The study of the science and technology of burning plasmas is a critical missing element in the restructured program of the Department of Energy’s Office of Fusion Energy Science (referred to in this report as the U.S. fusion program). The recent report from the National Research Council’s Fusion Science Assessment Committee (FUSAC) noted that experimental investigation of a burning plasma remains a grand challenge for plasma physics and a necessary step in the development of fusion energy.2 In light of the need to accomplish that step and of the significant advances over the last decade in the understanding of magnetically confined plasmas and in improved designs for burning plasma experiments, the committee recommends that the U.S. fusion program participate in a burning plasma experiment.
During the last decade, by focusing its reduced resources on plasma science, the U.S. fusion community has achieved notable advances in understanding and predicting plasma performance— particularly in the field of plasma theory and experimental work on small and intermediate physics experiments. These advances are documented in detail in the FUSAC report, which noted the “remarkable strides” in fusion science research. Of particular note is the ongoing effort to develop a fundamental understanding of the complex turbulent processes that govern the confinement of hot plasmas in magnetic fields. This effort has resulted in new theoretical models, large-scale computer simulations, new diagnostic techniques, and quantitative comparisons between theory and experiment. Application of these models gives added confidence to projections for the operation of a burning plasma experiment. There also has been progress in the understanding and control of a new class of large-scale magnetohydrodynamic (MHD) plasma instabilities, the neoclassical tearing mode, which has been a significant concern for the burning plasma regime. Progress in predicting, controlling, and mitigating fast plasma terminations has significantly reduced concerns about unacceptable electromechanical stresses in the proposed experiment. Experiments, both current and planned, and theory are bringing attractive advanced tokamak regimes with high pressure and self-driven currents closer to reality. These tokamak operating regimes may lead to a more economically attractive concept for a fusion reactor.
The progress made in fusion science and fusion technology increases confidence in the readiness to proceed with the burning plasma step. A modest reduction in mission and the incorporation of advanced design elements from the fusion science community have resulted in a more attractive proposal for ITER. These changes have reduced the estimated cost of such an experiment and allowed the development of advanced tokamak features in the burning plasma regime. The proposed design requires less extrapolation from present experiments, and the operating regime resides safely below established limits in plasma density, pressure, and current, making operational projections much more reliable. However, an additional and important goal of the burning plasma experiment is to explore operational regimes that are not so predictable and where instabilities are expected to arise in the self-heated burning plasma. Finally, experience with prototype components built as part of the design preparations for the ITER and IGNITOR experiments has increased confidence in the ability to build, assemble, and operate a burning plasma experiment.
Here, the committee offers two caveats: First, the fusion community is aging and has long range demographic problems. New people are required if the nation is to expand its efforts and make the program endure. The necessity of attracting graduate students and postdocs into the program requires the program to have a strong university-based component. Second, a technology program without a strong science base, or a science program without a strong technology base, will leave the United States in a position where it cannot build effectively on the developments coming from more advanced programs abroad. In its 1993 report Science, Technology, and the Federal Government: National Goals for a New Era, the National Academies’ Committee on Science Engineering and Public Policy (COSEPUP) said that the United States should be among the leaders in all major areas of science, and should maintain clear leadership in some of these areas so that it can take advantage of breakthroughs wherever they take