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3

Plasma Confinement Configurations

INTRODUCTION

A key goal of the U.S. fusion program is to answer important physics questions (as described in Chapter 4), with the aim of acquiring predictive capability (as described in Chapter 2). A crucial ingredient in this effort is the study of a range of confinement configurations, as described in this chapter. The hallmark of laboratory physics is its ability to provide an experimental configuration optimized to reveal the physics in question. In plasma physics, many basic questions are best illuminated by studying a family of configurations, each of which is capable of stable plasma confinement. Plasma behavior is determined in large part by the spatial structure of the confining magnetic field. The experimenter can manipulate the magnetic field to vary crucial properties such as curvature, field line pitch, field strength (relative to plasma pressure), and spatial symmetry. The result is a family of related confinement configurations that can be used for the controlled study of scientific issues.

Throughout most of the history of the fusion program, a variety of confinement configurations were investigated. However, in the past each configuration was largely viewed in terms of its suitability as a fusion reactor. Progress in a configuration was judged mostly in terms of progress toward reactor-relevant plasma parameters. Now, however, the restructured program has a stronger scientific focus and the variety of configurations under study is being broadened, motivated by the following rationale. First, a range of configurations is needed to examine the array of crucial plasma science issues. Indeed, even a configuration that does not prove to be a suitable source of fusion power may be of unique value for the study of a specific scientific issue. The choices of configurations for study are being driven more than before with science as a criterion, and research is being conducted with emphasis on the scientific coupling between configurations. Second, a particular configuration can be investigated for its potential as the core of a fusion power source; that is, the set of physical attributes possessed by a particular configuration may ultimately prove advantageous for a reactor. The optimal fusion configuration is not yet known—it may be a direct extrapolation from a present concept, an evolved hybrid of concepts presently under study, or even a concept not yet articulated.



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Page 45 3 Plasma Confinement Configurations INTRODUCTION A key goal of the U.S. fusion program is to answer important physics questions (as described in Chapter 4), with the aim of acquiring predictive capability (as described in Chapter 2). A crucial ingredient in this effort is the study of a range of confinement configurations, as described in this chapter. The hallmark of laboratory physics is its ability to provide an experimental configuration optimized to reveal the physics in question. In plasma physics, many basic questions are best illuminated by studying a family of configurations, each of which is capable of stable plasma confinement. Plasma behavior is determined in large part by the spatial structure of the confining magnetic field. The experimenter can manipulate the magnetic field to vary crucial properties such as curvature, field line pitch, field strength (relative to plasma pressure), and spatial symmetry. The result is a family of related confinement configurations that can be used for the controlled study of scientific issues. Throughout most of the history of the fusion program, a variety of confinement configurations were investigated. However, in the past each configuration was largely viewed in terms of its suitability as a fusion reactor. Progress in a configuration was judged mostly in terms of progress toward reactor-relevant plasma parameters. Now, however, the restructured program has a stronger scientific focus and the variety of configurations under study is being broadened, motivated by the following rationale. First, a range of configurations is needed to examine the array of crucial plasma science issues. Indeed, even a configuration that does not prove to be a suitable source of fusion power may be of unique value for the study of a specific scientific issue. The choices of configurations for study are being driven more than before with science as a criterion, and research is being conducted with emphasis on the scientific coupling between configurations. Second, a particular configuration can be investigated for its potential as the core of a fusion power source; that is, the set of physical attributes possessed by a particular configuration may ultimately prove advantageous for a reactor. The optimal fusion configuration is not yet known—it may be a direct extrapolation from a present concept, an evolved hybrid of concepts presently under study, or even a concept not yet articulated.

