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Fundamental Research in High Energy Density Science (2023)

Chapter: 2 Recent Progress and Opportunities

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Suggested Citation:"2 Recent Progress and Opportunities." 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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2
Recent Progress and Opportunities

This chapter describes a selection of recent discoveries in high energy density (HED) science, highlighting advances in basic science and opportunities for contributing to society (see Table 2-1). The intent is to give a sense of the vibrancy of the field. The committee did not attempt to be comprehensive, and many additional important contributions are described in the research literature.

The topics are broadly grouped into the following four categories: (1) physics and materials, (2) origin and evolution of planets and stars, (3) chemistry, and (4) technology with societal impact. However, there is considerable overlap among the examples given, as well as the associated methods and applications. For example, a contribution in materials chemistry may depend on a breakthrough in the underlying physics and have implications for understanding the origins or evolution of the solar system. Results from both theory and experiment are intertwined, with only the briefest mention of the techniques employed (see Appendixes B and C).

Finding: HED science has produced spectacular advances in basic science (see, e.g., Table 2-1 and Box 2-3), with potential applications ranging from providing energy (nuclear fusion) to transmitting it efficiently (superconductivity).

Suggested Citation:"2 Recent Progress and Opportunities." 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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TABLE 2-1 Research Advances: Illustrative Examples Noted in Chapter 2

Research Area References
Physics and Materials: Matter Manipulation on the Quantum Scale

Fluid hydrogen, deuterium metallized on multiple platforms with application to giant planets

PRL 76 (1996) 1860
Science 348 (2015) 1455
Science 361 (2018) 677

Semi-metallic crystalline hydrogen

Science 355 (2017) 715
Nature Physics 15 (2019) 1246
Nature 577 (2020) 631

Metallization of helium

PRL 104 (2010) 184503
PNAS 105 (2008) 11071

Crystalline metallic oxygen characterized

PRL 102 (2009) 255503

Metallization of carbon

Science 322 (2008) 1822

Metallic oxides, silicate planetary dynamos

PRL 97 (2006) 025502
Science 338 (2012) 1330
Science 347 (2015) 418

Transparent sodium

Nature 458 (2009) 182

Carbon taken to TFD regime

Nature 511 (2014) 330

TPa diamond-anvil cell (DAC) experiments

Sci Adv 2 (2016) 12
RSI 89 (2018) 111501

TPa calibration via laser- and magnetic-compression experiments

Science 372 (2021) 1063

First Gbar (10-100 TPa) laboratory measurements

Nature 584 (2020) 51
Origin and Evolution of the Solar System

Iron opacity measured, theory and astrophysics applications

Nature 517 (2015) 56

Laboratory evidence for the magnetic field of the universe

Nat Com 9 (2018) 591

Hydrogen-helium immiscibility: chemistry, planetary implications

Nature 593 (2021) 517

Superionic H2O documented, implications for “ice” giant planets

Nature Phys 14 (2018) 297

First-principles equation of state (FPEOS) database

PRE 103 (2021) 013203

Applications to planetary interiors, evolution

ApJ 669 (2007) 1279
Redefining Chemical Bonds

Kilovolt (core-electron) chemistry

PNAS 104 (2007) 9172
PRL 108 (2012) 055505
Technology and Societal Impact

Significant fusion burn achieved in laboratory (>1 MJ yield)

PRL 129 (2022) 075001

Indirect-drive experiments as supported by simulations

Nature 601 (2022) 542-548

Direct drive yield enhancement with stat. modeling

Nature 565 (2019) 581-586

Magnetized liner ICF

POP 22 (2015) 056306
Suggested Citation:"2 Recent Progress and Opportunities." 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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Fast ignition concept demonstrated

Nature 412 (2001) 798

Near room-temperature hydride superconductors at HED conditions (>1 Mbar)

Nature 525 (2015) 73
PRL 122 (2019) 027001
PRL 126 (2021) 117003
ARCMP 11 (2020) 57

