The text below is excerpted from National Research Council, Assessment of Inertial Confinement Fusion Targets (The National Academies Press, Washington, D.C., 2013).
In the fall of 2010, the Office of the U.S. Department of Energy’s (DOE’s) Under Secretary for Science asked for a National Research Council (NRC) committee to investigate the prospects for generating power using inertial fusion energy (IFE), noting that a key test of viability for this concept—ignition1—could be demonstrated at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in the relatively near term. In response, the NRC formed both the Committee on the Assessment of the Prospects for Inertial Fusion Energy (“the committee”) to investigate the overall prospects for IFE in an unclassified report and the separate Panel on Fusion Target Physics (“the panel”) to focus on issues specific to fusion targets, including the results of relevant classified experiments and classified information on the implications of IFE targets for the proliferation of nuclear weapons.
This is the report of the Panel on Fusion Target Physics, which is intended to feed into the broader assessment of IFE being done by the NRC committee. It
1 The operative definition of ignition adopted by the panel, “gain greater than unity,” is the same as that used in the earlier NRC report Review of the Department of Energy’s Inertial Confinement Fusion Program, Washington, D.C.: National Academy Press (1997).
consists of an unclassified body, which contains all of the panel’s conclusions and recommendations, as well as three classified appendices, which provide additional support and documentation.
Fusion is the process by which energy is produced in the sun, and, on a more human scale, is the one of the key processes involved in the detonation of a thermonuclear bomb. If this process could be “tamed” to provide a controllable source of energy that can be converted to electricity—as nuclear fission has been in currently operating nuclear reactors—it is possible that nuclear fusion could provide a new method for producing low-carbon electricity to meet U.S. and world growing energy needs.
For inertial fusion to occur in a laboratory, fuel material (typically deuterium and tritium) must be confined for an adequate length of time at an appropriate density and temperature to overcome the Coulomb repulsion of the nuclei and allow them to fuse. In inertial confinement fusion (ICF)—the concept investigated in this report2—a driver (e.g., a laser, particle beam, or pulsed magnetic field) delivers energy to the fuel target, heating and compressing it to the conditions required for ignition. Most ICF concepts compress a small amount of fuel directly to thermonuclear burn conditions (a hot spot) and propagate the burn via alpha particle deposition through adjacent high-density fuel regions, thereby generating a significant energy output.
There are two major concepts for inertial confinement fusion target design: direct-drive targets, in which the driver energy strikes directly on the fuel capsule, and indirect-drive targets, in which the driver energy first strikes the inside surface of a hollow chamber (a hohlraum) surrounding the fuel capsule, producing energetic X-rays that compress the fuel capsule. Conventional direct and indirect drive share many key physics issues (e.g., energy coupling, the need for driver uniformity, and hydrodynamic instabilities); however, there are also issues that are unique to each concept.
The only facility in the world that was designed to conduct ICF experiments that address the ignition scale is the NIF at LLNL. The NIF driver is a solid-state laser. For the first ignition experiments, the NIF team has chosen indirect-drive targets. The NIF can also be configured for direct drive. In addition, important work on laser-driven, direct-drive targets (albeit at less than ignition scale) is also under way in the United States at the Naval Research Laboratory and the OMEGA
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.
laser at the University of Rochester. Heavy-ion-beam drivers are being investigated at the Lawrence Berkeley National Laboratory (LBNL), LLNL, and the Princeton Plasma Physics Laboratory (PPPL), and magnetic implosion techniques are being explored on the Z machine at Sandia National Laboratories (SNL) and at Los Alamos National Laboratory (LANL). Important ICF research is also under way in other countries, as discussed later in this report.
SPECIFIC CONCLUSIONS AND RECOMMENDATIONS
The panel’s key conclusions and recommendations, all of them specific to various aspects of inertial confinement fusion, are presented below. They are labeled according to the chapter and number order in which they appear in the text, to provide the reader with an indicator of where to find a more complete discussion. This summary ends with two overarching conclusions and an overarching recommendation derived from viewing all of the information presented to the panel as a whole.
Targets for Indirect Laser Drive
CONCLUSION 4-1: The national program to achieve ignition using indirect laser drive has several physics issues that must be resolved if it is to achieve ignition. At the time of this writing, the capsule/hohlraum performance in the experimental program, which is carried out at the NIF, has not achieved the compressions and neutron yields expected based on computer simulations. At present, these disparities are not well understood. While a number of hypotheses concerning the origins of the disparities have been put forth, it is apparent to the panel that the treatments of the detrimental effects of laser-plasma interactions (LPI) in the target performance predictions are poorly validated and may be very inadequate. A much better understanding of LPI will be required of the ICF community.
CONCLUSION 4-2: Based on its analysis of the gaps in current understanding of target physics and the remaining disparities between simulations and experimental results, the panel assesses that ignition using laser indirect drive is not likely in the next several years.