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Page 46 In short, a broad family of concepts bound by common physics principles may be studied both to elucidate the fundamental physics and to stimulate the scientific innovation needed for fusion energy development. The U.S. fusion program is fruitfully becoming a program that recognizes the complementary, coupled contributions of different configurations. In addition, an optimal scientific program would contain experiments spanning a diversity of scales, with small experiments being optimal for some studies and large experiments necessary for others. In this chapter the motivation for the fusion concept program and the program's status are summarized. First, a small set of illustrative fundamental science issues is discussed, together with how they are best investigated in a variety of plasma configurations. Next, the impact of advances in these basic physics questions on the fusion energy goal is presented. The inertial confinement approach to fusion energy is also a large, active, and challenging endeavor, with important scientific underpinnings and opportunities. Although beyond the scope of this report, it, too, is discussed briefly. Another section touches on representative connections between the U.S. fusion program and the broader international program. The engineering challenges, which are discussed briefly, are also beyond the committee's charge. In the penultimate section, the metrics in place in the OFES program are discussed. Conclusions and recommendations regarding the fusion concept program are found in the last section. Appendix C contains brief descriptions of the various configurations being explored. IMPORTANT PHYSICS QUESTIONS MOTIVATING RESEARCH WITH VARIOUS CONFIGURATIONS The fusion research program is in part driven by a set of important physics issues that are being addressed through research using a diversity of plasma configurations. An example of one of these configurations, a tokamak, is shown in Figure 3.1. The major magnetic configurations now under study are described in detail in Appendix C. Many of these configurations are potential fusion concepts. Several of the important physics issues are briefly discussed, and for each the plasma configurations that are being employed to research the issues are identified. The issues can serve as organizing elements for the portfolio of configurations, as the committee recommends in the last section of this chapter, because the various plasma configurations under study follow naturally from them. The four physics challenges that the committee has selected are neither exhaustive nor necessarily optimal for planning purposes; rather, they are illustrative, and the actual development of a complete set of physics issues is left to the research community. Understand the Stability Limits to Plasma Pressure All plasma configurations possess an upper limit on the pressure (product of plasma density and temperature) beyond which the plasma becomes unstable and disassembles. In the parameter regime in which the plasma is well-described as a magnetized fluid, the theory of magnetohydrodynamics (MHD) can be used to evaluate the pressure limit for different magnetic structures and can also be used to model the detailed evolution of a plasma instability. Techniques to solve the MHD equations are well developed and can be used to accurately evaluate whether a plasma is stable to small perturbations (linear stability). Linear instabilities can, if sufficiently global in spatial extent, cause a rapid degradation of plasma confinement. Predictions of linear instability thresholds have been tested in small-scale plasma configurations. For example, the dependence of stability on magnetic curvature has been investigated through a range of configurations with varying curvature (beginning with magnetic mirror configurations). The

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Page 47 ~ enlarge ~ FIGURE 3.1 Components of the tokamak confinement configuration, one of the more advanced plasma confinement concepts. It uses a strong toroidal field created by external field coils (top) to stabilize the plasma while using a poloidal field created by a toroidal plasma current to confine the particles. The final configuration includes a large vacuum vessel to isolate the hot plasma from the surrounding environment (bottom). Courtesy of General Atomics and PPPL.

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Page 48 connection between magnetic curvature and macroscopic stability has been firmly established. More recently, the pressure limit to global instability observed in a range of tokamak experiments has been shown to agree with MHD theory. The continuing challenge is to develop a quantitative understanding of the nonlinear evolution of instabilities, of the relationship of stability to properties of the magnetic field (such as magnetic curvature and shear), and of additional effects on stability that cannot be treated within the MHD fluid model (such as the effects of large particle orbits, separate electron and ion dynamics, and energetic particles). Moreover, there are instabilities that arise only in the presence of a finite amplitude perturbation and that would not appear within a small amplitude linear theory. At the forefront of this endeavor is the study of plasma with pressure comparable to the magnetic pressure that confines the plasma. Plasma pressure is gauged in comparison to the magnetic pressure and is characterized by the dimensionless parameter β, the ratio of the plasma pressure to magnetic pressure. Plasma pressure in the tokamak configuration has been increased over the years, so that β values of about 10 percent are now readily obtained. Extreme pressures, with β close to unity, may be obtained by pushing various magnetic field properties—for example, toroidal curvature—to an extreme. The experimental and theoretical treatment of such configurations provides a testbed for MHD stability; conversely, a challenge to our understanding of plasma stability is to develop configurations with β close to unity. A large number of plasma configurations contribute to this goal. As one accentuates the curvature of a plasma torus (for example, by reducing the hole in the center of the torus), the curvature of the field becomes highly favorable for stability and the β limit is predicted to approach unity ( Figure 3.2). This configuration, the spherical torus, provides a test of stability at extreme toroidal curvature. In the stellarator family of configurations, the magnetic field is produced largely by external coils. The magnetic field properties can be varied controllably to isolate the impact of geometry on stability. Thus, although the β value of the stellarator is relatively modest, it can provide key input to the understanding of MHD stability. In addition, stellarators can operate with minimal plasma current, allowing us to distinguish the pressure and current contributions to the free energy source that drives instabilities. The reversed-field pinch (first explored in the United Kingdom) and spheromak configurations permit study of plasmas with unfavorable curvature but strong stabilizing shear (rate of twist) in the magnetic field lines. Finally, there are two toroidal configurations, the field-reversed configuration (FRC) and the dipole, in which the field points entirely in the poloidal direction—that is, the field that is directed the long way around the torus vanishes. The FRC is predicted by MHD theory to be unstable, yet experimental FRC plasmas are stable, persisting for times much longer than the growth times of predicted instabilities. This is an example of a plasma in which effects beyond the MHD model are needed to explain experimental results. Understand and Control Magnetic Chaosin Self-Organized Systems In configurations in which the confining magnetic field is weak, the plasma is less stable and the magnetic field can become turbulent. The result is that the magnetic field lines can fluctuate and wander chaotically in space, leading to the random transport of particles and loss of energy. On the other hand, magnetic chaos also causes the magnetic field to rearrange spontaneously (self-organization). The physics of this laboratory process is similar to that in the spontaneous generation of magnetic field in the Earth, stars, and galaxies, a process known as the dynamo. Self-organization also occurs through the process of magnetic reconnection, which is also prevalent in solar and astrophysical plasmas. Experimental and theoretical investigations are aimed at understanding the origin of magnetic self-organization, dynamo activity, and reconnection and the mechanisms by which the magnetic fluctuations