Quantum sensors measure stress tensor, magnetism at high P

Science 366 (2019) 1349, 1355, 1359

High-quality nanocrystalline diamond for ICF

PRE 106 (2022) 025202

Advanced X-ray and particle sources

Optica 4(10) (2017) 1298

Tunable laser plasma accelerator

Nature Physics 7 (2011) 862

NOTE: ApJ, Astrophysical Journal; ARCMP, Annual Review of Condensed Matter Physics; PNAS, Proceedings of the National Academy of Sciences; PRE, Physical Review E; PRL, Physical Review Letters; RSI, Review of Scientific Instruments; Sci Adv, Science Advances.

PHYSICS AND MATERIALS: MATTER MANIPULATION ON THE QUANTUM SCALE

Pressure Metallization and Un-metallization

One of the most dramatic effects of pressure is the transformation of many chemical elements and compounds into the metallic state, characterized by high electrical conductivity, as well as high opacity to and reflectivity of visible light. For example, hydrogen, helium, and oxygen—transparent, electrically insulating gases at ambient conditions—all transform to fluid, opaque (and reflective), electrical conductors at HED conditions of high temperatures and pressures.

Hydrogen is of special interest because it is the most abundant chemical element of the universe, and fluid metallic hydrogen is the primary constituent of stars and giant planets. The smallest atom of the Periodic Table, hydrogen has particular significance for theory as well as experiments, with the search for crystalline metallic hydrogen attracting considerable interest for the past 85 years (see Box 2-1).

Experiments using several distinct techniques, from gas-gun impacts and dynamically heated diamond cells to laser- and magnetic-driven compression, confirm that fluid hydrogen starts metallizing below 100 GPa at temperatures of about 3,000 K. Both experiments and theory indicated that the metallization pressure increases with decreasing temperature, and crystalline hydrogen has yet to be definitively metallized at room temperature, with recent room-temperature experiments now extending up to 500 GPa.

This last result is at odds with decades’ worth of theoretical expectation, which predicted that crystalline hydrogen should metallize more easily—not less easily—

Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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.
×

than fluid hydrogen. The discrepancy between theory and experiment appears now to be largely resolved, with modern theory in line with experimental results (see Table 2-1). However, the case of hydrogen illustrates both the challenge of theoretically predicting material properties, even for the simplest atom of the Periodic Table, and the necessity of leveraging theory and experiment together.

More generally, there is a close relationship between phenomena labeled as the metal-insulator transition of cold or warm-dense matter, continuum lowering in hot-dense plasmas, and electron localization of electrides in chemistry. The differences between these concepts and terminology have made it difficult to understand the relationships between these processes, and the transition between electrically insulating and conducting matter continues to be a rich topic for research in the high energy density realm.

Helium is important as the second-most abundant element of the universe, stars, and giant planets, and it is no-doubt the most challenging atom to metallize (its high ionization energy is a quantum effect). Still, fluid metallic helium has been produced and characterized in the laboratory, with theory revealing how its metallization differs from the high-temperature ionization associated with low-density helium plasmas. As discussed below, the more extreme pressure–temperature (P–T) conditions needed to metallize helium relative to hydrogen has important implications for planetary chemistry.

Diamond and even oxides (among the most common ceramic materials) are likewise found to transform to metals at terapascal pressures. The metallization of carbon, theoretically predicted and supported by experimental results, may limit diamond-anvil cells to maximum pressures of about 1 TPa.

The reason that pressure tends to metallize elements and compounds is that as atoms are squeezed together, their electrons tend to avoid each other, both because the negatively charged electrons repel one another and for quantum mechanical reasons (see Figure 2-1). The atomic orbitals are thereby smeared out, such that the atoms’ outer electrons can move from one ion to the next (electrons become “delocalized”). The material becomes electrically conducting, andas described belowcan even become superconducting.

Theory predicted that this simple explanation of metallization can be reversed, however, in that pressure can also cause some metals to become nonmetallic. The idea is that compression shapes the spatial distribution of electrons, such that electron clouds are piled up between the ions (see Figure 2-2). This change in electron distribution amounts to creating an ionic bond.

The theoretical prediction of pressure-induced electron “localization” was dramatically confirmed with the observation that sodiumnormally considered among the simplest of metals at ambient conditionsbecomes transparent and electrically insulating by 200 GPa pressure (see Figure 2-3). Effectively, Na transforms to Na-e (electride), analogous to the salt NaCl but with the “anion” being an

Suggested Citation:"2 Recent Progress and Opportunities." 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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Image
FIGURE 2-1 As atoms are brought together, their outer electron clouds touch (left) and join to form a metallic state (right). Each atom consists of a positively charged nucleus (dark blue point) surrounded by negatively charged electron orbitals (blue and green). At high enough compression, the outer orbitals are smeared into a sea of moving, “delocalized” electrons (green at right) that can carry electrical currents between the ions (blue).
SOURCE: Courtesy of R. Jeanloz.
Image
FIGURE 2-2 Compression causes electrons to pile up between ions (atoms with nuclei), thereby pinning down the electrons and transforming the metal to a non-metallic state. Each atom consists of a positively charged nucleus (dark blue point) surrounded by negatively charged electron orbitals (blue). At high enough compression, electrons become localized and concentrated between the ions (two clouds without nuclei, shown in blue near center), transforming the metal to an insulating electride (analogous to a salt).
SOURCE: Courtesy of R. Jeanloz.

electron cloud (e) rather than an ion (Cl). Theory predicts that other elements, such as aluminum, can exhibit similar transitions from metal to insulator and then transform back to metals again at yet higher compressions. In fact, such behavior might be ubiquitous for elements and compounds at sufficiently high pressures, opening a new chapter in HED matter research, with pressure being used to create and mold new chemical bonds, as guided by theory.

Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Image
FIGURE 2-3 Visible light can be seen passing through transparent sodium (Na) at 199 GPa (at right). Sodium is normally a metal opaque to visible light at ambient conditions and at lower pressures—for example, at 120 GPa (reflective of visible light) and 156 GPa (no longer reflecting light), respectively, in the images at left and center. The sample is observed through a diamond-anvil cell at room temperature, with the diamond tip being about 40 µm across. In the right image, light reflecting off some of the diamond facets is seen on the left side of the image.
SOURCE: Reprinted by permission from Springer Nature: Y. Ma, M. Eremets, A. Oganov, et al., 2009, “Transparent Dense Sodium,” Nature 458:182-185; © 2009.

Gigabar Compression and Ionization of Electron Shells

It was a major technical breakthrough nearly 50 years ago, when development of diamond-anvil cells made it possible to study materials at static pressures of 100 GPa in the laboratory. Dynamic-compression experiments were already probing the 0.1-0.5 TPa (100-500 GPa) range over time periods up to 1 µs, but now it was possible to create new materials and characterize them for arbitrarily long periods of time at the megabar (million-atmosphere) pressures that alter chemical-bonding energies; that is, at the onset of the HED regime.

Static experiments now reach 1 TPa (1,000 GPa), and the most accurate dynamic experiments use magnetic- or laser-driven planar compression to achieve 2-10 TPa, respectively. These experiments provide some of the first experimental checks of “statistical atom” theory that is widely used to understand the interiors of stars and giant planets, confirming as well as extending these atomic models (e.g., Thomas-Fermi-Dirac theory).

It is therefore a spectacular accomplishment that experiments have recently been extended to the 10-100 TPa (gigabar, or billion-atmosphere) realm by way of spherically convergent laser-driven experiments. This means that laboratory measurements are now being made at atomic-scale pressures (29 TPa), conditions overwhelming the quantum forces that determine atomic structure at ambient conditions (see Box 2-2). The combined P–T ionization of atoms’ different electron-

Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×

orbital shells can thus be quantified and directly compared with long-standing theoretical predictions (see Figure 2-4).

Finding: Many experimental methods can now access the HED regime exceeding 0.1 TPa (1 million atmospheres pressure) at which chemical bonding is changed, from static compression using diamond-anvil cells to mechanical impact and laser- and magnetic-driven dynamic compression.

Finding: High-quality measurements are possible well into the HED regime, to 0.5-1 TPa statically, and to 1-10 TPa through shock or ramp loading; pioneering laboratory measurements are exploring the 10-100 TPa (0.1-1 billion atmosphere) range of pressures at which atomic structure is altered and new types of chemical bonding emerge.

Image
FIGURE 2-4 Equation of state measurements toward 100 TPa (1 Gbar = 1,000 Mbar) conditions (red) show good agreement with modern theory for pressure-ionized matter (DFT or “Density Functional Theory”: solid black curve). Spherically convergent experiments on polystyrene significantly extend prior laboratory measurements (blue) and can discriminate between more modern theory (DFT curve) versus older theory (TFD or “Thomas Fermi Dirac”: dash-dot black curve) describing the effects of ionization, reflected here in the compression curve oscillating and turning back on itself at pressures above 10 TPa. AA and KS represent different methods—that is, “average atom” and “Kohn-Sham,” respectively.
SOURCE: Reprinted by permission from Springer Nature: A.L. Kritcher, D.C. Swift, T. Döppner, et al., “A Measurement of the Equation of State of Carbon Envelopes of White Dwarfs,” Nature 584:51-54; © 2020.
Suggested Citation:"2 Recent Progress and Opportunities." 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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ORIGIN AND EVOLUTION OF THE SOLAR SYSTEM

Iron Opacity and the Sun’s Composition

The properties of ions are important for many applications, from chemistry to astrophysics. A notable example is the absorption of light by such atoms that have lost some or all of their electrons, whether as a high-temperature plasma or as pressure-ionized matter (e.g., Figures 2-1 and 2-4). Although important, this ion opacity is difficult to calculate theoretically, and historically even more difficult to measure at HED conditions. The difficulty of calculations, which require not only extensive atomic structure but also dense-plasma effects, is compounded by the relatively few atomic physicists who focus on HED conditions.

The recent experimental measurement of iron opacity was therefore widely considered a major contribution, not only as a technical accomplishmentthe fact that the opacity of ionized iron could be successfully measuredbut also because it helps address a problem in understanding the Sun’s composition. (See Table 2-1.) Spectroscopic determinations of the solar iron abundance appeared to be in conflict with theoretical analysis of the Sun’s observed oscillations, unless previous estimates of opacity were too low. As it happens, the new laboratory measurements revise past estimates upward, but await confirmation—more measurements are surely warranted to follow up on this pioneering work—and underscore the importance of maintaining and even developing a variety of independent capabilities for both theory and experiments. Still, the new opacity measurement has had considerable impact across the scientific disciplines because there is a close linkage between the composition of a star and its planets, so the result can play a key role in advancing current understanding of planetary origins and evolution as applied to thousands of extra-solar planets.

Planet Formation and Evolution

The origins and evolution of Jupiter and Saturn are also of special interest because these planets are thought to have formed early in solar system history, potentially offering Earth protection from giant, planet-destroying impacts as the planet formed and grew.

One of the major open questions about the large planets concerns the rates at which Jupiter and Saturn lose heat and evolve over their multi-billion-year histories. These planets consist mainly of hydrogen and helium. Theories developed over the decades show that the cooling rates are influenced by the heavier element helium “raining” downward through the lighter hydrogen, thereby releasing gravitational energy that heats the deep interior as the planet rearranges itself with heavier elements concentrating toward the center.

Suggested Citation:"2 Recent Progress and Opportunities." 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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Astrophysicists cannot do experiments in space. On the other hand, HED experiments can be carried out under the same temperature and pressure conditions present in the cores of these large planets. In particular, experiments over the past 5 years show that there is a range of pressures (hence depths) within Jupiter and Saturn at which hydrogen is metallic but helium is not; due to the difference in chemical bonding, the two fluids do not mix but instead separate, like oil and water. The recent experiments provide laboratory evidence for the proposed unmixing and raining out of helium within the giant planets, confirming that this process plays an important role in the long-term evolution of the giant planets and explaining why Saturn puts out more heat than it gets from the Sun.

Another type of material chemistry is illustrated by the formation and characterization of a “superionic” form of H2O ice at high pressures in the laboratory. First predicted by theory, this form of ice has the protons (H+ ions) moving readily between rigidly packed oxygen ions. Ion conductivity is central to electrochemistry and the technology of modern batteries, and it is significant that such phenomena have now been experimentally documented at high compressions. Indeed, it appears that the superionic form of water ice is a major constituent of such planets as Neptune and Uranus.

High-pressure experiments have prompted theoretical efforts to systematically characterize materials in support of modeling the formation, evolution, and current constitution of planets in our solar system and beyond. The new First Principles Equation of State (FPEOS) compendium is a case in point of a new contribution from theory fostering powerful new collaborations between astronomers and the HED science community. (See Table 2-1.)

Another perspective on HED science addresses the role of impacts during the gravitational accumulation of mass as a planet forms. Typical orbital velocities around the Sun show that impacts associated with planet formation generate terapascal-scale pressures; that is, create HED conditions at the planet’s growing surface.

In short, planet formation is a violent process, with comparable-size bodies impacting each other as the planet grows. For example, it is thought that the Moon was splashed out of Earth by a large (roughly Mars-sized) body impacting our planet after it had largely formed, thereby melting much of the planet and all of the material from which the Moon was formed. The properties of the dense rock-metal plasma from which the Earth-Moon system then emerged can only be determined by HED theory and experiments.

Indeed, the Moon’s huge impact basins provide a record of major impacts during and shortly after the Earth-Moon system was formed (Earth’s ongoing geological activity has mostly wiped out evidence of early large impacts on the planet). The Moon’s craters and impact basins provide a record of the tail end of planet formation, and it is evident that Earth was subjected to large enough impacts

Suggested Citation:"2 Recent Progress and Opportunities." 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 0.5 billion years or more after its formation that early-formed oceans of water would have been boiled away, perhaps many times over.

Such “impact sterilization” seems likely for the early history of Earth and other planets that have the potential to harbor life. Evidence from geochemistry and other domains indicate that life may have already been forming on early Earth, leading to the hypothesis of impact “frustration” of life—that impact-induced HED conditions wipe out emerging life during planet formation. If so, the suggestion is that life emerged rapidly and perhaps often on Earth, despite the challenging HED conditions being commonly (and repeatedly) imposed on its surface. Thus, HED science has a role to play in understanding the early emergence of life on Earth, and perhaps for other planets more generally.

REDEFINING CHEMICAL BONDS

Low-Temperature Plasma and Electrochemistry

The electron distributions between atoms define the chemical bonds in a molecule, solid, liquid, or gas, determining the physical and chemical properties of matter. Not only do HED conditions of high pressure and temperature reshape the distribution of electrons between atoms, relative to ambient conditions, but the electromagnetic fields used to achieve these conditions in laboratory experiments also contribute to changing the electron clouds around atoms.

Large electric and magnetic fields applied in HED experiments move the electrons around within a sample, with an intense laser pulse, for example, tearing electrons away from the atoms in a form of ionization associated with preheat; light (electromagnetic radiation) incident on an atom is absorbed by the electron cloud around the atom’s nucleus, and the effect is to accelerate the electrons and leave ions behind.

The essence of HED conditions is thus to form and reshape the internal electron distribution within matter, leading to the electrons and ions behaving as plasmas that are to be controlled and molded. Indeed, the 2021 plasma decadal study1 highlights the potential for low-temperature plasma (LTP) to revolutionize chemistry, as electrons are torn off atoms and induced to form new chemical bonds. Low temperature in this case refers to the thousand-fold heavier ions moving slowly relative to the electrons, with the prospect of forming novel molecules and compounds through new kinds of chemical reactions.

The perspective of LTP chemistry offers an opportunity to use electron-ion separation in the HED regime as a means of creating new chemical bonds, and

___________________

1 National Academies of Sciences, Engineering, and Medicine, 2021, Plasma Science: Enabling Technology, Sustainability, Security, and Exploration, Washington, DC: The National Academies Press.

Suggested Citation:"2 Recent Progress and Opportunities." 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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therefore new materials. This is an entirely unexplored aspect of chemistry at extremes.

TECHNOLOGY AND SOCIETAL IMPACT

Fusion in the Laboratory

An extremely pressing issue facing humanity in the upcoming decades is that of generating enough energy for sustainable prosperity. The world is in need of a plentiful, carbon-free source of energy. Nuclear fusion, the same process powering the Sun, has the technical potential to offer such an energy source.

One pathway for insight into the fusion process may be through laboratory inertial confinement fusion (ICF), and evidence of significant energy being released in ICF experiments is among the most exciting recent breakthroughs in HED science. For the first time, more energy has been produced by nuclear fusion than was directly accessible to the material being compressed. That is, self-heating due to nuclear reactions was clearly demonstrated, and the ICF sample came close to the point of ignition, defined as yielding more energy than delivered to the whole target by the laser (see Figure 2-5). This breakthrough is the result of advances in scientific understanding, computer simulation, diagnostics, industry-led manufacturing, Discovery Science programs in basic research, and more.

Both laser- and magnetic-driven ICF have seen major progress over the past decade, to the point that the onset of nuclear ignition is now being approached. The amount of nuclear energy released increases rapidlyby a factor of 10 to

Image
FIGURE 2-5 Energy produced by laser-driven fusion experiments have dramatically increased over the last few years, as indicated by the size (area) of circles, with nuclear output in several experiments now matching or exceeding the amount of energy delivered to the sample material (200-300 kJ). Recent experiments at the National Ignition Facility have generated up to 1.3 MJ energy yield for a total laser input of 1.8 MJ (dashed circle), approaching the ignition criterion whereby energy output exceeds the total input of energy.
SOURCE: Courtesy of Lawrence Livermore National Laboratory.
Suggested Citation:"2 Recent Progress and Opportunities." 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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100once ignition is surpassed, thereby opening major new prospects for scientific research as well as for society.

In particular, through a renewed emphasis on fundamental science, the research community will be in a position to start optimizing ICF so as to explore its prospects as a source of energy for society. If successfully developed, fusion energy has the benefits of being carbon-free and based on readily available materials (i.e., the deuterium and tritium forms of hydrogen). It is well suited to an electric-based society, which many see as being the best aligned with future technologies and the most sustainable from resource, climate, and environmental perspectives.

Laboratory-based nuclear fusion would also provide intense sources of particles (e.g., neutrons) and a HED area of research. For example, 10-100 PPa = 1016-1017 Pa = 100-1,000 Gbar pressures are associated with nuclear fusion, up to 1 million-fold times the onset of the HED regime (see Figure 2-6). A new era of HED science and applications would thereby be initiated.

Finding: Laboratory-based ICF is experiencing major breakthroughs that can be enhanced by focusing on the underlying science.

Finding: NIF is a young facility for scientific research, with over half its diagnostics put in place in the past 5 years, showing that rapid progress can be made with suitable investment in experimental measurement and computational analysis.

Finding: NIF, Omega, and Z, the major NNSA HED laser and pulsed-power facilities, are producing breakthrough science, including through their external user programs for Discovery Science.

Targets

One of the key enabling technologies for performing HED science is the targetry, which is required in virtually all experiments. Target advancements can drive new scientific discoveries in and of themselves. Most recently, the target quality played a significant role in the ignition shot at the NIF. Highly specialized target fabrication laboratories are required, supplied by specific research groups or industrial partners such as General Atomics. Despite incredible advances in target design, fabrication, and metrology, a significant number of challenges exist for developing targets for the upcoming decade. Most pressing is the need to adapt to repetitive systems; current flagship experiments operate at a single-shot capacity, and nearly all aspects of targetry will need to be rethought. Additionally, materials such as low-density foams, amorphous high-density materials, and high-shock-impedance windows need to be developed to facilitate current experiments and enable future

Suggested Citation:"2 Recent Progress and Opportunities." 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 2-6 Energy produced by nuclear fusion is shown as a function of peak pressure achieved in inertial confinement fusion (ICF) experiments at the National Ignition Facility for different target configurations: “Low foot”: low initial laser-driven shock (light blue); “High foot”: high initial laser-driven shock (light green); HDC: high-density carbon (diamond) capsule (gold); Big foot: high initial laser-driven shock followed by large compression (purple); HyE: hybrid design with increased capsule radius (red, for 1050 and 1100 µm inner radius diamond ablators); Iraum: reshaped hohlraum design (purple). The energies are effectively measured, whereas the pressures are derived from indirect measurements and calculations (1 Gbar = 100 TPa = 1014 Pa).
SOURCE: O.A. Hurricane, P.K. Patel, R. Betti, et al., “Physics Principles of Inertial Confinement Fusion (ICF) and Status of the U.S. Program,” LLNL Report LLNL-JRNL-819317-DRAFT, submitted to Reviews of Modern Physics, 2022.

ones. Advances in fabrication, particularly in additive manufacturing, are set to solve numerous issues with complex, multi-component targets where joints and welds can introduce defects or leaks. Finally, metrology capabilities are needed on smaller spatial scales, and to see target composition and quality throughout the entire target. Just as there is a need for targets to be produced with ever smaller features and surface roughness, the tools required to characterize those targets also must increase in capability.

Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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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Finding: All major facilities have extensive requirements for future target—from design, to fabrication, to metrology—particularly to adapt to higher repetition rates.

Superconductivity

Superconductivity, fascinating to physicists because this quantum effect allows for the free flow of electric currents, was for many decades limited to low temperatures (see Figure 2-7). For society at large, superconductivity at easily accessible conditions would revolutionize the economy and technologies, transforming sectors ranging from energy production and transmission to medical imaging and transportation.

Room-temperature superconductors would be all-important for an electrified society of the future, so the race is on to find ways of synthesizing such metals at conditions near ambient (as has been done for diamond, for example, see Box 1-2). It is unknown if this is feasible, but the prospect exists that complex compounds containing many different elements in the types of intricate crystal structures often developed at high pressures could have the desired superconductivity and still be made in bulk quantities at low pressures.

The significance of HED science is that it greatly enhances the quantum regime of superconductivity, with several room-temperature superconductors having now been discovered at high pressures. In particular, hydrogen-rich compounds are found to exhibit superconductivity at the highest of temperatures, consistent with current HED understanding of hydrogen itself—these crystalline metal hydrides apparently exhibit some of the exotic quantum properties expected for metallic atomic hydrogen. HED experiments guided by theory and machine learning (to handle the multi-dimensional problem of characterizing complex compounds) hold promise for this research, which is at the cutting edge of both basic science and applied technologies.

Quantum Sensors

Recent developments of quantum sensors are supporting efforts to achieve novel conditions and make new materials in HED research. For instance, nitrogen-vacancy pairs (NV centers) in diamond can be used to measure the magnetism and detailed state of stress (stress tensor) in samples contained inside diamond-anvil cells at HED pressures. Magnetism can be a key indicator of superconductivity, thus helping to characterize the high-pressure materials of interest and to provide experimental data for comparison with theory.

That these sensors can map out the stresses across samples is exciting in its own right, because shear and normal stresses can now be quantified and potentially

Suggested Citation:"2 Recent Progress and Opportunities." 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 2-7 Materials exhibiting superconductivity—perfect conduction of electric currents—at room temperature have finally been discovered after more than a century of study. The figure shows experimentally measured superconducting transition temperatures as a function of time, with room-temperature superconductors having been discovered above 180 GPa pressure by 2020. Although currently synthesized at high pressures, research suggests that it is possible to produce room-temperature superconductors at or near ambient conditions.
SOURCE: C.J. Pickard, I. Errea, and M.I. Eremets, 2020, “Superconducting Hydrides Under Pressure,” Annual Review of Condensed Matter Physics 11:57-76.
Suggested Citation:"2 Recent Progress and Opportunities." 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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controlled as a function of pressure and deformation. For the first time, there is experimental access to determining the ultimate strength of materials under a wide range of loading conditions, raising the possibility of fundamentally advancing understanding of how and why materials deform and break.

ROLES OF THEORY, SIMULATION, AND EXPERIMENT

This chapter closes with a note about the complementary roles of theory, simulation, and experiment, all of which are necessary and depend on the integration of HED science featured throughout this chapter.

Suggested Citation:"2 Recent Progress and Opportunities." 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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Two primary goals of theory and micro-scale computer simulation are to interpret the results of experiments and to predict the existence and properties of new materials, including those potentially having important technological applications.

Rigorous experiments, coupled with the kinds of advanced diagnostics detailed in the National Diagnostics Plan,2 can validate these predictions and provide insights into how to improve theory and obtain more accurate results over a broad range of conditions—including those that are not yet experimentally accessible. There may also be cases in which experiments uncover gaps in theory, revealing unexpected new phenomena, states, or processes. In such instances, it is critical to validate and compare results from multiple theoretical and experimental approaches so as to improve existing theoretical approaches.

Rooted in quantum mechanics, atomistic theories and ab initio simulations provide a fundamental basis for advancing the scientific understanding of matter, as well as for developing new materials for society. But HED science is not only concerned with the atomic-scale properties of matter at extreme conditions. Multiscale, multi-physics simulations that model entire experiments and applications are critical to the field’s contributions to fundamental science and to applications ranging from laboratory nuclear fusion to planetary geophysics and astrophysics.

These simulations link experimental-scale predictions to the properties predicted by quantum mechanics at the microscale and to molecular dynamics, particle-in-cell, and kinetic models that can capture effects like mix, turbulence, and laser-plasma interaction. They may be hydrodynamic codes that depend on equations of states (EOS) to capture compression and bulk thermodynamic evolution of matter, and they may include radiation transport and/or non-local, non-equilibrium effects, or they may include descriptions of a material’s interactions with various sources of compression and heating, such as laser absorption or magnetohydrodynamics. The increasing fidelity, flexibility, and reliability of these critical toolsand the underlying experimentsis one of the triumphs of HED science.

Finding: The integration of approaches from theory, simulation, and experimentation is critical in the HED regimes, for which the multiplicity of time and lengths scales leads to challenges in understanding basic material properties and macroscale system behavior.

For more information about the tools and facilities used in HED science, as well as more technical information about ICF, see Appendixes A through C.

___________________

2 S. Ross, 2020, “The ICF National Diagnostic Plan (NDP) September 2020,” https://doi.org/10.2172/1671177.

Suggested Citation:"2 Recent Progress and Opportunities." 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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Page 19
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 20
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 21
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 22
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 23
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 24
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 25
Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 27
Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 29
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 30
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 31
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 32
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 33
Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 35
Suggested Citation:"2 Recent Progress and Opportunities." 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.
×
Page 36
Suggested Citation:"2 Recent Progress and Opportunities." 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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Suggested Citation:"2 Recent Progress and Opportunities." 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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Page 38
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High energy density (HED) science has critical applications for society from fusion energy to sustaining the US nuclear deterrent, while also contributing to broader scientific questions such as understanding planets and their origins.

The next decade of HED science will be instrumental to growing our understanding and in the development of new technologies and processes. Fundamental Research in High Energy Density Science identifies key challenges and science questions for the field for the coming decade and proposes ways to address them.

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