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3 Plasma Physics at High Energy Density INTRODuCTION High-energy-density (HED) plasma physics is the study of ionized matter at extremely high density and temperature. Quantitatively, HED physics is defined to begin when matter is heated or compressed (or both) to a point that the stored energy in the matter exceeds about 1010 J/m3, the energy density of solid material at 10,000 K (~1 eV), which corresponds to a pressure of about 105 atmospheres or a light intensity 16 orders of magnitude greater than the Sun’s intensity at Earth. By this definition, matter under HED conditions does not retain its structural integrity and cannot be sustained or contained by ordinary matter or vessels.1 Thus HED matter must be produced transiently in terrestrial laboratories, although it is common in high-energy astrophysics under both steady-state and rapidly chang- ing conditions. For example, the center of the Sun, where fusion reactions have been converting hundreds of millions of metric tons of hydrogen into helium each second for billions of years, is estimated to have an energy density of 2 × 1016 J/m3 (15 million K and 150 g/cm3). Supernova explosions are obvious examples of transient HED astrophysical plasmas. Small-laboratory HED plasmas include the nanometer-sized clusters irradiated by very high intensity lasers, and the ~1 µm, 10 million K, near-solid-density plasmas produced when dense plasma columns 1 The committee has chosen 1010 J/m3 as a reasonable lower limit of HED matter, even though it is an order of magnitude lower than the value chosen in the NRC report Frontiers in High Energy Density Physics: The X-Games of Contemporary Science (Washington, D.C.: The National Academies Press, 2003), in order to include solid-density material at 1 eV. 

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Plasma science  carrying a high current implode unstably to form short-lived micropinches. By contrast, the magnetic confinement fusion plasmas discussed in Chapter 4 of the current report are limited to perhaps 106 J/m3, which allows them to be confined by magnetic fields produced by steady-state electromagnets that are supported by common structural materials. What Constitutes HED Plasma Physics? The lowest temperature end of HED parameter space is condensed matter pushed beyond its limits, such as occurs when matter at room temperature is sub- jected to 1 million atm. At temperatures above a few thousand kelvin, any mate- rial becomes at least partially ionized, so HED physics is necessarily HED plasma physics. Such “warm dense matter” lies at the intersection of plasma science and condensed matter/materials science. At the opposite end of the parameter space are plasmas in which particles are at such high temperatures that relativistic effects must be considered, an exotic state of matter thought to exist in sources of extra- galactic gamma-ray bursts as well as in the plasmas produced by lasers focused to very high intensity (more than 1020 W/cm2) on solid surfaces. Some of these states are illustrated in Figure 3.1. This report has been prepared on the heels of two related reports that con- ducted extensive scientific assessments of high-energy-density physics. The just mentioned NRC report Frontiers of High Energy Density Physics: The X-Games of Contemporary Science is an important background reference for this chapter. That report, hereinafter referred to as The X-Games Report, essentially laid the ground- work for defining the HED field and identified key research topics. In 2004, the National High Energy Density Physics Task Force delivered the report Frontiers for Discovery in High Energy Density Physics to the Physics of the Universe Interagency Working Group of the White House Office of Science and Technology Policy (here- inafter called Frontiers for Discovery). Together, these reports provide an elegant and comprehensive survey of the field. The areas covered in this chapter are illustrative and intended to highlight selected research opportunities in HED physics, not to provide a complete summary of all of the compelling research thrusts. Additional research topics, including quark-gluon plasmas and some aspects of laboratory astrophysics, are discussed in more detail in those reports. Enabling Technologies and HED Science in Context As was discussed in The X-Games Report, the portion of plasma parameter space accessible to the scientific community in the laboratory has been expanding to higher and higher energy density because of new technologies developed and facilities built to study matter under conditions that are reached in nuclear explo-

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Plasma Physics high energy density  at R oom temperature 1,000 FIGURE 3.1 HED plasma space showing sample HED plasmas. sions. Some of the highest power facilities used for HED experiments in the United States are listed in Table 3.1. The widespread laboratory study of HED plasmas enabled by these facilities exemplifies the point made in Chapter 1—namely, that new plasma regimes have become the subject of plasma physics research in the past decade. These facilities, including powerful lasers and pulsed power machines, are 3.1 enabling plasma physicists, materials scientists, and atomic physicists to investigate states of matter that were not previously accessible for in-depth study in the labo- ratory. This trend will continue as facilities now under construction, also listed in Table 3.1, become operational during the next few years. The development of high-power lasers and pulsed power technology was originally driven by the quest for inertial confinement fusion (ICF) in the first case and laboratory-scale nuclear weapon effects testing in the second. At present and

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Plasma science  TABLE 3.1 Selected HED Facilities Peak Type of Energy Power/ Energy Repetition Facility Machine Delivered Current Delivery Rate Location Status Large-scale ~1 shot/ National Laser 1.8 MJ 500 TW Ultraviolet Lawrence To be Ignition photons 3 hr Livermore completed in Facility National 2009 Laboratory ~1 shot/ ZR Pulsed 3.5 MJ 350 TW/ Electric Sandia National To be power 26 MA; current; day Laboratories completed in 4 Mbar magnetic 2007 pressure ~30 kJ ~1 shot/ OMEGA/ Lasers 30 TW/ IR/ Laboratory Operational/EP OMEGA EP long pulse ultraviolet 3 hr for Laser to be completed 3 kJ short EP 1 PW photons Energetics, in 2008 pulse Rochester Linac X-ray free- 1 mJ 10 GW X-ray 120 Hz Stanford Linear To be Coherent electron photons Accelerator completed in Light laser Center 2009 Source Mid-scale ~1 shot/hr Titan Laser 200 J 400 TW Infrared Lawrence Operational photons Livermore National Laboratory ~1 shot/ Z-Beamlet/ Laser 1 kJ + 500 1 TW/1 Optical/IR Sandia National Operational/1 PW Z-Petawatt J short PW photons 3 hr Laboratories to be completed pulse in 2007 ~1 shot/hr Texas Laser 250 J 1 PW Infrared University of To be Petawatt photons Texas completed in 2007 ~1 Hz L’Oasis Laser 4J 100 TW Infrared Lawrence Under upgrade photons Berkeley National Laboratory ~1 shot/ Hercules Laser 20 J 800 TW Infrared University of To be photons min Michigan completed in 2008 ~100 kJ ~3 shots/ Cobra Pulsed 1 TW/1 MA Electric Cornell Operational power current day University Nevada Pulsed 100 kJ 2 TW/1 MA Electric 1 shot/day University of Operational; Terawatt power and 35 J laser +100 TW current + Nevada laser under laser laser IR photons construction NOTE: Not included in this table are several important 10- to 100-TW lasers in use and under development at university and national laboratory facilities—for example, the 100-TW Diocles laser facility at the University of Nebraska at Lincoln. ZR, Z refurbishment; EP, extended pulse; IR, infrared.

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Plasma Physics high energy density  at for the last decade, the principal purpose of the research carried out at such facili- ties has been to help assure the safe and reliable operation of our nation’s nuclear weapons stockpile in the absence of full-scale nuclear testing; this program of re- search is called the Stockpile Stewardship Program (SSP) and is sponsored by the National Nuclear Security Administration (NNSA). The vital connection between understanding HED states of matter and stockpile stewardship will be discussed in the next section. Inevitably, the accessibility of a whole new range of conditions of matter means that new experiments will produce unanticipated results, some of which will have important implications for stockpile stewardship and many others of which will find applications in basic physics and in practical applications far removed from direct relevance to stockpile stewardship. It is the excitement of entering unknown regions of parameter space with these facilities that engender the committee’s enthusiasm for HED plasma science, just as it did for the authors of The X-Games Report. Although that report is more comprehensive than this committee can be in its discussion of HED science opportunities, this committee will take advantage of the fact that this is a fast-moving field. Just since 2003, there has been great progress in several areas of HED plasma studies, including stockpile stewardship, ICF, and plasma wake field accelerators, as well as in basic HED science, some of which will be highlighted in this chapter. In addition to depending on the aforementioned new facilities, the advances discussed depend on recent developments in large-scale computer simulation ca- pability and the continuing development of diagnostic capabilities with exquisite temporal and spatial resolution. Although the state-of-the-art facilities and diag- nostic systems at the NNSA-sponsored laboratories (Table 3.1) are largely used for mission-oriented research, there is movement toward making them more available to the broad community of scientists interested in HED research. One approach is to reserve a small fraction of facility time for non-mission-oriented experiments. Another is to encourage university–national laboratory collaborations leading to novel experiments that can benefit both a laboratory mission and basic-physics- oriented university scientists. For example, at the Stanford Linear Accelerator Cen- ter (SLAC), a laboratory of DOE’s Office of Science, there are exciting new results on particle acceleration in laser- and particle-beam-driven, nonlinear wave–particle interaction experiments in HED plasmas. Continued progress on this front has the potential to shape future technology choices for the high-energy physics commu- nity. Such collaborations are potentially a good paradigm for NNSA to facilitate a broad range of HED science at its facilities. University-scale pulsed-power machines and high-intensity lasers (also listed in Table 3.1), albeit considerably lower power than those at the NNSA laboratories, already play an important role in broadening the progress of research in the HED science program. They enable testing novel ideas and carrying out non-mission-

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Plasma science 0 oriented HED plasma research, as well as training the next generation of HED plasma physicists, without having to interrupt the schedule of the larger NNSA- sponsored facilities. Both the larger facilities at the national laboratories and the smaller facili- ties at universities are providing a new window on nature by producing HED conditions that have not previously been studied, often leading to exciting, novel results. Quoting from The X-Games Report (p. x), “. . . research opportunities in this cross-cutting area of physics are of the highest intellectual caliber and are fully deserving of the consideration of support by the leading funding agencies of the physical sciences. . . . Such support . . . would greatly strengthen the ability of the nation’s universities to have a significant impact on this exciting field of physics.” Such support would also attract some of the brightest young scientists into the various subfields of HED plasma research and eventually to positions at the next generation of HED facilities that will soon be in operation. IMPORTANCE OF THIS RESEARCH HED plasma research in recent years has been largely driven by four applica- tions that can be represented as Grand Challenges: • Inertial confinement fusion (ICF). Can we achieve fusion ignition and, eventually, useful fusion energy from compressed and heated HED fusion plasma? • Stockpile stewardship. Can we understand the properties of the materials in nuclear weapons under weapon-relevant conditions, together with the operative physical processes, well enough to ensure that the safety, security and reliability of the nuclear weapons stockpile of the United States can be maintained in the absence of nuclear testing? • Plasma accelerators. Can we generate, using intense, short pulse lasers or electron beams interacting with plasmas, multigigavolt per centimeter electric fields in a configuration suitable for accelerating charged particles to energies far beyond the present limits of standard accelerators? • Laboratory plasma astrophysics. Can we better understand some aspects of observed high-energy astrophysical phenomena, such as supernova explo- sions or galactic jets, by carrying out appropriately scaled experiments to study the underlying physical processes and thereby benchmark the com- puter codes used to simulate both? Although these challenges will probably continue to be the main drivers for the research to be done in the coming decade, they have spawned many discoveries in several research areas in the last decade that provide additional research opportu- nities. These involve connections to a wide range of physics and technology areas,

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Plasma Physics high energy density 1 at including plasma, condensed matter, nuclear and atomic physics, laser and particle beam-physics and technologies, materials science, fluid dynamics and magneto- hydrodynamics, and astrophysics, all of which substantially enrich the intellectual content of HED plasma science. Economic and Energy Security The possibility of energy supplied by controlled fusion offers enormous po- tential economic security benefits through the energy-resource independence that would result for the United States and the rest of the world, as was discussed in Chapter 1. The ICF approach might offer a viable alternative to the international program in magnetic confinement fusion (see Chapter 4 and the discussion of ITER in Chapter 1). Although the enabling technologies for HED plasma generation were driven initially by research oriented to ICF and military applications and are now driven by the Stockpile Stewardship Program, areas as diverse as medicine and industrial manufacturing have been impacted by these technologies. For example, the unique way an intense femtosecond laser ablates material is now used in eye surgery and for cleaning out clogged arteries. In the realm of industrial manufacturing, intense laser ablation will soon be used to machine precise holes in jet turbine blades, laser-based extreme-ultraviolet light sources drive the latest generation of inte- grated circuit lithography, and the intense bursts of x-rays from laser-generated HED plasmas are now being used to characterize aerospace components. As the capabilities of short-pulse lasers become better known, it is likely that many more practical uses will be developed. National Security The study of HED plasmas has been an important element of the research port- folios of the nuclear weapons laboratories for 40 years or more. Until perhaps 20 years ago, when high-power laser facilities became available, essential parts of that research had to be carried out using underground nuclear tests; there was no alter- native method to address such physics issues as radiation transport and the physical properties of hot dense matter. In addition, an understanding of the effects of a nuclear explosion on nearby weapons and on both civilian and military electronics was achieved partially by testing components and subsystems of the weapons using high fluxes of x rays produced by pulsed power machines and partially by testing underground. The underground test moratorium 15 years ago was justified in part by the belief that rapidly advancing computer simulation capability, together with the anticipated new HED facilities, would make underground tests unnecessary for maintaining the safety, security, and reliability of our nuclear stockpile. NNSA’s SSP is intended to turn this expectation into reality. Some of the HED science issues

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Plasma science 2 that have been addressed in recent years as part of SSP are discussed in the next section. In the coming decade, with the continued aging of the nation’s nuclear stockpile and the continuing moratorium on testing them, HED science will play an even more important role in maintaining our national security. Intellectual Importance The coupling of HED plasma physics to several other subdisciplines of physics serves to broaden its intellectual impact well beyond its national security and ICF energy base. To summarize, • Studying the properties of warm dense matter brings together plasma re- search and condensed matter and materials research. • As temperatures increase, high atomic number HED plasmas bring plasma physicists together with atomic physicists to diagnose plasmas. • The fluid instabilities and turbulence in plasmas (ionized fluids) are very similar to their counterparts in ordinary fluids. • Nuclear physics contributes many diagnostic techniques to ICF, magnetic confinement fusion, and nuclear weapon studies. • Nonlinear wave–particle interaction in HED plasmas could lead to the next generation of high energy accelerators, affecting, in turn, high energy physics. • The principles of magnetohydrodynamics are central to understanding dense magnetized plasmas, such as wire-array z-pinches. • HED plasmas provide fundamental data required by astrophysicists and may be able to contribute to the interpretation of high energy astrophysical observations. The following paragraphs add more depth to these brief statements:2 • Atomic physics. HED drivers generate highly stripped, near-solid-density plasmas made of mid- and high-atomic-number atoms at temperatures of millions of degrees, with and without magnetic fields. Studying such plas- mas contributes to our understanding of atomic processes and structure in complex ions subject to the strong electric and magnetic fields. Under- standing dense radiating plasmas in the laboratory and in the interior of 2 For additional reading on the intersection of some of the frontiers of plasma science with those of atomic, molecular, and optical science, please consult the NRC report Controlling the Quantum World: The Science of Atoms, Molecules, and Photons, Washington, D.C.: The National Academies Press, 2007.

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Plasma Physics high energy density  at astrophysical objects often relies on our understanding of highly stripped atoms in extremely dense plasmas. HED plasmas enable theoretical pre- dictions of atomic energy levels, rate coefficients, and so on, to be tested experimentally. • Condensed matter physics and materials science. Studies of “warm dense matter” straddle the boundary between condensed matter and plasma physics. The kinds of equations of state and dynamic properties of mate- rials questions being addressed by experiments on warm, dense, partially ionized matter at high pressure connect to the questions being addressed by materials science studies. Thus physicists and materials scientists inter- ested in low-temperature matter at a pressure of 1 million atm do the same experiment on the same pulsed power machine to obtain relevant data as does the plasma physicist who is studying partially ionized matter at solid density and a few thousand degrees. • Nuclear physics. A potentially important connection between HED plasma science and nuclear physics will materialize if and when ignition is achieved in ICF experiments on the National Ignition Facility (NIF). Between 1017 and 1018 energetic neutrons will be emitted from a submillimeter source in less than 1 ns, offering the possibility of neutron-induced reactions in nearby target nuclei that have already been excited by a previous neutron interaction. • Accelerator physics and high-energy physics. The high cost and size associ- ated with conventional radio-frequency accelerator technologies has for nearly three decades been the main driver of a new approach to accelerat- ing charged particles. It has now been demonstrated that the interaction of powerful lasers and particle beams with plasmas can generate plasma waves with extremely large electric fields. Although the physics of plasmas and the physics of charged particle beams are distinct areas of research, there are important connections between the two disciplines in the areas of physical concepts, mathematical formulation, computational tools, applications, and terminology. • Pulsed x-ray sources for various applications. Laser-driven plasmas and ac- celerators produce electron bunches of very short duration that can be converted to ultrashort pulses of x-ray radiation. These radiation bursts are so short that they can be used as a strobe light to freeze-frame the motion of complex systems, such as materials being compressed by shock waves or molecules undergoing chemical reactions. By enabling this diagnostic capability, HED technology impacts material science, chemistry, biology, and medical sciences. • Fluid dynamics. There is close intellectual coupling between plasma physics and fluid dynamics through various hydrodynamic and magnetohydrody-

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Plasma science 4 namic instabilities and turbulence. This certainly applies to the instabilities present in imploding inertial fusion fuel capsules. • Astrophysics. Plasma and atomic physicists have collaborated for decades to make plasma spectroscopy a valuable tool for astrophysicists. HED plasmas are now being used to develop a database on equations of state, x-ray spec- tra, and radiation transport, all of which are also thought to be relevant to astrophysical observations. Whether HED plasmas can help to illuminate the dynamics in spatially distant cosmological events that take place on vastly different time and spatial scales is an open question. Role of Education and Training The national security of the United States requires the continuous presence of superior intellectual talent in all HED sciences at the DOE national laboratories. During the next 10 years or so, as the new facilities in Table 3.1 come into operation, it will be extremely important for HED university research programs to turn out bright, well-trained students to provide a pool of talent from which the national laboratories can draw. Although the changing character of the weapons complex will affect manpower needs, the age distribution of scientists at the weapon labo- ratories is such that retirements may also drive the need for new graduates. At present, a few universities have multiterawatt laser facilities and 100-kJ pulsed-power systems (see Table 3.1) that can be used for HED plasma research. Support of some of the exciting science described in the following section can be expected to attract some of the best students to carry out thesis research on those facilities. Making national laboratory facilities available part of the time to university teams, including students, could help assure interest in working on such facilities. RECENT PROgRESS AND FuTuRE OPPORTuNITIES This subsection begins with the main drivers of HED plasma physics research that were introduced in the last section. However, fundamental HED research is also driven by the access to new states of matter provided by pulsed-power machines and increasingly powerful short-pulse lasers. Opportunities to substantially expand fundamental HED research depend to a significant extent on the opening of the intermediate and large-scale facilities at the national laboratories to outside users part of the time. Some of the discoveries in HED science have already found practi- cal uses, as noted in the subsection on national security. Others remain a scientific puzzle or a curiosity. Several topics in the category of curiosities are mentioned here. For a more comprehensive discussion, please refer to The X-Games Report or Frontiers for Discovery. One breakthrough that has opened an entirely new window

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Plasma Physics high energy density  at on fundamental physics is highlighted in those two reports: the recent work with quark-gluon plasmas on the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. These novel quark-gluon phenomena are now thought to behave more like liquids than like plasmas per se. The committee hopes it is not missing too many of the ideas that will really blossom in the next decade, but then again, they would be welcome surprises. Inertial Confinement Fusion In the United States, ICF researchers have two goals: the use of laboratory-scale fusion explosions to acquire data for the U.S. nuclear weapons’ SSP and the harness- ing of these ICF explosions as a source of fusion energy. The vast majority of ICF research is funded by and directed at the former goal. However, it is the long-term opportunities associated with the second goal that motivate the enthusiasm for ICF of many of the researchers. To produce laboratory-scale energy release for either application, fuel capsules containing the hydrogen isotopes deuterium and tritium (DT) must be compressed to at least 200 g/cm3, hundreds of times the density of the solid, and heated to a high enough temperature, 100 million K, or about 10 keV, to induce a significant number of DT fusion reactions to occur before the fuel disassembles. This process is demonstrated at large scale by nuclear weapons and at astronomical scale by supernovas. By contrast, in magnetic confinement fusion, discussed in Chapter 4, strong magnetic fields confine the very hot plasma needed for a high fusion reac- tion rate in quasi-steady state. There are two main approaches to compressing the fusion fuel to the densi- ties required. The first, which has been the principal thrust of the SSP, is called indirect drive. In this approach, the energy provided by a very high power source (the “driver”)—such as an intense laser, a high current particle beam, or a HED imploding plasma from a pulsed power machine—is converted into x rays inside an enclosure, called a hohlraum, to assure symmetric irradiation of the fuel capsule contained within the hohlraum. That x-ray bath then causes an ablation-driven spherically symmetric implosion of the fuel capsule. In the second approach, direct drive, the capsule implosion is caused by spherically symmetric direct irradiation of the surface of the capsule by the driver. These two approaches are illustrated schematically in Figure 3.2. In both approaches to ICF, the energy absorbed by the fuel capsule surface layer, called the ablator, produces plasma that rapidly expands radially outward and acts like the exhaust of a rocket engine, driving the main mass of the fuel radially inward. The fuel is heated partway to the ignition temperature of 10 keV during the fuel compression by work done on the plasma to implode it. With conven- tional hot-spot ignition, the ignition temperature is reached in a central hot spot

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Plasma science 104 ablatively driven. In some of these experiments, radiative cooling has been achieved in the jets, allowing issues such as collimation to be studied. In other cases, the propagation of a jet through an ambient medium has been studied. Jet bending via the ram pressure of a crosswind has also been explored. Issues such as stability, collimation, and shock physics associated with jets might be addressed, but their relevance to astrophysical observations requires similarity of the physical situation, as determined by dimensionless parameters, and evidence that the scaling laws adequately connect the two hugely disparate situations. Morphological similar- ity between a laboratory plasma and an observation is not a particularly useful indication of relevance. Fundamental HED Research While the Grand Challenge applications of HED science discussed above have driven much of HED research in the past 10 years, the blossoming of this science outside the national laboratories has led to a series of exciting new research areas outside the scope of those applications. Much basic and applied HED research is being pursued in universities as well as in government laboratories and promises interesting opportunities in the coming decade. Research in many of these areas is of importance not only for intellectual reasons but also because the projects train students who ultimately become the leaders in the large projects that address national priorities. Advanced Computer Simulation of HED Plasmas Advances in predictive capability made possible by computer simulations are revolutionizing all areas of HED plasma research. Advanced computing has also been used to build complex physical models to yield detailed results that can be compared with experimental results. One such model involves the density func- tional theory calculations of the hydrogen equation of state. The challenge in the next decade for computational HED science will be in studies of plasma phenom- ena in which relevant physics occurs on very wide spatial and temporal scales. For example, the dramatic advances in PIC simulation capabilities that are being ap- plied to understanding a host of laser–plasma interaction problems are still limited to the submillimeter scale. The coming decade will see novel extensions of these codes using hybrid approaches spanning large spatial scales. HED Shock, Jet, and Ablation Hydrodynamics The past 10 years have seen quite remarkable progress in our ability to study in the laboratory various HED hydrodynamic phenomena, such as very high Mach number shock experiments. For example, high-power lasers have been used

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Plasma Physics high energy density 10 at to study Rayleigh-Taylor and other instabilities in shock waves at pressures well over 1 Mbar in solid density material. An example is shown in Figure 3.8. These experiments can now be performed at sufficiently high Reynolds number, Peclet number, and Mach number that the equations describing these shocks are similar to those that describe supernova dynamics. What’s more, our understanding of the hydrodynamics at the front of radiation-driven ablation has improved dramatically in the past decade. When a radiation field, such as from a laser, heats a plasma, material is ablated and the pressure exerted by this ablating material can drive instabilities. While achieving an understanding of ablation front hydrodynamics is critical to continued progress in ICF, this research also holds out hope of shed- ding light on ablation front instabilities found in such astrophysical situations as radiatively driven molecular clouds. Radiation Hydrodynamics Experiments in which radiation strongly affects the evolution of the plasma structure is also an area of active research. Most extensively studied are radiative shock waves in which the radiative fluxes exceed the material energy fluxes at the shock front and in which radiative losses are an important element of the dynamics. These experiments, which have been performed on facilities such as the OMEGA, JANUS, and Z-Beamlet lasers, have been useful in studying hydrodynamic in- stabilities and evolution in the radiative regime. There have also been some very exciting demonstrations of high Mach number plasma jets driven both by high- energy lasers and by z-pinches. Radiative dynamics often plays an important role in astrophysical jets, and the laboratory jet experiments are beginning to reach into this radiative regime. Atomic and Radiation Physics in HED Plasmas Atomic emission and absorption properties in hot dense plasmas are complex and are an active area of research. The past decade has seen the development of atomic structure and scattering codes that can compute details of the atomic quantum-level structure and kinetics, including ionization balance and level popu- lations in high atomic number plasmas. Experimentally, there have been important developments in spectroscopic diagnostic instrumentation in the past 10 years. Such diagnostic capability enables a comparison of theory and experiment that is sufficiently detailed to reveal plasma conditions as a function of space and time through comparison of observed and calculated spectra. The measurement ac- curacy is sufficient to check code calculations of spectral line energies. These new diagnostics coupled with a detailed understanding of atomic physics in dense plas- mas will lead to new ways of measuring and studying HED plasmas in the coming decade, including igniting ICF cores.

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Plasma science 10 FIGURE 3.8 Comparison of numerical simulations and experiment on multimode Rayleigh-Taylor in- stabilities. (a) A numerical radiograph from simulations. (b) Same as a, except with experimental noise added into the simulated output. (c) Experimental radiograph on strong shock-driven experiments done at the OMEGA laser. Courtesy of Laboratory for Laser Energetics (LLE), University of Rochester. SOURCE: A.R. Miles, D.G. Braun, M.J. Edwards, H.F. Robey, R.P. Drake, and D.R. Leibrandt, “Numeri- cal simulation of supernova-relevant laser-driven hydro experiments on OMEGA,” Physics of Plasmas 11: 3631 (2004). © 2004 American Institute of Physics.

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Plasma Physics high energy density 10 at Ultraintense Laser Generation of Bright Radiation Sources with HED Plasmas When an intense laser irradiates a solid target, energetic electrons are acceler- ated into the target and the electrons then generate x rays by various mechanisms. The past 10 years have seen the exploitation of this physics for the development of x-ray sources that are very bright and ultrafast (with pulse widths well under 1 psec). These ultrashort x-ray bursts driven by high-repetition-rate, multiterawatt lasers have found applications in a range of time-resolved x-ray spectroscopy experiments and dynamic probing experiments, such as those for the study of femtosecond condensed-matter dynamics, including melting and phonon propaga- tion in laser-excited crystalline materials. Other time-resolved x-ray spectroscopy techniques, such as x-ray absorption spectroscopy or x-ray scattering, are now be- ing implemented. These sources may soon be bright enough to probe the dynam- ics of chemical and biochemical reactions. At the petawatt level, isochoric heating experiments devoted to equation-of-state studies will be possible. Intense Femtosecond Laser Channeling in Air Over Long Distances Recent experiments have shown that an intense femtosecond laser of a few mil- lijoules to a joule in energy can self-channel in a gas, producing plasma filaments as long as a few kilometers. This allows laser spots of a few tenths of a millimeter to be delivered at great distances from the laser sources. It has also been observed that these plasma filaments are accompanied by strong terahertz emission. This self-channeling may lead to unique lidar systems that can detect atmospheric pol- lution or weaponized chemical and biological agents. Nonlinear and Relativistic Laser–Plasma Interactions Recent fundamental laser–plasma interaction research has concentrated in part on understanding such phenomena as the nonlinear saturation of the stimulated Raman scattering instability in a single hot spot and the use of optical mixing techniques to disrupt parametric instabilities, thereby providing some means of controlling these instabilities. In the coming decade, nonlinear effects such as so- called KEEN waves will be studied experimentally. What is more, with the recent development of laser technology capable of focused intensities over 1019 W/cm2, a wide range of relativistic laser–plasma phenomena, including novel nonlinear optical interactions and the creation of matter-antimatter plasmas, have become possible. Nonlinear optical phenomena attributable to the relativistic mass change of the electrons in the laser field lead to self-focusing and -channeling of the laser, or the generation of high-order harmonics in the laser field. The physics of how laser pulses interact with underdense plasma is critical in ICF and wake field ac- celerator research.

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Plasma science 10 University-Scale, Pulsed-Power-Driven HED Plasmas Kilovolt, near-solid-density plasmas can be produced routinely in the labora- tory by pulsed-power machines capable of as little as 50-100 kA with ~50- to 200- nsec pulse durations. In the last 10 years, such plasmas have been used to develop many x-ray diagnostics that are also useful on large-scale pulsed-power machines at the national laboratories using a variant of the exploding wire z-pinch called an x-pinch. This plasma yields x-ray sources as small as 1 µm that can be used for x-ray point-projection radiography with extremely high temporal and spatial resolution. At the 1 MA level, university machines have been used to generate plasma configu- rations that some believe are relevant to understanding astrophysical observations. As is the case with university-scale laser facilities, university-based pulsed-power systems offer opportunities to probe hot dense plasma in preparation for experi- ments on large-scale facilities, to benchmark computer codes, and to train students in the skills needed by the national laboratories. For example, wire-array z-pinch experiments at 1 MA university-scale machines (Figure 3.9) can test hypotheses about the origin of the instabilities observed in the wire-array z-pinches on the Z-machine. Rod-Pinch Development for Radiography Intense electron beams have been used to produce large amounts of 0.1-10 MeV radiation from 1-15 MV pulsed-power machines for simulating the effects of nu- clear weapons since the 1960s. However, the ability to focus a high current (~100 kA), multi-MeV beam to a ~1 mm spot for radiography has only recently been achieved. A sharp-pointed tungsten rod anode on an axis that extends through the hole of an annular cathode of a few megavolts pulsed-power machine has solved that problem. The cylindrical electron beam emitted from the cathode pinches down toward the rod and then propagates along the rod in such a way as to deposit its energy predominantly near the ~1 mm diameter tip. As a result, extremely fast hydrodynamics experiments, such as the subcritical plutonium materials science experiments being carried out as part of the SSP, can be performed with radiog- raphy having a resolution of a few millimeters using pulsed-power machines of modest size that produce only a few megawatts. ADDRESSINg THE CHALLENgES NNSA facilities are (legitimately) largely reserved for mission-oriented re- search. However, there are synergies between mission-oriented SSP science and fundamental HED science, and there are benefits to the cross-fertilization that occurs when university–national laboratory collaborations are developed. The

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Plasma Physics high energy density 10 at FIGURE 3.9 Laser shadowgraph image of an exploding wire z-pinch on the 1 MA COBRA generator at Cornell that started out with a cylindrical array of eight 12.7-µm Al wires at a radius of 8 mm. The anode (cathode) of the array is at the top (bottom). Notice the short wavelength structure in the plas- mas around each exploding wire. Also, note that there is a plasma forming on the array axis. Courtesy of the Laboratory of Plasma Studies, Cornell University.

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Plasma science 110 committee therefore applauds the NNSA Stewardship Sciences Academic Alliances program, which supports a broad range of HED science at universities and small businesses, as well as the new Stewardship Sciences Graduate Fellowship program. These will enable the research community to take advantage of more of the research opportunities offered by the HED field. Nascent efforts to develop user programs at NNSA’s intermediate- and large-scale facilities at the national laboratories are another step in this direction. Investigator-driven science can be facilitated by en- couraging two kinds of collaboration: • Dual-purpose (unclassified) experiments that involve collaborations be- tween university and national laboratory scientists, in which both parties benefit, the former by acquiring publishable data together and the latter by advancing stockpile stewardship science, and • Outside user programs on all major NNSA facilities, similar to the program at the National Laser User Facility (the OMEGA laser) at the University of Rochester, which sets aside perhaps 10-15 percent of the run time for investigator-driven research. A facility that is particularly in demand for investigator-driven research could increase its availability for a relatively small incremental cost by adding a shift each week, avoiding a reduction in the number of pulses available for mission-oriented research. As demand for intermediate-scale facility time increases, the HED re- search community and its sponsors should determine if HED research progress is significantly hampered by a lack of facilities dedicated to investigator-driven, peer-reviewed research. If that is happening, a case should be developed for the design, construction, and operation of a professionally managed, open-access, user- oriented facility similar to the synchrotron light sources operated for the materials science community by the DOE’s Office of Basic Energy Sciences. Finally, the committee observes that while a high-repetition-rate, 100-J laser for HED science experiments is not fully developed, both diode-pumped solid-state lasers and krypton fluoride lasers are approaching that level of capability. Several HED research areas could benefit from a shot-on-demand capability if at least one high-repetition-rate, 100-J laser is completed and the system turned into a user facility. Two such possibilities were mentioned at the end of the section on ICF, earlier in this chapter. CONCLuSIONS AND RECOMMENDATIONS FOR THIS TOPIC Conclusion: The remarkable progress in high-energy-density plasma sci- ence and the explosion of opportunities for further growth have been stimu-

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Plasma Physics high energy density 111 at lated by the extraordinary laboratory facilities that are now operating or soon will be completed. The laser and pulsed-power facilities that are now available, both very large and small enough to be called tabletop, enable the production and in-depth investiga- tion of matter in parameter regimes that were previously considered beyond reach. The applications of HED plasma science and the issues surrounding it that can be addressed by these facilities range from Grand Challenges of applied science to basic atomic, plasma, and materials physics. Connections to many other areas of the physical sciences, including condensed matter, nuclear, high energy and atomic physics, accelerators and beams, materials science, fluid dynamics, magnetohydro- dynamics, and astrophysics substantially broaden the intellectual impact of HED plasma research. As in all areas of plasma science, progress in HED research has benefited tremendously from advances in large-scale computer simulation capabil- ity and from newly developed diagnostic systems that have remarkable spatial and temporal resolution. The outcome of HED plasma research activities in the next decade will impact the SSP, our ability to interpret observed high-energy astrophysical phenomena, and our basic understanding of the properties of matter under extremes of density and temperature. In the longer term, the research highlighted here could lead to radically different particle accelerators with ultrahigh energies. It could also dem- onstrate the feasibility of inertial confinement fusion as a practical, inexhaustible energy source. Conclusion: The exciting research opportunities in high energy density (HED) plasma science extend far beyond the inertial confinement fusion, stockpile stewardship, and advanced accelerator missions of the National Nuclear Security Administration and the Department of Energy’s Office of High Energy Physics. The broad field of HED plasma science could ex- ploit the opportunities for investigator-driven, peer-reviewed HED plasma research better if it were supported and managed together with research encompassing all of plasma science. The NNSA provides by far the largest amount of research funding in the HED plasma area. The Stewardship Sciences Academic Alliances Program is a good start toward a healthy HED plasma research infrastructure outside the national labo- ratories. The Department of Energy’s Office of High Energy Physics (OHEP) and Office of Fusion Energy Sciences (OFES) provide additional support for some areas of HED plasma research. However, progress in many other areas of HED plasma science, such as warm dense matter, laboratory plasma astrophysics, and atomic physics in hot dense matter, is limited by the relatively narrow missions of the NNSA, the OHEP, and the OFES. Advances in investigator-driven, peer-reviewed

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Plasma science 112 HED research outside the scope of the mission-oriented agencies might develop much more rapidly if HED plasma research were integrated with the rest of plasma science in an organization whose mission included basic science. The announce- ment of a joint NNSA and OFES program in HED laboratory plasma physics is an important step forward.3 Conclusion: The cross-fertilization between the national laboratory pro- grams of the National Nuclear Security Administration (NNSA) and univer- sity research could be improved by increased cooperation between university and national laboratory scientists, facilitated by NNSA. User programs on the major NNSA facilities at the national laboratories that provide a significant amount of facility time for investigator-driven research are lacking. Intermediate-scale, user-oriented facilities, such as a petawatt laser facility comparable to the Rutherford Laboratory in the United Kingdom, are also lacking. The NNSA is beginning to foster user programs at its major facilities as well as collaborative experiments between university and national laboratory scientists, but such opportunities are not yet available to the broader scientific community. Successful examples are the National Laser Users Facility at the University of Roch- ester and, in magnetic confinement fusion research, the use of DIII-D at General Atomics. There are many other examples of this growing trend throughout the physical sciences. Conclusion: If the united States is to realize the opportunities for future energy applications that may come from the achievement of inertial fusion ignition, a strategic plan is required for the development of the related sci- ence and technology toward the energy goal. Currently no such plan exists at the Department of Energy. Perhaps more important, there are no criteria to guide the determination of when such a plan should be developed. The U.S. fusion program includes inertial fusion energy as a potential alter- native path to practical fusion energy in parallel with the magnetic confinement fusion approach. However, favorable results from ICF ignition experiments could change the landscape of and significantly impact DOE’s planning for the deploy- ment of fusion as an alternative energy resource for the United States. The large- scale introduction of commercial fusion reactors based on either magnetic or 3 This program was announced in February 2007 in the FY2008 presidential budget request. It includes individual investigators, research center activities, and user programs at national laser fa- cilities. The programmatic and scientific future of the program will be discussed in greater detail in the forthcoming report from the OSTP Task Force on High Energy Density Physics, a panel of the Physics of the Universe Interagency Working Group.

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Plasma Physics high energy density 11 at inertial confinement just a decade sooner will pay huge dividends to the United States economy and national security in the long term. Conclusion: Pursuit of some of the most compelling scientific opportuni- ties in HED physics requires facilities of an intermediate scale. The ability to propose, construct, and operate such facilities or to be granted access to existing facilities is quite constrained because the emerging scientific com- munity is supported primarily through NNSA and is constrained by the overarching NNSA mission. The emergence of HED physics as an intellectual discipline organized around compelling research topics was well articulated in The X-Games Report. As declared in that report and in the current report, the field is developing rapidly. In particu- lar, science topics such as laser–plasma interactions and warm dense matter could be explored at intermediate-scale facilities. Still largely embedded within NNSA, the scientists working on these topics do not have a mechanism for identifying, prioritizing, and managing a portfolio of small and intermediate-scale facilities. The committee notes that one symptom of this situation is the absence of com- petition among proposals for different facilities or even a discussion about them in the community. Recommendation: Existing intermediate-scale, professionally supported, state-of-the-art, high-energy-density (HED) science facilities at the national laboratories should have strong outside-user programs with a goal of sup- porting discovery-driven research in addition to mission-oriented research. To encourage investigator-driven research and realize the full potential of HED science, the research community and its sponsors should develop a ra- tionale for open-access, intermediate-scale facilities and should then design, construct, and operate them. Intermediate-scale facilities may be sited at universities or national laboratories; there are advantages to both. Intermediate-scale facilities have the flexibility and accessibilty to exploit opportunities that do not require the largest facilities (NIF, OMEGA-EP, and ZR), whose allocations of shots will be influenced by mission- oriented science. As such, existing intermediate-scale facilities could and should be shared by basic and programmatic science users. Provided operating costs can be funded, a broad user program at the existing facilities can enable new science while avoiding the capital costs of new construction. Small-scale facilities at universities complement intermediate- and large-scale facilities by testing novel ideas, developing diagnostic techniques, serving as staging grounds for experiments intended to be run on larger facilities, and providing criti- cal hands-on training for the next generation of HED experimentalists. Assuming

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Plasma science 114 the community clearly identifies the need, intermediate-scale user facilities should be built for HED science on the same basis as the DOE Office of Basic Energy Sci- ences provides user facilities for materials research. Finally, the committee notes that additional resources will be required to construct and operate any such new facilities. The DOE Office of Science should provide a framework for plasma science as a whole and play a role in managing a robust user program for broader science experiments at NNSA’s largest facilities.