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1 Exordium and Principal Findings and Recommendations INTRODUCTION The time is highly opportune for the nation’s scientists to develop a fundamental understanding of the physics of high energy density plasmas. Ground-based and space-based instruments for measuring astrophysical processes under extreme conditions are unprecedented in their sensitivity and detail, revealing a universe of titanic violence and continuous upheaval. In addition, a new generation of sophisticated laboratory systems (“drivers"), existing or planned, is capable of generating extreme high energy density conditions in matter, permitting a detailed exploration of physics phenomena under conditions never before accessible in the laboratory and approaching those in astrophysical systems. 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 and technology. Furthermore, learning to control and manipulate high energy density 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. Elucidating the physics of high energy density plasmas through experiment, theory, and numerical simulation is of considerable scientific importance in order to
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understand physical phenomena in laboratory-generated high energy density plasmas and astrophysical systems. Because the field is developing rapidly, a study of compelling research opportunities and synergies among related subfields is particularly pertinent. Recent advances in extending the energy and power of lasers, particle beams, and Z-pinch generators make extremely high energy density matter accessible in the laboratory. The collective interaction of this matter with itself, particle beams, and radiation fields is a rich and expanding field of physics termed high energy density (HED) physics. It is also a field rich in new physics phenomena and steeped with important applications. To illustrate the energy scale, let us briefly consider some of the systems (drivers) that deliver the energy in laboratory experiments. 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 W/cm2 on target. The present generation of lasers employed in inertial confinement fusion research (NIKE, OMEGA, and TRIDENT) deliver 1 to 40 kJ to a few cubic millimeters volume, in a few nanoseconds. The Z-pinch experiments at Sandia National Laboratories generate 1.8 MJ of soft x rays in a few cubic centimeters volume in 5 to 15 ns. With the planned upgrades of existing facilities and the completion of the National Ignition Facility (NIF) 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, there are increasing opportunities on these facilities to explore the basic aspects of high energy density physics. Although a sizable fraction of high energy density physics research is carried out at national laboratories engaged in inertial confinement fusion and nuclear weapons research, university involvement in physics investigations of high energy density plasmas is growing. University involvement has increased as a result of several factors, including the increased openness of national research facilities to collaborators and the development of relatively inexpensive short-pulse lasers and parallel computing clusters that are powerful enough to access high energy density physics regimes on university-scale facilities. High energy density experiments span a wide range of areas of physics, including plasma physics, laser and particle beam physics, material science and condensed matter physics, nuclear physics, atomic and molecular physics, fluid dynamics and magnetohydrodynamics, and astrophysics. While a number of scientific areas are
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represented in high energy density physics, many of the techniques used in high energy density research have grown out of ongoing research in plasma science, astrophysics, beam 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 interaction processes. DEFINITION OF HIGH ENERGY DENSITY The region of parameter space encompassed by high energy density physics includes a wide variety of physical phenomena. Simple estimates of high energy density conditions exhibited in different physical circumstances enable the overlap in conditions to be made readily apparent. Taking advantage of the synergies that can be developed among different areas of research has the potential to greatly increase the fundamental understanding of high energy density physics and to enhance the identification of compelling research opportunities. The energy density of common, room-temperature materials provides a starting point for a definition of high energy density conditions. Many of these materials (such as hydrogen, carbon, and iron) are ubiquitous in the universe. One definition of high energy density conditions is that these conditions exist when the external energy density applied to the material is comparable to the material’s room-temperature energy density. This can be thought of as the condition that exists when typical room-temperature materials become compressible—for example, if a shock wave is sent through the material. The energy density of a hydrogen