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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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1
High Energy Density Science: Understanding Matter at Extremes

OVERVIEW

High energy density (HED) science seeks to understand and control material at extreme conditions—at temperatures over 20,000°F (approximately 10,000°C, or 10⁴ K), pressures millions of times larger than atmospheric pressure,¹ or within electromagnetic or radiation fields that significantly alter the electronic structure of atoms. These conditions can be found deep inside planets and stars throughout the universe, and can also be produced in the laboratory by concentrating energy into small volumes. At these conditions, the forces between atoms, electrons, ions and even nucleons are profoundly modified, changing fundamental material properties, paving paths to the creation of entirely new forms of matter, and enabling conversion of matter into energy.

Traditionally, the domain of HED science is defined by pressures exceeding 1 million times atmospheric pressure (see Figure 1-1). This corresponds to an

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¹ A note on units: In the extreme conditions of high energy density (HED) science, familiar units become cumbersome. In the present report, units of temperature are usually reported in kelvin (K) or electronvolts (eV). The Kelvin scale is the same as Celsius (C) but starts at absolute zero temperature such that 0 K = –273°C. At high temperatures, the difference between Celsius and kelvin is often unimportant. An energy of 1 eV corresponds to a temperature of 11,606 K ≈ 10⁴ K, so that 1,000 eV = 1 keV translates to about 10⁷ K; room temperature is about 0.02 eV. Units of pressure are usually reported as multiples of either atmospheric pressure (1 atm = 1.013 bar) or the SI pressure unit Pascal (Pa), with 1 Pa = 1 J/m³. Earth’s average atmospheric pressure at sea level is close to 1 bar or 100,000 Pa = 10⁵ Pa; HED science considers materials at pressures above 100 GPa = 100 billion Pa, which is the same as 1 Mbar = 1 million atm.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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**FIGURE 1-1** The regimes of high energy density (HED) science. At Earth-surface pressures and at temperatures of order 100-1,000 (10²-10³) K, solids melt to form liquids and evaporate to become gases. At higher temperatures, in the 10³-10⁴ K range, electrons can escape individual atoms to form ions. With this ionization process, atoms are no longer electrically neutral, and the resulting mixture of ions and electrons—plasma—is an electrically conducting gas.¹ For condensed matter, this is the beginning of the HED regime at high temperatures and low pressures (or low material densities). Above about 10⁷ K, the collisions between atoms (ions) become frequent and violent enough that their nuclei begin to combine and react, leading to nuclear fusion. In HED science, “hot” is shorthand for the many kiloelectronvolts temperature that support nuclear fusion; “warm” indicates the many electronvolts (10³-10⁶ K) temperatures at which thermal effects are significant; and “cold” is limited to matter that is not thermally ionized (below 10³ K), where quantum behavior often dominates. At any given temperature, increasing material density leads to an increase in pressure that can also affect material on the atomic scale. Cold, electrically insulating materials can transform to metals under compression, and thence to “quantum matter” where collective quantum effects occur. In the regime of dense matter, both quantum and thermal effects exist simultaneously (e.g., pressure ionization and thermal ionization) and interact in complex ways. “Hot dense matter” characterizes the conditions of inertial confinement fusion (ICF), and of the interiors of the Sun and other stars, with densities that can be more than 100 times those of a typical terrestrial solid at ambient conditions. Note that QED is quantum electrodynamic.

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¹ Plasma as an ionized, high-temperature gas has nothing to do with blood plasma, and it is an unfortunate quirk of language that the same term is used for completely unrelated materials.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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energy density of 100 billion J/m³ (Pa), rivaling the quantum forces that determine material structure and properties (see Box 1-1). For perspective, to bring water to the HED regime requires about 100,000 times the energy needed to bring water to its boiling temperature at 1 atmosphere. As such large amounts of energy are hard to come by, MJ or more on laboratory scales (meters), most laboratory HED experiments use small sample volumes of microliters (mm³) or less.

This introduces a primary challenge in HED science, of controlling experiments and measurements within small volumes, and—for dynamic experiments—on short timescales, corresponding to dynamic times under a millionth of a second. Fortunately, there are multiple approaches to delivering energy in the laboratory: small samples can be compressed between the tips of diamonds; high-intensity optical lasers and high-electric-current, pulsed power can be used to both heat and compress material; and, for example, bright, X-ray free-electron lasers (XFELs) can strip electrons from atomic cores. XFELs represent an exciting frontier for HED science, and while this report focuses on opportunities related to National Nuclear Security Administration (NNSA) facilities, there are many opportunities in the broader Department of Energy (DOE) complex and wider university networks.

