4

Evaluation of ICF Targets

LASER-DRIVEN, INDIRECT-DRIVE TARGETS

Current Status

No laser fusion target has yet achieved ignition or breakeven,1 but current understanding leaves open the possibility that given time, funding, and the existence of alternative design options with sufficient margin for ignition and a gain of one, ignition might eventually be achieved.

The current U.S. program aimed at achieving ignition, the National Ignition Campaign (NIC), lays out a path via laser indirect drive (ID), and significant progress has been made along that path, although not enough either to demonstrate success or to conclude that ignition cannot be achieved. It is the understanding of this panel that the current program plan anticipates a demonstration of ignition sometime after the beginning of FY2013, although the planning document scheduled that event for the end of FY2012. The closest Level 1 milestone as of this writing is to achieve, in FY2012, significant alpha-heating of a capsule’s fuel. The expected signature of such an event is the production of at least 1016 deuterium-tritium (DT)-equivalent neutrons. The significance of this milestone is that it would indicate that fusion bootstrapping of the ion temperature in the capsule fuel had occurred—a prerequisite to achieving fusion ignition and energy gain.

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1 Breakeven occurs when fusion gain equals unity—that is, when the fusion energy released in a single explosion equals the energy applied to the target.



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4 Evaluation of ICF Targets LASER-DRIVEN, INDIRECT-DRIVE TARGETS Current Status No laser fusion target has yet achieved ignition or breakeven,1 but current understanding leaves open the possibility that given time, funding, and the exis- tence of alternative design options with sufficient margin for ignition and a gain of one, ignition might eventually be achieved. The current U.S. program aimed at achieving ignition, the National Ignition Campaign (NIC), lays out a path via laser indirect drive (ID), and significant ­progress has been made along that path, although not enough either to demonstrate success or to conclude that ignition cannot be achieved. It is the understanding of this panel that the current program plan anticipates a demonstration of igni- tion sometime after the beginning of FY2013, although the planning document scheduled that event for the end of FY2012. The closest Level 1 milestone as of this writing is to achieve, in FY2012, significant alpha-heating of a capsule’s fuel. The expected signature of such an event is the production of at least 1016 deuterium- tritium (DT)-equivalent neutrons. The significance of this milestone is that it would indicate that fusion bootstrapping of the ion temperature in the capsule fuel had occurred—a prerequisite to achieving fusion ignition and energy gain. 1  Breakeven occurs when fusion gain equals unity—that is, when the fusion energy released in a single explosion equals the energy applied to the target. 45

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46 Assessment of Inertial Confinement Fusion Targets The NIC Rev 5.0 target is designed to operate using indirect drive of a frequency- tripled (3ω) laser to reduce the negative effects of laser-plasma interactions (LPI) (see Box 4-1). Box 4-1 Laser-Plasma Interactions In laser-driven inertial confinement fusion (ICF), the capsule implosion is driven by thermal pressure.1 Thus, the incident laser energy must be absorbed by matter and thermalized, either in the outer shell of the capsule (direct drive) or in the inner walls of the hohlraum (indirect drive), which become plasmas. The variety of LPI that take place when an intense laser pulse hits matter have been studied for more than 50 years; they have been a key limiting factor in laser ICF, and are still incompletely understood. LPI that absorb and thermalize laser energy are desired. Undesirable, parasitic LPI include backscattering of laser light, which can result in loss of energy; cross-beam energy transfer among intersecting laser beams, which can lose energy or affect 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 are worse at longer laser wavelengths, so all modern drivers currently operate in the “blue” (3ω Nb:YAG at 353 nm) or ultraviolet (KrF at 248 nm). Moreover, lasers can be modu- lated so as to substantially ameliorate parasitic LPI by spectral broadening, spatially incoherent filtering, and/or polarization diversity, and great progress has been made over several decades on all the main kinds of laser drivers on such beam smoothing.2 Since LPI are threshold effects, target designers attempt to keep laser intensities below the threshold of major harm. However, neither fundamental understanding nor simulation are good enough to do so a priori; well- diagnosed experiments remain essential for LPI control.3 LPI are currently important in the National Ignition Campaign (NIC) indirect-drive targets. Overall, backscattered light losses appear to be 10-15 percent of the incoming laser energy; however, the inner beams backscatter more because of their greater path length in the hohlraum plasma. Stimulated Raman scattering (SRS) of the inner beams appears to play a significant role in causing drive asymmetry and hohlraum temperature deficits.4 The asymmetry has been controlled by the use of cross-beam energy transfer mediated by Brillouin scattering, but funda- mental understanding and simulation of this effect are incomplete, and its repeatability has not been established experimentally. Experiments so far are said to indicate that hot electrons are below the design threshold, but more diagnostics are needed, because hot electrons, if actually present, could explain the currently observed anomaly in capsule adiabat. Furthermore, other laser-produced sources of preheat, such as gold M-band emission, will require quantification in this new cross-beam environment. Rapidly increasing computer performance has enabled LPI calculations that were un- imaginable just 12 years ago, but full-scale National Ignition Facility (NIF) simulations remain beyond reach.5 The Lawrence Livermore National Laboratory (LLNL) typically performs single- or multiquad simulations using pF3D on the largest advanced simulation and computing (ASC) platforms. Improvements in hohlraum modeling have changed plasma conditions and the loca- tion of backscatter in LPI simulations, bringing them into better agreement with measurements. Recent simulations show that overlapping quads and spatial nonuniformities act to increase laser reflectivity. Simulations have suggested potential ways to mitigate the effect of overlap

