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INERTIAL CONFINEMENT FUSION 64 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 (0.1â1.0 MA/cm2, 1â12 MV, 1â5 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.
INERTIAL CONFINEMENT FUSION 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.) 65
INERTIAL CONFINEMENT FUSION 66 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 Âµm, irradiance 5 Ã 1013 W/cm2) strikes a small target (an aluminum disk 1 mm in diameter and 10 Âµm 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. 449â463 in Nonlinear Space Plasma Physics, ed. R.Z. Sagdeev, AIP Press, New York, 1993. Copyright Â© 1993 by the American Institute of Physics.)
INERTIAL CONFINEMENT FUSION 67 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
INERTIAL CONFINEMENT FUSION 68 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/Ã 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,