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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 6874 Appendix H: Summary from the Report of the Panel on the Assessment of 6875 Inertial Confinement Fusion (ICF) Targets (Unclassified Version) 6876 6877 The text below is excerpted from the prepublication version of the report of 6878 the National Research Council’s Panel on the Assessment of Inertial Confinement 6879 Fusion (ICF) Targets. 6880 Summary 6881 6882 In the fall of 2010, the Office of the U.S. Department of Energy’s (DOE’s) 6883 Under Secretary for Science asked for a National Research Council (NRC) committee 6884 to investigate the prospects for generating power using inertial fusion energy (IFE), 6885 noting that a key test of viability for this concept—ignition 1—could be demonstrated 6886 at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory 6887 (LLNL) in the relatively near term. In response, the NRC formed both the Committee 6888 on the Assessment of the Prospects for Inertial Fusion Energy (“the committee”) to 6889 investigate the overall prospects for IFE in an unclassified report and the separate 6890 Panel on Fusion Target Physics (“the panel”) to focus on issues specific to fusion 6891 targets, including the results of relevant classified experiments and classified 6892 information on the implications of IFE targets for the proliferation of nuclear 6893 weapons. 6894 This is the report of the Panel on Fusion Target Physics, which is intended to 6895 feed into the broader assessment of IFE being done by the NRC committee. It 6896 consists of an unclassified body, which contains all of the panel’s conclusions and 6897 recommendations, as well as three classified appendices, which provide additional 6898 support and documentation. 6899 BACKGROUND 6900 Fusion is the process by which energy is produced in the sun, and, on a more 6901 human scale, is the one of the key processes involved in the detonation of a 6902 thermonuclear bomb. If this process could be “tamed” to provide a controllable 6903 source of energy that can be converted to electricity—as nuclear fission has been in 6904 currently operating nuclear reactors—it is possible that nuclear fusion could provide a 6905 new method for producing low-carbon electricity to meet the U. S. and world 6906 growing energy needs. 1 The operative definition of ignition adopted by the panel, “gain greater than unity,” is the same as that used in the earlier National Research Council NRC report: Review of the Department of Energy's Inertial Confinement Fusion Program,Washington, D.C.: National Academy Press (1997). A-47
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 6907 For inertial fusion to occur in a laboratory, fuel material (typically deuterium 6908 and tritium) must be confined for an adequate length of time at an appropriate density 6909 and temperature to overcome the Coulomb repulsion of the nuclei and allow them to 6910 fuse. In inertial confinement fusion (ICF)—the concept investigated in this report 2—a 6911 driver (e.g., a laser, particle beam, or pulsed magnetic field) delivers energy to the 6912 fuel target, heating and compressing it to the conditions required for ignition. Most 6913 ICF concepts compress a small amount of fuel directly to thermonuclear burn 6914 conditions (a hot spot) and propagate the burn via alpha particle deposition through 6915 adjacent high-density fuel regions, thereby generating a significant energy output. 6916 There are two major concepts for inertial confinement fusion target design: 6917 direct-drive targets, in which the driver energy strikes directly on the fuel capsule, 6918 and indirect-drive targets, in which the driver energy first strikes the inside surface of 6919 a hollow chamber (a hohlraum) surrounding the fuel capsule, producing energetic X- 6920 rays that compress the fuel capsule. Conventional direct and indirect drive share 6921 many key physics issues (e.g., energy coupling, the need for driver uniformity, and 6922 hydrodynamic instabilities); however, there are also issues that are unique to each 6923 concept. 6924 The only facility in the world that was designed to conduct ICF experiments 6925 that address the ignition scale is the NIF at LLNL. The NIF driver is a solid-state 6926 laser. For the first ignition experiments, the NIF team has chosen indirect-drive 6927 targets. The NIF can also be configured for direct drive. In addition, important work 6928 on laser-driven, direct-drive targets (albeit at less than ignition scale) is also under 6929 way in the United States at the Naval Research Laboratory and the OMEGA laser at 6930 the University of Rochester. Heavy-ion-beam drivers are being investigated at the 6931 Lawrence Berkeley National Laboratory (LBNL), LLNL, and the Princeton Plasma 6932 Physics Laboratory (PPPL), and magnetic implosion techniques are being explored 6933 on the Z machine at Sandia National Laboratory (SNL) and at Los Alamos National 6934 Laboratory (LANL). Important ICF research is also under way in other countries, as 6935 discussed later in this report. 6936 SPECIFIC CONCLUSIONS AND RECOMMENDATIONS 6937 The panel’s key conclusions and recommendations, all of them specific to 6938 various aspects of inertial confinement fusion, are presented below. They are labeled 6939 according to the chapter and number order in which they appear in the text, to provide 6940 the reader with an indicator of where to find a more complete discussion. This 2 Inertial confinement fusion (ICF) is the process by which the target is heated and compressed by the driver to reach fusion conditions. Inertial fusion energy (IFE) is the process by which useful energy is extracted from ignition and burn of ICF fuel targets. A-48
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 6941 summary ends with two overarching conclusions and an overarching recommendation 6942 derived from viewing all of the information presented to the panel as a whole. 6943 6944 6945 Targets for Indirect Laser Drive 6946 6947 CONCLUSION 4-1: The national program to achieve ignition using indirect 6948 laser drive has several physics issues that must be resolved if it is to achieve 6949 ignition. At the time of this writing, the capsule/hohlraum performance in the 6950 experimental program, which is carried out at the NIF, has not achieved the 6951 compressions and neutron yields expected based on computer simulations. At present, 6952 these disparities are not well understood. While a number of hypotheses concerning 6953 the origins of the disparities have been put forth, it is apparent to the panel that the 6954 treatments of the detrimental effects of laser-plasma interactions (LPI) in the target 6955 performance predictions are poorly validated and may be significantly inadequate. A 6956 greatly improved understanding of laser-plasma interactions will be required of the 6957 ICF community. 6958 CONCLUSION 4-2: Based on its analysis of the gaps in current understanding 6959 of target physics and the remaining disparities between simulations and 6960 experimental results, the panel assesses that ignition using laser indirect drive is 6961 not likely in the next several years. As the panel understands it, the National 6962 Ignition Campaign (NIC) plan suggests that ignition is expected after the completion 6963 of the tuning program lasting 1-2 years that is presently under way and scheduled to 6964 conclude at the end of FY2012. While this success-oriented schedule remains 6965 possible, resolving the present issues and addressing any new challenges that might 6966 arise are likely to push the timetable for ignition to 2013-2014 or beyond. 6967 6968 Targets for Indirect-Drive Laser Inertial Fusion Energy 6969 6970 CONCLUSION 4-4: The target design for a proposed indirect-drive inertial 6971 fusion energy system (the laser inertial fusion energy or LIFE program 6972 developed by LLNL) incorporates plausible solutions to many technical 6973 problems, but the panel assesses that the robustness of the physics design for the 6974 LIFE target concept is low. 6975 • The proposed LIFE target presented to the panel has several modifications 6976 relative to the target currently used in the NIC—for example, rugby 6977 hohlraums, shine shields, and high-density carbon ablators—and the A-49
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 6978 effects of these modifications may not be trivial. For this reason, R&D and 6979 validation steps would still be needed. 6980 • There is no evidence to indicate that the margin in the calculated target 6981 gain ensures either sufficient gain for the LIFE target or its ignition. If 6982 ignition is assumed, the gain margin briefed to the panel, which ranged 6983 from 25 percent to almost 60 percent when based on a calculation that 6984 used hohlraum and fuel materials characteristic of the NIC rather than the 6985 LIFE target, is unlikely to compensate for the phenomena relegated to it— 6986 for example, the effects of mix—under any but the most extremely 6987 favorable eventuality. In addition, the tight coupling of LIFE to what can 6988 be tested on the NIF constrains the potential design space for laser-driven, 6989 indirect-drive IFE. 6990 6991 6992 Targets for Direct-Drive Laser Inertial Fusion Energy 6993 6994 CONCLUSION 4-6: The prospects for ignition using laser direct drive have 6995 improved enough that it is now a plausible alternative to laser indirect drive for 6996 achieving ignition and for generating energy. 6997 6998 • The major concern with laser direct drive has been the difficulty of 6999 achieving the symmetry required to drive such targets. Advances in beam- 7000 smoothing and pulse-shaping appear to have lessened the risks of 7001 asymmetries. This assessment is supported by data from capsule 7002 implosions (performed at the University of Rochester's OMEGA laser), 7003 but it is limited by the relatively low drive energy of the implosion 7004 experiments that have thus far been possible. Because of this, the panel’s 7005 assessment of targets for laser-driven, direct-drive IFE is not qualitatively 7006 equivalent to that of laser-driven, indirect-drive targets. 7007 • Further evaluation of the potential of laser direct-drive targets for IFE will 7008 require experiments at drive energies much closer to the ignition scale. 7009 • Capsule implosions on OMEGA have established an initial scaling point 7010 that indicates the potential of direct-drive laser targets for ignition and 7011 high yield. 7012 • Polar direct-drive targets 3 will require testing on the NIF. 3 In polar direct drive, the driver beams are clustered in one or two rings at opposing poles. To increase the uniformity of the drive, polar drive beams strike the capsule obliquely, and the driver energy is biased in favor of the more equatorial beams. A-50
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 7013 • Demonstration of polar-drive ignition on the NIF will be an important step 7014 toward an IFE program. 7015 • If a program existed to reconfigure NIF for polar drive, direct-drive 7016 experiments that address the ignition scale could be performed as early as 7017 2017. 7018 7019 7020 Fast Ignition 7021 7022 Fast ignition (FI) requires a combination of long-pulse (implosion) and short- 7023 pulse (ignition) lasers. Aspects of fast ignition by both electrons and protons were 7024 briefed to the panel. Continued fundamental research into fast ignition theory and 7025 experiments, the acceleration of electrons and ions by ultrashort-pulse lasers, and 7026 related high-intensity laser science is justified. However, issues surrounding low 7027 laser-target energy coupling, a complicated target design, and the existence of more 7028 promising concepts (such as shock ignition) led the panel to the following conclusion 7029 regarding the relative priority of fast ignition for fusion energy. 7030 7031 CONCLUSION 4-5: At this time, fast ignition appears to be a less promising 7032 approach for IFE than other ignition concepts. 7033 7034 7035 Laser-Plasma Interactions 7036 7037 A variety of LPI take place when an intense laser pulse hits the target capsule 7038 or surrounding hohlraum. Undesirable effects include backscattering of laser light, 7039 which can result in loss of energy; cross-beam energy transfer among intersecting 7040 laser beams, which can cause loss of energy or affect implosion symmetry; 7041 acceleration of suprathermal “hot electrons,” which then can penetrate and preheat the 7042 capsule’s interior and limit later implosion; and filamentation, a self-focusing 7043 instability that can exacerbate other LPI. LPI have been a key limiting factor in laser 7044 inertial confinement fusion, including the NIC indirect-drive targets, and are still 7045 incompletely understood. 7046 7047 CONCLUSION 4-11: Lack of understanding of laser-plasma interactions 7048 remains a substantial but as yet unquantified consideration in ICF and IFE 7049 target design. 7050 7051 RECOMMENDATION 4-1: DOE should foster collaboration among different 7052 research groups on the modeling and simulation of laser-plasma interactions. A-51
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 7053 7054 7055 Heavy-Ion Targets 7056 7057 A wide variety of heavy-ion target designs has been investigated, including 7058 indirect-drive, hohlraum/capsule targets that resemble NIC targets. Recently, the 7059 emphasis has shifted to direct-drive targets, but to date the analysis of how these 7060 targets perform has been based on computation rather than experiment, and the codes 7061 have not been benchmarked with experiments in relevant regimes. 7062 7063 CONCLUSION 4-12: The U.S. heavy-ion-driven fusion program is considering 7064 direct-drive and indirect-drive target concepts. There is also significant current 7065 work on advanced target designs. 4 This work is at a very early stage, but if 7066 successful, may provide very high gain. 7067 • The work in the heavy-ion fusion (HIF) program involves solid and 7068 promising science. 7069 • Work on heavy-ion drivers is complementary to the laser approaches to 7070 IFE and offers a long-term driver option for beam-driven targets. 7071 • The HIF program relating to advanced target designs is in a very early 7072 stage and is unlikely to be ready for technical assessment in the near term. 7073 • The development of driver technology will take several years and the cost 7074 to build a significant accelerator driver facility for any target is likely to be 7075 very high. 7076 7077 7078 Z-Pinch Targets 7079 7080 Current Z-pinch direct-drive concepts utilize the pressure of a pulsed, high 7081 magnetic field to implode deuterium-tritium fuel to fusion conditions. Simulations 7082 predict that directly using the pressure of the magnetic field to implode and compress 7083 the target can greatly increase the efficiency with which the electrical energy is 7084 coupled to the fuel as compared with the efficiency of indirect drive from Z-pinch X- 7085 ray sources. There is work under way on both classified and unclassified target 7086 designs. 7087 7088 CONCLUSION 4-13: Sandia National Laboratory is working on a Z-pinch 7089 scheme that has the potential to produce high gain with good energy efficiency, 4 Advanced designs include direct-drive, conical X-target configurations, see Chapter 2. A-52
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 7090 but concepts for an energy delivery system based on this driver are too 7091 immature to be evaluated at this time. 7092 It is not yet clear that the work at SNL will ultimately result in the high gain 7093 predicted by computer simulations, but initial results are promising and it is the 7094 panel’s opinion that significant progress in the physics may be made in a year’s time. 7095 The pulsed power approach is unique in that its goal is to deliver large energy (~10 7096 MJ) to targets with good efficiency (≥10 percent) and generate large fusion yields at 7097 low repetition rates. 7098 7099 7100 Target Fabrication 7101 7102 Current targets for inertial confinement fusion experiments tend to be one-off 7103 designs, with specifications that change according to the experiments being run. In 7104 contrast, targets for future IFE power plants will have to have standard, low-cost 7105 designs that are mass-produced in numbers as high as a million targets per day per 7106 power plant. The panel examined the technical feasibility of producing targets for 7107 various drivers, including limited aspects of fabrication for IFE. However, a full 7108 examination of the issues of mass production and low cost is the province of the NRC 7109 IFE committee study. 7110 7111 CONCLUSION 4-7: In general, the science and engineering of manufacturing 7112 fusion targets for laser-based ICF is well advanced and meets the needs of those 7113 experiments, although additional technologies may be needed for IFE. 7114 Extrapolating this status to predict the success of manufacturing IFE targets is 7115 reasonable if the target is only slightly larger than the ICF target and the process is 7116 scalable. However, subtle additions to the design of the ICF target to improve its 7117 performance (greater yield) and survivability in an IFE power plant may significantly 7118 affect the manufacturing paradigm. 7119 7120 7121 Proliferation Risks of IFE 7122 7123 Many modern nuclear weapons rely on a fusion stage as well as a fission 7124 stage, and there has been discussion of the potential for host state proliferation— 7125 particularly vertical proliferation 5—associated with the siting of an IFE power plant. 5 Vertical proliferation refers to the enhancement of a country’s capability to move from simple weapons to more sophisticated weapons. A-53
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 7126 The panel was asked to evaluate the proliferation risks associated with IFE, 7127 particularly with regard to IFE targets. 7128 7129 CONCLUSION 3-1: At present, more proliferation concerns are associated with 7130 indirect-drive targets than with direct-drive targets. However, the spread of 7131 technology around the world may eventually render these concerns moot. Remaining 7132 concerns are likely to focus on the use of classified codes for target design. 7133 CONCLUSION 3-2: The nuclear weapons proliferation risks associated with 7134 fusion power plants are real but are likely to be controllable. These risks fall into 7135 three categories: 7136 • Knowledge transfer, 7137 • Special Nuclear Material (SNM) production, and 7138 • Tritium diversion. 7139 7140 OVERARCHING CONCLUSIONS AND RECOMMENDATION 7141 While the focus of this panel was on ICF target physics, the need to evaluate 7142 driver-target interactions required considering driver characteristics as well. This 7143 broader analysis led the panel to the following overarching conclusions and a 7144 recommendation. 7145 OVERARCHING CONCLUSION 1: NIF has the potential to support the 7146 development and further validation of physics and engineering models relevant 7147 to several IFE concepts, from indirect-drive hohlraum designs to polar direct- 7148 drive ICF and shock ignition. 7149 • In the near to intermediate term, NIF is the only platform that can 7150 provide information relevant to a wide range of IFE concepts at 7151 ignition scale. So far as target physics is concerned, it is a modest step 7152 from NIF scale to IFE scale. 7153 • Targets for all laser-driven IFE concepts (both direct- and indirect- 7154 drive) can be tested on NIF. In particular, reliable target performance 7155 would need to be demonstrated before investments could confidently 7156 be made in development of laser-driven IFE target designs. 7157 NIF will also be helpful in evaluating indirectly driven, heavy-ion targets. It will be 7158 less helpful in gathering information relevant to current Z-pinch, heavy-ion direct 7159 drive, and heavy-ion advanced target concepts. A-54
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PREPUBLICATION COPY--SUBJECT TO FURTHER EDITORIAL CORRECTION 7160 OVERARCHING CONCLUSION 2: It would be advantageous to continue 7161 research in a range of IFE concepts, both because: 7162 • The challenges involved in the current laser indirect-drive approach 7163 in the single-pulse National Nuclear Security Administration program 7164 at the NIF have not yet been resolved and, 7165 • The alternatives to laser indirect drive have technical promise to 7166 produce high gain. 7167 In particular, the panel concludes that laser direct drive is a viable concept to 7168 be pursued on the NIF. SNL’s work on Z-pinch can mitigate the risk of NIF not 7169 operating as expected. This work is at a very early stage, but is highly complementary 7170 to the NIF approach, because none of the work being done at SNL relies on 7171 successful ignition at the NIF, and key aspects of the target physics can be 7172 investigated on the existing Z-machine. Finally, emerging heavy-ion designs could be 7173 fruitful in the long term. 7174 OVERARCHING RECOMMENDATION: The panel recommends against 7175 pursuing a down-select decision for IFE at this time, either for a specific concept 7176 such as LIFE, or for a specific target type/driver combination. 7177 Further R&D will be needed both on indirect drive and other ICF concepts, 7178 even following successful ignition at the NIF, to determine the best path for IFE in 7179 the coming decades. 7180 A-55