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E va l uat i o n of I CF T a r g e t s 47 beam intensity on SRS, including changing the hohlraum aspect ratio and changing the pointing of inner cone quads. Substantial computational and experimental resources are being devoted to LPI issues within the NIC. LPI for direct-drive targets are under experimental and theoretical study at the Laboratory for Laser Energetics (University of Rochester) (LLE);6 the most important effect appears to be cross-beam energy transfer, which results in 20 percent energy losses in capsule experiments on OMEGA. 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. OMEGA EP7 will be an important platform for studying direct-drive LPI issues at inertial fusion energy (IFE)-relevant plasma scale lengths. Naval Research Laboratory (NRL) is performing complementary LPI experiments at 248 nm on Nike.8 Two-plasmon decay experimental data seem to agree with thresholds calculated using simple plane-wave-based threshold formulas, confirming the classical wavelength scaling. In direct drive, the initial target aspect ratio can be modified to limit the intensity and mitigate LPI risk at the penalty of greater sensitivity to Rayleigh-Taylor hydroinstabilities. Increased LPI intensity thresholds and greater hydrodynamic efficiency for short wave- lengths should combine to give better overall stability in direct-drive implosions. The NRL baseline shock ignition target is above the two-plasmon decay threshold during compression (Liu and Rosenbluth, 1976). Extending the Nike laser to 20 kJ would provide a useful capability to study LPI and hydrodynamics at 248 nm in IFE-relevant scale-length plasmas and compare them with OMEGA extended performance and NIF data. Plasma physics, including LPI, involves many degrees of freedom on a huge range of length scales; moreover, nonlocal propagation by electromagnetic fields and fast electrons are important. For these reasons, a priori simulation of a full-scale target will be impossible for the foreseeable future, although impressive simulations are now feasible for fundamental processes and small-scale regions. Future development of subgrid and mesoscale modeling on full-scale systems would help to understand the experiments and support better target design, but would require a large effort to create and perfect. 1 Radiation pressure of the laser light itself is too small by many orders of magnitude. 2 D. Montgomery, LANL, “Overview of Laser Plasma Instability Physics and LANL Understanding,” pre- sentation to the panel on September 21, 2011. 3 M. Rosen, LLNL, “Understanding of LPI and Its Impact on Indirect Drive,” presentation to the panel on September 21, 2011. 4 Ibid. 5 D. Hinkel, LLNL, “State of the Art for LPI Simulation,” presentation to the panel on September 21, 2011. 6 D. Froula, LLE, “Laser-Plasma Interactions in Direct-Drive Implosions,” presentation to the panel on September 21, 2011. 7 OMEGA EP (extended performance) is an addition to OMEGA and extends the performance and capabili- ties of the OMEGA laser system. It provides pulses having multi­ ilojoule ­ nergies, picosecond pulse widths, k e petawatt powers, and ultrahigh intensities exceeding 1020 W/cm2. 8 A. Schmitt, NRL, “Assessment of Understanding of LPI for Direct-Drive (KrF),” presentation to the panel on September 21, 2011.

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48 Assessment of Inertial Confinement Fusion Targets Recent and Upcoming Work Recent work on indirect-drive laser fusion has brought the NIC program to the point where it has transitioned from preparation for the actual ignition campaign to the campaign itself. The latter involves optimization of a set of parameterized characteristics of the target and laser system in order to achieve conditions under which ignition could be anticipated to occur; the development of these “tuning parameters” has itself been one of the areas of development, in part because most of the tuning campaigns will require the use of specially designed capsules to enable data acquisition of the type and accuracy needed for that specific campaign. 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. These tuning campaigns are expected to use what are termed “keyhole” ­targets, backlit gas capsules, “symcap” capsules, and reemission capsules. Ignition is neither ­ expected nor desired in these types of capsules, although tritium-hydrogen-­ deuterium (THD) capsules, which are intended for use in many of the preignition integrated experiments, utilize the ignition design but incorporate less DT thermo- nuclear fuel in favor of the less reactive HD. The use of THD capsules is expected to allow collection of data with which to confirm or calibrate calculations of the nuclear performance of the optimized implosion system (laser pulse + hohlraum + capsule design). Calibration of the nuclear diagnostics is planned using capsules of the so-called “exploding pusher” design. The work mentioned thus far has all been accomplished at the NIF facility at LLNL. Additional preparations for optimization and testing of ignition cap- sules have been carried out at other laser facilities, notably the OMEGA laser at the University of Rochester’s (LLE). One aspect of this work has investigated some of the problematic aspects of LPI. Experiments at LLE have also facili- tated the development and porting of diagnostics to the NIF and have provided data on the operation of noncylindrical “rugby” hohlraums; 2 experiments are 2  Rugby hohlraums are shaped not like a cylinder but like a rugby ball, with a wall having a tapered curve.

