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detail to convey a sense of the science being done, the impact of program decisions on the science, and the scientific culture of the program, a few representative areas have been selected.
One complication in assessing the specific contributions of the U.S. Fusion Energy Sciences program to the overall fusion effort is the need to separate and compare the U.S. effort and the broader international effort. From the very beginning of the program, in the 1950s, there has been a close collaboration internationally on all aspects of magnetic confinement fusion (collaboration on inertial fusion was constrained by security issues). Large U.S. facilities have had international collaborators and vice versa. These close interactions often make it difficult to clearly separate U.S. contributions from international contributions. The science in this chapter generally refers to activities in which the United States has, at the least, played a very significant role. Where foreign programs clearly played the dominant role, this is noted.
Many of the important experimental and theoretical tools developed during the four-decade history of the program are now converging to produce a qualitative change in the program's approach to scientific discovery. Theoretical models are now sufficiently mature to describe much of the complex nonlinear dynamics of plasmas. Quantitative comparison with experimental observations is beginning to facilitate a first-principles understanding and interpretation of the behavior of plasmas. One consequence of the emerging scientific understanding of these systems is the development of techniques for manipulating turbulence and therefore controlling the energy-containment properties of the magnetic bottles. The suppression of small-scale turbulence and transport in a 100-million-degree medium is an accomplishment that is by any scientific standard a significant achievement and a sign of the high level of the science generally carried out under this program.
Significant advances have been made in each of the traditional foci of plasma physics research: equilibrium, stability, heating, and transport. Over the past decade, a high level of predictive capability has been developed in several key areas. The program is moving into a new era in which the tight integration of theoretical predictions and experimental observations is enabling the control of plasma dynamics, including the suppression of turbulence and transport.
Equilibrium and Heating
The theoretical and computational tools needed for studying plasma equilibria in complex magnetic containers are now well developed and extensively used in the design of new experiments and in the analysis of existing experiments. A number of techniques, including high-power ion beams and driven waves at frequencies from kilohertz to multigigahertz, generally referred to as radio-frequency waves, have been developed to heat plasmas and also to generate and sustain plasma currents. The basic propagation and absorption physics for beams and waves are well understood. These techniques are being used to control pressure, current, and flow profiles and thus to optimize plasma performance in present-day large experiments, but techniques applicable to future high-pressure plasmas require further development.
Diagnostics for remotely measuring important equilibrium-related quantities such as plasma density, electron and ion temperatures, and magnetic field are now available in major plasma experiments. Tools to measure electric fields and associated equilibrium flows, which are increasingly recognized as having an important influence on stability and transport, are less well developed. For future novel plasma configurations, these measurement techniques will have to be extended or new approaches invented.