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Burning Plasma: Bringing a Star to Earth 4 Program Structure and Balance INTRODUCTION From the discussions in this committee’s interim report (see Appendix E) and from the expanded analysis in the previous chapters, it is clear what can be learned from a burning plasma experiment and why the overall understanding achieved in the past decade makes a burning plasma experiment possible. On the basis of these considerations, and given the centrality of a burning plasma experiment to the development of fusion energy, the committee affirmed in December 2002 in its interim report and reaffirms here its recommendation that the U.S. fusion program participate in a burning plasma experiment. The committee also concludes that the best opportunity for the United States to pursue a burning plasma experiment is through participation in the International Thermonuclear Experimental Reactor (ITER) project. Subsequent to the issuance of the committee’s interim report, the U.S. government announced its decision to enter negotiations to participate in the ITER experiment. The U.S. and world fusion communities are already acting on this decision, and negotiations are in progress to define the possible roles of all potential participants in the ITER program. The discussion in this report has concentrated on issues directly related to participating in a burning plasma experiment. The previous two chapters focused on addressing the first two elements of the committee’s charge by discussing in detail the scientific and technical importance of a burning plasma experiment and the overall readiness of the fusion community to enter into such an experiment. This chapter addresses issues arising from the third element of the charge, which
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Burning Plasma: Bringing a Star to Earth 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 that apart from being presented with some short-term budget plans from the Office of Fusion Energy Sciences (OFES), progress reports on the state of the ITER negotiations, briefings on the activities and reports of the Fusion Energy Sciences Advisory Committee (FESAC), and reports on the status of the various elements of the current research program, the Burning Plasma Assessment Committee 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 this chapter. It should be noted that because the committee’s charge was limited to the consideration of magnetically confined burning plasmas, none of the inertial confinement fusion programs is considered here. Since the decision to reenter the negotiations on participation in ITER has been made by the U.S. government, it is necessary to consider the context and impact of this decision on the U.S. fusion program. The pursuit of a burning plasma experiment is a large undertaking that will necessarily require a significant shift in the distribution of activities in the U.S. fusion program. Even on a success-oriented schedule, experiments on ITER will not begin for approximately 10 years, and they will run for a decade or more. The Department of Energy’s fusion program must be designed both recognizing this timescale and addressing the importance of balancing the pursuit of the other critical issues of fusion science needed to establish the basis for fusion energy. In its interim report, the committee listed some 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).1 The committee reaffirms this conclusion. 1 The committee notes that the text in the interim report has a comma between the words “facility” and “consistent” in this quotation. Since publication of that report, the committee has become aware of the potential for the original formulation being interpreted in a manner inconsistent with the committee’s intent. Therefore, as shown in the Summary of the present report and in the list of recommendations later in this chapter, the committee has removed that comma. The removal of the comma reasserts the committee’s intended meaning, namely, that the U.S. role in producing the high-technology components of the facility be consistent with the size of the U.S. contribution.
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Burning Plasma: Bringing a Star to Earth With at least that level of participation in mind, the following question arises: What general areas of domestic research activity are required in anticipation and support of, and as a complement to, burning plasma experiments in ITER? To consider and answer this question in the interest of maximizing the scientific yield of the entire U.S. fusion science program, including a burning plasma experiment, the committee presents in this chapter a discussion of the domestic fusion science research program. The outstanding compelling scientific issues facing the program are considered in the following major section, entitled “Fusion Science Issues and Research Portfolio,” and how elements of the program will address these issues is discussed in the section after that, “Research Opportunities and Science and Technology Goals for the Domestic Fusion Program.” 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 and is thus relatively immature as an energy source. The ultimate success of producing an economically attractive new energy source is far in the future, and many outstanding scientific and technical issues have to be resolved before the path forward is well defined. Recognizing this, the 2001 study by the National Research Council’s Fusion Science Assessment Committee (FUSAC) recommended that the U.S. fusion program focus on addressing the compelling scientific issues and thereby strengthen the underlying science base of a fusion energy source.2 The committee agrees with this approach. This chapter focuses on the following issues: the critical science issues to be confronted by the U.S. fusion science program; research activities that could be undertaken over the next several years to prepare for experiments on ITER; fusion science issues to be addressed in a portfolio of smaller-scale research programs and specific goals to be pursued in those programs; the need for continuing efforts in theory and simulation; and concerns regarding education and workforce development relevant to achieving this overall program. The last two major sections of the chapter discuss the need for changing the structure of and setting priorities for the U.S. fusion program in the context of a decision to proceed with a burning plasma experiment. In formulating the rationale behind its recommendations, the committee focuses its discussion on research elements that will be important in the next few years and provides general guidance for the rest of the decade. The details for later 2 National Research Council, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program, Fusion Science Assessment Committee (FUSAC), Washington, D.C.: National Academy Press, 2001 (referred to as NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program), p. 3.