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E va l uat i o n of I CF T a r g e t s 49 planned to provide similar data on the efficacy of “P2” laser entrance hole (LEH) shields.3 If ignition can be achieved on the NIF, target simulations presented to the panel suggest that optimization of the tuning parameters and increases in the driver energy could result in gains of between 50 and 100 at some future facility. Evaluation and Discussion of Remaining R&D Challenges It is too early in the experimental campaign to evaluate the performance of the NIC ignition target design. However, information already in hand does indi- cate some potential problem areas, which could become showstoppers. 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. The fact that related quantities, such as capsule bang time, are likewise delayed compared to expectations confirms the interpretation of the velocity measurements. Possible explanations offered at the time the panel received its briefings are that the calibration of the hohlraum temperature measurement (Dante X-ray flux diagnostic) was incorrect, or that the opacity of the Ge dopant in the capsule wall (to reduce early-time heating of the interior portions of the capsule) was higher than expected. Plans are in place to explore these hypotheses by checking the calibration in question and testing capsules without that dopant for comparison. The principal means available to increase the implosion velocity is to increase the laser drive energy. Greater drive energy would, however, also increase the pre- heating from LPI, which, as discussed below, does not appear to be well understood. A path forward is thus not guaranteed. Implosion Symmetry The panel was told that there are some concerns about early-time imprinting of drive asymmetries based on observations of reemission targets. Furthermore, the overall implosion symmetry of baseline targets was routinely more prolate 3 P2 refers to the type of departure from sphericity that the shields are intended to reduce. A nearly spherical shape with azimuthal symmetry is often represented mathematically using Legendre polynomials, and P2 is the standard means of referring to the second Legendre polynomial, which is needed to describe a shape that has been described as a “sausage.”

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50 Assessment of Inertial Confinement Fusion Targets than predicted. Acceptable symmetry was obtained using interbeam energy trans- fer between outer and inner laser cones, but at present this process has not been successfully incorporated into the design simulations used to predict target perfor- mance. The consensus of the panel is that this situation may be a further indication of unknown LPI processes in the hohlraum or of other predictive inadequacies. Mix The prediction of mix across shocked interfaces and during convergent implo- sions has been a very active and controversial area of research in many technical communities for many years. Approximate simulations of mix are possible and are routinely included in some target simulations, but the calculated mix—and there- fore its calculated effects—is recognized to be unreliable. Moreover, data to validate calculations of the consequences of mix are thus far unavailable. It is therefore planned to compensate for the effects of mix empirically—that is, it is planned to design and engineer for sufficient margin in ignition conditions and gain to compensate for whatever degradation the mix may cause. The lack of a definitive, quantitative understanding of the origins and evolution of mixing has raised concerns that isolated bumps and defects in the capsule shell could give rise to spikes of wall material that would penetrate into the central fuel region. 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. Some workers have postulated that the disparities are due to inaccuracies in tabulated plastic ablator (CH) release isentropes, but there appears to be no technical evidence to support this hypothesis. LLNL briefings to the panel conveyed conviction that hot electron preheat from LPI in the NIF target has been adequately anticipated and that the implosion adiabat of the fuel can be managed by controlling shock heating. Nevertheless, the uncertainties concerning LPI processes within a target hohlraum (discussed below) and the strong sensitivity of a capsule’s gain to preheat make the understanding and management of a capsule’s implosion adiabat an area of concern to the panel. Laser-Plasma Interactions LPI diagnostics on an ID target assembly can only sample the small solid angle of light that is backscattered out of a hohlraum’s laser entrance holes. The processes occurring inside the hohlraum, including those that can produce hot electrons, are

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E va l uat i o n of I CF T a r g e t s 51 difficult to observe. These circumstances significantly decrease the effectiveness of efforts to ascertain the adequacy of simulations of LPI. Initial experiments on the OMEGA laser have shown disparities between modeling for both vacuum and gas-filled rugby hohlraums. Scattering of the inner beams entering a hohlraum is reported to be greater than predicted, providing specific evidence of simulation inadequacies. Current simulations approximate LPI using inverse Bremsstrahlung energy deposition models in which the power balance of the beams is input by the user, although rad-hydro modeling has apparently been improved through the use of nonlocal electron transport models and detailed configuration analysis (DCA). Cross-beam transfer is estimated via analytic models. There is a fluid model for LPI, called PF3D, which includes approximate models of kinetic effects; the use of similar models might improve LPI simulations for laser fusion applications. It appears to the panel that the current state of understanding and simulation capability of LPI presents a significant risk to both the NIC and the credibility of any indirect-drive IFE design concept, such as the Laser Inertial Fusion Energy (LIFE) initiative. The effects of LPI may be a central issue, contributing to observed disparities between measured and calculated implosion entropy, velocity, and shape in the NIC. Capsule Fabrication There is extensive experience in fabrication of NIC-style targets, and there is a high likelihood that the capsule and hohlraum system can be made to the desired specifications. 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 com- pressions 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 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 experi- mental results, the panel assesses that ignition using laser indirect drive is not likely in the next several years.

