2

Status and Challenges for Inertial Fusion Energy Drivers and Targets

A brief introduction to the concepts of drivers, targets, and implosion mechanisms was given in Chapter 1. In the first part of this chapter, the committee provides a more detailed discussion of alternative strategies for driving the implosion of targets and explains why terms such as “direct drive” and “indirect drive” are more accurate descriptors for some driver-target pairs than for others.

In the second part of this chapter, the committee takes up the status and future R&D needs of the three main driver candidates: lasers (which include diode-pumped, solid-state lasers and KrF lasers); heavy-ion accelerators; and pulsed-power drivers. This discussion of driver approaches is based on input received from proponents who are technical experts in the field.1 As such, the R&D challenges and investment priorities for moving each approach forward to a major test facility— Fusion Test Facility (FTF)—are discussed independently of one another—that is, as if a decision had been made to choose that particular approach as the best option for inertial fusion energy (IFE). The committee recognizes that a down-selection to one particular approach will have to be made and does not mean to suggest that all of the approaches should be funded simultaneously at the levels indicated in this chapter. A discussion of how these approaches might fit into an integrated program with down-selection decision points is given in Chapter 4. Throughout this chapter material is drawn from the report of the committee’s supporting Target Physics Panel (see the Preface); the Summary from the unclassified Target Physics Panel report appears as Appendix H.

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1 The experts who gave presentations to the committee are listed in Appendix C.



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2 Status and Challenges for Inertial Fusion Energy Drivers and Targets A brief introduction to the concepts of drivers, targets, and implosion mecha- nisms was given in Chapter 1. In the first part of this chapter, the committee pro- vides a more detailed discussion of alternative strategies for driving the implosion of targets and explains why terms such as “direct drive” and “indirect drive” are more accurate descriptors for some driver-target pairs than for others. In the second part of this chapter, the committee takes up the status and future R&D needs of the three main driver candidates: lasers (which include diode- pumped, solid-state lasers and KrF lasers); heavy-ion accelerators; and pulsed- power drivers. This discussion of driver approaches is based on input received from proponents who are technical experts in the field.1 As such, the R&D challenges and investment priorities for moving each approach forward to a major test facility— Fusion Test Facility (FTF)—are discussed independently of one another—that is, as if a decision had been made to choose that particular approach as the best option for inertial fusion energy (IFE). The committee recognizes that a down-selection to one particular approach will have to be made and does not mean to suggest that all of the approaches should be funded simultaneously at the levels indicated in this chapter. A discussion of how these approaches might fit into an integrated program with down-selection decision points is given in Chapter 4. Throughout this chapter material is drawn from the report of the committee’s supporting Target Physics Panel (see the Preface); the Summary from the unclassified Target Physics Panel report appears as Appendix H. 1  The experts who gave presentations to the committee are listed in Appendix C. 29

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30 An Assessment of the Prospects for Inertial Fusion Energy Conclusions and recommendations are given within the sections. General conclusions appear at the end of this chapter. METHODS FOR DRIVING THE IMPLOSION OF TARGETS A large number of target designs have been studied and proposed for IFE power plants. As explained in Chapter 1, these targets may be categorized according to the method used to drive the implosion (to compress the fuel to high density) and according to the method used to bring the fuel to the required ignition temperature. In addition, targets are sometimes categorized according to illumination geometry. For example, in some target designs, the incoming driver beams are arranged uni- formly around the target to approximate spherical illumination. At the National Ignition Facility (NIF), the beams are arranged in four cones that illuminate the inside wall of the hohlraum from two sides (the poles of the cylindrically sym- metric target). Historically, there have also been illumination geometries that more strongly illuminate the equatorial area of the target. Finally, for pulsed-power IFE systems, there may be no driver beams at all; the electrical energy is coupled directly to the target by the pressure of the magnetic field produced by the drive current. The two principal methods of driving laser implosions are indirect drive and direct drive (see Figure 1.4). For ion accelerators, there is nearly a continuum between indirect drive and direct drive. The three principal methods proposed to ignite the fuel are referred to as hot-spot ignition, shock ignition, and fast ignition. For indirect drive, there is some thermal inertia or heat capacity associated with the cavity surrounding the fuel capsule and with the ablator itself. It is more difficult to achieve the rapid rise in temperature and pressure with indirect drive because of the thermal i ­nertia of the hohlraum. Shock ignition requires rapidly rising drive pressure at the end of the drive pulse. Consequently, shock ignition is usually associated with direct drive. Hot-spot ignition and fast ignition are the main ignition modes for indirect drive. All three modes of ignition necessarily ignite only a small fraction of the fuel. The thermonuclear burn then propagates into the bulk of the fuel. Implosion Requirements A number of conditions must be satisfied to produce ignition and reactor-scale gain.2 These conditions are described in detail in Appendix A; in this section, the committee gives a brief overview. 2  R. Betti, University of Rochester, “Tutorial on the Physics of Inertial Confinement Fusion for Energy Applications,” Presentation to the committee on March 29, 2011.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 31 Symmetry Ideally, the final configuration of the imploded fuel should be nearly spheri- cal. For laser-driven and heavy-ion-driven implosions, this requirement imposes conditions on the uniformity of the light, X-ray, or ion flux driving the target, and also on the initial uniformity of the target itself. For example, if the target is driven more strongly near the poles, the final imploded configuration might be shaped like a pancake. If the equator is driven more strongly, the imploded configuration might resemble a sausage. The greater the convergence ratio3 of the target, the greater the precision required in direct drive—for example, in drive pressure or shell thick- ness. For most laser target designs, this convergence ratio lies between 20 and 40. Sausagelike, pancakelike, dumbbell-like, or even doughnutlike asymmetries are low-order asymmetries in the sense that the wavelength of the departures from spherical symmetry is comparable to the size of the compressed fuel configuration. Energy imbalance among the beams is one possible type of error leading to low- order asymmetries; beam misalignment is another. Fluid Instabilities In addition to the low-order asymmetries, higher-order asymmetries are also important. Small perturbations on the surfaces of the fuel and ablator shell can grow as the shell is accelerated. Unless the initial layer surfaces are very smooth (i.e., perturbations are smaller than about 20 nm), short-wavelength (wavelength comparable to shell thickness) perturbations can grow rapidly and destroy the compressing shell. Mix Similarly, near the end of the implosion, such instabilities can mix colder material into the spot that must be heated to ignition. If too much cold material is injected into the hot spot, ignition will not occur. Density Most of the fuel must be compressed to high density, approximately 1,000 to 4,000 times solid density. (In the case of hot-spot ignition, the central (gaseous) portion of the fuel is compressed to lower density.) Compression to such high den- sities demands that the fuel remain relatively cool during compression—technically, 3  For hot-spot ignition, the convergence ratio is usually defined as the initial target radius divided by the final hot-spot radius.

