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Executive Summary BACKGROUND Recent advances in extending the energy, power, and brightness of lasers, particle beams, and Z-pinch generators make it possible to create matter with extremely high energy density in the laboratory. The collective interaction of this matter with itself, particle beams, and radiation fields is a rich, expanding field of physics called high energy density physics. It is a field characterized by extreme states of matter previously unattainable in laboratory experiments. It is also a field rich in new physics phenomena and compelling applications, propelled by advances in high-performance computing and advanced measuring techniques. This report’s working definition of “high energy density” refers to energy densities exceeding 1011 joules per cubic meter (J/m3), or equivalently, pressures exceeding 1 megabar (Mbar). (For example, the energy density of a hydrogen molecule and the bulk moduli of solid materials are about 1011 J/m3.) The time is highly opportune for the nation’s scientists to develop a fundamental understanding of the physics of high energy density plasmas. The space-based and ground-based instruments for measuring astrophysical processes under extreme conditions are unprecedented in their accuracy and detail, revealing a universe of colossal agitation and tempestuous change. In addition, there is a new generation of sophisticated laboratory systems (“drivers”), existing or planned, that creates matter under extreme high energy density conditions, permitting the detailed exploration of physics phenomena under conditions not unlike those in astrophysical systems.
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A consensus is emerging in the plasma physics and astrophysics communities that many opportunities exist for significant advances in understanding the physics of high energy density plasmas through an integrated approach to investigating the scientific issues in related subfields. Understanding the physics of high energy density plasmas will also lead to new applications and benefit other areas of science. Learning to control and manipulate these plasmas in the laboratory will benefit national programs, such as inertial confinement fusion and the stockpile stewardship program, through the development of new ideas and the training of a new generation of scientists and engineers. Furthermore, advanced technologies in the areas of high-speed instrumentation, optics (including x-ray optics), high-power lasers, advanced pulse power, and microfabrication techniques can be expected to lead to important spin-offs. High energy density experiments span a wide range of areas of physics including plasma physics, laser and particle beam physics, materials science and condensed matter physics, nuclear physics, atomic and molecular physics, fluid dynamics and magnetohydrodynamics, intense radiation-matter interaction, and astrophysics. While a number of scientific areas are represented in high energy density physics, many high energy density research techniques have grown out of ongoing work in plasma science, astrophysics, beam physics, accelerator physics, magnetic fusion, inertial confinement fusion, and nuclear weapons research. The intellectual challenge of high energy density physics lies in the complexity and nonlinearity of the collective interaction processes that characterize all of these subfields of physics. It should be emphasized that while high energy density physics is a rapidly developing area of research abroad, particularly in Europe and Japan, the primary focus of this report is on assessing the present capabilities and compelling research opportunities in the United States. To illustrate the energy scale of the high energy density regime, some of the systems that deliver the energy in high energy density laboratory experiments in the United States can be considered. Typical state-of-the-art short-pulse lasers and the electron beams generated at the Stanford Linear Accelerator Center can be focused to deliver 1020 watts per square centimeter (W/cm2) on target. The present generation of lasers employed in inertial confinement fusion (on the NIKE facility at the Naval Research Laboratory, on OMEGA at the Laboratory for Laser Energetics at the University of Rochester, and at the TRIDENT laser laboratory at Los Alamos National Laboratory) deliver 1 to 40 kilojoules (kJ) to a few cubic millimeters volume in a few nanoseconds. In Z-pinch experiments on the Z-machine at Sandia National Laboratories, 1.8 megajoules (MJ) of soft x rays are delivered to a few cubic centimeters volume in about 5 to 15 nanoseconds (ns). With the planned upgrades of existing facilities and the completion of the National Ignition Facility (NIF) at the Lawrence
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Livermore National Laboratory in the early 2000s, the parameter range of high energy density physics phenomena that can be explored will expand significantly. Complementary technologies, such as gas guns, explosively driven experiments, and diamond anvils, can also generate physically interesting high energy density physics conditions in the laboratory. While the primary purpose of the major facilities sponsored by the Department of Energy’s National Nuclear Security Administration (NNSA) is to investigate technical issues related to stockpile stewardship and inertial confinement fusion, increasing opportunities on these facilities are also available for exploring the basic aspects of high energy density physics. These state-of-the-art facilities allow repeatable experiments and controlled parameter variations to elucidate the important underlying physics. Elucidating the physics of high energy density plasmas through experiment, theory, and numerical simulation presents exciting science opportunities for understanding physical phenomena in laboratory-generated high energy density plasmas and in astrophysical systems. Because the field is developing rapidly, a study of the compelling research opportunities and synergies in high energy density plasma physics and its related subfields is particularly pertinent at the present time. ASSESSING THE FIELD In carrying out this assessment, the National Research Council’s Committee on High Energy Density Plasma Physics found high energy density physics (pressure conditions exceeding 1 Mbar, say) to be a rapidly growing field of physics with exciting research opportunities of high intellectual challenge spanning a wide range of physics areas. Opportunities for exploring the compelling questions of the field have never been more