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Burning Plasma: Bringing a Star to Earth (2004)

Chapter: 4 Program Structure and Balance

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Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.”

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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-

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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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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-

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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-

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

also contribute to controlling these high-pressure plasmas. These are significant areas of current research. 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 advanced tokamak and spherical torus configurations.

Taking a different approach, stellarators produce the magnetic field completely or dominantly by external coils (with the remnant due to the bootstrap current). Stellarators are compatible with steady-state operation and robust, as the magnetic configuration is maintained as long as the coils are energized. Theoretically, the pressure limit in stellarators can be relatively insensitive to the detailed profiles of pressure and the bootstrap current. This compatibility with steady state is a significant motivation for investigating stellarator plasmas for fusion energy.

Energetic Particles in Plasmas

A number of experiments have investigated how energetic particles—often beams of particles—excite waves and instabilities in plasmas. For example, the excitation of plasma waves, lower hybrid waves, and whistler waves by beams has been studied extensively. The theory of nonlinear wave–particle interaction has advanced considerably in the past 20 years and has been extensively validated against experiments. In burning plasmas, the excitation of Alfvén waves by the energetic fusion alpha particles is of significant concern. Different magnetic configurations can be more or less stable to these waves, offering opportunities for improvement. An outstanding issue is that of exploring the properties of these waves in the different configurations and developing a predictive understanding to guide the design of fusion configurations beyond any initial burning plasma experiment.

Plasma Behavior When Self-Sustained by Fusion

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

RESEARCH OPPORTUNITIES AND SCIENCE AND TECHNOLOGY GOALS FOR THE DOMESTIC FUSION PROGRAM

In considering the scale of effort needed to achieve a strategically balanced fusion science program and motivate its support, it is useful to identify specific goals to be addressed and activities to be pursued. The previous section considered the nonburning fusion program from the perspective of compelling scientific issues that must be addressed to make progress on the fusion program goals. In this section, the committee considers how the fusion goals can be addressed from a programmatic perspective. The questions addressed include these: 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? and What is the importance of developing fusion technologies? To address such questions, a range of opportunities for fusion science research over the next decade or so is presented.

This report is not the first effort to identify the opportunities for the U.S. fusion program as it prepares to incorporate a burning plasma experiment. The recent DOE Integrated Program Planning Activity5 and the Snowmass studies by the fusion community itself6 have described challenges and research opportunities for nonburning plasma fusion science. The DOE Integrated Program Planning Activity plan for the fusion program is organized around a detailed set of scientific issues and objectives. Together, the discussions that led to these reports established a range of science and technology goals for the fusion science program for the next 5 to 15 years.

From an examination of the studies referred to above, the NRC FUSAC review,7 other community reviews, and presentations to this committee, the committee identified key areas in which ongoing U.S. research and development (R&D) are recommended for the domestic fusion science program. It should be noted that this list is strictly representative and not meant to be exhaustive. The actual choice of which opportunities to pursue must be determined through the usual federal government process, advised by the fusion community (as described later

5  

Integrated Program Planning Activity for the DOE’s Fusion Energy Sciences Program, December 2000; available online at http://vlt.ucsd.edu/IPPAFinalDec00.pdf. Accessed May 1, 2003. This plan established objectives at 5-year intervals, with detailed objectives for 2005, and envisioned a review at approximately that time.

6  

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. Accessed September 1, 2003.

7  

NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

in this chapter, in the section entitled “Setting Priorities to Strike the Balance”), and must include consideration of the U.S. fusion program goals and international fusion activities. Nevertheless, the committee agrees that, generally, the aggregate level of activity implied below 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.

Directly Support the Burning Plasma Program on ITER

ITER is a tokamak plasma-confinement device. A wide range of topics can be addressed in the domestic and world tokamak programs to prepare for and improve concepts for the operation of the ITER experiments. The preparation for and execution of a burning plasma experiment will be a multidecade activity. While there is every confidence that the ITER effort 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. This section identifies 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. While these opportunities are discussed in the context of ITER, they are generally relevant to all burning plasma experiments.

  • “Pedestal” profiles in high-confinement plasmas. Many of the highest-performance tokamak discharges operate in the high-confinement, or H-mode, regime, in which there is a steep gradient, or “pedestal,” in both the temperature and density near the plasma edge (see Figure 4.2). Projections of both the stored energy and the fusion gain, Q, depend strongly on the height of this pedestal. Transport models are able to predict the thermal transport and resulting plasma temperature only if the pedestal height is taken from experiment observations. Work is needed to develop a first-principles theoretical understanding of this phenomenon.

  • Edge-localized modes. The pedestal height in the H-mode is limited by so-called edge-localized modes (ELMs), which produce rapid bursts of heat and particles that can damage plasma-facing components. Mitigating these effects is an important topic for continuing research. Possible solutions now under study include new operating regimes with reduced or no ELM activity and ergodization of the edge magnetic field to control the pedestal. However, more experimental and theoretical work will be required before these techniques can be applied in the burning plasma regime.