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Page 49 ~ enlarge ~ FIGURE 3.2 Examples of the magnetic topologies of several related toroidal configurations with increasing curvature and varying stability characteristics. The tokamak (left) uses a strong external toroidal field to provide robust stability against pressure- and current-driven instabilities. The spherical torus uses a weak toroidal field in a compact configuration to allow access to higher β values than obtained in the tokamak. The spheromak (right) uses internal plasma currents only to provide the confining poloidal field plus a weak toroidal field. A larger safety factor indicates a higher level of protection from current-driven instabilities. Courtesy of M. Peng (PPPL). Reprinted by permission from Physics of Plasmas, 2000, vol. 7, pp. 1681-1692. drive transport. This understanding can be used to develop techniques to control the chaos and transport. Control techniques can be used to test our understanding of magnetic fluctuations and transport, as well as to enhance confinement for the fusion application. These issues are critical to, and best studied in, the class of configurations with low magnetic field, such as the reversed-field pinch ( Figure 3.3) and the spheromak. Magnetic self-organization also can occur in high field configurations under special situations, such as in a tokamak during the reconnection process that underlies a sawtooth (relaxation) oscillation, as discussed in Chapter 2. Understand Classical Plasma Behavior and Magnetic Field Symmetry The transport of particles through the plasma arising only from collisional interactions between particles (two-particle correlations) is referred to as classical transport. The effect of instabilities and turbulence is neglected. Some aspects of plasma behavior can be dominated by classical processes, even in the presence of turbulence. The classical processes are determined, in part, by underlying spatial symmetries in the magnetic field. To confine particle orbits, it is advantageous for the magnetic field to have a direction of symmetry. The presence of a symmetry coordinate favorably constrains the motion of particles in such “two-dimensional” systems so they do not drift out of the magnetic container. A class of confinement concepts

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Page 50 ~ enlarge ~ FIGURE 3.3 A magnetic confinement concept such as the reversed-field pinch (top) is a relatively self-organizing configuration that is subject to turbulent magnetic field structures. The magnetic topology includes a reversal of the toroidal field inside the plasma owing to plasma currents. Under normal inductive current drive, the magnetic field lines can readily become chaotic, as indicated by a puncture plot of the field lines as they traverse a poloidal plane (bottom left). With finer control of the plasma currents, well-defined flux surfaces are restored (bottom right). Courtesy of S. Prager (University of Wisconsin at Madison).

