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Plasma Science: From Fundamental Research to Technological Applications 3 Inertial Confinement Fusion INTRODUCTION AND BACKGROUND The goal of fusion research is to develop a reliable alternative to the present burning of fossil fuels for energy. In the inertial confinement approach to fusion, high-intensity laser or charged-particle beams are used to compress and heat the fusion fuel to the density and temperature required for fusion of the nuclei. The fusion of deuterium and tritium is schematically illustrated in Figure 3.1. The pursuit of inertial confinement fusion (ICF) depends on many phenomena associated with plasma science. The interaction of radiation with matter in the plasma state and the subsequent energy transport and high-density compression leading to thermonuclear burning of the plasma fuel must be optimally balanced. Nonlinear collective effects must be understood and accommodated. The international goal is to achieve an environmentally improved source of electrical power generation. The majority of the program continues to be implemented in the nuclear weapons laboratories, the Naval Research Laboratory, and the Laboratory for Laser Energetics at the University of Rochester. New large-scale facilities and facility upgrades are currently envisioned with funding authorizations at various stages. Support for the underlying basic plasma science and the breadth of the involved community should be strengthened. The role of basic plasma research within the ICF program may be at a crossroads, requiring timely reexamination.
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Plasma Science: From Fundamental Research to Technological Applications FIGURE 3.1 Schematic diagram of the basic fusion process in which deuterium and tritium nuclei combine to form 3He and a neutron. For electric power applications, the energy from this reaction is transformed into heat and then converted into electrical energy. RECENT ADVANCES Laser Fusion In the past decade, significant progress has been made in the understanding of high-energy-density plasmas created by intense lasers and particle beams. An ICF hohlraum irradiated by the Nova laser is shown in Plate 3. With several notable exceptions, this work has been carried out under the auspices of inertial confinement fusion research with large lasers (i.e., having energies greater than 1 kJ). Direction, progress, and accomplishments within the ICF program have been subject to frequent national review.1 Experiments and computer simulations during the past decade have led to a 1 For example: National Research Council, Second Review of the Department of Energy's Inertial Confinement Fusion Program, Final Report, National Academy Press, Washington, D.C., 1990.
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Plasma Science: From Fundamental Research to Technological Applications quantitative understanding of the Rayleigh-Taylor instability in hot, ablating plasmas. Future work will examine the transition to turbulence and address Rickmyer-Meshkov and Kelvin-Helmholtz-like instabilities. Two-dimensional hydrodynamic simulations have modeled successfully the linear and early nonlinear evolution of the Rayleigh-Taylor instability in ablating plasmas with a range of initial sources, Attwood numbers, and accelerations. Photon and electron energy deposition lead to finite density gradients and mass removal, which can substantially reduce the Rayleigh-Taylor growth rate from its classical value. Fokker-Planck codes embedded in hydrodynamic simulations have been developed to model better the nonlocal electron energy transport. These simulations describe phenomena such as thermal filamentation and thermal conduction, and influence our basic understanding of the details of the Rayleigh-Taylor instability. Using the Nova laser facility, successful experiments were conducted that addressed the physics of interpenetrating materials at accelerated interfaces (an area of hydrodynamics critical to both ICF and weapons research). Several of the ''ignition physics milestones" described in the NRC's 1990 review of the ICF program were achieved, including experimental confirmation of the LASNEX simulation code predictions for Rayleigh-Taylor instability growth rates in the presence of ablation and density gradients for both radiation-driven and electron-conduction-driven planar foils. New diagnostic techniques were demonstrated, including large neutron scintillator arrays; a single-hit scintillator array neutron spectrometer; and a high-energy, ring-aperture x-ray microscope. The Nova target chamber is shown in Figure 3.2. There has been significant progress in the ability to measure and calculate the radiation properties of complex, partially stripped ions over a wide range of plasma conditions. The recent measurement of iron opacity in dense (ne > 1020 cm-3), warm (Te ≥70 eV) plasmas in local thermodynamic equilibrium illustrates these advances. These conditions are also relevant for astrophysical plasmas. The demonstration of nickel-like gold plasma x-ray lasers operating at a wavelength of 33 Å is an example of the present capability to model non-LTE plasmas. Sophisticated opacity codes for dense plasmas composed of multi-electron ions have been developed. These codes describe the complex absorption and emission features of these ions in terms of unresolved transition arrays and super-transition arrays, and have led to improvements in modeling radiant energy flow in high-density plasma of interest in both inertial fusion and astrophysical applications. Laboratory experiments have been performed that validate these codes. Ion-Beam Fusion An equally robust and successful track record of accomplishments exists for the light- and heavy-ion ICF efforts, which represent alternative "driver" ap-