The National Ignition Campaign (NIC) plan—as the panel understands it— suggests that ignition is planned after the completion of a tuning program lasting 1-2 years that is presently under way and scheduled to conclude at the end of FY2012. While this success-oriented schedule remains possible, resolving the present issues and addressing any new challenges that might arise are likely to push the timetable for ignition to 2013-2014 or beyond.
Targets for Indirect-Drive Laser Inertial Fusion Energy
CONCLUSION 4-4: The target design for a proposed indirect-drive IFE system (the Laser Inertial Fusion Energy, or LIFE, program developed by LLNL) incorporates plausible solutions to many technical problems, but the panel assesses that the robustness of the physics design for the LIFE target concept is low.
- The proposed LIFE target presented to the panel has several modifications relative to the target currently used in the NIC (e.g., rugby hohlraums, shine shields, and high-density carbon ablators), and the effects of these modifications may not be trivial. For this reason, R&D and validation steps would still be needed.
- There is no evidence to indicate that the margin in the calculated target gain ensures either its ignition or sufficient gain for the LIFE target. If ignition is assumed, then the gain margin briefed to the panel, which ranged from 25 percent to almost 60 percent when based on a calculation that used hohlraum and fuel materials characteristic of the NIC rather than the LIFE target, is unlikely to compensate for the phenomena relegated to it—for example, the effects of mix—under any but the most extremely favorable eventuality. In addition, the tight coupling of LIFE to what can be tested on the NIF constrains the potential design space for laser-driven, indirect-drive IFE.
Targets for Direct-Drive Laser Inertial Fusion Energy
CONCLUSION 4-6: The prospects for ignition using laser direct drive have improved enough that it is now a plausible alternative to laser indirect drive for achieving ignition and for generating energy.
- The major concern with laser direct drive has been the difficulty of achieving the symmetry required to drive such targets. Advances in beam-smoothing and pulse-shaping appear to have lessened the risks of asymmetries. This assessment is supported by data from capsule implosions (performed at the University of Rochester’s OMEGA laser), but it is limited by the relatively low drive energy of the implosion experiments that have thus far been possible. Because of this, the panel’s assessment of laser-driven, direct-drive targets is not qualitatively equivalent to that of laser-driven, indirect-drive targets.
- Further evaluation of the potential of laser direct-drive targets for IFE will require experiments at drive energies much closer to the ignition scale.
- Capsule implosions on OMEGA have established an initial scaling point that indicates the potential of direct-drive laser targets for ignition and high yield.
- Polar direct-drive targets3 will require testing on the NIF.
- Demonstration of polar-drive ignition on the NIF will be an important step toward an IFE program.
- If a program existed to reconfigure the NIF for polar drive, direct-drive experiments that address the ignition scale could be performed as early as 2017.
Fast ignition (FI) requires a combination of long-pulse (implosion) and short-pulse (ignition) lasers. Aspects of fast ignition by both electrons and protons were briefed to the panel. Continued fundamental research into fast ignition theory and experiments, the acceleration of electrons and ions by ultrashort-pulse lasers, and related high-intensity laser science is justified. However, issues surrounding low laser-target energy coupling, a complicated target design, and the existence of more promising concepts (such as shock ignition) led the panel to the next conclusion regarding the relative priority of fast ignition for fusion energy.
CONCLUSION 4-5: At this time, fast ignition appears to be a less promising approach for IFE than other ignition concepts.
A variety of LPI take place when an intense laser pulse hits the target capsule or surrounding hohlraum. Undesirable effects include backscattering of laser light, which can result in loss of energy; cross-beam energy transfer among intersecting laser beams, which can cause loss of energy or affect implosion symmetry; acceleration of suprathermal “hot electrons,” which then can penetrate and preheat the capsule’s interior and limit later implosion; and filamentation, a self-focusing instability that can exacerbate other LPI. LPI have been a key limiting factor in laser inertial confinement fusion, including the NIC indirect-drive targets, and are still incompletely understood.
CONCLUSION 4-11: The lack of understanding surrounding laser-plasma interactions remains a substantial but as yet unquantified consideration in ICF and IFE target design.
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.
RECOMMENDATION 4-1: DOE should foster collaboration among different research groups on the modeling and simulation of laser-plasma interactions.
A wide variety of heavy-ion target designs has been investigated, including indirect-drive, hohlraum/capsule targets that resemble NIC targets. Recently, the emphasis has shifted to direct-drive targets, but to date the analysis of how these targets perform has been based on computation rather than experiment, and the codes have not been benchmarked with experiments in relevant regimes.
CONCLUSION 4-12: The U.S. heavy-ion-driven fusion program is considering direct-drive and indirect-drive target concepts. There is also significant current work on advanced target designs.4 This work is at a very early stage, but if successful may provide very high gain.
- The work in the heavy-ion fusion (HIF) program involves solid and promising science.
- Work on heavy-ion drivers is complementary to the laser approaches to IFE and offers a long-term driver option for beam-driven targets.
- The HIF program relating to advanced target designs is in a very early stage and is unlikely to be ready for technical assessment in the near term.
- The development of driver technology will take several years, and the cost to build a significant accelerator driver facility for any target is likely to be very high.
Current Z-pinch direct-drive concepts utilize the pressure of a pulsed, high magnetic field to implode deuterium-tritium fuel to fusion conditions. Simulations predict that directly using the pressure of the magnetic field to implode and compress the target can greatly increase the efficiency with which the electrical energy is coupled to the fuel as compared with the efficiency of indirect drive from Z-pinch X-ray sources. There is work under way on both classified and unclassified target designs.
CONCLUSION 4-13: Sandia National Laboratories is leading a research effort on a Z-pinch scheme that has the potential to produce high gain with good energy
4 Advanced designs include direct-drive, conical X-target configurations (see Chapter 2).
efficiency, but concepts for an energy delivery system based on this driver are too immature to be evaluated at this time.
It is not yet clear that the work at SNL will ultimately result in the high gain predicted by computer simulations, but initial results are promising and it is the panel’s opinion that significant progress in the physics may be made in a year’s time. The pulsed-power approach is unique in that its goal is to deliver a large amount of energy (~10 MJ) to targets with good efficiency (≥10 percent) and to generate large fusion yields at low repetition rates.
Current targets for inertial confinement fusion experiments tend to be one-off designs, with specifications that change according to the experiments being run. In contrast, targets for future IFE power plants will have to have standard, low-cost designs that are mass-produced in numbers as high as a million targets per day per power plant. The panel examined the technical feasibility of producing targets for various drivers, including limited aspects of fabrication for IFE. However, a full examination of the issues of mass production and low cost is the province of the NRC IFE committee study.
CONCLUSION 4-7: In general, the science and engineering of manufacturing fusion targets for laser-based ICF are well advanced and meet the needs of those experiments, although additional technologies may be needed for IFE. Extrapolating this status to predict the success of manufacturing IFE targets is reasonable if the target is only slightly larger than the ICF target and the process is scalable. However, subtle additions to the design of the ICF target to improve its performance (greater yield) and survivability in an IFE power plant may significantly affect the manufacturing paradigm.
Proliferation Risks of IFE
Many modern nuclear weapons rely on a fusion stage as well as a fission stage, and there has been discussion of the potential for host state proliferation— particularly vertical proliferation—associated with the siting of an IFE power plant. The panel was asked to evaluate the proliferation risks associated with IFE, particularly with regard to IFE targets.
CONCLUSION 3-1: At present, there are more proliferation concerns associated with indirect-drive targets than with direct-drive targets. However, the spread of
technology around the world may eventually render these concerns moot. Remaining concerns are likely to focus on the use of classified codes for target design.
CONCLUSION 3-2: The nuclear weapons proliferation risks associated with fusion power plants are real but are likely to be controllable. These risks fall into three categories:
- Knowledge transfer,
- Special nuclear material (SNM) production, and
- Tritium diversion.
OVERARCHING CONCLUSIONS AND RECOMMENDATION
While the focus of this panel was on ICF target physics, the need to evaluate driver-target interactions required considering driver characteristics as well. This broader analysis led the panel to the following overarching conclusions and a recommendation.
OVERARCHING CONCLUSION 1: The NIF has the potential to support the development and further validation of physics and engineering models relevant to several IFE concepts, from indirect-drive hohlraum designs to polar direct-drive ICF and shock ignition.
- In the near to intermediate term, the NIF is the only platform that can provide information relevant to a wide range of IFE concepts at ignition scale. Insofar as target physics is concerned, it is a modest step from NIF scale to IFE scale.
- Targets for all laser-driven IFE concepts (both direct-drive and indirect-drive) can be tested on the NIF. In particular, reliable target performance would need to be demonstrated before investments could confidently be made in the development of laser-driven IFE target designs.
The NIF will also be helpful in evaluating indirectly driven, heavy-ion targets. It will be less helpful in gathering information relevant to current Z-pinch, heavy-ion direct drive, and heavy-ion advanced target concepts.
OVERARCHING CONCLUSION 2: It would be advantageous to continue research on a range of IFE concepts, for two reasons:
- The challenges involved in the current laser indirect-drive approach in the single-pulse National Nuclear Security Administration program at the NIF have not yet been resolved, and
- The alternatives to laser indirect drive have technical promise to produce high gain.
In particular, the panel concludes that laser direct drive is a viable concept to be pursued on the NIF. SNL’s work on Z-pinch can serve to mitigate risk should the NIF not operate as expected. This work is at a very early stage but is highly complementary to the NIF approach, because none of the work being done at SNL relies on successful ignition at the NIF, and key aspects of the target physics can be investigated on the existing Z-machine. Finally, emerging heavy-ion designs could be fruitful in the long term.
OVERARCHING RECOMMENDATION: The panel recommends against pursuing a down-select decision for IFE at this time, either for a specific concept such as LIFE or for a specific target type/driver combination.
Further R&D will be needed on indirect drive and other ICF concepts, even following successful ignition at the NIF, to determine the best path for IFE in the coming decades.