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52 Assessment of Inertial Confinement Fusion Targets The 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. 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. How high the gain at the NIF could be will be better understood by follow-on experiments once ignition is demonstrated. At this writing, there are too many unknowns to project a potential gain. • Achieving ignition will validate the assumptions underlying theoretical predictions and simulations. This may allow a better appreciation of the sensitivities to parameters important to ignition. USE OF LASER-DRIVEN, INDIRECT-DRIVE TARGETS IN A PROPOSED IFE SYSTEM The proposed—and de facto—baseline model for a laser ID power plant is the LIFE initiative of LLNL. The discussions in this section are therefore based on that design as presented to the panel. The current target design for LIFE was derived from the current baseline NIC design, with subtle but distinct differences. Modification was necessary to increase the calculated gain for IFE. Other modifications were to enable rapid, affordable fabrication in bulk, because the current plan for LIFE envisions firing approxi- mately 1 million targets per day. The developers of LIFE plan to accommodate errors in the calculated target performance by adopting a design that is calculated to produce 125 percent of the gain for which LIFE was designed. The 25 percent surplus gain is viewed as a margin that would be eroded by the combined effects of inaccuracies in target design, fabrication, insertion, drive (shape, intensity, smooth- ing, and aiming), and LPI. As discussed above, in evaluating the current NIC target, issues relating to the target implosion velocity, implosion symmetry, mix, the implosion adiabat, and LPI must be addressed. In spite of the modifications to the NIC target design that adapt it for use in LIFE, sufficient similarities persist that the preceding issues apply fully, unless and until optimization and other research conducted under

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E va l uat i o n of I CF T a r g e t s 53 the NIC program lead to a favorable resolution of the underlying uncertainties. The differences between the NIC and LIFE targets also raise additional issues, as discussed below. Modifications to Increase Gain The design approach to increasing the gain of the IFE capsule stems from an approximate analytical expression in which capsule yield is proportional to Ecapsule5/3, where Ecapsule is the energy absorbed by the capsule. The strategy is to increase the implosion energy primarily by increasing the drive temperature in the target hohlraum. The drive temperature is increased by increasing the laser driver energy and decreasing losses. The laser energy is to be increased from a maximum energy of 1.8 MJ at the NIF to 2.2 MJ for LIFE. A hohlraum shaped like a rugby ball has been designed to more efficiently par- tition the drive energy; the redesign includes reducing the case-to-capsule diameter ratio to 2.0-2.4. The energy lost by reradiation from the hohlraum is to be reduced by the use of P2 LEH shields, and the conversion of absorbed energy to implosion energy is to be increased by using a high-density carbon (HDC) shell to increase the ablation efficiency. An illustration of the LIFE target design is shown in Figure 4-1. FIGURE 4-1  The LIFE target design. Modifications from the NIC target design include the curved (“rugby”) inner wall of the hohlraum, the high-density carbon ablator, the LEH shields, and the P2 shine shields. SOURCE: M. Dunne, LLNL, presentation to the panel on July 7, 2011.

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54 Assessment of Inertial Confinement Fusion Targets Modifications for Production Operation The proposed manufacturing process of the LIFE target is a significant exten- sion of the well-proven process for manufacturing targets for the NIC. Capsule Fabrication There is extensive experience in capsule fabrication, and it appears likely that the capsule can be made to the desired specifications. The technical challenges are (1) to demonstrate the formation of a uniformly thick, low-density (20 mg/cc) foam wall inside the diamond shell using a technique that is suitable for mass pro- duction and (2) to develop a cost-effective manufacturing process that can process more than 1 million targets per day through multiple steps where each target is individually handled. Proponents assert that automation can achieve the required throughput for an indeterminate capital and development cost; the bigger issue is whether the manufacturing can be done for the required per-item cost (estimated to be in the range of 20-40 cents).4 The method proposed for forming a uniformly thick fuel layer is a radical departure from the method used for making targets for the NIF. 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. The thickness uniformity of the fuel layer is expected to be provided by the 20 mg/cc CH foam wall, the inter- facial liquid surface tension, and a controlled thermal profile along the surface of the hohlraum. This process has to be demonstrated. A critical technical milestone is to demonstrate that the DT liquid can be supercooled sufficiently to achieve the required vapor pressure, a property that has not been observed in cryogenic fluids.5 A second technical challenge will be to preserve the uniformity of the liquid fuel when the capsule is accelerated to a velocity of 250 m/s into the target chamber. The low mechanical stiffness of the low-density foam and the low viscosity of the liquid will make the uniformity of the fuel layer thickness susceptible to the high acceleration loads. Neither of the traditional methods of introducing fuel into the capsule—a capsule fill tube or diffusion filling—is feasible for power plant targets. A method would have to be developed to seal the capsules with a plug of some appropriate material after filling them with DT. 4  D.T.Goodin, General Atomics, presentation to the main IFE committee on January 29, 2011. 5 Different IFE target designs exist for different methods of achieving compression. Only one target design proposes supercooled DT liquid. If this step turns out to be physically impossible, then alternative designs will be explored.