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Page 51 has been discovered by theoretical methods in which the magnetic field is fully three-dimensional (having no direction of symmetry) but appears to be nearly two-dimensional from the viewpoint of a moving particle in the plasma (see Figure 3.4). The design of such highly nonintuitive, “quasi-symmetric” systems has just recently become possible with the advent of new computational techniques. Thus, it is now feasible to study the new symmetry principle by investigating the relation of particle orbits (and associated diffusion) to magnetic field symmetry. Such configurations are within the stellarator class of magnetic containers. In plasmas that are asymmetric in at least two coordinates, it has been discovered that an electric current can flow along the third coordinate direction in the absence of an electric field. This is a classical effect—a thermoelectric effect—in which the current is driven by a combination of a pressure gradient across the magnetic field and an effective viscosity arising from the nonuniformity of the magnetic field. The viscosity channels the electron or ion flows along the symmetry direction. A relatively complete equilibrium plasma kinetic theory exists to describe this self-generated “bootstrap current.” A magnetic configuration that exhibits a very large bootstrap current, such as a toroidal plasma with a very strong poloidal asymmetry but with toroidal symmetry, is needed to test the theory and explore the robustness of the self-driven current. The spherical torus possesses these characteristics. Theory predicts that the spherical torus, when heated to high pressure, can approach the limit in which nearly all the plasma current is bootstrap-driven. The magnetic symmetry properties of a stellarator can be adjusted continuously so that experiments in this configuration offer opportunities for controlled tests of the theory. Understand Plasmas Self-Sustained by Fusion (“Burning” Plasmas) In all existing magnetic fusion experiments the plasma is heated by external sources. The energy input is required to overcome the inevitable energy loss from the plasma. In a plasma containing fusion reactions, a state can be reached in which the plasma is self-sustaining: the alpha particles produced in the fusion reaction deposit their energy back into the plasma at a rate sufficient to keep the plasma at a fixed temperature. In such a burning plasma, the external heating can be turned off and the plasma will undergo fusion burn continuously until the fuel is exhausted. The presence of alpha-particle dynamics in the plasma introduces at least three physics issues: alpha-particle heating, alpha-particle transport, and alpha-particle-generated instabilities or turbulence. Alpha particles will transfer their energy to the background plasma, either classically through collisions or through more rapid processes involving turbulence or instabilities. The spatial profile of the heating by alpha particles can be different from that for externally heated plasmas and can alter the behavior (such as the transport) of the background plasma. Whether the alpha particles within the plasma are transported at the same rate as the background plasma, or whether the transport depends on the particle energy, is a key issue. For example, the large gyration orbits of energetic alpha particles cause them to sample the background plasma turbulence differently from the small-orbit particles that constitute the bulk plasma—transport properties may therefore differ from the bulk. Finally, the alpha particles represent an additional source of free energy, so they can introduce new plasma instabilities or turbulence, which can in turn affect confinement of either the alpha particles or the background plasma. Since the alpha-particle dynamics depends on background plasma properties that are themselves influenced by the alpha particles, a burning plasma is a type of self-organized system, organized in part by the alpha-particle dynamics. Isolated aspects of alpha-particle effects can be treated theoretically. However, calculation of the dynamics of the plasma, including the coupled alpha-particle effects, is a complex, nonlinear problem whose complete solution remains undetermined.

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Page 52 ~ enlarge ~ FIGURE 3.4 The stellarator concept uses complex three-dimensional coil and magnetic flux surfaces to create a quasi-symmetric configuration in which the magnetic field appears to be only two-dimensional in the frame of reference of a moving particle in the plasma. The conventional stellarator (a) has relatively simple helical symmetry and multiple harmonics in the field strength along a field line (b), which in turn gives rise to large particle losses. In contrast, the quasi-symmetric stellarator (c) eliminates the harmonics and produces a field line with single harmonic symmetry (d), effectively eliminating toroidal curvature (i.e., the long-period feature in (b)) and dramatically improving particle confinement. Courtesy of D.T. Anderson (University of Wisconsin at Madison).

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Page 53 Two types of experiments have been performed to investigate alpha-particle effects. First, some alpha-particle effects have been simulated in experiments through the production of energetic particles, for example by heating with radio-frequency waves. Second, experiments have been performed in weakly burning plasmas, in which the alpha-particle production rate is small but finite. Both series of experiments produced valuable information on specific alpha-particle effects but were unable to investigate the full, integrated physics of a burning plasma. Thus, experimental investigation of a burning plasma remains a grand challenge for plasma physics and a necessary step in the development of fusion energy. The tokamak is a configuration that is sufficiently developed to provide access to a near-term burning plasma experiment, as indicated in Figure 3.5. The ITER, Fusion Ignition Research Experiment (FIRE), and Ignitor designs and other conceptual design studies of burning plasma experiments are thus necessarily based on the tokamak. The optimal route to a burning plasma experiment depends on judgements made about strategic issues such as its time urgency, the likelihood of achieving burn conditions with present designs, and the transferability of results to configurations other than that of the experiment (and, of course, the cost of the installation). The determination of the optimal route to a burning plasma experiment is beyond the scope of the committee's charge; rather, the route should be decided in the near term by the fusion community. However, based on its interviews, as well as its observations of the Snowmass planning process, the committee believes that there is considerable evidence that the existing program is marking time on burning plasma physics issues (alpha-particle confinement and dynamics)—it is, for example, making do with paper studies until the time is ripe for the deployment of a burning experiment. The most recent planning documents available to the committee reinforce this impression. For example, the FESAC panel report 1 recommends preparation for participation in a burning plasma experiment in a 5- to 10-year time frame but gives no further specifics, and the committee found no evidence for contingency plans in case no burning experiment is undertaken by the ITER partners (Europe, Japan, and the Russian Federation). At this point, the U.S. fusion program is not demonstrating leadership in this area, apparently because Congress does not want to consider a burning plasma experiment at this time (given the history of U.S. participation in ITER, this is perhaps understandable). However, an optimal fusion science program needs two components: (1) experiments in nonburning plasmas to explore the large range of critical science issues that do not require a burning plasma and (2) experiments in burning plasmas. The first component, which would attack a broad range of issues, should not be sacrificed for the second component and can lead to scientific progress in the absence of the second component. Nevertheless, it is clear from the discussion of physics issues in Chapter 2 and in this chapter that a burning plasma experiment is required to address key issues that cannot be fully explored by the present portfolio of experiments. Thus, the United States must soon explore options for pursuing alpha-particle physics issues, possibly as a part of an international team. REACTOR DESIGN FEATURES MOTIVATING FUSION CONCEPT DEVELOPMENT In the last section, physics questions were discussed that motivate research employing a variety of plasma configurations and that can serve as organizing elements for a program of multiple confinement configurations. Although it may be obvious that a fundamental understanding of plasma behavior in 1 Department of Energy (DOE), Fusion Energy Sciences Advisory Committee, Panel on Priorities and Balance. 1999. Report of the FESAC Panel on Priorities and Balance. Washington, D.C.: DOE.

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Page 54 ~ enlarge ~ FIGURE 3.5 Improvements in plasma stability and confinement obtained in magnetic confinement configurations should allow the study of burning fusion plasmas in the near future. The Lawson fusion parameter is the product of the plasma density, ion temperature, and average confinement time and represents a simple figure of merit for proximity to conditions for fusion ignition. Courtesy of PPPL.

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Page 55 magnetic fields will lay a foundation for progress in fusion energy, this section briefly describes the connection between the basic physics goals of the preceding section and the development of a fusion system with attractive design features (such as small size, simple magnets, continuous operation, absence of sudden terminations, and low recirculating power): Understand the stability limits to plasma pressure. The fusion reaction rate of a plasma is proportional to p2, where p is the plasma pressure. Hence, the higher the plasma pressure (or β), the more attractive, generally speaking, is a fusion energy system. For example, a plasma configuration with a high β limit can operate with either a weaker magnetic field (simplifying the magnet requirements) or a smaller size. Understand and control magnetic chaos in magnetically self-organized systems. The class of configurations with weak toroidal magnetic field suffers from large energy transport generated by the processes associated with magnetic self-organization. If the transport can be controlled, then the reactor advantage of the weak magnetic field requirement may be realized. The development of an understanding of magnetic self-organization may also clarify the causes of the sudden termination of the plasma, which plagues some current-carrying plasma configurations. The disruption in a tokamak is the most prominent example. Since disruptions can do substantial impulsive damage to a fusion energy system, their elimination is highly desirable. Understand classical plasma behavior and the role of magnetic field symmetry. The magnetic field within plasmas that lack symmetry can be produced without externally driven current. Such plasmas can, accordingly, form the basis for steady-state operation, a desirable feature for a fusion energy system. In addition, they may be less susceptible to disruptions. Similarly, systems in which the magnetic field structure has been designed to optimize the self-driven bootstrap current can also operate in steady state with minimal need for external current drive (which then minimizes the recirculating power). Understand plasmas self-sustained by fusion. Since the presence of alpha particles can affect nearly all aspects of plasma behavior, their dynamics can have a very large impact on the realization of a fusion reactor. The influence of alpha particles on plasma transport and their effectiveness in heating the plasma directly affect the size of the plasma core of reactor. Moreover, if they were very poorly confined or if they greatly enhanced transport, they could render a confinement configuration unfeasible for fusion energy production. INERTIAL FUSION ENERGY CONCEPT DEVELOPMENT Inertial confinement offers an entirely distinct approach to fusion energy—one with its own set of scientific and engineering challenges as well as scientific opportunities. An inertial fusion energy (IFE) system will have three components: the target, the driver, and the fusion chamber. Just as with magnetic fusion energy (MFE) systems, there are a variety of different concepts under study to serve as implosion drivers (e.g., direct versus indirect drive) and high-repetition-rate drivers (e.g., heavy ion beams, solid state lasers, krypton fluoride lasers) and for the fusion chamber (e.g., solid walls, liquid walls). The physics issues associated with the target dynamics are immensely challenging; they include hydrodynamic stability, the equation of state of dense matter, radiation transport, and the laser-plasma interaction. At the present time, nearly all the research associated with inertial confinement is supported by DOE Defense Programs (DP). It is particularly noteworthy that in the coming years, burn propagation physics is planned to be studied in the National Ignition Facility. In addition to developing a knowledge of the target physics, inertial fusion energy will require the development of high-repetition-rate lasers, inexpensive target fabrication techniques, and suitable confinement chambers. The Office of Fusion

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Page 56 Energy Sciences presently supports the development of heavy-ion-beam drivers. While the involvement of OFES in IFE development is growing, strongly leveraged by the very large DP effort, the OFES program is still heavily weighted toward MFE. Since OFES is the sole steward of MFE, this situation will probably persist for the foreseeable future. Accordingly, the committee was constituted to focus its effort on science within the MFE program, so it leaves treatment of the rich science under way within the IFE effort to another committee. LINKAGES WITH INTERNATIONAL PROGRAMS The dollar total of international research programs in fusion greatly exceeds the annual U.S. budget in this area. In addition, in recent years both the European Union and Japan have been more willing than the United States to commit themselves to major experimental projects that will advance the fusion effort. A major stellarator experiment (the Large Helical Device) has just come on line in Japan, and a machine of comparable scale is being constructed in Germany. The JET tokamak experiment, which is presently the only machine in the world able to operate with a mixture of deuterium and tritium and therefore to explore processes involving energetic alpha particles produced during fusion reactions, continues to operate in the United Kingdom under funding provided by the European Union. There are recent indications from Japan that the JT60-U facility may be upgraded. No facilities of a comparable scale exist or are close to approval in the United States. As discussed in the introduction to Chapter 2 , however, the United States, because of its investment in diagnostics and theory and computation, continues to play a leadership role in developing the science base for fusion by linking experimental observations with a fundamental understanding of the physical processes controlling the plasma dynamics. As a consequence, the impact of the United States program exceeds what the budget figures alone would suggest. There have long been and continue to be extensive collaborations at all levels between international participants in the quest for controlled fusion. For example, the United States provided the neutral beams used to reach the record values of the plasma β in the spherical torus experiment at the Culham Laboratory in the United Kingdom. Given the strength of the international program and the impressive facilities that are available, it is essential that the United States maintain these collaborations. The expertise in diagnostics, theory, and computation from the United States is especially suitable for maximizing the return on these foreign experiments, and our country should continue to promote these collaborations as beneficial to all parties. To develop an understanding of the impact of alpha-particle dynamics in regimes with dominant alpha-particle heating, it will be essential to collaborate with international programs because of the high costs of constructing devices that can reach the parameters required and can handle the radiation problems safely. Should scientists in Europe decide to conduct further deuterium-tritium experiments, the United States should seek to participate as appropriate to maximize the physics return on the experiments. As mentioned earlier, the United States should begin to reestablish its international leadership role by defining an affordable burning plasma experiment that could be constructed with financial contributions from several international partners. ENABLING TECHNOLOGIES FOR PLASMA CONFIGURATION DEVELOPMENT This report focuses on the scientific aspects of fusion concept development. However, significant advances in engineering sciences are also critical. Two types of engineering advances are necessary— those that enable plasma experiments and those that enable a fusion power system. Often these needs

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Page 57 overlap. Examples of the former type include the development of neutral beams to heat plasmas, radio-frequency sources for current drive and heating, pellet injectors for plasma fueling, and high field magnets for plasma confinement. Numerous engineering advances are needed to realize fusion power. Materials research is a major challenge: a fusion reactor must be made of materials that can, among other things, withstand intense heat fluxes, intense neutron fluxes, and acceptable tritium breeding. Ideas are under investigation for new alloys and for flowing liquid walls. Other challenges include the development of fueling techniques, high field magnets, low activation materials, and remote maintenance techniques. In addition, as the physics of the concepts described in the section on reactor design features moves forward, engineering constraints become the limiting factor. For example, concepts aimed at developing compact plasmas face the physics hurdle of achieving good confinement. However, if that physics goal is achieved, the burden shifts toward the development of materials that can sustain the intense neutron bombardment, a necessary consequence of compact systems. The enormous engineering challenges of fusion power, and their contributions to engineering science, are beyond the scope of this report. CURRENT METRICS FOR FUSION CONCEPT DEVELOPMENT FESAC has defined three stages for the experimental development of a fusion concept, beginning with the concept exploration stage (initial experiments to investigate, at a small scale, isolated physics features of a concept), the proof-of-principle stage (with medium-sized experiments aimed at investigating the broad range of key physics issues), and the performance extension stage (where experimental parameters are brought closer to conditions in a reactor). 2 The performance extension stage will be followed by a burning plasma experiment. As this report is being written, in the United States the tokamak is being explored in facilities at the performance extension stage. At the proof-of-principle stage, only the spherical torus is under full investigation. The reversed-field pinch has been recommended by FESAC to proceed to the proof-of-principle stage; reversed-field-pinch research is now transitioning from the concept exploration stage to the proof-of-principle stage. Several configurations are under experimental investigation at the concept exploration stage, including the quasi-symmetric stellarator, spherical torus, spheromak, field-reversed configuration, magnetized target fusion, dipole configuration, electrostatic confinement, and other emerging concepts. This is an evolving set of configurations, some of which will graduate to the next level and others of which will terminate as new results unfold. These various concepts are described in Appendix C, while Figure 3.6 shows examples of experimental facilities at each stage of concept development. In 1999, a FESAC subpanel was convened to establish criteria, goals, and metrics for the fusion program. 3 It discussed two forms of metrics: those to judge whether a fusion concept is ready to move to the next stage of development and those to judge whether the overall fusion program is properly balanced. For the former judgements, 10 criteria were described: the quality of the research, the confidence for the next step, the plasma science and technology benefit, the issue resolution capabilities, the degree to which the research is at the cutting edge in its area, the energy vision of the concept, the programmatic issues of the proposed work (cost, adequacy of resources, etc.), the influence of the 2 Department of Energy (DOE), Fusion Energy Sciences Advisory Committee, Alternate Concepts Review Panel. 1996. Alternative Concepts: A Report to the Fusion Energy Sciences Advisory Committee. Washington, D.C.: DOE. 3 Department of Energy (DOE), Fusion Energy Sciences Advisory Committee, Panel on Criteria, Goals, and Metrics. 1999. Report of the Panel on Criteria, Goals, and Metrics. Washington, D.C.: DOE.

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Page 58 ~ enlarge ~ FIGURE 3.6 Examples of the stages of experimental development of plasma configuration concepts: (a) a large advanced tokamak experiment (DIII-D) at the performance extension stage exploring plasma parameters approaching those of a reactor; (b) a mid-sized reversed-field pinch experiment exploring a range of issues at the proof-of-principle stage; and (c) a smaller spherical torus dedicated to exploring a particular type of current drive in a concept exploration experiment. Courtesy of (a) General Atomics, (b) S. Prager (University of Wisconsin at Madison), and (c) T. Jarboe (University of Washington).

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Page 59 proposal on the overall fusion concept research portfolio, the general science and technology benefit, and the adequacy of milestones. These criteria are meant to be applied to proposals for research at each stage of development—concept exploration, proof of principle, performance extension, and fusion energy development. However, the weighting of the criteria varies from stage to stage. For example, the energy vision gains in importance at the later stages, while contributions to science in general decrease in importance at and beyond the performance extension stage. The FESAC report also discusses the need for balance in the distribution of research among the various stages of development and in the coverage of the various scientific elements. The number of research programs at the earlier stages of development is expected to be large and then to decrease steadily from stage to stage. However, since the cost of an experiment increases with its stage of development, the funding would probably be weighted toward the more expensive, advanced stages of development—even in a program with a large base of small experiments. The report also articulates the various program elements that should be active in a well-balanced scientific program. The program elements included plasma science and technology, confinement physics, confinement configurations, fusion technology, and systems analysis. General metrics for each program element are described. A continuing peer review process carried out by panels is also advocated to allow rebalancing the program as needed. In addition to the FESAC metrics described above, ongoing systems analyses of fusion power plants serve to assess the probability that specific confinement approaches will be achieved in the plant's reactors. These system analyses incorporate a conceptual design for a fusion power plant based on assumptions about the physics and technology. Usually these assumptions are intended to be judicious extrapolations from present knowledge, so the systems studies can be best executed for the most highly developed fusion concepts. For emerging concepts that are not well understood, a complete power plant study is more speculative. Such system studies serve to provide insight into the different fusion concepts embodied by the reactors, to reveal which physics and technology are most likely to increase the attractiveness of a fusion power plant, to provide a basis for comparing different fusion concepts, and to evaluate the role of fusion within the full portfolio of energy sources. The results of such studies are often encapsulated in a single number, the cost of electricity. However, such a simplification, which involves many uncertainties in economics and other areas, is inherently inaccurate. The more enduring value of systems studies is the guidance they provide for research. The above categorization of the various stages of reactor development (concept exploration, proof of principle, performance extension) has been useful in establishing a program to systematically evaluate innovative concepts. It has also provided a framework for peer review. However, the categories mainly relate to the progress of individual confinement concepts toward a fusion power reactor and not to progress on understanding fundamental, cross-cutting science issues. Thus, alongside these FESAC categories of concept development, a parallel set of scientific questions should be developed, as was proposed earlier in this chapter. This would give more weight to the scientific contributions from experiments and would more openly allow for experiments in configurations not suitable for a reactor that would advance important fusion science issues.

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Page 60 FINDINGS AND RECOMMENDATIONS Findings 1. A fusion research program must investigate a range of confinement approaches. Such a wide-ranging program would allow fusion science issues to be examined in ways not possible in a single configuration. It would also allow developing the optimal configuration for fusion energy application. 2. The fusion program benefits from experiments covering a range of scales. Some issues are best addressed at small scale, some at large scale. In the past, fundamental discoveries have emerged from both small and large experiments. 3. In the past several years, the OFES program has effectively broadened the spectrum of confinement configurations under study. At least six new experiments in nontokamak configurations have been initiated at the concept exploration level, and two have been initiated at the proof-of-principle level. In just the past year, several additional concept exploration experiments have been initiated. 4. FESAC has defined three stages for the experimental development of individual fusion concepts toward the fusion energy goal, along with metrics to assess whether a particular concept is ready to advance to the next stage of development. These categories of progress and metrics have been employed in the peer review process, which has led to the present program of innovative concepts. While the categories are effective in assessing the progress of a given experimental concept toward the fusion energy goal, they are not effective in defining or promoting the solution of essential cross-cutting science issues. Given that most of the concepts supported by the program will not, ultimately, manage to achieve reactor capability, this categorization underplays the broader scientific importance that such experiments could have for the program. Recommendations The confinement configuration program should be specified in terms of scientific questions. The primary contribution of exploring a variety of confinement configurations is the elucidation and discovery of plasma science relevant to fusion. An individual scientific question or issue may warrant experimental investigation in a variety of plasma configurations. The scientific question should determine the needed experimental scale. Some issues are best investigated at small scale, others require a large scale. Alongside the FESAC categories for stage of concept development, a parallel set of categories should be developed to assess how the research in the program is organized in terms of science issues. The OFES budget should also be justified according to these thematic scientific categories. The present set of categories (concept exploration, proof of principle, performance extension) describes the progress of individual fusion concepts towards a fusion power reactor but does not reflect progress in cross-cutting scientific issues. Whereas criteria for assessing concept development and the peer review process strongly emphasize scientific contributions, the above categories do not sufficiently reflect such contributions. A roadmap for the fusion program should be drawn up that shows the path to answering the major scientific questions, as well as the progress so far in the development of fusion concepts. The development of a roadmap for a fusion-based energy source is essential to aid in the long-term planning of the fusion program. The roadmap should show the important scientific questions, the

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Page 61 evolution of confinement configurations, the relation between these two features, and their relation to the fusion energy goal. Solid support should be developed within the broad scientific community for U.S. investment in a fusion burning experiment. Such an experiment is scientifically necessary and is also on the critical path to fusion energy. Determining the optimal route to a burning plasma experiment is beyond the scope of the committee's charge; rather, the route should be decided in the near future by the fusion community. Resources above and beyond those for the present program will be required. The U.S. scientific community needs to take the lead in articulating the goals of an achievable, cost-effective scientific burning experiment and to develop flexible strategies to achieve it, including international collaboration. The committee agrees with the SEAB report that “... development both of understanding of a significant new project and of solid support for it throughout the political system is essential.” 4 However, since the U.S. fusion energy effort is now positioned strategically as a science program, advocacy by the larger scientific community for the next U.S. investments in a fusion burning experiment now becomes even more critical to developing that support. For this reason alone, the scientific isolation of the fusion science community needs to be lessened. There should be continuing broad assessments of the outlook for fusion energy and periodic external reviews of fusion energy science. A planned sequence of independent external reviews should replace the current pattern of multiple program reviews of different provenance (e.g., this review and recent SEAB and FESAC reviews). These reviews should be open, independent, and independently managed. They should involve a cross section of scientists from inside and outside the fusion energy program. The manifest independence of the review process will help ensure the credibility of the reviews in the eyes of Congress, OMB, and the broader scientific community. The scientific, engineering, economic, and environmental outlook for fusion energy should be assessed every 10 years or so in a process that draws on the expertise of fusion scientists, other scientists, engineers, policy planners, environmental experts, and economists, from the United States and elsewhere. The assessment should examine from multiple perspectives the progress in the critical interplay between fusion science and engineering in light of the evolving technological, economic, and social contexts for fusion energy. Consonant with its charge, the committee has not taken up the many critical-path issues associated with basic technology development for fusion or the engineering of fusion energy devices and power plants, yet it is the combined progress made in science and engineering that will determine the pace of advancement toward the energy goal. Moreover, much of fusion science research is undertaken in the expectation that it will contribute to the energy goal. Regular, formal assessment of the progress towards fusion energy is one important way in which a fusion science program can be made accountable to the long-range energy goal. 4 Department of Energy (DOE), Secretary of Energy Advisory Board, Task Force on Fusion Energy. 1999. Realizing the Promise of Fusion Energy: Final Report of the Task Force on Fusion Energy. Washington, D.C.: DOE, p. 2.