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EXECUTIVE SUMMARY 24 BEAMS, ACCELERATORS, AND COHERENT RADIATION SOURCES Until now, progress in this area has been driven by many important defense applications. Given recent changes in world politics, the need for such programs has greatly decreased. Military applications aside, this area of plasma science has a wide variety of important technological applications. Examples of uses of intense electron beams include the bulk sterilization of medical products and food, toxic waste destruction via oxidation, the processing of advanced materials, and new welding techniques. Free-electron laser radiation sources offer the possibility of providing intense, tunable sources of electromagnetic radiation in virtually all parts of the electromagnetic spectrum from the far infrared to x-ray wavelengths. They have a variety of potential applications in medicine and industry. The development of x-ray lasers is in its infancy but holds promise for many important practical applications. To effectively pursue such applications will require a coordinated research and development effort. The panel recommends that beams, accelerators, and coherent radiation sources be given high priority for "defense conversion" funding. SPACE PLASMAS Space plasma physics is concerned with the observation and understanding of naturally occurring solar-system plasmas. It is an evolutionary field, and progress has been achieved incrementally. The space plasma medium extends from the ionosphere of Earth to the far reaches of the solar system and encompasses plasmas of many types. Portions of this domain, such as the magnetosphere of Uranus, have experienced only brief, exploratory coverage, while others, like Earth's ionosphere and magnetosphere, have been investigated relatively thoroughly. In the case of the former, we are still at the stage of trying to deduce gross plasma structure from limited data; with the latter, we are in the process of understanding specific mechanisms that are responsible for the observed morphology. Occasionally, an entirely new physical situation is encountered, unlike anything previously observed in either space or in the laboratory, and this opens new scientific vistas. One example is the dusty plasmas of comets and planetary rings that are dominated by the dynamics of charged macroparticles for which gravitational and electromagnetic effects are of comparable importance. There are many applications of plasma physics to space science, ranging from the development of plasma thrusters for spacecraft propulsion to "space weather" prediction in the magnetosphere and ionosphere, which has important consequences for physical phenomena on Earth, such as global communications. To some extent, space plasma physics draws upon the vast body of knowledge accrued through the laboratory program for the analysis, interpretation, and modeling of phenomena. Frequently, however, the parameters and the nature of
EXECUTIVE SUMMARY 25 boundary conditions are such as to render the space plasma physics unique, and necessitate entirely new theory or computer modeling. Thus, space plasma physics also contributes to the expansion of our knowledge of basic plasma physics. For example, our extensive understanding of collisionless shocks is based largely on space plasma studies of the Earth's bow shock. In the past, space plasmas have also been used as media in which to study phenomena of both applied and intrinsic interest and importance. However, these aspects of space plasma physics have now been deemphasized programmatically to the point of virtual extinction. Observation is central to space plasma physics. Although observations are expensive to make, especially those requiring space flight, advancement in the field will continue to rely heavily on carefully planned and judiciously selected experiments to provide data that underlie new and refine old ideas. Technological improvements in detection systems and data handling capabilities can be expected to provide increasingly complete and accurate data on which to base models and theories. Recent progress in this area has been impressive, and the prospects for the future are very good. The ambient space plasma can be modified by a number of techniques, including the injection of waves and particle beams, the injection of plasma and neutral gas, and perturbation by space vehicles. Such perturbations provide opportunities to isolate and study space plasma effects in detail and to create space plasmas relevant to other regions of space. Of particular concern to the panel is the fact that programs in this area of active, space plasma experimentation have recently been deemphasized by the funding agencies, and the panel recommends that this trend be reversed. Given the spatial and temporal intermittencies of space plasma measurements, a program in laboratory experiments to study space plasma phenomena could be of great benefit. Such experiments have been supported in the past only to an extremely limited degree, due in large part to the fact that the design of experiments with appropriate scaling to space conditions is difficult in laboratory-sized devices. Advances in laboratory plasma experimentation have now progressed to the point that relevant plasma processes can be investigated in the laboratory with a degree of control, precision, and repeatability not achievable in situ. The panel recommends that an initiative be created for the support of laboratory experiments relevant to space plasmas. Understanding space plasma phenomena frequently requires a combination of extensive data analysis, theory, modeling, and laboratory experiments, in addition to in situ observation. There is concern that in response to the pressure of escalating costs for observations, support for these other aspects of space plasma science has shrunk to unhealthy levels. The panel recommends that NASA and NSF fund a vigorous observational program, including both in situ and ground-based facilities, properly balanced with complementary programs of theory, modeling, and laboratory experiments.
