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1 Overview plasÂ·ma: â²plaz-m (noun) [German, from late Latin, something molded, from Greek, e from plassein, to mold]: the most common form of visible matter in the cosmos, consisting of electrically charged remnants of atoms in the form of electrons and ions, moving independently of each other; as a result of their motion, these charged particles generate electric and magnetic fields that, in turn, affect the plasmaâs be- havior. Definition of the Field Plasmas seem simple enough. Theyâre a collection of free electrons and ions governed largely by physical laws known to late 19th-century physicists. Yet the sophisticated and often mysterious behavior of plasmas is anything but simple. This is strikingly evident in, for instance, the dramatic images of solar flaresâsudden plasma eruptions from the surface of the Sun. Plasma is found almost everywhere on Earth and in space; indeed only the invisible dark matter is more abundant. The vast regions between galaxies in galaxy clusters are filled with hot magnetized plasmas. Stars are dense plasmas heated by fusion reactions. Computer processors are fabricated using cold chemically reacting plasmas. Powerful lasers make relativ- istic plasmas in laboratories. And the enormously varied list goes on. None of these plasmas are quiescent; they wriggle and shake with instabilities and turbulence, and sometimes they erupt with spectacular force (Figure 1.1). One of the great achievements of plasma science has been to show that the bewildering variety and complexity of plasmas is understandable in terms of some
Plasma Science FIGURE 1.1â Exploding plasma on the Sun. X-ray image of one of the most dramatic of natural phe- nomena, the solar flare, caused by the sudden destabilization of the magnetized plasma in the Sunâs outer atmosphere (the corona). The eruption is lifting plasma above the Sunâs surface. The bright lines are the illumination of some of the complicated magnetic field lines by plasma emission. Courtesy of Transition Region and Coronal Explorer (TRACE), a mission of the Stanford-Lockheed Institute for Space Research and part of the NASA Small Explorer program. very elemental ideas that bind the field together (Figure 1.2). This is not to say that all questions have been answeredâthey have not. Rather, it confirms that the sci- ence is evolving rapidly and that there are fundamental principles that organize our knowledge. Much of plasma science seeks to explain the plasmaâs highly nonlinear behavior and the order and chaos that result. Plasma science has, therefore, much in common with many areas of modern complex system research, from climate modeling to condensed matter studies. Indeed, plasma scientists have played a pivotal role in the development of nonlinear dynamics and chaos theory, which have a multitude of applications to complex systems. Plasma science has made enormous advances in the last decade. Rapid progress in our ability to predict plasma behavior has been fueled by new diagnostics that
Overview 1010 100,000,000,000,000 Plasma Accelerators Black Hole Accretion Plasma Relativistic 1,000,000,000 Temperature (eV) Non-Relativistic 105 ITER Magnetic NIF Inertial Temperature (oK) Fusion Fusion Magneto -sphere Ionized Low Temperature White Partially Plasmas Dwarf 10,000 100 Ionized Ionosphere Stars ed d Neutral lat Dominated rre elate co r Un Cor al Anti-matter Qu sic m Plasmas as tu an Cl Pure Ion Plasmas 0.1 10â5 1 1010 1020 1030 Density (particles per cm3) FIGURE 1.2â New regimesânew physics. Plasma science is expanding into new territory and discover- ing new phenomena. Diagram shows some of the range of plasma phenomena. Regimes that are new areas of study since 1990 are indicated in gray, including the future regimes of the National Ignition fig 1-2 Facility (NIF) and the International Thermonuclear Experimental Reactor (ITER). replacement observe and measure an unprecedented level of detail and by computations that 10/30 resolve most of the essential physics. In many areas, from fusion plasma science to the manufacture of computer chips, science-based predictive models are replacing empirical rules. What is notable in the research examined for this report, further- more, is that plasma science is moving beyond the understanding of complicated but isolated phenomena and is entering an era in which plasma behavior will be understood and described as a whole. Growth in fundamental understanding has led to new applications and improved products such as the large-area plasma panel televisions now found in many homes. This report discusses the scientific highlights of the past decade and opportuni- ties for further advances in the next decade. Detailed analyses are contained in five chapters representing the subfields: low-temperature plasma science and engineer- ing; high-energy-density (HED) plasma science; magnetic fusion plasma science; space and astrophysical plasmas; and basic plasma science. Chapters 2 through 6, the topical chapters, contain in their final sections the committeeâs conclusion(s) and recommendation(s) pertaining to the particular topic. The remainder of this chapter, the Overview, summarizes key issues raised by these analyses. The next section shows that plasma research is an essential part of
Plasma Science the nationâs science and technology enterprise and that its importance is growing. Six scientific highlights of the past decade and the opportunities they create are featured in the section after that. While these examples by no means constitute a comprehensive survey, they give a flavor of the breadth and depth of the field. The fourth section discusses the growth in predictive capability and the emergence of new plasma regimes, two scientific themes that pervade recent advances. Further progress on many applications is predicated on a better understanding of some key plasma processes. These fundamental processes demonstrate the unity of the field by cutting across the applications and the topical areas. They are addressed briefly in the penultimate section, and they appear repeatedly in the topical chapters. The last section of this chapter presents the principal conclusion and the principal recommendation of the entire report. Importance of Plasma Science and Engineering The link between scientific development and increased prosperity, security, and quality of life is well documented. Advances in plasma science have contributed enormously to current technology and are critical to many future developments. An effective national research enterprise must have breadth because scientific discovery in any one area is often highly dependent on discovery in other areas. Plasma science is an important part of the web of interdependent disciplines that make up our essential core knowledge base. It contributes to at least four areas of national interest: â¢ Economic security and prosperity.â In the past decade, new plasma technolo- gies have entered the home. Many families view entertainment on plasma display televisions and illuminate their homes with plasma lighting. How- ever, the enormous role plasma technologies play in manufacturing remains largely hidden from view (Figure 1.3). Microelectronics devices would not exist in their advanced state if not for the tiny features etched onto semicon- ductor wafers by plasma tools. Surfaces of materials are hardened, textured, or coated by plasma processes. The value of all this economic activity is hard to estimate, but one small example is that displays and televisions built by plasma tools and lit by special plasma (fluorescent) lights will be a $200 billion market by 2010. The worldwide $250 billion semiconductor âSee, for example, the National Academies report Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future, Washington, D.C.: The National Academies Press, 2007. â Alfonso Velosa III, research director for semiconductors, Gartner, Inc., âSemiconductor manu- facturing: Booms, busts, and globalization,â presentation to the National Academy of Engineering, September 2004.
