Assessment of Inertial Confinement Fusion Targets (2013) / Chapter Skim
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4 Evaluation of ICF Targets
4 Evaluation of ICF Targets
Pages 45-86
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program aimed at achieving ignition, the National Ignition Campaign (NIC) , lays out a path via laser indirect drive (ID)
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. Box 4-1 Laser-Plasma Interactions In laser-driven inertial confinement fusion (ICF)
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The relatively short beam paths in coronal plasma suggest that other LPI, and hot elec trons, may be controllable in the extrapolation to ignition targets for direct drive, though most of the key experiments remain to be done. However, the greater laser intensities needed for shock ignition may cause harmful LPI; this must be studied.
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Four key input variables are to be optimized in the NIC tuning campaigns: • The implosion adiabat (usually designated α) , which strongly affects the resistance of the capsule to implosion; • The implosion velocity V; • The amount of capsule material involved in mixing across the single inter face characteristic of this class of capsule designs, M; and • The overall shape of the implosion, which is characterized by a dimension less parameter S
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They are discussed individually below. Implosion Velocity Perhaps the most critical discrepancy is that the measured implosion velocity of nonoptimized capsules is ~10 percent lower than the calculated velocity, even early in the implosion.
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The potential for such an occurrence clearly is related to the precision of target fabrication; some target fabrication technology issues are discussed below. Implosion Adiabat Measurements indicate the existence of disparities between the calculated and actual adiabats on which NIF capsules implode.
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CONCLUSION 4-2: Based on its analysis of the gaps in current understanding of target physics and the remaining disparities between simulations and experi mental results, the panel assesses that ignition using laser indirect drive is not likely in the next several years.
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CONCLUSION 4-3: Ignition of a laser-driven, indirect-drive capsule will pro vide opportunities for follow-up work to improve understanding of the poten tial for IFE. • If ignition is achieved with indirect drive at the NIF, then an energy gain of 50-100 should be possible at a future facility.
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FIGURE 4-1 The LIFE target design. Modifications from the NIC target design include the curved ("rugby")
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The reason for this new concept is to reduce the time required to form the fuel layer and thereby reduce the tritium inventory for the power plant. The design is for the fuel layer to be maintained as a supercooled liquid at a temperature sufficiently below the freezing point to achieve the required vapor pressure.
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The differences between the NIC and LIFE targets and drives also raise additional issues, which are discussed below. This section on the LIFE design concludes with an evaluation of the robustness of the LIFE target design.
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The outlook for carryng out this plan is clearly independent of the target design, i but any compromise in achieving this energy goal could severely reduce the likeli hood of achieving sufficient gain for a power plant to be feasible. Calculations indicate that a redesign of the target hohlraum from the cylinder shape used thus far at the NIF to a rugby shape can increase the drive temperatures for the enclosed capsule.
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Only once the target is demonstrated to meet the specifica tions can the feasibility of mass-producing these targets for the desired cost be accurately assessed. The plan to form the outer fuel layer of a LIFE target capsule by wicking liquid DT into a layer of nanoporous foam is a radical departure from the method used for making targets for the NIF.
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The critical point from the standpoint of target design is that a compromise on any target specification or other aspect of fabrication quality would be likely to significantly reduce target gain. Additional Considerations The combination of extreme conditions that exist in a power plant reaction chamber and the very tight specifications that must be maintained for an IFE power plant to function result in an unusually tight coupling between the target design and some of what would typically be considered the separable engineer ing aspects of a power plant design.
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The Sombrero Power Plant study (Sviatoslavsky et al., 1992) was used as a starting point.1 The HAPL program was based on laser-driven, direct-drive targets because of their poten tial for higher drive efficiency, simpler target fabrication, lower estimated cost, and smaller inventory for material recycling.
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An assessment of this effect has not been presented to the panel. Some unspecified portion of the gain margin calculated for the LIFE target has been allocated to compensating for nonoptimum insertion, but turbulence or other irregularities in the Xe gas through which the targets must pass could lead to suf ficient inaccuracy not only to overwhelm that margin, but also to preclude capsule ignition.
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The main factors leading to this assessment are the following: • Ignition of a fusion target operating in the physics regime of laser-driven ICF has never been observed, but a robust design would have to reliably produce a large gain under much less controlled conditions than are normal in laboratory experiments. Moreover, the parameter space over which simu lations predict adequate gain for the LIFE target capsule is relatively small, and the optimization of several parameters, an integral part of NIC, can be expected to further narrow the parameter space over which sufficient gain might be obtained; • Significant departures from predicted operation have been observed on implosion experiments pertinent to the LIFE target design.
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SOLID-STATE-LASER-DRIVEN, DIRECT-DRIVE FUSION Current Status The leader in direct-drive inertial confinement fusion with solid-state lasers is the Laboratory for Laser Energetics (LLE) at the University of Rochester, which operates the OMEGA Laser Facility (OMEGA and OMEGA EP)
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However, it is not clear that the final assembly procedure strictly adhered to this principle. Reconfiguring the NIF to symmetric direct drive geometry represents the low est target physics risk but the highest facility cost, and it would disrupt weapons physics experiments using hohlraums.