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E va l uat i o n of I CF T a r g e t s 55 Hohlraum The rapid capsule insertion necessary for a power plant will require structurally rigid support for the capsule and the LEH shields. The hohlraum-capsule structure is a delicate and intricate design with tight assembly tolerances on how precisely the capsule needs to be positioned inside the hohlraum. In addition, there are two internal shine shields that need to be positioned precisely inside the hohlraum using a low-mass support structure so that neither the thermal profile nor the X-ray radiation flux within the hohlraum is excessively perturbed. Further work is required to define a construction that meets these requirements and will also survive the high acceleration loads experienced when the assembly is injected into the target chamber. The hohlraum walls in the LIFE design are to be of a lead alloy that is optimized for high opacity at the capsule drive temperature. Current hohlraums are con- structed either entirely of gold, or of gold-plated uranium. The latter are impracti- cal for a high production rate. As an example, a firing rate of 10 Hz translates to 8.6 × 105 capsules fired per day. With a hohlraum mass of 3 g, 2.6 metric tons of lead must be collected and recycled per day. Using lead rather than solid gold will reduce both the start-up cost and the security requirements for the crucial processes of hohlraum material recycling and target fabrication. Evaluation In evaluating the current NIC target, issues relating to the target implosion velocity, implosion symmetry, mix, the implosion adiabat, and LPI were discussed above. The modifications to the NIC target design that adapt it for use in LIFE leave it fully vulnerable to the issues surrounding the performance of the NIC cap- sule, unless and until optimization and other research conducted under the NIC program lead to a favorable resolution of the underlying issues. 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. Modifications to Increase Gain The credibility of the effectiveness of the target design changes from NIC to LIFE is directly related to obtaining and understanding the desired performance of the NIC Rev 5.0 design and understanding its operation. The seriousness of the issues discussed in this section can be expected to become more apparent as the ignition campaign unfolds. Many of these changes are scheduled for study on OMEGA, the NIF, or both.

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76 Assessment of Inertial Confinement Fusion Targets deposition.16 The briefing the panel received on heavy-ion target design at the July 2011 meeting17 focused on the much newer X-target. The X-target is a HIF- motivated design that uses single-sided illumination by three sequential beam pulses and has features that offer new opportunities in accelerator driver technol- ogy, chamber technology, and driver-chamber interface. Two preliminary target designs were presented to the panel at its Rochester meeting: (1) a 1-D Lasnex design of a DD target requiring 3 MJ of 3 GeV Hg+1 ions, giving a gain of ~150, and (2) a single-sided direct-drive X-target also utilizing 3 MJ of ions with a calculated 2-D gain of between 50 and 400 (see Figure 2-6). 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. Uranium beams of 80 GeV are already focused to <300 µm (full-width at half maximum) at GSI in Germany (transverse emittance sufficiently low), but beam current and space charge effects are small, and the bunch pulse durations are too long for fast ignition (>100 ns). Experiments at LBNL (NTX and NDCX-I) have shown that intense beam space charge can be neutralized with preformed target chamber plasma much greater than beam density. However, plasma neutraliza- tion cannot prevent the spread of the focal spot size due to chromatic aberrations (random momentum spread in the beam). The sole LBNL target designer is continuing to evolve the X-target calculations in 2-D using the LLNL HYDRA code. Evaluation and Discussion of Remaining R&D Challenges The limitation of present accelerators in energy and focal intensity means that there are only a few data on ion-stopping powers in warm dense matter and no ICF target data. The PD and X-target performance estimates are purely based on rad-hydro code simulations that need to be greatly increased in sophistication and resolution to deal with all of the issues in a computational sense. The entry-level price of a heavy-ion target physics facility is sufficiently high that it is unlikely to be constructed by the DOE/NNSA program in the near or medium term. Integrated 3-D Target Design The 3-D nature of the HIF targets and highly sheared flows will require increas- ingly sophisticated simulations at very high resolutions (massively parallel). 16  L.J. Perkins, LLNL, “Targets for Heavy Ion Fusion Energy,” presentation to the panel on February 16, 2011. 17  B.G. Logan, LBNL, “Heavy-Ion Target Design,” presentation to the panel on July 7, 2011.