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Burning Plasma: Bringing a Star to Earth years are necessarily more general, because the understanding of phenomena such as turbulence, transport, and stability will deepen through theory, simulation, and experiments on existing and planned facilities. These advances are likely to change the course of the ITER program and other experiments in significant ways. Plans will evolve as understanding grows—as new ideas and priorities for the experimental plan itself are put forward, as new ways of interpreting experiments (and the tools to do this) are developed, and as confidence grows about the extrapolation of results. FUSION SCIENCE ISSUES AND RESEARCH PORTFOLIO As discussed earlier, the mission of the U.S. fusion science program is to advance “the knowledge base needed for an economically and environmentally attractive fusion energy source.”3 As noted in the goals of the U.S. fusion program, this requires advances in the fusion science of plasma confinement and fusion technology. For magnetic confinement, the key overarching goals for achieving attractive fusion energy are these: Maximize the plasma pressure, Maximize the plasma energy confinement, Minimize the power needed to sustain the plasma configuration, and Simplify and increase reliability of the overall system. The first three of these goals directly address increasing the economic appeal of fusion energy by increasing the efficiency of utilizing the magnetic field, increasing the power density, and decreasing the recirculating power. The fourth goal relates to overall system attractiveness and feasibility. The tokamak configuration of magnetic fields has made the greatest progress in advancing these goals and is thus capable of exploring burning plasmas. A burning plasma experiment would enable a large step forward by confronting these goals in a strongly fusing environment for the first time. As discussed in Chapter 2, there is a highly nonlinear interaction between the plasma and the magnetic field during plasma confinement. As a consequence, there are many arrangements of the magnetic field that confine plasma and offer possible advantages on these goals over the conventional tokamak. The various 3 U.S. Department of Energy, Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program, DOE/ER-0684, Washington, D.C., August 1996, p. 3.
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Burning Plasma: Bringing a Star to Earth configurations differ primarily by the degree to which the magnetic field is controlled externally or is self-organized by the plasma and plasma currents (see Figure 4.1 and the sidebar entitled “Magnetic Fusion Research Configurations”). The U.S. fusion program is focused on innovation and concept optimization, based on developing predictive understanding of the underlying physics. Accomplishing the program goals requires the investigation of the following issues: Plasma turbulence and turbulent transport, Stability limits to plasma pressure, Stochastic magnetic fields and self-organized systems, Plasma confinement with different types of magnetic field symmetry, Control of sustained high-pressure plasmas, Energetic particles in plasmas, and Plasma behavior when self-sustained by fusion (burning). A burning plasma experiment is a crucial step for the development of fusion science and technology. It will offer exciting opportunities to study the burning plasma physics issues, as discussed in Chapter 2. It is appropriate to ask what other FIGURE 4.1 Comparison of the main experimental configurations for magnetic fusion research. The various configurations are displayed relative to their level of self-organization and the strength of their toroidal magnetic field. NOTE: ST—spherical torus; RFP—reversed-field pinch; FRC—field-reversed configuration; Q—fusion gain factor. Individual images courtesy of the Max-Planck-Institut fuer Plasmaphysik; M. Peng, Oak Ridge National Laboratory; Lawrence Livermore National Laboratory; A. Hoffman, University of Washington, Redmont Plasma Physics Laboratory; M. Mauel, Columbia University.