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32 An Assessment of the Prospects for Inertial Fusion Energy very nearly Fermi-degenerate. Otherwise, too much energy is required to achieve the required density. This requirement in turn places stringent constraints on the pulse shape driving the target. The drive pressure must initially be relatively low (on the order of 1 Mbar); otherwise the initial shock wave that is created will heat the fuel to an unacceptable level. The pressure must then increase to produce a sequence of carefully timed shock waves to compress and ignite the fuel in the hot spot. Moreover, if the beam–target interaction produces too many energetic electrons or photons that can penetrate into the fuel and preheat it, efficient com- pression is not possible. Fuel compression is related to an important quantity, the product of fuel den- sity and fuel radius (rr). This quantity is important for two reasons. The first is related to ignition. Ignition occurs when the rate of energy gain in the fuel exceeds the rate of energy loss. The igniting fuel gains energy as the fuel is shocked and compressed, but it must also gain energy by capturing its own burn products; spe- cifically, in the case of deuterium-tritium fuel, it must capture the alpha particles that are produced. In this case, the rr of the hot spot must exceed approximately 0.3 g/cm2, the stopping range of an alpha particle in igniting fuel.4 The second reason that rr is an important quantity is because it determines the fraction of fuel that burns. This fraction is approximately given by rr /(rr + 6), where rr is given in g/cm2. To achieve high target energy gain needed for laser inertial fusion energy (IFE), the rr of the entire fuel, not just the hot spot, must be of the order of 3 g/cm2. It is noteworthy that if one were to achieve such a rr with uncompressed fuel, the fuel mass would be of the order of 1 kg. Heating 1 kg to 10 keV requires about 1012 J (~200 tons of high explosive equivalent) delivered to the fuel, and the resulting fusion yield would be 100 kton. These are perhaps the most important reasons why a small mass of fuel, typically 1 to 10 mg, must be compressed to high density. Implosion Velocity As noted above, ignition occurs when the rate of energy gain in the fuel exceeds the rate of energy loss. For hot-spot ignition, an implosion velocity on the order of 300 km/s is required to provide adequate self-heating of the fuel. It is fortunate that this velocity corresponds to a specific energy that is more than adequate to compress the fuel to the required density. However, since the ignition velocity exceeds the velocity needed for compression, it may be possible to improve target performance by separating the compression and ignition processes. This possibility is the reason for considering fast ignition and shock ignition. 4  R. Betti, University of Rochester, “Tutorial on the Physics of Inertial Confinement Fusion for Energy Applications,” Presentation to the committee on March 29, 2011.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 33 Laser Targets, Direct and Indirect Drive As discussed above, there are two principal ways to drive laser targets, direct drive and indirect drive. Both have advantages and disadvantages. Choosing between the two approaches has been, and remains, one of the most thoroughly (sometimes hotly) debated issues in inertial fusion. The choice is complicated because it involves not only target physics but also issues associated with target fabrication, reactor chamber geometry and wall protection, target injection, align- ment tolerances, and target debris. Moreover, target performance depends on the wavelength and bandwidth of the laser light used to illuminate the target. Tradi- tionally this dependence has coupled the choice of direct vs. indirect drive to the choice of laser, further complicating the scientific issues. It is important that the laser–target interaction does not produce energetic photons or electrons that can preheat the fuel and prevent proper compression. A number of laser–plasma instabilities are known to produce preheat. The product of laser intensity (power per unit area) and wavelength squared is a measure of the importance of such instabilities. The instabilities are less important at lower intensities and shorter wavelengths. Consequently, as explained later in this chapter, solid-state lasers that typically produce light with a wavelength of 1 µm employ frequency doubling, tripling, or quadrupling to obtain wavelengths that are more compatible with target requirements. KrF lasers intrinsically produce light with a wavelength of 0.25 µm and do not require frequency multiplication. Even at shorter wavelengths, important concerns and uncertainties remain, especially because the targets required for inertial fusion power production must be larger than the targets that have been experimentally studied. Instabilities are expected to be worse in the larger plasma scale lengths associated with these larger targets. The high efficiency of coupling laser energy to the imploding fuel is usually considered the most important advantage of direct drive. In the case of indirect drive, a substantial fraction of the laser energy must be used to heat the hohlraum wall. Typically less than half the laser energy is available as X-rays that actually heat the ablator. On the other hand, the calculated efficiency of X-ray ablation is usually somewhat higher than the efficiency of direct ablation, partially offsetting the ­ ohlraum losses. Nevertheless, the higher coupling efficiency of direct drive is h reflected in the target gain curves (target energy gain vs. laser energy) shown to the committee. Specifically, for hot-spot ignition, the calculated target gain for direct drive at the same drive energy is roughly 3 times higher, or, alternatively, 1.5 times higher at two-thirds of the drive energy. (Higher gain and lower driver energy lead to improved economics for IFE.) If shock ignition (described below) turns out to be feasible for direct drive but not indirect drive, the difference in gain between direct and indirect drive for a given driver energy will be more pronounced.