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Plasma Science: From Fundamental Research to Technological Applications FIGURE 3.2 Photograph of the inside of the Nova 10-beam target chamber at Lawrence Livermore National Laboratory. An essential part of Nova is its diagnostic capability, which includes optical, x-ray, and neutron measurement techniques to study the performance of the ICF targets. These diagnostics surround the target, which is positioned in the center of the chamber on the end of a rod descending from the top of the picture. (Courtesy of Lawrence Livermore National Laboratory.) proaches, and they are being pursued concurrently with the laser program. A principal aim of heavy-ion fusion accelerator research is to gain an understanding of the dynamics of intense, space-charge-dominated beams in accelerator structures. These beams, which are effectively nonneutral plasmas, exhibit collective behaviors in addition to those bulk motions driven by the externally applied fields. The beam plasma frequency is comparable to the frequency associated with motion in the applied fields. Analytic theory originally predicted a multitude of instabilities; however, most of these were not observed. Computer simulations resolved the disagreement for transverse modes by elucidating the nonlinear behavior and early saturation of the instabilities. Resolution of this disagreement represents a major step in understanding these nonneutral plasmas.
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Plasma Science: From Fundamental Research to Technological Applications The light-ion ICF program is based on the robust and cost-effective intense ion beam technology developed over the past 20 years. The program has many significant achievements. The ability to transport megajoules of electromagnetic energy at megavolt-per-centimeter electric fields in pulse power accelerators and over several meters of transmission line has been achieved only after careful study of electron insulation in high-voltage devices. Electrons emitted from surfaces are confined by the self-magnetic field of the transverse electromagnetic (TEM) waves, and the behavior of relativistic, diamagnetic electron flow in strong electromagnetic fields is a large extrapolation beyond the physics of magnetrons. The successful generation of multi-kiloampere-per-square-centimeter light-ion beams in magnetically insulated ion diodes has resulted from careful study of the physics of collective effects in electron and ion streams in strong electric fields. This reduction in beam divergence has come from two- and three-dimensional computer codes that model the electromagnetic and particle physics in realistic geometry supported by analytical studies. Important developments in the physics of the propagation of intense ion beams to the target in an ionized background gas have occurred both experimentally and theoretically. Unique diagnostic systems have been developed capable of measuring ion beams and plasmas on very short time scales and in extremely hostile radiation environments, including the measurement of high ion beam intensities (0.1–1.0 MA/cm2, 1–12 MV, 1–5 TW/cm2) using elastic scattering and characteristic x-ray line emission. Diagnostic techniques developed include visible spectroscopy, VUV spectroscopy, ion pinhole cameras, ion movie cameras, Rutherford magnetic spectrographs, and nuclear activation and nuclear track detectors with automatic track counting capability. Stark shift measurements using visible spectroscopy diagnostics on the Particle Beam Fusion Accelerator II (PBFA II) have demonstrated electric fields as high as 9 MV/cm, the largest electric field ever measured using this technique. Fluorescence spectroscopy using dye lasers has emerged as an important technique for measuring the ion-beam divergence in high-powered ion diodes. SCIENTIFIC AND TECHNOLOGICAL OPPORTUNITIES New facilities, such as the Omega Upgrade and the National Ignition Facility (shown in Figure 3.3), will enable the creation of plasmas with densities in excess of 1026 cm-3 and pressures exceeding 200 Gbar. Energy transport (including fusion by-products), the equation of state, and radiative properties can also be studied with these facilities. For example, plasma conditions will be well matched to the study of electron energy transport that is either nonlocal or not dominated by collisions. Configurations can also be established for magnetic fields to play a role. Dense plasmas formed by isochoric heating by penetrating electrons formed with intense short-pulse laser irradiation also may be appropriate for transport studies in which hydrodynamic expansion can be minimized.
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Plasma Science: From Fundamental Research to Technological Applications FIGURE 3.3 An artist's rendition of the proposed 192-beam National Ignition Facility (NIF). The NIF is designed to demonstrate an important ICF milestone, ignition and subsequent self-heating of the fusion fuel. Nearly 2 MJ of laser energy (40 times more than the existing Nova facility can produce) will create high-energy-density plasmas relevant to a broad range of research and applications. (Courtesy of Lawrence Livermore National Laboratory.)
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Plasma Science: From Fundamental Research to Technological Applications These facilities will also enable quantitative experiments that examine hydrodynamic stability properties. Mixing of materials originally separated by interfaces, including the fully turbulent phase and subsequent energy, mass, and momentum transport, will be studied. The development of three-dimensional codes that incorporate turbulent mix models will complement these experiments. A laser-plasma experiment illustrating several relevant multidisciplinary phenomena is illustrated in Figure 3.4. There will continue to be significant advances in numerical simulations and FIGURE 3.4 Images of a laser-produced plasma expanding into a gas. An intense, shortpulse laser (pulse duration 3 ns, wavelength 1.05 µm, irradiance 5 × 1013 W/cm2) strikes a small target (an aluminum disk 1 mm in diameter and 10 µm thick, mounted on a stalk), creating an energetic, expanding plasma. The plasma initially moves to the left, and accelerated bulk target material propagates to the right. Two superimposed dark-field shadowgrams, which are sensitive to density gradients, image the evolution of a very strong shock wave (Mach number greater than 100) propagating into atmospheric pressure (left side of target stalk) and into a turbulent plasma (right side of target stalk). Such techniques permit physical effects that occur in plasmas with ultrahigh energy densities, such as space plasmas, supernovae, and nuclear explosions, to be readily studied in the laboratory. (Reprinted, by permission, from B.H. Ripin, J. Grun, C.K. Manka, J. Resnick, and H.R. Burris, "Space Physics in the Laboratory," pp. 449–463 in Nonlinear Space Plasma Physics, ed. R.Z. Sagdeev, AIP Press, New York, 1993. Copyright © 1993 by the American Institute of Physics.)
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Plasma Science: From Fundamental Research to Technological Applications the experimental capability to describe the radiative properties of complex, high-energy-density plasmas. The development of non-LTE codes and expansion of the experimentally accessible parameter space will be areas of future research emphasis. Within the heavy-ion fusion accelerator research area, additional work is needed on longitudinal modes and on the interaction between longitudinal and transverse motions. Because some processes are inherently three-dimensional, advanced three-dimensional simulations need to be developed and applied. The light-ion fusion program provides science opportunities for creating high-density and high-temperature plasmas for materials and plasma science, for developing new computer codes to investigate ion-beam transport in vacuum and plasmas, and for developing a new class of soft x-ray diagnostics that can operate in harsh radiation environments to provide accurate measurements of both Planckian and non-Planckian radiation. Furthermore, the modification of surface properties of materials under bombardment of intense low-energy ion beams can be studied. Important areas for further research include study of stimulated Raman and Brillouin scattering in laser plasmas at higher intensities, nonlinear interaction of plasma instabilities, plasma opacities and equations of state (EOS), and plasma hydrodynamic stability and mixing. Research objectives within these areas include determining thresholds, saturation levels, and scaling relationships; normalizing high-temperature theoretical opacity models; obtaining high-pressure EOS data and standards for programmatically relevant materials; and characterizing phenomenology associated with plasma compression in excess of a factor of 100. There also exists a related set of research areas that reflects the interdisciplinary nature of plasma science, when compared with the more formally recognized academic fields of study such as atomic physics, optical physics, condensed matter physics, and fluid mechanics. Consider the basic area of radiation-plasma interaction. ICF programs have used laser light to compress and heat thermonuclear fuel, magnetic fusion energy programs have considered electromagnetic radiation as a supplemental plasma heating source, and programs have been conducted to modify the ionosphere with high-power, high-frequency radiation. All exhibit basic common physical phenomena (such as instabilities, nonlinearity, turbulence, particle acceleration, and heating) and effects of spatial inhomogeneity (such as mode conversion, modification of instabilities, and wave propagation). Phase conjugation and wave mixing in plasmas acting as the nonlinear medium are examples of nonlinear optics phenomena whose analogues in optical media have led to previous technological applications. The recent development of ultrashort-pulse lasers provides an opportunity to address basic science questions that cross over the descriptive boundaries of ICF plasma physics, atomic physics and condensed matter physics. For example, potential studies include behavior of atoms, ions, and molecules in the strong
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Plasma Science: From Fundamental Research to Technological Applications electromagnetic fields of the ultra-short-pulse laser; the properties of hot, solid-state plasmas; and radiation-matter interactions in all density-temperature regimes. Early in research on inertial confinement fusion, laser-plasma experiments revealed many interesting plasma physics phenomena, including several types of parametric instabilities, superthermal particle acceleration, and spontaneous magnetic field generation. Ultrafast-pulse laser technology has now progressed to the point where extremely nonlinear phenomena are accessible. Incident electric fields as high as 10 kV/Å transform bound electrons instantly into relativistic free electrons. These so-called tabletop terawatt lasers are university-scale facilities. Numerous new phenomena will be available for study. These include relativistic self-focusing, relativistic penetration into overdense plasmas, coupling that leads to energetic electrons and ions (E < 1 MeV), severe ponderomotive pressure modification of the plasma hydrodynamics, and coherent radiation generation. Large-amplitude plasmons can be created by several different processes (beat wave, wake field, etc.), and their evolution from coherent to chaotic structures can now be studied. Electron heating by these large-amplitude, high-phase-velocity waves can be examined, as well as the subsequent wave-particle interactions. The short-pulse aspect (<1 ps) also enables studies of high-energy-density plasmas created through rapid energy deposition (~ 1030 W/g may be possible). The pulse duration may also be matched to characteristic time scales of the plasma, such as the period of an electron plasma oscillation for electron densities exceeding 1017 cm-3. The fundamental properties of electromagnetic wave interaction change qualitatively, and numerous processes can lead to the acceleration of particles to high energy. High-irradiance, short-pulse lasers may be able to generate extremely large magnetic fields (B > 100 MG) in plasmas. Diagnostic techniques, such as subpicosecond Faraday rotation, hold the promise of measuring the evolution of such fields. Plasma resistivity can be controlled by the judicious choice of experimental parameters (including preformed plasmas), and fields initially containing more than 10% of the laser energy may be possible. Significant plasma flows can be created with high-brightness lasers. These plasma flows, which can have velocities approaching 109 cm/s, can be directed across externally applied magnetic fields or used to form counterstreaming plasmas. Numerous plasma phenomena can be studied. The ultrafast regimes are particularly amenable to computer simulation and are an ideal test bed for improving the understanding of strongly driven plasmas. Research in this area could lead to new compact sources of tunable radiation and to a new class of compact particle accelerators. New diagnostic tools could be made available for material, biological, and electronic applications. Finally,
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Plasma Science: From Fundamental Research to Technological Applications improved understanding of strongly driven plasmas will benefit fusion and astrophysical research, as well as advance nonlinear science. The equation of state of plasmas formed over a wide range of thermodynamic pathways can be studied at pressures exceeding 1 Gbar. High-irradiance, short-pulse lasers and high-intensity lasers can be utilized in these studies. The creation of strongly coupled plasmas and the near-isentropic compression of plasmas by lasers to densities exceeding 1025 cm-3 at temperatures less than the Fermi energy are now possible. The properties of such plasmas, of interest to inertial confinement fusion, astrophysics, and condensed matter physics, can now be studied in the laboratory. Experiments have been conducted demonstrating ultrahigh-pressure shocks exceeding 700 Mbar. Pressures readily in excess of 1 Gbar are now possible, enabling the equation of state and other aspects of condensed matter physics to be studied in regimes previously unattainable in the laboratory. Finally, high-energy-density, non-LTE plasmas play a major role in several multidisciplinary endeavors, including optimized ICF target and driver design, x-ray plasma diagnostic spectroscopy, and x-ray lasers. Energy balance, hydro-dynamic behavior, and radiation transfer are all affected by the detailed atomic states and kinetics of the plasma. Experiments would further normalize the computational ability to model and simulate plasma behavior and would further improve the spectroscopic ability to measure and characterize the plasma state. CONCLUSIONS AND RECOMMENDATIONS Certain trends are already evident in light of the evolving national priorities triggered by the commonly acknowledged end of the Cold War. First, a healthy process of consolidation and collaboration is evident. Increasingly, joint efforts addressing design, experiment, analysis, and facility issues collectively involve the Livermore, Sandia, Los Alamos, Naval Research, and University of Rochester laboratories. Technology collaboration and transfer arrangements with the industrial sector and a renewed emphasis on the quality and commitment to education are being discussed. Collaborative teaming and partnering among and within government, industrial, and academic organizations are being nurtured all the while in a fiscal environment focused on national deficit and debt reduction. It is in this context that recommendations regarding the funding and implementation of basic plasma science research impacting ICF are made. A segment of the ICF community holds the view that the health of the field is adversely affected by the priority allocation of resources to facilities and operating costs, at the expense of support for basic plasma research aimed at a fundamental understanding of relevant phenomena. This resource allocation is reflective of an ICF program that relies primarily on computer simulation and full-scale experimental results. It also is suggested that the historical association between the ICF and nuclear weapon programs of the Department of Energy (DOE) has
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Plasma Science: From Fundamental Research to Technological Applications limited the size, nature, and degree of involvement of the basic plasma research community doing related work. Past classification and facility access policies compounded this problem. However, recent DOE plans to declassify large portions of the ICF program provide a major opportunity to involve the basic plasma research community. Given budgetary constraints, capital-intensive full-scale experimentation can constrain support for more fundamental theoretical and experimental scaling research and modeling. Although full-scale experimentation is essential, the inclusion of a basic plasma science research component within the fusion energy program can lead to a more timely achievement of the basic goals. The ability to conduct basic plasma science research in ICF, as described above, depends critically on the accessibility of facilities and the availability of equipment, independent of the organizations and personnel involved. A dual approach is suggested. The previous policy of developing facilities for full-scale experimentation has put in place large numbers of components, subsystems, and equipment. The reconfiguration and recommissioning of smaller-scale research facilities should be considered to make effective use of existing equipment and capabilities. Consideration should be given to providing the opportunity for a broader representation of participating organizations. Interested universities, small businesses, and corporate America could participate competitively, while at the same time offering cost-sharing opportunities. Existing federal programs, such as the National Laser User Facility (NLUF), the Small Business Innovation Research (SBIR) program, and Cooperative Research and Development Agreements (CRADAs) between industry and the national laboratories, could be helpful in this effort. Consideration should be given to allocation of funding within the inertial confinement fusion program to support more related basic research and use of major ICF facilities as national user facilities. Where appropriate, ICF facility use should be encouraged in support of nonfusion programs. If no additional funding is available, basic plasma science research judged to be the most important could be funded from large facility accounts.
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