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Burning Plasma: Bringing a Star to Earth MAGNETIC FUSION RESEARCH CONFIGURATIONS The main experimental configurations for magnetic fusion research can be usefully listed in order of the increasing fraction of magnetic field from external coils or, equivalently, in order of the decreasing degree of self-organization of the plasma configuration (see Figure 4.1). They include the field-reversed configuration (FRC), the spheromak, and the reversed-field pinch (RFP), all of which explore low-magnetic-field plasma configurations that rely on strong self-organization of plasma currents. These devices potentially offer more compact and more efficient confinement configurations but face formidable issues of plasma stability and sustainability. As the fraction of externally imposed magnetic field is increased, improved plasma stability and confinement are obtained, and fusion-grade plasma conditions are accessible. The devices that operate in this way range from the spherical torus (ST) to the tokamak and advanced tokamak, and, finally, the stellarator. The ST and advanced tokamak experiments use geometrical variations and increasingly sophisticated active control tools to optimize the performance and confinement efficiency of the plasma. These two types of devices are stabilized by relatively strong external magnetic fields, but also include significant plasma current and some self-organizing features of plasma behavior. The stellarator uses magnetic fields almost completely generated by external coils and, through three-dimensional shaping of the configuration, provides stable steady-state operation in the fusion regime without requiring plasma currents. The dipole configuration uses a relatively small superconducting ring floating within a large vacuum chamber to confine a hot plasma. It has the possibility of being steady state with classical confinement and high beta. Compared with a tokamak, the dipole configuration would not require current drive; however, the internal floating ring provides a technical challenge. More details on these confinement configurations are presented in Appendix F, “Fusion Reactor Concepts.”
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Burning Plasma: Bringing a Star to Earth activities are needed in order to investigate and resolve the full range of issues in fusion science. In order to maximize progress toward the goal of developing an attractive fusion energy source, how should the program be balanced between a program of burning plasma studies and a program of non-burning-plasma studies addressing other critical issues of fusion science and basic plasma physics? The proposed burning plasma experiment (ITER) is a tokamak; its design uses the best current understanding of accessible confinement. The committee concludes, in its interim report and in this report, that the fusion community is ready to take the step of proceeding with a burning plasma experiment. However, ITER is not a demonstration fusion reactor; significant further improvements will be required in order to develop an attractive fusion system—these improvements would need to include increasing plasma pressure, efficient stable sustainment to steady state, and higher generated fusion power density. The magnitude of the improvements needed can be estimated by comparing the ITER design with the Advanced Reactor Innovation Evaluation Study (ARIES) designs for projected attractive fusion energy systems.4 The ARIES studies generally assume that significant progress on each of the issues mentioned above achieves higher performance than has been demonstrated experimentally. These studies provide targets for the development of fusion energy systems and the associated fusion science experimental program. Table 4.1 compares the characteristics of ITER and the ARIES-RS (Reverse Shear) and ARIES-AT (Advanced Tokamak) studies, in which the normalized pressure is the ratio of the average plasma pressure to the vacuum magnetic pressure at the horizontal midpoint of the plasma. The ARIES designs project to economically attractive performance by producing 4 to 5 times more fusion power in less than half the plasma volume of ITER. They assume that the normalized pressure can be increased by a factor of 2 to 3 and that the plasma current can be sustained almost entirely by the pressure-generated bootstrap current, increasing the power gain (Q) of the reactor. One focus of the ongoing program is to achieve this level of plasma performance. The U.S. fusion program today is pursuing several research avenues to develop an understanding of the outstanding and compelling scientific issues, pursue the goals of the program, and thereby achieve such improvements. Some efforts—referred to as advanced tokamak research—involve modifications to the tokamak, leading to improved steady state. In addition, the current program includes re- 4 The ARIES program is a national, multi-institutional research activity for performing advanced integrated design studies of the long-term fusion energy embodiments to identify key research and development directions and to provide visions for the fusion science program. This research is funded by the DOE Office of Fusion Energy Sciences.