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34 An Assessment of the Prospects for Inertial Fusion Energy Another potential advantage of direct drive is the chemical simplicity of the target. Laser direct-drive targets usually contain little high-Z material. In contrast, indirect-drive targets require a hohlraum made of some high-Z material such as lead. For this reason the indirect-drive waste stream (from target debris) contains more mass and is chemically more complex than the direct-drive waste stream. This issue is discussed more fully in Chapter 3. Indirect drive also has a number of advantages. For indirect drive, the beams do not impinge directly on the capsule but rather on the inside of the hohlraum wall (see Figure 1.4). The radiation produced at any point illuminates nearly half the surface area of the target. Moreover, the radiation that does not strike the target is absorbed and reemitted by the hohlraum wall. Thus, there is a significant smoothing effect associated with indirect drive. Consequently, beam uniformity, beam energy balance, and beam alignment requirements are less stringent than they are for direct drive. For example, for direct drive, a typical beam align- ment tolerance might be 20 µm. The NIF baseline indirect-drive target, however, can tolerate a beam misalignment of about 80 µm. Furthermore, although the h ­ ohlraum complicates the waste stream from the target, it also provides thermal and mechanical protection for the target as it is injected into the hot chamber. This protection enables the use of chamber wall protection schemes (e.g., gas protection) that are not available to direct drive; for instance, gas in the chamber produces u ­ nacceptable heating of bare, direct-drive targets. Moreover, the smoothing effects of the ­ ohlraum allow greater flexibility in beam geometry (chamber design) than h is the case for direct drive. Specifically, polar illumination is suitable for indirect drive. It is likely suitable for direct drive as well, but for direct drive it degrades performance relative to spherical drive. A final advantage of indirect drive is not a technical advantage at all, but rather a programmatic advantage. Much of the capsule physics of indirect drive is nearly independent of the driver. Therefore significant amounts of the informa- tion learned on laser indirect-drive experiments carry over to indirect drive for ion-driven targets. As for interactions with the chamber wall, direct-drive targets and indirect- drive targets have very different output spectra in terms of the fraction of energy in exhaust ions compared to the fraction of energy in X-rays. Specifically, for indirect drive a substantial fraction of the ion energy is converted to X-rays when the ions strike the hohlraum material. Partly because of the difference in spectra, different wall protection schemes are usually adopted for the two target options. For example, magnetic deflection of ions is an option that is being considered for direct drive while gas or liquid wall protection to absorb X-rays is usually favored for indirect drive. The issues of output spectra, target debris, chamber options, and target fabrication costs are discussed more fully in Chapter 3.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 35 The NIF houses the world’s largest operating laser.5 The NIF team has selected indirect drive with hot-spot ignition and polar illumination for its first ignition experiments. Without modification, the NIF could also be used to study some aspects of direct drive such as the behavior of laser beams in plasmas having large scale lengths. With modifications to improve beam smoothness, NIF is also able to study polar direct drive with and without shock ignition.6 Such modifications are estimated to take 4 or more years to complete and cost $50 million to $60 million (including a 25 percent contingency added by this committee; see Chapter 4).7 In summary, both direct drive and indirect drive have advantages. The current uncertainties in target physics are too large to determine which approach is best, particularly when one includes all the related issues associated with chambers, tar- get fabrication and injection, wavelength dependence, and so on. This conclusion leads to Recommendation 2-1, below. Laser-Driven Fast Ignition In laser-driven fast ignition the target is compressed to high density with a low implosion velocity and then ignited by a short, high-energy pulse of electrons or ions induced by a very short (a few picoseconds) high-power laser pulse.8 Fast ignition has two potential advantages over conventional hot-spot ignition: higher gain, because the target does not need to be compressed as much, and relaxed sym- metry requirements, because ignition does not depend on uniform compression to very high densities. The fast-ignition concept for inertial confinement fusion (ICF) was proposed with the emergence of ultrahigh-intensity, ultrashort pulse lasers using the chirped-pulse-amplification (CPA) technique. The target compression can be done by a traditional driver: direct-drive by lasers or ion beams; or indirect drive from X-rays using a hohlraum driven by nanosecond lasers, ion beams, or a Z-pinch or magnetically imploded target. The ignition is initiated by a converting a short, high-intensity laser pulse (the so-called “ignitor pulse”) into an intense electron or ion beam that will efficiently couple its energy to the compressed fuel. A number of different schemes for coupling a high-energy, short-pulse laser to a compressed core have been examined. The “hole-boring” scheme involves 5  E.I.Moses, 2011, The National Ignition Facility and the promise of inertial fusion energy, Fusion Science and Technology 60: 11-16. 6  J. Quintenz, NNSA, and M. Dunne, LLNL, Two presentations to the committee on February 22, 2012 (see Appendix C). 7  “Polar Drive Ignition Campaign Conceptual Design,” TR-553311, submitted to NNSA in April 2012 by the Lawrence Livermore National Laboratory (LLNL) and revised and submitted to NNSA by the Laboratory for Laser Energetics (LLE) in September 2012. 8  R. Betti, University of Rochester, “Tutorial on the Physics of Inertial Confinement Fusion for Energy Applications,” Presentation to the committee on March 29, 2011.

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36 An Assessment of the Prospects for Inertial Fusion Energy two short-pulse laser beams, one having a ~100 ps duration to create a channel in the coronal plasma surrounding the imploded dense fuel, through which the high-intensity laser pulse that generates the energetic electrons or ion beams would propagate.9 An alternative design uses a hollow gold cone inserted in the spherical shell,10 as illustrated in Figure 2.1. In this scheme, the fuel implosion produces dense plasma at the tip of the cone, while the hollow cone makes it possible for the short-pulse-ignition laser to be transported inside the cone without having to propagate through the coronal plasma and enables the generation of hot electrons at its tip, very close to the dense plasma. A variant cone concept uses a thin foil to generate a proton plasma jet with multi-MeV proton energies. The protons deliver the energy to the ignition hot spot, with the loss of efficiency in the conversion of hot electrons into energetic protons balanced by the ability to focus the protons to a small spot.11 As is the case for hot-spot ignition, the minimum areal density for ignition at the core (ρr ~ 0.3 g/cm2 at 10 keV) is set by the 3.5-MeV alpha particle range in deuterium-tritium (DT) and the hot-spot disassembly time. This must be matched by the electron energy deposition range. This occurs for electron energy in the ~1 to 3 MeV range. The minimum ignition energy, Eig, is independent of target size and scales only with the density of the target; the greater the mass density, the less the beam energy required for ignition (about 20 kJ of collimated electron/ion beam energy is required for a ~300 g/cm3 fuel assembly).12 The optimum compressed-fuel configuration for fast ignition is an approxi- mately uniform-density spherical assembly of high-density DT fuel without a cen- tral hot spot. High densities can be achieved by imploding thick cryogenic DT shells with a low-implosion velocity and low entropy. Such massive cold shells produce a large and dense DT fuel assembly, leading to high gains and large burn-up fractions. Experimental investigations of the fast-ignition concept are challenging and involve extremely high-energy-density physics: ultraintense lasers (>1019 W cm–2); pressures in excess of 1 Gbar; magnetic fields in excess of 100 MG; and electric fields in excess of 1012 V/m. Addressing the sheer complexity and scale of the problem inherently requires the high-energy and high-power laser facilities that are now becoming available (OMEGA Extended Performance and NIF’s Advanced 9  M. Tabak, J. Hammer, M.E. Gilinsky, et al., 1994, Ignition and high gain with ultrapowerful lasers, Physics of Plasmas 1: 1626. 10  R. Kodama, P.A. Norreys, K. Mima, et al., 2001, Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition, Nature 412: 798. 11  M.H. Key, 2007, Status of and prospects for the fast ignition inertial fusion concept, Physics of Plasmas 14: 5. 12  R.R. Freeman, C. Anderson, J.M. Hill, J. King, R. Snavely, S. Hatchett, M. Key, J. Koch, A. MacKinnon, R. Stephens, and T. Cowan, 2003, High-intensity lasers and controlled fusion, European Physics Journal D 26: 73-77.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 37 FIGURE 2.1  In this fast ignition approach, a hollow gold cone inserted in the spherical shell is used to couple energy to the compressed core. SOURCE: H. Azechi, Osaka University, “Inertial Fusion Energy: Activities and Plans in Japan,” Presentation to the committee on June 15, 2011. Radiographic Capability, among others) as well as the most advanced theory and computer simulation capability available. Laser-Driven Shock Ignition As in fast ignition, shock ignition separates the compression of the thermo­ nuclear fuel from the ignition trigger. The ignition process is initiated by a spheri- cally convergent strong shock (the “ignitor shock”) launched at the end of the compression pulse. This late shock collides with the return shock driven by the ris- ing pressure inside the central hot spot and enhances the hot-spot pressure.13 Since the ignitor shock is launched when the imploding shell is still cold, the shock propagation occurs through a strongly coupled, dense plasma. If timed correctly, the shock-induced pressure enhancement triggers the ignition of the central hot spot. In laser direct-drive shock ignition, the capsule is a thick wetted-foam shell14,15 driven at a relatively low implosion velocity of ~250 km/s. The compression pulse consists of a shaped laser pulse designed to implode the capsule with low entropy to achieve high volumetric and areal densities. The fuel mass is typically greater for shock ignition than for hot-spot ignition. The large mass of fuel leads to high fusion-energy yields and the low entropy leads to high areal densities and large burn-up fractions. These conditions lead to high predicted gain. The ignitor shock 13  R. Betti, C.D. Zhou, K.S. Anderson, L.J. Perkins, W. Theobald, and A.A. Solodov, 2007, Shock ignition of thermonuclear fuel at high areal density, Physical Review Letters 98: 155001. 14  Ibid. 15  J. Sethian and S. Obenschain, Naval Research Laboratory, “Krypton Fluoride Laser Driven Inertial Fusion,” Presentation to the committee on January 29, 2011.

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38 An Assessment of the Prospects for Inertial Fusion Energy FIGURE 2.2  Shock ignition power input. SOURCE: J. Sethian and S. Obenschain, Naval Research Laboratory, “Krypton Fluoride Laser-Driven Inertial Fusion,” Presentation to the committee on Janu- ary 29, 2011. is required because at low velocities the central hot spot is too cold to reach the ignition condition with the conventional ICF approach. The ignitor shock can be launched by a spike in the laser intensity on target or by particle beams incident on the target surface (see Figure 2.2). Recent numerical simulations suggest that it may be possible to achieve gains exceeding 100 at laser energies smaller than 500 kJ.16 Although the intensity of the final shock ignition pulse exceeds the threshold for laser–plasma instabilities, there are grounds to believe that target preheat by fast electrons may not be a problem.17 Laser Beam–Target Interaction In order to achieve any of the conditions needed for ignition and thermo- nuclear burn, it is essential that the beams interact properly with the target. For 16  A.J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalasek, and D.E. Fyfe, 2010, Shock ignition target design for inertial fusion energy, Physics of Plasmas 17: 042701. 17  Ibid.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 39 example, if too large a fraction of the beam energy is reflected or refracted away from the target, it is not possible to achieve high energy gain. Also, as noted above, the beam–target interaction must not produce a sufficient number of energetic electrons or photons to preheat the fuel so that it cannot be adequately compressed. For indirect drive, the beam energy must efficiently convert into X-rays, and for direct drive, the ablation process must efficiently drive the implosion. Despite extensive theoretical and experimental work, beam-target interactions are still not fully understood. The beam-target interaction for ion beams will be discussed in a later section. For laser beams, effects such as laser–plasma instabilities depend on the size of the plasma. While there is considerable experimental information at scale sizes that are too small to achieve ignition and burn, these instabilities are an important concern for both direct drive and indirect drive for fusion-scale targets, especially because the available experimental data are limited. Furthermore, the instabilities become more deleterious with increasing wavelength and increasing laser intensity. The scaling with wavelength is the reason that current target experi- ments are usually performed with frequency-tripled 351 nm light from solid-state lasers or the 248 nm ultraviolet light from KrF lasers. The intensity scaling means that laser–plasma instabilities are greater during the brief shock-ignition pulse than during hot-spot ignition, although hot-spot ignition may be more vulnerable to the hot electrons produced by laser–plasma instabilities over the long drive pulse. OMEGA, Nike, and the NIF are valuable national assets that are continuing to elucidate the unknown features of laser–plasma interactions. Status of Laser-Driven Target Implosion Research The NIF laser, commissioned in March 2009, is a unique facility for exploring IFE physics and validating target design and performance. It is the only facility that may be able to demonstrate laser-driven ignition during the next several years. It can deliver up to ~1.8 MJ of UV (351 nm) energy with 30-ps timing precision. The NIF laser has met a 95 percent availability level for requested shots, and more than 300 shots were commissioned through 2012. Critical ignition physics studies took place during the National Ignition Campaign (NIC) program, which concluded on September 30, 2012. The goal of this program was to achieve ignition, to com- mission targets, and to understand the physics necessary for successful, reliable ignition. Recent target shots have led to improved symmetry and a measured yield of 5-9 × 1014 neutrons at 1.4-1.6 MJ drive energy. To put this in perspective, alpha particle heating of dense fuel surrounding the hot spot is confirmed at a yield of ~1016 neutrons and breakeven ignition at ~5.6 × 1017 neutrons on a threshold curve

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78 An Assessment of the Prospects for Inertial Fusion Energy all aspects of this approach owing to a paucity of relevant experimental data on target physics and ignition and a lack of in-depth design studies on inertial fusion reactors at the proposed multi-GJ yield and ~0.1 Hz repetition rate called for by the advocates. In addition to MagLIF, there other promising approaches to pulsed- power fusion energy, including one called magnetized target fusion (MTF). While MagLIF operates on the 100-ns timescale, is ~1 cm in size, and involves open magnetic field lines, MTF operates on a ~1 µs timescale, is tens of centimeters in size, and involves closed (field-reversed) magnetic field lines. A pulsed-power fusion reactor system would be very different from both laser- and heavy-ion fusion systems. As such, the technological or economic failure modes are likely to be very different. Historical Background The use of <100-ns-pulse-duration, intense electron beams driven by pulsed- power generators for ICF was first discussed in the mid-1960s at Physics Interna- tional Company as pulsed-power generators capable of hundreds of kiloamperes and ~10 MeV were being developed there and elsewhere.112 F. Winterberg appears to have the earliest full publications on the subject.113 Sandia National Laboratories (SNL) initiated a research program on pulsed-power-driven IFE with intense elec- tron beams in the early 1970s.114 This became the light-ion fusion program in 1979, when the advantages of intense light-ion beams relative to electrons were recog- nized and it became possible to produce intense light-ion beams efficiently.115 Some progress on the generation of adequately intense light-ion beams using pulsed- power generators was made by the middle 1990s.116 However, the demonstration of efficient coupling of electrical energy into magnetic energy and then to soft X-rays (through the intermediary of imploding cylindrical wire-array Z-pinches with hundreds of fine tungsten wires)117 deflected the pulsed-power-driven inertial fusion community in the direction of radiation-driven (indirect-drive) fuel-capsule 112  F.C. Ford, D. Martin, D. Sloan, and W. Link, 1967, Bulletin of the American Physical Society 12: 961. 113  F. Winterberg, 1968, The possibility of producing dense thermonuclear plasma by an intense field emission discharge, Physical Review 174: 212-220. 114  G. Yonas, J.W. Poukey, and K.R. Prestwich, 1974, Electron beam focusing and application to pulsed fusion, Nuclear Fusion 14: 731-740. 115  See, for example, J.P. VanDevender, 1986, Inertial confinement fusion with light ion beams, Plasma Physics and Controlled Fusion 28: 841-855. 116  J.P. Quintenz, T.A. Mehlhorn, R.G. Adams, G.O. Allshouse, et al., 1994, Progress in the light ion driven inertial confinement fusion program, Proceedings of 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research 3: 39-44. 117  T.W.L. Sanford, G.O. Allshouse, B.M Marder, et al., 1996, Improved symmetry greatly increases X-ray power from wire-array Z-pinches,” Physical Review Letters 77: 5063-5066.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 79 implosions. The even higher potential efficiency of magnetically driven (direct- drive) ignition of magnetized fusion fuel—MagLIF—and recent favorable com- puter simulation results for this concept have caused MagLIF to become a leading candidate for pulsed-power fusion energy.118 Imploding a magnetized, field-reversed target plasma in a solid or liquid liner by a pulsed external magnetic field is a 1970s (or earlier) idea that has been pushed from the millisecond to the microsecond timescale in the present embodiment, MTF.119 This approach is very properly described as a hybrid of magnetic and inertial confinement fusion, since the magnetic field configuration is a closed- confinement geometry. However, the duration of confinement—should fusion reactions be ignited—is determined by the inertia of the imploding liner. Status The necessary high-efficiency, 0.1-1 pulse-per-second pulsed-power technol- ogy is close to being in hand, and the cost per joule of energy delivered to the fusion target load is projected to be substantially lower than for all other drivers. Proof of principle that the necessary driver for a fusion reactor can be built for an acceptable price is possible within 6 years, according to the advocates.120 Thus far, target physics for MagLIF has been addressed only through computer simulations.121 However, current research program plans at SNL include addressing many target physics issues using existing facilities as part of the NNSA-sponsored (single-pulse) ICF program.122 On the reactor side, the present MagLIF approach as proposed by SNL involves extremely high-yield pulses (~10 GJ) at a repetition rate on the order of 1 per 10 s (~0.1 Hz). This makes some of the proposed reactor challenges unique, such as the requirement for power delivery to the fusion fuel by a recyclable transmission 118  M. Cuneo et al., SNL, “Pulsed Power IFE: Background, Phased R&D, and Roadmap,” Presentation to the committee on April 1, 2011; M.E. Cuneo et al., SNL, Response to the committee, submitted in March 2011; S.A. Slutz, M.C. Herrmann, R.A. Vesey, et al., 2010, Pulsed-power-driven cylindrical implosions of laser pre-heated fuel magnetized with an axial magnetic field, Physics of Plasmas 17: 056303. 119  G. Wurden and I. Lindemuth, LANL, “Magnetio-Inertial Fusion (Magnetized Target Fusion),” Presentation to the committee on March 31, 2011. 120  M. Cuneo, et al., SNL, “Pulsed Power IFE: Background, Phased R&D, and Roadmap,” Presentation to the committee on April 1, 2011; M.E. Cuneo et al., SNL, Response to the committee, submitted in March 2011. 121  S.A. Slutz, M.C. Herrmann, R.A. Vesey, et al., 2010, Pulsed-power-driven cylindrical implosions of laser pre-heated fuel magnetized with an axial magnetic field, Physics of Plasmas 17: 056303. 122  M. Cuneo et al., SNL, “Pulsed Power IFE: Background, Phased R&D, and Roadmap,” Presentation to the committee on April 1, 2011; M.E. Cuneo et al., SNL, Response to the committee, submitted in March 2011.