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E va l uat i o n of I CF T a r g e t s 77 Mix The sheared flows in the X-target with high-Z slide surfaces make mix with the DT fuel a serious concern. Acceleration Compression Physics It will be very challenging to reach the 200 ps/200 µm radius goals of the accelerator physics program. Ultimately, the limits of focusing and compression are determined by Liouville’s theorem. The NDCX-II experiments will explore more intense beam compression and focusing physics related to subnanosecond heavy-ion shock ignition and fast ignition. Neutralized Ballistic Focusing The conceptual X-target designs assumed neutralized ballistic focusing of heavy ions through a background chamber plasma as simulated by the IBEAM systems code (Barton et al., 2005). Some panel members question the maturity of the models for dynamic charge state; the degree of neutralization in the reactor chamber environ- ment; and the potential impact of beam space charge on the final focus. This is a transport issue that is unique to heavy-ion fusion and will require further research through detailed simulations and validation by experimental data (Sharp et al., 2004). Potential for Use in an IFE System All three heavy-ion target physics options are intended to use multiple-beam linac drivers with thick liquid-protected chambers to mitigate material neutron damage risks. The liquid-protected chamber technology is synergistic with some aspects of the pulsed-power approach to IFE. In principle, the injection of targets into the reactor chamber for heavy ions has the same features as laser fusion. Light-gas-gun or magnetic-slingshot systems developed for laser fusion should be applicable. If the heavy-ion chamber uses a liquid lithium protection for the first wall, there may be some differences in injec- tion system implementation and the specifics of cryogenic layer survivability in the reactor environment, which would be accounted for in a detailed system study. All of the DD heavy-ion fusion target concepts are at a very early stage. Simi- larly, the proposed novel accelerator techniques for compressing heavy-ion beams to 200 ps with focusing to 200 µm radius are challenging and at an early stage of research. 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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78 Assessment of Inertial Confinement Fusion Targets 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.18 This work is at a very early stage, but if suc- cessful may provide very high gain. • The work in the 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. Z-PINCH TARGETS Description of Current U.S. Efforts The main research in Z-pinch-driven ICF is performed at Sandia National Lab- oratories in Albuquerque, New Mexico. After the conversion of the PBFA-II accel- erator to “Z” in 1997 to increase the radiated power from its wire-array Z-pinches, Sandia transitioned its ICF research from light-ion beam drivers to Z-pinches. The initial ICF concepts utilized thermal radiation from Z-pinches to indirectly drive ICF capsules. For example, the double-ended hohlraum concept drew heavily from ID ICF design experience at the NIF. Initial experiments on this concept demon- strated control of radiation symmetry via backlit capsule implosions; however, calculations showed that significant fusion experiments required much higher currents than achievable on Z (60 MA for high yield versus 20 MA Z capability). After completion of the Z Refurbishment Project in October 2007 (26 MA peak current), the NNSA issued guidance that the primary mission of Z should be to support the Science Campaigns within its Stockpile Stewardship program, espe- cially in the areas of dynamic materials and nuclear weapons effects. Presently, the limited portion of the Z experimental program that is devoted to ICF research is focused on concepts utilizing the DD of high magnetic field pres- sure to implode DT fuel to fusion conditions, citing an estimated 25-fold increase in theoretical efficiency for direct magnetic drive versus indirect X-ray drive. The Magnetized Liner Inertial Fusion (MagLIF) concept (see Figure 2-7) has been theoretically developed, and initial experiments to study the stability of the shell during magnetic implosion have been completed. Future experiments will add laser 18  Advanced designs include DD, conical X-target configurations (see Chapter 2).

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E va l uat i o n of I CF T a r g e t s 79 preheat to the magnetic implosions, with the eventual goal of G = 1 laboratory breakeven (DT fusion yield equals energy delivered to fuel). Quantitatively, this translates to ~100 kJ DT yields, although D2 experiments will initially be performed for simplicity. High-yield (GJ-class), high-gain (>500) target designs are under development. Much of the relevant physics can be tested on Z. R&D Challenges and Requirements Some Z-pinch IFE system concepts were developed several years ago during a brief period when limited funding for IFE technology was provided within the NNSA ICF program. The concept of a recyclable transmission line (RTL) was explored as part of this technology project, although it was intended for use with the ID target designs that were being studied at that time. Extrapolated calculations of Z-pinch target designs typically require around 60 MA of current to be delivered from the pulsed-power driver to the implosion system to achieve high fusion yields. In contrast to laser and heavy-ion targets, which receive their energy from beams that are transported either in a vacuum or through small amounts of gas within the reactor chamber, the RTL directly connects the driver to the Z-pinch fusion target. This energy delivery strategy leads to a unique set of challenges and requirements for achieving the Z-pinch fusion system performance. The economics of this system ­ design favor a low repetition rate and a high fusion target yield. Technical and program managers at Sandia indicated to the panel that they perceive that ICF target research is not considered a high priority given the exten- sive funding necessary for the NIC and DOE’s current prioritization of high- energy-density physics experiments on Z (e.g., the plutonium equation of state). Nevertheless, the existing program recently accommodated a modest amount of scientific work that shows significant promise for IFE. However, magnetically driven ICF ultimately needs to achieve robust fusion burn conditions, just as laser or heavy-ion ICF do. It has unique features that appear to the panel to provide an alternative risk-mitigating path to fusion energy. 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. The following summarizes the overall program status: • Single-shot, magnetically driven fusion target designs, funded by the NNSA, are being investigated on the Z accelerator. • The MagLIF concept has been developed to exploit the favorable ignition requirements that, in theory, apply to target designs with magnetized and

