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EXECUTIVE SUMMARY 8 rather than focusing only on the demands made on plasma science by applications. In turn, a greater understanding of the fundamentals of plasma science can be expected to advance significantly its successful application to the needs of society. Progress will be greatly inhibited without a strong experimental and theoretical research program directed at the fundamental principles of plasma science and not constrained to focus only on near-term applications. In particular, although theoretical and computational studies have spearheaded many of the advances in plasma physics in the past, well-planned and precisely controlled experiments will be crucial to further progress. The panel was charged with the assessment of specific areas of plasma science that it refers to as topical areas. These include low-temperature plasmas, nonneutral plasmas, inertial and magnetic confinement fusion, beams, accelerators, and coherent radiation sources, and space and astrophysical plasmas. These areas vary in size, the nature of the scientific efforts, and the key scientific and organizational challenges facing them. Part II contains assessments of these topical areas with conclusions and recommendations specific to each. The panel was also charged with the assessment of broad areas of plasma science: basic plasma experiment, theory and computational plasma physics, and plasma science education; this is done in Part III. Although research and development in the topical areas is proceeding reasonably well, the panel's conclusion is that maintaining the vitality of basic plasma science faces severe difficulties unless there is concerted action by both the funding agencies and the scientific community. Because of the importance of present and potential applications of plasma science to our society, much benefit would be gained by a coherent program of support for basic plasma science. Much of the remainder of this overview chapter is devoted specifically to this issue, and the chapter concludes with a summary of the central messages of the report and the panel's general conclusions and recommendations. THE ROLE OF PLASMA SCIENCE IN OUR SOCIETY Plasma science impacts daily life in many significant ways. Low- temperature plasmas, in which electric fields in the plasma can impart significant energy to the electrons and ions but the plasmas are still cool enough to support a multitude of chemical reactions, are critical to the processing of many modern materials. This method of ''plasma processing" is an enabling technology in the fabrication of semiconductors. Important applications include the plasma etching of semiconductors and the surface modification and growth of new materials. A recent National Research Council report,1 which highlights the importance 1 National Research Council, Plasma Processing of Materials: ScientificOpportunities and Technological Challenges, National Academy Press, Washington, D.C., 1991.
EXECUTIVE SUMMARY 9 of plasma processing in the electronics industry, indicates that the worldwide sale of plasma reactors alone amounted to $1 billion dollars in 1990 and is expected to double in the next five years. Other important uses of low- temperature plasmas include the "cold" pasteurization of foods, the sterilization of medical products, environmental cleanup, gas discharges for lighting and lasers, isotope separation, switching and welding technology, and plasma-based space propulsion systems. Coherent radiation sources and particle accelerators rely on plasma concepts. Uses of intense electron beams include the bulk sterilization of medical products and food, toxic waste destruction via oxidation, processing of advanced materials, and new welding techniques. Free-electron-laser radiation sources have a variety of potential applications in medicine and industry, and they offer the possibility of intense, tunable sources of electromagnetic radiation in virtually all parts of the electromagnetic spectrum. Nonneutral plasmas in electromagnetic traps have application as ultraprecise atomic clocks and as a method to confine and manipulate antimatter such as positrons and antiprotons. Plasma science is central to the development of fusion as a clean, renewable energy source. In order to control the fusion process, which is the source of energy of the Sun and the stars, we must learn to create hot, dense plasmas of deuterium and tritium in the laboratory. Great progress has been made toward this goal. Fusion-plasma confinement times have increased by a factor of more than 100 in the last two decades, and achievable temperatures have increased by a factor of 10. There is now in place an international collaboration to design the first prototype fusion power reactor, the International Thermonuclear Experimental Reactor (ITER). However, the continued refinement of the fusion concept and the optimization of fusion as a power source will require improved understanding of methods of confining and heating plasmas, as well as the development of techniques to lessen the damage to material walls due to the close proximity of the fusion-temperature plasmas. The leverage on investment in this area is tremendous. All major industrial nations have experienced a steady increase in the use of electricityâit is the energy type of choice. Nuclear fission plants are aging, fossil fuels continue to be of concern due to the production of greenhouse gases, and fusion offers the potential of large-scale electricity generation with abundant fuel supply and attractive environmental features. We live in the 1% or so of the universe in which matter is not ionized, so plasmas are not readily apparent in our daily lives. However, as illustrated in Figure S.1, plasmas occur in many contexts, spanning an incredible range of plasma densities and temperatures. The most common examples of plasmas that we can actually see are the gas discharges in neon lights and the discharges in bolts of lightning. Most of the observable matter in the universe is in the plasma state (i.e., in the form of positively charged ions and negatively charged electrons). Plasma science provides one of the cornerstones of our knowledge of the Sun, the stars, the interstellar medium, and galaxies. We cannot understand such
EXECUTIVE SUMMARY 10 FIGURE S.1 Plasmas that occur naturally or can be created in the laboratory are shown as a function of density (in particles per cubic centimeter) and temperature (in kelvin). The boundaries are approximate and indicate typical ranges of plasma parameters. Distinct plasma regimes are indicated. For thermal energies greater than that of the rest mass of the electron (kBT>mc2), relativistic effects are important. At high densities, where the Fermi energy is greater than the thermal energy (EF>kBT), quantum effects are dominant. In strongly coupled plasmas (i.e., nλD3<1, where λD is the Debye screening length), the effects of the Coulomb interaction dominate thermal effects; and when Ef>e2n1/3, quantum effects dominate those due to the Coulomb interaction, resulting in nearly ideal quantum plasmas. At temperatures less than about 105 K, recombination of electrons and ions can be significant, and the plasmas are often only partially ionized.