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80 An Assessment of the Prospects for Inertial Fusion Energy FIGURE 2.10  Recyclable transmission line concept with liquid wall chamber. SOURCE: M. Cuneo, SNL, Presentation to the committee on April 1, 2011. line (RTL) (see Figure 2.10).123,124 There has been some analysis, and some small- scale experiments have been carried out that address how such high yields might be sustained repetitively in a reactor chamber.125 Single-pulse tests of MTF are being done now with the Shiva Star facility at the Air Force Research Laboratory at 6 MA. Next-generation tests are proposed that would use explosively driven, high-magnetic-field generation to drive the implo- sion, but IFE would require a high-repetition-rate pulsed-power driver. Reactor considerations for this concept have not been developed in detail to the commit- tee’s knowledge. 123  The recyclable transmission line is destroyed during each shot. Because it contains a considerable mass of material, economical operation dictates that this material be recycled. 124  See M. Cuneo et al., SNL, “Pulsed Power IFE: Background, Phased R&D, and Roadmap,” Presentation to the committee on April 1, 2011; M.E. Cuneo et al., SNL, Response to the committee, submitted in March 2011; and J.T. Cook, G.E. Rochau, B.B. Cipiti, et al., Z-inertial fusion energy: Power plant final report FY06, Sandia National Laboratories report SAND2006-7148. 125  See J.T. Cook, G.E. Rochau, B.B. Cipiti, C.W. Morrow, S.B. Rodriguez, C.O. Farnum, et al., 2006, Z-inertial fusion energy: Power plant, SAND2006-7148, Sandia; M. Sawan, L. El-Guebaly, and P. Wilson, 2007, Three dimensional nuclear assessment for the chamber of Z-pinch power plant, Fusion Science and Technology 52: 753; S.B. Rodríguez, V.J. Dandini, V.L. Vigíl, and M. Turgeon, 2005, Z-pinch power plant shock mitigation experiments, modeling and code assessment, Fusion Science and Technology 47: 656; S.I. Abdel-Khalik and M. Yoda, 2005, An overview of Georgia Tech studies on the fluid dynamics aspects of liquid protection schemes for fusion reactors, Fusion Science and Technology 47: 601; S.G. Durbin, M. Yoda, and S.I. Abdel-Khalik, 2005, Flow conditioning design in thick liquid protection, Fusion Science and Technology 47: 724.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 81 Scientific and Engineering Challenges and Future R&D Priorities for Pulsed-Power IFE Applications Implosion of magnetized plasma inside a conducting cylinder on open field lines to achieve fusion ignition depends on magnetic inhibition of radial energy transport and effective fusion burn before the hot plasma can run out the ends. MagLIF would achieve this with a ~100 ns implosion time and a few centimeters of high-density plasma confined by open magnetic field lines. Thus, the major target physics challenges that are to be addressed in the near term on Z are the following: • Demonstrating that the predicted high-efficiency energy transfer from electrical energy to hot magnetized fusion fuel plasma compressed by magnetic-field-driven implosion of a cylindrical conducting liner occurs in experiments. Determining plasma conditions inside the imploding liner is a major part of this challenge. • Demonstrating that the energy-loss rate of the compressed plasma is much less than that of an unmagnetized plasma. Understanding how the magnetic field affects the transport coefficients is a necessary part of this research to allow validating the design codes. The MTF version of the two items is to demonstrate at 6 MA that a sufficiently well-confined plasma can be produced to warrant explosively driven experiments that have a much higher cost than the pulsed-power experiments. As in MagLIF, diagnostic access to the plasma if it is not generating the predicted number of n ­ eutrons is very limited, again making determination of the plasma condition inside the liner a part of this challenge. The biggest early technology challenge for pulsed-power IFE is establishing the technical credibility of the proposed low-repetition-rate (~0.1 Hz), ~10 GJ yield- per-pulse reactor concept. The recyclable transmission line approach for delivering the current from the pulsed-power system to the fusion-fuel-containing target must be demonstrated to be technically feasible. Technical issues that must be addressed for the transmission line include these: what material to use, how thick it must be, and how to recycle it economically; how best to load the assembly in the reactor chamber (bearing in mind that the fusion-fuel-containing load—possibly requiring cryogenics—must be attached to it); and how to assure that the assembly makes a good electrical connection to the pulsed-power system. Demonstrating the engineering feasibility of a thick-liquid-wall reactor cham- ber is a challenge that pulsed-power shares with other possible approaches, particu- larly heavy-ion fusion. However, pulsed-power fusion, as most recently proposed, is alone in requiring compatibility of the reactor chamber with recyclable trans- mission lines and with ~10 GJ yield per pulse (the equivalent of 2.5 tons of high