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14 J Fernandez, LANL, "Inertial Confinement Fusion (ICF)
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Initial shock ignition designs for the NIF have 1-D gains of 70 at 680 kJ, with about half of that total energy in the shock generation pulse. PD diagnostic commissioning targets using existing ID phase plates are being imploded on the NIF (Cok et al., 2008)
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Continued development of these models -- including the effects of nonlocal transport -- is important, especially for PD beam geometries. Shock Ignition Fully integrated 2-D point designs for the NIF PD shock ignition targets are required in order to plan for experimental campaigns on the NIF.
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Polar-Drive Physics Understanding of the following areas of polar-drive target physics need to be improved: • Deposition in low-density plasma by oblique beams at equator, including three-dimensional (3-D) laser ray trace algorithms that are compatible with PD geometry.
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This should include experiments to explore the performance of shock ignition targets on the NIF. To date, the LLE ICF program has been focused on the development of laser beam-smoothing technologies and single-shot ICF target physics experiments, which is the appropriate scope of the NNSA program.
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Further, LLE has much to contribute in target physics and target fabrication if KrF lasers prove more attractive as the laser driver in a DD power plant. Additional Considerations Target Injection A key issue here is the repeatability of any phenomena that significantly perturb the target's trajectory.
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These are issues that need to be thoroughly addressed early in any future IFE program. KRYPTON FLUORIDE LASER-DRIVEN, DIRECT-DRIVE FUSION The leader in DD inertial confinement fusion with KrF lasers is NRL in W ashington, D.C., which operates the Nike and Electra lasers.
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This includes utilizing the NIF-equivalent OMEGA EP beam parameters to study LPI at plasma scale lengths that are relevant to ignition high-yield DD IFE targets. A series of DD IFE target designs have been studied with the goal of maximiz ing target gain while minimizing laser energy.
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The growing collaboration will allow 15 M Rosen, LLNL, "Understanding of LPI and Its Impact on Indirect Drive," presentation to the panel on September 21, 2011.
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The theory and simulation efforts included the development of better physics models for the FAST code, improved two- and three-dimensional hydroimplosion simulations, and improved ability to perform LPI simulations. NRL also proposed the development of one KrF IFE beam line that was capable of delivering ~20 kJ on target to study target interaction and LPI physics at IFE-relevant intensity and plasma scale lengths.
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The final decision between 351 and 248 nm should be based on a total system performance analysis, including laser efficiency, durability, power plant integration issues, and overall target gain and performance. At this point, it would seem that an overall collaboration in direct-drive target physics and a competition between driver technologies at the beamline level would be a prudent technology maturation path.
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Historically, the mainline heavy-ion fusion (HIF) target design was developed to leverage the NIF experiments to demonstrate hot-spot ignition of an indirect drive target.
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. There are plans to extend the DD target design to 2-D design to incorporate a PD illumination geometry as well as a tamper and shock ignition assist.
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While heavy ions may represent a promising long-term option for effi cient, reliable, repetitive fusion power plants, they probably represent a second- or third-generation capability.
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• 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.
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The Sandia Z100 program has been developed to address some of the key target physics issues in pulsed-power ICF. The pulsed-power technology program within the NNSA Science Campaigns is developing some of the next-generation technologies that would advance the pulsed-power driver issues of a fusion energy technology program.
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is now under way to continue the development of the RTLs. Numerous issues surrounding target physics, driver technology, and fusion power system parameters stand between the current state of technology and mag netic IFE.
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This will require additional calculations, target fabrication techniques, and experimental implementation. While providing improved performance, it also makes the fabrication and fielding logistics in a fusion power plant more complicated.
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Therefore, the overall credibility of the energy delivery system and the ICF target performance cannot be quantitatively evaluated. 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 efficiency, but concepts for an energy delivery system based on this driver are too immature to be evaluated at this time.
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The HAPL program studied the spectrum of threats to the first wall posed by direct drive targets and developed candidate mitigation strategies and materials. It should be noted that while 14 MeV neutrons and 3.5 MeV a-particles are the universal products of the DT fusion reaction, the different target material and configurations for direct drive and indirect drive produce different threat spectra at the reactor chamber first wall.
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These problems are not presently understood but appear to require a thick liquid-wall chamber. TARGET FABRICATION The primary concern of this panel with regard to ICF target fabrication relates to the technical feasibility of various proposed fabrication methods and the remain ing technical risks and uncertainties associated with these methods.
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seal-coat onto the capsule once the fuel is loaded. TWO OVERARCHING CONCLUSIONS AND A RECOMMENDATION Based on the discussion in this chapter, the panel reached the following over arching conclusions and makes 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.
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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 NNSA program at the NIF have not yet been resolved, and • The alternatives to laser indirect drive have technical promise to produce high gain.
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