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Burning Plasma: Bringing a Star to Earth TABLE 4.1 Comparison of the Characteristics of the International Thermonuclear Experimental Reactor (ITER) and Two Advanced Reactor Innovation Evaluation Studies (ARIES)—Reverse Shear (ARIES-RS) and Advanced Tokamak (ARIES-AT) Parameter ITERa Pulsed ITERa Steady State ARIES-RSb ARIES-ATc Radius (m) 6.2 6.4 5.5 5.4 Plasma volume (m3) 831 770 351 329 Normalized pressure (percent) 2.8 2.8 5 9.2 Normalized confinement (H98y,2) 1.0 1.6 1.5 1.8 Pressure-driven current fraction (percent) Not available 48 88 91 Magnetic field strength (T) 5.3 5.2 8.0 5.6 Fusion power (GW) 0.5 0.36 2.17 1.76 Q (fusion power/power supplied) 10 6 22 49 NOTE: The normalized pressure is the ratio of the average plasma pressure to the vacuum magnetic pressure at the horizontal midpoint of the plasma. aFrom “ITER Technical Basis,” available online at http://www.iter.org/ITERPublic/ITER/PDD4.pdf. Accessed June 1, 2003. bFrom “Overview of the ARIES-RS (Reverse Shear) Tokamak Fusion Power Plant,” available online at http://aries.ucsd.edu/LIB/REPORT/CONF/ISFNT4/najmabadi.pdfandhttp://aries.ucsd.edu/ARIES/DOCS/ARIES-RS/RS6/output.html. Accessed June 1, 2003. cFrom “ARIES-AT: An Advanced Tokamak, Advanced Technology Fusion Power Plant,” available online at http://aries.ucsd.edu/LIB/REPORT/CONF/IAEA00/najmabadi.pdfandhttp://aries.ucsd.edu/miller/AT/output.html. Accessed June 1, 2003. search on innovative magnetic configurations that change the interaction of the plasma with the magnetic field. These concepts have developed and tested our understanding of improving fusion performance. There are many elements to consider when addressing how the current portfolio of research activities in the OFES program should evolve as the nation undertakes to participate in a burning plasma experiment at the same time that compelling scientific issues remain to be addressed. In the following pages, these scientific issues are considered in more detail. The discussion here focuses on the importance of these issues to the progress of the understanding of fusion science from the perspective of a non-burning-plasma program. How a burning plasma experiment, such as ITER, might address some of these questions was discussed in Chap-
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Burning Plasma: Bringing a Star to Earth ter 2 (see the section entitled “Scientific Importance of a Burning Plasma for Fusion Energy Science and the Development of Fusion Energy,” p. 54). 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. In addition, there has been recognition that sheared flows can be generated by the turbulence, establishing its saturated amplitude and transport level. Experiments directly testing the theoretical understanding of turbulence suppression are in progress on fusion experiments and smaller basic laboratory experiments. These experiments, together with continued progress in theory and simulation, will lead to improved predictive understanding. In particular, there is an acute need for improved understanding of electron turbulence and its effect on transport, as well as of edge transport and its influence on energy. Building on improved understanding, new magnetic configurations have been designed to facilitate the suppression of ion turbulence. In the advanced tokamak and stellarator, “reversed” or weak shear of the magnetic field’s helical twist weakens the turbulence drive, lowering the threshold for suppression. Turbulence suppression has been observed in such advanced tokamak experiments and is generally consistent with theoretical simulations. The spherical torus is predicted to have large-enough pressure-driven flow shear to suppress ion turbulence directly. This is being tested in ongoing experiments. Further improvements in the understanding of plasma turbulence will enable better configuration designs. 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. The experimentally observed stability limit in tokamaks is in reasonable agreement with theoretical predictions. Methods to increase the stability limit have been developed and incorporated in the advanced tokamak configurations—these methods include the use of a highly elongated and triangular plasma shape, modifications of the plasma current or magnetic shear profiles, and the stabilization of pressure-limiting instabilities using active feedback or close-fitting conducting structures. The spherical torus configuration was designed, building on the understanding of tokamak stability, to have a very high normalized pressure limit. This in-
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Burning Plasma: Bringing a Star to Earth creased limit has been demonstrated experimentally and is a significant motivation for investigating spherical torus plasmas for fusion energy. Stability pressure limits in stellarators and in reversed-field pinch (RFP) have not been experimentally observed. Experiments are under way to search for these limits and to compare theoretical predictions with observed behavior. In stellarators, however, the achieved pressures already significantly exceed theoretically predicted instability thresholds, and improved nonlinear models are being investigated. New experiments, designed using current understanding, will explore the theory at higher pressure levels and will evaluate access to normalized pressures more attractive in terms of stability. The experimentally observed normalized pressure in RFPs is already high enough (approximately 10 percent) to motivate investigation of that configuration. Stochastic Magnetic Fields and Self-Organized Systems In configurations in which plasma currents dominantly produce the magnetic field, or in which the plasma is unstable owing to tearing (or reconnection) instabilities, the magnetic field can become stochastic or turbulent. In this case, the motion of the plasma along these magnetic field lines can lead to a loss of particles and energy. Such systems can also self-organize, owing to nonlinearities in the plasma dynamics, as is observed in the RFP. An experimental understanding of the magnetic turbulence observed in RFPs has been used to develop methods to suppress the turbulence, improving the plasma confinement. The basic method is to carefully adjust the current profile near the plasma edge using external current drive. This method reduces the free energy driving the instabilities and is calculated to return the magnetic field to a nonturbulent state. The magnetic topology can also change as a result of local magnetic reconnection. This phenomenon is being investigated in several research groups in a concerted attempt to understand the fundamental mechanisms of the process. A number of experiments to investigate magnetic reconnection have clarified, although not yet completely illuminated, the physical mechanisms. Detailed measurements of the reconnection process have been performed. The magnetic structure of the region where the field lines break and reconnect is observed to be flattened, so the reconnection flows are not fast. Inside this region turbulence accelerates the reconnection process. The generation of this turbulence and the effect on the rate of reconnection are now partially understood. The experimental effort is complemented by a large coordinated effort to simulate reconnection using high-performance computing and supporting theoretical analysis. The computations have revealed the role of turbulence within the reconnection region. The combined experiment, theory, and simulation program has not reached the point
