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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).
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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