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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 Sunâs 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. 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 Ã 1016Â 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 Âµm, 10 million K, near-solid-density plasmas produced when dense plasma columns âThe 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. 75
76 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 âwarm dense matterâ 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-
Plasma Physics at High Energy Density 77 R oom temperature 1,000 FIGURE 3.1â 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 1ânamely, 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
78 Plasma Science TABLE 3.1â Selected HED Facilities Peak Type of Energy Power/ Energy Repetition Facility Machine Delivered Current Delivery Rate Location Status Large-scale National Laser 1.8 MJ 500 TW Ultraviolet ~1 shot/ Lawrence To be Ignition photons 3 hr Livermore completed in Facility National 2009 Laboratory ZR Pulsed 3.5 MJ 350 TW/ Electric ~1 shot/ Sandia National To be power 26Â MA; current; day Laboratories completed in 4 Mbar magnetic 2007 pressure Omega/ Lasers ~30 kJ 30 TW/ IR/ ~1 shot/ 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 Titan Laser 200 J 400 TW Infrared ~1 shot/hr Lawrence Operational photons Livermore National Laboratory Z-Beamlet/ Laser 1 kJ + 500 1 TW/1 Optical/IR ~1 shot/ Sandia National Operational/1 PW Z-Petawatt J short PW photons 3 hr Laboratories to be completed pulse in 2007 Texas Laser 250 J 1 PW Infrared ~1 shot/hr University of To be Petawatt photons Texas completed in 2007 LâOasis Laser 4J 100 TW Infrared ~1 Hz Lawrence Under upgrade photons Berkeley National Laboratory Hercules Laser 20 J 800 TW Infrared ~1 shot/ University of To be photons min Michigan completed in 2008 Cobra Pulsed ~100 kJ 1 TW/1 MA Electric ~3 shots/ 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 facilitiesâfor example, the 100-TW Diocles laser facility at the University of Nebraska at Lincoln. ZR, Z refurbishment; EP, extended pulse; IR, infrared.
Plasma Physics at High Energy Density 79 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 nationâs 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 committeeâs 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 universityânational 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 DOEâs Office of Science, there are exciting new results on particle acceleration in laser- and particle-beam-driven, nonlinear waveâparticle 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-
80 Plasma Science 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), â.Â .Â .Â 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.Â .Â .Â . Such supportÂ .Â .Â .Â would greatly strengthen the ability of the nationâs universities to have a significant impact on this exciting field of physics.â 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: â¢ Inertial confinement fusion (ICF).â Can we achieve fusion ignition and, eventually, useful fusion energy from compressed and heated HED fusion plasma? â¢ Stockpile stewardship.â 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? â¢ Plasma accelerators.â 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? â¢ Laboratory plasma astrophysics.â 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,
Plasma Physics at High Energy Density 81 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. NNSAâs SSP is intended to turn this expectation into reality. Some of the HED science issues
82 Plasma Science 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 nationâs 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, â¢ Studying the properties of warm dense matter brings together plasma re- search and condensed matter and materials research. â¢ As temperatures increase, high atomic number HED plasmas bring plasma physicists together with atomic physicists to diagnose plasmas. â¢ The fluid instabilities and turbulence in plasmas (ionized fluids) are very similar to their counterparts in ordinary fluids. â¢ Nuclear physics contributes many diagnostic techniques to ICF, magnetic confinement fusion, and nuclear weapon studies. â¢ Nonlinear waveâparticle interaction in HED plasmas could lead to the next generation of high energy accelerators, affecting, in turn, high energy physics. â¢ The principles of magnetohydrodynamics are central to understanding dense magnetized plasmas, such as wire-array z-pinches. â¢ 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: â¢ Atomic physics.â 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 âFor 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.
Plasma Physics at High Energy Density 83 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. â¢ Condensed matter physics and materials science.â Studies of âwarm dense matterâ 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. â¢ Nuclear physics.â 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. â¢ Accelerator physics and high-energy physics.â 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. â¢ Pulsed x-ray sources for various applications.â 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. â¢ Fluid dynamics.â There is close intellectual coupling between plasma physics and fluid dynamics through various hydrodynamic and magnetohydrody-
84 Plasma Science namic instabilities and turbulence. This certainly applies to the instabilities present in imploding inertial fusion fuel capsules. â¢ Astrophysics.â 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
Plasma Physics at High Energy Density 85 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 weaponsâ 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 âdriverâ)âsuch as an intense laser, a high current particle beam, or a HED imploding plasma from a pulsed power machineâis 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
86 Plasma Science a b FIGURE 3.2â Schematic diagrams of the indirect drive and direct drive versions of inertial confinement fusion (ICF). fig 3.2 a, b as rapidly converging fuel stagnates in the center just moments after the tempera- ture is increased by converging shock waves. An alternative approach, called âfast ignition,â utilizes a second ultra-high-power, short-pulse external heating source impinging on the compressed fuel to locally increase the temperature to 10 keV. These alternatives are illustrated in Figure 3.3. The first ignition experiments will be performed at the NIF, a laser that will deliver 1.8 MJ of ultraviolet light in 192 convergent laser beams. (See Table 3.1 for further information about the NIF and other HED facilities named in this section.) Completion of the NIF is scheduled for 2009, with initial fusion ignition experi- ments planned for the following year. Based on data obtained on the nova laser prior to its closure in the late 1990s, those first NIF experiments will utilize the most highly developed path to ICF, indirectly driven hot-spot ignition. Advances in the ability to carry out large-scale two- and three-dimensional computer simulations on ICF target designs, together with technology developments and high-quality experiments carried out using the largest available laser and pulsed power systems, OMEGA and Z, have all contributed to the forward momentum of the ICF effort during the last 10 years. The development of exquisite diagnostics enabling mean- ingful comparison of experiments with simulations has been key to this progress. As a result, the completion of the NIF in 2009 is generating great optimism that ignition in the laboratory will soon be within reach. The main benefit of directly driven fuel implosions using lasers is reduced driver energy requirement if a sufficiently high level of capsule irradiation sym-
Plasma Physics at High Energy Density 87 FIGURE 3.3â Hot-spot ignition vs. fast ignition of compressed deuterium-tritium fuel. Courtesy of Laboratory for Laser Energetics, University of Rochester. metry and adequate hydrodynamic stability can be achieved during the implosion. The simplicity of the required target (only a fuel capsule is necessary) could be a substantial benefit for inertial fusion energy. Exquisitely diagnosed experiments utilizing the OMEGA and NIKE lasers, together with supporting computer simula- tions aimed at developing the direct drive approach, have generated optimism for this approach in recent years. Research also continues on the possibility of using pulsed power to produce the x-ray source for indirect drive using imploding plasmas that start out as a cy- lindrical array of hundreds of very fine tungsten wires. The pulsed-power-based x-ray source, while less well developed than laser-based indirect-drive ICF, is an intriguing alternative because of the high efficiency (>10%) with which electric energy is converted to x rays. It offers the possibility of achieving high yield with a facility only a few times bigger than the soon-to-be-commissioned ZR machine. The presence of ultrahigh (megagauss) magnetic fields in an imploding plasma may suppress thermal transport across the field lines and thereby facilitate the creation of thermonuclear plasmas suitable for fusion-energy development. There is also a parallel development path for indirect-drive ICF that makes use of pulses of charged particles. This driver option will be discussed in the context of the inertial fusion energy.
88 Plasma Science Challenges to the Achievement of ICF Ignition There are many technical challenges to achieving ignition of fusion reactions in an ICF fuel capsule, and these will be important areas for HED plasma research in the next few years. The critical issues for fuel assembly and ignition are cap- sule implosion symmetry, which applies to all variants of ICF, and interaction of the laser beams with plasma in the hohlraum, which applies to the laser-driven, indirect-drive approach that will begin at the NIF in 2010. Thanks to modern computer simulation capabilities, many refinements have been developed for ex- actly how to best utilize the available laser power and how to avoid unacceptable growth of instabilities. For example, employing mixtures of elements on the inside of hohlraum walls instead of just elemental gold can improve the conversion ef- ficiency from laser energy to low-energy x rays inside the hohlraum by 10-15Â per- cent. Another example is the ability to design beryllium or plastic fuel capsule ablator shells with specific dopant profiles to mitigate hydrodynamic instabilities. Continuing to develop such refinements will be essential to the long-term success of ICF (and inertial fusion energy). State-of-the-art computer simulations of the latest hohlraum-plus-fuel-capsule designs imply that if the experiments go as predicted, as little as 50 percent of the NIF laser design energy will be needed to achieve ignition (ignition is defined as the ratio of fusion energy released to laser energy absorbed in the hohlraum). Controlling the Implosion.â A fundamental challenge to ICF is that the radius of a spherical shell of fuel surrounding a much lower-density deuterium/tritium-gas- filled sphere must be compressed by a factor of 30-40 for central hot-spot ignition. Furthermore, the fuel should be compressed nearly adiabatically, that is, with the least possible increase in thermal energy consistent with achieving a hydrodynami- cally stable implosion. This requires an incredibly uniform squeeze over the entire outside surface to assure a symmetric implosion that does not squirt much of the spherical shell of fuel into the low-density central region as a result of hydrody- namic instabilities. The necessary ingredients for implosion symmetry in indirect drive are that the laser irradiation of the hohlraum must be nearly uniform (this necessitates many beams) and that the radiation driving the ablation of the fuel capsule must be smoothed to near-perfect spherical symmetry by multiple absorp- tions and re-emissions of the radiation inside the hohlraum. Increasing the central hot-spot temperature to 10 keV depends on the strength and timing of the shocks propagating through the target. Any energy delivered by photons or energetic electrons that heats the fuel before it is fully compressed is detrimental to capsule performance. LaserâPlasma Interactions.â Before the laser beams can be converted to x rays by striking the inside wall of the hohlraum, they must pass through substantial
Plasma Physics at High Energy Density 89 amounts of plasma that is coming off the hohlraum walls. The interactions of the laser light with these plasmas can drive waves in plasmas that could lead to a multitude of phenomena, many of them detrimental to the goal of creating smoothly distributed x rays in the hohlraum. As laser beams propagate through a high-temperature plasma they can (1) break into small filaments and spray out at an angle, (2) undergo significant energy transfer between crossing beams, (3) scatter back out of the hohlraum, and/or (4) generate high-energy electrons via a variety of instabilities involving either electron plasma waves (the stimulated Raman instability and the two-plasmon decay instability) or ion-acoustic waves (the stimulated Brillouin instability). These phenomena could be disastrousâfor example, energetic electrons could preheat the cold fuel or the waves could scatter a significant fraction of the laser energy back out of the hohlraum. Considerable progress toward understanding and controlling these phenomena has been made in recent years. For example, computer simulations and experiments suggest that the effect of some of these instabilities can be reduced by using mixtures of gases filling the hohlraum to damp the waves and by smoothing the laser beamsâ energy profile. However, substantial uncertainties still remain, so that understanding laserâplasma interaction will be the subject of many near-future experiments and computer simulations. Fast Ignition The alternative and less-well-developed fast ignition approach to heating the compressed fuel would be applicable, in principle, to any method by which the fuel might be compressed. The basic principle of fast ignition is that a small portion of unstructured, fully compressed fuel is heated to the ignition temperature by a short-pulse laser in 10-30 psec. As a result, hydrodynamic mixing cannot quench the burn, and the fuel is far from pressure equilibrium. This allows the main fuel to be less dense than for hot-spot ignition. Recent experimental and computational research on coupling ultra-high-power laser energy into compressed fuel suggests dramatically favorable driver energy consequences for the fast ignition approach. More fuel is predicted to undergo fusion reactions for a given driver energy, and the total laser energy that must be delivered to a fuel capsule to achieve the high gain needed for inertial fusion energy is predicted to be an order of magnitude less for fast ignition than for hot-spot ignition. The coupling of laser energy into the compressed fuel depends on the genera- tion and control of extremely large currents (~109 A) of electrons or subsequently produced ion beams. These flows, together with the incident laser fields, generate enormous electric and magnetic fields. Magnetic fields, for example, can exceed 1,000 T. These extreme conditions lead to very rich physics that needs to be under- stood, implying fertile areas for research during the next 10 years if experimental
90 Plasma Science facilities are available. For example, ideas have been put forward to shorten the distance between (1) the point beyond which light cannot propagate in the target plasma and (2) the compressed fuel that is to be heated by the fast ignition pulse energy. One possibility investigated experimentally on a facility in Japan is the use of an evacuated cone in the capsule to open a path for the short pulse laser. Simula- tions of implosions with this geometry are in good agreement with experimental implosions (Figure 3.4). Much more work is needed to determine the viability of fast ignition but is being seriously hampered by the lack of domestic facilities at the necessary laser power and energy. As a result, fast ignition experiments at the petawatt power level (e.g., 1 kJ in 1 psec), as well as other experiments requiring similar power levels, must be carried out abroad using the Vulcan laser (Rutherford Laboratory, U.K.) or the Gekko petawatt laser (Institute of Laser Engineering, Osaka, Japan) until U.S. facilities come on line (see Table 3.1). Inertial Fusion Energy Achieving fusion ignition in a single fuel capsule at the NIF is both the first step for SSP applications and the proof-of-principle step for the development of ICF as a practical path to the inexhaustible energy source that many believe fusion a b FIGURE 3.4â Comparison of a computer simulation and an experiment addressing fast ignition. Re- printed with permission from R. Stephens, S. Hatchett, M. Tabak, C. Stoeckl, H. Shiraga, S. Fujioka, M. Bonino, A. Nikroo, R. Petrasso, T. Sangster, J. Smith, and K. Tanaka, âImplosion hydrodynamics of fast ignition targets,â Physics of Plasmas 12: 7 (2005). Â© 2005, American Institute of Physics. fig 3.4 a, b
Plasma Physics at High Energy Density 91 will eventually be. Achieving ignition will demonstrate a practical understanding of a broad variety of HED physical processes, such as laserâplasma interaction, hydrodynamic instabilities, and radiation transport, in tandem. This intellectual milestone will then have to be followed by major developments in high-repetition- rate drivers, large-scale fuel capsule manufacturing, and other technologies that are required for practical fusion energy based on ICF. Issues such as developing materials that can tolerate the high neutron flux of a fusion reactor and tritium handling are common to both ICF and magnetic confinement fusion. Laser development paths for inertial fusion energy (IFE) have already been staked out for diode-pumped solid-state lasers and krypton fluoride gas lasers. Both approaches have been exploited to demonstrate 5- to 10-Hz lasers deliver- ing between 50 and 100 J per pulse over extended periods of time. These systems still require extensive development to reach the ~10 kJ per beam level needed for a reactor laser system. However, it is noteworthy that even a small additional step forwardâfor example, a 100- to 1,000-J laser system that could produce pulses as rapidly as a researcher could use them (i.e., as rapidly as gas puffs or new targets can be put in position)âcould provide the opportunity to revolutionize the way some classes of data are collected, for example, in laser-wake field accelerator studies or x-ray spectroscopy research on highly stripped high-atomic-number materials. Pulsed-power-driven IFE is now projected to involve 0.1-Hz ârecyclable trans- mission lineâ repetitive pulse systems, which are still at the stages of conceptual design and technology development. The heavy-ion-driver approach to IFE benefits from the fact that high current heavy-ion-beam technology is being developed with the high repetition rate ca- pability that is common for high-energy accelerators. At present, the capability of heavy-ion beams to deliver a power pulse to a target is many orders of magnitude away from a proof-of-principle demonstration. However, recent experiments on space-charge-neutralized beam transport using a preionized plasma have enabled a potassium beam with a head-to-tail velocity ramp imposed on it to be longitu- dinally compressed by a factor of 50 (in peak current) to a few nanoseconds in duration. Radial focusing by a factor of 200 in intensity was also achieved in a plasma. Both results were in good agreement with the results of particle-in-cell computer simulations. Although these beams are still at the level of a few amperes and a few hundred keV, at present intensities they can already be used for studies of warm dense matter that take advantage of the fact that energetic ions deposit their energy deep within a target. Stockpile Stewardship The goal of the SSP is to assure the safety, security, and reliability of the U.S. nuclear weapons stockpile without carrying out full-scale nuclear weapons tests.
92 Plasma Science The task includes assessing the weapons for safety and reliability as they age and modifying them as necessary to extend their lives. HED plasma science is a critical component of the SSP for testing materials, for validating computer codes, and so on. The complexity of these weapons and the wide range of physical processes and extreme states of matter involved when one is detonated make stockpile steward- ship an exceedingly challenging task. To achieve the goals of the SSP requires a fundamental understanding of many different materials under conditions ranging from room temperature to millions of degrees, with both ends of this range well within the HED range. The SSP experimental component must provide accurate fundamental materi- als data for many different materials over the wide range of densities and tempera- tures that occur in nuclear weapon explosions. For example, data on equations of state, materials strength, and radiative properties are essential for accurate calcula- tions by nuclear weapon codes. Thus stockpile stewardship is the driver for much of the HED materials research described below. The experimental program also must include well-diagnosed dynamic HED plasma experiments that will be able to validate computer simulations of how a specific configuration of materials will respond if it is rapidly heated from room temperature to the weapons-relevant re- gime. Finally, the experimental program must carry out complex experiments that involve several, if not all, of the physical processes that are important in a nuclear weapon explosion, albeit not with all the same materials and not necessarily at the same temperatures, in order to illuminate their interaction. This class of experi- ments includes, for example, radiation transport in a multimaterial ICF capsule ablation layer in the presence of shock waves and hydrodynamic instability growth. Understanding the results of such experiments and validating the computer codes used to predict their outcome obviously go hand in hand. Stockpile stewardship clearly also requires the ability to carry out large-scale computer simulations of very complex processes in HED matter in three dimen- sions. For example, three-dimensional computer codes are being developed that include models of material microphysics, intermediate-scale turbulence, radiation transport, and the like. To be credible, the computer codes must be validated and extensively benchmarked by analytic theory and laboratory experiments as just discussed. (These codes can be benchmarked against the underground test database as well as against laboratory experiments.) ICF is a key element of the SSP for several reasons. First, with the heavy reli- ance of ICF target design on computer simulation capability, the achievement of fusion ignition in an ICF fuel capsule will be a major integrated test of the predic- tive capability of multidimensional computer simulation codes that model self- consistently the many physical processes relevant to nuclear weapon explosions. In addition, achieving ignition of an ICF fuel capsule will greatly extend the range of temperatures, densities, shock strengths, etc. over which weapons-relevant materi-
Plasma Physics at High Energy Density 93 als and certain aspects of a weapon detonation can be studied. Finally, the exciting scientific challenge of achieving the near-term goal of fusion ignition in the labora- tory, followed by the equally exciting and even more challenging goal of developing practical inertial fusion energy, will draw some of the brightest young minds into the HED plasma field, talent needed to maintain a robust SSP in the future. An alternative approach to carrying out HED experiments relevant to stockpile stewardship is provided by the generation of intense x-ray bursts using wire-array z-pinches driven by pulsed-power machines. This approach involves delivering mil- lions of amperes of current to a cylindrical array of fine wires. The current-carrying plasmas that form around each wire are all attracted to the cylindrical axis by the total magnetic field, where they form a hot, dense plasma radiation source. Such plasmas were used to produce many kilojoules of soft x rays starting in the 1970s, but the last decade has seen a dramatic advance in the x-ray power that can be produced by these machines. The breakthrough that enabled z-pinches to achieve extremely high peak power (over 200 TW) and energy (nearly 2 MJ) x-ray pulses was the use of hundreds of wires in a cylindrical array instead of the small number of wires used in earlier, lower current experiments. Such high x-ray yields have led to the Z-machineâs being used for important stockpile stewardship experiments related to the aging of stockpile weapons. The achievement of such high x-ray powers and energies has also led to the serious consideration of using z-pinches for indirect-drive ICF. Exciting proof-of- principle experiments with a deuterium-containing fuel capsule have yielded over 1013 fusion neutrons (eclipsing the best fusion yield ever produced on the Nova laser). A major effort is under way to understand the physical processes that un- derlie the behavior of wire-array z-pinches in order to enable the optimum design of experiments on the refurbished Z-machine, called ZR. ZR will be capable of delivering 26 MA into wire-array z-pinch loads. Materials and radiation flow ex- periments important to stockpile stewardship are planned, including experiments relevant to hot-spot-ignition-based and fast-ignition-based ICF. Although basic science is not a mission of NNSA, the need for a pool of talented young HED scientists to staff its new facilities and the need to promote innovation has motivated NNSA to establish the Stewardship Sciences Academic Alliances program, followed by the Stewardship Sciences Graduate Fellowship Program (see http://www.krellinst.org/ssgf/). Both programs are important for the health and development of the HED plasma science field. Properties of Warm Dense Matter and Hot Dense Matter An important aspect of HED plasma research is the study of the fundamental properties of dense matter subject to extremes of pressure and temperature. How compressible is it? How much does the plasma radiate, and how opaque to radiation
94 Plasma Science is it? What is its electrical conductivity? How viscous is it? These properties, which are well understood for material normally encountered at room temperature or for hot plasmas that are tenuous, are not well understood for many HED plasmas. In- deed, much of the underlying physics that defines such quantities as compressibility and opacity cannot be simply described using well-developed physical theories. For example, when a solid material is heated to 10,000 K, the electrons and ions cannot be treated as if they were constrained in a lattice structure, as they are in a room temperature solid, but neither are they are governed by Debye shielding, as are most low-density plasmas. Such plasmas, described as âstrongly coupled,â are characterized by the fact that the electrostatic (coulomb) potential energy between neighboring charged particles exceeds the mean kinetic energy, and the electrons are at least partially degenerate. Some of the studies of fundamental aspects of strong coupling are discussed in Chapter 6. The atomic physics of dense plasmas is similarly complicated. As the tempera- ture at solid density is driven up to perhaps 10 million K, the matter becomes fully singly ionized or even multiply ionized if it is a high atomic number material. The electrostatic potential energy between particles remains high, assuring complicated atomic physics if there are still bound electrons on the atoms. Now that we can make the HED plasmas routinely using lasers and pulsed-power machines, we are beginning to understand them. Figure 3.5 illustrates the density-temperature regimes of particular interest here. At the lower temperatures, the physics of these warm dense matter states join with the physics of dense low-temperature plasmas that are finding many new applications (see Chapter 2). In the past decade, many advances have been made toward understanding the properties of warm and hot dense matter, examples of which follow: â¢ Equation of state (EOS).â An EOS attempts to describe the relationship between temperature, pressure, density, and internal energy for a given substance or mixture of substances. In experiments starting with dense, room-temperature materials, ultra-high-power lasers can drive shock waves or can heat the matter so fast that no expansion can take place during the heating pulse (this is known as isochoric heating). The Z-pulsed power ma- chine has been used for isentropic compression experiments. An example of results from an isochoric heating experiment is shown in Figure 3.6. The data from such experiments can help differentiate among complicated EOS models. As another example, experiments were carried out on the Nova laser to determine the EOS for shock-compressed deuterium. A small but important disagreement between the experiments and theoretical calcula- tions was found over a parameter range of importance to ICF. Later experi- ments on the Z-machine and then on OMEGA obtained experimental EOS results that differed significantly from the Nova results and are closer to the calculated EOS.
Plasma Physics at High Energy Density 95 FIGURE 3.5â Phase diagram illustrating the regimes of warm and hot dense matter. Note that this diagram expands beyond the HED range of parameters. â¢ Radiative properties.â Much progress has been made in computational methods for determining the radiative and opacity properties of dense plasmas. Experiments have been important in validating these calculations, as was illustrated in a pioneering Z-machine experiment on the opacity of iron, which is important for understanding the structure of the Sun. Agreement between theoretical modeling and experiments implies we are beginning to understand the properties of ions, electrons, atoms, and even molecules in dense plasmas.
96 Plasma Science 100 eV t = â3 psec t = 6 psec t = 14 psec FIGURE 3.6â Time-resolved image of a short pulse laser isochorically heating a fused silica target. The transparent target was heated by a picosecond infrared laser pulse from the top. A radiative heat wave travels in over the course of ~10 psec and heats the solid density material to a temperature approach- ing 106 K. The images were taken by probing the target edge with a second picosecond pulse and imaging the shadow that the opaque heated material makes. These data were taken at Imperial Col- lege. Reprinted with permission from T. Ditmire, E.T. Gumbrell, R.A. Smith, L. Mountford, and M.H.R. Hutchinson, âSupersonic ionization wave driven by radiation transport in a short pulse laser produced plasma,â Physical Review Letters 77: 498 (1996). Â© 1996 by the American Physical Society. â¢ Electrical properties.â In the past decade, it was learned that for matter with temperatures below a few eV, both electrical and thermal conductivities depend markedly on plasma density. This behavior has important ramifica- tions for the initiation of wire-array z-pinch implosions. Major advances in theoretical understanding of electrical properties have been achieved through the medium of molecular dynamics calculations. Short-pulse laser experiments have been particularly effective in deriving conductivity data on solidlike-density plasma heated on a femtosecond timescale. â¢ Dynamic properties.â The properties discussed above are usually defined for materials in equilibrium. However, in some practical situations, the time scale required to reach equilibrium is incommensurate with the dynamics of the system under investigation. This leads to an added level of complex- ity. Many recent shock physics simulations have begun to address these issues. Time-resolved experiments such as the recent use of short bursts of x-ray from intense lasers to image shocks propagating in solid density mate- rials have begun to yield dynamic information on rapidly heated plasmas. The committee foresees several exciting research opportunities for the next decade, including the topics discussed below.
Plasma Physics at High Energy Density 97 Warm Dense Matter Warm dense matter (WDM) is a particularly intriguing subset of the HED regime, as it refers to a regime of heated dense matter that is neither solid, fluid, nor traditional plasma. On the one hand, it refers to states from near-solid density to much greater densities with temperatures comparable to the Fermi energy. It also refers to those plasmalike states of matter that are too dense and/or too cold to admit to standard solutions used in plasma physics, the strongly coupled regime referred to earlier, in which theories based on only two particles interacting by cou- lomb interaction forces at a time fail. WDM, therefore, refers to a region between condensed matter and plasmas. The accessibility of WDM has grown dramatically in recent years thanks to high-intensity, short-pulse lasers and pulsed-power ma- chines, but studies have only just begun. There will be many intellectually exciting opportunities for research in this regime in the coming decade. One profound fun- damental question that needs to be answered is whether matter can be transformed into new phases at high density and pressure or if it undergoes a metal-insulator transition. The answer to a question like this can impact our understanding of the cores of the giant planets as well as many areas of applied science: ICF implosions, exploding wires, detonators, z-pinch wire-array dynamics, x-ray laser sources, laser machining and fabrication, and high-velocity impacts. Making WDM does not require the largest, most energetic drivers. Ion-beam accelerators, university-scale pulsed-power machines, and subpicosecond, 100- TW lasers that are small enough to be described as tabletop size can also generate interesting WDM. However, while the intermediate-scale facilities at the NNSA laboratories are needed for many of the most interesting experiments, a strong outside usersâ program exists only on the OMEGA laser system. Rapid progress in this research area would benefit substantially from a significant level of user ac- cess to some of the other NNSA facilities. A particularly exciting opportunity will exist at the Linac Coherent Light Source (LCLS), to be built at SLAC, where rapid energy deposition with deposition lengths long compared to target thicknesses will produce uniformly heated, uniform-density samples that can then be probed rapidly with LCLS x-ray pulses. The application of ion-beam drivers to WDM, discussed at some length in Frontiers for Discovery, benefits from the uniform energy deposition rate of en- ergetic ions in matter near the Bragg peak. Thus studies of the strongly coupled plasma physics of warm dense matter between 0.1 and 1 eV can be carried out even with relatively low energy beams that are made available in the inertial fusion energy program by uniformly heating thin foil targets. The experimental advances strongly suggest that interesting WDM plasmas can be studied with ion-beam drivers in the next few years. Longitudinal beam compression by factors of 50 or more to ~2 nsec was achieved by applying a voltage ramp to the beam. Beam radial
98 Plasma Science focusing in a space-charge-neutralizing plasma was also demonstrated. Both results confirmed computer simulations, underscoring the importance of the increases in predictive capabilities. Radiative Properties in Extreme Magnetic Fields While great progress has been made in the study of the radiative properties of dense plasmas without embedded magnetic fields, much less is known about hot dense matter with very strong magnetic fields. Such information would help us to understand some astrophysical phenomena, laserâtarget interaction experiments, and z-pinch implosions. For example, observations show that white dwarf stars can have surface magnetic field strengths up to 100,000 T. Magnetic fields in laser- target plasmas and pulsed-power experiments can easily exceed 1,000 T, with one recent short-pulse laser experiment reaching 5 Ã 104 T. Such fields can significantly modify radiative properties in these HED plasmas. The motion of atoms or ions that are not fully stripped in a strong magnetic field affects their atomic structure to the point where radiative transitions become very broad, causing substantial changes in opacity of the matter and eliminating standard features in emission spectra. Opportunities for curiosity-driven experimental and theoretical research abound in this area. Hot Dense Matter Hot dense matter refers to the regime of high temperatures and densities (e.g., 107 K and 100 g/cm3) similar to those found at the center of the Sun and in the cores of ICF implosion experiments. Even for the relatively simple situation of the Sunâs core, our ability to simulate the radiation outflow that leads to the solar radiation we observe is enormously challenging. Conditions that approach this regime are produced when certain wire-array z-pinch configurations called XÂ pinches implode unstably to form near-solid-density, 10 million K metal plasmas. Understanding the plasma dynamics and atomic physics properties of 20 to 40 times ionized high- atomic-number atoms in a solid density plasma with magnetic fields of perhaps 10,000 T is a challenging undertaking, again providing inspiration for curiosity- driven research. Plasma-Based Electron Accelerators The latter half of the 20th century witnessed remarkable advances in our un- derstanding of the elementary constituents of matter thanks to the development of ever more powerful and ingenious particle accelerators. As we enter the new century, continued progress in unraveling the most fundamental questions of our
Plasma Physics at High Energy Density 99 time is threatened because accelerators at the energy frontier have become too big and expensive for any one nation to build. As was discussed in Chapter 1, new physical mechanisms that enable extremely large electric fields must be invented and developed for these accelerators. Plasma-based accelerators might enable the next giant leap forward, because the magnitude of the electric field in a plasma is not limited by the electrical breakdown strength of any solid material, eliminating the major limitations on the electric field at the position of an accelerating particle bunch. Because the mechanism of plasma wake field accelerators was discussed in Chapter 1, here we simply summarize. An ultra-high-intensity laser or electron beam propagating through a plasma creates a high-gradient, large-amplitude plasma wave that moves with the speed of light in the wake of the beam. This wake field, in turn, can be used to trap and accelerate a trailing bunch of charged particles to relativistic energies (Figure 1.5). The accelerating fields in the plasma wave structures can, in principle, reach gradients that are many orders of magni- tude greater than present radio frequency (RF) accelerator technology. Highlights Based on research carried out since the late 1970s and spurred on by recent de- velopments in laser technology and multidimensional computer simulation capa- bility, laser-based wake field accelerator experiments in 2004 by three independent groups achieved accelerated beams of electrons at the ~100 MeV energy level. Ac- celerating gradients of ~50 GeV/m were achieved, three orders of magnitude greater than were achieved with conventional RF accelerator technologies. Beam charac- teristics were a transverse emittance of less than 2 mm-mrad, an energy spread of 2Â or 3 percent, and pulse length of less than 50 fsec (see Figure 3.7). The charge per pulse was about 0.3 nC. These performance characteristics are comparable to the performance of state-of-the-art photocathode RF guns. The results were chosen as one of the top 10 discoveries of the year by Nature. More recently, a high-quality electron beam with 1 GeV energy was produced by channeling a 40-TW peak power laser pulse in a 3.3-cm-long, gas-filled capillary discharge waveguide. Electron-beam-driven plasma wake field accelerator research has its roots even further in the past, with the theory having been worked out in the 1960s by Veksler and Budker in the former Soviet Union. At SLAC in 2005, a self-ionized, beam-driven plasma wake field accelerator accelerated particles by >2.7 GeV in a 10-cm-long plasma module. A 28.5-GeV electron beam with 1.8 Ã 1010 electrons was compressed to 20 Âµm longitudinally and focused to a transverse spot size of 10 Âµm at the entrance of a 10-cm-long column of lithium vapor with a density of 2.8 Ã 1017 atoms/cm3. The electron bunch fully ionized the lithium vapor to create a plasma and then expelled the plasma electrons. These electrons returned one-half
100 Plasma Science FIGURE 3.7â (a) Laser wake field accelerator experiments demonstrated production of low-energy spread electron beams using plasma channels to extend the interaction distance beyond the diffraction distance. Beams up to 150 MeV were observed using a 9-TW laser. (b) Particle simulations show that the important physics is trapping in the first wake period behind the laser, with termination of trapping due to wake loading. (c) Concentration of the particles in energy at the dephasing point when they outrun the wake. The predicted final energy is near the experimental observation. Courtesy of W.P. Leemans, Lawrence Berkeley National Laboratory (LBNL). SOURCE: C.G.R. Geddes et al., Physics of Plasmas 12: 056709 (2005). Â© 2005 American Institute of Physics.
Plasma Physics at High Energy Density 101 plasma period later, driving a large-amplitude plasma wake that in turn accelerated particles in the back of the bunch by more than 2.7 GeV. In February 2006, after fabrication of 1-m-long plasma source and beamline modifications, the same col- laboration demonstrated doubling of the energy of some of the 30-GeV electrons in a plasma accelerator, a significant advance in demonstrating the potential of plasma accelerators. The research opportunities for this field over the next decade will clearly focus on answering the question asked in the second section of this chapter: 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? Additional research and development steps must be taken. For example, the plasma through which the laser or particle beam propagates must be tailored so as to maximize the peak electric field and length over which acceleration takes place. It will also be necessary to optimize the laser or electron beam pulse intensity pro- file and the plasma profile so as to minimize the emittance and energy spread of the accelerated beam for the beam to be as useful as possible for particle physics experiments. Laser wake field accelerator issues associated with long-distance propagation and acceleration include optical guiding, instabilities, electron dephasing, and group velocity dispersion, all of which can limit the acceleration process. As an example, the scale length for laser beam diffraction is too short to allow reaching GeV electron energies, so optical guiding mechanisms like relativistic focusing and ponderomotive channeling, as well as preformed plasma channels, are necessary to increase the acceleration distance. Recent high-intensity experiments have demon- strated guidance over 10 diffraction lengths by a plasma channel. Combining such guiding techniques with an injector geometry that allows controlled acceleration of monoenergetic beams will be a key step in the development of laser wake field accelerators. Understanding the interplay among the nonlinear physical processes in plasma wake field accelerators requires numerical simulations. Particle-based models, such as fully explicit particle-in-cell (PIC) algorithms, which allow the self-consistent treatment of particle trajectories in their electromagnetic fields, are essential. Recent advances in algorithms and high-performance computing have enabled the development of highly efficient, fully parallelized, fully relativistic, three- dimensional PIC models that are used for the self-consistent modeling of full-scale wake field experiments, giving results such as that shown in Figure 1.5. Experiments are under way to demonstrate the production of GeV-class femto- second electron beams in distances of a few centimeters. Such a device could serve
102 Plasma Science as the first building block in future high-energy physics accelerators, but it might also lead to significant advances in accelerator-based light sources as well. A key challenge will be the development of high repetition, femtosecond laser systems with high (multi-kilowatt) average power. Plasma-based accelerators have clear connections to many fields of science. Laser-driven accelerators produce electron bunches of very short duration that can be converted to ultrashort radiation pulses. Therefore, in addition to impacting high-energy physics, significant impact is expected in materials science, nuclear science, chemistry, biology, and medical sciences through the use of intense radia- tion produced from the femtosecond electron bunches covering a wide range of the electromagnetic spectrum, from the terahertz regime to gamma rays, or directly from the electron beams. It is important to point out that these results are built on nearly 30 years of university research on plasma wakefield accelerators that was consistently spon- sored over the years by the NSF and the DOEâs Office of High Energy Physics. As described in Chapter 6, important progress on these research questions is often made in smaller-scale experiments, especially with the development of short-pulse lasers (see the section on laser-produced and high-energy-density plasmas in Chap- ter 6 for details). The value of continuous support for high-risk but promising ideas over decades until definitive results are obtained is clear. Laboratory Simulation of Astrophysical Phenomena The universe has become the subject of much more probing studies in recent years because of new telescopes that cover the entire electromagnetic spectrum. They have permitted phenomenally high-energy events to be observed but not understood. Can we possibly do HED experiments in the laboratory that can il- luminate these dramatic but spatially and temporally distant events? How can we test hypotheses concerning the physics of an observation that took place millions or even billions of light years away? The NRC report Connecting Quarks with the Cosmos says that the goal of laboratory plasma astrophysics is to discern the physical principles that govern extreme astrophysical environments through the laboratory study of HED physics. The challenge here is to develop physically credible scaling relationships that enable, through the intermediary of a computer code, laboratory experiments on the scale of centimeters or meters to illuminate physical processes taking in a distant part of the universe over enormous length scales (see, for ex- ample, Figure 1.14). There is general agreement that laboratory experiments can and do provide atomic physics, equations of state, and other data on HED states of matter similar to those hypothesized to exist in distant objects. Laboratory plasma physicists, atomic physicists, and astrophysicists have, in fact, collaborated for many decades to
Plasma Physics at High Energy Density 103 make plasma spectroscopy a valuable tool for astrophysicists. The fresh twist is that laboratory experiments now allow experimentalists to investigate macroscopic vol- umes of HED plasma in states that are thought to be relevant to astrophysics and to determine equations of state, x-ray spectra, and radiation transport coefficients. The use of HED laboratory experiments to investigate physical processes thought to be operative in astrophysical phenomena is a relatively new and con- troversial endeavor. It is generally believed that laboratory experiments cannot directly simulate an astrophysical situation even if some of the relevant dimen- sionless parameters are on the same side of some critical value, whatever that might be, in both the laboratory and the cosmos. However, the new generation of laboratory HED facilities can investigate matter under conditions that enable some of the physical processes that are thought to underlie observed phenomena to be studied. Examples of processes and issues that can be experimentally addressed in the laboratory under conditions that may be relevant to a range of astrophysical phenomena are compressible hydrodynamic mixing, strong-shock phenomena, magnetically collimated jets, radiative shocks, radiation flow, complex opacities, photoionized plasmas, equations of state of highly compressed matter, and rela- tivistic plasmas. The laboratory experiments can, therefore, be used to validate the computer codes that are being used by astrophysicists to try to understand the observations, assuming that the scaling laws imply that the experimental regime scales in some reasonable way to the astrophysical phenomenon. Thus, although the growing capacity of experimental studies has potentially opened new windows on cosmic plasmas and their behavior, it is not yet clear that these experiments will one day become standard tools for addressing issues of astrophysical plasmas. Many complex large-scale structures observable in the universe result from the nonlinear evolution of flows emanating from compact objects. Astrophysical plasma jets are a prime example of this class of phenomena and are also a good example of how laboratory experiments might contribute to an understanding of astrophysical observations. These collimated flows range over size scales from the 0.1 parsec associated with planetary nebulae and young stellar objects to the kilo- parsec jets driven by active galactic nuclei. The most pressing questions concerning these flows center on the processes responsible for their formation and collimation as well as their interaction with ambient media. In particular, the effects of radiative cooling, magnetic fields, and intrinsic pulsing on jet structures have received much attention in the literature. In addition to the examples cited, during the last stages of a massive starâs evolution, jets arising during gravitational collapse may play an important role in the explosion of some types of supernovas. Experiments designed to be relevant to these astrophysical phenomena are performed using high intensity lasers and conical wire arrays on pulsed-power facilities. The laboratory jets are formed hydrodynamically in these experiments, in some cases through converging conical flows that were either shock-driven or
104 Plasma Science 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 laserâplasma 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
Plasma Physics at High Energy Density 105 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. Whatâs 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.
106 Plasma Science FIGURE 3.8â 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, âNumeri- cal simulation of supernova-relevant laser-driven hydro experiments on OMEGA,â Physics of Plasmas 11: 3631 (2004). Â© 2004 American Institute of Physics.
Plasma Physics at High Energy Density 107 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 LaserâPlasma Interactions Recent fundamental laserâplasma 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 laserâplasma 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.
108 Plasma Science 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 Âµm 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 0.1-10Â 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 universityânational laboratory collaborations are developed. The
Plasma Physics at High Energy Density 109 FIGURE 3.9â 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-Âµm 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.
110 Plasma Science 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 NNSAâs 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: â¢ 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 â¢ 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 DOEâs 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:â The remarkable progress in high-energy-density plasma sci- ence and the explosion of opportunities for further growth have been stimu-
Plasma Physics at High Energy Density 111 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:â 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 Energyâs 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 Energyâs 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
112 Plasma Science 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. Conclusion:â 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:â 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 DOEâs 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 âThis 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.
Plasma Physics at High Energy Density 113 inertial confinement just a decade sooner will pay huge dividends to the United States economy and national security in the long term. Conclusion:â 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 laserâplasma 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:â 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
114 Plasma Science 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 NNSAâs largest facilities.