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80 Assessment of Inertial Confinement Fusion Targets preheated fuel. The MagLIF design is to be investigated in near-term valida- tion experiments and simulations. • Benchmark experiments on Z have shown excellent agreement between magneto-Rayleigh-Taylor simulations and observations. • Development of an overall system for pulsed-power IFE was supported from 2004 to 2006 by modest (~$10 million) internal research funding. Sandia has indicated that internally funded research ($700,000) 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. These issues include the following: • Liner dynamics — btain requisite velocities with suitable shell integrity. O — emonstrate sufficient control over the fuel adiabat during the implo- D sion (e.g., pulse shaping). — emonstrate tolerable levels of mixing at stagnation. D — emonstrate required level of axial asymmetry. D — emonstrate required level of azimuthal asymmetry. D • Fuel assembly — emonstrate the required stagnation pressure. D — emonstrate required confinement time. D —Compress sufficient current to a small radius to create extreme conditions. — ompress magnetic flux in the stagnating plasma. C • Driver scaling —Determine the driver parameters required for ignition and/or high yield. —Demonstrate scientific breakeven and support target approach with vali- dated simulations. — evelop robust, high-yield targets designs in state-of-the-art 2-D and D 3-D simulations. — emonstrate a repetitive coupling with an RTL system. D — esign a system for reliably creating, handling, and utilizing repetitive, D high fusion yield with high availability. Some additional specific technical issues still need to be explored: • The MagLIF target design benefits from short implosion times; that is, the final density of the imploded fuel varies as (100 ns)/implosion time. However, the cost and the complexity of the pulsed-power driver have the opposite scaling. It was also stated that some target designs might be able

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E va l uat i o n of I CF T a r g e t s 81 to operate at longer implosion times. This would obviously be a huge lever arm on the total system that requires further investigation. • The MagLIF performance scaling simulations have been primarily per- formed in 1-D, with limited exploration of 2-D Rayleigh-Taylor instability issues. However, the physics of thermal conduction and transport in mag- netized plasmas is fully 3-D in nature and requires exploration in greater detail. 1-D simulations provide ideal energy scaling; 2-D begins to bring in Rayleigh-Taylor instabilities. Magnetized performance, however, will require 3-D studies. • As stated by Sandia, “batch burn” (volume ignition) will result in a low yield, and a “levitated fuel” layer should give better performance. 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. • Traditional magnetized target fusion concepts have not been shown to scale to high yield and gain. Sandia states that it has recently calculated high-yield performance with MagLIF targets. However, the additional cost of the magnets and optics that would be destroyed on each shot and the complexity of transporting the heater laser through the thick-liquid-wall chamber environment must both be accounted for in the system economics and design. • References from the 2005 Sandia IFE program discuss potential issues of operating RTLs if the final radius and gap become too small. At that time the baseline power flow was relatively large wire-array Z-pinches. It will be important to study the compatibility of the RTL concept with the smaller diameter of direct magnetic-drive targets. Potential for Use in an IFE System Concepts for IFE systems using Z-pinch targets were presented to the panel,19 but sufficient uncertainties remain that it would be premature to attempt an evaluation at this time. As presently envisioned, each 3-GJ fusion energy pulse would require the insertion, connection, and energizing of an RTL and fusion target assembly at a 0.1 Hz repetition rate. The assembly comprises an evacuated RTL system that contains the cryogenically cooled Z-pinch target at its center. The details of this concept are complex and will require extensive research and development if Z-pinches are pursued as an IFE technology. It is too early in both the target physics and fusion technology research programs to evaluate the target 19  M. Cuneo et al., Sandia National Laboratories, “The Potential for a Z-pinch Fusion System for IFE,” presentation to the panel on May 10, 2011.

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82 Assessment of Inertial Confinement Fusion Targets fabrication and economic issues quantitatively, but the material and fabrication costs of the expended portions of the system will certainly be a factor in Z-pinch power plant economics. Because of the limited ICF target physics database, incom- plete validation of the design tools and methodologies, and related lack of an integrated, high-yield target design, a consistent set of requirements and solutions for the pulsed-power driver, RTL, and ICF target cannot be articulated at this time. 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. The Z-pinch scheme is completely different from the NIF and HIF approaches and therefore serves as risk mitigation for the ICF and IFE programs. It is not yet clear that the work at SNL will ultimately result in the high gain predicted by com- puter 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. CONCLUSION 4-14: The target manufacturing and delivery processes that are proposed for direct-drive heavy-ion and pulsed-power fusion energy are less developed conceptually and technically than the targets for laser-based fusion energy. This is primarily because the priority has been to emphasize the implo- sion physics and driver issues (pulsed-power and linear accelerators). The pulsed- power target appears to be straightforward to manufacture, difficult to field, and challenging to reprocess after the thermonuclear event. In contrast, the heavy-ion targets possess many synergies with the laser-based target, but because a final target design is far from being defined, potential manufacturing complexities cannot be accurately assessed. The target delivery method for pulsed-power fusion is more conceptual than for laser- or heavy-ion-based fusion and presents very different problems—for example, a very much larger mass (~1,000 times larger), a slower replacement frequency (~100 times slower), and potentially a greater radioactive waste disposal problem.

