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Plasma Science: From Fundamental Research to Technological Applications PART I Overview
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Plasma Science: From Fundamental Research to Technological Applications This page in the original is blank.
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Plasma Science: From Fundamental Research to Technological Applications 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 10,000°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,
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Plasma Science: From Fundamental Research to Technological Applications 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: Scientific Opportunities and Technological Challenges, National Academy Press, Washington, D.C., 1991.
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Plasma Science: From Fundamental Research to Technological Applications 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 electricity—it 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
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Plasma Science: From Fundamental Research to Technological Applications 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.
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Plasma Science: From Fundamental Research to Technological Applications phenomena as sunspots, the formation of stars from interstellar gas clouds, the acceleration of cosmic rays, the formation and dynamics of energetic jets and winds from stars and galaxies, or the interaction of supernova remnants with interstellar gas, without the concepts of plasma science. Plasmas are central to many aspects of space science. The space plasma medium extends from the ionosphere surrounding the Earth to the far reaches of the solar system. "Space-weather" prediction in the ionosphere and magnetosphere is important for global communications, and the properties of space plasmas are important in determining the capabilities and longevity of spacecraft. Thus, although it is often not readily apparent, plasma science affects our society in a myriad of ways. THE DISCIPLINE OF PLASMA SCIENCE Common Research Themes The panel has concluded that plasma science is frequently viewed, not as a distinct discipline, but as an interdisciplinary enterprise focused on a large collection of applications. The underlying, common, and critical feature of plasma science as a discipline is that its goal is to understand the behavior of ionized gases, and this requires fundamentally different techniques from those applicable to uncharged gases, fluids, and solids. This coherence of plasma science as a discipline is apparent when one considers some of the challenging intellectual problems, central to plasma science, that span applications in many of the topical areas. The impact of four such problems on the topical areas assessed in this report are summarized in Table S.1. Wave-Particle Interactions and Plasma Heating Understanding the interaction of plasma particles with the collective plasma oscillations and waves is a fundamental question with many practical applications. Basic scientific issues involve the trapping of particles in waves, the nonlinear saturation of wave damping, chaotic behavior induced in particle orbits, and particle acceleration mechanisms. Wave heating is an important method of heating fusion plasmas to the required temperatures for fusion. Waves can be used to drive electrical currents in plasmas. One promising scheme for a steady-state tokamak fusion reactor is to use waves to drive electrical currents to confine the plasma, instead of the present method of driving pulsed currents inductively. Wave-particle interactions are central to the operation of free-electron lasers and other coherent radiation sources, to many advanced accelerator concepts, and to methods of creating and heating low-temperature plasmas for plasma processing applications. Wave-particle processes are important in Earth's magnetosphere and ionosphere, and shock waves are the dominant production mechanism for cosmic rays of astrophysical origin.
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Plasma Science: From Fundamental Research to Technological Applications TABLE S.1 Applications of Basic Plasma Research, Illustrating the Commonality of Scientific Issues Across Topical Areas Scientific Issue Topical Area Wave-Particle Interactions and Plasma Heating Chaos, Turbulence, and Transport Sheaths and Boundary Layers Magnetic Reconnection and Dynamos Low-temperature plasmas Magnetrons Plasma sprays Instabilities in plasma processing Plasma processing Lighting Plasma torches MHD drag reduction Nonneutral plasmas ICR mass spectrometry Precision clocks Fluid flows Antimatter storage Switches Diodes — Inertial confinement fusion Parametric instabilities Preheating Turbulent mixing Rayleigh-Taylor instability Plasma-driver interface ICF plasma magnetic fields Magnetic confinement fusion rf current drive rf plasma heating Energy and particle loss Divertor operation Plasma-limiter interaction Sawteeth and current profile dynamics in tokamaks Reversed-field pinches
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Plasma Science: From Fundamental Research to Technological Applications Beams, accelerators, and coherent radiation sources FELs Advanced accelerators Instabilities in FELs and advanced accelerators Cathodes — Space plasmas Magnetosphere Ionosphere Cometary and planetary atmospheres Solar wind Magnetospheric and ionospheric boundaries plasma-satellite interaction Solar interior and corona Magnetopause Astrophysical plasmas Cosmic-ray acceleration Accretion disks Dynamo viscosity Pulsar magnetospheres Accretion disk boundaries Solar and stellar magnetic fields NOTE: ICR = ion cyclotron resonance rf = radio-frequency FEL = free-electron laser MHD = magnetohydrodynamic ICF = inertial confinement fusion