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82 An Assessment of the Prospects for Inertial Fusion Energy explosive). Some analyses of fatigue and nucleonics limits of possible chamber materials and some experimental studies relevant to thick liquid wall reactor cham- bers have been carried out,126 but much work is yet to be done here. Design and execution of a hydrodynamically equivalent experiment that could be conducted in a smaller “scaled” chamber at a much-reduced energy level should be part of the Phase 1 research program (see Table 2-3). This research would benefit heavy-ion fusion as well. If there is no technically viable solution to the reactor chamber prob- lem at 10 GJ that is also economically viable, then pulsed-power fusion researchers will have to reoptimize their system design at a lower energy per pulse and a higher repetition rate than 0.1 Hz. Thus, the technical and economic feasibility of the 10 GJ yield system should be evaluated as early in Phase 1 as possible. Given the state of development of linear transformer drivers (LTDs) (see Fig- ures 2.11 and 2.12),127 the technology challenges associated with the pulsed-power system appear to be much less daunting than those discussed above. Nevertheless, the technology must still be demonstrated to be extremely reliable, as there would be hundreds of thousands of switches and a million capacitors in a pulsed-power reactor driver.128 Furthermore, the driver must be demonstrated to be compatible with using recyclable transmission lines, including their potential failure modes (e.g., sparking due to poor connections). Many of the scientific issues having to do with MagLIF target physics can be addressed using existing facilities in the next 5 years, and many will be investi- gated as part of the NNSA-sponsored (single-pulse) ICF program at SNL. It is anticipated that this program will be funded at an estimated level of $6.8 mil- lion to $8.5 million per year through 2017.129 All pulsed-power approaches call for recyclable transmission lines and extremely high-yield pulses at a repetition rate of ~0.1 Hz, and these requirements make some of the necessary research and development for pulsed-power IFE unique. The high repetition rate driver technology needed for fusion via pulsed power is currently receiving development funding at the rate of $1.5 million to $3.3 million per year,130 and steady progress is being made. The engineering feasibility challenges of MagLIF should be addressed early in the program, along with the target physics, to assess the viability of pulsed-power fusion. To do this, new funding would be required starting in 2013 at the level 126  Ibid. 127  W. Stygar, SNL, “Conceptual Design of Pulsed Power Accelerators for Inertial Fusion Energy,” Presentation to the committee on April 1, 2011. 128  J.T. Cook, G.E. Rochau, B.B. Cipiti, C.W. Morrow, S.B. Rodriguez, C.O. Farnum, et al., 2006, Z-inertial fusion energy: Power plant, SAND2006-7148. 129  M. Cuneo, personal communication to committee member D. Hammer on November 2, 2011. 130  Ibid.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 83 FIGURE 2-11  Pictorial representation of a side section of an annular LTD cavity where the load now is the coaxial line formed by the inner cylindrical surface of the cavity and the central (cathode) cylin- drical electrode. The red arrows show the current direction in each conductor. Each unit consists of two capacitors charged to ±100 kV, a 200-kV switch, and a portion of the annular ferrite cores that assure that the pulse is delivered to the load until the cores saturate. There are many such units in parallel around the annular cavity in order to produce the desired output current. SOURCE: Copied with permission of the first author from M.G. Mazarakis, W.E. Fowler, A.A. Kim, et al., 2009, High current, 0.5-MA, fast, 100-ns, linear transformer driver experiments, Physical Review Special Topics- Accelerators and Beams 12: 050401. FIGURE 2-12  Top view of 20 units in parallel in an annular LTD cavity. SOURCE: Copied with permis- sion of the first author from M.G. Mazarakis, W.E. Fowler, A.A. Kim, et al., 2009, High current, 0.5-MA, fast, 100-ns, linear transformer driver experiments, Physical Review Special Topics-Accelerators and Beams 12: 050401.

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84 An Assessment of the Prospects for Inertial Fusion Energy TABLE 2.3  Elements of a Pulsed-Power Inertial Fusion Energy Program Phase 1 Phase 2 Phase 3 (FTF) MagLif target physics Target physics: achieve ignition Build and test an FTF that operates on a single-pulse facility with in burst mode and is capable of Validate codes repetition-rate-capable pulsed- achieving breakeven. power technology. LTD technology development Achieve multigigajoule yield per Establish the viability of a pulse. RTL engineering studies 0.1 Hz, 10-GJ-yield IFE facility through analysis and scaled Reactor chamber engineering hydrodynamics experiments. studies Demonstrate RTL engineering Infrastructure planning (targets, feasibility in burst mode. etc.) Design an FTF for pulsed-power IFE. of $8 million to $10 million per year if a Technology Readiness Level of 6 (see Chapter 4) by 2018 is to be achieved for many of the elements of the reactor.131 Conclusion 2-11: The promise of MagLIF as a high-efficiency approach to inertial confinement fusion is largely untested, but the program to do so is in place and is funded by NNSA. Conclusion 2-12: There has been considerable progress in the development of efficient pulsed-power drivers of the type needed for inertial confinement fusion applications, and the funding is in place to continue along that path. Conclusion 2-13: The physics challenges associated with achieving ignition with pulsed power are being addressed at present as part of the NNSA- sponsored (single-pulse) inertial confinement fusion program. Recommendation 2-2: Physics issues associated with the Magnetized Liner Inertial Fusion (MagLIF) concept should be addressed in single-pulse mode during the next 5 years so as to determine its scientific feasibility. 131  M. Cuneo et al., SNL, “Pulsed Power IFE: Background, Phased R&D, and Roadmap,” Presentation to the committee on April 1, 2011, and M.E. Cuneo et al., SNL, Response to the committee, submitted in March 2011.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 85 Conclusion 2-14: The major technology issues that would have to be resolved in order to make a pulsed-power IFE system feasible—the recyclable trans- mission line and the ultra-high-yield chamber technology development— are not receiving any significant attention. Recommendation 2-3: Technical issues associated with the viability of recyclable transmission lines and 0.1 Hz, 10-GJ-yield chambers should be addressed with engineering feasibility studies in the next 5 years in order to assess the technical feasibility of MagLIF as an inertial fusion energy system option. Assuming the necessary milestones are achieved in both target physics and engineering feasibility, a second phase that would last about 10 more years could be undertaken starting around 2018 to develop the necessary reactor-scale technology and industrial capacity for an FTF. Some of the necessary technology infrastructure—specifically, production of the recyclable transmission line—may be close enough to standard large-scale industrial manufacturing that development costs and schedule can be projected with reasonable confidence without major demonstration projects. The fact that the cylindrical fusion fuel-containing targets for MagLIF will be inserted into the reactor chamber as part of the recyclable transmission line assembly is a potential simpli- fication compared to other IFE approaches, assuming viable engineering solutions for the line’s fabrication, emplacement, contact, and recycling problems are found. The MTF has a 3-year target physics program plan using Shiva Star at $2.8 mil- lion per year, which is to be followed by explosively driven implosion tests in Nevada at about $100 million per year for 2 years. Path Forward for Pulsed-Power Inertial Fusion Energy The plan for pulsed-power IFE that follows is based on information provided to the committee by SNL. Near Term (≤5 Years, Initially Using NNSA Funding) • Target physics. Using existing facilities, validate the magnetically imploded cylindrical target concept to the point of achieving scientific breakeven (fusion energy out = energy delivered to the fuel). This requires develop- ing tritium-handling capability on Z. Also, develop IFE target requirements experimentally and theoretically, which requires validating computer codes. • Pulsed power. Demonstrate the capability of Linear Transformer Driver pulsed-power technology to deliver the necessary power, energy, and