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Burning Plasma: Bringing a Star to Earth at which the rate of reconnection can be reliably predicted. However, progress is rapid, and the results are already changing the interpretation of reconnection events in fusion experiments. 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. This symmetry is important, as it ensures confinement of plasma-particle orbits and low damping of the plasma flow in the toroidal direction. Theoretical studies in the 1980s demonstrated that good particle orbit confinement could 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. These configurations are called quasi-symmetric. The quasi-symmetry can be chosen to be in a toroidal, helical, or poloidal direction. Such configurations have low flow-damping in the quasi-symmetric direction and can be designed to have orbit confinements as good as or better than a similar tokamak. Recently, the first quasi-symmetric (helical) experiment began operation. It has already observed signatures of confinement improvement with quasi-symmetric magnetic fields. New stellarator experiments are under construction to test quasi-toroidal and quasi-poloidal symmetry. They are designed to have excellent orbit confinement, while also optimizing the magnetic field distribution to increase the stability pressure limit. These experiments will determine whether three-dimensional magnetic field configurations can produce economically attractive fusion systems. 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. Toroidally symmetric configurations—including the tokamak, spherical torus, and reversed-field pinch—create part or most of the magnetic field using plasma current. This current must be generated either by the plasma pressure (the bootstrap current for the tokamak and spherical torus) or driven externally. Externally driven plasma current requires the injection of energy, which decreases the power gain of a fusion system. Thus, the advanced tokamak and spherical torus attempt to minimize the external current drive requirements by maximizing the pressure-driven bootstrap current. However, the profile of the pressure and current within the plasma must also be controlled to obtain stability for high plasma pressure. Feedback stabilization techniques may
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Burning Plasma: Bringing a Star to Earth FIGURE 4.6 Trends in the fusion technology workforce and budget since 1985. The trend shows that the fusion technology workforce has sharply declined since the mid-1990s, roughly coincident with the deemphasis of technology when the United States left the ITER project. Not only is this population aging, but there is a concern that it may fall below the number of staff needed to optimize participation in a burning plasma experiment and gain maximum benefit from participation. burning plasma device. It will also require a renewed and sustained effort to train and retain the highly specialized personnel necessary to create burning plasmas and to study fusion physics in them. These personnel must be trained not only in the fundamentals of basic plasma science, but also in technical areas specific to the study of burning plasmas. PROGRAM STRUCTURE AND ITS EVOLUTION Considering the previous discussions in this chapter and in Chapter 3, the committee believes it to be clear that, in order to look at the broad range of fusion science issues, the U.S. fusion program needs to support both the study of burning
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Burning Plasma: Bringing a Star to Earth plasmas and a portfolio of non-burning-plasma, smaller-scale research efforts. These two thrusts are tightly coupled, and pursuing one at the expense of the other seriously weakens the entire enterprise. A strategically balanced fusion program must include theory programs, computer simulations, experiments with existing facilities, advanced diagnostic development, technology development, and support for alternate configurations, not only as support for the ITER effort, but also as the means of continuing to look toward the larger goal of developing the foundations for fusion energy. This need for a U.S. fusion program that pursues burning plasma studies and addresses science issues beyond the burning plasma experiment itself has been affirmed by the fusion community’s 2002 Snowmass study, by reviews from the DOE’s Fusion Energy Sciences Advisory Committee (FESAC), and by outside reviews of the U.S. fusion program. Recognizing the diversified and balanced approach of the current program, the NRC FUSAC report says: An optimal fusion science program needs two components: experiments in non-burning plasmas to explore the large range of critical science issues which do not require a burning plasma; and experiments in burning plasmas….14 While concluding that fusion science is on a par with other fields of physical science, the FUSAC study 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 research.15 As noted previously in this report, this committee reaffirms these recommendations as guiding principles for embarking on a burning plasma experiment. The initiation of burning plasma experiments at a large facility will impact all levels of the U.S. fusion program. The ITER experiment, or indeed any burning plasma experiment, represents a significant new commitment by the United States to the development of fusion energy science. 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. The discussion in this section addresses the breadth and structure of the fusion program that will be necessary to support the development and operation of a burning plasma experiment on ITER and to achieve a program in which the critical elements are in reasonable balance for the purposes of attaining the long-range 14 NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program, p. 53. 15 NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program, p. 3.