Overview FIGURE 1.3â Plasmas in the kitchen. Plasmas and the technologies they enable are pervasive in our everyday life. Each one of us touches or is touched by plasma-enabled technologies every day. Products from microelectronics, large-area displays, lighting, packaging, and solar cells to jet engine turbine blades and biocompatible human implants either directly use or are manufactured with, and in many cases would not exist without, plasmas. The result is a better quality of life and economic competitiveness. NOTE: CVD, chemical vapor deposition; HID, high-intensity discharge; LED, light- emitting diode; LCD, liquid crystal display. industry is built on plasma technology. In the absence of plasma technolo- gies, the $2 trillion telecommunications industry would arguably not exist. (See Chapter 2 for a more detailed discussion of this area of plasma science and its many applications.) â¢ Energy and environmental security.â Our prosperity and lifestyle rest on a ready supply of moderately priced energy, but it is well known that fossil fuel resources are limited and the environmental impact of their long-term
10 Plasma Science use is problematic. The search, therefore, for new and sustainable energy sources and new technologies that can reduce energy consumption is, and will remain, a high-priority research goal. Fusion energy has unparalleled potential to meet the need. Deployment of fusion (the fusing of hydrogen nuclei to make helium nuclei, a neutron, and energy) as an alternative energy resource should remain a priority for the nation. The challenge of fusion is that it requires plasmas with temperatures greater than those at the center of the Sun. Plasma science has made great strides in controlling and confining such plasmas (see Chapter 4 for a discussion of the science). ITER, which exploits some of these achievements, aims to explore fusion burning plasmas at the end of the next decade. This is a key and indeed essential step on the path to fusion energy. Research in alternative paths to fusion is also proceeding rapidly. In the meantime, plasma science has contributed to near-term innovations in energy efficiency. For example, the more than 1 billion light sources in operation in the United States use 22Â percent of the nationâs electrical energy budget. Consumers are switching to the more efficient plasma (fluorescent) lighting as innovations improve the quality of the light and the life expectancy of the lamp. Plasmas also aid the efficient combustion of fuels and the manufacture of materials for solar cells, and they improve the efficiency of turbines and hydrogen production. There is a small but growing use of plasmas to ensure a clean and healthy environment. New applications exploit the ability of plasmas to break down harmful chemicals and kill microbes to purify water and destroy pollutants. (See Chapter 2 for a detailed discussion of the science.) â¢ National security.â HED plasma science is central to Science-Based Stockpile Stewardship (SBSS), the DOE program that ensures the safety and reliability of the nationâs nuclear stockpile. The study of HED plasma physics has been greatly enhanced by the remarkable progress in producing such plasmas (and copious amounts of x rays) by passing large currents through arrays of wires in Sandia National Laboratoriesâ Z machine. In the next decade, the NIF (the worldâs most powerful laser facility) at Lawrence Livermore National Laboratory will create plasmas of unusually high energy densities and seek to ignite pellets of fusion fuel. These facilities and experiments are central to the stockpile stewardship program (see Chapter 3 for a discussion of the science). It is perhaps less widely appreciated that plasma technology is also critical to the manufacture of many conventional weapons systems. For example, the turbine blades in the engines of high-performance fight- ers are coated by a plasma deposition technique to substantially improve their performance. Recently developed plasma-based systems for destroy- ing chemical or biological hazards are answering homeland security needs.
Overview 11 Atmospheric pressure plasma sources are being employed as âplasma hosesâ to decontaminate surfaces after a chemical spill or attack. â¢ Scientific discovery.â Plasma science raises and answers scientific questions that contribute to our general understanding of the world around us. Un- raveling the complex and sometimes strange behavior of plasmas is in itself an important scientific enterprise. The intellectual challenge of explaining the intricacies of collective behavior continues to inspire serious scholar- ship. Our current understanding is being stretched by, for example, the properties of the curious forms of matter formed when plasmas become correlated at extremely low temperatures (see Chapter 6 for a discussion). Because most of the visible matter in the universe is plasma, many of the great questions in astrophysics and space physics require a detailed un- derstanding of plasmas. For example, while currents in the cosmic plasma must create the magnetic fields that pervade much of the universe, it is not known when these fields and currents first appeared in the universe or how they were generated (see Box 1.1 and Chapter 5 for discussion). The scientific challenges posed by these important goals are being addressed by a large but diffuse U.S. community of plasma scientists and engineers. The plasma research effort is global, however (see Box 1.2). Selected Highlights of Plasma Science and Engineering The committee now describes six highlights of the scientific frontiers of plasma research and development. This is neither an exhaustive survey nor a list of the greatest discoveriesâit is, rather, a sampling of exciting and important work. While these examples demonstrate the enormous diversity in plasma research they also illustrate the unity of the underlying science. Fundamental plasma processes are the common threads that weave through all these applications. âIn the United States, many plasma scientists participate in divisional meetings of the American Physical Society (APS), the American Geophysical Union, the American Vacuum Society, and the Institute for Electrical and Electronics Engineers. In 2006, the membership of the APS Division of Plasma Physics numbered about 2,500; at about 5.5 percent of the entire membership, the Plasma Physics Division is the fourth largest. Of course, there are at least as many plasma researchers who are not members of the APS. For more information about the demographics of the plasma science and engineering community, especially the fusion community, please see Fusion Energy Sciences Advisory Committee, Fusion in the Era of Burning Plasma Studies: Workforce Planning for 2004-2014, DOE/SC-0086, Washington, D.C.: U.S. Department of Energy, 2004; and E. Scime, K. Gentle, and A. Hassam, A Report on the Age Distribution of Fusion Science Faculty and Fusion Science Ph.D. Produc- tion in the United States, Washington, D.C.: University Fusion Associates, 2003.
12 Plasma Science BOX 1.1 Living and Working Inside a Plasma In 2000, an important human milestone came to pass quietly: Our species became a permanent inhabitant of space. Since then, the human presence in low Earth orbit has been continuous and uninterrupted on board the International Space Station (ISS). Humans now inhabit Earthâs ionosphere, where the rain is meteor showers and the wind is plasma, a place of awesome beauty and unforgiving hazards (Figure 1.1.1). The plasma environment surrounding the ISS is itself a hazard since electrons from the plasma charge up the structure. The pressurized modules of the ISS tend to act as large capacitors storing electrical energy hazardous to space-walking astronauts. Electrical shocks and arcs caused by the charge buildup could puncture spacesuits or damage critical instrumentation with catastrophic consequences. Recent measurements have also shown that the charge buildup varies significantly from day to day as the spacecraft moves from equatorial to polar regions and from daytime to nighttime. The charge buildup is neutralized (and the astronauts protected) by devices called âplasma contactors,â which serve the same function as grounding rods in well-designed homes on Earth. The ISS plasma contactors spray elec- trons into the surrounding ionosphere by hollow cathode discharges fueled by xenon gas. The rate of electron spray is sufficient to maintain the electrical ground of the station (its metal frame) at the same electrical potential as the surrounding ionosphere. Space plasma physics knowledge gained in the last few years through our continuous activities in space is teaching us much about the environment in which our planet functions and the important plasma processes that affect our life on the ground. Biotechnology and Health Care Dental patients might be surprised to know that their dentist is using a tiny plasma to treat their teeth. Yet the use of plasmas in biological applications is an emerging field that ranges from the surface treatment of human implants to plasma-aided surgery. These applications exploit the fact that plasmas are uniquely dry, hot, and cold, all at the same time. Plasma is dry in that the working medium is a gas and not a liquid, so less material goes into and comes out of the process. The hot electrons can drive high-temperature chemistry while the gas and surface remain near room temperature. â¢ Biocompatibility of surgical implants.â Plasma treatment is routinely used to make surgical implants such as joints and stents biocompatible by either
