Next Steps for the Fusion Science Program
The search for a means to control thermonuclear fusion has led to the development of magnetic and inertial plasma confinement systems and to the study of high-temperature plasma physics in general. Fusion research, carried out in the United States under the sponsorship of the Department of Energy’s (DOE’s) Office of Fusion Energy Sciences (OFES) and referred to herein as “the U.S. fusion program,”1 has made remarkable progress in recent years and has passed several important milestones. A large element in this program is that focused on the science of magnetic fusion, in which hot fusion plasmas are confined by large magnetic fields.
Significant progress has been made in understanding and controlling turbulence and instabilities in high-temperature plasmas; this in turn has led to improved plasma confinement. Theory and modeling are now able to provide useful insights into turbulence and to guide experiments. Experimental diagnostics can extract detailed information about the processes occurring in high-temperature plasmas. It is widely perceived in the plasma physics community that the next
large-scale step in magnetic fusion research and high-temperature plasma physics is to create a burning plasma—one in which alpha particles from the fusion reactions provide the dominant heating of the plasma. The objective of doing so is to understand the physics of the confinement, heating, and stability of a burning plasma as well as to explore the technical problems connected with the development of a power-producing fusion reactor. A burning plasma experiment is a key scientific milestone on the road to the development of fusion power.
The first mildly burning plasma experiments were achieved in the 1990s at the Tokamak Fusion Test Reactor (TFTR) in the United States and at the Joint European Torus (JET) in the United Kingdom. The plasmas in these experiments generated up to 16 MW of fusion power for about 1 s; 80 percent of this power was in the form of 14 MeV neutrons, which escaped from the plasma, and 20 percent was in the form of 3.5 MeV charged alpha particles (helium nuclei) that were confined within the plasma. These alpha particles heat the plasma through Coulomb collisions with the other particles within the plasma—the fraction of transient alpha-particle heating in TFTR was about 5 percent and in JET about 15 percent. Nevertheless, in both cases alpha-particle-induced heating of electrons near the plasma core was clearly measured. These experiments began the exploration of the burning plasma regime.
Several strongly burning plasma experiments have been proposed, including the International Toroidal Reactor (INTOR), the U.S. Compact Ignition Tokamak (CIT), the U.S. Burning Plasma Experiment (BPX), the Italian IGNITOR experiment, the International Thermonuclear Experimental Reactor (ITER), and, most recently, the U.S. Fusion Ignition Research Experiment (FIRE) (see Appendixes C and F for additional information on proposed experiments and fusion reactor concepts). The experimental goal in each of these experiments is to reach a plasma state in which the alpha-particle self-heating is the dominant energy source for the plasma.2 The creation of such plasmas is a necessary but not sufficient condition for the development of a practical energy-producing magnetic fusion power plant.
The study of the science and technology of burning plasmas is a critical missing element in the OFES program. The recent report from the National Research Council’s Fusion Science Assessment Committee (FUSAC)3 noted that experi-
mental investigation of a burning plasma remains a grand challenge for plasma physics and a necessary step in the development of fusion energy. In light of the need to accomplish that step and in view of the significant advances over the past decade in the understanding of magnetically confined plasmas and in improved designs for burning plasma experiments, the committee recommended in its interim report that the U.S. fusion program participate in a burning plasma experiment.4
During the past decade, the fusion community has achieved notable advances in understanding and predicting plasma performance—particularly in comparing the results of theoretical and numerical calculations with the results of experiments on small and intermediate physics experiments. These advances are documented in detail in the FUSAC report, which noted the “remarkable strides” in fusion science research. Of particular note is the ongoing effort to develop a fundamental understanding of the complex turbulent processes that govern the confinement of hot plasmas in magnetic fields. This effort has resulted in new theoretical models, large-scale computer simulations, new diagnostic techniques, and quantitative comparisons between theory and experiment. The application of these models gives added confidence to projections for the operation of a burning plasma experiment.
Progress has also been made in the understanding and control of a new class of large-scale magnetohydrodynamic (MHD) plasma instabilities, the neoclassical tearing mode, which has been a significant concern for the burning plasma regime. Progress in predicting, controlling, and mitigating fast plasma terminations has significantly reduced concerns about unacceptable electromechanical stresses in the proposed experiment. Experiments, both current and planned, and theory are bringing attractive advanced tokamak regimes with high pressure and self-driven currents closer to reality. These tokamak operating regimes may lead to a more economically attractive concept for a fusion reactor. The progress made in fusion science and fusion technology increases confidence in the readiness to proceed with the burning plasma step.
The incorporation of advanced design elements from the fusion science and technology community has resulted in more attractive proposals for the burning plasma experiment. These changes have reduced the estimated cost of such an experiment and have allowed the investigation of advanced tokamak features in the burning plasma regime. The designs require less extrapolation from present
National Research Council, Letter Report of the Burning Plasma Assessment Committee, Burning Plasma Assessment Committee (BPAC), Washington, D.C.: National Academies Press, 2002. (The text of this interim report is reproduced in Appendix E and is available online at http://books.nap.edu/openbook/NI000487/html/index.html.)
experiments, and the operating regime resides safely below established limits in plasma density, pressure, and current, making operational projections much more reliable. However, an additional and important goal of the burning plasma experiment is to explore operational regimes that are not so predictable, where instabilities are expected to arise in the self-heated burning plasma. Undertaking a burning plasma experiment within the U.S. fusion program is a great challenge to the fusion community and to the program itself. It is a step that requires careful strategic planning—a requirement that led to detailed consideration of such a strategy through “the Snowmass process”5 and lengthy deliberations of the DOE’s Fusion Energy Sciences Advisory Committee (FESAC). In addition, DOE charged the Burning Plasma Assessment Committee to conduct an assessment of the importance of a burning plasma experiment, an analysis of the readiness to undertake a burning plasma experiment, and an assessment of DOE’s plan for a burning plasma experimental program. The committee was also asked to make recommendations on the program strategy to maximize the output of such a program for the future development of fusion as an energy source. (See Appendix A for the complete charge to the committee.)
The present report answers this charge. Chapter 1 describes the context of the entire discussion and lays out a description of the committee’s reasoning; the chapter concludes with recommendations and guidance. Chapters 2 and 3 then describe in more detail the compelling scientific importance of and readiness for a burning plasma experiment, respectively. Chapter 4 discusses the overall structure of the nation’s fusion program in light of the general comments made in Chapter 1.
The issues raised in the later chapters are summarized in the following sections of this chapter as a synopsis of the rationale behind this committee’s findings, which motivated a number of well-defined conclusions. These findings and conclusions are the foundation for the recommendations presented at the end of this chapter.
This report focuses on the charge to the committee by assessing the scientific readiness for and benefits from participation in a burning plasma experiment. It is important to note that many additional issues and activities are critical to achieving practical fusion energy through magnetic confinement, but they are outside the purview of this committee. These include issues such as the qualification of
nuclear materials for long-life operation under high neutron fluences, the development of low-activation materials, the qualification of near-full-scale power technologies such as chamber components, high duty factor testing, and fuel breeding and management. The modeling and testing of the effects of fusion-produced neutrons on materials constitute an area of considerable scientific challenge and interest in itself. The proposed burning plasma experiment will allow some initial examination of several of these fusion technology issues, but their more complete development for practical fusion energy will require consideration at future dedicated facilities beyond ITER. This report focuses on the merits of the proposed experiment to elucidate the scientific and technological issues of a burning plasma.
PREPARING FOR A BURNING PLASMA EXPERIMENT
Although developing any energy source is a long and difficult task, the international fusion community has concluded that the critical next step toward fusion energy is to build a facility capable of achieving a burning plasma.6 Demonstrating a burning plasma is the experiment necessary for continuing to develop the scientific and technological understanding to proceed toward the development of controlled fusion energy.
A number of experiments, ranging from a reactor-scale device using superconducting magnets, to compact, high-field copper-magnet devices, have been considered for implementing a burning plasma experiment (see Appendix C for a discussion of the three currently proposed burning plasma projects—ITER, FIRE, and IGNITOR). On the global scale the greatest effort has been put into realizing the International Thermonuclear Experimental Reactor, an international facility that is designed to demonstrate the scientific feasibility of fusion as an energy source and to develop and test key features of the technology that will be required for a fusion power plant.7 A cutaway figure of the device is shown in Figure 1.1.
Several reports have considered this issue (see Appendix D for some fusion community efforts in this regard). The National Research Council has also addressed the subject of burning plasmas, saying, most recently, “(The) experimental investigation of a burning plasma remains a grand challenge for plasma physics and a necessary step in the development of fusion energy” (NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program, p. 53).
Before its withdrawal in 1998, the United States was a member of the ITER team. Following consecutive budget cuts in the U.S. fusion program (from $365 million in FY 1995 to $225 million in FY 1997) and its restructuring from a schedule-driven development strategy into a science-driven program in 1996, the U.S. Congress mandated withdrawal from the ITER program following the completion of the ITER Engineering Design Activity. Since 1998 the remaining ITER partners have continued with the development of a redesigned and improved ITER machine, and negotiations on the choice of a site and other important decisions are well under way.
The ITER project has benefited greatly from the expertise and scrutiny of fusion-plasma researchers throughout the world. The present design is the result of a decade of effort, which included one major redesign that lowered the anticipated cost by a factor of 2 by reducing the size and eliminating some of the capability to test fusion power components and technologies. The engineering design of ITER is well developed, and prototypes for many of the systems have been built. ITER provides excellent opportunities to address key physics issues. ITER has been designed to accommodate a range of heating and current drive technologies and to have the most complete set of plasma diagnostics of the three currently proposed burning plasma experiments. The long pulse capability, the range and flexibility of heating and current drive technologies, and the extensive diagnostic set provide the capability to explore and evaluate advanced, steady-state operating regimes. In addition, the present ITER design would demonstrate integrated operation of some of the important technologies for fusion power.
The U.S. fusion community has asserted that a burning plasma experiment is an essential milestone on the road to practical fusion energy and has identified its
readiness to proceed to ITER as the desired platform for investigating burning plasma behavior (see Appendix D regarding recommendations of the fusion community). The community’s near unanimity is based on important advances in understanding the behavior of large-scale hot plasmas. These advances come from experiments on a host of tokamaks around the world, on theory and computer simulations to understand and predict the results of experiments, and on the development of technologies that have made advanced facilities and diagnostics available.
With the foregoing assertion of the community in mind, the U.S. fusion program was considering reentering the negotiations on the ITER program when this committee was established. At the committee’s first meeting, on September 17, 2002, Raymond Orbach, director of the DOE Office of Science, asked the committee to report, by December 2002, on two aspects of its charge and to comment on whether the United States should reenter the ITER negotiations. The resulting interim report (see Appendix E) was issued on December 20, 2002, in response to that urgent request.
The interim report, expanded upon in the later chapters of this report, makes clear what can be learned from such a burning plasma experiment and why the overall understanding achieved in the past decade makes a burning plasma experiment achievable. These findings are summarized below.
Scientific Value and Interest
Fusion energy holds the promise of providing a significant part of the world’s long-term, environmentally acceptable energy supply. At the center of all schemes to make fusion energy is a plasma—an ionized gas that, like the center of the Sun, is heated by fusion reactions. A burning plasma experiment would address for the first time the scientific and technological questions that all magnetic fusion schemes must face. The scientific importance of such an experiment is discussed in Chapter 2 and summarized here. In addition to enabling the next steps in research on plasma confinement and heating, a burning plasma experiment will present new scientific challenges with a plasma that is mainly self-heated by fusion reaction products. The nonlinear behavior of magnetically confined plasmas at high temperature and pressure, a behavior that in turn may be modified by the alpha-particle heating, is of fundamental interest. In addition, burning plasmas used for energy production will be significantly larger in volume than present experiments, affecting the plasma confinement, and they may therefore be expected to show new phenomena and changes in previously studied behavior.
An extrapolation from present experiments to the effective size of an energy-producing reactor entails substantial uncertainty, which can, however, be reduced
by studying a burning plasma experiment. The increase in effective plasma size at high plasma temperature is predicted to modify many phenomena that can determine the level of fusion power produced in a reactor. Understanding these effects is not feasible in the smaller-scale8 fusion experiments that are available to the scientific community today.
In particular, it can be expected that a burning plasma experiment, owing to its unique plasma parameters and its ability to study these issues in the burning state, will make critical contributions to understanding the following:
Plasma behavior when self-sustained by fusion (burning),
Fusion-plasma turbulence and turbulent transport,
Stability limits to plasma pressure,
Control of a sustained burning plasma, and
Power and particle exhaust.
In addition to its scientific importance to fusion energy science, a burning plasma experiment may also make contributions to plasma science and science in general. Basic plasma physics is the study of fundamental processes in the plasma state of matter and is relevant to a variety of fields, including space plasmas, industrial plasmas, astrophysics, and fusion. A burning plasma experiment is designed specifically to investigate the burning plasma state and cannot replace experiments that are purpose-built to directly address the broader set of basic plasma issues. However, a burning plasma experiment and the scientific program that leads to and supports it may make useful contributions to the basic understanding of plasmas on issues such as these:
Magnetic field line reconnection,
Abrupt plasma behavior, and
Energetic particles in plasmas.
In considering the potential for even broader impact, the committee notes that progress in plasma physics, and fusion-plasma physics in particular, can lead to progress in other subfields of physical science. A burning plasma experiment will likely lead to progress in new regimes. There will undoubtedly be unexpected discoveries. Only a few examples of such connections are mentioned here. For
instance, burning plasmas will generate the highly energetic ions and large temperature gradients that characterize many astrophysical systems and provide the opportunity to study enhanced transport under these more realistic conditions. In addition, a burning plasma experiment may offer a chance to learn about self-organization of a complex physical system with strong drivers and weak constraints, which occurs in many astrophysical, space, and geophysical settings. Self-organization is characterized by phenomena on small spatial scales acting in concert to produce phenomena on large scales.
Technological Value and Interest
Depending on its scale, a burning plasma experiment could offer the opportunity for beginning the development of essentially all technologies needed for a fusion reactor. These include components and systems unique to fusion’s energy goal; plasma technologies such as divertors; heating, current drive, and fueling systems; hardened diagnostics; remote handling and maintenance capabilities; and superconducting coils of unprecedented size and energy. A burning plasma experiment will provide an integrated demonstration of the reliability and effectiveness of these technologies. In addition, by operating safely, reliably, and within the structural code requirements used by the nuclear industry, a burning plasma experiment can demonstrate some of the favorable safety characteristics of fusion power.
A burning plasma experiment could provide the opportunity to test and evaluate blanket designs—the blanket being the physical system surrounding the hot plasma; it provides shielding and absorbs fast neutrons, converts the energy into heat, and produces tritium. A breeding blanket—that is, a nuclear system that creates tritium via interaction of the fusion-produced 14-MeV neutrons with lithium—is a key fusion nuclear technology. Fusion reactors must operate with more tritium produced and recovered than is burned. A burning plasma experiment provides the first opportunity to evaluate test blanket modules.
A burning plasma experiment will contribute to developing the technology for tritium processing. Most of the fuel injected in a fusion reactor will not be burned in a single pass. Unburned fuel will be continuously transported to the plasma edge, where it must be collected, separated from impurities, and then reinjected. The demonstration of an integrated steady-state reprocessing capability in a burning plasma experiment would show that the technology can be extrapolated to the scale needed for a reactor. A related issue is to show that the tritium inventory in a fusion reactor can be kept to an acceptably low level.
The behavior and integrity of materials in a fusion system are of great importance to the long-term viability of fusion energy. The high flux of energetic neu-
trons poses a serious materials problem that will require substantial testing, some of which may be done on a burning plasma experiment and the rest of which may require a separate materials test facility. Burning plasma experiments will need to develop high-heat-flux components and will serve as testbeds in which to evaluate the performance of the components in a reactor-like fusion environment. The heat loads on components in a burning plasma experiment will be comparable to those expected in a reactor and as a research issue will require the application of state-of-the-art high-heat-flux technology using materials that satisfy requirements of tritium retention, safety, structural integrity, lifetime, and plasma compatibility. While some materials testing may be initiated, an evaluation of material lifetimes under expected fusion reactor neutron fluence will not be possible with the low fluence expected in this first burning plasma experiment.
In summary, a burning plasma experiment would be of technological interest particularly with regard to the following issues:
Breeding blanket development,
High-heat-flux component development, and
Remote handling technology.
Readiness to Pursue a Burning Plasma Experiment
Having asserted the scientific and technical interest in a burning plasma experiment, it is prudent to ask if the fusion community is ready to undertake such an experiment. Specifically, the question is whether an experiment designed and constructed with present knowledge can achieve a burning plasma state so that new phenomena present only in such a state can be explored. In assessing readiness, the committee found it useful to define 12 specific scientific and technical criteria—6 in each category—that it judged to be necessary (and sufficient) components of any path to a burning plasma experiment. The committee then assessed the readiness of current science and technology against each criterion. These criteria are discussed in more detail in Chapter 3 and are summarized here.
Following are the six criteria for defining scientific readiness:
Confinement projections. Reaching the burning plasma regime depends critically on the rate at which energy is lost from the plasma. It is possible to predict accurately the energy-loss rate in existing tokamak experiments through confinement scaling studies; the present level of uncertainty in these projections is acceptable.
Operational boundaries—plasma pressure and current. Tokamak operation is constrained by limits on the plasma pressure and current. The present operational boundaries and other constraints, including limits on plasma pressure and current, are sufficiently well understood and amenable to control to proceed.
Mitigation of abnormal events. Burning plasma experiments are designed to handle safely abnormal events such as disruptions, should they occur. While there is confidence that these and other abnormal events can be avoided or mitigated, further research is needed to develop operating regimes that present less stringent heat loads to plasma-facing components.
Maintenance of plasma purity. Impurities in the plasma—either helium from fusion reactions or from sputtered first-wall materials—can significantly degrade plasma performance. There is confidence that the required plasma purity can be obtained by helium removal and the inhibition of impurity influx from the first wall and divertor.
Characterization techniques. Techniques are available to adequately characterize and evaluate most of the important parameters in a burning plasma.
Plasma control techniques. Plasma control techniques are needed that are adequate to produce and evaluate burning plasma physics and to explore steady-state advanced operational regimes. Such techniques have been developed.
Following are the six criteria that define technical readiness:
Fabrication of necessary components. The required techniques for fabricating components have been successfully demonstrated with prototypes. The components for a burning plasma experiment can be manufactured and assembled, including the required magnetic field coils, the vacuum vessel, divertor, and first-wall components.
Component lifetime in a nuclear environment. The lifetime of the various parts of a working fusion reactor must be able to minimize the vulnerability to damage from operating in a nuclear environment. There is sufficient assurance that major components can survive in the required nuclear environments.
Lifetime of plasma-facing components. Prototype designs of plasma-facing components have been tested for normal heat-flux conditions, and it has been demonstrated that the mechanical designs can accommodate the projected disruption forces.
Tritium inventory control. Safety analyses have found that the proposed
burning plasma devices meet fusion safety standards, and none of the devices requires an evacuation plan beyond the site boundary. The required tritium inventory can be handled safely, but further research is required to develop plasma-facing components that can reduce the tritium inventory.
Remote maintenance. The required remote maintenance has been demonstrated in operational fusion experiments.
Fueling, heating, and current drive control. The injection of frozen pellets of deuterium-tritium is a proven method to fuel fusion plasmas. The use of various heating and current drive control systems is well established.
In essence, significant progress has been made in the development of the scientific and technological foundations needed to implement a fusion machine of the scale and nature of ITER. It is clear that ongoing research can be expected to adequately address issues requiring continued attention, but no issues remain that would undermine the fusion community’s assertion that it is ready to undertake a burning plasma experiment.
The Next Step?
On the basis of its consideration of the interest in and readiness for a burning plasma experiment, and given the centrality of implementing a burning plasma experiment to the development of fusion energy, the committee affirmed in December 2002 and reaffirms here that the U.S. fusion program should participate in the ITER program. The committee notes that since the issuance of its interim report, the U.S. government has joined the ITER negotiation process as recommended in that report.
Notwithstanding the progress at the ITER negotiations, even on a success-oriented schedule, experiments on ITER could not begin for another 10 years or so. The DOE must consider how to structure its fusion program so that it remains vibrant and positioned to optimize its scientific progress in this time frame and beyond. This effort will be a challenge, as was recognized in the committee’s interim report, which included the following recommendation:
A strategically balanced fusion program, including meaningful U.S. participation in ITER and a strong domestic fusion science program, must be maintained, recognizing that this will eventually require a substantial augmentation in fusion program funding in addition to the direct financial commitment to ITER construction [see Appendix E, p. 157, in this report].
This need was affirmed by DOE Secretary Spencer Abraham in January 2003 in a talk at the Princeton Plasma Physics Laboratory.
Our decision to join ITER in no way means a lesser role for the fusion programs we undertake here at home. It is imperative that we maintain and enhance our strong domestic research program—at Princeton, at the universities and at our other labs. Critical science needs to be done in the U.S., in parallel with ITER, to strengthen our competitive position in fusion technology.9
The preparation for and execution of a burning plasma experiment will be a multidecade activity. The scientific and technological payoff from this experiment will be greatly enhanced by a domestic fusion research program that both supports and complements the ITER program effort, to progress toward the long-term fusion energy goal. These goals can only be achieved through a balancing of the U.S. fusion science program in a dynamic way.
The next section examines the various elements required in a strategically balanced fusion program in some detail. It focuses on the critical science issues to be confronted by the fusion science program, on research activities that could be undertaken over the next several years to prepare for experiments on ITER, on fusion science issues to be addressed in a portfolio of smaller-scale research programs and on the specific goals to be pursued therein, on the need for continuing efforts in theory and simulation, and on considerations of education and workforce development relevant to achieving this overall program.
The goal of the U.S. fusion program is to develop the scientific and technological knowledge base for practical fusion energy production. It is thus characterized as a science program with an energy goal. A distinguishing feature of the U.S. fusion program has been the development of understanding at a fundamental level of the physical processes governing observed plasma behavior—a feature that was formalized with the 1996 restructuring of the fusion program. Studies and reports on the program have repeatedly pointed to the science focus of the fusion program as being critical to its success as a source of innovation and discovery for the international fusion energy effort.
Developing any energy source is a long and difficult task. Typically, the time from concept to facility is more than three decades after the basic concept has been proven. Fusion has not reached the stage for building a successful demonstration reactor. A decision to participate in the ITER burning plasma experiment represents a commitment to invest in a large experiment that will advance our scientific
Remarks of Secretary Abraham are available online at http://www.pppl.gov/common_pics/secretary_remarks.pdf. Accessed May 1, 2003.
and technical understanding in pursuit of the energy goal of the U.S. fusion program. The decision will clearly require the direction of a large amount of resources in the fusion program to support this effort. The ITER project, no matter how successful, is not an end in itself, but only a major step on the road to a larger goal—practical fusion energy. Even on a success-oriented schedule, experiments on ITER will not begin for approximately 10 years. It is natural to ask, therefore, how the DOE fusion program should be designed, recognizing both this timescale and the importance of balancing the pursuit of the critical issues of fusion science needed to establish the basis for fusion energy.
The discussion in the following subsections addresses the breadth of the fusion program necessary to support the development and operation of the ITER facility and to achieve a program in which the critical elements are in balance for reaching the long-range program goals. In addressing these issues, the committee responds to the third element of its charge, which asks for “an independent review and assessment of the plan for the U.S. magnetic fusion burning plasma experimental program … [and] recommendations on the program strategy aimed at maximizing the yield of scientific and technical understanding as the foundation for the future development of fusion as an energy source” (see Appendix A). The committee notes, however, that apart from being presented with some short-term budget plans from the Office of Fusion Energy Sciences, progress reports on the state of the ITER negotiations, briefings on the activities and reports of the Fusion Energy Sciences Advisory Committee, and reports on the status of the various elements of the current research program, it was not presented with a coherent and singular strategy for the OFES program. The committee strives to present a foundation for such a strategy in this report, as detailed in Chapter 4.
The U.S. fusion program is formally defined by its mission:
[To] advance plasma science, fusion science, and fusion technology—the knowledge base needed for an economically and environmentally attractive fusion energy source.10
The program has defined three goals to achieve in pursuit of this mission:
Advance plasma science in pursuit of national science and technology goals;
Develop fusion science, technology, and plasma confinement innovations as the central theme of the domestic program; and
Pursue fusion energy science and technology as a partner in the international effort.11
A strong domestic fusion program necessarily supports all three of these goals, but as with any dynamic and vital research program, the distribution of activities in pursuit of the goals must evolve to reflect current program priorities. The first two elements are often referred to as the core program or base program and include most of the research activities being pursued at present. These efforts provide a foundation for the fusion science program by investigating a range of key fusion science issues. The third element of the program includes participation in an international burning plasma experiment.
In carrying out its analysis of the fusion program, the committee did not find the common characterization of the U.S. fusion program as consisting of a “base program” and a burning plasma program to be particularly useful. The committee found it more important to view the program as a unified, science-driven effort that pursues the fusion energy goal and is composed of a diverse set of complementary efforts.
The U.S. fusion program’s pursuit of its three goals has defined the balance of the fusion program. During the past decade the program achieved notable advances in understanding and predicting plasma performance—particularly in the field of plasma theory and experimental work in comparing the results of theoretical and numerical calculations with experiment. Important parts of the evaluation of the scientific and technical basis for an attractive fusion reactor concept can be accomplished in smaller-scale activities. These activities, plus modest support of basic plasma science itself, encompass a wide range of experimental and theoretical investigations. This is referred to as the portfolio approach to fusion science and technology development.
The fusion research portfolio addresses issues of importance to developing the knowledge base for fusion energy. It involves studies of plasma properties across a range of different magnetic configurations to test basic understanding of magnetically confined plasmas, to improve reactor concepts, and to establish the science base that underlies the large-tokamak and burning plasma experimental programs. This portfolio includes programs in theory and computation, advanced diagnostic development, and enabling technology.
The importance of this diversified approach has been affirmed by past outside
reviews. The advances made by the portfolio approach fusion program are documented in detail in the NRC’s FUSAC report, which noted the “remarkable strides” in fusion science research. Recognizing the diversified and balanced approach of the current program, the FUSAC report says, “An optimal fusion science program needs two components: experiments in nonburning plasmas to explore the large range of critical science issues which do not require a burning plasma; and experiments in burning plasma….”12
While concluding that fusion science “is on a par with the quality in other leading areas of contemporary physical science,” the FUSAC study also noted that “a strong case can also be made that a program organized around critical science goals will also maximize progress toward a practical fusion reactor.”13
The FUSAC report recommended that “increasing our scientific understanding of fusion-relevant plasma should become a central goal of the U.S. fusion energy program on a par with the goal of developing fusion energy technology” as the appropriate approach to fusion energy.14 This committee reaffirms these findings as guiding principles while embarking on a burning plasma experiment.