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Burning Plasma: Bringing a Star to Earth fusion goal. Since the negotiations that will define the U.S. commitment to ITER are not complete, it is difficult to be precise now about the scale and distribution of the program elements. Nevertheless, some general principles are clear. They are presented below to define the structure of a fusion program including a burning plasma facility. Present Structure When considering the distribution, or balance, of activities in the fusion research program, it is instructive first to examine the program’s present structure, which was defined by its restructuring into a science-based program in the mid-1990s. The goal of the U.S. fusion program is to develop the scientific and technological knowledge base for practical fusion energy production. This goal was formally enunciated in the program’s mission statement: “Advance plasma science, fusion science, and fusion technology—the knowledge base needed for an economically and environmentally attractive fusion energy source.”16 The program has defined three goals to achieve in pursuit of this mission: “(1) Advance plasma science in pursuit of national science and technology goals; (2) Develop fusion science, technology, and plasma confinement innovations as the central theme of the domestic program; and (3) Pursue fusion energy science and technology as a partner in the international effort.”17 Pursuing all three of these goals 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—carried out through the study of plasma behavior across a portfolio of advanced tokamak and non-tokamak confinement considerations. The activities range from relatively large national experiments on advanced tokamak and the related spherical torus configuration, to small, university-scale experiments studying a range of non-tokamak confinement concepts. The larger facilities, which are 16 U.S. Department of Energy, Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program, DOE/ER-0684, Washington, D.C., August 1996, p. 3. 17 U.S. Department of Energy, Strategic Plan for the Restructured U.S. Fusion Energy Sciences Program, DOE/ER-0684, Washington, D.C., August 1996, p. 3.
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Burning Plasma: Bringing a Star to Earth well diagnosed, pursue simultaneous studies of a wide range of fusion science topics in near-reactor conditions; the smaller devices are typically focused on a specific topic, 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. Given the program’s budgetary constraints and the 1998 withdrawal of the United States from the original ITER consortium, several reviews—both internal18 and external19—endorsed this program structure and strategy. A few additional characteristics of the present program structure should be mentioned. With the restructuring to a science-based program in the mid-1990s and the subsequent U.S. withdrawal from the original ITER program, the technology programs in the U.S. fusion community shrank considerably. What remained of technology efforts was directed to supporting enabling technology for existing experimental programs—a Next Step Options design effort that led to the FIRE design—and relatively modest efforts at reactor-system design evaluations and some reactor-chamber research. A second trait of the present program is that some separation exists between the university fusion research community and the larger national laboratory efforts. There are, of course, very productive collaborations between selected groups or individuals from universities and the large laboratory programs. Nevertheless, the bulk of activity in the universities is centered on research in smaller facilities constructed under the DOE Innovative Confinement Concepts program and located on campuses. The larger facilities at the national laboratories generally pursue research activities that are carried out as directed programs staffed mainly by laboratory staff and full-time, on-site collaborators from other laboratories and universities. Required Elements of a Balanced Program Recognizing the need to optimize the scientific output of all elements of the present U.S. fusion program, the distribution of activities among the elements of 18 R. Bangerter, G. Navratil, and N. Sauthoff, 2002 Fusion Summer Study Report, 2003, available online at http://www.pppl.gov/snowmass_2002/snowmass02_report.pdf; Fusion Energy Sciences Advisory Committee, A Restructured Fusion Energy Sciences Program, Washington, D.C.: U.S. Department of Energy, 1996, available online at http://wwwofe.er.doe.gov/more_html/PDFFiles/FEACREPORT.pdf, accessed September 1, 2003. 19 Secretary of Energy Advisory Board, Realizing the Promise of Fusion Energy, Task Force on Fusion Energy, Washington, D.C.: U.S. Department of Energy, 1999. Available online at http://www.fusionscience.org/FETfinal.pdf. Accessed September 1, 2003.