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E va l uat i o n of I CF T a r g e t s 83 OUTPUT SPECTRUM FROM VARIOUS IFE TARGETS The fusion reaction of each type of IFE target produces a spectrum of threats (X-rays, ions, neutrons, and debris) to the first wall of the reaction chamber. 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. An IFE engineering test facility could be an intermediate step, before full-scale electrical power production, wherein fusion material issues could be studied. Indirect Drive The high-Z hohlraum materials used in ID absorb most of the a-particles and radiate more energy as X-rays. The actual threat spectrum is dependent on the details of the hohlraum design. For an ID, heavy-ion target, calculations show that 69 percent of the energy is in neutrons, 25 percent is in X-rays (500 eV peak), and 6 percent is in ions.20 For the LIFE target, the X-ray fraction is about 12 percent, the ion fraction about 10 percent, and the remainder in neutrons.21 X-rays are the dominant threat to the first wall for ID targets. The Osiris heavy-ion target chamber uses walls wetted by liquid lithium to mitigate the X-ray threat, while LIFE uses Xe gas to protect a dry solid wall. Direct Drive DD targets for both KrF and DPSSL systems produce the same threat spectrum, where approximately 1.3 percent of the energy is released in X-rays (4 keV peak) that produce surface deposition in less than the first 1 μm; 24 percent is in ions that have subsurface deposition in less than 5 μm, and the remainder is in neutrons that have volumetric deposition. Ions produce the greatest first wall heating for direct drive, and the implantation of a-particles presents a helium retention chal- lenge. The HAPL program studied both of these challenges, combining ­ odeling m with experiments using lasers, ions, and plasma arc lamps to test thermomechanical cyclic stresses. The helium retention issue was similarly ­modeled, and experiments were performed on both the Van de Graff and the Inertial Electrostatic Confinement 20  L.J. Perkins, LLNL, “Targets for Heavy Ion Fusion Energy,” presentation to the panel on Febru- ary 16, 2011. 21  M. Dunne, LLNL, “LIFE Target System Performance,” presentation to the panel on July 7, 2011.

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84 Assessment of Inertial Confinement Fusion Targets fusion devices at the University of Wisconsin. A nanoengineered tungsten wall material showed an encouraging ability to mitigate helium retention. Experiments showed that cyclic heating in the IFE chamber mitigates helium retention. Z-Pinch The spectrum output issues associated with the RTL/Z-pinch system are unique to this approach. The mass of material in this assembly is much greater than in any other concept, leading to greater recycling requirements. Further, the interaction of the fusion output with the RTL structure could lead to unique problems with the formation of shrapnel and debris. 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. The question of whether the targets can be made cost-efficiently for a power plant is beyond the purview of this panel and is addressed by the National Research Council’s IFE committee. Some promising approaches are discussed below. Microfluidic Methodologies for Manufacturing Targets The polymer shell that contains the DT fuel for DD laser and heavy-ion-beam fusion is proposed to be manufactured using a microfluidic droplet formation method.22 This is an established technology that is used to make ICF capsules for current DD and ID experiments. The principle is to flow three immiscible fluids coaxially through two nozzles where the Rayleigh-Plateau instability that occurs in the region where they intersect produces individual droplets. Each droplet is an emulsion consisting of a thin shell of water surrounding a spherical oil droplet; these droplets are collectively immersed in oil. The thin shell of water contains the polymer precursors that form the plastic capsule. The final phase of the production process is to remove the fluids using supercritical drying. This process has a very high production rate that is needed for a fusion energy program. However, the repeatability and precision of the process must be improved if the process is to be a viable option for an energy program. (The repeatabil- ity of the current process does not ensure that each capsule meets the required 22  A. Nikroo, General Atomics, “Technical Feasibility of Target Manufacturing,” presentation to the panel on July 8, 2011; see also Utada et al., 2007.

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E va l uat i o n of I CF T a r g e t s 85 specifications, so each capsule is individually measured to determine its suitability; this raises the cost of the targets, which is acceptable for ICF experiments but not for an IFE program.) In all other aspects, this production process offers a potentially viable method for producing targets cost-effectively. One modification to the current microfluidic method that may improve the reliability is to introduce electromechanical control into the process (Cho et al., 2003). This process, referred to as “lab-on-a-chip,” has demonstrated the feasibil- ity and benefits of using electric fields and electronics to control important steps in the target production process (Bei et al., 2010; Wang et al., 2011). This concept can potentially reduce the production time and physical size of a target production facility and address the precision and reliability concerns with the existing process. Further development of the process is needed. The lab-on-a-chip concept is being evaluated as a method to accomplish the cryogenic operation of loading the DT fuel into the capsule.23 Preliminary proof- of-concept experiments show that it is possible to form individual droplets of liquid deuterium of the correct size and wick them into a foam capsule in a short period of time. This would have the benefit of simplifying the target fueling pro- cess and shorten the process time, which would reduce the tritium inventory that is required by an IFE plant. Additional work is required to further develop this concept—specifically, to demonstrate that the process works with tritium and that it is practical to apply a condensed gas (argon, neon, or xenon) 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. • 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. 23  R. McCrory, LLE, “Target Fabrication for IFE Reactors: A Lab-on-a-Chip Methodology Suited for Mass-Production,” submission to the panel on July 6, 2011.

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86 Assessment of Inertial Confinement Fusion Targets • 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 NNSA 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 pursu- ing 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 research and development 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.