Overview 13 FIGURE 1.1.1â Left: Committee member Frank- lin Chang-DÃaz conducting assembly tasks outside the International Space Station (ISS) in June 2002. Courtesy of NASA. Right: Aurora australis photographed during a spacewalk on mission STS 111 in June 2002. The ISS routinely flies through the auroral plasma. Courtesy of NASA. depositing material or modifying the surface characteristics of the material (Figure 1.4). â¢ Sterilization.â The goal fig plasma sterilization is to destroy undesirable of 1.1.1 a, b biological activity with absolute confidence. The current workhorse of sterilization is the autoclave, in which medical instruments are exposed to superheated steam for 15 minutes. Autoclaves can damage even metal instruments and cannot be used on many thermosensitive materials. Fur- ther, like any single treatment method, it is not universally effective and in fact has been questioned for emerging threats like the prions associated with Creutzfeldt-Jakob (mad cow) disease. Plasmas provide two agents that destroy biological actvity: reactive neutral species and ultraviolet light. Gaseous neutrals can diffuse into complex biological surfaces, whereas
14 Plasma Science BOX 1.2 Plasma Research Goes Global The past decade has seen an acceleration of foreign research, investment, and discoveries in plasma physics. Increasing foreign participation testifies to the compelling scientific opportunities. The committee conducted a primitive exercise to crudely gauge the level of U.S. participation in the global plasma science enterprise. The 200 most frequently cited papers over the past decade from each of six major journals were reviewed and the proportion of foreign-based lead authors was tabulated. The results were as follows: Nuclear Fusion, 68 percent foreign; Plasma Physics and Controlled Fusion, 78 percent foreign; Physics Review E (selecting the plasma-related articles by keyword), 75 percent foreign; Physics of Plasmas, 39 percent foreign; Plasma Sources Science and Technology, 72 percent foreign; Physical Review Letters (selecting the plasma-related articles by keyword), 54 percent foreign. Twenty years ago, the U.S. share would have been much greater. While these results could be taken to mean that U.S. activity in plasma research is decreasing, the real cause is the large surge in research activities overseas. There are not fewer U.S. papersâthere are more and more foreign ones! Because it puts the smallest proportion of U.S. papers at 22 percent, this exercise does after all support the notion that the United States has a globally significant community in basic plasma science and HED physics. ultraviolet photons can travel only along the line of sightâcombined, they could lead to efficient local sterilization techniques. Ongoing research aims to improve the effectiveness of plasma sterilization while minimizing instrument damage through careful selection of the working gas composi- tion and plasma conditions. â¢ Plasma-aided surgery.â While plasma sterilization is only beginning to be- come a commercial process, surgery is already being performed with plasma instruments. It is entirely routine to cut and cauterize tissue with plasma. Emergingâand already in some useâare new plasma âknivesâ that gener- ate nonequilibrium plasma âstreamersâ (like miniature lightning bolts) in conducting liquids (saline). These streamers explosively evaporate water in bubbles to cut soft tissue. Here is the convergence of almost all the themes in low-temperature plasma science: selectivity to generate the desired spe- cies; interaction with exceedingly complex surfaces; stochastic behavior and multiphase media (bubbles in liquids); and the generation and stability of high-density microplasmas. Most surgical procedures still aim to cut and remove tissue, not modify it in a constructive way. However, there are indications that more selective and constructive processes are possible. For example, plasmas can change the metabolic behavior of cells and trigger cell detachment. The potential for plasmas in health care might best be viewed as an analog to their use in semiconductor manufacturing. Four-bit microprocessors were manu-
Overview 15 FIGURE 1.4â Plasmas and biology. Using low-temperature, reactive plasmas, the surface of polymers may be functionalized and patterned to allow the cells to adhere. In this example, amine functional groups were patterned on a polymer, resulting in a predetermined network of adhering cells. Courtesy of INP Greifswald, Germany. factured in liquid acid baths. Plasmas entered the scene and made possible 8- and 16-bit computers with megahertz clock speeds and kilobytes of memory. Today, after two decades of research and development, desktop computers are 64-bit, with gigahertz speeds and gigabyte memories, all enabled by plasmas. If this same physical and chemical precision can be brought to plasmas in health care, will the benefits be any less dramatic? Accelerating Particles with Plasma Wake Fields When an electron bunch moves at nearly the speed of light through a plasma, the electrostatic repulsion of the bunch on the stationary plasma electrons pushes them aside, punching a hole in the plasma electron density. The unbalanced posi- tive charge in the hole attracts the plasma electrons back into the hole, setting up
16 Plasma Science plasma oscillations. These plasma oscillations and the hole keep pace with but trail the bunch, providing a plasma wake field that also moves near the speed of light. Some electrons sitting just at the back of the hole are accelerated forward to- ward the bunch. These âsurfingâ electrons can reach energies greater than the ener- gies of the electrons in the driving bunch: This is the principle of the plasma wake field accelerator. An alternative approach employs a laser to excite the plasma in place of the initial electron bunch. The laserâs radiation pressure expels the plasma electrons from the pulse. The chief advantage of plasma wake field accelerators is the enormous accelerating force on the electronsâcurrently greater than 50 GV/m, or a thousand times the force in a conventional accelerator. From the very beginning of research in plasma accelerators, high-resolution multidimensional computer simulations have helped identify and resolve the sci- entific issues. Modern massively parallel computer simulations of wake field ac- celeration (Figure 1.5) are steering the experimental program. The standard com- FIGURE 1.5â A computer simulation of laser wake field acceleration. The laser pulse is moving for- ward, followed by a deficit of electrons, a hole in the electron density. The green sheet represents the electron density with holes colored blue and peaks red. The accelerated electrons are shown and the height above the sheet indicates energy. Most of the accelerated electrons are in the first trailing hole, but some can be seen in the later holes. Courtesy of Tech-X Corp.; simulation, J. Cary; visualization, P. Messmer.
Overview 17 putational tool is particle simulation that follows electrons and ions in the electric and magnetic fields created by the currents and charges of the particles themselves. These simulations have been improved by the theoretical development of new algo- rithms that exploit the ultrarelativistic nature of the problem. The close interaction of theory, simulation, and experiment in this area has been remarkably productive. Indeed it is a model of the way modern physics (especially plasma science) relies on all three components. Continuing progress in high-energy physics is hampered by the limits set by conventional accelerator technology. The enormous accelerating fields in a plasma wake field accelerator suggest a path to compact accelerators at a lower cost. Such compact accelerators would have many uses as sources of high-energy particles and photons. However, for the wake field accelerator to be useful, the accelerated electrons must be unidirectional and have uniformly high energy. Rapid progress in the last few years suggests that these criteria are achievable. In 2004, three in- dependent groups demonstrated that laser-driven, plasma-based accelerators are capable of producing high-quality, intense beams with very little angular spread and performance characteristics comparable to state-of-the-art electron sources for accelerators. Within the past 2 years at the Stanford Linear Accelerator (SLAC), a beam-driven plasma wake field accelerator first accelerated particles by over 2.7Â GeV in a 10-cm-long plasma module and now has demonstrated doubling of the energy of some of the 42-GeV electrons in a 1-m-long plasma (Figure 1.6). While recent progress in plasma wake field accelerators has been extraordinary, there are many questions to be answered. For example, what is the optimum shape of the driving electron bunch or laser pulse? How should the background plasma be shaped to produce the best acceleration and beam quality? Can the present success be scaled to much longer plasmas, taking the particles to much higher energies? Fusion Burning Plasmas in a Magnetic Bottle The pursuit of a nearly limitless, zero-carbon-emitting energy source through the process of nuclear fusion has been an inspiration to many plasma researchers (Box 1.3). In the magnetic confinement approach to fusion, a 100-million-degree deuterium-tritium plasma is contained in a magnetic bottle where the nuclei collide many times and eventually fuse. The high-energy neutrons born from the fusion reactions are captured in the reactor walls, producing heat that could be converted into electricity. A principal goal of magnetic confinement fusion is to build magnetic field configurations that contain the plasma stably for long times without much leakage â With an energy of 100 MeV, an energy spread of 2 to 3 percent, and a pulse length of less than 50Â femtoseconds. The charge per pulse was on the order of 0.3 nanocoulombs.
18 Plasma Science FIGURE 1.6â Demonstration of energy doubling of 42-GeV electrons in a meter-scale plasma wake field accelerator at Stanford Linear Accelerator Center (SLAC). (a) The energy spectrum of the dispersed electron beam after traversing an 85 cm long, 2.7 Ã 1017 cmâ3 lithium plasma. (b) The comparison between the measured and simulated energy spectrum. Reprinted by permission from Macmillan Publishing Ltd.: Nature 445: 741-744 Â© 2007. of heat to the walls through turbulence (Figure 1.7). Electrons and ions spiral along magnetic field lines, staying inside the plasma. The helium nucleus produced in the fusion reaction is also contained by the magnetic field, and each one deposits its 3.5 MeV of energy in the plasma. Plasmas begin to burn when the self-heating from fusion alpha particles provides most of the heat necessary to keep the plasma hot. Ignition is when the self-heating is sufficient to provide all the heat necessary to keep the plasma hotâthat is, enough to balance the heat lost through plasma collisions, turbulence, and radiation. In the last decade, the Tokamak Fusion Test Reactor (TFTR) at Princeton and then the Joint European Torus (JET) in the United Kingdom provided the first real taste of fusion. These experiments produced substantial fusion powerâ10 MW in the TFTR and 16 MW in the JET (Figure 1.8). But neither TFTR nor JET had significant heating from the fusion alpha particles and were therefore not in
Overview 19 BOX 1.3 Nuclear Fusion The easiest fusion reaction to initiate is the fusion of two isotopes of hydrogen, deuterium and tritium, to make a helium nucleus (an alpha particle) and a neutron (Figure 1.3.1). Fusion reactions are hard to initiate because the positively charged nuclei repel until they come close enough for the nuclear force (the strong force) to pull them together and fuse. The nuclei must be slammed together at energies of 100 million degrees, six times the temperature at the center of the Sun, to overcome the repulsion and fuse. The basic process of nuclear fusion is what releases energy in the Sun, causing it to shine and radiate the energy that warms Earth. FIGURE 1.3.1â The deuterium-tritium fusion reaction. The helium nucleus (alpha particle) is released with 3.5 MeV and the neutron with 14 MeV. A 1-GW power station would use 250 kg of fuel per year. Published with permission of ITER. the burning plasma regime. This was, nonetheless, an important milestone on the road to fusion power. Another key achievement of the tokamak program in the last decade was to develop operating regimes that can be extrapolated to a burning plasma experiment. This reflects confidence in the predictive tools and the science that made them possible. It is clear that the next critical step in the development of fusion power is a burning plasma experiment. ITER is that step. It is a large toka-
20 Plasma Science FIGURE 1.7â Plasma confinement in the tokamak magnetic configuration. This type of configuration has produced plasmas at fusion temperatures and densities. The confined plasma is illustrated as the semitransparent pink doughnut-shaped volume. This is the configuration chosen for ITER. Courtesy of the Joint European Torus (EFDA-JET). mak experiment using superconducting long-pulse magnets that is being built in southern France by an international consortium that includes the United States. ITER is designed to produce enough alpha-particle heating to replace two- thirds of the heat lost by turbulent transport. It is projected to generate about 500Â MW of fusion power. These projections are based on conservative regimes where plasma behavior is well understood. Recent research has uncovered new regimes, called advanced tokamak regimes, where turbulent transport is reduced and the plasma current is driven by the pressure gradient. This has been one of âThe detailed argument for the United States joining this experiment was laid out in the NRC report Burning Plasma: Bringing a Star to Earth, Washington, D.C.: The National Academies Press, 2004. The structure of the project is summarized briefly in Appendix B of the current report.
Overview 21 JET 15 (1997) TFTR (1994) Fusion power (MW) 10 JET 5 (1997) JG97.565/3c 0 0 1.0 2.0 3.0 4.0 5.0 6.0 Time (s) FIGURE 1.8â First fusion. Left: Fusion power versus discharge time for the U.S. experiment TFTR in 1994 and two discharges for the European experiment JET in 1997. Center: Confining alpha particles. Gamma rays reveal the spatial distribution and temperature of alpha particles in JET. Right: The calculated alpha particle trajectory. 1.8 left Courtesy of the Joint European Torus (EFDA-JET). most remarkable successes of fusion research in the last decade. If ITER can reach such regimes, the performance may considerably exceed expectations, perhaps even approach ignition. ITER is an experiment and it will investigate important science questions. How does the plasma behave when a substantial fraction of the heating is from fusion? Can it be controlled? Do the alpha particles change the turbulence and/or drive new instabilities? Does the large size of ITER change the physics and scaling of heat and particle transport? Can the walls handle the bursts of heat from edge-localized explosive plasma instabilities and disruptions? Can these explosive events be con- trolled or minimized? Are there new long-time-scale physical processes that will be revealed in the long pulses of ITER? Do the sophisticated computer models of the turbulence developed in the last decade successfully predict ITERâs turbulence? Can the turbulence be reduced and the confinement improved? What is the limit on the plasma pressure in the burning regime? The scientific advances that ITER will enable will considerably improve our ability to predict the behavior of burning plasmas in all kinds of configurations. But to become economical, fusion power will require developments beyond ITERâ perhaps refinements in the magnetic configuration will be needed and certainly it will be necessary to develop the engineering and technology of the first generation of fusion reactors. The importance of hastening the removal of remaining scientific barriers to magnetic fusion power will only grow as the limitations of fossil fuels become ever more apparent.
22 Plasma Science Magnetic Reconnection and Self-Organization The magnetic field protruding from the surface of the Sun into the surround- ing coronal plasma is impressively complex (Figure 1.9). Nonetheless, the scientific challenge is to explain why it is not far more tangled. The plasma in the Sunâs corona is sufficiently electrically conducting that, to a very good approximation, the field lines are frozen into the plasmaâthat is, the lines move, bend, and stretch with the plasma motion. The turbulent bubbling of the Sunâs surface randomly braids the field lines by moving their ends. To break a line and reconnect it to another lineâa process called magnetic reconnectionâthe plasma must slip across the field. This happens most effectively in narrow regions, where the field changes abruptly and oppositely directed components of the field are brought close together. In the solar corona, the random braiding of field lines proceeds until narrow dissipative regions are formed and reconnection releases the magnetic energy stored in the tangled field. Early estimates of the rate and effectiveness of reconnection suggested that the Sunâs field should be considerably more tangled than is observed. These same estimates also failed to explain the extremely rapid rates of magnetic reconnection in Earthâs magnetosphere and in fusion experiments. However, in the last decade, processes that enable fast magnetic reconnection have been discovered and illumi- a b FIGURE 1.9â Magnetic reconnection. Left: Image of the Sunâs coronal plasma from the TRACE satellite. The striations indicate the direction of the magnetic field. Sometimes TRACE observes coronal loops that are wrapped around each other (generally once, rarely more). Courtesy of Transition Region and Coronal Explorer (TRACE), a mission of the Stanford-Lockheed Institute for Space Research and part of the NASA Small Explorer program. Right: Cartoon of red field line reconnecting with oppositely directed blue field line in a narrow region. Outflow removes the field lines from the reconnection region. fig 1-9 a, b
Overview 23 nated by new experiments, observations, and a concerted program of theory and simulation. Although magnetic reconnection occurs in many different plasmas, the process has been profitably abstracted from the context, and universal features have been identified. Simulations of the narrow dissipation region have shown that a key to fast reconnection is the difference in the coupling of ions and electrons with field lines due to the Hall effect. When a field line is forced into the narrow region, it first decouples from the ions and then, in a much narrower region, decouples from the electrons. Field lines reconnect in the narrower electron-decoupling region. Recon- nected field lines exit the narrow region dragging plasma outflows (Figure 1.9b). Initially, they move rapidly because they only have to drag the lighter electrons. The ion outflow is slower and over a much wider flaring region. The current in the electron outflow produces a characteristic quadrupole field. This field has been identified in experiments purpose-built to study reconnection (Figure 1.10) and in observations of magnetospheric reconnection. It is clear that the Hall reconnection mechanism does lead to a dramatic increase in the speed and effectiveness of reconnection. However, laboratory ex- periments also show that the narrow layers are highly turbulent and that the tur- (a) (b) c/Ï â3 44 pi 4x10 (T) 3 42 3 2 40 1 38 R (cm) 0 2 36 â1 34 â2 1 32 â3 â4 30 â5 28 0 â6 â10 â8 â6 â4 â2 0 2 4 6 8 10 Z (cm) c/Ï â2 â1 0 1 2 pi FIGURE 1.10â Hall mechanism for fast magnetic reconnection, the smoking gun. (a) Results from a recent labo- ratory experiment showing color contours of the out-of-plane quadrupole magnetic field (definitive signature of the two-fluid Hall currents that produce the reconnection) superposed on vectors of the magnetic field in the reconnection region. Field lines flow in toward 1.10line R = 38 and outflows are along this line. Ion decoupling the begins at a distance of about 2 c/Ïpi above and below R = 38, whereas electron decoupling begins at about Â±0.8Â c/Ïpi. (b) Three-dimensional plot of reconnecting the field lines showing the way in which they are dis- torted; color projections are the quadrupole components. Courtesy of M. Yamada, Princeton Plasma Physics Laboratory (PPPL).