molecule and the bulk moduli of solid-state materials are similar, that is, about 1011 J/m3. Table 1.1 lists some of the ways of expressing the energy density corresponding to 1011 J/m3. At this energy density, the pressure is 1 Mbar. The energy density of electromagnetic radiation can be considered either as an effective intensity or as a blackbody radiation temperature. An intense radiation pulse interacting with matter can ablate material, generating a pressure wave in the material. The x-ray and laser drives required for a 1-Mbar ablation pressure wave are shown in Table 1.1. The magnetic and electric field strengths that correspond to this energy density are also shown. In a plasma with a specified electron number density, the temperature required to give an energy density corresponding to 1011 J/m3 is shown in Table 1.1. These different ways of expressing the same energy density facilitate comparisons of different physical conditions and identify similarities and potential synergies. Figure 1.1 shows a plot in temperature-density space indicating regions encompassed by different physical processes and conditions. Regions that are accessible in various high energy density laboratory facilities are indicated in the figure, and the
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TABLE 1.1 Static High Energy Density Definition: Various Quantities That Correspond to an Energy Density of 1011 J/m3 Energy Density Parameter Corresponding to ~1011J/m3 Value Pressure 1 Mbar Electromagnetic Radiation Electromagnetic wave (laser) intensity (I) (p~I) 3×1015 W/cm2 Blackbody radiation temperature (Trad) (p~Trad1/4) 4×102eV Electric field strength (E) (p~E2) 1.5×1011 V/m Magnetic field strength (B) (p~B2) 5×102 T Plasma Pressure Plasma density (n) for a thermal temperature (T) of 1 keV (p~nT) 6×1026 m−3 Plasma density (n) for an energy per particle (temperature) (T) of 1 GeV (p~nT) 6×1020 m−3 Ablation Pressure Laser intensity (I) at 1 µm wavelength (λ) (p~(I/λ)2/3) 4×1012 W/cm2 Blackbody radiation temperature (Trad) (p~Trad3.5) 75 eV NOTE: The scaling of the pressure with the appropriate physical quantity is shown parenthetically in the first column. 1-Mbar contour is shown. As is evident from the figure, a wide variety of physical and astrophysical processes and objects have energy densities greater than 1 Mbar. High energy density systems exhibit a variety of physical properties that can be useful in characterizing such systems. Some of these are summarized below. Nonlinear and collective responses. One of the defining characteristics of high energy density conditions is their collective response to external stimuli. Examples include wave motions in plasmas and the response of a metal to a strong shock wave. High energy density systems often have a significant nonlinear response to an applied energy source. An electromagnetic wave propagating in a plasma generates many nonlinear responses from the plasma, including stimulated (parametric) instabilities such as Raman and Brillouin scattering, and relativistic instabilities generated at higher intensities. At still higher intensities, the vacuum itself can become nonlinear. (This nonlinear response is discussed in Chapter 4.) Full or partial degeneracy. High energy density systems can be driven to such extremely high density that their pressure is determined by the Pauli exclusion principle rather than by their temperature. The response of such systems is determined by their quantum-mechanical properties. Many astrophysical and laboratory high energy density systems are partially or fully degenerate. These include the centers of large planets, brown dwarfs, white
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FIGURE 1.1 Overlap between high energy density experimental range and astrophysical conditions. The horizontal axis is logarithmic density (lower axis in grams per cubic centimeter, upper axis in number per cubic meter). The vertical axis is logarithmic temperature (left in degrees Kelvin, right in electronvolts). All shaded regions correspond to the high energy density (HED) regime. The rectangular boundaries in the center enclose the high energy density regimes now accessible on experimental facilities such as OMEGA and Z (smaller, tan box); the National Ignition Facility, or NIF (larger, light-yellow box); and intermediate stages of NIF as it begins operations (orange region). The tan and yellow elliptical regions correspond to short-pulse, ultrahigh-intensity lasers, present and future. A density-temperature plot is only one of many ways to parameterize laboratory systems, and it ignores the trade-offs among the utility of different systems depending on the choice of experiment. Magnetic fusion experiments probe comparable temperatures, but much lower densities. The rectangles overlap most of the extreme conditions for typical stars (stars of 60 and 1 solar mass are shown; the 60-solar-mass star is both burning helium [at the center] and hydrogen [in a shell], while the 1-solar-mass star is a model of the present-day Sun), and a significant fraction of the extreme conditions found in the interior of giant planets and brown dwarfs.