numerous. The many excellent high energy density facilities— together with a new generation of sophisticated diagnostic instruments, existing or planned, that can measure properties of matter under extreme high energy density conditions—permit laboratory exploration of many aspects of high energy density physics phenomena in exquisite detail under conditions of considerable interest for the following: basic high energy density physics studies, materials research, understanding astrophysical processes, commercial applications (e.g., extreme ultraviolet lithography), inertial confinement fusion, and nuclear weapons research. Furthermore, a revolution in computational capabilities has brought physical phenomena within the scope of numerical simulations that were out of reach only a few years ago. Numerical modeling is now possible for many aspects of the complex nonlinear dynamics and collective processes characteristic of high energy density laboratory plasmas and for the extreme hydrodynamic motions that exist under astrophysical conditions. The first phase of advanced computations at massively parallel facilities such as those developed in the Advanced Strategic Computing
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Initiative (ASCI) is reaching fruition with remarkable achievements, and a unique opportunity exists at this time to integrate theory, experimentation, and advanced computations to significantly advance the fundamental understanding of high energy density plasmas. Exciting new discoveries in astrophysics have occurred along with dramatic improvements in measurements by ground-based and space-based instruments of astrophysical processes under extreme high energy density conditions. Using the new generation of laboratory high energy density facilities, macroscopic collections of matter can be created under astrophysically relevant conditions, providing critical data on hydrodynamic mixing, shock phenomena, radiation flow, complex opacities, high-Mach-number jets, equations of state, relativistic plasmas, and possibly, quark-gluon plasmas characteristic of the early universe. A highly cost-effective way of significantly extending the frontiers of high energy density physics research is to upgrade and/or modify existing and planned experimental facilities to access new operating regimes. Such upgrades and modifications of experimental facilities will open up exciting research opportunities beyond those which are accessible with existing and planned laboratory systems. These opportunities range, for example, from the installation of ultrahigh-intensity (petawatt) lasers on inertial confinement fusion facilities to create relativistic plasma conditions relevant to gamma-ray bursts and neutron star atmospheres, to the installation of dedicated beamlines on high energy physics accelerator facilities for carrying out basic high energy density physics studies, such as the development of ultrahigh-gradient acceleration concepts and unique radiation sources extending from the infrared to gamma-ray regimes. In reviewing the level of support for research on high energy density physics provided by federal program agencies, the committee found that the level of support by agencies such as the NNSA, the nondefense directorates in the Department of Energy, the National Science Foundation, the Department of Defense, and the National Aeronautics and Space Administration has lagged behind the scientific imperatives and compelling research opportunities offered by this exciting field of physics. The NNSA’s establishment of the Stewardship Science Academic Alliances program to fund research projects at universities in areas of fundamental high energy density science and technology relevant to stockpile stewardship is commendable and important, particularly because the nation’s universities represent a vast resource for developing and testing innovative ideas in high energy density physics and for training graduate students and postdoctoral research associates. The committee is convinced that research opportunities in this crosscutting area of physics are of the highest intellectual caliber and that they are fully deserving of the consideration of support by the leading funding agencies of the physical sciences. A broad federal support base for research in high energy density physics, including
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plasma science, and the encouragement of interagency research initiatives in this very interdisciplinary field would greatly strengthen the ability of the nation’s universities to have a significant impact on this field. THE KEY QUESTIONS In developing a unifying framework for the diverse areas of high energy density physics and identifying research opportunities of high intellectual value, the committee found it useful to formulate key scientific questions ranging from the very basic physics questions to those at the frontier of the field. These are questions that, if answered, would have a profound effect on our understanding of the fundamental physics of matter under high energy density conditions. The following list of questions is not intended to be complete but rather to be illustrative of important questions of high intellectual value in high energy density physics: How does matter behave under conditions of extreme temperature, pressure, density, and electromagnetic fields? What are the opacities of stellar matter? What is the nature of matter at the beginning of the universe? How does matter interact with photons and neutrinos under extreme conditions? What is the origin of intermediate-mass and high-mass nuclei in the universe? Can nuclear flames (ignition and propagating burn) be created in the laboratory? Can high-yield ignition in the laboratory be used to study aspects of supernovae physics, including the generation of high-Z elements? Can the mechanisms for formation of astrophysical jets be simulated in laboratory experiments? Can the transition to turbulence, and the turbulent state, in high energy density systems be understood experimentally and theoretically? What are the dynamics of the interaction of strong shocks with turbulent and inhomogeneous media? Will measurements of the equation of state and opacity of materials at high temperatures and pressures change models of stellar and planetary structure? Can electron-positron plasmas relevant to gamma-ray bursts be created in the laboratory? Can focused lasers “boil the vacuum” to produce electron-positron pairs? Can macroscopic amounts of relativistic matter be created in the laboratory and will it exhibit fundamentally new collective behavior?
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Can we predict the nonlinear optics of unstable multiple and interacting beamlets of intense light or matter as they filament, braid, and scatter? Can the ultraintense field of a plasma wake be used to make an ultrahigh-gradient accelerator with the luminosity and beam quality needed for applications in high energy and nuclear physics? Can high energy density beam-plasma interactions lead to novel radiation sources? These questions cut across the boundaries of this field, and answering them will require new approaches to building a comprehensive strategy for realizing the exciting research opportunities. With this in mind the committee makes the following recommendations. RECOMMENDATIONS a.Recommendation on external user experiments at major facilities It is recommended that the National Nuclear Security Administration continue to strengthen its support for external user experiments on its major high energy density facilities, with a goal of about 15 percent of facility operating time dedicated to basic physics studies. This effort should include the implementation of mechanisms for providing experimental run time to users, as well as providing adequate resources for operating these experiments, including target fabrication, diagnostics, and so on. A major limitation of present mechanisms is the difficulty in obtaining complex targets for user experiments. b.Recommendation on the Stewardship Science Academic Alliances program It is recommended that the National Nuclear Security Administration continue and expand its Stewardship Science Academic Alliances program to fund research projects at universities in areas of fundamental high energy density science and technology. Universities develop innovative concepts and train the graduate students who will become the lifeblood of the nation’s research in high energy density physics. A significant effort should also be made by the federal government and the university community to expand the involvement of other funding agencies, such as the National Science Foundation, the National Aeronautics and Space Administration, the Department of Defense, and the nondefense directorates in the Department of Energy, in supporting research of high intellectual value in high energy density physics.
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c.Recommendation on maximizing the capabilities of facilities A significant investment is recommended in advanced infrastructure at major high energy density facilities for the express purpose of exploring research opportunities for new high energy density physics. This effort is intended to include upgrades, modifications, and additional diagnostics that enable new physics discoveries outside the mission for which the facility was built. Joint support for such initiatives is encouraged from agencies with an interest in funding users of the facility as well as from the primary program agency responsible for the facility. d.Recommendation on the support of university research It is recommended that significant federal resources be devoted to supporting high energy density physics research at university-scale facilities, both experimental and computational. Imaginative research and diagnostic development on university-scale facilities can lead to new concepts and instrumentation techniques that significantly advance our understanding of high energy density physics phenomena and in turn are implemented on state-of-the-art facilities. e.Recommendation on a coordinated program of computational-experimental integration It is recommended that a focused national effort be implemented in support of an iterative computational-experimental integration procedure for investigating high energy density physics phenomena. f.Recommendation on university and national laboratory collaboration It is recommended that the Department of Energy’s National Nuclear Security Administration (NNSA) continue to develop mechanisms for allowing open scientific collaborations between academic scientists and the NNSA laboratories and facilities, to the maximum extent possible, given national security priorities. g.Recommendation on interagency cooperation It is recommended that federal interagency collaborations be strengthened in fostering high energy density basic science. Such program collaborations are
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important for fostering the basic science base, without the constraints imposed by the mission orientation of many of the Department of Energy’s high energy density programs. To summarize, the committee believes that now is a very opportune time for major advances in the physics understanding of matter under extreme high energy density conditions. A sustained commitment by the federal government, the national laboratories, and the university community to answer the important questions of high intellectual value identified by the committee and to implement the recommendations of this report will contribute significantly to the timely realization of these exciting research opportunities and the advancement of this important field of physics.
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