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86 An Assessment of the Prospects for Inertial Fusion Energy repetition rate with a long operational lifetime and the anticipated high efficiency. Design the reactor driver. • Recyclable transmission line. Develop an engineering design of a recyclable (magnetically insulated) transmission line (RTL) and demonstrate its engi- neering feasibility experimentally at high power (low repetition rate). • Reactor chamber. Carry out a detailed design study of the presently favored, multigigajoule, thick-liquid-wall, low-repetition-rate (~0.1 Hz) reactor con- cept; develop the conceptual design of a credible demonstration power plant in partnership with industry; initiate necessary technology R&D. Design and, if warranted, implement a hydrodynamically equivalent test of the via- bility of a thick-liquid-wall chamber to contain repeated 10 GJ yield fusion explosions. Determine with industrial partners if such a low-repetition-rate, high-yield system is the optimum solution for pulsed power in light of target physics, recyclable transmission line, and pulsed-power ICF/IFE develop- ments in Phase 1 (Table 2.3). • Industrial infrastructure planning. In partnership with industry, design pro- duction lines and delivery systems needed for RTLs, targets, etc. • Next facility design. Determine the necessary new facility for ignition experi- ments (defined as fusion alpha-particle heating of the fuel exceeding energy delivered to the fuel by the driver) and high yield (up to 100 MJ), from which the fusion burn can be scaled to the ~10 GJ yield per target needed by the reactor. (See ZFIRE in the pulsed-power IFE roadmap in Figure 2.13.) New funding between $8 million and $10 million per year is needed to under- take the last four engineering development tasks.132 Medium Term (5-15 Years, Assumes All Milestones in Phase 1 Are Achieved) • Target physics: Ignition. Achieve ignition in a new, repetitive-pulse-capable Linear Transformer Driver pulsed-power facility (ZFIRE); fully validate design codes needed to scale to full reactor yield. This would be an NNSA facility that can be used for weapon physics and weapon effects testing. • Recyclable transmission line engineering. Demonstrate operation of an RTL at ~100 TW and 0.1 Hz (burst mode), with ignition for one or more “single pulses.” • Reactor chamber. Establish by analysis and demonstrate key technolo- gies associated with the thick liquid wall IFE reactor chamber needed for ~10 GJ, 0.1 Hz operation (vacuum system, liquid wall recovery, and so on). This technology could also be beneficial for heavy-ion fusion. 132  M. Cuneo et al., op. cit.

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S tat u s & C h a l l e n g e s for Inertial Fusion Energy Drivers & Targets 87 FIGURE 2.13  Pulsed-power roadmap. SOURCE: M.E. Cuneo, M.C. Herrmann, W.A. Stygar, et al., from the document submitted to the committee in response to the committee’s Second Request for Input, p. 6, received March 24, 2011. • Target design and fabrication for inertial fusion energy. Determine optimized target design and target fabrication requirements for an FTF and a dem- onstration power plant. • Fusion Test Facility design. With industry, develop an engineering design for an FTF for pulsed-power fusion, including factories to build RTLs, ­ argets, t and other components that must be replaced with each pulse; tritium breeding and handling systems; and all balance-of-plant systems. Design must include full resource requirement and safety and reliability analyses. An economically “competitive” cost of electricity must be projected or this approach cannot go to the demonstration stage. There are two aspects to the cost of electricity: the amortized capital cost of the plant, the estimate of which is likely to be better than a factor of two only at the end of Phase 2 (see Table 2.3), and the cost of plant operation. Included in the latter is fuel cost, including operation of the tritium recovery system. Let us assume that is the same for all of the potential reactors. The dominant additional operating cost for pulsed-power fusion energy is likely to be manufacturing and recycling the RTLs. At present it is not known how that will compare with, for example, the actual

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88 An Assessment of the Prospects for Inertial Fusion Energy costs incurred by laser-driven systems for replacing optical components or heavy- ion fusion for replacing final focusing magnets. This kind of operating cost will not be known very well until the end of Phase 2 for any of the approaches to IFE. Long Term (>20 Years): Build and Operate a Fusion Test Facility Assuming all milestones in the medium-term program are met, an FTF would be designed to achieve facility breakeven in initial operation (fusion yield of 100-200 MJ) in repetitive pulse operation but for “bursts” of limited duration. Upgrades would enable this facility to increase its yield to ~2 GJ or more. It is too early to provide a credible estimate for the cost of an FTF (see ZFUSE in the Roadmap, below) as the cost of the reactor chamber and recyclable transmission line factory are likely to be dominant and they will not be established until the end of Phase 2. A conceptual roadmap for implementing the R&D program for pulsed power inertial fusion is shown in Figure 2.13. GENERAL CONCLUSIONS There are a number of technical approaches, each involving a different com- bination of driver, target, and chamber, that show promise for leading to a viable IFE power plant. These approaches involve three kinds of targets: indirect drive, direct drive, and magnetized target. In addition, the chamber may have a solid or a thick-liquid first wall that faces the fusion fuel explosion, as discussed in Chapter 3. Substantial progress has been made in the last 10 years in advancing most of the elements of these approaches, despite erratic funding for some programs. Nonetheless, substantial amount of R&D will be required to show that any par- ticular combination of driver, target, and chamber would meet the requirements for a demonstration power plant. In all cases, the drivers may build on decades of research in their area. In all technical approaches there is the need to build a reactor-scale driver module for use in an FTF. The timing for this step is discussed in Chapter 4. As discussed in Chapter 4, development of an FTF and the upgrade to a dem- onstration plant requires an integrated system engineering approach supported by R&D at each stage. This statement is true regardless of which driver-target combination is chosen. It also requires involvement and support from the user community (utilities), from the facilities engineering community (large engineer- ing firms), and the government (national laboratories) to conduct R&D and risk reduction programs for laser drivers, target physics, target manufacturing and commissioning, reactors, and balance-of-plant systems. In addition, work must address licensing and environmental and safety issues.