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dwarfs, neutron stars, and various phases of an inertial confinement fusion implosion. In the case of the neutron star, the degeneracy of the neutrons determines the system characteristics. (These properties are discussed further in Chapters 2 and 3.) It is noteworthy that some low energy density systems exhibit degeneracy at extremely low temperatures, such as those in single-component plasmas and Bose-Einstein condensates. Dynamic systems. The Reynolds and Mach numbers serve as yardsticks for hydrodynamic instabilities. At high values of the Reynolds number, turbulence ensues: the ultimate nonlinear response. The Mach number measures compressibility, the ratio of kinetic to thermal energy, and the ability of the flow to form and sustain shocks. The transition to turbulence in a high energy density medium is probably the least understood high energy density condition, either experimentally or theoretically. Figure 1.2 illustrates the various hydrodynamics regimes encountered in high energy density conditions as a function of Reynolds number and Mach number. Compressible turbulence regimes map in the upper-right-hand sector, that is, Re>104 and Ma>0.5, and are relevant to larger-scale and, in particular, astrophysical phenomena, as discussed in Chapter 2. These regimes are likely to be within the experimental reach of future facilities such as the National Ignition Facility. The conditions that are described above are not unique in achieving energy densities of order 1011 J/m3. For example, the ionization of individual atoms or molecules in intense laser fields occurs at similar energy densities. These latter systems do not, however, demonstrate a collective response and are therefore outside the scope of this report. The high energy density interactions with individual atoms are discussed in the National Research Council (NRC) report entitled Atomic, Molecular, and Optical Science—An Investment in the Future.1 PHYSICAL PROCESSES AND AREAS OF RESEARCH By way of introduction, this section outlines some, but certainly not all, of the physical processes that are normally included under the descriptor “high energy density physics.” This section briefly gives a sense of the field, and subsequent chapters provide considerably more detail, as well as identifying research opportunities of high intellectual challenge. 1 National Research Council, Atomic, Molecular, and Optical Science: An Investment in the Future, Washington D.C., National Academy Press, 1994.
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FIGURE 1.2 Mach number-Reynolds number plane indicating various hydrodynamic regimes encountered in high energy density phenomena. The range of astrophysical interest is large. In one event, a Type Ia supernova, the Mach numbers range from less than 0.01 at thermonuclear ignition to more than 100 at emergence of the explosion shock at the stellar surface. The Reynolds number scales with size, so that astrophysical events generally involve much larger Reynolds numbers than those accessible by HED experiments. Astrophysical phenomena generally lie above the top of the graph, at Reynolds numbers greater than 1 million. The experiments sample the region now being explored by direct numerical simulation and are relevant to understanding the tools that will be used to explore more extreme conditions. High Energy Density Astrophysics During the past decade, a new subfield of laboratory astrophysics has emerged, made possible by current and planned high energy density experimental facilities, such as large laser facilities and Z-pinch generators. On these facilities, macroscopic collections of matter can be created under astrophysically relevant conditions and their properties measured. Examples of processes and issues that can be experimentally addressed include compressible hydrodynamic mixing, strong-shock phenomena, magnetically collimated jets, magnetohydrodynamic turbulence,
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radiative shocks, radiation flow, high-Mach-number jets, complex opacities, photoionized plasmas, equations of state of highly compressed matter, and relativistic plasmas. These processes are relevant to a wide range of astrophysical phenomena, such as supernovae and supernova remnants, astrophysical jets, radiatively driven molecular clouds, accreting black holes, planetary interiors, and gamma-ray bursts. There has been a concomitant increase in observational and simulation capabilities that allow a more direct connection between laboratory and astrophysical high energy density conditions. Laser-Plasma Interactions The nonlinear optics of intense lasers in plasmas is an exciting area of forefront research. Ultrahigh-power, short-pulse lasers are generating extraordinary fluxes of very energetic electrons and ions and the highest electric and magnetic fields produced on Earth. Another challenging area of research is the collective interaction of multiple, interacting, long-pulse laser beams as they filament, braid, and scatter. Research on laser-plasma interactions impacts plasma physics, astrophysics, inertial confinement fusion, and stockpile stewardship. It may also lead to ultrahigh-gradient particle accelerators, novel light sources, advanced diagnostics, and new approaches to fusion. Beam-Plasma Interactions Short-pulse electron beams with densities greater than the plasma electron density can, like the laser pulses described above, be used to drive electron-accelerating plasma waves. In this case, the electron beam ejects all plasma electrons from the propagation channel, and in turn the ion channel that has been formed constitutes a very powerful charged particle lens. High-power charged-particle beam interactions with plasmas produce a wealth of high energy density physics and are of long-standing interest in inertial fusion energy. In addition to external beams interacting with plasmas, interesting electron and ion beams can be generated from within the plasma itself. Relativistic electron beams of unprecedented peak currents, exceeding the Alfvén current by orders of magnitude, have been produced in petawatt-laser-solid-target interaction experiments. High-brightness ion beams with energies exceeding 5 MeV per nucleon have also been produced by petawatt lasers. Their development remains an exciting physics challenge. Ion beams of unprecedented peak current, exceeding the Alfvén current for the ions by orders of magnitude, have been produced in petawatt-laser-solid-target interaction experiments.