When materials are compressed to HED pressures without heating, the distance between atoms shrinks and chemical bonds are significantly modified. The trends of the Periodic Table of chemical elements are thus fundamentally transformed in the HED regime, with new materials, properties, and processes being observed. Hydrogen and helium, the most abundant chemical elements in the universe, transform to fluid metals at HED conditions, for example, with liquid metallic hydrogen being the predominant constituent of stars and giant planets (see Figure 1-2).

Diamond, a high-pressure form of carbon, is emblematic of how even modest forays toward the HED regime can lead to materials with extreme properties, including strength, transparency, and conduction of heat (see Box 1-2). “Quantum matter” is the cold, high-density regime at which atoms behave “collectively” rather than near-randomly, exhibiting such properties as superconductivity or superfluidity (see Figure 1-3).

When material is heated to high temperatures, whether by direct irradiation with optical lasers or X rays, or kinetically heated by a high-velocity pressure wave, the electrons that are usually bound to atoms gain enough energy to escape, ionizing the atoms. This leads to the formation of an HED plasma, a high-density gas of charged ions and electrons characterized by strong interactions with external and internal electric, magnetic, and radiation fields. For example, the boundary of the radiation and convection zones of the Sun is highly dependent on material properties in the HED regime, for instance the X-ray opacities of iron and oxygen—that is, how effectively those ionized elements absorb radiation from the stellar core (see Chapter 3).

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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BOX 1-1
Technical Characteristics of the High Energy Density Limit

The pressure-volume (P–V) work associated with onset of the high energy density (HED) regime, –PΔV for P = 100 GPa = 10¹¹ J/m³, is of order 1 eV (96.5 kJ/mole), the energy of chemical bonds and of the atom’s outermost electrons; constant-volume (isochoric) heating yields similar magnitudes of pressure and energy changes for condensed matter (see Figure 1-1-1).

Characteristic values for the dimension of an atom (1 ångström = 1 Å = 10^–10 m) and for chemical-bond energies give an energy density of 1 eV/Å³ = 160 GPa, near the lower limit of HED conditions. Multiplying the HED onset (10¹¹ J/m³) by typical values of atomic volume (Å³ = 10^–30 m³) and atom-dynamics times (1 fs = 10^–15 s) gives the quantum of action, the reduced Planck constant: 100 GPa Å³ fs = 1 × 10^–34 J s = h/2π = ħ. In short, ångströms and femtoseconds are length and time scales for atoms and molecules combining or reacting, such that 100 GPa corresponds to the quantum of chemistry.

The TPa = 1,000 GPa pressures now accessible in a variety of laboratory experiments (see Chapter 2) are comparable to the quantum mechanical forces determining the structure of the atom: ħ²/(4πm_ea₀⁵) = 2.3 TPa is the quantum pressure that keeps the electron from spiraling into the nucleus of the Bohr atom, and ħ²/(m_ea₀⁵) = 29 TPa is the atomic unit of pressure at which not only chemical properties but the structure itself of the atom is profoundly altered (m_e and a₀ are the mass of the electron (9.109 × 10^–31 kg) and the Bohr radius (53 pm), respectively). At the kiloelectronvolt energies reached under 10-100 TPa pressures (0.1-1 Gbar), core electrons participate in chemical bonding: these are the conditions of kilovolt chemistry.

FIGURE 1-1-1 Energy change on compression (blue) or constant-volume heating (red) to the 100 GPa = 10¹¹ J/m³ pressure range is comparable to chemical-bonding energies (eV), with 1 eV ≈ 10⁴ K.
SOURCE: Courtesy of R. Jeanloz.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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**FIGURE 1-2** Many elements and compounds become metals at high pressures. The reflecting sample at the center of the image is a crystal of metallic oxygen at 125 GPa and room temperature inside a diamond-anvil cell. The oxygen (~20 µm in size) is surrounded by helium (gray), contained inside a rhenium foil (outer mottled-reflecting region).
SOURCE: Courtesy of G. Weck, French Alternative Energies and Atomic Energy Commission.

In the warm dense matter (WDM) regime, a combination of high density and significant temperature leads to complex interactions of quantum and classical phenomena. Here, pressure ionization (due to overlapping of the electron clouds bound to atoms) occurs alongside thermal ionization (atoms losing electrons due to heating), and ions can simultaneously exhibit properties pertinent to both fluids and solids. WDM also makes up much of the planets and stars, and allows for the creation of new materials. While it is experimentally accessible with even low-energy experimental drivers (such as XFELs, and small optical lasers and pulsed power devices), the WDM regime is theoretically and computationally challenging.

Yet more extreme HED conditions can be reached by simultaneously heating and compressing material, as in the spherical implosions of laser inertial confinement fusion (ICF) or the cylindrical implosions of pulsed power magneto-inertial fusion (MIF) (see Appendix A for more detail). These experiments can heat hydrogen to temperatures of tens of millions of kelvin (energies of many kiloelectronvolts) that can support nuclear fusion reactions: collisions of light atoms that convert matter into energy. These are the processes that power the Sun’s heat and light, and that are now reached in laboratory experiments.

The committee notes that strong magnetic or electric fields and high gravitational forces at planetary or astrophysical scales can also produce HED-relevant conditions, offering promising and relatively new directions of study. However, this report emphasizes recent laboratory experiments and computer simulations and theory, which tend to concentrate on high densities and temperatures.

While the atomic-scale properties of materials at extreme conditions are fundamental to HED science, the behavior and performance of HED systems is often most relevant to applications. All HED experiments and astrophysical objects access an enormous range of materials, conditions, length scales, and time scales over the

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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BOX 1-2
High Energy Density Material: Diamond

Its strength, stiffness, transparency to light over a wide range of wavelengths (from infrared to X ray), and high thermal conductivity make diamond an important technological material, with applications ranging from micro-cutting blades in the clinic to windows for detectors in space, and from quantum sensors in the laboratory to heat sinks for computer chips (see Figure 1-2-1). In nature, diamond is formed inside planets and during planetary impacts, at pressure-temperature conditions close to the high energy density (HED) limit (10¹⁰ J/m³). That is, the conditions for diamond to be stable are mainly in the HED regime. However, ingenious methods of chemical vapor deposition (CVD) allow its synthesis at near-ambient conditions in the laboratory, thereby making it possible to manufacture high-quality diamond in large quantities. The global market for synthetic diamonds is estimated to be about $18 billion per year.

Early HED research guided the way to a widely available class of diamond products, from nanocrystals to gems. HED experiments use gem-quality diamonds to compress materials into the 0.1-1 TPa pressure range, and capsules of nanocrystalline diamond containing solidified hydrogen isotopes (deuterium and tritium or “DT” ice) are used as targets for inertial confinement fusion (ICF) experiments.

FIGURE 1-2-1 Diamond as used in high energy density science. Left: Large (up to 1 cm diameter, 2.4 carat) synthetic diamond anvils used for high-pressure research. Right: 2.6 mm-diameter nanocrystalline diamond inertial confinement fusion capsules. Inset: X-ray image of a capsule (75 µm wall thickness) that can hold a deuterium and tritium ice layer at 18 K for inertial confinement fusion experiments
SOURCES: Left: T. Irifune and R.J. Hemley, 2012, “Synthetic Diamond Opens Windows into the Deep Earth,” EOS 93(7):65-66. Copyright 2012 by the American Geophysical Union. Right: J. Biener, D.D. Ho, C. Wild, et al., 2009, “Letter: Diamond Spheres for Inertial Confinement Fusion,” Nuclear Fusion 49(11):112001.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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**FIGURE 1-3** Important quantum properties of matter such as free-flowing electric currents and liquids appear at high energy density (HED) conditions. Quantum matter at low temperatures (low–T: *left*) or at high pressure (high compression: *right*) is characterized by the separation between nearest-neighbor atoms (*a_nn*) being less than the quantum wavelengths of the atoms themselves (de Broglie wavelength, *λ_db*), *λ_db* > *a_nn*; whereas normal matter (*center*) is characterized by the de Broglie matter waves having shorter crest-separation than the distances between atoms, *λ_db* < *a_nn*. (Here, 1 bohr = 0.529 Angstroms.) Quantum effects include “collective” behavior of atoms (superfluidity: fluid flow without any viscous resistance; Bose-Einstein condensation: not labeled) or electrons (superconductivity: electric flow with zero resistivity). Atoms themselves may become quantum-indistinguishable as their separation becomes smaller than the de Broglie wavelength, here given as *λ_db* = h/√(3mk_BT), where h is Planck’s constant, m is the mass of the atom, *k_B* is Boltzmann’s constant, and T is temperature in kelvin. Low-temperature atomic, molecular, and optical physics traditionally focuses on low-temperature conditions (*left*), and HED science is now characterizing the complementary HED regime (*right*).
SOURCE: Courtesy of G. Collins.

course of their evolution. Thus, it is not sufficient to understand the microphysics of HED science: we must also understand how samples interact with external fields; how inhomogeneous radiation distributions influence plasma evolution; and how instabilities form, grow, and evolve into turbulence and mix.

This variety of phenomena—from the quantum mechanics of chemical bonding, to collective plasma effects, to strong coupling of matter with radiation, to thermonuclear processes, along with the enormous ranges of relevant length and time scales, from the atomic to the astrophysical—leads to some of the fundamental excitement of HED science. And learning how to manipulate and diagnose HED matter can unlock entirely new states of matter, enable the efficient production of exotic materials, generate fusion energy, and reveal the conditions by which planets form and evolve over time.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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Today, HED science spans many topics and disciplines, from astrophysics and chemistry, to materials science and physics (see Boxes 1-3 and 1-4). Computational science brings the tools of atomistic simulations and artificial intelligence, as well as well as fluid dynamics, optics, and condensed-matter and plasma theory (e.g., laser-plasma interactions). And HED research engages academic, national laboratory, and industry partners worldwide at the frontier of scientific discovery and its societal applications.

Progress in fundamental HED research at national laboratories and universities, and through international collaboration, has been critical in refining our understanding of these applications, with investments in this domain already yielding important returns. Continued investment will therefore be important for technology applications as well as the broader field of HED science.

In anticipation of continued advances in both computational and experimental capabilities, the committee prepared this report to assess the accomplishments, opportunities, and challenges of basic research in HED science.

In addition, HED science has applications in core mission areas of the NNSA, including stewardship of the nation’s nuclear weapons stockpile, countering proliferation of the associated technologies, and development of nuclear fusion–based energy capabilities: areas in which the basic research described in the text by the

BOX 1-3
The Language of High Energy Density Science

Atomic pressures. Pressures at or above the atomic unit of pressure, roughly 30 TPa (see Box 1-1), at which the external forces begin to overwhelm the internal forces of atoms, changing the nature of atoms themselves.

Cold matter. Matter that is not ionized by temperature, typically solids or fluids at temperatures below 10²-10³ K (= 0.01-0.1 eV energies), and often implying quantum properties.

Condensed matter. Solids and fluids comprising much of the matter that we interact with daily and characterized by significantly higher densities than gases. At ambient or Earth-surface conditions (10⁵ Pa, 300 K), condensed matter has densities of about 1-20 g/cm³, depending on chemical composition (the density of water is 1 g/cm³, for example), whereas gases have densities of about 10^–3-10^–1 g/cm³.

Degenerate matter. Matter for which the density is high enough and the temperature low enough that quantum statistics are required to determine material properties.

Dense matter. Matter under pressure, hence compressed to higher densities than at ambient (Earth-surface) conditions, for instance to densities 10-, 100-, or even 1,000-fold higher than ambient.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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Dense plasma. Dense fluid at high enough temperatures to be ionized, hence electrically conducting. Coulomb jelly(or jellium) and Coulomb fluid are sometimes used to convey the same meaning.

High energy density matter. Matter at pressures above 10¹¹ Joules/m³ = 100 GPa, conditions for which external forces begin to overwhelm chemical forces of ordinary matter on Earth.

Hot matter. Matter at high enough temperatures to sustain nuclear fusion (typically above 10⁷ K ~1 keV).

Nuclear fusion. The process of atomic nuclei combining with a release of energy and particles (e.g., neutrons), typically at temperatures above 10⁷ K = 1 keV. Nuclear burning refers to enough energy and particles (especially helium nuclei) being released by the nuclear reactions to trigger nuclear fusion in surrounding material. Nuclear ignition refers to more energy being released by the nuclear reactions than was required to create the conditions for nuclear fusion.

Nuclear matter. Matter at pressures so great as to engage the strong and weak nuclear forces, overwhelming the Coulomb repulsion of ions.

Plasma. Matter at high enough temperatures to be an ionized gas (typically at or above 10³-10⁶ K = 10^–1-10² eV): electrons released (ionized) from the atoms cause the gas to be electrically conducting.

Quantum matter. Matter exhibiting quantum behavior, often characterized by the atom or electron wavelengths (de Broglie wavelengths) exceeding the separation between atoms. This can occur at very low temperatures or very high densities, or both low temperatures and high densities (see Figure 1-3).

Radiative plasma. Ionized matter for which the radiation pressure is comparable to or greater than the thermal pressure.

Relativistic plasma. Ionized matter for which the electron velocities (thermal velocity at high temperature, or Fermi velocity at high density) are a significant fraction of (e.g., more than ~1/10) the speed of light.

Warm matter. Matter typically at “classical” conditions, neither quantum nor thermonuclear, roughly at temperatures of 10²-10⁶ K ≈ 1-100 eV energies. Warm dense matter is the regime of this matter above ~100 GPa at which several energy scales are comparable, including the Coulomb, thermal, Fermi, and plasmon energies. Traditional approximations used in plasma or condensed-matter physics may not be reliable in this regime.

Weakly coupled plasma. Plasma for which the electrostatic (Coulomb) energies of the ionized gas are less than the thermal and/or quantum statistical (Fermi) energies. Strongly coupled plasma indicates the opposite condition of electrostatic energies exceeding thermal and Fermi energies.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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BOX 1-4
Scientific Disciplines of High Energy Density Science

High energy density science lies at the intersection of many research domains, combining the concepts and terminology of the following and other scientific disciplines.

Astrophysics
Atomic and molecular physics
Chemical physics
Computational chemistry, physics, and materials science
Condensed-matter chemistry and physics
Electromagnetism
Fluid dynamics
High-pressure research
Laser science
Low-temperature physics
Magnetohydrodynamics
Materials chemistry and physics
Mineral physics
Nuclear physics
Optics
Planetary geophysics
Plasma physics
Pulsed power
Quantum mechanics
Radiation hydrodynamics
Statistical/thermal mechanics

committee is essential to (1) advancing the underlying science and (2) assuring that the workforce continues to maintain its high level of expertise in the future. A complementary, congressionally mandated study on HED research for stockpile stewardship was separately prepared in response to a request in the 2019 National Defense Authorization Act (NDAA). The present study is entirely about open science, engineering, and technology and did not consider classified topics or materials.

STATEMENT OF TASK AND IMPLEMENTATION

This study is in response to Section 3137 of the 2020 NDAA (Public Law 116-92) requesting that the NNSA engage the National Academies to produce an unclassified, publicly available assessment of recent advances and the current status of research in the field of HED physics. The statement of task as determined by the NNSA and the National Academies follows.

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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Statement of Task

The National Academies shall establish a committee that will articulate the recent advances, status, and future directions of high energy density (HED) physics in the United States. The committee will consider HED physics as the physics of matter and radiation at energy densities exceeding 1 × 10¹¹ J/m³ or other temperature and pressure ranges within the warm dense matter regime. It will include a particular focus on HED material phases, plasmas atypical of astrophysical conditions, and conditions of interest to the National Nuclear Security Administration (NNSA). The committee will then develop a report that will:

Assess the progress and achievements in HED physics over the last decade, including theory, computation, modeling and simulation, driver development, instrument development, emerging technologies, analytical methods, and target fabrication.
Identify major scientific gaps and potential new directions in areas of modeling, simulation, instrumentation, and target fabrication that offer the most promising near- and mid-term investment opportunities.
Identify challenges that the field may face over the next decade in realizing those opportunities and offer guidance for addressing them, including, but not limited to, investment level, interagency collaboration, research tools, and infrastructure.
Evaluate the role of HED physics in developing an expert workforce for NNSA and assess whether changes in resources, scientific focus, access to experimental facilities, or funding levels are necessary to meet nuclear security workforce needs in the coming decade.
Assess the state and recent advances made by other countries in HED physics and discuss the relative standing of the United States.

Implementation

To ensure that it would address the issues of the sponsor’s interest, the committee’s first meeting included a discussion with staff from Congress and the NNSA regarding the statement of task. (Appendix E summarizes the committee’s activities.) These groups, which the committee treated as sponsors, reaffirmed their original intent for an unclassified study resulting in a publicly available report that assesses fundamental HED science, complementing a classified study of programmatic efforts in HED science that had been previously completed for the NNSA (as requested in the 2019 NDAA).

Therefore, the committee studied the topics described in the statement of task, including “HED material phases, plasmas atypical of astrophysical conditions, and

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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conditions of interest to the National Nuclear Security Administration,” in the broader context of the fundamental science, rather than limiting itself to NNSA’s current programs. This context is reflected in the committee’s analysis of HED science as a whole, including areas of physics, chemistry, materials science, planetary science, astrophysics, plasma physics, and various technological applications. These are but representative examples of research directions that are currently having a high impact.

Similarly, in response to gaps recognized in the current programs, capabilities or research domains, the committee did its best to identify the most promising directions for future research that can fill those gaps and provide new scientific perspectives. This focus on actionable opportunities is needed to advance the science as rapidly and effectively as possible.

REPORT CONCLUSIONS AND RECOMMENDATIONS

The committee organized this report to contain leading and major recommendations, as well as more general recommendations, conclusions, and findings.

Leading recommendations, such as those in the Executive Summary (and immediately below), present the committee’s main, overarching vision for the future. Major recommendations, listed in the following subsection, represent key initiatives for advancing HED science and are further elaborated in the following chapters. The remaining recommendations are important for growth of the field in terms of collaborations, capacity, and culture that can sustain the NNSA mission and serve the nation into the future. Findings (typically, findings of fact) and conclusions (inferences based on information the committee has gathered) are also noted.

Key Conclusion and Leading Recommendations

Key Conclusion: An overarching challenge facing the NNSA is retention and recruitment of its expert workforce. The rapidly expanding influence of the private sector, developments around the world, and challenges to workplace climate put at risk the approach and laboratories in HED science research that have served the nation well since World War II.

Leading Recommendation: To strengthen its global leadership in high energy density (HED) science and address future national needs, the NNSA should exploit and enhance the capabilities of its flagship HED facilities (e.g., the National Ignition Facility, Z Pulsed Power Facility, and Omega Laser Facility) by establishing plans over the next 5 years for (1) extending, upgrading, or replacing those facilities; (2) increasing the promotion of forefront technology development, including in high-intensity lasers; (3) enhancing academic

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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capabilities and mid-scale facilities; and (4) broadening remote access to its major experimental and computing facilities.

Leading Recommendation: To enhance career pathways for high energy density science research at NNSA facilities, the NNSA should (1) broaden its current programs for achieving excellence through diversity, equity, and inclusion while improving workplace climate and (2) develop a strategic plan for balancing security and proliferation concerns with openness and accessibility, such as for collaborators internationally, and with academia and the private sector.

Major Recommendations

Major Recommendation: The NNSA should work with the academic and national laboratory user community, relevant government agencies, and industry to develop a high-performance computing (HPC) strategy for high energy density science over the next 2 years. This strategy should include benchmarking and the verification and validation of codes, code comparisons, the close integration of simulations using HPC with experiments, co-development of hardware and software for the research community, open-source documentation of codes and experimental results in a standardized open format (e.g., to enhance use and effectiveness of machine learning and artificial intelligence tools), and an industry-relevant implementation plan.

Major Recommendation: The NNSA and the national laboratories should, in coordination with partner science agencies (e.g., including the Department of Energy’s Office of Science and the National Science Foundation), academia, and industry, set expectations for rigorous benchmark experiments that can provide solid foundations for multi-scale high energy density simulations. Particular emphasis should be given to characterizing material properties under extreme and non-equilibrium conditions, including conditions accessible at university- and mid-scale facilities, and develop a new generation of diagnostics that can take advantage of modern technology such as higher repetition rate (e.g., compact light sources) that access a range of time and length scales.

Major Recommendation: The inertial confinement fusion community should redouble efforts to focus on the underlying basic science to (1) achieve robust ignition and the maximum yield with optimal efficiency, (2) establish the best uses of laboratory burning plasmas, and (3) help identify the best path

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Suggested Citation:"1 High Energy Density Science: Understanding Matter at Extremes." National Academies of Sciences, Engineering, and Medicine. 2023. Fundamental Research in High Energy Density Science. Washington, DC: The National Academies Press. doi: 10.17226/26728.

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for future experimental and computational facilities. In particular, the sustainment of innovation and breakthrough research will require a careful balance between yield-producing and non-ignition experiments. Additionally, the NNSA should work with the relevant agencies (e.g., the Department of Energy’s Fusion Energy Sciences and Advanced Research Projects Agency–Energy and the National Science Foundation) and private industry to leverage research in inertial fusion energy and—where possible—partner in research areas of mutual interest.

READER’S GUIDE AND NOTE ON APPENDIXES

The report is organized into five chapters and several appendixes. Mirroring the study’s statement of task, Chapter 2 summarizes recent progress in the field; Chapter 3 points to opportunities to pursue, which also reflect current gaps; Chapter 4 considers the HED science workforce; and Chapter 5 addresses the international HED science landscape.

In seeking a balance with material of interest to a general audience, the committee placed into the appendixes those matters judged to be more relevant to specialists in the field. The committee considers the appendixes as integral to the report.