24 Plasma Science bulence is changing the reconnection dynamics. New, probably intermediate-scale experiments that achieve a larger separation of scales are required to distinguish the contributions of the turbulent and Hall dynamics. Furthermore, several im- portant features of reconnection in space and in fusion experiments are not yet seen in the small-scale reconnection experiments or predicted by the theory. For example, reconnection is thought to be responsible for some of the most dramatic and explosive events in nature such as solar flares, magnetic substorms, and certain tokamak disruptions. If reconnection were always fast and effective, however, it would be impossible to store significant energy in the field. That is because recon- nection would remove energy as soon as it is built up. Thus reconnection must be triggered, but how or when is not known. Many of the most energetic recon- nection events result in a large fraction of the magnetic energy being converted to energetic particlesâagain it is not clear how. How reconnection works in fully three-dimensional configurations (like the solar corona) is also not yet understood. Extending the advances of the past decade to address these outstanding issues is a difficult but exciting challenge. Clearly, there is an opportunity to make progress on a fundamental problem that has confounded plasma scientists for 50 years. Such progress would enhance predictive capability in a huge number of plasma applications, from fusion to astrophysics. Fusion Ignition in an Exploding Pellet In 2009, the 1.8-MJ NIF laser system will begin full-power operation at Law- rence Livermore National Laboratory in California. Its goal is to compress and heat a tiny capsule filled with a deuterium-tritium mixture to the point that fusion burning takes place. In this process a significant fraction of the fuel must react and burn before the capsule expands and cools. This process is called inertial confine- ment fusion. The data obtained from the experiments at NIF will provide critical information to ensure the safety and reliability of the nationâs nuclear stockpile. The tiny thermonuclear explosions are initiated by squeezing the capsule of fuel by a factor of 20-30 in radius (Figure 1.11). As is obvious to anybody who has tried FIGURE 1.11â Images of the last stage of compression of a capsule by the Omega laser at the University of Rochesterâs Laboratory for Laser Energetics. These x-ray images from argon emission are spaced 35 psec apart and magnified 87 times. This experiment achieved a 15-fold compression in radius. Courtesy of R.E. Turner, Lawrence Livermore National Laboratory (LLNL).
Overview 25 to squeeze a balloon by a factor of two, squeezing a pellet 20- to 30-fold demands a remarkably symmetric and precise squeeze. This can be achieved by very uniform ablation of the surface of the capsule that, by the rocket effect, compresses the cap- sule. This challenge has driven a deeper understanding of HED plasma science and the development of modern computational tools to design the fuel capsules and to study the many physical processes involved in delivering the laser energy. The NIF will deliver its 1.8 MJ of energy using 192 convergent laser beams to power the ablation. For the indirect-drive approach, the laser beams will irradiate the inside surface of an enclosure (called a hohlraum) surrounding the capsule, producing a bath of x rays that heat and ablate the capsule surface. In the direct- drive approach, the beams shine on the capsule itself. In both approaches, the basic concept is to heat a central hot spot in the imploded fuel hot enough to initiate fusion reactions that will spread to the surrounding denser but cooler fuel layers. Innovative variants of the basic idea of inertial confinement fusion have been introduced in the last decade. For example, it was shown that the capsuleâs fusion could be greatly enhanced by delivering a very sudden injection of energy to initiate reactions at the point of maximum compression. This energy might be delivered into the capsule by, for example, relativistic electrons generated by a very short pulse laser. Modeling and experiments have confirmed that this process, called âfast ignition,â can indeed significantly improve performance. Additional innovations that will increase the efficiency of inertial confinement fusion are likely to appear once the NIF is in operation. The huge energy and power of the NIF laser will allow access to many new HED plasma regimes. For example, in some cases the nonlinear interaction of NIF beams with diffuse plasma is expected to produce highly nonlinear (perhaps turbulent) laserâplasma interaction. Ultrashort high-energy laser pulses such as would be needed for fast ignition experiments will accelerate dense beams of relativistic particles and produce novel plasma states. The NIF will also be able to probe the dynamics and stability properties of radiation-dominated plasmas, including processes that, at present, can be seen faintly only in distant astrophysical objects. Finally, the achievement of ignition will release ~1018 neutrons in a frac- tion of a nanosecond from a submillimeter spot, potentially enabling the study of nuclear processes involving more than one neutron. Understanding some of these phenomena does not directly advance the mission of NIF, but it will certainly open new avenues for fundamental research. Plasma Physics and Black Holes Black holes are among the most remarkable predictions of theoretical phys- ics. So much mass is compressed into such a small volume that nothing, not even light, can escape. Currently, a black hole can be detected by either its gravitational
26 Plasma Science influence on surrounding matter or the electromagnetic radiation produced when plasma falls toward the black hole and heats up as it is accelerated to nearly the speed of light (Figure 1.12). There has been a growing recognition over the past 35 years that black holes are ubiquitous and play an essential role in many of the most fascinating and energetic phenomena in the universe. Massive stars that have exhausted their nuclear fuel collapse to form black holes with masses about 10 times that of our Sunâthere are perhaps 10 million such black holes in a galaxy like our own. There is now compelling evidence that nearly every galaxy contains, in addition to these roughly solar-mass objects, a much more massive black hole at its centerâthese range in mass from a million to a billion solar masses. Accreting black holes power the most energetic sources of radiation in the universe and produce powerful outflows. The central difficulty in understanding N 0.2" E S0-1 S0-2 S0-4 S0-5 S0-16 S0-19 S0-20 Keck/UCLA Galactic Center Group 1995-2006 FIGURE 1.12â Left: Detecting a black hole by its influence on the orbits of nearby stars. Infrared image of stars in the central 0.1 light-year of our galaxy, a region comparable in size to our solar system. Every star in the 1.12L image has been seen to move over the past decade. For approximately a dozen stars, this motion can be well fitted by orbits around a central 3.6 Ã 106 solar mass black hole (indicated by the star at the center of the im- age). Courtesy of Keck/UCLA Galactic Center Group; based on data from A. Ghez et al., Astrophysical Journal 620: 744 (2005). Right: Detecting the emission from plasma falling toward a black hole. X-ray image of the central 10 light-years of our galaxy showing diffuse emission from hot plasma and a number of point sources. Some of the ambient hot plasma is gravitationally captured by the black hole at the center of the galaxy. As it falls toward the black hole, this plasma heats up and produces a bright source of radiation. The point source at the lower left of the central three sources is coincident with the location of the massive black hole from the left panel. Courtesy of NASA/MIT/PSU.
Overview 27 black holes as sources of radiation and outflows lies not in understanding the physics of the black holes themselves (as predicted by general relativity) but rather in understanding the physics of the accreting plasma that produces the observed radiation. Further progress on understanding general relativistic plasma phys- ics (i.e., plasma physics in curved space-time) is essential both for interpreting observations of black holes in nature and for achieving the long-sought goal of using such observations to test general relativityâs predictions for the strong grav- ity around black holes. In general, inflowing plasma does not fall directly onto the black hole but instead, because it has angular momentum, orbits the black hole. The orbiting plasma forms a disc called an accretion disc, such as that shown in the numerical simulation in Figure 1.13. FIGURE 1.13â Left: Radio images of the galaxy M87 at different scales (1 kpc = 3,260 light-years) show, top left, giant, bubblelike structures on the scale of the galaxy as a whole, where radio emission is powered by relativistic outflows (âjetsâ) from the galaxyâs central black hole; top right, the jets coming from the core of the galaxy; and bottom, an image of the region close to the central black hole, where the jet is formed. The small circle labeled 6RS shows six times the radius of the event horizon for the galaxyâs black hole (about 10 times the distance from the Sun to Pluto). Courtesy of National Radio Astronomy Observatory (NRAO)/Associated Universities, Inc. (AUI)/National Science Foundation (NSF); based on data from Junor, Biretta, and Livio, Nature 401: 6756. fig 1.13 a, b Right: The inner regions of an accretion disk around a black hole, as calculated in a general relativistic plasma simulation. The black hole is at coordinates (0,0). The accretion disk rotates around the vertical direction (the axis of the nearly empty funnel region). Its density distribution is shown in cross section, with red representing the highest density and dark blue the lowest. Above the disk is a tenuous hot magnetized corona, and between the corona and the funnel is a region with ejection of mildly relativistic plasma that may be related to the forma- tion of the jets seen in the left panel. Image based on work that appeared in de Villiers et al. (2003), Â© American Astronomical Society.
28 Plasma Science Unlike the planets orbiting the Sun, plasma is subject to frictional forces that redistribute angular momentum and allow the plasma to flow inward. In the past decade, it has been realized that magnetic fields in accretion disks are amplified by a powerful instability known as the magnetorotational instability. Such mag- netic fields provide the necessary viscous angular momentum transport in most accretion disks and also help generate powerful outflows such as those seen in Figure 1.13. Much remains to be understood about plasma physics in the vicinity of black holes. What determines the inflow rate of plasma in an accretion disc? How much of the energy of the inflowing plasma is radiated away, ejected in outflows, or swallowed by the black hole? How are jets launched, and why do only some black holes, some of the time, have jets? In addition to progress on the theoretical front, observations are rapidly improving and are providing information about the condi- tions very close to the event horizon of black holes, by means of both direct images of plasma near the event horizon (e.g., the picture of M87 in Figure 1.13) and the indirect but powerful information about the velocity of the plasma provided by spectral lines. Given the wealth of observational information and the diversity of exciting and difficult problems, black hole plasma physics will remain a vibrant research area in the coming decade. Key Themes of Recent Scientific Advances This section examines the overall trends in plasma research. Two themes frame recent advances: â¢ Plasma science is developing a significant predictive capability. â¢ New plasma regimes have been found that expand the scope of plasma research and applications. Both themes are illustrated by the six examples of cutting-edge science in the pre- ceding section. More complete descriptions of the scientific advances and questions are contained in the ensuing topical chapters. Prediction in Plasma Science The recent growth of predictive capability in plasma science is perhaps the greatest indicator of progress from fundamental understanding to useful science- based models. It is due primarily to two factors: (1) advances in diagnostics that can probe the internal dynamics of the plasma and yield much greater quantitative understanding and (2) theoretical and computational advances that have led to models that can accurately predict plasma behavior. Good examples are the pre-
Overview 29 dictive modeling of turbulence in fusion plasmas, the modeling of reconnection dynamics, and the modeling of industrial plasma processes. The cost of develop- ment via an Edisonian approach, where multiple designs and prototypes are tried, is prohibitive for many plasma science applications, notably but not exclusively fusion. Predictive models provide a basis for steering investigation and ultimately reduce development cost and time. Nonetheless, our understanding of many fun- damental aspects of plasma behavior remains rudimentary, and further increases in predictive capability require progress in understanding the basic plasma processes outlined in the next section. That is, the next generation of improvements in pre- dictive capabilities will probably be driven by theoretical insights. New Plasma Regimes New facilities and experimental techniques have revealed new plasma regimes. The highly relativistic plasma physics in the beam plasma interaction at SLAC is a good example (see the preceding section). The power of the SLAC beam has opened up this regime to study. Another example is the very cold, highly correlated plasmas being studied in basic experiments made possible by the development of new techniques for cooling the plasma. Low-temperature microplasmas that blur the distinction between the solid, liquid, and plasma states are being created to explore novel plasma chemistry. In studying accretion discs, astrophysicists are considering the behavior of plasmas in the curved space around black holes. These new regimes are revealing unexpected new phenomena, challenging and extending our understanding. In the next decade, more new regimes are expected. For example, ITER will begin studying magnetically confined plasmas heated by alpha particles produced in fusion reactionsâthe burning plasma regime. The NIF will seek to produce a fusion burn in a pellet compressed by lasers. Common Intellectual Threads of Plasma Research Plasmas occur over a fantastic range of temperatures, densities, and magnetic fields. However, there are a number of issues that are pervasive, and much of plasma behavior can be characterized in terms of universal processes that are, at least partially, independent of the particular context being considered. Some of these processes have been well understood and the behavior can be predicted with near certainty. The propagation of weak electromagnetic waves through plasmas, such as radio waves through the ionosphere, is one example where predictive capability has risen to a level of considerable certainty in the last decade. However, six critical plasma processes are not well understood. They yield some of the great questions of plasma science. Progress on any one of them would
30 Plasma Science advance many areas of plasma science simultaneously. Indeed they define the re- search frontier. â¢ Explosive instability in plasmas.â Some of the most striking events in plasmas are the explosive instabilities that spontaneously rip apart plasmas. Such instabilities give rise to a massive and often destructive release of energy and accelerated particles. For example, disruptions in magnetically confined fusion plasmas can deposit large fractions of the plasma energy (tens of megajoules) on the solid walls of the experiment in less than a millisecond. Solar flares convert magnetic energy equivalent to billions of nuclear weap- ons to plasma energy in 10 to 1,000 seconds. It is not understood when and how plasmas explode. â¢ Multiphase plasma dynamics.â Multiphase plasmasâplasmas that are in- teracting with nonplasmas (such as neutral gas, solid surfaces, particulates, and liquids)âare widespread. For example, low-temperature multiphase plasmas are used to perform tasks such as emitting light of a particular color, destroying a pollutant or sterilizing a surface. A host of basic ques- tions about these plasmas have at best been only partially answered. â¢ Particle acceleration and energetic particles in plasmas.â In supernova shocks, laserâplasma interactions, the wakes of particle beams, solar flares, and many other phenomena, we observe the acceleration of some plasma par- ticles to very high energies. Particles may be accelerated by surfing on waves in the plasma or by being randomly scattered by moving plasma irregulari- ties. It is still not clear how nature accelerates particles so effectively or what can be learned from this behavior in the laboratory. â¢ Turbulence and transport in plasmas.â Magnetic fusion plasmas, accretion discs around black holes, Earthâs magnetosphere, laser-heated plasmas, and many industrial plasmas are permeated with turbulence that transports heat, particles, and momentum. The effects of this turbulence often domi- nate these plasmas, yet many aspects are not understood. For example, can we reduce and control turbulence? â¢ Magnetic self-organization in plasmas.â In many natural and laboratory plasmas, the magnetic field and the plasma organize themselves into a structured state. For example, although it is not known how, the Sunâs turbulent plasma produces an ordered magnetic field that cycles with an al- most constant 22-year period. Laboratory plasmas often seek out preferred configurations called relaxed states. Magnetic reconnection is almost always a key part of the relaxation processes that lead to self-organization. â¢ Correlations in plasmas.â In cool, dense plasmas, the electrostatic forces between the ions and electrons begin to dominate the motion of the par- ticles. This induces ordering and structure into the particle positions. The
Overview 31 behavior of such plasmas in stars, HED systems, laboratory experiments, and industry is of great current interest. Unraveling the properties of highly correlated plasmas is an ongoing challenge. It is notable that each of these six processes plays a role in four or more of the (five) topical areas treated in Chapters 2-6. A variety of approaches are needed to advance our knowledge of these processes. Some phenomena must be studied at a large scale and therefore can only be addressed in the context of (well-funded) applications or space/astrophysics research. Other phenomena can be best under- stood through a series of small-scale laboratory experiments whose objectives are to peel back the layers of complexity. Clearly, progress on understanding these six fundamental processes will benefit a broad range of applications. Such advances in understanding will lead (via modeling and simulation) to improvements in predictive capability. The reportâs principal Conclusion and Principal Recommendation Plasma science is on the cusp of a new era. It is poised to make significant breakthroughs in the next decade that will transform the field. For example, the international magnetic fusion experimentâITERâis expected to confine burn- ing plasma for the first time, a critical step on the road to commercial fusion. The NIF plans to ignite capsules of fusion fuel with the goal of acquiring the knowl- edge necessary for maintaining the safety, security, and reliability of the nuclear stockpile. Low-temperature plasma applications are ushering in new products and techniques that will change everyday lives. And plasma scientists are being called upon to help crack the mysteries of exotic plasmas in the cosmos. This dynamic future will be exciting and challenging for the field. It will demand a well-organized national plasma science enterprise. Principal Conclusion:â The expanding scope of plasma research is creating an abundance of new scientific opportunities and challenges. These oppor- tunities promise to further expand the role of plasma science in enhancing economic security and prosperity, energy and environmental security, na- tional security, and scientific knowledge. Plasma science has a coherent intellectual framework unified by physical pro- cesses that are common to many subfields. Therefore, and as this report shows, plasma science is much more than a basket of applications. The committee believes that it is important to nurture fundamental knowledge of plasma science across all of its subfields to advance the science and to create opportunities for a broader range of science-based applications. These advances and opportunities are, in
32 Plasma Science turn, central to the achievement of national priority goals such as fusion energy, economic competitiveness, and stockpile stewardship. The vitality of plasma science in the last decade testifies to the success of some of the individual federally supported plasma science programs. However, the emergence of new research directions necessitates a concomitant evolution in the structure and portfolio of programs at the federal agencies that support plasma science. The committee has identified four significant research challenges that the federal plasma science portfolio as currently organized is not equipped to exploit optimally: â¢ Fundamental low-temperature plasma science.â The many emerging appliÂ cations of low-temperature plasma science are challenging and even out- stripping fundamental understanding. A basic research program in low- temperature plasma science that links the applications and advances the science is needed. Such a government-sponsored program of long-range research would capitalize on the considerable benefits to economic com- petitiveness offered by key breakthroughs in low-temperature plasma sci- ence and engineering. No such program or federal steward for the science exists at present. The detailed scientific case for this program is presented in Chapter 2. â¢ Discovery-driven, HED plasma science.â Fueled by large new facilities and breakthroughs in technologies that have enabled access to previously un- explored regimes, our understanding of the science of HED plasmas has grown rapidly. Mission-driven HED plasma science (such as the advanced accelerator program in the DOE Office of High-Energy Physics or the Iner- tial Confinement Program in the National Nuclear Security Administration [NNSA]) is thriving. New regimes revealing new processes and challenging our fundamental understanding of plasmas will be discovered in the next decade at the new HED facilities (such as NIF and upgrades elsewhere). It is very likely that some of the science that will emerge in these new regimes and new processes cannot be adequately explored by the current suite of facilities given the specificity of their purposes. By extension, discovery- driven research in HED plasmas cannot grow inside the facilitiesâ parent programs that are dedicated to specific missions. However, there is no other home for this research in the present federal portfolio. âThisscience is discussed in Chapter 3 in the NRC report Frontiers of High Energy Density Physics: The X-Games of Contemporary Science, Washington, D.C.: The National Academies Press, 2003; and Frontiers in High Energy Density Physics, July 2004, prepared by the National Task Force on High En- ergy Density Physics for the Office of Science and Technology Policyâs (OSTPâs) interagency working group on the physics of the universe.
Overview 33 â¢ Intermediate-scale plasma science.â Some of the most profound questions in plasma science are ripe for exploitation right now and are best addressed at the intermediate scale. These questions can only be studied in facilities that are intended for groups larger than single-investigator groups. They do not, however, require the very large national and international ex- perimental facilities on the scale of NIF and ITER. For example, magnetic reconnection research would be advanced significantly by an experiment at an intermediate scale, where the collisionless physics is dominant. Such intermediate-scale facilities might be sited within national laboratories or at universities. The current mandates of the mission-driven programs of the NNSA and OFES do not provide for the development of intermediate- scale facilities that pursue discovery-driven research directions in plasma science that are not clearly applicable to their missions. The discoveries that intermediate-scale facilities would foster are unlikely to happen within the current paradigm of federal support for plasma science. â¢ Crosscutting research.â Federal stewardship of plasma research is disaggre- gated and dispersed across four main agenciesâthe Department of Energy, the National Science Foundation, the Department of Defense, and the National Aeronautics and Space Administrationâand within those, across many offices (e.g., magnetic fusion is primarily supported through DOEâs Office of Science, and inertial confinement fusion is primarily supported through DOEâs NNSA). This dispersion hinders progress in many areas of plasma science because it does not allow for an intellectual juxtaposi- tion of disparate elements that will force dialogue on common issues and questions. There are significant opportunities at the interfaces between the subfields, but the current federal structure fails to exploit them. Notwithstanding the success of individual federal plasma science programs, the lack of coherence across the federal government ignores the unity of the science and is an obstacle to overcoming many research challenges, to realizing scientific opportunities, and to exploiting promising applications. The committee observes that the stewardship of plasma science as a discipline will likely expedite the appli- cations of plasma science. The need for stewardship was identified in many reports over two decades. The evolution of the field has only exacerbated the stewardship problem and has driven this committee to conclude that a new, integrated way of managing the federal support for the science is necessary. âSeeNRC, Plasma and Fluids, Washington, D.C.: National Academy Press, 1986; NRC, Plasma Sci- ence: From Fundamental Research to Technological Applications, Washington, D.C.: National Academy Press, 1995; and NRC, An Assessment of the Department of Energyâs Office of Fusion Energy Sciences Program, Washington, D.C.: National Academy Press, 2001.
34 Plasma Science The committee considered a wide range of options to provide stewardship without disrupting the vigor and energy of the ongoing plasma research. Recogniz- ing the potentially far-reaching consequences of any recommendation to integrate research programs in plasma science, the committee considered four options in great detail: â¢ Option 1:â Continue the current structure of federal plasma science programs unchanged.â It is apparent that many plasma science programs were very successful in the past and some continue to be successful. Certainly, the pace of discovery would remain high in many areas if the system remains unchanged. However, the status quo option does not position the nation to exploit the emerging new directions in plasma science and their potential applications. Even now, the committee judges, the structure is impeding broad progress in plasma science. â¢ Option 2:â Form a plasma science interagency coordinating organization.â In- teragency working groups have facilitated crosscutting science and technol- ogy initiatives such as nanotechnology and information technology. With some of the fundamental questions in plasma science being investigated by as many as three agencies (and several offices within those agencies) it is clear that a coordinated effort that is supported at the highest levels within the government would be beneficial. However, while such an approach might stimulate some crosscutting research, it would not, in itself, create research initiatives in fundamental low-temperature plasma science and discovery-driven, HED plasma science. An interagency task force cannot facilitate the development of intermediate-scale facilities for the emerging science if those facilities are all within one large agency. Furthermore, an interagency advisory panel cannot directly provide stewardship nor can it provide advice on coordination if the roles and responsibilities of the par- ticipating agencies are too diffuse. Arguably, the future of plasma science requires more than a coordinating effort. â¢ Option 3:â Create an office for all of plasma science, pulling together programs from DOE, NSF, NASA, DOD, and other government agencies.â Such an office would centrally manage all plasma science and engineering in the federal portfolio. It would naturally emphasize the unity of plasma science and the commonality of the physical processes. Certain efficiencies would be realized through common administration and management. However, this move would uproot many successful activities, separating flourishing programs from their applications and isolating others from their related areas of science. It might create more problems than it would solve. â¢ Option 4:â Expand the stewardship of plasma science at DOEâs Office of Science.â Since the heart of the science at stake resides within DOE, this op-
Overview 35 tion would address directly the four problems identified by the committee. As the home of many large plasma science applications (fusion, stockpile stewardship, and so on), DOE has abundant interest in the effective devel- opment of the science. It has also successfully nurtured basic plasma science through the NSF-DOE partnership. Furthermore, DOE has experience (and success) in operating large and intermediate-scale science facilities as part of broader research programs. An expanded stewardship of plasma science in the Office of Science would not, however, exploit all the connections that the science presents. Nonetheless, by linking together a large part of the core science, the Office of Science could coordinate effectively with other offices and agencies on common scientific issues. Thus a stewardship focused in the Office of Science would be at the heart of a balanced strategy that would bring coherence without sacrificing connections to applications and the broader science community. The scientific advantages of the fourth option are compelling to the committee. After careful assessment, this is the route the committee recommends. Assessing the bureaucratic and managerial issues involved in effective pursuit of this option, however, is beyond this committeeâs charge. Principal Recommendation:â To fully realize the opportunities in plasma research, a unified approach is required. Therefore, the Department of En- ergyâs Office of Science should reorient its research programs to incorporate magnetic and inertial fusion energy sciences; basic plasma science; non- mission-driven, high-energy-density plasma science; and low-temperature plasma science and engineering. The new stewardship role for the Office of Science would extend well beyond the present mission and purview of the OFES. It would include a broader portfo- lio of plasma science as well as the research OFES presently supports. Two of the thrusts would be new: (1) a non-mission-driven, HED plasma science program and (2) a low-temperature plasma science and engineering program. These changes would be more evolutionary than revolutionary, starting modestly and growing with the expanding science opportunities. The committee recognizes that these new programs would require new resources and perhaps a new organizational structure for the Office of Science. However, the scale and extent should evolve naturally from community proposals and initiatives through a strategic planning process such as outlined below and the usual budget and operation planning within the government. The committeeâs intention is not to replace or duplicate the plasma science programs in other agencies. Rather, it would create a science-based focal point for federal efforts in plasma-based research. Space and astrophysical plasma research
36 Plasma Science would remain within the space and astrophysical research programs in NASA and NSF. The NSF-DOE partnership in basic plasma science would continue. HED programs in plasma accelerators would remain in the DOE Office of High Energy Physics. Inertial confinement fusion research enabling the stockpile stewardship mission of DOEâs NNSA would remain there. With a renewed and expanded re- search focus, the Office of Science would also be naturally positioned to accept a lead scientific role in interagency efforts to exploit HED physics. Finally, current programs at NIST and NSF wrestling with the engineering applications of low- temperature plasma science would continue. In fact, they would be substantially enhanced by the inception of the new DOE plasma science programs that could provide directed scientific inquiry on key issues as well as coordination and com- munication of the most compelling breakthroughs in the basic research. The committee is aware that there are substantial challenges and risks associ- ated with its chief recommendation. A comprehensive strategy will be needed in order to ensure a successful outcome. The planning should do the following: â¢ Develop a structure that integrates the scientific elements, â¢ Initiate a strategic planning process that not only spans the field but also provides guidance to each of the subfields, and â¢ Identify the major risks and develop strategies to avoid them. The committee recognizes that there is no optimal strategy without risk. In- deed, the status quo is not without considerable risk. Some things could be done, however, to mitigate the most obvious risks: â¢ Strong leadership to achieve these ambitious goals and inspire the elements of the program to rise above their particular interests. â¢ Careful consultation among the communities, their sponsors, and constitu- encies to build trust and a strong consensus. â¢ An advisory structure that reflects the breadth and unity of the science. â¢ Scientific and programmatic connections to related disciplines in the broader physical sciences and engineering. DOEâs magnetic fusion and inertial fusion programs are currently focused on large developing facilities (ITER, NIF, and Z). The next decade will see these facilities mature into vibrant and exciting scientific programs. Looking beyond that âUnder the direction of the National Science and Technology Councilâs interagency working group on the physics of the universe, an ad hoc National High Energy Density Physics Task Force has been formed to coordinate federal activities in HED physics. A report from this group was expected by mid-2007.
Overview 37 phase, however, the committee has two observations. First, NNSAâs support for HED science will become uncertain when NIF and Z complete their stockpile stew- ardship missions. Yet, by that time, HED science will have flowered and expanded in many directions. Second, if ITER is successful and 15 years from now the nation is actively pursuing the development of fusion energy, DOEâs fusion science program is likely to have changed dramatically. The fusion energy effort may move outside the Office of Science. Which entity will then become the de facto steward of plasma science? The committee concludes that the Office of Science would naturally fill this role. A broad-based plasma science program within the Office of Science would explicitly include (among other research programs) the science of magnetic fusion and the science of inertial fusion. Indeed, the Office of Science will steward plasma science long after the current large facilities have come and gone. There is a spectacular future awaiting the United States in plasma science and engineering. But the national framework for plasma science must grow and adapt to new opportunities. Only then will the tremendous potential be realized.