  • Stabilizing neoclassical tearing modes. At high plasma pressures, tokamak

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

FIGURE 4.2 Temperature pedestal from a high-confinement mode regime discharge in the DIII-D tokamak. The increase in Te across the last 4 cm at the outboard midplane is comparable to the temperature at the central density. NOTE: Te is the temperature of the plasma, R is the radius from the center, and R–Rsep is the distance from the edge of the plasma. Courtesy of General Atomics.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

plasmas are susceptible to instabilities known as neoclassical tearing modes. These instabilities reduce the plasma confinement and projected fusion power output. It has been shown experimentally that these instabilities can be stabilized by injecting microwave power to drive currents at the location of the instability (see Figure 3.4 in Chapter 3).8 To expand the ITER operating regime to higher pressure, techniques to determine quickly and reliably the location of these instabilities and to control the feedback current must be developed.

  • Steady-state and advanced tokamak operating regimes. The tokamak would be much more attractive as a fusion energy source if it were able to operate in steady state. Developing the physics basis for long pulses before the initiation of ITER experiments would permit more effective use of ITER. Consideration is being given to hybrid operating scenarios that have improved confinement and stability limits. Successful demonstration of advanced tokamak scenarios would further expand stability limits, and additional current drive could permit discharges to be driven in true steady state, limited only by the cooling requirements of the device.

  • The density limit and high-density operation. Modeling indicates that the energy gain and fuel purity of burning plasmas are favorably affected by increasing the plasma density. However, in present-day tokamaks, a limit to the plasma density that is proportional to the plasma current is observed. Very near this limit, confinement in H-mode plasmas is often observed to decrease, although some discharges with good confinement at densities significantly exceeding this limit have also been observed. Good progress is being made, both experimentally and theoretically, in understanding this limit. Continued research to understand this limit and the development of methods to exceed this limit would provide significant benefit to a burning plasma experiment.

  • Turbulent transport. Understanding the transport in H-mode discharges and discharges with internal transport barriers could lead to large increases in energy gain in ITER and/or could permit operation at reduced values of

8  

R.J. LaHaye, S. Günter, D.A. Humphreys, J. Lohr, T.C. Luce, M.E. Maraschek, C.C. Petty, R. Prater, J.T. Scoville, and E.J. Strait, “Control of Neoclassical Tearing Modes in DIII–D,” Phys. Plasmas 9, 2051 (2002); G. Gantenbein, H. Zohm, G. Giruzzi, S. Günter, F. Leuterer, M. Maraschek, J. Meskat, and Q. Yu, “Complete Suppression of Neoclassical Tearing Modes with Current Drive at the Electron-Cyclotron-Resonance Frequency in ASDEX Upgrade Tokamak,” Phys. Rev. Lett. 85, 1242 (2000).

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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plasma current and magnetic field. Understanding plasma turbulence is a key unsolved problem and one of the grand challenges in plasma physics. Exciting progress has occurred in this area over the past two decades. A working model of ion turbulence and the associated plasma transport has been developed. It is capable of reproducing the general characteristics of the turbulence and the resulting temperature profiles, but requires detailed testing by experiments. In contrast, no such model exists for turbulent electron transport, particle transport, and momentum transport. Associated with these phenomena is the need to understand the generation of electric fields in the plasma—these can either be spontaneously generated or externally driven—since they can profoundly affect the turbulence and thus the resulting plasma confinement. Theoretical models and experimental measurements for short-wavelength turbulence, which is predicted to play the most important role in electron transport, are just beginning to be developed. Similar efforts are under way with respect to turbulence in the important plasma edge region. Further progress in this area will also require additional theoretical and computational efforts and new measurements of the properties of the turbulence.

  • Tritium retention in plasma-facing components. The present ITER design uses carbon-composite materials in the divertor, but the erosion of carbon and the deposition of tritium-laden carbon could make unusable much of the tritium inventory. Currently, two approaches are being pursued to address this issue. One approach is to better understand the erosion, transport, and redeposition of carbon and to devise mechanisms to remove the tritium from co-deposited carbon. The other approach involves the development of tungsten (or similar high-Z) plasma-facing components capable of both withstanding large pulsed-heat loads and producing plasmas with low levels of high-Z impurity radiation. Further research on this problem is needed before deuterium-tritium (D-T) plasmas are studied in ITER.

  • Disruption avoidance and mitigation. Disruptive plasma terminations can occur as a consequence of exceeding magnetohydrodynamic (MHD) stability limits or through control or hardware failure. Research has now been successful in developing a disruption-mitigation technique using the injection of high-pressure noble gas.9 Further research will extend the applicability of these results to larger devices. A related issue is that of determining

9  

D.G. Whyte, T.G. Jernigan, A. Humphreys, A.W. Hyatt, C.J. Lasnier, P.B. Parks, T.E. Evans, M.N. Rosenbluth, P.L. Taylor, A.G. Kellman, D.S. Gray, E. M. Hollmann, and S.K. Combs, “Mitigation of Tokomak Disruptions Using High-Pressure Gas Injection,” Phys. Rev. Lett. 89, 055001 (2002).

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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safe limiting values for the plasma operating parameters. Reliable triggers are under development to initiate disruption mitigation in the case of an unexpected abnormal event.10

  • Divertor development. The capabilities of ITER depend on divertors that can handle large heat and particle fluxes while maintaining plasma purity. The current ITER divertor is designed to operate at relatively high plasma densities. To explore alpha physics and steady-state operating scenarios, divertor solutions at lower plasma densities with improved heat-flux capabilities should be developed using techniques to cool the edge plasma through seeded-impurity radiation.

  • Plasma-facing components. Plasma-facing components are one of the key issues for additional R&D. Designs that have been proven on small scales must be further developed for fabrication using large-area manufacturing techniques. Further testing will be needed to verify that these techniques are reproducible and reliable. This R&D should be done in the 5 years or so before the components are fabricated.

  • Diagnostic development. The ITER program calls for a sophisticated set of measurement techniques capable of surviving in a hostile radiation environment. More diagnostic design is needed in order to integrate diagnostics into the ITER plan while maintaining the shielding requirements within the ports. Engineering R&D is needed to ensure the reliability of materials (ceramics and optical and insulating materials) and components (bolometers, probes, mirrors, and shutters) in the ITER radiation environment. New measurement techniques must also be developed; for example, a method is needed to measure the confined and escaping alpha-particle distributions in the burning plasma. These techniques must be developed and tested on ongoing experiments to avoid costly delays in undertaking burning plasma experiments.

  • Tritium breeding blankets. To ensure a sufficient tritium supply for follow-on devices, it is highly desirable to initiate research on tritium breeding on the ITER device. Since the tritium-breeding test blanket module for ITER will be a first-of-a-kind device, significant R&D is needed to verify its design and to predict breeding performance accurately. It would be advantageous

10  

D. Wroblewski, G.L. Jahns, and J.A. Leuer, “Tokamak Disruption Alarm Based on a Neural Network Model of the High-Beta Limit,” Nucl. Fusion 37, 725-741 (1997); D.G. Whyte, T.C. Jernigan, D.A. Humphreys, A.W. Hyatt, C.J. Lasnier, P.B. Parks, T.E. Evans, P.L. Taylor, A.G. Kellman, D.S. Gray, and E.M. Hollmann, “Disruption Mitigation with High-Pressure Noble Gas Injection,” J. Nucl. Mater. 313-316, 1239-1246 (2003).

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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to start R&D on the test blanket module immediately after the ITER negotiations are completed.

The committee believes that the activities described above will play a central role in the domestic fusion program, in coordination with the international partners, in supporting the preparation for and 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 following subsections address the role of the four largest concept-optimization research programs along with other key research activities and summarize specific scientific goals for each of them.

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 is a leading candidate for a first-generation design of a fusion reactor. It 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. In addition, these experiments will expand to investigate wider ranges of plasma shape and stability limits so as to test the fundamental understanding of possible AT regimes.

In summary, the major goals of the advanced tokamak program are these:

  • To demonstrate integrated advanced tokamak scenarios with current sustained dominantly by the bootstrap current and enhanced confinement at high pressure, and to develop predictive understanding of AT regime accessibility and control;

  • To develop techniques to control plasma current, pressure, flow, and transport profiles while maintaining plasma stability in this highly nonlinear, self-organizing regime;

  • To develop radiative divertor operation regimes that can minimize power deposition and maintain helium pumping in low-density AT operational regimes compatible with external current drive;

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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  • To test theories of MHD instability control and develop techniques to allow the active avoidance of unstable boundaries resulting from resistive wall modes and neoclassical tearing modes; and

  • To demonstrate techniques to ameliorate the effects of abrupt plasma disruptions if boundaries are breached.

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. The ST plasmas near these limits are characterized by the following: stable access to very high normalized plasma pressure (plasma pressure comparable to magnetic field pressure), suppressed electrostatic turbulence due to strong rotation shear, plasma of very high dielectric constant strongly affecting wave–plasma interactions, and high particle trapping near the plasma edge. The ST may provide a reduced-cost path to the development of fusion energy if the central induction solenoid can be eliminated through the development of start-up and sustainment techniques.

In summary, the major goals of the spherical torus program are these:

  • To test MHD stability theory at conditions of extreme toroidicity in order to elucidate physics of very high normalized plasma pressure and high fraction of self-generated (bootstrap) currents, strong magnetic shear, and strong plasma rotation relative to the Alfvén velocity;

  • To validate turbulence theory in the extreme condition of high pressure with possible electromagnetic effects—using unique features of the ST, such as strong field line curvature, strong and reversed-field gradients (magnetic well), and high edge magnetic shear to test fundamental theories of turbulence and transport;

  • To explore the interactions of strongly supra-Alfvénic energetic particles and MHD instabilities such as the toroidal Alfvén eigenmodes with spectral characteristics different from those found in tokamaks;

  • To extend the understanding of plasma edge instabilities and transport to regimes of high particle trapping and strong field line expansion; and

  • To demonstrate plasmas dominantly sustained by the bootstrap current and initiated without an internal transformer.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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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 cause the toroidal field to change direction near the plasma edge region (hence the name). The equilibrium results from a self-relaxation of the plasma to this reversed-field state; the relaxation is driven, to date, by a dynamo effect. This phenomenon 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; several areas of investigation are required in order to evaluate its potential for fusion and to provide laboratory tests of self-organizing plasmas with relevance to astrophysical phenomena.

In summary, the major goals of the reversed-field pinch program are these:

  • To demonstrate the generation of RFP equilibria without a dynamo driven by large-scale MHD instabilities, using efficient current sustainment techniques;

  • To evaluate the confinement properties of the RFP in the absence of large-scale MHD fluctuations;

  • To investigate the ability to improve the RFP via control of the plasma geometry and/or profiles and via control of the spectral properties of fluctuations;

  • To investigate the stability limit of the plasma pressure and to develop methods to increase it using feedback stabilization; and

  • To improve the understanding of the physics that is common to the RFP and astrophysical plasmas.

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. This allows very efficient steady-state operation and, potentially, greatly reduced susceptibility to current-driven instabilities. Advanced stellarator concepts suggest that confinement properties at least comparable to those of tokamaks can be achieved with underlying symmetries in

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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the magnetic field coordinate system. The near-term focus is to test benefits predicted with magnetic symmetry using three-dimensional shaping, examine more-compact stellarator configurations, and explore plasma shapes that are predicted to be able to operate at high normalized plasma pressures.

In summary, the major goals of the stellarator program are these:

  • To test theory of MHD stability boundaries in three-dimensional plasmas, varying the contribution from plasma currents, and to explore the sensitivity of the plasma pressure stability limit to strong three-dimensional shaping;

  • To test the understanding of current-driven disruptive instabilities in stellarators;

  • To demonstrate the predicted ability to achieve tokamak-like confinement properties in stellarators with magnetic symmetry;

  • To test theories of turbulence-driven transport in three-dimensional magnetic configurations of varying symmetry; and

  • To explore the ability to access improved confinement regimes in stellarators—the strong rotational damping, which is drastically reduced in stellarators with symmetry, provides a test of the mechanisms of turbulence suppression.

Explore Novel and Emerging Fusion Science and Technology Concepts

Small-scale experiments can address some unique fusion research issues that 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. The concepts promise engineering simplifications (for example, simpler plasma-wall interfaces) and potentially more-compact fusion systems that could be compatible with novel chamber technologies such as lithium metal walls. Many of these systems are small enough to reside in university laboratories; thus they efficiently contribute as workforce recruitment and training facilities as well as being research devices.

One class of such investigations addresses configurations with external topology that is spherical rather than toroidal; no electromagnets penetrate the plasma volume. The spheromak and field-reversed configuration (FRC) are in this class. Similar to the reversed-field pinch, they rely on self-organizing properties to establish closed flux surfaces for confinement, and they are susceptible to large-scale MHD instabilities. Fusion science research opportunities in this area include the following: exploring stability and confinement characteristics of spheromak plas-

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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mas in a regime in which the electron collisional path length is comparable to the plasma dimensions; developing an understanding of the physics of using linked magnetic flux tubes to form and sustain these strongly self-organized plasmas; and determining the origin of experimentally observed stability in the FRC at low collisionality.

A second class of small experiments addresses novel, less-developed fusion and plasma confinement concepts that expand the knowledge base of basic plasma stability and confinement and offer specific advantages for speculative new fusion concepts. The issues under investigation naturally evolve over time, but they include these, among others: the study of high-pressure plasmas in a simple magnetic-dipole configuration, the use of the magnetic compression of physical liners to compress and heat small FRC plasmas to thermonuclear conditions on a pulsed basis, and the use of strongly flowing and/or rotating plasmas to stabilize simple cylindrical plasma configurations.

Develop Fusion Technologies to Enable Innovative Fusion Science Experiments and Provide Attractive Long-Term Reactor Concepts

As discussed earlier, the pursuit of a burning plasma experiment requires the development of new technologies to produce and study burning plasmas in ITER and facilitates the testing of critical fusion technologies in a reactor-scale environment. 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 degree 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 development of low-activation materials that can survive in a fusion environment is a critical issue for the long-term suitability of fusion as an energy source. Such materials are not critical to the success of the ITER experiment, but the availability of appropriate materials impacts the performance, safety, and overall costs of an eventual fusion system. Consequently, this is an active area of research in the international program. Relative to Japan and Europe, the United States has a relatively small fusion technology program with concentration in low-activation materials and high-heat-flux components. Opportunities exist for the U.S. program in collaboration with international partners to make significant contributions to evaluating the properties of varying alloys and composites.

To realize the advantages of compact confinement systems that are being in-

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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vestigated for future fusion systems, novel plasma chamber technologies may be required to handle very high heat loads. Innovative chamber technologies using flowing liquid walls and high-power-density solid walls are under investigation.

Partner with International Collaborators

It is important to recognize that R&D in the U.S. fusion program needs to be coordinated with the international partners of the United States and with the ITER process. U.S. tokamak programs are already loosely integrated with equivalent and larger facilities in the European Union and Japan through the International Tokamak Physics Activity (ITPA), which identifies and promotes areas of cross-fertilization and comparative experiments. Recently there has been significant international planning of “joint experiments” to address critical scientific issues identified by ITPA groups. These other international tokamak programs—that is, in addition to the ITER program—are also pursuing many of the issues discussed above.

The stellarator, spherical torus, reversed-field pinch, and tokamak programs all have International Energy Agency agreements for international coordination and collaboration. Each of these respective communities holds regular meetings. In each of these cases, there is a high degree of sharing of personnel and tools between the U.S. and non-U.S. programs.

The U.S. support of the ITER endeavor and the entire U.S. domestic program will require tighter coordination and collaboration. The ITPA efforts will provide a natural bridge to coordinate the U.S. tokamak activity with related international efforts and thus will optimize the return to the United States from its investment in the ITER program. Increased international interactions could also benefit the configuration-optimization research programs and should be strongly encouraged.

THEORY AND COMPUTATION

One important goal of a burning plasma experiment is to use the knowledge gained to predict performance in other toroidal confinement devices (i.e., potential candidates for subsequent steps toward useful fusion energy). However, transferring burning plasma knowledge to these configurations 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 participation in ITER, it will be necessary to develop a first-principles understanding of the phenomena that determine ITER’s performance. This understanding 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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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continued program of experiment, theory, and modeling, including a strong experimental program on ITER itself.

The progress of fusion science has relied heavily on the development of theory and extensive numerical computation and simulation. It has long been recognized that the complexity of the burning plasma problem was so great that purely analytical methods are not capable of yielding the desired fidelity. Computer models of parts of the entire system were developed (the so-called reduced description), allowing a piecemeal simplification of the complex physics. This approach has led to a new level of understanding and has served the fusion program well. Much of the work has been carried out by individual investigators or small teams and has benefited from access to computational resources ranging from workstations to supercomputers.

Recent efforts along these lines have played an essential role in the decision to move forward with rejoining the ITER negotiations. Indeed, simulations in both fluid and kinetic regimes were able to demonstrate instability control or avoidance in substantial agreement with experiment. A critical lesson drawn from these efforts is the importance of tight coupling of theory, experiment, and computation.

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, the region at the outer boundary of the plasma, is one of rapidly varying density; it strongly influences stability. For a proper treatment of turbulence, an understanding of this region is necessary, and it determines divertor design. The plasma edge is not adequately treated in the current, simplified models. This defect stems from the need to deal with a kinetic-theory description in which the mean free paths vary dramatically, spatial gradients are large, boundary-condition fixation is essential but often incompletely known, and complicated chemistry and wall effects prevail.

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 emergence of grid computing may be an enabler of this kind, although progress in numerical algorithms can be as fruitful as improvements in hardware in dealing with large problems. Since many of these developments are expected to arise from university-based research programs, these activities require continued support. Emerging from these efforts will be new insights and algorithms that will improve the simulations which will eventually have

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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to be done on the largest of the supercomputers. One daunting goal is the development of integrated programs that reliably model in detail most of the fusion machine. The computation and simulation part of the fusion program will need attention and possible expansion for the ITER program.

It may be that other areas of science, heavily dependent on computation, have developed tools that can be adopted for the progress of fusion science. In particular, the struggle to improve weather forecasting by even one day has given rise to techniques of ensemble averaging, reanalysis, treatment of mesoscale and synoptic regions, and data assimilation to drive models. The approach used by the climate community has also been successful in permitting widely separated research groups to utilize common models as well as providing a testbed for new developments.

In the field of computation and simulation relating to fusion technology development, one area of potential promise is the marriage of nanoscience techniques and advanced computation to help in the development of materials modifications such as dispersion strengthening, which could allow for higher-temperature operation. Modeling material damage from energetic fusion neutrons is an especially challenging problem that involves molecular dynamics, mesoscale modeling, self-healing, and other areas, and combines the physics of different characteristic timescales.

WORKFORCE READINESS

In the era of a burning plasma experiment, the recruitment, training, and retention of scientific and technical talent constitute a crucial element of the fusion and plasma research and development effort. The nation’s research universities and national fusion facilities will play a critical role in filling these personnel needs. The decision to participate in a large burning plasma experiment such as ITER carries with it an increased level of commitment to an extended program in fusion research and development. Since the preparation for ITER and the execution of its experimental program are expected to cover more than two decades, the technical personnel activities associated with this effort must be sustained and ongoing. With any increased U.S. investment in fusion in the era of a burning plasma experiment, the development and maintenance of the most highly qualified personnel in plasma and fusion science and engineering become even more important than they have been until now. Training the plasma and fusion workforce has two related components: a broad university education in basic science and engineering and the more specialized training in technical areas specific to fusion and the burning plasma experiment.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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Aging Workforce and Dwindling Supply

New personnel will be needed not only for a burning plasma experiment but also to maintain the supporting educational and research programs in the universities and national laboratories. The current demographics of the fusion science and plasma physics workforce point to potentially significant problems.

The NRC FUSAC report noted that the fusion and plasma science workforce in the universities and at large fusion facilities is aging, with too few young people entering the field. The same report also noted that the nation’s fusion and plasma science programs are concentrated in relatively few universities.11 Responding to the FUSAC report and to earlier studies, the Office of Fusion Energy Sciences took important actions that will help to increase the talent pool and ensure the vitality of the basic plasma research efforts in the universities. It established a Principal Young Investigator program in plasma science, as well as supporting several new small-scale experimental programs through the Innovative Confinement Concepts activity. It also took a leading role in creating the Department of Energy/ National Science Foundation (DOE/NSF) program in basic plasma physics. In view of the need for supplying a sufficient workforce as the U.S. fusion program enters the burning plasma era, these issues are discussed briefly here.

The rate of plasma science Ph.D. production is summarized in Figure 4.3. The production rate of Ph.D.’s in plasma and fusion science shows a decline since the mid-1980s. The decline shown in Figure 4.3 generally tracks—although it starts approximately 3 years after—the onset of a similar decline in the funding level of the U.S. Office of Fusion Energy Sciences Program. In contrast, the rate of Ph.D. production over all fields of physics shows no such decline, consistent with approximately constant funding for physics as a whole. The flattening of the fusion budget over the past several years suggests that plasma and fusion Ph.D. production may soon flatten, or even rise in response to the increased number of university research initiatives started in the past decade. Nonetheless, the trend continues to be worrisome.

Of course not all new entrants into the field need come from university plasma programs, and in fact it is desirable to have an influx of new scientists from other areas of science and technology as the field moves forward. The U.S. fusion program has a long history of attracting talented scientists and engineers who were educated in other fields, such as high-energy physics or nuclear engineering. Such cross-fertilization from other technical fields provides valuable infusions of talent

11  

NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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FIGURE 4.3 (a) Total plasma science Ph.D. production per year from 13 institutions with major plasma science programs (red and violet) over the past 20 years. A decline of approximately 50 percent in the past decade is observed. Data from 1997 and later (shown in red) include all responding institutions. Pre-1997 data from some institutions are incomplete. The violet shows Ph.D. production from those institutions with data. Ph.D. production for the remaining institutions (open blocks) was assumed to equal the level for the most recent year with data. This assumption likely underestimates the pre-1997 Ph.D. production. (b) Funding level of DOE Office of Fusion Energy Sciences program in constant FY00 dollars. SOURCE: E. Scime, K. Gentle, and A. Hassam, Report on the Age Distribution of Fusion Science Faculty and Fusion Science Ph.D. Production in the United States, College Park, Md.: University Fusion Association, 2003. Courtesy of the University Fusion Association.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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and diverse approaches to fusion problems. Likewise, not all students who do Ph.D. research in plasmas and fusion pursue careers in fusion. It is estimated that these two fluxes tend to cancel one another, although hard data are not available at the present time.

The age distribution of U.S. fusion science faculty as compared with that of physics faculty in all fields is shown in Figure 4.4. The ratio of faculty in the 55-to-75 age bracket to faculty in the 30-to-50 age bracket is about 1.5 for the fusion science faculty and 1.1 for all physics faculty. As shown in part (a) of the figure, this aging of fusion science faculty is most pronounced when the older, more established, and larger institutions are considered alone. Current hiring plans will not remedy this situation. To quote the University Fusion Association report, “Hiring trends at [these] larger institutions suggest that recent and projected fusion science hiring at larger institutions is down…. [T]he hoped-for hiring in fusion science over the next five years indicates a hiring-to-retirement ratio of at most two hires for every three retirements.”12

As shown in Figure 4.5, the age distribution of the scientific and engineering workforce at the nation’s three largest fusion laboratories—General Atomics, the Princeton Plasma Physics Laboratory, and the Massachusetts Institute of Technology—is similarly skewed toward older ages. Replacing this demographic bulge in the fusion community as the program moves into the burning plasma era will place significant demands on workforce development.

The available data indicate that the scientific and technical workforce in plasma and fusion science is aging markedly. There is a possibility that too few young people will be entering the field. In the worst case, it is possible that a significant fraction of the U.S. participants in the ITER effort will be near the end of their careers. Predictions over the long range are uncertain in that they depend on overall program development, but it is clear that the situation merits deeper investigation and continuing scrutiny to ensure a sufficiently large, high-quality workforce in the fusion science program.

Recruitment and Basic Scientific and Technical Education

At least two factors affect the recruitment of new personnel into the plasma and fusion science workforce. The first is the relatively small number of U.S. plasma and fusion programs, discussed above. Increased educational and research opportunities in plasma science and the continued expansion of outreach efforts

12  

E. Scime, K. Gentle, and A. Hassam, Report on the Age Distribution of Fusion Science Faculty and Fusion Science Ph.D. Production in the United States, College Park, Md.: University Fusion Association, 2003, p. 1.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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FIGURE 4.4 (a) The age distribution of fusion science faculty at 23 institutions with active plasma and fusion science programs. Shown in red are data from six major centers of plasma physics (Massachusetts Institute of Technology, University of Maryland, University of Wisconsin at Madison, University of Texas, University of California at San Diego, and University of California at Los Angeles). (b) The age distribution of physics faculty in all fields in U.S. colleges and universities. SOURCE: E. Scime, K. Gentle, and A. Hassam, Report on the Age Distribution of Fusion Science Faculty and Fusion Science Ph.D. Production in the United States, College Park, Md.: University Fusion Association, 2003. Courtesy of (a) the University Fusion Association and (b) the American Institute of Physics.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

FIGURE 4.5 Age distribution of the scientific and engineering staff at the nation’s three largest fusion facilities: Princeton Plasma Physics Laboratory, General Atomics, and Massachusetts Institute of Technology. This population comprises roughly one-half of the professional research staff supported by the fusion science program, excluding the university population, and is reasonably representative of the community as a whole.

by the fusion community (for example, at the undergraduate level) would help. The aim of these efforts should be to provide a high-quality education in the broad range of areas relevant to fusion science and technology and to attract excellent talent to the field.

The second factor affecting recruitment is the availability of challenging job opportunities. Scientific and technical talent gravitate toward exciting opportunities. In other words, new initiatives and sustained efforts attract talent. With a time lag of 5 to 7 years from the start of new initiatives to the first students’ completion of their training, personnel needs take time to fill. This development time also argues for a sustained long-term commitment. Because of the expected scale of a burning plasma experiment, the ITER effort could provide such an opportunity

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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for a national initiative that would help attract and sustain talent, drawing in personnel from areas of science and engineering beyond traditional plasma and fusion science.

The breadth and quality of training will also be important. The more than two decades of future activity on a burning plasma experiment will be accompanied by significant changes in science and technology. The well-trained fusion scientist or engineer of the coming decades will require knowledge of concepts and techniques that do not now exist. The hardware and techniques for engineering and scientific research can be expected to change in fundamental ways. Examples involve expected advances in computational techniques, laser and other radiation sources for probing plasmas, sensors, measurement techniques, materials, manufacturing techniques, interfacing of computers to experiments, and so on. Furthermore, many of the scientific concepts used to describe physical phenomena will be qualitatively more sophisticated a decade or two hence. Examples of areas currently undergoing dramatic changes include the modeling of nonlinear processes ranging from plasma heating to magnetic reconnection and models of plasma turbulence and turbulent transport. These and many other areas are likely to change dramatically in the decades of the burning plasma experiment. Thus, the basic training of fusion scientists and engineers in broad areas of physical science and engineering must continue to be an integral part of the fusion program.

Increases in funding for university programs potentially can have a disproportionately large impact in various ways. Such increases can have an impact on recruiting new talent, on providing broad training of fusion scientists and engineers, on expanding the ties between the fusion community and other areas of science and technology research, and on leveraging more effectively the U.S. investment in burning plasma R&D to generate new ideas and exploit progress made in other fields.

The committee believes that the U.S. fusion program should make a focused effort to analyze and address personnel needs required for the following: (1) revitalizing the fusion workforce, (2) building a burning plasma device, and (3) conducting burning and non-burning-plasma experiments (see the subsection entitled “The Role of the Universities: Research, Education, and the Fusion Workforce” in Chapter 1). If a dearth of personnel is found, the fusion program could consider several possible actions that would aid in resolving this problem. Options might include highlighting a program of nationally competed, prestigious fellowships in fusion science and technology to attract outstanding Ph.D.’s to the field. To infuse new talent into the aging university plasma and fusion faculties, the fusion program could consider providing increased matching salary and start-up funds for new assistant professors in plasma and fusion science. Expanded use of DOE Office of Fusion Energy Sciences fellowships at the national fusion facilities

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

could encourage the participation of graduate students in larger-scale activities in fusion science and technologies. Similarly, establishing sizable university-based user groups to collaborate on national facilities could increase university involvement and offer unique graduate study opportunities. Finally, broadening the available talent pool and expanding training opportunities for students and postdoctoral researchers could be aided by increased support for the NSF/DOE plasma science initiative13 and the DOE/NSF Fusion Science Center Program that is scheduled to begin with a first center in FY 2004.

Ensuring the continuing vitality of the fusion science and engineering research activities in the universities is critically important. Projects that have traditionally been the major source of trained personnel for the fusion program include smaller-scale confinement experiments, diagnostics development, theory and modeling, and technology research. Recognizing that much of fusion science research is moving to team-oriented research on larger, shared facilities, it is also important that the university community have the opportunity to become integrally involved in these regional, national, and international fusion research activities.

Specialized Training in Fusion Technology

Fusion and plasma physics of the future, and particularly the burning plasma experiment, will involve highly specialized technical endeavors. In many areas, the traditional doctoral degree in plasma physics or engineering will need to be augmented by training at a fusion-related facility (for example, a large tokamak or related facility, or ITER itself). This more specialized training will be required for work both on the burning plasma experiment and at other fusion-grade plasma research and development facilities. Specialized training is needed in all areas. Examples include tokamak operation and control, specialized diagnostics, and specific research topics such as fusion alpha physics, Alfvén modes, transport, and magnetohydrodynamic stability. This training can best be done by making full use of the range of U.S. and international plasma and fusion facilities.

The committee finds an immediate and critical need for technically trained personnel to begin to build the burning plasma experiment. The fact that there has

13  

The NSF/DOE initiative is currently funded at a level of $4 million per year. The 1995 NRC plasma science study recommended the establishment of this program at $15 million per year, and the NRC FUSAC report endorsed this recommendation. (See National Research Council, Plasma Science: From Fundamental Research to Technological Applications, Washington, D.C.: National Academy Press, 1995, p. 3; and NRC, FUSAC, An Assessment of the Department of Energy’s Office of Fusion Energy Sciences Program, p. 5.)

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

been only one fusion device, of modest size, built in the past decade has led to a critical shortage of trained fusion engineers. While the R&D effort associated with the ITER Engineering Design Activity helped bridge this gap, U.S. involvement in this activity ended 5 years ago. The fusion environment presents unique and challenging technical problems—for example, spatially and temporally varying magnetic fields, large transient electromechanical stresses, copious amounts of atomic hydrogen, high heat fluxes, a limited range of suitable materials that minimize plasma contamination, and significant fluxes of high-energy neutrons. The harsh and demanding burning plasma environment requires training personnel with highly specialized skills so that they are capable of developing practical engineering solutions and affordable components for the burning plasma experiment.

The bidding process for the ITER work packages is now under way. It nominally requires proven experience in the technologies and devices being bid. Owing to the recent deemphasis of fusion technology, the United States does not now have the desired level of proven experience in most areas. As shown in Figure 4.6, the number of personnel involved in fusion technology R&D in the United States has declined by about 50 percent since the mid-1990s, along with the budget for fusion technology. Specialized facilities at universities and national laboratories have been constructed for technology research, but they are currently underutilized. If the United States is to make the most of full partnership in ITER, significant new activity must be supported to reinvigorate the U.S. fusion technology enterprise and to enable the United States to participate effectively in the construction of components for ITER. Such activity will also help position the United States to play a leading role in the follow-on steps toward useful fusion energy.

Consideration might be given in the U.S. fusion program to creating internships in fusion technology for established scientists and engineers in order to jump-start the training of new fusion personnel. The program could also consider increasing its involvement in industries that provide fusion-relevant technology. This type of increased involvement could benefit the discovery of new technology—such developments are more likely in an environment in which fusion-relevant hardware is developed and constructed on a regular basis. New hardware presents new technical challenges and stimulates new solutions to this type of forefront problem.

In summary, careful attention must be paid to the training of scientific and technical personnel for the fusion and plasma physics work in the foreseeable future. This will require increased outreach to talent pools and additional connections to the broader academic, scientific, and technical communities. It will require immediate attention to the training and retaining of fusion engineers capable of designing and building the many intricate components necessary for a

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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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:

  1. What is the priority of current programs relative to the emerging requirements associated with participation in the ITER effort?

  2. What is the future for U.S. tokamak research programs? What are the relative priorities of these programs?

  3. What should be the scope, pace, and composition of the investigations regarding alternative and innovative configurations? Which approaches should have high priority?

  4. What educational priorities should be set, and how should the presence of fusion science in academe be expanded?

  5. How should the U.S. fusion program be linked to current and planned international fusion research programs?

  6. 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

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
×

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.

Suggested Citation:"4 Program Structure and Balance." National Research Council. 2004. Burning Plasma: Bringing a Star to Earth. Washington, DC: The National Academies Press. doi: 10.17226/10816.
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Next: Appendix A: Charge to the Burning Plasma Assessment Committee »
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Significant advances have been made in fusion science, and a point has been reached when we need to decide if the United States is ready to begin a burning plasma experiment. A burning plasma—in which at least 50 percent of the energy to drive the fusion reaction is generated internally—is an essential step to reach the goal of fusion power generation. The Burning Plasma Assessment Committee was formed to provide advice on this decision. The committee concluded that there is high confidence in the readiness to proceed with the burning plasma step. The International Thermonuclear Experimental Reactor (ITER), with the United States as a significant partner, was the best choice. Once a commitment to ITER is made, fulfilling it should become the highest priority of the U.S. fusion research program. A funding trajectory is required that both captures the benefits of joining ITER and retains a strong scientific focus on the long-range goals of the program. Addition of the ITER project will require that the content, scope, and level of U.S. fusion activity be defined by program balancing through a priority-setting process initiated by the Office of Fusion Energy Science.

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