is anticipated to cost about $360 million 29 Argonne National Laboratory, Technical P/arming Activ;tY: Fi- (including diagnostic equipment and associated nal Report, op. cit., table S. 12, p. 44 and table S. 16, p. 48. R&D) and will take advantage of existing equip- JOU .s, Congress, House Committee on Science and Technology, ment at the site. Operation is scheduled to be- Task Force on Science Policy, Science Policy Study Background Report No. 4: World Inventory of “Big Science” Research lnstru- rnents and Facilities, Serial DD, prepared by the Congressional Re- search Service, Library of Congress, 99th Cong., 2d sess., Decem- 31 Argonne National Laboratory, Technical P/arming Act;v;~y: Fi- ber 1986, pp. 111, 121, and 127. na/ Report, op. cit., p. 79.

96 . Starpower: The U.S. and the International Quest for Fusion Energy

Figure Preliminary Design

SHIELD

SOURCE: Princeton Plasma Physics Laboratory, 1987. gin in 1993, and annual operating cost is esti- back as 1977, proposals were made for compact, mated at about $75 million. According to a review high-magnetic-field tokamak devices using high- by a panel of the Magnetic Fusion Advisory Com- performance copper magnets; this is the approach mittee (M FAC)32, CIT will be able to study most that was selected for CIT. Other proposals, which of the effects that generation of alpha particles were ultimately not adopted, called for long-pulse might have in a fusion plasma. tokamaks using superconducting magnets and costing well over over $1 billion. The CIT design Ever since TFTR was designed in the mid- 1970s, is intended to focus on scientific aspects of the the design for a successor device has been a topic fusion process, and it will not necessarily form of active interest in the fusion community. As far the engineering basis for future fusion reactors:

Committee is a committee ClT’s copper magnets consume amounts of elec- of scientists and engineers from academia, national labora- tricity that would be prohibitive for commercial tories, and industry that advises DOE on technical matters purposes, they cannot operate for longer than a fusion research. An (Panel XIV, chaired by Dale few seconds at a time without overheating, and Meade of Princeton Plasma Physics Laboratory) reviewed plans for in the Report on Assessment Phenomena their compact size does not provide enough Feb. 10, 1986. The re- room for a blanket to recover fusion energy or port was reviewed by the full in Advisory breed tritium. But CIT will have the ability to re- Committee on Compact Experiments (Charge XIV), submitted to Dr. Alvin Director, Off Ice of Energy Re- solve critical physics uncertainties sooner and at search, Department Energy, Washington, DC, February 1986. lower cost than an experiment having more of Ch. 4.—Fusion Science and Technology ● 97

the engineering features that a reactor wouId re- fer in energy and effects from fusion-generated quire. Moreover, although specifics of the CIT de- neutrons; tests of materials in fission reactors have sign may not be applicable to future reactors, the to be carefully arranged in order to provide overall approach of high-field, high-performance meaningful data on fusion neutron effects. magnets in fact may be relevant if such magnets Eventually, a high-intensity source of 14-million- can be made with superconducting technology. electron-volt (14-MeV) neutrons will be required CIT is being designed through a national effort to evaluate the lifetime potential of most materi- with wide-based technical support, and the proj- als. To accelerate the effects of aging, the test fa- ect has been endorsed by MFAC as a “cost-ef- cility must generate neutron irradiation levels sig- fective means for resolving the technical issues nificantly higher than those expected in a reactor. of ignited tokamak plasmas.”33 MFAC determined Such levels could be provided by a device such that the “existing tokamak data base is adequate, as a driven fusion reactor, which would gener- with credible extrapolation, to proceed with the ate fusion neutrons but consume more power design of the ClT,” and that by fiscal year 1988 than it generated. Such a device would be com- “we shouId have acquired sufficient information pletely impractical as an energy source, but could from present large machines to support proceed- bean effective method of generating high fusion ing with the construction of the CIT.”34 MFAC neutron intensity over a small volume (1 O to also stated that CIT “should be part of a balanced 1,000 cubic centimeters). overall fusion program, “ implying that it should TPA estimates that a materials irradiation test not drain funds away from “other essential ele- facility would cost from $150 million to $250 mil- ments . . . in the DOE Magnetic Fusion Program lion and would take about 4 years to build. Ma- Plan.”35 terials testing would also require several addi- Long-term burn issues, which cannot be ad- tional facilities each costing $10 million or less. dressed by CIT, will require another device in the future. Even if constructed solely to study physics Fusion Nuclear Technology issues, such a device would probably cost at least Fusion nuclear technologies are those involved $1 billion. If other functions such as nuclear tech- with the recovery of energy from the fusion re- nology testing were incorporated, the cost would action and the breeding and recovery of tritium be even higher. needed to replace the tritium consumed in the reaction. Most of the nuclear technology func- Fusion Materials tions of a fusion reactor are incorporated in the According to TPA, “the ultimate economics first wall, blanket, and shield. and acceptability of fusion energy, as with most The first wall/blanket test program is currently other energy sources, will depend to a large ex- at a very early stage. However, the characteris- tent on the limitations of materials for the vari- tics of the required experiments have been de- ous components.”36 Addressing the specific ma- fined in the FINESSE study .37 A number of exper- terial issues identified earlier in the chapter iments, each costing about $5 million, will be requires faciIities for testing and evaluating can- important for guiding the future of the program. didate materials. Some of this testing can be done Several larger experiments will be needed to fol- by exposing materials to neutrons in fission re- low upon the earlier ones; each of these will cost actors. Fission-generated neutrons, however, dif- up to tens of millions of dollars to build and $1 million or more a year to operate .38 33Magnefic Fusion Advisory Committee Report on compact ig- nition Experiments (Charge X/V), letter from MFAC chair Fred L. j~he FINESSE study IS a 3-year study done to r DOE Ink o I \ I ng Rlbe to Dr. Alvin Trlvelpiece, Director, Office of Energy Research, organizations and scientists from the U n ited States, ELI rope a nci Department of Energy, Feb. 24, 1986. Japan. The study produced many pub]lcatlons; one example IS ‘A ]~1 bid, Study of the Issues and Experiments (or Fusion Nuclear Technol- 3jlbid. ogy,” by M. A. Abdou, et al., Fwon Technology 8, November 1985. 16 Argonne N~tlona [ Laboratory, Techrr/ca/ ~/.3flfl;~g Act/~;ty: FI- IBArgonne Nationa I Laboratory, Technic a/ f’/.? nngng Ac~l~’lty. FI - rra/ Report, op. cit., p, 259, na/ Reporf, op. cit., table 4.13, pp. 234-236. 98 ● Starpower: The U.S. and the International Quest for Fusion Energy

After individual facilities enable blanket con- If a nuclear-technology-only device is built in- cepts and components to be designed and the stead of an ETR, an additional device would be number of options to be reduced, a large-scale required specifically to study long-term burn. This nuclear technology demonstration facility will be additional experiment would probably cost more required to integrate these components and test than $1 billion. Further expense would come later them under fusion reactor conditions. Such a when the tested nuclear technology systems were large-scale device must not only produce enough scaled to reactor-level and integrated with a fusion power to provide a realistic environment reactor-sized plasma. Thus, the cost of an ETR for nuclear technology testing, but it must also cannot be compared only to the cost of a nuclear incorporate development and construction of the technology device plus a long-term burn device. individual nuclear technology systems themselves. Although DOE recognizes the need for an ETR If this device is also intended to address the or equivalent, it has no plans to build one. The physics issues associated with long-term fusion Japanese and European programs each have burns, it will become still more complex and ex- plans to design such a device independently, but pensive. neither has yet committed to its construction. The One possibility, identified by the FINESSE study Soviet and the U.S. fusion programs, on the other and reviewed and adopted by TPA, is building hand, appear to prefer international collabora- a nuclear technology demonstration facility that tion on such a facility. The U.S. Government has does not simultaneously serve as the long-burn proposed to the other major fusion programs that physics test facility. A device built solely to study conceptual design of an international engineer- nuclear technologies would need a source of ing test reactor, called the International Ther- fusion-generated 14-MeV neutrons, but this source monuclear Experimental Reactor (ITER), be jointly would not need to be an ignited plasma. Such undertaken. The U.S. proposal does not extend a device could test scaled-down versions of nu- to multinational construction of such a device. clear technology components, provided that However, at the conclusion of the conceptual de- these results could be applied with confidence sign effort, the parties could decide whether they to reactor-sized versions. TPA estimated that such wanted to proceed with construction, either in- a nuclear-technology-only device would cost dependently or jointly. Possible avenues of fu- about $1 billion and would take 5 to 6 years to ture international collaboration in fusion research build. TPA did not estimate operating costs for are discussed in chapter 7. this device. TPA identified as a second possibility an engi- Resource Requirements neering test reactor (ETR) that would include both Schedule long-term burn physics and nuclear technology studies. Such a device would require an ignited TPA identified six major decision points that de- or near-ignited plasma, making it big enough to termine the course and schedule of future fusion accommodate full-size nuclear technology com- research, leading up to the overall assessment of ponents. Both the more stringent physics require- fusion’s potential. In figure 4-16, each decision ments and the need for full-scale nuclear tech- is pegged to a key technical issue and to a year. nology components would make an ETR much more expensive than a nuclear-technology-onIy The ratio of annual operating expense to capital expense for TFTR facility. TPA estimated the cost to build an ETR is about 15 percent, and that projected for CIT is about the same if the value of existing facilities at Princeton Plasma Physics Lab- at about $3 billion. It did not estimate operating oratory is included in ClT’s capital cost. Applying this 15 percent costs but said that the experience with TFTR ratio to an engineering test reactor would predict annual operat- would suggest $150 million annually.39 ing expenses of close to $500 million. However, fusion scientists argue that there is no reason to be- lieve the ratio of operating expense to capital expense to be the WIt is not clear how this estimate of $150 million is obtained; it same for an ETR as it is for significantly smaller devices. They agree is not computed merely by assuming the annual operating expense that the more a device costs to build, the more it will cost to oper- of a device to be a specific fraction of its capital cost. Were the ate. However, they maintain that there is no reason to expect cap- estimate made in this manner, it would be considerably higher. ital and operating costs to increase at the same rate,

Ch. 4.—Fusion Science and Technology “ 99

Figure 4-16.-Top Level Decision Points in the Magnetic Fusion Program

SOURCE: Argonne National Laboratory, Technical Planning Activity: Final Report, commissioned by the U.S. Department of Energy, Office of Fusion Energy, ANL/FPP-87-1, January 1987, figure S.8, p. 23.

TPA did not examine all the possible scenarios assured of success. Rather, it shows that an ex- resulting from different timings and outcomes for haustive process of technical review has not un- these decisions but instead adopted a “reference covered any inconsistencies. scenario” believed to reflect current DOE plan- ning. The years in figure 4-16 are taken from the cost reference scenario, which is shown in greater de- tail in figure 4-17. TPA estimates that the worldwide research re- quired between 1987 and 2005 under the refer- In the reference scenario, the decision to pro- ence scenario will cost in the range of $20 bil- ceed with CIT as the short-term burn experiment lion. This cost does not include an integrated is made in 1987. By about 1990, the decision is fusion facility, demonstration reactor, or any made to combine nuclear technology studies and other facility constructed after the assessment of long-term burn physics issues in a single engineer- fusion in 2005. Total operating cost worldwide ing test reactor, possibly ITER. At about the same is judged to be relatively constant at about $800 time, decisions are made concerning the nature million annually, and total yearly funding for fu- of a materials testing facility and the selection of sion research increases to about $1.5 billion in confinement concepts to be tested at the proof- the mid-1990s when construction cost is added. of-principle scale. By 1997, certain alternate con- The total construction budget is estimated at finement concepts successfully showing proof- about $6 billion, half of which is required for an of-principle are selected for reactor-scale dem- engineering test reactor. onstration, and the overall assessment of fusion is targeted for 2005. TPA does not conclude that TPA acknowledges that the ground rules used the reference scenario is fastest, cheapest, or most for cost projection could have been applied more

January 1987, figure S.1O, p. 27. uniformly, that no iterative review of the cost esti- estimated total cost is higher than an actual man- mates was undertaken, and that no effort was ager would spend when faced with fiscal con- made to estimate the resources required for alter- straints. Similarly, these critics complain that any nate research scenarios. Nevertheless, the report study done by experts from a single field has an states that the information gathered is sufficient inherent bias towards overestimating that field’s to present “a broad view of the resources re- research requirements. Researchers in a field are quired for a full assessment of the commercial presumed to have an interest in maintaining or potential of fusion.”40 increasing their field’s funding, these critics ar- gue, and the researchers would not have any in- Some critics of TPA’s cost estimates argue that, centive to prepare a study underestimating the since TPA was not charged with designing, man- cost of research if such a study might be used as aging, and executing an actual research strategy— justification for cutting the field’s research budget. indeed, TPA was forbidden from performing such programmatic planning, which is strictly DOE’s Other critics, however, feel that any bias in domain–the total cost represents a sum of “wish TPA’s estimated total is likely to be in the direc- lists” rather than a realistic budget. According to tion of underestimating the total rather than in- these critics, without the requirement to conduct flating it. Since each technical aspect of the study the politically difficult task of eliminating research was analyzed by experts in the field, some de- that, although useful, may not be necessary, the gree of technical optimism is probably inherent at each stage; unanticipated difficulties would Laboratory, drive the cost of the research program above Report, op. cit., p. 28. TPA’s estimates. Moreover, these critics suggest, Ch. 4.—Fusion Science and Technology ● 101

it would not be in the collective interest of the Characteristics and prospects of fusion as a com- fusion community to estimate a higher total cost mercial energy source are discussed in chapter 5. than necessary, since continued support of the fusion program depends on perceptions that its Findings benefits are worth its cost. Overestimating the to- Estimates of the annual worldwide funding re- tal cost could threaten the program’s support. quired to evaluate fusion’s potential early in the The process by which TPA estimated the to- next century are several times today’s annual U.S. tal cost of future fusion research involved a wide fusion research budget. The estimated annual degree of fusion community participation, and worldwide funding is, however, on the order of OTA can find no evidence that the estimate is the amount now spent each year by all the world’s flawed. However, OTA also recognizes that while major fusion programs put together. The fund- the researchers in any technical field are the ing estimates suggest three possibilities to U.S. most qualified to estimate costs of experiments policy makers for continuing the U.S. fusion re- in that field, they are also the beneficiaries of search program: support given to the field. Therefore, their esti- 1. With funding levels several times their pres- mates may be influenced by non-technical fac- ent level and with a significant measure of tors, although it is not clear whether the esti- technical success, the U.S. fusion program mate would be too high or too low as a result. can decide on its own whether or not to be- gin the demonstration and commercializa- Summary tion of fusion power in the early part of the next century. Probability of Success 2. If the major world fusion programs can col- It seems likely that at the conclusion of the re- laborate and plan their research efforts to search program, fusion’s technological feasibil- complement each other and eliminate dupli- ity—the ability to use fusion power to generate cation, and if the effort has a significant electricity—can be shown. The fusion program measure of technical success, a collective has made steady progress over the last 35 years assessment to proceed with fusion’s devel- on the key technical issues. It is still possible that opment could be made early in the 21st cen- fusion’s scientific feasibility will be impossible to tury. In this case, only modest increases in demonstrate, due to surprises in the behavior of funding would be required for each of the a plasma that generates substantial amounts of world’s fusion programs, with the exact fusion power. However, successfully attaining ig- amount of the increases depending on how nition in CIT will resolve most of the scientific un- well the programs were able to avoid dupli- certainties. cation of research. 3. If major international collaboration is not at- Most of the subsequent scientific and engineer- tained, and if the U.S. fusion budget is not ing challenges in designing and building a reactor increased to the point where the necessary have been identified. Once scientific feasibility research can be carried out domestically, the is established, a concerted and well-funded re- United States cannot assess fusion’s poten- search effort should be able to develop a reactor tial until later in the next century. that produces fusion power. However, it can- not yet be determined whether or not such a These possibilities form the basis of the policy fusion reactor will be commercially attractive. options discussed in greater detail in chapter 8,