It is clear that the commitment to move to a burning plasma experiment will require a substantial reconfiguration of the distribution of activities among the major elements of the domestic U.S. fusion program. In addition to the new activities required to prepare for participation in the ITER program, it will be necessary to substantially refocus many existing activities in support of the burning plasma ITER program. In addition, the balance between science and technology activities is critical. As the committee noted in its interim report: “[A] technology program without a strong science base, or a science program without a strong technology base, will leave the United States in a position where it cannot build effectively on the developments coming from more advanced programs abroad” (see Appendix E, p. 158).
This need for a broad science program has also been recognized by the DOE Office of Science; an occasional paper released by the Office of Science before the decision to reenter the ITER negotiations stated:
If the U.S. chooses to join ITER, it will be imperative to continue and strengthen the basic elements that have provided the insights leading to the improved ITER design in the first place. The core U.S. strengths in theory and modeling, diagnostics,
advanced and innovative concepts, and plasma and fusion technologies will be needed to ensure the success of ITER and the pathway to fusion energy.15
The balance of the research portfolio of the U.S. fusion program has been successful; it is now clear that a major part of the fusion program will be affected by the U.S. role in ITER. While the negotiations that will define the U.S. commitment are not complete, some general principles are clear. Accepting the need for a major investment in ITER, it is essential to consider the issues that will affect the balance of the portfolio of the U.S. fusion program following this significant change. The following discussion is framed in the context of the next few years. It provides only general guidance for the rest of the decade, because increased understanding of phenomena such as turbulence, transport, and magnetic reconnection is likely to change in very significant ways the course of ITER experiments.
Primary Issues of Fusion Science Research
The pursuit of the three fusion program goals, as detailed above, supports the development of the knowledge base for an attractive energy source and has effectively defined a balanced fusion program. The third element of the program encompasses participation in international burning plasma experiments, an element that was considerably deemphasized upon the withdrawal of the United States in 1998 from the original ITER program. The first two elements include most current research activities on non-burning-plasma issues—such as plasma stability, nonlinear turbulence, self-organizing systems, magnetic field symmetry, and plasma sustainability at high pressure—by studying plasma behavior across a portfolio of advanced tokamak and non-tokamak confinement considerations. The activities range from relatively large national experiments on advanced tokamaks and the related spherical torus configuration to small university-scale experiments studying a range of non-tokamak confinement concepts. The larger facilities are well diagnosed and pursue simultaneous studies of a wide range of fusion science topics in near-reactor conditions, while the smaller devices are typically focused on specific topics, which can be addressed in detail with less overall capability and diagnostic coverage. This program rests on a foundation of research in theory and simulation, advanced diagnostic development, and enabling technology developments.
The U.S. fusion program is focused on innovation and optimization, based on
“Fusion Energy—Bringing a Star to Earth,” available online at http://www.sc.doe.gov/Sub/Occasional_Papers/6-Occ-Bringing-a-Star-to-Earth.PDF. Accessed May 1, 2003.
developing predictive understanding of the underlying physics (see Chapter 4 for a more in-depth discussion of the program). Accomplishing the program goals has required and continues to require the investigation of the following primary and compelling issues:
Plasma turbulence and turbulent transport. A key to high fusion performance in burning plasmas is the suppression of turbulence and the transport of pressure and particles that it generates. Over the past two decades, a number of methods to suppress ion turbulence have been discovered, including stabilization by sheared flows. These experiments, together with continued progress in theory and simulation, will lead to improved predictive understanding of turbulence suppression.
Stability limits to plasma pressure. Increasing the plasma pressure that can be confined stably is key to developing more attractive fusion energy. Consequently, all of the research on magnetic configurations seeks to increase the maximum stable pressure limit.
Stochastic magnetic fields and self-organized systems. For configurations in which plasma currents dominantly produce the magnetic field, or those in which the plasma is unstable owing to tearing (or reconnection) instabilities, the magnetic field can become stochastic or turbulent, leading to a loss of particles and energy. A number of experimental efforts to investigate magnetic reconnection—along with complementary theory and simulation programs—have clarified, although not yet completely illuminated, the physical mechanisms involved.
Plasma confinement with different types of magnetic field symmetry. In tokamaks and most of the other magnetic configurations, the magnetic field does not vary in the toroidal direction and thus is toroidally symmetric. Theoretical studies have demonstrated that good particle orbit confinement can be achieved in three-dimensional stellarator magnetic configurations by making the magnitude of the magnetic field strength be constant along a specified direction in a suitable flux coordinate system. The resulting quasi-symmetric (helical) configurations have already begun operation and observed signatures of confinement improvement.
Control of sustained high-pressure plasmas. Steady-state operation greatly increases the economic appeal of fusion systems. Efficiently sustaining and controlling high-pressure plasmas therefore constitute a critical issue. While theoretically optimized solutions have been found, experiments have not yet observed steady-state-compatible high-pressure plasmas consistent with low amounts of external current drive. These investigations are crucial for establishing the benefits of the various fusion configurations.
Energetic particles in plasmas. A number of experiments have investigated how energetic particles—often beams of particles—excite waves and instabilities in plasmas. The theory of nonlinear wave-particle interaction has advanced considerably in the past 20 years and has been extensively validated against experiments. Different magnetic configurations can be more or less stable to these waves, offering opportunities for improvement.
Plasma behavior when self-sustained by fusion (burning). In a burning plasma, the dominant heat source arises from the fusion-produced fast alpha particles. This is fundamentally a nonlinear process, which will combine with the turbulent transport processes to modify the plasma equilibrium and stability properties. In addition, the fast alpha particles can directly generate fluctuations in the plasma and thereby influence the confinement of the alpha particles and possibly the background thermal plasma itself. The net result is a highly nonlinear plasma regime with strong elements of self-organization. Plasma regimes with the relevant population of fast alpha particles in a reactor-relevant size of experiment are accessible only in the proposed burning plasma experiments.
Having considered the primary and compelling issues facing the U.S. fusion program as it pursues the program goals, it is also appropriate to consider what the opportunities are for the fusion program as it prepares to incorporate a burning plasma program. In particular, the committee considered the following questions in its analysis: What are the needs of the burning plasma program on ITER? What are the goals of the concept-optimization programs? What role is there for novel concepts? What is the importance of developing fusion technologies? These issues are addressed below, followed by a discussion about the workforce and education issues that face the fusion program and the fusion community.
Research Opportunities and Science and Technology Goals for the Next Decade: Direct Support of the Burning Plasma Program on ITER
The preparation for and execution of a burning plasma experiment will be a multidecade activity. While there is every confidence that ITER will be a successful scientific endeavor, a number of scientific and technological issues must be addressed to prepare for and make the best use of a burning plasma experiment.
ITER is a tokamak confinement device, and a wide range of issues can be addressed in the domestic and world tokamak programs to prepare for and improve concepts for operation of the ITER experiments. From an examination of recent studies, the NRC FUSAC review, other community reviews, and presentations to this committee, the committee has identified key areas in which ongoing
U.S. research and development can make significant contributions in order to gain the maximum benefit from participation in a burning plasma experiment. The committee believes that these activities will be a significant part of the domestic program—in coordination with the international partners—to support and prepare for the operation of a burning plasma experiment. These activities define a substantial part of the role that tokamaks can play—with associated theory, diagnostic, and technology development—as ITER is constructed and operates. The issues to be addressed in support of the burning plasma program are discussed in detail in Chapters 2 and 3. A short summary is given here:
Theoretical understanding and modeling. This area includes the development of improved models of the edge plasma and pedestal, density limits, core confinement, and MHD instabilities.
“Pedestal” profiles in high-confinement plasmas. Work is needed to develop a first-principles theoretical understanding of this phenomenon in order to allow fully predictive transport models from the edge to the hot core region.
Turbulent transport. Understanding the transport in high-confinement mode (H-mode) discharges could lead to increases in energy gain and/or to operation at reduced current and magnetic field.
Edge-localized modes (ELMs). Understanding of these modes is needed in order to mitigate their effects on plasma-facing components, especially in the burning plasma regime.
Stabilizing neoclassical tearing modes. Controlling these high-pressure instabilities will expand the operation space of burning plasmas.
Advanced tokamak operating regimes. Developing the physics basis for long pulses before the initiation of ITER experiments would enable more effective use of ITER.
The density limit and high-density operation. The energy gain and purity of burning plasmas are favorably affected by increasing the plasma density.
Tritium retention in plasma-facing components. Additional research on materials and tritium transport, together with the development of alternative plasma-facing components, can be used to ameliorate this issue, thereby decreasing the potential for ITER downtime.
Disruption avoidance and mitigation. The extension of new gas-injection suppression techniques to ITER scale will reduce the effects of disruptive plasma terminations.
Divertor development. Divertor solutions at lower plasma densities with improved heat-flux capabilities are needed for exploring alpha physics and steady-state operating scenarios.
Plasma-facing components. The improvement of these components is a key
issue for ITER research and development. New designs must be further developed for fabrication with large-area manufacturing techniques.
Diagnostic development. The deployment of complex measurement techniques in a hostile radiation environment requires their careful integration into the facility design; a burning plasma requires new measurement capabilities for analysis and control.
Radio-frequency heating and current drive technology. Robust antenna designs and sources are needed to provide heating and current drive capabilities in a burning plasma.
Tritium breeding blankets. Research on tritium breeding using ITER is necessary to secure sufficient fusion fuel supplies for follow-on fusion devices.
Research Opportunities and Science and Technology Goals for the Next Decade: Concept-Optimization Research
In addition to the goals of the burning plasma program on ITER, the committee considered roles for the four largest concept-optimization research programs. Its specific scientific goals for each of these programs are summarized below:
Develop an understanding of paths to advanced tokamak regimes. The advanced tokamak (AT) is a variation of the tokamak confinement configuration. It uses active profile optimization and MHD mode stabilization to provide, in principle, steady-state operation at high pressure and enhanced confinement, with the self-generated bootstrap current sustaining almost the entire plasma current. The AT employs active control of accessible plasma profiles (e.g., heating, density, pressure, and so on) to provide this enhanced performance. The integration of these varied tools and characteristics into a self-consistent scenario is a major focus of research. AT experiments in smaller facilities with a range of control tools and plasma-shape capabilities will complement and guide the AT studies in the burning plasma program and in ITER itself.
Test the effects of extreme toroidicity in the spherical torus. The spherical torus (ST) is attained when the toroidal aspect ratio of a tokamak is reduced toward its absolute lower limit (i.e., the hole in the center of the torus is reduced to a small fraction of the plasma radius). The study of ST plasmas is of interest because it challenges tokamak-based physics understanding at the limits of toroidicity and shaping and provides access to plasmas of very high relative pressure and high fraction of self-generated currents. The ST may also provide a reduced-cost path to the development of fusion energy.
Investigate sustainment and enhanced confinement in the reversed-field pinch. The reversed-field pinch (RFP) is a toroidally symmetric configuration in which the magnetic fields are generated mainly by internal plasma currents. These currents result in the toroidal field’s changing direction near the plasma edge region (hence the name). The RFP provides a laboratory test of nonlinear plasma relaxation properties found in nature and the laboratory. An RFP reactor may present attractive properties, arising from low magnetic fields and high plasma pressure (relative to the magnetic pressure). The RFP is at a level of development considerably less mature than that of the tokamak.
Explore the potential for passive stability and steady-state operation in three-dimensional stellarators with underlying magnetic symmetry. The stellarator is a toroidal configuration in which the magnetic fields needed for plasma confinement and stability are generated by twisting the shape of external coil sets to produce closed magnetic-flux surfaces. The stellarator does not require externally driven plasma current—allowing very efficient steady-state operation and, potentially, greatly reduced susceptibility to current-driven instabilities. The near-term focus is to test the benefits predicted with magnetic symmetry using three-dimensional shaping, to examine more compact stellarator configurations, and to explore plasma shapes that are predicted to be able to operate at high normalized plasma pressures.
Explore novel and emerging fusion science and technology concepts. Small-scale experiments can address some unique fusion research issues, which may be relevant to near-term applications of fusion science and technology or allow the study of speculative, emerging concepts for advanced fusion systems. These experiments, and their associated theory efforts, address basic issues of formation, equilibrium, and stability. They promise potentially more compact fusion scenarios. The spheromak and field reversed configuration (FRC) are in this class—both are somewhat similar to if less mature than the reversed-field pinch.
Develop fusion technologies to enable innovative fusion science experiments and provide attractive long-term reactor concepts. The pursuit of a burning plasma experiment requires the development of new technologies to produce and study burning plasmas in ITER. In addition to developing those technologies related to the burning plasma program, the domestic fusion program, in collaboration with international partners, must advance the knowledge base for fusion energy by addressing issues in three main areas: plasma technologies in support of advanced fusion science experiments, plasma chamber technologies, and fusion materials. Regardless of the de-
gree of commitment to developing a fusion reactor in any specific time frame, research activity in these areas supports the long-range goal of developing attractive fusion concepts.
The committee agrees that, generally, the aggregate level of activity discussed above is needed both to support the move to a burning plasma program and to maintain a vibrant, productive domestic research program that is making progress toward the long-range goal of establishing the knowledge base for fusion energy. The committee notes that the range of activities presented here is strictly representative and is not meant to be proscriptive. The choice of which opportunities to pursue—including consideration of the U.S. fusion program goals and international fusion activities—must be determined by the usual federal government process, advised by the fusion community, as described later in this report.
Theory, Simulation, and Computation
Transferring knowledge of burning plasmas to other elements of the fusion program will require a detailed theoretical understanding of the fundamental physical processes involved. If the U.S. magnetic fusion program is to take full advantage of ITER, it will be necessary to develop a first-principles understanding of the phenomena that determine ITER’s performance. This will require the development of improved models of the edge plasma, transport barriers, density limits, core confinement, and MHD instabilities. Success in this endeavor will require a continued program of experiment, theory, and modeling, including a strong experimental program on ITER itself.
It has long been recognized that the complexity of the burning plasma problem precludes the use of purely analytical methods to yield the desired fidelity. Computer models of parts of the entire system were developed instead. This approach has led to a new level of understanding and has served the fusion program well. However, significant near-term challenges remain in the areas of plasma edge physics, turbulence on transport timescales, global macroscopic stability, and their extensions to a burning plasma regime. The problem of modeling systems with widely disparate time and space scales has been dealt with so far by the use of reduced descriptions, but at some stage of investigation the coupling between the reduced regimes becomes important and presents formidable challenges. An example of the complexity involved is what is called plasma edge physics. The plasma edge region, at the outer boundary of the plasma, is one of rapidly varying density, and it strongly influences stability.
Going forward, a program in theory and simulation must rely on a marriage of advances in information technology, plasma science, applied mathematics, and
future developments in software. The computation and simulation part of the fusion program will need attention and possible expansion for the ITER program.
The Role of the Universities: Research, Education, and the Fusion Workforce
The role of the universities in the fusion program is manyfold. The universities train the students who will fulfill the future workforce needs of the field. The universities serve as centers for research with long-term perspectives, in both experiment and theory. University research generates and nurtures new scientific and technological ideas, and it leverages new knowledge from other fields of science. University theoretical efforts make connections with concepts from other fields, such as fluid dynamics, plasma astrophysics, and materials-related plasma science. Local experimental facilities are testbeds for new ideas, and they give students immediate, hands-on experience in plasma and fusion science. University user groups play important roles in experiments at larger facilities. As fusion devices become larger and experiments are further coordinated on the worldwide stage, this trend—which has long been standard in astronomy, high-energy physics, and nuclear physics—can be expected to become even more important.
The ramp-up to a burning plasma experiment poses special challenges in meeting workforce needs, particularly in light of the workforce demographics in fusion and plasma science and engineering. Extending beyond the needs of the burning plasma experiment is a pressing need to replace aging personnel in fusion and plasma sciences in the universities and the national laboratories.16 In comparison with other fields, university fusion and plasma sciences faculty members are older than their counterparts, with comparatively fewer new hires in the field.17 The situation is similarly critical at the nation’s three largest fusion science laboratories, where there is a significant bulge in the scientific workforce in the 50- to 60-year-old age group.18 Meeting these personnel needs is a key function of the university fusion programs. As expressed in the committee’s interim report, “New people are required if the nation is to expand its [fusion] efforts and make the program endure. The necessity of attracting graduate students and postdocs into
the program requires that it have a strong university-based component” (see Appendix E, p. 158). If support is not available for faculty and graduate students in plasma and fusion science, scientists and engineers will move to other areas of concentration.
Recent assessments of university plasma and fusion programs reveal another challenge to training new fusion personnel. The 1995 NRC plasma science study19 and 2001 NRC FUSAC report found the fusion community to be relatively isolated from other fields of science and engineering. This isolation has many detrimental effects, including reduced appreciation for fusion science, decreased support for faculty appointments in fusion science, and reduced access to the broad population of science and engineering students. The University Fusion Association’s recent survey of university plasma and fusion science programs shows a decline of fusion science positions in the most highly ranked academic institutions in the United States.20 These programs tend to be the largest, most visible university fusion programs. The University Fusion Association’s survey of 10 of these large institutions indicates that 15 out of 66 faculty will reach retirement age in the next 5 years, while their institutional plans call for hiring at the very most 9 faculty members over the next 5 years. The conclusion is that the presence of fusion science research in the top 25 physics and engineering programs is declining just as the program is attempting to move toward ITER and the study of burning plasmas. This decline also raises the danger of further isolation of the fusion community from the larger scientific community.
New personnel with special technical training—beyond the conventional science and engineering degrees—will be needed to design and build the burning plasma experiment. The current pool of technical personnel is inadequate to fill this need. This shortage is due in part to the fact that the United States has built only one major fusion device in 20 years. With the redirection of the fusion program to a science program in 1996, the number of U.S. fusion technology personnel decreased by 50 percent, and support for specialized technology research facilities was reduced. Full participation in the burning plasma experiment will require that specific attention be paid to revitalizing the fusion technology workforce.
The potential payoff of a broad and freely structured program of long-term
university research requires that it continue to be an important part of the U.S. fusion program. There will continue to be a need for small-scale plasma and fusion programs with single or small groups of principal investigators. Maintaining a concentration of funding at only a few major facilities, pushing small-scale projects aside, makes the withering of these programs a real possibility.21 Similarly, there is a danger that a concentration of theory funding for only tokamak and burning plasma problems will lead to the evaporation of support for other important areas. There is much to be gained by maintaining innovative smaller programs in terms of both generating new ideas and attracting new talent.
The federal fusion program must be the steward of plasma science in order to maintain the flow of new ideas and new talent into the field of fusion science. Although the fusion program has made important contributions to basic physics knowledge in areas such as fluids and nonlinear dynamics,22 plasma research does not stand out as a priority in long-range planning among physics and engineering departments. Beyond basic plasma research, important university efforts include smaller-scale tokamak and alternate-concept experiments, as well as participation in the larger national programs. While the specific projects to be pursued will change as the fusion program evolves, the important role of university research in the U.S. fusion program will continue throughout the era of the burning plasma experiment and beyond.
Prior to the recent U.S. decision to rejoin the ITER negotiations, the Office of Fusion Energy Sciences took several important actions that help to increase the talent pool and ensure the vitality of the basic plasma research efforts in the universities. OFES established a Principal Young Investigator program in plasma science and several small-scale experimental programs via the Innovative Confinement Concepts activity. It also took a leading role in creating the DOE/National Science Foundation program in basic plasma physics. The level of support for these programs, and other measures to revitalize the fusion workforce, should be responsive to the research and personnel needs in the era of the burning plasma experiment.
The material presented here indicates that many plasma and fusion science faculty and fusion laboratory personnel are approaching retirement and that there may be a serious shortage of professionals in the future as ITER develops and the
program expands. However, as OFES funding improves, this outlook may become more positive. It is appropriate for OFES to initiate a review of the demographics with respect to this problem, utilizing historical time lags between funding, staffing, and graduate student enrollment. Expanding the percentage of funds going to university programs could attract more plasma and fusion science students and postdoctoral researchers and should increase the visibility of fusion science in the universities. OFES should examine the benefits of such a strategy as well as the negative effects on the non-university programs and staffing. OFES also should address how the large facilities can become more effective user facilities to integrate a larger university contribution, similar to the modes of other facilities supported by the DOE Office of Science.
The ITER Negotiations and Program Contingency
The pace of the ITER program will be decided by the participants through the negotiating process. The U.S. component will be settled as the negotiations proceed and as procurement packages are assigned and construction preparations commence. Those negotiations will determine the U.S. financial contribution to ITER construction and will determine the role for and demands on the U.S. fusion program as an ITER partner.
In its interim report, the committee listed a minimal level of participation in the ITER program to which the U.S. fusion program should commit in order to gain sufficient benefit from this opportunity to study burning plasmas. It said, “The United States should pursue an appropriate level of involvement in ITER, which at a minimum would guarantee access to all data from ITER, the right to propose and carry out experiments, and a role in producing the high-technology components of the facility, consistent with the size of the U.S. contribution to the program” (see Appendix E, p. 157).23 The committee reaffirms this conclusion. Involvement in high-technology components is important in order to challenge and sustain the domestic program’s vitality; without this type of activity, the U.S. readiness for fusion power will not be sufficiently leveraged off ITER.
Recognizing the importance of the negotiating process to the future of the U.S. program, the committee also made some recommendations in its interim report to the Department of Energy. Specifically, the committee recommended that in entering the ITER negotiations, the Department of Energy should take several actions:
Develop an estimated total cost of full participation in the ITER program, using standard U.S. costing analysis methods and considering the potential full scope….
Analyze several scenarios for U.S. involvement.
Assess the impacts of U.S. participation in ITER on the core fusion science program, including opportunities to increase international leverage in the core program as well.
Develop other options for a burning plasma experiment in case ITER construction is not approved by the negotiating parties.
Establish an independent group of experts to support the U.S. ITER negotiating team on scientific and technical matters [see Appendix E, pp. 160-161].
The committee was pleased to learn that a preliminary and successful review of the construction costs for ITER was conducted and considers this an important first step in understanding the potential costs of the ITER program for the United States. Furthermore, the committee understands that the DOE is carrying out an analysis of the various work packages that will be of primary interest to the U.S. fusion program and that it has engaged the fusion community in this effort through the establishment of a Burning Plasma Program Advisory Committee and the holding of an ITER forum for community input. The negotiating process remains critical to defining the future of the U.S. fusion program. With this in mind, the committee reaffirms the DOE actions recommended in its interim report and quoted above.
Notwithstanding the goodwill of all of the negotiating parties and the significant progress made to date, it is important to recognize that the ITER negotiations could be unsuccessful, and reasonable contingency planning for that eventuality is prudent until a decision on ITER is reached. In the case of failure to proceed with ITER, the world community would naturally reassess and look for an alternative approach to a burning plasma experiment that most likely would become an international collaboration. All potential participants would want a role in the choice of parameters and the final design of such an experiment. The FIRE concept represents one possible contingency that could be considered in this context. Depending on the circumstances, the partners would need to reassess the optimal path for the development of a burning plasma experiment. Because a burning plasma experiment is a key step on the necessary scientific critical path toward fusion energy, any delays in realizing such an experiment—such as failure in the ITER negotiations—will necessarily delay the domestic program’s ability to address and under-
stand fusion science questions that must be answered before practical fusion power can be developed.
STRIKING THE BALANCE
Summary of Findings and Discussion
The U.S. fusion program, after many years of research, is poised to take a major step toward its energy goal. It is clear that a burning plasma experiment is a necessary step on the road to fusion energy and of scientific and technical interest to the U.S. fusion program and beyond.
It can be expected that a burning plasma experiment will make critical contributions to understanding fusion science and fusion technology issues such as the following: behavior in a self-sustained burning plasma burn, fusion-plasma turbulence and turbulent transport, stability limits to plasma pressure, control of a sustained burning plasma, power and particle exhaust challenges, breeding blanket development, tritium processing, magnet technology, high-heat-flux component development, and remote handling technology. In addition, a burning plasma experiment may make useful contributions to the basic understanding of plasmas on issues such as magnetic field line reconnection, plasma turbulence, abrupt plasma behavior, and energetic particles in plasmas.
Recent studies inside and outside the fusion community agree that the U.S. fusion effort is scientifically and technically ready to undertake such an experiment and that ongoing research can be expected to adequately address issues requiring continued attention. The critical issues on which confidence is now high are these: confinement projections; operational boundaries—plasma pressure and current; the mitigation of abnormal events; the maintenance of plasma purity; characterization techniques; plasma control techniques; fabrication of necessary components; component lifetime in a nuclear environment; the lifetime of plasma-facing components; tritium inventory control; remote maintenance; and fueling, heating, and current drive control.
Having considered the options for a burning plasma experiment, members of the fusion community arrived at a consensus that the United States should seek to join the ITER program. Preparations for the ITER project are well advanced, and the U.S. government began participating in the ITER negotiations in January 2003.
The pursuit of a burning plasma experiment is a large undertaking that will necessarily require a major shift in the distribution of activities in the U.S. fusion program, not only now but as the ITER program evolves and develops. A large portion of the U.S. fusion program will focus directly on the burning plasma
experiment as a centerpiece of the program, including activities needed to support the development and operation of the ITER facility.
Considering the discussions earlier in this chapter and in the remainder of the report, the committee has found that the broad range of fusion science studied on a burning plasma experiment and by non-burning-plasma smaller-scale research efforts are complementary and tightly intertwined. Pursuing one at the expense of the other seriously weakens the entire enterprise.
The list of compelling basic plasma physics questions that will define the U.S. commitment to ITER is not complete. However, once the decision is made, fulfilling the international commitment to help construct the ITER facility and participate in the ITER program will necessarily become the highest priority in the program. Given the magnitude of this step and the need to support it in full, it is clear that a new balance will need to be struck among the elements of the U.S. fusion program. This rebalancing is required especially because finite funding resources cannot be expected to support all possible interests of the community. The restructured program may be considered an evolutionary change from the present structure, but nonetheless it will require changes across the whole fusion program.
This evolution in the program must be accompanied by the recognition of the strong interconnection among all elements of the expanded program. The often-cited distinction between an existing base program and a separate burning plasma program is no longer relevant or useful, and indeed it impedes the development of a unified rationale for the required broad-based program and undermines the support for the constituent parts of the program. As the burning plasma program elements move forward, they will be necessarily integral parts of an overall balanced program. Decisions on programmatic priority should be guided by the goal of optimizing the scientific output of the entire program, with due recognition for other program needs—for example, workforce development.
Compelling basic plasma physics questions remain to be addressed. In addition, and because of the need to continually maintain a plasma-physics-literate workforce, another element of the restructured program will need to be the continued support for stewardship of the field of basic plasma science. Although this need commands a relatively small fraction of actual resources in the U.S. fusion program, it is a critical component of any U.S. fusion program structure. Finally, the program requires a fusion technology component whose scale is commensurate with the level of commitment and the timing required to achieve the fusion energy goal. However, the technology programs at this point will be those focused on technologies that will enable a successful burning plasma experiment, that is, primarily those technologies important for the development of ITER.
The endorsement of the merits of these varied activities in the program by this committee does not mean that every activity can or even should be supported
unconditionally. Under any funding scenario that can be reasonably expected, decisions will need to be made about the relative priority of activities to pursue at any given time. Since the fusion program is a science-based program, these priorities need to be based on a discussion of scientific opportunities and goals. The need for setting priorities is discussed in the subsection below, “Setting Priorities to Strike the Balance.”
Implications for the Fusion Community
The guiding principle in preparing for U.S. participation in the ITER program is the need to position the U.S. fusion community to optimize the scientific output of its activities in the burning plasma program. This need has been addressed so far in this report by considering a technical level of participation. It is important for participation in the ITER program, and indeed for the entire U.S. fusion program, that the community consider changes in the way that it operates in order to position itself to provide the intellectual leadership of chosen areas of research and to optimize the return on its investment.
The choice of major research thrusts will be determined by the government with significant input from the fusion community; examples might include elements of advanced tokamak development, stabilization of large-scale MHD instabilities, turbulence and transport studies, and so on. This approach requires the organization of the community around campaigns that are based more on scientific issues than on the operation of individual facilities. Such an approach appears to be working well in the European program for the operation of JET.
A transition to collaborative research based on scientific issues is a model to be considered for the entire U.S. program as it moves forward. Organizing the research efforts on the larger domestic facilities—the advanced tokamaks, spherical torus, stellarator, and reversed-field pinch—in a similar manner will support the transformation of the community to more of a user-group model and will more effectively engage the research community in these efforts.
While the nature of fusion science research has its unique features, the community can profitably learn how to coordinate dispersed national and international collaborations from other areas of “big science,” such as the high-energy and astrophysics communities. This will both optimize the large investments needed in the domestic program and provide practical experience for participation in the ITER program.
This transformation of the culture of the program will take time and could even be somewhat demographically driven to minimize disruption. However, it is important to start now in making this transformation so that a vibrant domestic
research program with a sufficient workforce for fusion-grade facilities is available, and so that the community is intellectually and sociologically positioned to optimize its participation in the ITER program as well as optimally exploiting its domestic faculties.
As stated in its interim report, the committee recognizes that pursuing participation in ITER with a balanced program “will eventually require a substantial augmentation in fusion program funding in addition to the direct financial commitment to ITER construction” (see Appendix E, p. 157). However, the incremental funding requirements for the recommended program likely will be relatively small in the initial years, which should minimize the competition for funds within the overall federal research budgets.
Since the negotiations on U.S. participation in ITER are just starting, it is not possible to estimate the exact level of funding needed to pursue a viable research program at ITER.
The committee is concerned about the pressures on the U.S. fusion program as the United States moves into the ITER program if there is no increase in funding for the OFES. It is important to recognize that the costs of fabricating ITER and its components during the construction phase do not provide any significant support for the science and technology workforce in the fusion research community. While much of the research and development to support ITER has been done, a modest increase in technology and engineering support must be made available to support the negotiations and address some remaining issues, as well as to help mitigate technical risks during ITER construction. Most of the funds for ITER construction will go to those companies that will actually manufacture the components.
A flat budget for the OFES will degrade the scientific research support in the fusion program, inevitably leading to decay in facilities and a decline in research opportunities. A constriction of the U.S. fusion program to pay for ITER participation will disproportionately weaken the presence of the fusion program in academia; it will also further erode connections to the wider scientific and engineering community while reducing the career prospects for critically needed new young talent. In a similar vein, reduced effort on all of the large national facilities will reduce the critical activities needed by the U.S. community both to allow significant contributions to the planning of ITER research and to pursue configuration optimization. Such a reduced effort, in turn, will increase the risk that the United States will play a following instead of a leading role in the ITER scientific program. Similar considerations are clearly relevant for theory and simulation and
for technology. Overall, this approach weakens the very structures needed to optimize the benefits of the investment in the ITER program. A clear example of this kind of weakening has already been set in motion, as much of the fusion technology program that was focused on developments beyond ITER was eliminated in the FY 2004 budget. Overall, this is precisely the wrong approach and should not be taken.
A funding trajectory that avoids these risks would provide the support to capture the long-term benefits of joining the international ITER collaboration while retaining a strong scientific focus on the long-range goal of the U.S. fusion program. This approach would support the fusion research field as a vibrant and exciting field with opportunities for attracting outstanding young talent into the field, as well as increasing the connections of fusion research to the other fields of science and engineering in academia. As important, such an approach will position the U.S. contingent in the ITER project to be leaders in significant fractions of the overall program.
Estimates of the funding level needed to maximize the benefit from participation in ITER within the context of a balanced fusion energy program can vary significantly, depending on the areas of U.S. contributions to the ITER program that will be determined in the negotiations. Additional funding for burning-plasma-related support activities and augmentation of the core science program were estimated by FESAC and the DOE Office of Science in briefings to the committee at $50 million to $100 million per year, without elaboration.
It is clear that, at a minimum, in order to capture the benefits of a burning plasma experiment, augmenting the U.S. fusion program to cover all of the U.S. ITER construction and operating costs would be required.
In addition, for the committee’s recommendations to be implemented, several elements of the resulting program will require increased investment:
The U.S. share of ITER fabrication and experimental operation,
Investigations on present facilities and diagnostic development that directly support preparation for ITER,
Support for university programs and for theory and simulation,
An increased technology program, and
Increased utilization of programmatically relevant, larger national experimental facilities.
These areas of increased investment need to be balanced against currently ongoing and planned activities. The balancing process also could be guided by a multiyear budget-planning path that projects funding growth, within the broad ranges described above.
The committee has concluded that a prioritization process is needed to decide on the appropriate programmatic balance, given the science opportunities identified and the budgetary situation of the time.
Setting Priorities to Strike the Balance
The elements and thrusts of the U.S. fusion program are complementary and intertwined. However, a constrained federal budget environment is likely to continue during the period of ITER implementation, and arguably this will be the greatest influence on the building of a balanced U.S. fusion program that includes participation in the ITER program. Notwithstanding the success of the current portfolio approach to the U.S. fusion program, the budget stress facing the program is real and ongoing. The investment in ITER will be significant and must be accounted for in pursuit of a balanced U.S. fusion program. The OFES and the fusion community will have to make serious judgments with respect to priorities in determining the activities at all stages of the fusion program.
The endorsement of the merits of the program activities outlined in this report does not mean that every activity can or even should be supported unconditionally. Any funding scenario that can be reasonably expected will necessitate deciding the relative priority of activities to pursue at any given time. As the U.S. fusion program rebalances its priorities in light of commencing burning plasma studies, some lean years may be expected. The choice of which opportunities to pursue—and which program activities not to pursue—must be determined by the usual federal government process, advised by the fusion community and cognizant of international fusion efforts.
Active planning has been undertaken by the U.S. fusion community in recent years. However, the addition of so major a new element as the ITER program requires that, in order to ensure the continued success and leadership of the U.S. fusion program, the content, scope, and level of U.S. activity in fusion should be defined through a prioritized balancing of the program. A rigorous evaluation of the program priorities should be undertaken by the OFES, with broad-based input from the fusion community. This priority-setting process should be guided by the stated objective of maintaining a balanced program, as discussed in this report.
The committee concludes that in order to develop a balanced program that will maximize the yield from participation in a burning plasma project, the prioritization process should be organized with the following program objectives in mind:
Advance plasma science in pursuit of national science and technology goals;
Develop fusion science, technology, and plasma confinement innovations as the central theme of the domestic program; and
Pursue fusion energy science and technology as a partner in the international effort.
Through the prioritization process, the fusion community should identify and prioritize the critical scientific and technology questions to address in concentrated, extended campaigns, similar to the planning done for other areas of science such as for high-energy physics. A prioritized listing of those campaigns, with a clear and developed rationale for their importance, would be very helpful in generating support for their pursuit, while also developing clear decision-making processes in the fusion research community. Further discussion of possible models for this process and the types of questions that participants in the process might ask in the making of real priorities are presented in Chapter 4.
This prioritization process represents a reasonable form of risk management in the overall planning of a fusion program that stretches over several decades. It requires the identification of those issues that may be most uncertain and/or will have the greatest impact on decisions of future directions and investments. Addressing and resolving such issues will help maintain program focus and will continually improve the case for viable fusion energy.
Any future development of larger domestic experiments and any definition of future program needs will be driven by the parallel evolution of related activities in the international community. The international coordination of large science efforts can avoid duplication and exploit opportunities to perform leading-edge research on the best facilities in a cost-effective manner. It is thus important that consideration be given to coordinating with the global fusion program the broad range of fusion activities, including non-ITER-related programs, as appropriate.
Finally, the committee is convinced that the implementation of a process of program prioritization will go a long way toward ensuring the best balance of the U.S. fusion program and its continued vitality and leadership.
CONCLUSIONS AND RECOMMENDATIONS—ELEMENTS OF A STRATEGICALLY BALANCED FUSION PROGRAM
Conclusion: Participation in a burning plasma experiment is a critical missing element in the U.S. fusion science program.
The committee concludes that the scientific and technological case for adding
a burning plasma experiment to the U.S. fusion program is clear. During the past decade, the portfolio of activities within the U.S. fusion program has achieved notable advances in understanding and predicting fusion-plasma performance. Because of the progress made in fusion science and fusion technology, there is now high confidence in the readiness to proceed with the burning plasma step. It is also clear that progress toward the fusion energy goal requires the program to take this step and that the tokamak is the only fusion configuration ready for implementing such an experiment.
Conclusion: Participation in the International Thermonuclear Experimental Reactor (ITER) program provides the best opportunity for the United States to engage in a burning plasma experiment.
Of the choices proposed for U.S. participation in a burning plasma experiment, ITER, with the United States as a significant partner, is the best choice for a burning plasma experiment. It is the most mature design and, in the committee’s view, is both sound and carefully planned. It is sufficiently conservative in design to provide great confidence in achieving burning plasma conditions, while flexible enough to test critical advanced tokamak operating regimes in steady-state burning plasma conditions. It also allows tests of several fusion-relevant technology issues. Participation in the ITER program also leverages very effectively the U.S. investment in a burning plasma experiment. However, participation in ITER is a major modification to the U.S. fusion program, and the U.S. fusion effort requires a strategically balanced program in the context of meaningful participation in ITER to optimize the scientific output of this investment.
Conclusion: The fusion effort requires a strategically balanced program in the context of U.S. participation in ITER in order to optimize the scientific output of this effort and to maintain the readiness to exploit the outcomes of the fusion program as a whole.
Conclusion: In developing the U.S. fusion science program with participation in ITER, it will be important to maintain the diversified character of the U.S. program. In particular, the vitality of the U.S. program requires a diverse range of activities in the domestic and the international suite of current and planned tokamak and non-tokamak facilities.
When considering the balance of the U.S. fusion program, it is essential to analyze the program as a unified, science-driven effort in pursuit of the fusion energy goal and composed of complementary and diverse efforts. All three ele-
ments of the U.S. fusion program, outlined above in the section entitled “Striking the Balance,” are essential. The committee concludes that the strength of the U.S. program is in its science-based foundation. It will, therefore, be essential to maintain a strong program in fusion and plasma science as a companion to a major facility program such as ITER.
The outcomes of the negotiations to join ITER are critical to the future development of the U.S. fusion effort. It is therefore vital that the U.S. delegation to the negotiations strive for the best outcome for the program and the nation. As ITER negotiations commence, it will be necessary for the OFES, working with the fusion community, to reexamine all elements of the present and desired fusion program and to work through the difficult, often contentious, but vital process of prioritizing all parts of the program. In the absence of such a process, budget pressures and commitments to ITER could severely unbalance the program.
While the ITER process and the outcomes of the negotiations will determine a large part of the U.S. effort, this is not the only determinant when striking a new balance for the U.S. program. For instance, it is clear that a technology program without a strong science base, or a science program without a strong technology base, will leave the United States unable to build effectively on the developments coming from more advanced programs abroad as well as from ITER.
Although not directly related to a burning plasma experiment in a tokamak, some scientific issues of importance to the long-range development of the U.S. fusion program will be best addressed on nonburning facilities in tokamak and non-tokamak machines. The U.S. fusion program must continue an effort parallel to the ITER project focused on developing the scientific base for promising fusion reactor concepts.
The internationalization of fusion research is increasing with the development of the ITER project. However, the international effort is not limited to ITER, or indeed to collaborations on the large tokamaks in the global fusion portfolio. International partnerships on developing alternative fusion configurations have been and will continue to be important.
Throughout this report, the committee provides analysis of the compelling and key scientific, technical, and programmatic issues that will need to be balanced as the U.S. program progresses.
Conclusion: A robust program of theory and simulation, coupled with experimental verification, is required in order to maximize the yield of scientific and technical understanding from a balanced fusion program.
Theory and simulation are essential components of understanding large-scale fusion systems and have significantly contributed to progress in understanding the
behavior of fusion plasmas—for example, in the area of turbulence and nonlinear physics. Going forward, a program in theory and simulation must rely on a marriage of advances in experimental fusion science, information technology, plasma science, applied mathematics, and future developments in software.
Conclusion: The recruitment, training, and retention of scientific and technical talent are crucial elements of the U.S. fusion science program.
The success of the U.S. fusion effort will depend on strong programs in plasma and fusion science. Universities have and will continue to play several critical roles, including those of maintaining the workforce supply and serving as research centers that can generate and nurture new scientific and technological ideas and that can leverage extensively the latest knowledge from other fields of science.
The committee concludes that the ramp-up to a burning plasma experiment will pose critical workforce challenges for the U.S. fusion effort. Indeed, the scientific and technical workforce in plasma and fusion science and engineering in the universities and at large fusion facilities is aging, with too few young people entering the field. There is an immediate need for technically trained personnel to build a burning plasma experiment. It is clear, therefore, that the U.S. fusion program will have to take steps to meet these critical needs.
There is a related issue regarding the viability and vitality of the university programs. These projects provide many of the new ideas and techniques and the continuing influx of talented personnel that will be needed for a burning plasma experiment and beyond in the quest for useful fusion energy. The specific projects to be pursued in the universities will change as our understanding increases, new ideas are developed, new facilities come online, and strategies involving specific concepts evolve. Nevertheless, the role that university programs play in meeting personnel needs and providing new ideas and training opportunities can be expected to continue, throughout the era of the burning plasma experiment and farther along the path to practical fusion energy.
Recommendations for a Program Strategy
The committee offers its conclusions as guiding principles for the Department of Energy as it plans to maintain a strategically balanced fusion program in support of the ITER project, aimed at maximizing the scientific and technical understanding and providing the foundation of fusion as an energy source.
It is clear that there are many unknowns as the fusion community embarks on this great scientific challenge. The elements required for the long-term health and vitality of this part of the U.S. research enterprise are not crystal clear, but this
report strives to provide a strategy for the balancing of the program through its elucidation of the key scientific, technical, and programmatic issues that need to be addressed in the coming years. What is clear is that whatever strategy is adopted, it should be flexible, innovative, and inclusive in striking the required balance for success. It is with this objective in mind that the committee offers the following recommendations:
The United States should participate in a burning plasma experiment.
The United States should participate in the International Thermonuclear Experimental Reactor (ITER). If an international agreement to build ITER is reached, fulfilling the U.S. commitment should be the top priority in a balanced U.S. fusion science program.
The United States should pursue an appropriate level of involvement in the ITER program, which at a minimum would guarantee access to all data from ITER, the right to propose and carry out experiments, and a role in producing the high-technology components of the facility consistent with the size of the U.S. contribution to the program.
If the ITER negotiations fail, the United States should continue, as soon as possible, to pursue the goal of conducting a burning plasma experiment with international partners.
A strategically balanced U.S. fusion program should be developed that includes U.S. participation in ITER, a strong domestic fusion science and technology portfolio, an integrated theory and simulation program, and support for plasma science. As the ITER project develops, a substantial augmentation in fusion science program funding will be required in addition to the direct financial commitment to ITER construction.
The U.S. fusion science program should make a focused effort to meet the need for personnel who will be required in the era of the burning plasma experiment. This effort should have the following goals: to attract talent to the field; to provide broad scientific and engineering training, specialized training, and training on large devices, as required; and to revitalize the fusion workforce.
Although active planning has been undertaken by the U.S. fusion community in recent years, the addition of so major a new element as ITER requires that, to ensure the continued success and leadership of the U.S. fusion science program, the content, scope, and level of U.S. activity in fusion should be defined through a prioritized balancing of the program. A prioritization process should be initiated by the Office of Fusion Energy Sciences to decide on the appropriate programmatic balance, given the science opportunities identified and the budgetary situation of the
time. The balancing process also could be guided by multiyear budget planning that projects funding growth and should involve significant community input. The prioritization process should be organized with three elements of the fusion program in mind:
To advance plasma science in pursuit of national science and technology goals;
To develop fusion science, technology, and plasma confinement innovations as the central theme of the domestic program; and
To pursue fusion energy science and technology as a partner in the international effort.
These program elements are indeed the three goals of the U.S. fusion program as outlined by the OFES in 1996. The committee affirms these elements as substantive and appropriate for a strategically balanced program.
The committee notes that the development of a scientifically and programmatically balanced program for fusion energy research and development must be matched with a credible and achievable funding plan. The plan should have a multiyear focus and must be cognizant of overall federal budgetary issues and likely spending constraints. With this in mind, the committee offers the following observations on the budget implications of the strategy recommended herein:
Undertaking a burning plasma experiment cannot be done on a flat budget.
A funding trajectory for the U.S. fusion program should be developed to provide support for capturing the long-term benefits of joining the international ITER collaboration while retaining a strong scientific focus on the long-range goal of the program.
A flat budget for the Office of Fusion Energy Sciences (OFES) will degrade the scientific research support in the fusion program, inevitably leading to decay in facilities and a decline in research opportunities. Overall, this approach weakens the very structures needed to optimize the benefits of the investment in the ITER program.
At a minimum, in order to capture the benefits of a burning plasma experiment, augmenting the U.S. program to cover all of the U.S. ITER construction and operating costs would be required.
The OFES and the fusion community will have to make serious judgments with respect to priorities in determining the activities at all stages of the U.S. fusion program.
The committee concludes that the United States is ready to take the next critical step in fusion research and recommends that participation in a burning plasma experiment be implemented through participation in the ITER project as part of a strategically balanced fusion program. As the following chapters show, the opportunity for advancing the science of fusion energy has never been greater or more compelling, and the fusion community has never been so ready to take this step.