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Plasma Science: From Fundamental Research to Technological Applications Chaos, Turbulence, and Transport Most plasmas of interest are nonuniform in density and temperature, which results in the excitation of turbulent waves and fluctuations. These fluctuations in turn produce the transport of particles and energy that tend to drive the plasma toward a more uniform state. Very generally, turbulence and turbulent transport are not understood. The recent renaissance in nonlinear dynamics and studies of phenomena such as chaos provide new tools with which to attack these problems, and in fact, plasmas offer a convenient and often unique medium in which to study turbulent phenomena. Many of the scientific issues are now clear, and the plasma applications are many. For example, turbulent transport is the dominant mechanism for energy and particle transport in tokamak fusion plasmas. Turbulent transport is frequently the dominant particle and energy loss mechanism in low-temperature plasmas. It is important in the Earth's magnetosphere, in stellar convection zones, and in astrophysics in settings such as the interstellar medium. Plasma Sheaths and Boundary Layers Understanding the boundaries of plasmas (called sheaths) is a well-defined problem with many practical consequences. In magnetically confined fusion plasmas, the hot plasma cannot be allowed to contact the material walls. The result is that there must be large gradients in plasma temperature and density in the plasma and, frequently, non-equilibrium particle distributions. The precise character of these boundary layers can greatly influence the character of the bulk plasma and the rate at which wall damage occurs. Plasma sheaths are important in the plasma processing of materials. This sheath is adjacent to the material surface to be processed; therefore, the properties of this layer determine the characteristics of the plasma-matter interaction. A similar phenomenon occurs in space plasmas, where a plasma sheath separates a satellite from the surrounding space plasma, and the properties of this sheath determine the interaction of the plasma particles with the satellite. This is relevant for considerations such as surface damage and electrical phenomena. A type of boundary layer called a "double layer" can separate regions of plasma with distinctly different properties. Double layers are known to occur both in laboratory and in space plasmas, where they play an important role in determining the global configuration of the plasma. Related electrode-sheath phenomena are the least well understood aspects of lighting plasmas, and learning to control them better would lead to more efficient and longer-life products. Magnetic Reconnection and Dynamo Action The behavior of a magnetized plasma is determined largely by the configuration of the magnetic field in the plasma. Currents and plasma flows can induce
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Plasma Science: From Fundamental Research to Technological Applications changes in the topology of the field by breaking and reconnecting the magnetic field lines. This process occurs in (magnetohydrodynamic) "sawtooth" oscillations in tokamak plasmas, in sun spots, and in many astrophysical plasmas. Dynamo action is the process by which a flowing plasma converts mechanical energy into magnetic field energy. This process is thought to be the origin of Earth's magnetic field, and it is likely to be one of the mechanisms for producing astrophysical magnetic fields. Very little is understood about magnetic reconnection and dynamo action, yet new techniques are now available to address these problems, by analytic methods, by computer simulation, and by suitably designed laboratory experiments. These and other forefront problems in plasma science are described in Part III, and the relationship of this research to specific topical areas and applications is discussed in Part II. Research and Education in Plasma Science The findings and conclusions regarding the three broad areas of plasma science assessed in Part III are discussed in this section. Basic Plasma Experiments Of any of the topics in the panel's study, basic plasma experiments constitute the area of greatest concern. Progress in the physical sciences has relied historically on the close interplay between theory and experiment. Perhaps nowhere is this more true than in many-body physics, which naturally includes plasma physics. Physical phenomena can be identified, isolated, and studied most efficiently, quickly, and economically in experiments specifically tailored for this purpose. There are many advantages of basic experiments, compared to experiments done in settings determined by other considerations such as particular applications. These advantages include the flexibility to choose the setting to isolate a particular physical phenomenon, the ability to explore the broadest range of plasma parameters, and the ability to make experimental changes quickly, guided by the internal logic of the underlying science and by new results as they unfold. Despite the importance of basic plasma experiments to plasma science, there have been clear warning signs for more than a decade of a deficiency in this area. This was expressed clearly in the Brinkman report, Physics Through the 1990s, written almost a decade ago.2 The finding of the panel is that this situation has worsened since the Brinkman report was issued. There are several causes of this 2 National Research Council, Plasmas and Fluids, in the series Physics Through the 1990s, National Academy Press, Washington, D.C., 1986.
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Plasma Science: From Fundamental Research to Technological Applications low-temperature plasma phenomena are poorly understood. Yet modern techniques are available to address a range of problems of fundamental importance that have important practical applications. Examples include the physics and chemistry of plasmas at material boundaries (i.e., plasma "sheaths"), the creation of plasmas by electrodeless discharges, and the stability and reproducibility of plasma discharges. There are many important applications of low-temperature plasma science, yet there is no structure in place to support the fundamental research in this area that will be required to systematically develop these applications. Federal agencies have traditionally had only modest efforts in low-temperature plasma research; and recently, they have deemphasized these programs. Industry has usually engaged only in projects for which there is a short-term payoff. In contrast, there is an active effort in this area in Japan, and a large effort in low-temperature plasma research has recently been created in France. The crucial problem regarding low-temperature plasma science in the United States is the lack of a coordinated governmental program in this technologically important area. The panel recommends the creation of a coordinated support structure for fundamental research in low-temperature plasma science. Nonneutral Plasmas Nonneutral plasmas include pure electron plasmas and pure ion plasmas in electromagnetic and electrostatic traps, electron beams, and ion beams. Examples of applications of nonneutral plasmas are electron beams and plasmas for the generation of electromagnetic waves, pure ion plasmas in traps for atomic clock applications, advanced concepts for particle accelerators, and the confinement of antimatter such as positrons and antiprotons. Nonneutral plasmas are more easily confined than neutral plasmas. Consequently, they can be more easily controlled and studied. Important questions that have recently been addressed include issues of plasma confinement, the creation of thermodynamic equilibrium states and controlled departures from equilibrium, and the mechanisms for the transport of particles and energy. Many of the concepts developed in the study of nonneutral plasmas have wider applications to understanding the physics of neutral plasmas and to fluid dynamics and atomic physics. Nonneutral plasma physics is one area of basic plasma science that has progressed dramatically in the past two decades, and questions of fundamental importance have been addressed that are also relevant to technological applications. This was due, in large part, to a program of dedicated support for research in this area by the Office of Naval Research. This successful support of experimental and theoretical research on nonneutral plasmas should be used as a model for a program of renewed support of basic experiments in neutral plasmas that is recommended elsewhere in this report.
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Plasma Science: From Fundamental Research to Technological Applications Nonneutral plasma research will be an important area for the foreseeable future, both from the point of view of fundamental plasma science in neutral and nonneutral plasmas and in exploiting this knowledge for technological applications. The panel recommends that continued support be given to research in this area and to the development of technological applications. Inertial Confinement Fusion The goal of inertial confinement fusion is to harness fusion power using intense lasers or ion beams to compress fusionable material, such as deuterium and tritium. The required plasma parameters for inertial confinement fusion are densities as much as 100 times that of ordinary matter and temperatures in excess of 100 million Kelvin. Much progress has been made in this area in the last decade. Important, new diagnostic techniques have been developed. The largest laser project, Nova, has conducted experiments to study the important problem of the physics of interpenetrating materials at accelerated interfaces. Computer simulations have clarified the role that fluid dynamical instabilities play in the dynamics of target compression. However, many challenging problems remain to be addressed. Examples include understanding stimulated Raman and Brillouin instabilities in laser-plasma interactions, particularly at high laser intensities, and understanding nonlinear plasma instabilities and the equations of state and opacity of matter at high densities and temperatures. Many of the outstanding problems in this area have a high degree of commonality with important problems in other areas of science. Examples relevant to space physics and fusion include questions of plasma turbulence, particle acceleration and heating by electromagnetic radiation, and the effects of spatial inhomogeneities on wave propagation and mode conversion. Other important problems have much in common with optical science. Research in inertial confinement fusion can also benefit other fields. Examples include the development of short-pulse and x-ray lasers. While progress toward inertial confinement fusion has been good, continued emphasis on programmatic milestones could leave unaddressed fundamental scientific questions crucial to the achievement of future goals. It is important for the program to reemphasize a broad-based program of support for relevant areas of basic research. The commonality of scientific problems with other areas of science could be used to facilitate progress, both in the inertial fusion program and in related areas. Much of the fusion target program in the United States had been classified for security reasons. Much benefit should be gained by declassification currently being done by the Department of Energy. Given the commonality of problems in this area with those in other areas of plasma science and the importance of basic research in related fields to the achievement of fusion, the panel recommends that some resources be reallocated within the program to support
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Plasma Science: From Fundamental Research to Technological Applications the study of basic science relevant to inertial confinement fusion. Further, the panel recommends that the use of inertial confinement fusion facilities by scientists, working outside the program but on relevant problems, be encouraged. Magnetic Confinement Fusion Magnetic confinement fusion continues to be the largest driver for the intellectual development of plasma science. Central to the achievement of fusion in magnetically confined plasmas is the ability to confine hot plasmas (i.e., those with temperatures of more than 100 million Kelvin). Since these plasmas must eventually come in contact with material boundaries, this program also involves important considerations concerning low-temperature plasma science. Much progress has been made in this field over the past two decades. Confinement times of fusion plasmas have increased by a factor of more than 100, and achievable temperatures have increased by a factor of 10. Progress has been made in the development of new diagnostics of plasma behavior, and these diagnostics have, in turn, led to a deeper understanding of the behavior of fusion plasmas. New methods have been developed to heat fusion plasmas and to drive electrical currents in these plasmas noninductively using intense neutral beams and radio-frequency electromagnetic waves. These methods of current drive could eventually permit the operation of a steady-state fusion reactor. New operating regimes with improved plasma confinement have been discovered, such as the so-called "high-confinement" and "very-high-confinement" modes. There has been progress in the understanding of plasma stability as well as in understanding the interface between the plasma edge and the material walls of the confinement vessel. A key element in the magnetic confinement fusion program is the development of the International Thermonuclear Experimental Reactor (ITER). This device will be designed to test elements of reactor-relevant plasma science not possible by other means such as the physics of ignition. However, there are many other plasma processes relevant to controlled fusion that will not be able to be addressed effectively by the ITER program. The physics of the edge plasmas in tokamaks needs to be better understood. Advanced modes of tokamak operation at very long pulse lengths will be studied in the Tokamak Physics Experiment (TPX), now planned as a national facility for such studies. Finding improved methods of removing large quantities of heat from the plasma edge is an immediate problem. The efficient production of self-generated plasma currents by high plasma pressures (so-called "bootstrap currents") is an important goal of advanced tokamak configurations that is not likely to be studied efficiently in the ITER program. Experiment and theory should continue in the search for optimized geometries and operating conditions to improve reactor efficiency and power-handling capabilities.
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Plasma Science: From Fundamental Research to Technological Applications Crucial to the operation of a fusion device is the transport of particles and energy by plasma turbulence, and turbulent transport has been the dominant transport mechanism in all magnetically confined fusion plasmas to date. There is, as yet, only an extremely limited first-principles understanding of the turbulence in fusion plasmas and the resulting transport. Any predictive capability that does exist is based on empirical "scaling laws" that must be validated when applied outside the operating parameter range of present and past fusion devices. A quantitative understanding of this transport and the ability to control it could potentially lead to improved reactor performance and reduced size and cost. This fundamental base of plasma science is crucial not only for the efficient development of a successful fusion reactor, but also for quantitative understanding of fusion-related plasma science, which will continue to be important in maximizing the competitiveness of fusion power in the decades to follow. The panel recommends that there be established a coordinated research program in fusion-relevant plasma physics. This will require a range of project sizes, in order to optimize the particular experiments to study the relevant plasma processes. Experimental research is most efficiently done on the smallest scale possible. This allows the greatest flexibility in making changes, as required by new results and discoveries, as well as the greatest exploration of the relevant parameter ranges at minimum cost. Many fundamental questions in basic plasma science should be addressed by small experiments that, in many cases, are specifically designed for a particular purpose. Other questions can be addressed only in larger devices. To study the effects of fusion products (e.g., alpha particles at energies of a few million electron volts) on fusion plasmas, reactor-sized devices, such as the Tokamak Fusion Test Reactor or the Joint European Torus, are required. Thus, a coordinated program of fusion plasma research will require a range of devices and programs, from small, basic experiments that isolate and address fundamental questions in plasma science to experiments on the largest fusion devices. If the present trend toward large experiments continues without adequate attention paid to a broader base of experimental research facilities, a dangerous gap will develop in our ability to address the wide range of questions important to fusion-relevant plasma physics. Many important questions in fusion plasma physics might be more appropriately addressed by smaller, long-term research programs dedicated to isolating and studying fundamental plasma phenomena in a more complete and systematic manner. The panel recommends that the program in magnetic confinement fusion include support for a range of projects, with the sizes chosen to best suit the particular plasma problem. Provision needs to be made for research on fusion-relevant basic plasma science. The details of this recommendation are given below in Part II.
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Plasma Science: From Fundamental Research to Technological Applications 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
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Plasma Science: From Fundamental Research to Technological Applications 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.
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Plasma Science: From Fundamental Research to Technological Applications Astrophysical Plasmas Plasma physics is relevant to almost every area of astrophysics, ranging from stellar and interstellar plasmas to star clusters and galaxies. The potential contribution of plasma physics to astrophysics has been limited by the fact that plasma physics is not yet part of the standard graduate astrophysics curriculum, and this should be changed. Also of concern to the panel is the fact that plasma astrophysics does not have a distinct home in any of the federal funding agencies. Examples of plasma astrophysics where there has been significant progress include models of magnetized accretion disks and associated jets and winds, including the effects of relativity, strong magnetic fields, rapid rotation, and magnetohydrodynamic waves and instabilities. Mechanisms of particle acceleration in plasma shockwaves have been clarified that are relevant to the acceleration of cosmic rays in the interstellar medium. Models have also been developed to describe the convective fluid motion in stars, including crucial effects arising from the presence of strong magnetic fields on the flow of stellar material. While there has been progress in plasma astrophysics, many fundamental problems need to be addressed. These include the description of dense stellar plasmas with temperatures in excess of 20 million Kelvin and densities 10 to 100 times solid density. Other important problems involve turbulent plasmas, the origin of the magnetic fields in the universe, and magnetic field line reconnection. Plasma astrophysics is not yet recognized as a coherent discipline by any federal funding agency. Yet, plasma astrophysics deals with phenomena that are important to virtually all aspects of astronomy and astrophysics, and many of these problems are central to basic plasma physics. The panel recommends that there be established interdisciplinary programs in the National Aeronautics and Space Administration and in the National Science Foundation to fund research in plasma astrophysics, including research on basic plasma processes relevant to astrophysical systems but not tied to any particular application. CENTRAL MESSAGES OF THIS REPORT The panel was charged with assessment of the state of plasma science in the United States and evaluation of its potential to contribute to the technology base of our society. It was further charged with assessing the institutional infrastructure in which plasma science is conducted, identifying changes that would improve the research and educational effort, and making recommendations to federal agencies and to the community to address these issues. The theme of this report is that, although plasmas are pervasive in nature and many of the applications of plasma science are being pursued and exploited effectively, plasma science is not adequately recognized as a discipline. Consequently,
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Plasma Science: From Fundamental Research to Technological Applications there is not an effective structure in place to develop the basic science that underlies its many applications. The potential contributions of plasma science to our society would benefit greatly by a coordinated effort of support for fundamental research, not tied to specific programs but designed to establish this basic scientific foundation. This report describes many of the developments in theory and experiment that have led to important industrial applications, significant commercial and residential uses, and a deeper understanding of the universe. While delineating the many successes and identifying the exciting and potentially critical challenges, this report is an expression of concern. Applications depend on development, which in turn depends upon a fundamental understanding of the underlying science—the sine qua non of new development. It is the view of experts in the field that this fundamental understanding is, in most cases, best obtained by individual and small-group experimental and theoretical efforts, typical of university-scale programs. The problem is not that there is a gross imbalance in the total funding going into plasma science and technology, but that there has been a gradual, long-term decrease in support for fundamental research in plasma science. The result is that there is now a clear need for such support, particularly in the areas of small-scale, basic plasma experiments and complementary small-group theoretical efforts. As discussed above, a wide variety of programs pursue the applications of plasma science, including those in space, fusion, and the plasma processing of materials; yet there is great commonality of the underlying science. Even in the context of a particular application, there are often several programs, frequently spread across more than one federal agency. More often than not, this diversity is justified and healthy. However, coordination of research efforts is vital to eliminate duplication, to make the most effective use of resources by maintaining complementary programs, and to ensure that all of the critical problems are being addressed.4 A prime example of the existing lack of coordination in plasma research is the fact that no agency or agencies have yet assumed the responsibility for basic research in plasma science. Thus, the central messages of this report are threefold: Small-scale research provides much of the fundamental base for plasma science; Such individual-investigator and small-group research is in need of support; and 4 One beneficial effort of this type is the recent, informal coordination of experimental space plasma research by the National Aeronautics and Space Administration, the Office of Naval Research, and the National Science Foundation.
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Plasma Science: From Fundamental Research to Technological Applications There is a need for increased coordination of federally funded plasma science research. These conclusions coincide with the principal findings and recommendations of the Brinkman report,5 which was an overall assessment of the future of plasma physics in the United States: Direct support for basic laboratory plasma-physics research has practically vanished in the United States. The number of fundamental investigations of plasma behavior in research centers is small, and only a handful of universities receive support for basic research in plasma physics. A striking example is the minimal support for basic research in laboratory plasmas by the National Science Foundation.… Because fundamental understanding of plasma properties precedes the discovery of new applications, and because basic plasma research can be expected to lead to exciting new discoveries, increased support for basic research in plasma physics is strongly recommended. If anything, the state of basic plasma science has worsened in the nine years since the Brinkman report was published. This situation can be remedied only by the creation of a coherent and coordinated plan for the support of the basic plasma science. CONCLUSIONS AND RECOMMENDATIONS Ongoing research and development programs in the United States have produced important advances in plasma-related science and technology. Plasma science holds promise of further progress in the future, including advanced methods of processing materials, better methods for cleaning up environmental hazards and mitigating the effects of deleterious chemicals, new methods of accelerating particles and producing electromagnetic radiation, progress toward fusion energy, and improved understanding of our space environment and the astrophysical media of the universe. Thus, plasma science can have a significant impact on many disciplines and technologies, including those directly linked to industrial growth. This impact, however, is critically dependent on the support of basic plasma science. It will be important to effect some shift of research funds to this area because of its close relation to applications. To properly pursue its potential, the United States must create and maintain a coherent and coordinated program of research and technological development in plasma science. Currently, support for basic plasma science is mostly for small programs, found in many agencies, and not coordinated among agencies. The Department of Energy has large programs in the development of fusion energy, by both magnetic and inertial confinement schemes, but it has no unit in 5 See footnote 2, p. 15.
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Plasma Science: From Fundamental Research to Technological Applications the Basic Energy Sciences Division to provide support for the study of the fundamentals of plasma science. The National Science Foundation (NSF) is viewed as the supporter of basic science in universities, and there are several quite small plasma programs scattered throughout the agency. However, basic plasma science has no identified home in NSF and, thus, no specified coordinating and review point and no sponsor. Below, the panel recommends increased support of university-scale experimental research in basic plasma science in the amount of $15 million per year. The justification for this amount is discussed above and in Chapter 8, Basic Plasma Experiments. Although it may seem that this could have only a small influence on a field with an annual budget in excess of $400 million, the expenditure on other than the largest applications, fusion and space plasmas, is less than 10% of this amount. Consequently, an investment of $15 million on basic experiments can be expected to provide an important stimulus to the entire field. It can also be expected to have a multiplicative effect in that the results in basic plasma research will provide the foundation for research more closely related to all of the applications, including space and fusion. While many successful programs in plasma science are currently under way, there is a lack of support for the basic aspects of plasma research, particularly where the payoff to a specific program cannot be justified in the near term. The development of plasma science would be improved substantially by its recognition as a scientific discipline. Given these findings and conclusions, the panel recommends the following six actions: To reinvigorate basic plasma science in the most efficient and cost-effective way, emphasis should be placed on university-scale research programs. To ensure the continued availability of the basic knowledge that is needed for the development of applications, the National Science Foundation should provide increased support for basic plasma science. To aid the development of fusion and other energy-related programs now supported by the Department of Energy, the Office of Basic Energy Sciences, with the cooperation of the Office of Fusion Energy, should provide increased support for basic experimental plasma science. Such emphasis would leverage the DOE's present investment in plasma science and would strengthen investigations in other energy-related areas of plasma science and technology. Approximately $15 million per year for university-scale experiments should be provided, and continued in future years, to effectively redress the current lack of support for fundamental plasma science, which is a central concern of this report. Furthermore, individual-investigator and small-group research, including theory and modeling as well as experiments, needs special help, and small amounts of funding could be life-saving. Funding for these
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Plasma Science: From Fundamental Research to Technological Applications activities should come from existing programs that depend on plasma science. A reassessment of the relative allocation of funds between larger, focused research programs and individual-investigator and small-group activities should be undertaken. The agencies supporting plasma science should cooperate to coordinate plasma science policy and funding. Members of the plasma community in industry and academe should work aggressively for tenure-track recognition of plasma science as an academic discipline, and work with university faculty and administrators to provide courses in basic plasma science at the senior undergraduate level.
Representative terms from entire chapter: