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1 Introcluction en c! Executive Summary GENERAL FINDINGS AND RECOMMENDATIONS Findings The Panel has the following general findings concerning plasma physics and fluid physics: · In each area reviewed by the Panel, the level of basic understand- ing increased significantly during the past decade. · Although most plasma and fluid research is motivated by applica- tions such as defense, fusion, space, communications, and atmospheric modeling, the associated fundamental physics is at the very forefront of knowledge and is characterized by high intellectual challenge. · All matter under physical conditions with energies exceeding 1 electron volt per atom above the ground state involves plasma-physics phenomena. Plasma physics combines concepts from electromagne- tism, fluid physics, statistical mechanics, and atomic physics into a unified methodology for the study and practical use of the nonlinear collective interactions of charged particles with one another and with electric and magnetic fields. · The most important applications of plasma physics are to fusion and space research, which have stimulated many recent advances in plasma science. Other applications include new types of particle 1
2 PLASMAS AND FLUIDS accelerators and coherent radiation sources, isotope separation, and astrophysics. · The technology needed to study hot plasmas in the laboratory and natural plasmas in space has been developed largely over the past 25 years. As a result, plasma physics has become a well-developed scientific discipline. · The 1970s saw the extensive development of coherent and turbu- lent nonlinear plasma physics, which is proving to be fundamental. Many nonlinear concepts and mathematical methods developed in plasma physics, such as solitons and stochasticity, have found appli- cations in other areas of physics. · The above theoretical advances aided by rapid developments in precision plasma diagnostics and data-acquisition techniques have led to significant technical advances in the laboratory. These include, for example, the demonstration of rf current drive in tokamaks, observa- tion of plasma solitons and cavitons, and magnetic braiding and the development of new coherent radiation sources such as the free- electron laser, the gyrotron, and the x-ray laser. · The increasing precision of measurements, numerical modeling, and theory applied to space plasma problems amounts to a revolution in technique relative to 10 years ago. As a result, the study of space plasmas has become one of the primary motivations and experimental arenas for basic plasma research. . · The concepts and techniques of fluid physics find widespread use In plasma physics, atmospheric science, oceanography, solid-earth geophysics, astrophysics, biology, and medicine; in problems in laser physics, combustion, and pollution control; and in the engineering of transportation and defense systems, among others. · The understanding of turbulent fluid motion and the ability to control it have increased dramatically during the past decade. The transition from organized to chaotic fluid flow has become a principal arena for testing recent conceptual advances in nonlinear mechanics. · The effectiveness of fluid-physics And plasma-physics research is being revolutionized by the use of large-scale numerical computations to investigate and solve previously intractable theoretical problems, as well as to analyze and correlate large arrays of data. Recommendations In addition to the specific recommendations summarized in the following chapters, the Panel makes the following general recommen- dations:
INTR OD UCTI ON AND EXEC UTI VE S UMMAR Y 3 · To advance our understanding of fusion and space plasmas, and to maintain and extend U.S. excellence in plasma physics, we recom- mend that the federal government proceed with the next generation of major projects in fusion and space research that are identified later in this report and in other reports referenced herein. · To enhance progress in fluid-physics research, we recommend two national research initiatives: one would develop and deploy the simul- taneous, multipoint flow instrumentation described later in this report; the other would expand programs for research access to major com- putational and experimental fluid-physics facilities. · In view of the significant advances in plasma physics and fluid physics during the past decade, we recommend a renewed commitment by the federal government to basic research in these subjects. An adequate level of basic research, free from short-term, application- oriented goals, should be established in order to provide the founda- tions for future scientific advances and new technologies. In addition, the Panel makes the following recommendation to the academic community: · In view of the increasing precision of the experimental and theoretical techniques of plasma and fluid physics, and their many applications, we strongly recommend that senior-level courses in plasma physics and fluid physics become a required part of university physics curricula. INTRODUCTION The Emergence of Plasma Physics With the rise of electrical science in the nineteenth century came intimations of what are recognized today to be plasma effects. In the 1830s, Michael Faraday created electrical discharges to study the chemical transformations induced by electrical currents. These dis- charges exhibited unusual structured glows that were manifestations of a new state of matter. It was impossible to go further until the discovery of the electron by J.J. Thomson in 1895 and the elucidation of the atomic theory of matter by N. Bohr, E. Rutherford, and others shortly thereafter. By the early twentieth century, the subjects of electromagnetism, fluid mechanics, statistical mechanics, and atomic physics had been clearly defined. These would eventually be assem- bled into a unified methodology for the study of the nonlinear collective interactions of electrically charged particles with one another and with
4 PLASMAS AND FLUIDS electric and magnetic fields, i.e., plasma physics. The realization that plasma is the fourth physical state of matter was a major achievement reserved for twentieth-century physics. Advances in understanding plasmas in the laboratory, in space, and in astrophysics occurred in parallel throughout the twentieth century. In the 1920s, I. Langmuir discovered collective plasma oscillations in the laboratory, and G. Breit and M. Tuve first reflected radio waves from the ionosphere the very edge of space. Between 1930 and 1950, the foundations of plasma physics were created, largely as a by-prod- uct of ionospheric, solar-terrestrial, and astrophysical research, moti- vated by such diverse concerns as understanding how radio waves propagate in the ionosphere, how solar activity leads to auroral displays and magnetic storms on Earth, and the role of magnetic fields in the behavior of stars, galaxies, and the interstellar medium. H. Alfven, E. Appleton, S. Chandrasekhar, S. Chapman, T. Cowling, M. Saha, L. Spitzer, and many others contributed to this research. During this period, laboratory gas-discharge experiments multiplied in number and efficacy. In 1946, L. Landau developed the first theory of the interaction between waves and resonant particles in a plasma without collisions. By the l950s, it was clear that the collision-free nature of hot plasmas was an essential property that highlights the collective inter- actions fundamental to plasmas. Modern plasma physics began in the l950s. Two events symbolizing the deeper intellectual currents of those years were the first successful launch of an artificial Earth satellite by the Soviet Union and the revelation, through declassification, that both the United States and the Soviet Union had been trying to harness the energy source of the Sun- thermonuclear fusion for peaceful purposes. Then, as now, the obstacles to achieving controlled fusion lay not in our ignorance of nuclear physics, but of plasma physics. In 1958, the terrestrial radiation belts were discovered and in 1960, the solar wind, both by spacecraft. These discoveries showed that our exploration and future understand- ing of the Earth and Sun's space environment would also be couched in terms of plasma physics. Two powerful motivations stimulated the growth of plasma physics after 1960. Fusion research seeks a source of energy accessible to human use that will last for a time comparable with the present age of the Earth. Space research seeks useful comprehension of nature's processes on a global and, indeed, solar-system scale, in recognition of Man's dependence on his environment, as well as his curiosity about the cosmos.
INTRODUCTION AND EXECUTIVE SUMMARY 5 The international effort to achieve controlled thermonuclear fusion has been the primary stimulus to the development of laboratory plasma physics. As early as 1958, the theta-pinch configuration produced fusion temperatures at high plasma densities. However, the energy confinement time was orders of magnitude lower than that required for net energy production. The simultaneous achievement of high temper- atures, densities, and confinement times similar to the plasma condi- tions at the centers of stars required significant improvements in forming and understanding plasmas confined by magnetic fields or by inertial techniques. It became possible to diagnose fusion plasmas with increasing precision, and theoretical plasma physics was stimulated to explain observations made possible by more detailed and complete measurements. The technology needed to create fusion plasma condi- tions in the laboratory high magnetic field, large-volume supercon- ducting magnets, intense energetic neutral beams, powerful lasers, vacuum and surface techniques, and high-power radio-frequency sources spanning a wide range of frequencies was systematically assembled. The scientific feasibility of controlled fusion will likely be demonstrated in the coming decade, an event that we expect will invigorate research in plasma physics, just as the emergence of the tokamak as an attractive confinement approach in the late 1960s led to strong growth of the fusion program in the 1970s. As the science and related experimental techniques developed, other applications of plasma physics came into view. One example, among many, is the free-electron laser. The free-electron laser, which can generate coherent radiation from microwaves to optical frequencies and perhaps even into the x-ray range, will find applications in many branches of physics, other sciences, industry, and medicine. Using collective plasma effects, it may also be possible to create a new generation of accelerators, such as the beat-wave accelerator, operat- ing at the frontiers of high-energy particle physics. A new and challenging application of plasma physics is to the separation of stable and unstable isotopes, for nuclear fuels, for medical research and diagnostics, for agricultural research, for tracking the motion of environmental pollutants, and for other uses. Many subtle problems of plasma physics, plasma chemistry, and plasma- surface interactions arise in isotope-separation research. It is significant that the same discipline of physics plasma physics- defines the basic language now used both in fusion research and in solar-system plasma physics. The experimental diagnosis and theoret- ical interpretation of many space plasma processes now match in precision the best of current laboratory practice. As a result, the study
6 PLASMAS AND FF UlDS of space plasmas has become one of the primary motivations and experimental arenas for basic plasma research. Moreover, the plasma phenomena in the solar system have proven to be examples of general astrophysical processes. Not only does plasma physics describe both solar-system and astrophysical phenomena, but the solar system has become a laboratory in which astrophysical processes of great gener- ality can be studied in situ. The study of plasmas beyond the solar system has developed more slowly than space plasma physics for a fundamental reason: the microscopic plasma processes that regulate the behavior of distant astrophysical systems cannot be observed directly, as they can in space and in the laboratory. Now, however, the modern theoretical and computational techniques developed to understand fusion, labora- tory, and space measurements have opened the door to modeling of the plasmas in the still larger and more exotic environments of astrophys- ics, ranging from stellar atmospheres to quasars. Such numerical modeling, which is at the cutting edge of computer physics, is applied to statistical mechanics, nonlinear dynamics, fluid turbulence, and elementary-particle physics, as well as to plasma physics. In plasma physics, it has made quantitative the study of complex magnetohydrodynamic systems, and it has clarified the nonlinear collective processes that regulate plasma transport in such systems. Developed scientific disciplines are characterized by deep philosoph- ical motivations, a unified body of powerful theoretical and experimen- tal techniques, and a wide range of applications. It is our conviction that with the growing integration of laboratory, fusion, space, and astrophysical plasma research, plasma physics is becoming a well- developed scientific discipline. When a scientific discipline matures, technological innovation follows. Plasma physics, the only major branch of physics to come largely into being in the past generation, is just beginning to have its impact. Classification of Plasmas The plasmas encountered in nature and studied in the laboratory can be classified as tenuous or dense, classical or quantum. Given the wide range of plasma types and phenomena examined in this report, it is useful to display this diversity in a single plot of plasma temperature T (in kelvins) versus density n (per cubic centimeter) as shown in Figure 1.1. Evidently, plasmas range from the extremely hot, relativistic, classical, tenuous plasmas encountered in the magnetospheres of
INTRODUCTION AND EXECUTIVE SUMMARY 7 Pulsar Magnetospheres 10 10 ye - 5 10 0~ Dens ity n ( cm~3 ) 1,,,, 1 2o FIGURE 1.1 Classification of plasmas by temperature and density. RELATIVIST I C PLASMA Magnetic Fusion Solar Co rona Solar Wind IDEAL CLASSICAL PLASMA Inertial Fus ion kBT~E ~Discharges ~i ~nX3 = I NO NOTICEABLE IONIZATION 1~1 1 , , , 1 , , /EF = kBT / DEGENERATE QUANTUM PLASMA lo, ,, Electron ,, Gas in Metals EF=e2/n l/3 Wh its Dwa rfs _ ~ 1 103° pulsars to the extremely dense, cold, degenerate quantum electron plasmas in white dwarfs. As a guide to Figure 1.1, we consider a plasma with average number density n and mean kinetic energy (3/2)kBT per particle. (Here, kB is Boltzmann's constant.) The average distance between neighboring charged particles is rO ~ n- ~/3. Therefore, the average Coulomb interaction energy between neighboring particles is (~> ~ e2/n-~/3, where e is the electron charge. Assuming a warm plasma with kBT exceeding the ionization energy Ei in Figure 1.1, then the plasma is classified as an ideal classical plasma provided kBT >> e2/n-~/3, i.e., provided the thermal kinetic energy is large in comparison with the average Coulomb interaction energy. If both sides of this inequality are raised to the 3/2 power, this condition can also be expressed as nA3 >> 1, where the Debye length AD = (kBT/4nTne2~/2 is the characteristic shielding distance of the Coulomb interaction potential in a plasma. It is evident from Figure 1.1 that an extensive region of (n,T) parameter space is located above the curves nA3 = 1 and kBT~ Ei, corresponding to ideal classical plasmas. Although they have widely different densi- ties and temperatures, such classical plasmas include pulsar magnetospheres and other astrophysical systems, the solar corona, the
8 PLASMAS AND FLUIDS solar wind, and planetary magnetospheres, as well as a wide range of laboratory plasmas characteristic of magnetic and inertial confinement ~ . fusion. As the plasma density is increased, the average distance between neighboring particles becomes very small, and quantum effects become important when n-~/3 is comparable to the thermal de Broglie wave- length ~/me(2kgTe/me)~/2 of an electron. The characteristic scale of electron kinetic energy in such a quantum plasma is the Fermi energy EF ~ ~2~31r2n)2/3/2me, where 2~h is Planck's constant and me is the electron mass. Referring to Figure 1.1, quantum effects become important when EF > kBT, i.e., when the Fermi energy exceeds the classical thermal energy kBT. If the Fermi energy EF also exceeds the average classical Coulomb interaction energy e2/n-~/3, the quantum plasma is ideal and weak-interaction models can be used to describe such degenerate quantum systems. An example is the degenerate electron gas in white dwarfs. On the other hand, in the region kBT < EF < e2/n-~/3, which includes the electron gas in metals in Figure 1.1, the quantum plasma is nonideal. Finally, the small triangular region in Figure 1.1 bounded by nA3 = 1, kBT ~ Ei, and kBT = EF is referred to as strongly coupled plasma. Such plasmas are classical (since kBT > EF), but the Coulomb interactions are strong since e2/n-~/3 is typically larger than kBT in this region. Unlike ideal classical plasmas (where e2/n-~/3 << kBI), the correlations due to Coulomb interactions are strong, and such systems are modeled by numerical simulation on high-speed computers. Fluid Physics* Fluid physics, which is among the oldest branches of the physical sciences, continues to fascinate scientists and engineers with an eclectic collection of elegant problems. Our need to understand the world of flow around us, encompassing the nature of transport across biological membranes to the appearance of solitary waves in planetary atmospheres, remains a constant stimulation and adventure. Fluid motion, which can exhibit the apparent randomness of turbu- lent flow as well as much larger-scale coherent structures, provides one *The term "fluid physics" is appropriate in the context of this report. However, owing to the broad range of interests of its practitioners, it uses several names, each of which is proper in its own context. Therefore, terms such as "fluid mechanics," "gasdynamics," and "biofluid mechanics" are often used to describe special branches.
INTRODUCTION AND EXECUTIVE SUMMARY 9 of the premier testing grounds for new developments in nonlinear dynamics. Wavelike fluid-mechanic teleconnections transmit informa- tion about the Earth's tropical oceans over vast distances to alter patterns of global atmospheric circulation. Swimming creatures are governed by the laws of efficient underwater travel, providing insights into the evolutionary pathways stimulated by changing environments or vacant biological niches. In common with many other branches of physics, fluid physics also finds a driving force in the existence of important problems in engi- neering. The pacing element for advances in many applications such as the efficiency of flight, the effectiveness of heat engines, and the pro- ductivity of chemical processing systems is our understanding of the fundamentals of fluid motion. There are striking examples in the machines of engineering as they exist today, compared with even the recent past, that measure the magnitude of the advances in our under- standing of fluid physics. As it is beyond the scope of this report to delineate all of these advances, only a few are mentioned as examples. The modern transport plane, with swept wings and quiet engines, reflects the progress in the last few decades of our understanding of high-speed flows. These configurations have been derived by a com- bination of originally empirical and, more recently, theoretical and conceptual constructs made possible by advances in our understanding of the physics of flow. The gas turbine engine of today, although superficially similar to its historical counterpart, includes major im- provements made possible by extensive efforts in fluid physics. The increased fundamental knowledge of combustion and heat transfer, which was obtained with so much difficulty through research, has led to lower exhaust pollution and longer life of the critical engine components. Today's chemical engineering plants have a throughput and an efficiency increase manyfold over those of only a decade ago, brought about by careful analysis of fluid mixing and heat transfer. These examples illustrate that basic knowledge in fluid physics moves quickly from research in flow physics to application because of the intense competitiveness of today's technological society. In summary, fluid physics remains intellectually stimulating owing to the elegance, widespread natural occurrence, and importance of its problems. In addition, new levels of understanding of complex phe- nomena have further vitalized this field. Much of this understanding has been created in the last decade by the development of powerful new tools that enable us to investigate the nature of complex phenom- ena that heretofore appeared to be intractable mysteries. Thus, the study of turbulence, complex high-speed flows, biological flows, and
1O PLASMAS AND FLUIDS geological phenomena has been paced by new developments in pow- erful computational and instrumentation techniques. We look forward to the next decade as a time of excitement and adventure. The impli- cations of mastering many important practical problems so necessary for the well-being of the nation and the world serve as further stimulus. PRINCIPAL FINDINGS AND RECOMMENDATIONS We summarize here selected major findings and recommendations that pertain to the areas reviewed by the Panel on the Physics of Plasmas and Fluids. General Plasma Physics · The fundamental studies carried out during the past two decades solidified the relatively new science of plasma physics. The under- standing of small-amplitude wave propagation and of fluctuation phe- nomena achieved in the 1960s is a necessary prerequisite for many plasma applications. · The 1970s saw the extensive development of coherent and turbu- lent nonlinear plasma physics, which is proving to be even more fundamental. The development and widespread use of advanced com- putational techniques provided an important link between theory and experiment. · 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 pre- cedes 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. The physics community should be encouraged to submit high-quality proposals for basic research in laboratory plasmas. · A dedicated study of plasma physics can be expected to lead to important new research techniques and technological opportunities. For example, dense nonneutral plasmas composed mainly of high- energy electrons may become available with new types of accelerators, taking advantage of collective processes. In a related area, new types of particle accelerators using collective effects will contribute to
INTRODUCTION AND EXECUTIVE SUMMAR Y 11 nuclear and high-energy physics, energy-related applications, and defense. New coherent radiation sources based on plasma technology, particularly x-ray lasers and generators of submillimeter microwaves, will have applications in materials research, medicine, defense, and no doubt in other areas not yet perceived. · The impact of plasma physics on related sciences and on technol- ogy has continued to grow since the birth of modern plasma physics in the late 1950s and will continue to grow for the foreseeable future, provided a strong research base for plasma physics is maintained by an adequate level of support. Fusion Plasma Confinement and Heating We divide the major findings and recommendations into those that pertain to magnetic confinement, in which strong externally applied magnetic fields are used to confine a high-temperature fusion plasma, and those that pertain to inertial confinement, in which a solid pellet is imploded to ultra-high densities. MAGNETIC CONFINEMENT · In all the main approaches to the magnetic confinement of fusion plasmas, the principal measures of performance plasma density, temperature, and confinement time-improved by more than an order of magnitude as a result of intensified fusion research in the 1970s. One approach the tokamak has already come within a modest factor of meeting the minimum plasma requirements for energy breakeven in deuterium-tritium plasmas. These achievements have been made pos- sible by rapid advances in plasma science. · The techniques used for plasma control and heating, the technol- ogy of high-power heating sources, and the precision of plasma measurements all improved dramatically during the past decade. There were equally rapid advances in plasma theory and numerical modeling, which are now able to explain much of the observed dynamical behavior of magnetically confined plasmas. The establishment of the National Magnetic Fusion Energy Computer Center (NMFECC) made many of these advances in theoretical modeling and data interpretation possible. · A particular strength of the U.S. fusion program is its broad base, which includes research on several alternatives to the mainline con- finement concepts, to ensure that the maximum potential of fusion is ultimately realized.
12 PLASMAS AND FLUIDS · A new generation of magnetic fusion facilities, coming into oper- ation worldwide, will, in the mid-1980s, extend experimental plasma parameters to reactorlike densities, temperatures, and confinement times. · However, if the United States' preeminent position in the world- wide fusion program is to be maintained into the 1990s in the face of aggressive Japanese and European competition, the pace of new- device authorization that characterized the early 1970s will have to be restored soon. · A vigorous base research program that emphasizes both increased scientific understanding and research on improved reactor concepts (advanced tokamaks, tandem mirrors, and other approaches) is essen- tial. The purpose of concept improvement activities should be both to increase scientific understanding and to improve reactor prospects. Historically, the interplay between these two lines of effort has led to our most creative physical insights and concepts. Such a program is essential to technical progress and to the training of talented new people. · The demonstration and experimental study of an ignited fusion plasma is the obvious next research frontier after attainment of scientific breakeven. Indeed, the achievement of fusion requires that this goal be attained. While the scientific understanding of many key plasma phenomena can best be gained on moderate-size experimental facilities, ultimately plasma-confinement properties must be investi- gated under conditions of intense alpha-particle heating, which will require an ignited plasma core. The fusion program is at the point where consideration of such experiments can proceed with some degree of realism. Obviously, ideas will continue to evolve rapidly as results from experiments, particularly the TFTR tokamak, become available in the next several years. In the near term, studies for a Burning Core Experiment should emphasize maximum scientific out- put with minimum project cost, consistent with the recent recommen- dations of the Magnetic Fusion Advisory Committee (MFAC).* · A vigorous research program is essential to technical progress in mainline tokamak and mirror research. Moderate-size experimental facilities are the primary sources of the scientific and technological innovations required to develop fusion to its fullest potential. *Magnetic Fusion Advisory Committee Recommendations on the Tokamak Fusion Core Experiment, Department of Energy (July 1984).
INTRODUCTION AND EXECUTIVE SUMMAR Y 13 · Continued research on alternative fusion concepts is essential to advance basic understanding of plasma confinement and to foster the development of approaches that show significant promise of improved reactor configurations. · Intensive research must continue on the theoretical and computa- tional descriptions of magnetically confined plasmas and on supporting experiments in basic plasma physics. These have been a source of many promising new concepts in fusion research. · Continued strong university involvement will be essential to fusion research for the foreseeable future. Universities augment fusion re- search in the national laboratories in several unique and important ways. They educate and train professional fusion researchers, they provide the fusion program access to a breadth of talent and intellect in the sciences and engineering, and their research is a major source of innovative ideas and scientific and technological advances. INERTIAL CONFINEMENT The United States has maintained world leadership in inertial- confinement fusion research since its inception in the late 1960s. Its near-term applications are military, with promising long-term applica- tions to energy production. An inertial-confinement fusion reactor would have a relatively small containment volume, and its operation, maintenance, and repair may be relatively simple. An issue important to inertial-confinement fusion research is that classification limits access to technical information. During the past decade, a vigorous international research effort was established to investigate the inertial-confinement approach to fusion. An impressive array of experimental facilities was developed, includ- ing neodymium-glass and CO2 lasers and light-ion accelerators, which has resulted in considerable scientific progress. Investigations of laser-coupling physics over a wide range of intensity and wavelength showed that lasers with wavelengths of a micrometer and shorter have very good coupling. Deuterium-tritium fuel was heated to thermonu- clear temperatures in laser-irradiated implosions. Shells were ablatively accelerated to speeds above 107 calls, with velocity nonuniformities of less than 5 percent. In implosions, final fuel densities of 100 times the liquid density of deuterium-tritium were achieved with fuel temperatures of about 5 million degrees. These fuel densities are within a factor of 10 of the compression needed for a high-gain target.
14 PLASMAS AND FLUIDS On the basis of the significant scientific and technological progress, we recommend the following near-term emphasis and strategy for inertial-confinement fusion research: · Use present driver facilities to determine the physics and scaling of energy transport and fluid and plasma instabilities to regimes characteristic of high-gain targets. · Use the new generation of drivers under construction to implode deuterium-tritium fuel mixtures to 1000 times liquid density required for high-gain targets and to implode scale models of high-gain targets to the density and temperature of the full-scale target. · Identify and develop cost-effective, multimegajoule driver ap- proaches. Timely execution of this strategy will provide the basis for a decision in the late 1980s on the next generation of experimental facilities. Drivers in excess of a megajoule would allow demonstration of high-gain targets for both military and energy applications. As a final point, research in inertial-confinement fusion is carried out primarily in the national laboratories, with smaller but prominent efforts at several university, industrial, and other research laboratories. In view of the importance of basic physical processes in inertial fusion, and the requirements for advanced technology development, there should be continued emphasis on maintaining and strengthening the breadth of institutional participation. Space and Astrophysical Plasmas · Many practical systems, both civilian and defense, must operate in the highly variable and potentially hostile plasma environment of the Earth and solar system. Plasma processes in this environment also influence and even disrupt important ground-based systems over local and regional scales. · The solar system, including the Sun itself, is the primary labora- tory in which astrophysical processes of great generality can be studied in situ; these processes include magnetic reconnection, plasma heating and particle acceleration, magnetohydrodynamic wave generation and propagation, magnetoconvection, magnetoturbulence and turbulent magnetic-field diffusion (including spatially intermittent magnetic fields), collisionless shock formation, wave-particle interactions, and the interaction of plasmas with radiation. · Because of the wealth of pertinent information flowing from solar-system plasma physics, and continuing advances in large-scale
INTRODUCTION AND EXECUTIVE SUMMARY 15 numerical modeling, magnetohydrodynamics and plasma physics are becoming essential to the interpretation of many astronomical obser- vations. · Studies of plasma behavior in extreme astrophysical environ- ments, such as pulsars, enriches basic theory and may suggest future laboratory investigations. · Cosmic-ray observations provide important information about space and astrophysical plasmas. The plasma physics of cosmic-ray acceleration and transport made especially rapid progress in the past decade. The improved precision of cosmic-ray composition measure- ments now makes possible quantitative tests of theories of nucleosyn- thesis and galactic chemical evolution. · A vigorous program of observations, measurements, and experi- ments, in space and on the ground, is key to productive research in space and astrophysical plasma physics. We recommend implementa- tion of the comprehensive research strategy outlined in the Space Science Board publication Solar-System Space Physics in the 1980's: A Research Strategy (National Academy of Sciences, Washington, D.C., 19801. These programs, including the International Solar Terrestrial Program, are the primary ones that will explicitly contribute to our knowledge of the physical processes in large-scale plasmas. We endorse the programs proposed in the Astronomy Survey Committee report Astronomy and Astrophysics for the 1980's (National Academy Press, Washington, D.C., 1982), because they will make significant contributions to many problems in plasma astrophysics. · We recommend establishment of a national computational pro- gram dedicated to basic plasma physics, space physics, and astrophys- ics that will maintain the state of the art in the technology appropriate to large-scale theoretical models and simulations and provide access to users on the basis of peer review. The implementation of such a program should be studied forthwith by a committee drawn from the affected disciplines. · In view of the increasing precision of its experimental and theo- retical techniques, and in view of its wide applicability to space physics, astrophysics, and technology, we recommend that plasma physics become a regular part of the university science curriculum. · In view of the many common processes underlying both labora- tory and space plasmas, such as reconnection of magnetic field lines and particle acceleration by plasma waves, there should be an ex- panded effort to simulate space- and astrophysical-plasma processes in the laboratory.
16 PLASMAS AND FLUIDS Fluid Physics The diverse research field of fluid physics is vital to the country's economic health and defense because of its direct impacts on trans- portation and chemical and material processing systems and its central role in the understanding of many geophysical and astrophysical phenomena. We summarize here our findings and recommendations related to research support and education in fluid physics. · Access to major computational and experimental research facili- ties in fluid physics is limited. The computer has emerged as a significant tool whose applications range from the rapid organization of data and its subsequent analysis and display to the direct numerical simulation of the major features of limited volumes of turbulent flow. In many applications of computational fluid dynamics, such as aircraft design, there is aggressive competition from Europe with indications of similar efforts in Japan. In addition, there are unique national experi- mental facilities that are unavailable for basic fluid-physics research. · It is strongly recommended that unique national fluid-physics facilities, both computational and experimental, be made available to the university and nongovernment communities for basic research. Direct allocations of time and other resources will be necessary in order to maintain an appropriate balance between basic research and development and to assure steady operational funding of these facili- ties. · Many of the recent advances in fluid-physics research have originated with new instrumentation techniques coupled with an in- creasing ability to analyze larger quantities of data, as for example in the study of turbulent flow. · We recommend a concerted research effort to devise instrumen- tation for essentially simultaneous, time-resolved, multipoint measure- ments of flow properties throughout large volumes. The instruments might be based on laser holographic methods, on multiprojection (tomographic) techniques, or on a combination of these and other as yet unexplored methods. This will require the acquisition of major hardware components, as well as identification of sufficient resources for their development and deployment. · The mathematical techniques that have been developed and re- fined during the past 15 years have become increasingly important tools in advancing fundamental understanding of complex flow and in
INTR OD UCTI ON AND EXEC UTI VE S UMMAR Y 17 improving numerical methods, as well as in testing the results of numerical simulation. · Given the increasing importance of numerical simulation tech- niques, we recommend that these techniques not be allowed to obscure the fundamental importance of analytical methods. · Funding for basic research in fluid physics comes from a wide variety of sources, but the field lacks an individual national identity. Despite the common technical threads that bind fluid physics as a scientific discipline, its basic research support is poorly organized and lacks quantity, continuity, and the early recognition of significant new opportunities. · We recommend that a continuing survey of research support in fluid physics be established and monitored by an appropriate agency. The information developed by this survey would help to coordinate research and would identify basic areas that offer significant new opportunities or are neglected because they are not included within the mission of a support agency. Areas that receive excessive overlapping support could also be identified. · Education and university research in fluid physics is conducted primarily in engineering and applied mathematics departments in the United States. During 1983, there were more than 50 times as many fluid-dynamics-related Ph.D. theses in engineering departments as there were in physics departments. Physics research in the United States has been deprived thereby of the opportunity to participate in many fundamental and applied problems involving fluid physics. · The use of fluid physics is pervasive in many areas of modern science and technology. We strongly recommend that physics depart- ments consider requiring an undergraduate course in fluid physics. We similarly recommend that engineering schools consider a required upper-division undergraduate course in modern physics. These impor- tant steps would enhance collaboration between the physical and . . . engineering sciences. · The need for dedicated, separate funding for modern laboratory equipment in fluid mechanics is at least as severe as the well-publicized needs in other branches of physics. Fluid-flow instrumentation, espe- cially optical, will continue its recent rapid progress. Unfortunately, as a result, university laboratory equipment will become even more out of date. · We recommend that this deficiency in instrumentation be ad- dressed in a timely manner.
18 PLASMAS AND FLUIDS RECENT ACCOMPLISHMENTS AND FUTURE RESEARCH OPPORTUNITIES In this section, we summarize selected significant research accom- plishments during the past decade and future research opportunities in those areas reviewed by the Panel on the Physics of Plasmas and Fluids. Considerably more detail is given in subsequent chapters. General Plasma Physics SIGNIFICANT RECENT ACCOMPLISHMENTS Selected significant accomplishments in general plasma physics during the past decade include the following: · Major advances in understanding the physics of nonlinear (i.e., large-amplitude) plasma phenomena. These include experimental, the- oretical, and computational studies of large-amplitude plasma waves, including the trapping of particles in the potential troughs of these waves; the breaking of waves; and the passage toward turbulent behavior. Also included is the discovery of two different types of plasma solitons (Langmuir and ion-acoustic). Solitons are highly localized, self-perpetuating wave entities with remarkable properties of coherence. Other developments include the elucidation of the ponderomotive forces (radiation pressure) exerted by large-amplitude waves in plasmas, the understanding of "parametric" instabilities (two coexisting plasma waves amplifying a third wave), and the prediction and confirmation of radio-frequency driven do currents in tokamak magnetic confinement devices. · There have also been major advances in computer capabilities. These led to realistic computer simulation experiments that follow thousands of "particles" and model the collective behavior of real plasmas. Another important advance was the use of on-line computers to analyze, correlate, and immediately display synoptic data from the plasma diagnostics on laboratory experiments. · Development of high-voltage, high-current accelerators that pro- duce 10- to 20-million-megawatt pulses of power with applications to the generation of intense charged-particle beams and x-ray pulses. · Development of highly collimated intense beams of energetic electrons, at the million-megawatt level, as in the Livermore Advanced Test Accelerator (ATA), and the study of beam propagation in vacuum and in low-pressure gases.
INTRODUCTION AND EXECUTIVE SUMMAR Y 19 · Development of tunable, coherent "free-electron" radiation sources that generate intense monochromatic radiation at wavelengths ranging from centimeters to micrometers (microwaves, infrared and visible light). These sources include relativistic magnetrons, gyrotrons (cyclotron masers), and free-electron lasers. · Major advances in atomic physics, including measurements of electron-ion ionization and recombination processes, understanding of radiation losses, interpretation of x-ray lines, and x-ray laser experi- ments. New spectroscopic diagnostics making use of magnetic dipole transitions and charge exchange were introduced into fusion research. · Minimum energy states have been predicted for a class of plasma- field configurations, subject to the constraint of global helicity. The tendency for reversed-field pinch and spheromak configurations to relax toward these states has now been confirmed experimentally, allowing unique approaches to configuration sustainment and current drive. Understanding the basic physics of relaxation to a minimum energy state under the constant-helicity constraint may have a wide range of applications, e.g., to the magnetosphere as well as to magnetic fusion configurations. FUTURE RESEARCH OPPORTUNITIES Particularly promising research areas are summarized below: · Development of new particle accelerators of two types: first, high-current accelerators that necessarily involve collective effects, that is, electric and magnetic fields generated by charged particles; second, accelerators employing the intense electromagnetic fields generated by high-power lasers to achieve ultrahigh particle energies. · Further development of free-electron radiation sources, to pro- duce tunable radiation sources, some of exceptionally high-power output. Applications include spectroscopy, condensed-matter re- search, isotope separation, communications and radar, and biology and . . met lame. · Advances in understanding strongly coupled plasmas, i.e., dense ionized matter where coulombic correlations among particles deter- mine bulk and dynamic properties. Such plasma states occur in inertial fusion targets, large planets, and red giant and neutron stars. The use of large computers and novel computer simulation techniques is expected to lead to major developments in this important field, which is central to the understanding of matter at ultrahigh densities.
20 PLASMAS AND FLUIDS · Developments in the physics of nonneutral plasmas. A nonneutral plasma is composed mainly of one species of charge (i.e., electrons or ions). For example, a pure electron plasma confined by external electric and magnetic fields has many unique properties, including great stability. Indeed, confinement times of hours have been observed for such plasmas. Theoretical predictions indicate that nonneutral plasmas can be cooled to the point that the electrons enter a liquid or even a crystalline state. · Advances in the understanding of the excitation, ionization, and recombination of multiple ionized atoms. Quantitative plasma spec- troscopy is needed to test the validity of the atomic data base and reaction kinetics, as are more precise wavelengths. Self-consistent calculations of radiative and transport properties of dense and hot matter continue to pose difficult challenges to many-body theory. · Further advances in computers and their use in plasma simulation to the point where realistic three-dimensional plasma simulations can be envisaged. Fusion Plasma Confinement and Heating SIGNIFICANT RECENT ACCOMPLISHMENTS MAGNETIC CONFINEMENT The last decade saw greater progress than any previous decade in fusion's history. Many of the most significant accomplishments were achieved within the U.S. magnetic fusion program. Among these are the following: · In the tokamak approach, the achievement of well-confined plas- mas at SO-million-degree ion temperatures, approaching those needed for fusion. Before these results were obtained, the theory of microinst- abilities in tokamaks had predicted that, at such temperatures, new forms of fine-scale turbulence might arise, associated with the entry of the ions into a previously unexplored low-collisionality regime. A1- though enhanced fluctuations were indeed observed in the experi- ments, energy transport was not affected significantly. These results have led to a re-evaluation of, and fundamental improvements in, the theory of anomalous transport due to microinstabilities in tokamaks. · Also in the tokamak, the demonstration of energy confinement of dense plasma (at a lower but significant temperature) for values of the Lawson nine parameter almost equal to those needed in a full-scale
INTRODUCTION AND EXECUTIVE SUMMARY 21 fusion reactor, and the identification of empirical energy-confinement scalings that are favorable for reactor sizing. According to one empir- ical scaling, which fits the data from ohmically heated tokamaks over a wide range of parameters, the confinement time varies with the cube of a plasma linear dimension, as would be expected for a diffusive process in which the transport coefficient depends on gradient-induced anomalous processes. · The successful use, in accordance with theoretical prescriptions, of radio-frequency waves to drive plasma currents in tokamaks, thereby permitting confining magnetic fields to be steady state, a property of importance to the practicality of tokamak reactors. Exper- iments on radio-frequency current drive have exhibited a hot-electron population of current carriers in agreement with theory and have verified the predicted dependence of current-drive efficiency on plasma density. · The achievement of beta values (ratio of plasma pressure to magnetic pressure) in tokamak plasmas approaching those required for an economic reactor. · In the mirror the principal alternative U.S. approach to magnetic fusion the progressive elimination of high-frequency instabilities that had been an obstacle to progress. It has been the development of a detailed theoretical understanding of the dependence of these high- frequency instabilities on the velocity distribution of mirror-confined ions that has led to their suppression by a variety of techniques in present-day experiments and to their predicted elimination in a ther- mal-barrier tandem mirror. · The introduction of the tandem-mirror concept and the demon- stration that tandem mirrors can effectively suppress plasma leakage through the ends, thereby pointing the way toward mirror systems adequate for fusion applications. A tandem-mirror reactor would have mirror end cells and an axial thermal-barrier potential profile to suppress end losses from a large-volume, high-beta, ignited central-cell plasma. · The development and application of multimegawatt neutral-beam sources that can heat both mirror and tokamak plasmas to fusion temperatures. Neutral-beam heating experiments have verified that beam ions deposit their energy in tokamak and mirror plasmas by means of well-understood classical processes. · Major advances in plasma theory and computation, which are now able to describe in detail most large-scale phenomena of confined
22 PLASMAS AND FLUIDS plasmas and which are beginning to provide valid understanding of microscopic phenomena. Accomplishments of particular note include (i) the successful description of the nonlinear regime of resistive instabilities and the circumstances leading to disruptions in tokamaks, (ii) the detailed delineation of stability limits on beta in a tokamak for a wide variety of plasma profiles and cross sections, and (iii) the accurate identification and characterization of the loss-cone microin- stabilities that are responsible for end losses in simple mirror configu- rations. · Major advances in the suppression of gross plasma instabilities by such diverse means as hot electron rings (Elmo Bumpy Torus), relaxation to minimum-energy-state profiles (reversed-field pinch), induced currents in nearby conductors (compact toroids), and helical windings to provide confining fields in a torus (stellarator). FUTURE RESEARCH OPPORTUNITIES MAGNETIC CONFINEMENT Now that magnetic confinement has been shown to be a viable fusion concept, at least qualitatively, the future emphasis of magnetic fusion research will be on quantitative questions: What are the precise laws governing the fine-scale stability and rate of transport of hot plasma from practically configured magnetic confinement systems, and how are such systems to be optimized? To answer such questions will require increased understanding of all aspects of the plasma state, together with increased experimental sophistication. The goal: a pre- dictive capability that will lead to the identification of magnetic confinement systems satisfying the rigorous requirements of practical fusion energy applications. Among the specific opportunities for future U.S. magnetic fusion research are the following: · In the tokamak approach, the identification and understanding of physical scaling laws for confinement times. This will require well- diagnosed and carefully analyzed experiments over a wide range of parameters on tokamaks with both ohmic and auxiliary heating, especially on the new generation of larger devices, which will provide definitive information on size scaling. Although the fundamental phys- ical processes affecting confinement in tokamaks are clearly very complex, sufficiently so to have defied accurate theoretical character
INTRODUCTION AND EXECUTIVE SUMMARY 23 ization to date, the development of a broader experimental data base, accompanied by advances in the theoretical description of large- and fine-scale turbulence in tokamaks, is almost certain to lead eventually to a sound fundamental understanding of these processes. · Confirmation of tokamak scaling laws by means of the simulta- neous achievement of fusion temperatures, densities, and confinement times, early in the second half of the 1980s as the new facilities now coming into operation are brought up to full performance. · Optimization of the plasma beta (plasma pressure relative to magnetic pressure) in tokamaks, to approach the limit predicted by theory (about 10 percent). Optimization of the tandem mirror so as to achieve the projected beta values of about 40 percent. Achievement of these beta values will require sound guidance from theory and compu- tation as to the detailed magnetic designs and the plasma profiles and shapes that provide the greatest protection against various types of gross plasma instability. · Detailed understanding of the physics of self-generated electric potentials within plasmas, essential to the operation of tandem-mirror systems. This will require well-diagnosed experiments on tandem mirrors, aimed at determining the relationship between the axial potential profile and the power applied to heating the electrons in the thermal-barrier region. · Scale-up of the most promising alternative magnetic-confinement concepts to experimental facilities capable of achieving reactorlike plasma parameters. This will require: (i) the scale-up of reversed field-pinch experiments to test sustainment of, and confinement in, stable high-beta configurations at higher plasma temperature, (ii) the improvement of formation techniques and the determination of energy- confinement scaling as temperatures are increased beyond the present 100-electron-volt range into the collisionless regime in compact toroids, and (iii) the demonstration of stability and favorable confine- ment at moderate beta values in stellarators at low plasma collisional- ity. All of these advances depend critically on improved understanding of the dynamics of confined plasmas. · Increased fundamental understanding of hot plasmas made possi- ble by, and contributing to, these advances in fusion research. This will require well-diagnosed experiments on the major facilities, a vigorous program in plasma theory and computer simulation, and carefully designed small-scale experiments that can be devoted exclusively to detailed studies of fundamental plasma processes.
24 PLASMAS AND FLUIDS SIGNIFICANT RECENT ACCOMPLISHMENTS INERTIAL CONFINEMENT Scientific advances in inertial fusion research were made by eluci- dating basic physics, by conceptual breakthroughs, and by the devel- opment of advanced technologies. They include the following: · Advances in pellet design, which have high leverage, since they can raise potential pellet gains, increase efficiency, and thereby reduce driver and system requirements (and cost). For example, the idea of using tailored laser pulses to increase gain and reduce the required driver energy gave inertial confinement research its initial impetus. The x-ray hohlraum approach to implode pellets may significantly reduce the beam illumination uniformity requirements. · Driver technology advanced steadily during the past decade. Lasers progressed from the 10joule range to present-day drivers (NOVA, ANTARES, and PBFA II) with outputs of many tens of kilojoules. Megajoule-class drivers, needed for reactors, are now within technological reach. · Light-ion driver technology progressed rapidly; highly efficient ion diodes were developed; ion beams were transported more than 1 meter from these diodes and focused to more than 1 terawatt per square centimeter. Recent developments in ultra-fast plasma-erosion switches are likely to lead to a new generation of high-power electron and light-ion accelerators. ~ The laser-target coupling processes were identified at 1- and 10-,um laser wavelengths. Although there are still many gaps, the understand- ing gained thus far favors the shorter laser wavelengths, where more efficient laser-plasma coupling was demonstrated. · Cold targets were accelerated to speeds above 100 kilometers per second with nonuniformities below 3 percent, close to the conditions required for high-gain inertial fusion. Pellet shells must be uniformly imploded at high speeds and yet remain cold to compress the fuel properly. · Deuterium-tritium fuel in laser-irradiated pellets was heated to thermonuclear temperatures early in inertial fusion research. More recently, ablatively imploded pellets compressed fuel to very high density (100 times solid density). Simultaneous fuel heating and high-density compression are needed for significant energy yields. · There have been major advances in the development of the scientific tools needed to understand the physics of the high-energy- density and short-time-regime characteristic of inertial fusion. Espe
INTRODUCTION AND EXECUTIVE 5UMMAR Y 25 cially noteworthy were advances in experimental diagnostics and computer modeling. · Parametric instabilities of the laser radiation in strongly inhomo- geneous plasmas, causing Raman and Brillouin scattering and conver- sion to plasma and ion waves, were predicted by theory, confirmed by experiment, and shown to be important processes affecting laser- plasma coupling. FUTURE RESEARCH OPPORTUNITIES INERTIAL CONFINEMENT Many scientific and technological issues will be addressed with the hundred-kilojoule drivers that will be available in the mid-1980s, while others await megajoule systems. Future research opportunities in inertial fusion include the following: · Detailed tests of the improved laser-plasma coupling with short- wavelength light will be made. It is especially important that this information be obtained under plasma coronal conditions modeling those in a reactor. · Experiments defining the hydrodynamic stability of accelerated targets and imploding pellets will provide information essential to high-gain pellet design, which will continue to have high leverage and will, in large part, define future technological requirements. · There are several approaches to making the deposition of driver-beam energy on the pellet acceptably uniform. One promising approach, converting the beam energy to x rays in a hohlraum, which in turn drives the implosion, will be tested experimentally in detail. · Several innovative ways of making laser beams much more uniform, such as the induced spatial incoherence technique and precision beam control, will be tested in large systems. Highly sym- metric beam illuminations would allow beam energy to be used directly for pellet implosion. · Driver technology will continue to advance toward a high-energy, high-repetition-rate, efficient driver suitable for energy applications. One promising system under development is the krypton-fluoride excimer laser; megajoule-class glass-laser designs are also being eval- uated. Heavy- and light-ion beam drivers offer high potential efficiency and repetition rates. · There will be opportunities to study the properties of hot matter at densities 100-1000 times solid densities.
26 PLASMAS AND FLUIDS Space and Astrophysical Plasmas SIGNIFICANT RECENT ACCOMPLISHMENTS Selected significant accomplishments in space and astrophysical plasma physics are summarized below: · The first spacecraft studies of the magnetospheres of Mercury, Venus, Jupiter, and Saturn occurred during the past decade. The concepts used to understand the Earth's magnetosphere were success- fully extended to these magnetospheres, and significant new physical phenomena were discovered. The generality of the concept of a magnetosphere in solar-system and astrophysical environments was recognized. · Rigorous models of magnetic-mirror confinement, radial diffusion, and turbulent pitch-angle scattering of energetic ions and electrons were created and successfully tested by observations in the magnetospheres of Earth, Jupiter, and Saturn. · Rigorous analytical theory and numerical simulation established the correct magnetohydrodynamical description of reconnection the conversion of configurational magnetic energy to plasma kinetic and thermal energy. Theoretical understanding of the more fundamental collisionless description was consolidated. Experimental evidence for reconnection was provided by laboratory measurements and by obser- vations in the Earth's magnetosphere. · A coherent program of active and passive radar experiments, chemical releases, rocket measurements, analytical theory, and numer- ical simulations devoted to the equatorial ionosphere led to the most complete analysis of the nonlinear development of the Rayleigh-Taylor instability in plasma physics. · Detailed observations of the solar surface have forced a re- evaluation of our current theoretical understanding of hydrodynamic and magnetohydrodynamic flows: in consequence, sophisticated ana- lytical and numerical models that aim to describe the observed highly intermittent magnetic fields on the solar surface have been initiated. · The detection by the Einstein Observatory spacecraft of stellar coronal x rays proved that solarlike magnetohydrodynamic and plasma processes are central to the physics of the atmospheres of all stars that have convecting outer layers. · A clear understanding of electron heat transport in the solar wind was achieved by systematic measurements of superthermal electrons. Quantitative studies of the conduction of heat between the solar corona
INTRODUCTION AND EXECUTIVE SUMMAR Y 27 and chromosphere, which promise to make interpretation of chromo~nheric and transition-region line emissions more secure. were -A Or--- initiated. · Synoptic measurements of the dependence of the Earth's bow- shock structure on the properties of the upstream solar wind substan- tially increased the basic understanding of collisionless shocks. · Measurements of the energetic particles and plasma turbulence associated with interplanetary shocks and planetary bow shocks began to be used to test self-consistent shock acceleration theories. These results are providing a solid basis for theories of the acceleration of cosmic rays by supernova shocks. · Measurements of the isotopic composition and elemental abun- dances of cosmic rays defined the lifetime of cosmic rays in the galaxy and suggested that they are accelerated directly out of the interstellar medium after being produced by the stellar nuclear-burning cycle. · Detection of an x-ray line, plausibly at the electron cyclotron frequency, provided the first experimental indication that neutron stars have superstrong magnetic fields, of order 10~2 gauss a fundamental hypothesis of pulsar and galactic x-ray source theories. · Energetic plasma jets were found to occur in a wide range of astronomical objects, from compact stars to active galaxies and quasars. · Plasma theory, numerical simulation, and laboratory experiments provided a basic explanation of the Alfven critical flow velocity criterion for the rapid ionization of the neutrals in a plasma-neutral gas mixture. FUTURE RESEARCH OPPORTUNITIES During the next decade, the expected research opportunities and accomplishments include the following: · The first stage in the exploration of solar-system plasmas, includ- ing planetary magnetospheres and the large-scale heliosphere, will be nearly completed by the Voyager encounters with Uranus and Neptune, by the Galileo mission to Jupiter, and by the International Solar-Polar Missions to high heliographic latitudes. The most impor- tant remaining exploratory objective will be in situ measurements of the solar corona. · The plasma environment of the Earth will be subjected to con- trolled study, and, perhaps, to a measure of control, through the
28 PLASMAS AND FLUIDS systematic use of active experiments and by synoptic observations made by the International Solar-Terrestrial Physics Program. · High-spatial-resolution observations in the optical region by the Solar Optical Telescope; in the radio region by the VLA, VLBI, and the planned VLBA; and in the UV and x-ray regions by other planned space experiments, such as the Advanced Solar Observatory, will provide essential information for defining quantitative models of solar- surface convection, surface magnetic fields and dynamics, solar flares, and coronal heating, thereby creating the basis for general understand- ing of stellar activity. · The growing ability to make a series of detailed high-resolution observations in many wavelength ranges (such as in the x-ray range by the Advanced X-Ray Astrophysics Facility) will render many astro- physical objects increasingly subject to theoretical models that explic- itly take plasma processes into account. · Understanding of many space-plasma processes will be sufficiently quantitative to make them reliable components of models of large-scale space and astrophysical systems. · The first generation of large-scale numerical models of space and astrophysical systems will be completed. Such models will likely make plasma physics central to the interpretation of many astronomical observations and motivate new and different kinds of observations. Fluid Physics SIGNIFICANT RECENT ACCOMPLISHMENTS During the past decade, significant research accomplishments in fluid physics included the following: · The revolutionary development of computational fluid dynamics, which has made possible the solution of problems that previously defied theoretical analysis and experimental simulation, such as con- vection and circulation within the Sun and planetary atmospheres, and the nonequilibrium flow surrounding the Space Shuttle on re-entry. The time and expense required to design aircraft wings, internal combustion engines, nuclear fusion and fission devices, and surface and undersea naval vehicle components were reduced. Computational fluid dynamics has increased the understanding of flows in the presence of combustion, chemical reactions, and multiple phases. · An improved basic understanding and ability to model and com- pute turbulent flows. These improvements include basic insight into
INTRODUCTION AND EXECUTIVE SUMMARY 29 how mechanical systems display chaotic behavior; a better apprecia- tion of the role, organization, and interaction of fluid structures of all sizes; new diagnostics and methods of interpreting data; and the use of large-scale computing. · Single and multiphoton excitation, as well as scattering tech- niques, were developed to study the energy budgets of severe gas- dynamic environments such as flames, permitting the first detailed investigations of complicated chemically reacting flows. · Large-scale turbulent and coherent fluid-dynamic structures were identified in the Earth's oceans and atmosphere and the atmospheres of Jupiter, Saturn, and Venus. Their successful simulation using eddy- resolving computer models gives a new view of laboratory turbulence and the general circulation, storms, and weather of the atmosphere and deep ocean. · Fluid-dynamic modeling led to basic new knowledge of our cardiovascular, reproductive, and urinary systems and many other internal organs of the human body. It has also illuminated the locomo- tion of biological organisms, from a single ciliate cell to the humming- bird and the tuna. Fluid-dynamic principles proved vital to the design of artificial organs, cardiovascular implants, prostheses, and the devel- opment of new clinical diagnostic methods. · Important advances were made in understanding the collective behavior of dilute particulate and aerosol suspensions. New mathemat- ical methods were devised for treating large-amplitude droplet defor- mation and the strong interaction between three or more particles, which have potential application to denser systems. These advances have led to new insight into the behavior of clouds, fluid separation phenomena, geological magma chambers, climate dynamics, and fluids with complex theologies, such as blood. · The central unifying idea of modern geology is the fluid-convection interpretation of the motion of the Earth's upper mantle. The implica- tions for planetary evolution, earthquakes, volcanism, and mineral and petrochemical resource exploration were made clear in the last decade. · Dimensional analysis and recent theoretical understanding of jet noise, acoustic damping, and turbulent flows led to a thousandfold reduction in the energy of acoustic emissions from aircraft, resulting in major reductions in perceived noise levels. · An accelerated pace of accomplishment in understanding high- speed flows has been made possible by improved analytical tools, numerical simulation, and new experimental techniques. These tech- nical advances inspired significant improvements in the efficiency of
30 PLASMAS AND FLUIDS commercial transport, in the effectiveness of high-performance air- craft, and in manned re-entry from space. · Simple wavelike connections were discovered in terrestrial cli- mate studies. Circulation changes, like E1 Nino of the tropical Pacific, are communicated across the globe and have large effects on rainfall and winds. · Noninvasive instrumentation techniques, such as those that detect blood-flow-initiated acoustic emissions from the human body, or neutrally buoyant probes that track, via satellite, the transient and mean circulation of the oceans, significantly advance the understanding of many fluid-flow phenomena. · New constitutive models based on molecular physical structure led to a better understanding of the striking flow properties of non- Newtonian fluids, such as polymer solutions and drag-reducing agents. · The synergistic interaction of chemical, fluid, and optical physics has resulted in the new continuous high-power laser. This success led to the identification of fluid phenomena important to the performance of electric discharge and other gas-media lasers. · The development of numerical simulation techniques has permit- ted the study of molecular motion in transition-regime gas flows, which occur between the limits of collision-dominated continuum flow and collisionless, free-molecular motion. FUTURE RESEARCH OPPORTUNITIES During the next decade, the expected research opportunities and accomplishments in fluid physics include the following: · Rapid advancement in the basic understanding of the characteris- tics and origins of turbulence, including investigations of the connec- tion between the routes to chaos found for systems with a finite number of degrees of freedom and the continuous instability that is fluid- dynamic turbulence. · Improvements in the ability to control turbulent flows will lead to novel drag and noise-reduction techniques; increased combustion efficiency; and control of separation, spreading, and mixing. Major advances in technology will result from the ability to predict flow with turbulent zones. · Continued rapid development of advanced computational tech- niques in fluid dynamics, together with the next generation of comput- ers, will provide the opportunity to calculate and obtain a new level of physical understanding of complex three-dimensional compressible ~,
INTRODUCTION AND EXECUTIVE SUMMARY 31 viscous flows. It will then be possible to optimize more effectively the design of high-performance aircraft, improve the forecasting of severe storm formation, attempt to predict global seasonal and annual climate changes, and realistically simulate and model planetary and astrophys- ical fluid-dynamical behavior. · Development of powerful laser-based optical techniques for the rapid, multipoint measurement of flow-field properties, in conjunction with numerical techniques, will provide new types of information and increase the usefulness of large experimental facilities. · In many technologically important fluid machines the flow is either separated or unsteady or both. With the help of modern instrumenta- tion and computerized data analysis, we are beginning to understand the physics of such flows and how, often in combination, they can be used to improve technological devices ranging from heart valves to aircraft. These possibilities will present a major research challenge in the coming decade. · The subjects of combustion and reacting flows are likely to yield new applications in the near future. Control of soot and other pollut- ants will result from the understanding of their production mecha- nisms. Basic studies of the interaction between chemical kinetics and fluid instabilities will result in an understanding of deflagration and the transition to detonation. Applications range from improved fuel econ- omy to fire safety. · We expect to see major advances in the understanding of multiphase flow systems, including the macroscopic and microscopic interface phenomena of interest in both industrial and geological processes, for example, the stability of the liquid-liquid interface leading to fingering in oil recovery, convective processes in the ocean, and the formation of layered structures in magma chambers. · There will be increased interest in denser particulate systems, from the multiparticle interaction of finite clouds of particles to, more generally, the flow through porous media and filters based on the hydrodynamic interaction with their microstructure. · Interdisciplinary study of basic cellular level biofluid dynamic processes in the presence of molecular forces will expedite explana- tions of such diverse phenomena as electrokinetic behavior in pores and membranes, the microstructure of osmosis, cell division, cellular transport function, gel hydration, and fluid motion in the intracellular tissue matrix. This will lead to a better understanding of basic cellular physiological function. · Increased computational and data-handling capability will permit assimilation and understanding of the massive data sets required to
32 PLASMAS AND FF UlDS describe complex natural flow phenomena. For example, global-scale investigation of the oceans and climate dynamics is now possible using satellites and shipborne instruments. Using Lagrangian mathematical techniques and instruments that move with the fluid, we anticipate new views of turbulent dispersion; of the interaction between waves, turbulence, and mean flow in boundary layers; and in ocean- atmosphere circulations. · The development of Monte Carlo computational techniques, which account for molecular motion in gas flows, will be extended to higher-density flows, permitting meaningful modeling of highly nonequilibrium, chemically reacting flow systems. FUNDING AND MANPOWER RESOURCES The present funding levels for the areas of research described in Chapters 2-5 are summarized in Table 1.1, including a breakdown by government agency. The funding level in Table 1.1 corresponds to a total of approxi- mately 8600 professional researchers, assuming an average expendi- ture of approximately $150,000 per researcher. The incremental fund- ing and manpower required to carry out the recommended research programs over the next 5 years are delineated in subsequent chapters. Specific areas in which there is a critical manpower shortage are also identified (e.g., coherent radiation generation, atomic physics, basic experimental plasma physics, computational plasma physics and fluid dynamics, and plasma astrophysics). INSTITUTIONAL INVOLVEMENT The subjects of plasma-physics and fluid-physics research cover an extremely wide range of interests and applications, from basic astro- physics to such applications as fusion energy and long-range weather prediction. Correspondingly, the institutional structure within which the studies are carried out spans the entire range of research institu- tions from academe to industry. In what follows, we give a brief description of the institutional makeup of the subfields covered in this report. General Plasma Physics The funding for general plasma physics comes almost exclusively from government agencies, with major contributions from defense
INTRODUCTION AND EXECUTIVE S UMMAR Y 33 TABLE 1.1 Fiscal Year 1984 Funding Summary (in $ Millions) DOE NSF DOD NASA NOAA Total General plasma physics2.5 3.467.6 73.5a Fusion plasma confinement and heating (a) Magnetic471 - - 47 lb (b) Inertial170 - l70C Space and astrophysical2 30 5 100 2 1394 plasmas Fluid physics25 80e 170 110 42 427f a DOD funding total includes: $4 million, ONR; $36 million, DARPA; $6 million, AFOSR; $1.6 million, ASD (Wright-Patterson); and $20 million funding of the Naval Research Laboratory Plasma Physics Division. The $36 million DARPA total includes $10.5 million for operation of the ATA facility at LLNL. The $2.5 million DOE total is for the Division of Advanced Energy Projects. b Includes $88 million for fusion development and technology, $98 million for operation and modification of the TFTR tokamak facility, and $55 million for construc- tion (and supporting R&D) of the MFTF-B tandem mirror. The FY 1985 funding for magnetic fusion Is $437 million. c Includes $85.1 million for glass-laser, $46.9 million for gas-laser, $23 million for pulse-power research programs, and $1.8 million for university research; it also includes $12.9 million for construction of the PBFA-II light-ion facility. The FY 1985 funding for inertial fusion is $169 million. ~ This total does not include launch vehicles and tracking of space vehicles; it is an estimate of the funding for satellite plasma instrumentation and associated data analysis, theory, and numerical simulation. e This total includes $5 million for basic fluids research related to engineering support. The remaining is for atmospheric sciences and oceanography. f These are estimates of FY 1983 funding. Includes $230 million for national facility operation and research field measurements; it does not include the substantial funding used for the development testing of flight articles. related agencies. The National Science Foundation provides only a small fraction of the support for this research area. Most of the applied research in general plasma physics is performed at government laboratories, such as the Naval Research Laboratory, at national laboratories, or at industrial laboratories. A small portion of the research (approximately 15 percent) is performed at universities. The university effort, however, contains a major part of the innovative and basic components of the research. Plasma Confinement and Heating Since plasma fusion research is a long-range, energy-related topic, its support (except for relatively small industrial components) derives
34 PLASMAS AND FLUIDS entirely from the Department of Energy (DOE). Correspondingly, the major part of magnetic confinement research is carried out at national laboratories (Livermore, Los Alamos, and Oak Ridge), at the Plasma Physics Laboratory at Princeton University, and at the GA Technol- ogies industrial laboratory. Although representing a smaller fraction of the effort (approximately 10 percent), the universities represent a very important component, providing not only innovative ideas and major technical advances but also manpower training. Prominent among the universities involved in magnetic-confinement research are Columbia University, the Massachusetts Institute of Technology, New York University, the University of California at Los Angeles, the University of California at Berkeley, the University of Maryland, the University of Texas at Austin, and the University of Wisconsin. The Magnetic Fusion Advisory Committee Report on the Long-Term Role of Univer- sities in the Fusion Program, Department of Energy (August 1983) provides a detailed delineation of the university involvement (approx- imately 26 institutions) in magnetic fusion research. Similarly, the major part of inertial confinement research is carried out at national laboratories (Livermore, Los Alamos, and Sandia), at the Naval Research Laboratory, and at the KMS Fusion industrial laboratory. There is also a major research effort in inertial confinement fusion at the University of Rochester, with smaller but prominent research activities at Cornell University, the University of Arizona, the University of California at Davis, the University of California at Los Angeles, and the University of Maryland. The present pre-eminence of U.S. research in magnetic-confinement fusion and inertial-confinement fusion is evidence favoring the present mix of institutions involved. However, as fusion comes closer to its goal of energy applications, the involvement of industry, currently largely limited (except for GA Technologies, TRW Systems, and KMS Fusion) to a technological support role, would be expected to increase. A strong industrial participation in fusion research and development is essential to eventual commercialization of fusion and spinoff applica- tions. Space and Astrophysical Plasmas Support for space and astrophysical plasma research comes almost exclusively from three government agencies, The National Science Foundation (NSF), the National Aeronautics and Space Administra- tion (NASA), and the Department of Defense (DOD), with some DOE and small industrial components. The research itself is performed at
INTRODUCTION AND EXECUTIVE SUMMARY 35 various government and national laboratories, at a few industrial laboratories, and at many universities. A large fraction of the scientists engaged in this research are university based, yet much of the research is performed using the facilities of national and government laborato- ries, a circumstance that can be understood in terms of the obvious, necessary involvement of NASA. Although this division of effort has thus far been successful, especially when viewed in the light of the many recent accomplishments in space and astrophysical plasma research, the decreasing frequency of flight opportunities presents serious problems for university research, a situation that now requires urgent attention. A more consistent policy of support for university participation in space science is needed. Fluid Physics Fluid physics, consistent with the unusual breadth of its applications (aeronautics, weather, and oceanography, for example), derives its support from a variety of agencies, notably NSF, NASA, the Air Force Office of Scientific Research, the Office of Naval Research, the National Oceanic and Atmospheric Administration (NOAA), and DOE. Major funding is received by institutions such as the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and the NOAA atmospheric-science activity at Princeton University (weather prediction), which in turn supports field studies and other related activities. Universities are involved, at lower support levels, in a wide range of activities ranging from fluid-related biological research to aerodynamics to advanced gas-laser research. Fluid-physics re- search is at the forefront of many current problems in applied physics. Thus its institutional composition is very broad.