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Beam-Laser Interactions Colliding high-brightness picosecond relativistic electron beams with ultrahigh-power, short-pulse lasers enables the study of fundamental electromagnetic radiation processes in the laboratory. These collisions provide a testing ground for relativistic quantum electrodynamics, where the nonlinear quantum electrodynamic production of electron-positron pairs has been observed. They also produce copious quantities of Compton x rays. These experiments can lead to compact, high-spectral-brightness x-ray sources that may be brighter than current synchrotron light sources for materials science research and medical applications. Free Electron Laser Interactions The use of relativistic electron beams as a lasing medium is attractive, as it allows the production of very high photon energy densities and operation at nearly arbitrary tunable wavelengths. The development of an x-ray free electron laser is now a major focus of the synchrotron radiation community, as it will provide very powerful “fourth-generation” x-ray sources that are highly coherent. Coherent, high-power imaging is expected to revolutionize molecular, biological, and materials research. Such an x-ray free electron laser at 1 angstrom (Å) is expected to be so powerful that at its focus it may produce conditions that “boil the vacuum” to produce electron-positron pairs. These high-flux x-ray sources can be used to volumetrically heat large amounts of solid-density matter to fully or partially degenerate conditions and/or to probe their properties. The electron beam needed to drive the x-ray free electron laser is of unprecedented brightness, and its production and diagnosis are a frontier research effort. High-Current Discharges Research on high energy density, magnetically confined, radiation-dominated plasmas is rapidly advancing because of the availability of pulsed-power sources that can deliver up to 20 mega-amperes (MA) in 100-ns pulses. Discharges through wire-arrays produce copious amounts of soft x rays—as much as 1.8 MJ at the Z-machine at the Sandia National Laboratories—in times of 5 to 15 ns. Soft x-ray fluxes are used to study radiation-matter interaction. High-Mach-number jets of plasma in discharges produced by the MAGPIE pulsed-power facility in the United Kingdom simulate astrophysical jets. This research also plays an important role in stockpile stewardship and inertial confinement fusion.
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Radiation-Matter Interaction, Hydrodynamics, and Shock Physics High-energy laboratory drivers are used to obtain opacities and to study radiation transport. Useful opacity information for astrophysics can be obtained in the laboratory. New experimental techniques have been developed for understanding Rayleigh-Taylor, Richtmyer-Meshkov, and other hydrodynamic instabilities in the linear and nonlinear regimes. High-power laser experiments study shock wave propagation and interaction especially at high Mach numbers Ma~15 to 20. These experiments access the regime where the shock pressures exceed the material’s bulk modulus. Instabilities of shock waves, shocked matter, compressible turbulent mixing, and radiation interaction with shocks are also being studied. These measurements benchmark codes used in supernova research, inertial confinement fusion, and in defense applications. Equation-of-State Physics Recent laser-driven and pulsed-power-driven compression experiments provide new and unexpected data on the equations of state of hydrogen and its isotopes at high pressures (>1 Mbar), and of other materials, such as copper and iron. High energy density drivers create novel states of matter such as metallic hydrogen or complex carbon states. These experiments supply critical data for stellar and planetary interior calculations. Atomic Physics of Highly Stripped Atoms High energy density drivers generate highly stripped, near-solid-density, mid-Z and high-Z plasmas at kiloelectronvolt temperatures, with and without magnetic fields. Such plasmas can be used to benchmark atomic physics codes and may contribute to our basic understanding of atomic processes in complex ions in strong electric and magnetic fields. Theory and Advanced Computations High energy density physics phenomena are difficult problems to analyze theoretically. The high degree of nonlinearity and complexity of multiple scales make many traditional approaches difficult at best. Advances in scientific computation and computing technology are being utilized with considerable success in modeling many of these systems. The knowledge obtained from laboratory experiments can be used to verify and develop theoretical models needed to more fully understand the fundamental physics and to model astrophysical objects for which we have limited observational data.
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Inertial Confinement Fusion Many of the high energy density physics areas described above are relevant to the development of inertial fusion as an energy source, using either intense heavy ion beams or high-repetition-rate lasers as drivers. The current goal of the inertial confinement fusion program is to achieve ignition in the laboratory, where more energy is produced in fusion reactions than is incident on the imploding deuterium-tritium fusion pellet. It is anticipated that ignition will be achieved on the National Ignition Facility, currently under construction. Conditions in ignited plasmas mimic those in stars. Inertial fusion energy has the further goal of using ignited targets to drive an economical electric power plant. FINDINGS AND RECOMMENDATIONS Subsequent chapters in this report describe the compelling research opportunities and questions of high intellectual challenge in high energy density astrophysics (Chapter 2), high energy density laboratory plasmas (Chapter 3), and laser-plasma and beam-plasma interactions (Chapter 4). The research opportunities range from investigating the very largest cosmological systems to exploring at the very smallest scales, with questions such as these—Can astrophysical jets be simulated in laboratory experiments? and Can focused lasers “boil the vacuum” to produce electron-positron pairs? The questions deal with the properties of matter under extreme high energy density conditions, including matter in stars, at the beginning of the universe, and in inertial confinement fusion experiments. During the course of this assessment of research opportunities and national capabilities in high energy density physics, the Committee on High Energy Density Plasma Physics reached a number of important conclusions that are included here as the principal findings and recommendations of the report. In many respects, the field of high energy density physics is still in its infancy. As would be expected, several of the recommendations identify compelling research opportunities in which an increase in the level of federal research support would lead to significant advances in physics understanding and new discoveries, particularly in research areas in which there are strong synergies among related subfields of high energy density physics. The findings and recommendations presented here follow not only from the content of the subsequent chapters, but also from the data provided to the committee in the course of this study and the extensive interactions of the committee with the many members of the research community identified in the Preface of this report. In formulating its findings and recommendations, the committee recognized that setting priorities and indicating a quantitative scale (in dollars) for recommended initiatives may be necessary to implement these changes and effectively realize the science opportunities identified in this report. However, the committee considered this
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d.Finding on new opportunities in understanding astrophysical processes The ground-based and space-based instruments for measuring astrophysical processes under extreme high energy density conditions are unprecedented in their sensitivity and detail, revealing an incredibly violent universe in continuous upheaval. Using the new generation of laboratory high energy density facilities, macroscopic collections of matter can be created under astrophysically relevant conditions, providing critical data and scaling laws 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. There has been an explosion of discoveries in astrophysics as well as dramatic improvements in measurements from new space-based and ground-based instruments. The discoveries include these: evidence for the natural cosmological distance of gamma-ray burst sources, discoveries of giant planets and brown dwarfs, measurements of Type Ia supernovae at cosmological distances, discovery of relativistic jets in stellar mass microquasars, high-resolution observations of radiative shocks, and measurements of rapidly rotating neutron stars with ultrastrong magnetic fields. A proliferation of physical and theoretical problems accompanies these new discoveries. Many of these problems parallel the scientific issues arising in recent and planned high energy density experiments and advanced simulation studies. Laboratory high energy density experiments and simulation studies can provide important new understanding regarding, for example, the equations of state and opacities of high-pressure matter in giant planets and brown dwarfs, nuclear reactions and element formation in stars and supernovae explosions, the behavior of high-Mach-number shocks and the acceleration of charged particles in shocks, the behavior of high-Reynolds-number turbulent plasma flows with and without magnetic fields, the behavior of electron-positron explosions that mimic processes of gamma-ray bursts, and the radiative dynamics of hypersonic flows. e.Finding on National Nuclear Security Administration support of university research The National Nuclear Security Administration recently established a 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. The National Nuclear Security
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Administration is to be commended for initiating this program. The nation’s universities represent an enormous resource for developing and testing innovative ideas in high energy density physics and for training graduate students and postdoctoral research associates—a major national resource that has heretofore been woefully underutilized. The National Nuclear Security Administration’s establishment of the Stewardship Science Academic Alliances program comes at a highly opportune time. Following presentations at its meetings on the science opportunities in the field of high energy density physics, and from numerous contacts with the research community on an individual basis, through a town meeting, and through a public solicitation effort for input to the study, the committee concluded that 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 sub-fields. Students and postdoctoral associates have a unique opportunity to be trained on a wide range of new sophisticated laboratory systems, existing and planned, that produce extreme high energy density conditions in matter. Numerous universities have infrastructures that can support high energy density experiments—for example, facilities based on lasers in the multiterawatt to petawatt power range, and 100-kJ pulsed-power systems. Studies of high energy density physics have been proposed or performed at university facilities, at national laboratory facilities supported by the National Nuclear Security Administration, and at nondefense laboratory facilities of the Department of Energy. The Stewardship program is positioned to support this exciting research, which may attract some of the best young minds to the field of high energy density physics and produce a pool of talented scientists for the National Nuclear Security Administration laboratories. f.Finding on the need for a broad, multiagency approach to support the field of high energy density physics The level of support for research on high energy density physics provided by federal agencies (e.g., the National Nuclear Security Administration, 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. An important finding of this report is that the research opportunities in this crosscutting area of physics are of the highest intellectual caliber and are fully
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deserving of consideration of support by the leading funding agencies of the physical sciences. Agency solicitations in high energy density physics should seek to attract bright young talent to this highly interdisciplinary field. The rapid progress in scientific understanding, creative concepts, and new high energy density physics tools, both experimental and computational, has created an explosion of opportunity for advances in high energy density science that now far outpaces the level of support available to capitalize on them. As noted above, the research opportunities in this crosscutting area of physics are of the highest intellectual caliber and are fully deserving of consideration of support by the leading funding agencies of the physical sciences, such as the nondefense directorates of the Department of Energy, the National Science Foundation, the Department of Defense, and the National Aeronautics and Space Administration. The lack of investment in high energy density physics is particularly apparent through the small representation of plasma physicists on the faculty of U.S. universities. A broad federal support base for research in high energy density physics, including plasma science, and the encouragement of interdepartmental research initiatives in this very interdisciplinary field would greatly strengthen the ability of the nation’s universities to have a significant impact on this exciting field of physics. The astrophysics, plasma physics, and other scientific communities contributing to high energy density physics are highly international, and mechanisms for encouraging open scientific collaborations in high energy density physics on National Nuclear Security Administration facilities should be encouraged to the maximum extent possible, consistent with national security priorities. g.Finding on opportunities to upgrade existing facilities Through upgrades and modifications of experimental facilities, exciting research opportunities exist to extend the frontiers of high energy density physics beyond those that are accessible with existing laboratory systems and those currently under construction. These opportunities range, for example, from the installation of ultrahigh-intensity (petawatt) lasers on inertial confinement fusion facilities for creating 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 high energy density physics studies, such as the development of ultrahigh-gradient acceleration concepts and unique radiation sources ranging from the infrared to the gamma-ray regimes.
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A highly cost-effective way to significantly impact and extend the horizons of high energy density research is to upgrade and/or modify existing and future beam and laser experimental facilities to achieve extreme high energy density conditions. As one example, a significant opportunity exists in utilizing one or more of the beamlines of the National Ignition Facility to produce a single or several 0.1 to 10 picosecond (ps) laser pulses employing chirped pulse amplification technology. The characteristics of such a laser pulse would be unprecedented, opening the path to truly unique and exciting new physics. The power levels would be in the multipetawatt range, perhaps approaching the exawatt level; the pulse energy would be between 10 and 100 kJ, with focused intensities as high as 1023 to 1024 W/cm2. Significant high energy density experiments could be carried out with this “one of a kind” national laser facility in a number of areas. Examples include fast ignition for inertial confinement fusion research, simulation of properties of black holes, and ultrahigh-gradient acceleration of electrons and protons. In addition to the NIF laser, the 30-kJ OMEGA laser facility could also be modified to produce multipetawatt short laser pulses for exciting high energy density physics experiments under extreme high energy density conditions. As another example, the Stanford Linear Accelerator Center (SLAC) electron/ positron gigaelectronvolt-class beams also provide unique opportunities for high energy density science. At a relatively modest cost, a dedicated SLAC beamline could be made available for high energy density experiments. With such a beamline, pioneering high energy density experiments would extend our basic physics understanding in such frontier research areas as ultrahigh-gradient wakefield acceleration, novel x-ray radiation sources, and ultrastrong-focusing plasma lenses. h.Finding on the role of industry There are active partnerships and technology transfer between industry and the various universities and laboratory research facilities that are mutually beneficial. Industry both is a direct supplier of major hardware components to the high energy density field and has spun off commercial products utilizing concepts first conceived for high energy density applications. Further, it is to be expected that industry will continue to benefit from future applications of currently evolving high energy density technology and that high energy density researchers will benefit from industrial research and development on relevant technologies. As the direct supplier of laser glass, accelerator components, power supplies, and many other products, industry is critically coupled to the field and benefits from
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the research and development performed. Many of the relevant fabrication processes and designs were first developed at universities and national laboratories and then transferred to industry for production. Products first developed in this manner often find diverse application in other areas. Additionally, chirped pulse amplification lasers and certain nonlinear optical elements, originally developed for high energy density applications, are now commercially available elements of laser systems. As a result, the technology transfer process has come full circle, and economical tabletop terawatt laser systems are commercially available for university research and development in chemistry and many other fields. Indirectly, industrial research and development contributes significantly to high energy density physics applications. As examples, rapid advances in diode-pumped laser efficiency and in lithographic manufacturing processes have been driven by the multibillion-dollar research and development investment of the telecommunications and semiconductor industries. Yet these advances are key enablers for recent high energy density schemes for laser acceleration of particles. Similarly, the high average power laser source developed for extreme ultraviolet lithography (EUVL) is ideally suited to meet the high photon flux needed for fluid turbulence diagnostics. It is to be expected that some of the ongoing research and development will generate new spin-off opportunities for industry. Among these could be advanced laser and optical components such as compact, high-performance optical parametric oscillators, dielectric gratings, more effective and economical radioisotope production or miniature advanced accelerators for medical or material processing applications. Many other applications of high-power lasers can rightly be considered spin-offs to industry from the field, although their performance parameters fall below the threshold definition of high energy density utilized in this report. Examples include solid-state, chemical, and free-electron lasers for defense applications, and “ultrafast” material processing lasers. 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.
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Throughout this study, the committee was told that one of the most positive developments during the past decade has been the engagement of the academic community in pursuing frontier experimental science on major high energy density physics facilities. The scientific return on the investment for about 15 percent of usage time at the national facilities has been enormous. Elegant scale transformations now link strong-shock laboratory experiments with supernova explosions and other large-scale phenomena. The transition to turbulence has been theoretically and experimentally demonstrated in strong-shock-driven hydrodynamic flows. Lattice diagnostics probe the solid-state response of matter to heating and compression in exquisite detail on subnanosecond time scales. The equation of state of hydrogen has been measured under conditions of extreme pressure and density, relevant to the interior of Jupiter. The opacities of plasmas under conditions relevant to Cepheid variable pulsating stars have been measured. The collective behavior of ultrarelativistic electron beams interacting with ambient plasma has been demonstrated. These and many other impressive scientific achievements have resulted from the university outreach programs. The value to major programmatic missions within the NNSA, both near term and far term, is significant and clear. The committee commends the National Nuclear Security Administration for its engagement of the academic community over the past decade on frontier scientific phenomena accessible on its high energy density physics facilities, and it recommends an expansion of this program for the coming decade to include both existing and future Department of Energy facilities. One limitation in the academic outreach programs brought to the attention of the committee by the outreach participants has been that of obtaining sophisticated targets and potentially unique diagnostics to carry out the experiments on facilities such as the Nova and OMEGA lasers and the Z magnetic pinch facility. Once facility time is awarded through a rigorous proposal selection process, it is recommended that there be a firm commitment to providing the necessary support (target fabrication, diagnostic modification and/or development, and so on) to maximize the scientific return and ultimate value of this program to the National Nuclear Security Administration. 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
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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. The arguments for investment in university high energy density research are compelling. First, the productive lifetime of major new facilities, such as the National Ignition Facility, is sufficiently long that the ultimate success of these facilities may well depend on the talent and creativity of a generation of scientists who are yet to enter college, let alone graduate school. Moreover, the degree to which these facilities accomplish their important scientific missions will likely depend on attracting, retaining, and inspiring the very best students and researchers. These students will need to be broadly educated in high energy density science, and the talents of experimentalists, theorists, and modelers will be needed. The field of high energy density science is rich with exciting topics to attract such students, and many of these fundamental problems are tightly coupled to applications of great societal and scientific importance. Second, the freedom to focus broadly and to pursue curiosity-driven research makes universities a natural home for creativity. Such freedom comes historically from the support of such agencies as the National Science Foundation and the nondefense directorates of the Department of Energy, which seek as their primary criterion research proposals that are innovative. Finally, the strong connections to astrophysics made in this report argue for support for this field from the National Aeronautics and Space Administration, the agency that traditionally has been the leader for such support. 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.
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The major high energy density facilities, both existing and planned, are built to pursue specific missions. Though they can also be used directly to pursue new opportunities in basic high energy density science outside their original mission (see “Recommendation on external user experiments at major facilities,” above), some of the most exciting scientific opportunities may lie slightly outside the original capability of the facility. Incremental funding to enhance these facilities through added diagnostics and capabilities to access new parameter regimes (such as shorter pulse lengths) would open whole new vistas for high energy density science. Examples of particular interest are the addition of a high-intensity (pulse-compressed) laser to high energy density laser facilities and the addition of a dedicated short-pulse beamline at a major high energy particle beam facility. The addition of short-pulse petawatt lasers are interesting in their own right for the extreme relativistic plasma conditions they can create. However, when synchronized to a high energy density facility, unique new regimes of target physics open up, with possible relevance to aspects of gamma-ray bursts and stellar atmospheres. Similarly, the addition of a beamline at a particle beam facility with the flexibility to customize it to plasma science experiments would open new parameter regimes of potential interest for laboratory astrophysics, novel radiation sources, and particle acceleration techniques. 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. The primary source of new scientists in high energy density physics is the universities. They educate and train graduate students and postdoctoral research associates in this area of research, and they attract faculty members to this exciting field. While the existence of state-of-the-art high energy density facilities (large lasers, pulsed-power machines, and powerful computers) provide high visibility and facilitate exciting advances in the field, they are not sufficient. It is difficult to attract graduate students to high energy density physics without on-site university facilities. These facilities provide critical training in high energy density physics and can lay the groundwork for research on the state-of-the-art national facilities.
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University research in high energy density physics is not limited to the direct study of high energy density physics phenomena—for example, exploiting recent technological innovations such as tabletop high-intensity lasers and/or advanced computational facilities. It also includes the study of phenomena that occur under high energy density conditions but which can be explored in the laboratory at low-to-moderate energy densities. Imaginative research in these areas leads to important insights that significantly advance our understanding of high energy density physics phenomena. 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. The first phase of the Advanced Strategic Computing Initiative and other large-scale computational programs have catalyzed and continue to facilitate remarkable achievements. Recent experimental successes and paradigm shifts in terms of facilities and diagnostics discussed elsewhere in this report are responsible for equally significant contributions. Integration of these contributions into a coordinated program is recommended. Such a coordinated program can be expected ultimately to yield as great a revolution—perhaps even a greater one—in the ability to simulate high energy density phenomena as that generated by the recent increases in computing technology. It would combine experimental components to continue to explore, elucidate, and facilitate the development of physics-based models of the dynamics of matter in high energy density conditions; the incorporation of these models in the next-generation simulation codes; further developments of needed computational-science components; and the closure of the experiment-physics-simulation loop through a concerted validation effort. Only through such quantitative comparisons can there be continued significant advances in understanding the underlying physics. 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 laborato-
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ries and facilities, to the maximum extent possible, given national security priorities. The joint endeavor between the NNSA and the academic community to pursue basic scientific questions on major Department of Energy facilities has been a particularly successful and rewarding development. The positive outcomes of this partnership—novel ideas, high-quality scientific articles, new talent sources for the laboratories—drastically outweigh any difficulties or inconveniences this program may cause. For the science resulting from this effort to flourish, in a manner consistent with the scientific method, it is necessary to discuss the scientific results at periodic meetings and workshops of all interested scientists, regardless of nationality. Such workshops are vital to extracting world-class science and understanding from the basic physics data acquired on these facilities. These workshops should be open to all interested scientists (independent of nationality) and should be supported by the NNSA both philosophically and, where appropriate, monetarily. 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 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. A number of federal agencies besides the Department of Energy have overlapping interests in high energy density plasma physics as well as in related or supporting disciplines. In a few instances, the Department of Energy has engaged in developing jointly funded programs, such as the plasma science program conducted jointly by its Office of Fusion Energy Sciences and the National Science Foundation. As stated, such program collaborations are 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. The committee applauds these efforts and encourages both the strengthening of existing programs and the initiating of new interagency collaborations. Examples of exciting research opportunities exist especially in astrophysics, where the high energy density physics explored in the Department of Energy’s programs have immediate applications to key astrophysics problems.
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To summarize, the committee believes that now is a very opportune time to make 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 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.
Representative terms from entire chapter: