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In short, a broad family of concepts bound by common physics principles may be studied both to elucidate the fundamental physics and to stimulate the scientific innovation needed for fusion energy development. The U.S. fusion program is fruitfully becoming a program that recognizes the complementary, coupled contributions of different configurations. In addition, an optimal scientific program would contain experiments spanning a diversity of scales, with small experiments being optimal for some studies and large experiments necessary for others.

In this chapter the motivation for the fusion concept program and the program's status are summarized. First, a small set of illustrative fundamental science issues is discussed, together with how they are best investigated in a variety of plasma configurations. Next, the impact of advances in these basic physics questions on the fusion energy goal is presented. The inertial confinement approach to fusion energy is also a large, active, and challenging endeavor, with important scientific underpinnings and opportunities. Although beyond the scope of this report, it, too, is discussed briefly. Another section touches on representative connections between the U.S. fusion program and the broader international program. The engineering challenges, which are discussed briefly, are also beyond the committee's charge. In the penultimate section, the metrics in place in the OFES program are discussed. Conclusions and recommendations regarding the fusion concept program are found in the last section. Appendix C contains brief descriptions of the various configurations being explored.

IMPORTANT PHYSICS QUESTIONS MOTIVATING RESEARCH WITH VARIOUS CONFIGURATIONS

The fusion research program is in part driven by a set of important physics issues that are being addressed through research using a diversity of plasma configurations. An example of one of these configurations, a tokamak, is shown in Figure 3.1. The major magnetic configurations now under study are described in detail in Appendix C. Many of these configurations are potential fusion concepts. Several of the important physics issues are briefly discussed, and for each the plasma configurations that are being employed to research the issues are identified. The issues can serve as organizing elements for the portfolio of configurations, as the committee recommends in the last section of this chapter, because the various plasma configurations under study follow naturally from them. The four physics challenges that the committee has selected are neither exhaustive nor necessarily optimal for planning purposes; rather, they are illustrative, and the actual development of a complete set of physics issues is left to the research community.

Understand the Stability Limits to Plasma Pressure

All plasma configurations possess an upper limit on the pressure (product of plasma density and temperature) beyond which the plasma becomes unstable and disassembles. In the parameter regime in which the plasma is well-described as a magnetized fluid, the theory of magnetohydrodynamics (MHD) can be used to evaluate the pressure limit for different magnetic structures and can also be used to model the detailed evolution of a plasma instability. Techniques to solve the MHD equations are well developed and can be used to accurately evaluate whether a plasma is stable to small perturbations (linear stability).

Linear instabilities can, if sufficiently global in spatial extent, cause a rapid degradation of plasma confinement. Predictions of linear instability thresholds have been tested in small-scale plasma configurations. For example, the dependence of stability on magnetic curvature has been investigated through a range of configurations with varying curvature (beginning with magnetic mirror configurations). The



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