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EXECUTIVE SUMMARY 7 INTRODUCTION Plasma science is the study of the ionized states of matter. Plasmas occur quite naturally whenever ordinary matter is heated to a temperature greater than about 10,000Â°C. The resulting plasmas are electrically charged gases or fluids. They are profoundly influenced by the long-range Coulomb interactions of the ions and electrons and by the presence of magnetic fields, either applied externally or generated by current flows within the plasma. The dynamics of such systems are complex, and understanding them requires new concepts and techniques. Plasma science includes plasma physics but aims to describe a much wider class of ionized matter in which, for example, atomic, molecular, radiation- transport, excitation, and ionization processes, as well as chemical reactions, can play significant roles. Important physical situations include partially ionized media and the interaction of plasmas with material walls. Thus plasma science draws on knowledge and techniques from many areas of science, including chemistry, fluid dynamics, and large-scale numerical computation, to achieve an accurate description of plasma behavior. The goal of plasma physics is to describe elementary processes in completely ionized matter. In common with such fields as chemistry, condensed matter physics, and molecular biology, plasma physics is founded on well- known principles at the microscopic level. Description of plasmas typically involves use of Maxwell's equations for the electromagnetic fields and the Liouville or Boltzmann equations to model the dynamics of the electrons and ions, which are treated as point charges. Simpler approximations based on fluid descriptions for the electrons and ions (e.g., magnetohydrodynamics) are also used. The plasma medium is inherently nonlinear because the charged particles composing the plasma interact collectively with the electromagnetic fields produced self-consistently by the charge density and currents associated with the plasma particles. Much of the basis for analyzing and treating plasmas has now been laid out, and a number of important advances in our understanding have been made. However, we are far from being able to make quantitative predictions of plasma behavior in many, if not most, of its manifestations. The intellectual challenge in plasma physics is to develop principles for understanding the complex macroscopic behavior of plasmas, given the known principles that govern their microscopic behavior. The development of plasma science in the past three decades has been propelled by applications such as fusion energy, space science, and the need for a strong national defense, and this support has resulted in significant progress. Yet, by necessity, only those aspects that appeared to be more or less directly pertinent to applications received the lion's share of attention. Plasma science has benefited greatly from this support, but the field has now reached a level of maturity where many basic issues have been identified and remain to be resolved. Further progress will depend eventually on addressing these basic issues,