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Burning Plasma: Bringing a Star to Earth the program must be substantively reconfigured with a commitment to a burning plasma experiment. This rebalancing is especially required because finite funding resources cannot be expected to support all possible interests of the fusion community. A newly restructured program may be considered an evolutionary change from the program as currently structured, but changes will nonetheless be required across the whole fusion program. One urgently needed change in the fusion community is the recognition, and the integration into program planning, 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 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 elements move forward, they will be necessarily integral parts of a balanced overall program. The distinction between a base program separate from the burning plasma activity, and vice versa, is no longer relevant or useful. 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, such as workforce development. The committee agrees that the rationale for a vigorous and broad program of research with both a burning plasma element and a domestic program of fusion science centered on understanding and concept optimization is compelling. However, this rationale must be dynamic, flexible, continuously developed, and enunciated clearly in order to maintain support. The issue, then, is how to strike the relative balance of activities across a tightly integrated program that addresses, as much as possible, all of the critical fusion science issues. As the balance is clearly influenced by available funding, conditions could lead to the suppression of activity in one area or another, which occurred when the pursuit of a burning plasma experiment was halted in the late 1990s. As the U.S. fusion community enters into the burning plasma era, the scale of the burning plasma experiment sets a new scale for other activities. In this respect, all other facilities—even in the largest national domestic programs—become smaller-scale focused (or “niche”) programs that are designed to explore issues complementary to those in the centerpiece burning plasma program. This change continues the evolution of the fusion program to a smaller number of larger-scale experiments—but experiments that are still small compared with the single burning plasma facility—both on the national and international scales. This shift to “bigger science” has implications for all areas of the U.S. fusion community; they include the optimal role of universities and laboratories, the setting of priorities, the role of technology, and so on. While a large portion of the program efforts will focus directly on the burning
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Burning Plasma: Bringing a Star to Earth plasma experiment as centerpiece of the program, the actual level of effort in that area is dependent on the U.S. role in the ITER program. The pace of the ITER program will be decided by the international participants. The U.S. component of that program will be settled as the negotiations proceed. A U.S. role in producing high-technology components is important, however, because of the need to keep the domestic fusion science and technology program involved in the compelling science questions. Those negotiations will determine the U.S. budget contribution to ITER construction; it is important to allocate sufficient engineering resources to support the ITER negotiations. Vigorous programs of experiments on existing facilities, theory, and computer simulation have brought the U.S. fusion program to the present level of understanding of the confinement of high-temperature plasma and readiness to pursue a burning plasma study. There is much to learn through a continuing experimental program that will directly impact ITER’s performance. Major existing tokamaks and a new Korean machine20 will be the workhorses of the program during ITER construction. Such experiments not only contribute to a deeper understanding of plasma physics, but also allow the testing of advanced diagnostic instrumentation that will be necessary for ITER itself. Some particular issues that these smaller tokamak experiments and theory can address in support of a burning plasma experiment were discussed earlier in this chapter (see the section entitled “Research Opportunities and Science and Technology Goals for the Domestic Fusion Program”). All of these facilities are useful now, and a subset should be kept running at least until ITER operates successfully. The second major component of the U.S. fusion program is the investigation of fusion science issues on innovative magnetic configurations (other than the standard tokamak) to improve future fusion systems. The research goals and opportunities of this program, as summarized in the previous major section of this chapter, represent a reasonable level of effort for this component of the program. The investigations of these toroidal configurations require sufficient supporting programs in theory, diagnostic development, and enabling technologies. The composition of this portfolio will necessarily evolve over time, reflecting the completion of specific campaigns and the generation of new ideas for furthering the exploration of fusion science and improving confinement configurations. 20 The Korean Superconducting Tokamak Reactor (KSTAR) project is a long-pulse, superconducting tokamak being designed to explore advanced tokamak regimes under steady-state conditions. A team of U.S. national laboratories, universities, and industrial participants (including the Massachusetts Institute of Technology, Lawrence Livermore National Laboratory, Oak Ridge National Laboratory, Princeton Plasma Physics Laboratory, and General Atomics) is supporting the Korean National Fusion Program in the design of KSTAR.
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Burning Plasma: Bringing a Star to Earth As is evident from the discussion here and in Chapter 2 of the compelling basic plasma physics questions that remain to be addressed, 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 effort commands a relatively small fraction of the 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, the scale of which is commensurate with the level of commitment and timing required to achieve the fusion energy goal. However, the technology programs at the present time will be those focused on enabling a successful burning plasma experiment—that is, focused primarily on those technologies important for the development of ITER. The endorsement of the merits of these varied activities in the U.S. fusion 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 regarding 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 section below, “Setting Priorities to Strike the Balance.” Integration of Program Activities The need to pursue the broad range of activities in the program as described above requires the participation of the entire fusion research community. As the program progresses inevitably to larger and more expensive facilities to access fusion-grade plasma parameters and phenomena, the need to integrate the research community into large-scale collaborative teams will grow. The community will be challenged by an increasing concentration on large facilities, similar to the situation in many other areas of physical science research. The entry into the ITER program is the most obvious evidence of this trend, but it holds true also for the present and future domestic program activities. The guiding principle in preparing for 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 thus far in this report by recommending a technical level of participation. It is just as important for participation in the ITER program, and indeed for the entire U.S. fusion program, that the community consider fundamental changes in the way 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.
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Burning Plasma: Bringing a Star to Earth It is reasonable to assume that the assignation of operating time to particular experiments on ITER will be determined in large part by the scientific merit of particular proposals. To optimize the position of the U.S. community in such an environment, teams of researchers need to be organized. These teams, composed of researchers from all parts of the community, should be focused on particular topical areas of high scientific interest. Organizing these teams quickly would help inform the U.S. negotiators about desired participation areas and would facilitate preparations for U.S.-team-based research at ITER. These collaborative teams would concentrate national expertise, positioning it to scientifically lead and effectively pursue chosen areas of research in the ITER program. The choice of major research thrusts will need to be determined by the community itself. Some examples may 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 the Joint European Torus. Another important element of this approach is to employ the technological means and to develop the sociological infrastructure for participation in large-scale programs by a dispersed community of researchers. Remote communications should be exploited to allow remote access to all data, real-time participation in experiments from remote sites, and active, real-time communication for joint planning, scientific interactions, and so on. This transition to collaborative research based on scientific issues, coupled with a strong commitment to remote interactions, is a model required for the entire U.S. fusion 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 those efforts. It will provide opportunities to engage the universities in the critical research topics of the program, strengthening them and the entire U.S. fusion effort and better coupling the fusion science program to the physical science and technology communities. In order for this approach to be effective, the large domestic facilities will need to support collaborative teaming through the shared governance of the research programs and planning. 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. Such coordination and collaboration will both
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Burning Plasma: Bringing a Star to Earth optimize the large investments needed in the domestic program and give practical experience for participation in the ITER program. The transformation of the culture of the program described here will take time, and it could even be somewhat demographically driven so as to minimize disruption. However, it is important to start making this transformation now so that a vibrant domestic research program with a sufficient workforce for fusion-grade facilities is available, and the community is intellectually and sociologically positioned to optimize its participation in ITER as well as to optimally exploit its domestic facilities. 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 implementation of ITER, and arguably this will be the greatest influence on the building of a balanced U.S. fusion program that includes participation in the ITER effort. 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 its activities at all stages of the fusion program. 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. This is especially true in the present context of expected lean budgets. Subsequent to a decision to construct and participate in a burning plasma experiment, the DOE should initiate a rigorous evaluation of the program priorities. This priority-setting process should be guided by the stated objective of maintaining a balanced program and a focus on fusion science, 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 three 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.
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Burning Plasma: Bringing a Star to Earth 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 a clear decision-making process in the fusion research community. The types of questions that could be used to guide the prioritization process would include these: What is the priority of current programs relative to the emerging requirements associated with participation in the ITER effort? What is the future for U.S. tokamak research programs? What are the relative priorities of these programs? What should be the scope, pace, and composition of the investigations regarding alternative and innovative configurations? Which approaches should have high priority? What educational priorities should be set, and how should the presence of fusion science in academe be expanded? How should the U.S. fusion program be linked to current and planned international fusion research programs? What will be the impact of closing selected existing U.S. facilities to enable new research thrusts? What would be an appropriate transition strategy? The prioritization process could follow the model of the budget planning and prioritization process used by the DOE High Energy Physics Advisory Panel. This panel’s process has provided important input to DOE during the transitioning of ongoing research programs and facilities as new initiatives are implemented. The implementation of such a process will go a long way toward ensuring the best balance of the U.S. fusion program and its continued vitality and leadership. Finally, while the U.S. fusion program is currently planning on integrating its burning plasma activity into the international fusion program, the committee notes that a reasonably high level of international cooperation is already in place—through formal planning activities, regular workshops, and some personnel exchanges for the four largest programs in the United States. The global fusion effort is moving toward a deepening of the international effort with the realization of the ITER project. Any future development of larger domestic experiments, and any definition of future program needs, will be driven by the parallel evolution of
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Burning Plasma: Bringing a Star to Earth 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 all non-ITER-related activities discussed here with the global fusion program, as appropriate.
Representative terms from entire chapter: