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1219 2 STATUS AND CHALLENGES FOR INERTIAL FUSION
1220 ENERGY DRIVERS AND TARGETS
1221 A brief introduction to the concepts of drivers, targets, and implosion mechanisms
1222 was given in Chapter 1. In the first part of this chapter, we provide a more detailed
1223 discussion of alternative strategies for driving the implosion of targets and explain
1224 why terms such as “direct drive” and “indirect drive” are more accurate descriptors
1225 for some driver-target pairs than for others.
1226 In the second part of this chapter, we take up the status and future R&D needs of the
1227 three major driver candidates: lasers (which include diode-pumped, solid-state lasers
1228 and krypton fluoride lasers); heavy-ion accelerators; and pulsed-power drivers. This
1229 discussion of driver approaches is based on input received from proponents who are
1230 technical experts in the field. 1 As such, the R&D challenges and investment priorities
1231 for moving each approach forward to a major test facility (fusion test facility, or FTF)
1232 are discussed independently of one another; i.e., as if a decision had been made to
1233 choose that particular approach as the best option for IFE. The committee recognizes
1234 that a down-selection to one particular approach will have to be made and does not
1235 mean to suggest that all of the approaches should be funded simultaneously at the
1236 levels indicated in this chapter. A discussion of how these approaches might fit into
1237 an integrated program with down-selection decision points is given in Chapter 4.
1238 Throughout this chapter is material drawn from the report of the Committee’s
1239 supporting Target Physics Panel (see the Preface); the Summary from the unclassified
1240 Target Physics Panel report appears in Appendix H.
1241 Conclusions and recommendations are given within the sections. General conclusions
1242 appear at the end of this chapter.
1243 METHODS FOR DRIVING THE IMPLOSION OF TARGETS
1244 A large number of target designs have been studied and proposed for inertial fusion
1245 energy power plants. As explained in Chapter 1, these targets may be categorized
1246 according to the method used to drive the implosion (i.e., to compress the fuel to high
1247 density), and according to the method used to bring the fuel to the required ignition
1248 temperature. In addition, targets are sometimes categorized according to illumination
1249 geometry. For example, for some target designs, the incoming driver beams are
1250 arranged uniformly around the target to approximate spherical illumination. At the
1251 National Ignition Facility (NIF), the beams are arranged in four cones that illuminate
1252 the inside wall of the hohlraum from two sides (the poles of the cylindrically
1253 symmetric target). Historically, there have also been illumination geometries that
1254 more strongly illuminate the equatorial area of the target. Finally, for pulsed-power
1255 IFE systems, there may be no driver beams at all; the electrical energy is coupled
1256 directly to the target by the pressure of the magnetic field produced by the drive
1257 current.
1
A list of the experts who gave presentations to the committee is in Appendix C.
2-1
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1258 The two principal methods of driving laser implosions are indirect drive and direct
1259 drive (see Fig. 1.4). For ion accelerators, there is nearly a continuum between indirect
1260 drive and direct drive.
1261 The three principal methods proposed to ignite the fuel are referred to as hot-spot
1262 ignition, shock ignition, and fast ignition. For indirect drive, there is some thermal
1263 inertia or heat capacity associated with the cavity surrounding the fuel capsule, and
1264 with the ablator itself. It is more difficult to achieve the rapid rise in temperature and
1265 pressure with indirect drive because of the thermal inertia of the hohlraum. Shock
1266 ignition requires rapidly rising drive pressure at the end of the drive pulse.
1267 Consequently, shock ignition is usually associated with direct drive. Hot-spot ignition
1268 and fast ignition are the main ignition modes for indirect drive. All three modes of
1269 ignition necessarily ignite only a small fraction of the fuel. The thermonuclear burn
1270 then propagates into the bulk of the fuel.
1271 Implosion Requirements
1272 A number of conditions must be satisfied to produce ignition and reactor-scale gain. 2
1273 These conditions are described in detail in Appendix A; in this section, we give a
1274 brief overview:
1275 Symmetry
1276 Ideally, the final imploded fuel configuration should be nearly spherical. For laser-
1277 driven and heavy-ion-driven implosions, this requirement imposes conditions on the
1278 uniformity of the light, x-ray, or ion flux driving the target, and also on the initial
1279 uniformity of the target itself. For example, if the target were driven more strongly
1280 near the poles, the final imploded configuration might be shaped like a pancake. If the
1281 equator were driven more strongly, the imploded configuration might resemble a
1282 sausage. The level of precision required in direct drive (e.g., in drive pressure or shell
1283 thickness) is greater, the greater the convergence ratio 3 of the target. For most laser
1284 target designs, this convergence ratio lies between 20 and 40.
1285 Sausage-like, pancake-like, dumbbell-like, or even doughnut-like asymmetries are
1286 “low-order” asymmetries in the sense that the wavelength of the departures from
1287 spherical symmetry are comparable to the size of the compressed fuel configuration.
1288 Energy imbalance among the beams is one possible type of error leading to low-order
1289 asymmetries; beam misalignment is another.
1290 Fluid Instabilities
1291 In addition to the low-order asymmetries, higher-order asymmetries are also
1292 important. Small perturbations on the surfaces of the fuel and ablator shell can grow
1293 as the shell is accelerated.
2
R. Betti, “Tutorial on the Physics of Inertial Confinement Fusion for Energy Applications,”
presentation to the committee, March 29, 2011.
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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1294 Unless the initial layer surfaces are very smooth (i.e., perturbations smaller than about
1295 20 nm), short-wavelength (wavelength comparable to shell thickness) perturbations
1296 can grow rapidly and destroy the compressing shell.
1297 Mix
1298 Similarly, near the end of the implosion, such instabilities can mix colder material
1299 into the spot that must be heated to ignition. If too much cold material is injected into
1300 the hot spot, ignition will not occur.
1301 Density
1302 Most of the fuel must be compressed to high density, approximately 1000−4000 times
1303 solid density. (In the case of hot-spot ignition, the central (gaseous) portion of the fuel
1304 is compressed to lesser density.) Compression to such high densities demands that
1305 the fuel must remain relatively cool during compression—technically, very nearly
1306 Fermi-degenerate. Otherwise, too much energy is required to achieve the required
1307 density. This requirement in turn places stringent constraints on the pulse shape
1308 driving the target. The drive pressure must initially be relatively low (of the order of 1
1309 Mbar); otherwise the initial shock wave that is created will heat the fuel to an
1310 unacceptable level. The pressure must then increase to produce a sequence of
1311 carefully timed shock waves to compress and ignite the fuel in the hot spot.
1312 Moreover, if the beam-target interaction produces too many energetic electrons or
1313 photons that can penetrate into the fuel and preheat it, efficient compression is not
1314 possible.
1315 Fuel compression is related to an important quantity, the product of fuel density and
1316 fuel radius (ρr). This quantity is important for two reasons. The first is related to
1317 ignition. Ignition occurs when the rate of energy gain in the fuel exceeds the rate of
1318 energy loss. The igniting fuel gains energy as the fuel is shocked and compressed, but
1319 it must also gain energy by capturing its own burn products; specifically, in the case
1320 of deuterium-tritium fuel, it must capture the alpha particles that are produced. In this
1321 case, the ρr of the hot spot must exceed approximately 0.3 g/cm2, the stopping range
1322 of an alpha particle in igniting fuel. 4 The second reason that ρr is an important
1323 quantity is because it determines the fraction of fuel that burns. This fraction is
1324 approximately given by ρr /(ρr + 6) where ρr is given in g/cm2. To achieve high
1325 target energy gain needed for laser inertial fusion energy, the ρr of the entire fuel, not
1326 just the hot spot, must be of the order of 3 g/cm2. It is noteworthy that if one were to
1327 achieve such a ρr with uncompressed fuel, the fuel mass would be of the order of 1
1328 kg. Heating 1 kg to 10 keV requires about 1012 Joules (~200 tons of high explosive
1329 equivalent) delivered to the fuel, and the resulting fusion yield would be 100 ktons.
1330 These are perhaps the most important reasons why a small mass of fuel, typically 1 to
1331 10 mg, must be compressed to high density.
1332
4
R. Betti, op. cit.
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1333 Implosion Velocity
1334 As noted above, ignition occurs when the rate of energy gain in the fuel exceeds the
1335 rate of energy loss. For hot-spot ignition, implosion velocity of the order of 300 km/s
1336 is required to provide adequate self-heating of the fuel. It is fortunate that this
1337 velocity corresponds to a specific energy that is more than adequate to compress the
1338 fuel to the required density. However, since the ignition velocity exceeds the velocity
1339 needed for compression, it may be possible to improve target performance by
1340 separating the compression and ignition processes. This possibility is the motivation
1341 for considering fast ignition and shock ignition.
1342 Laser Targets, Direct and Indirect Drive
1343 As discussed above, there are two principal ways to drive laser targets, direct drive
1344 and indirect drive. Both methods have advantages and disadvantages. Choosing
1345 between the two approaches has been, and remains, one of the most thoroughly
1346 (sometimes hotly) debated issues in inertial fusion. The issue is complicated because
1347 it involves not only target physics but also issues associated with target fabrication,
1348 reactor chamber geometry and wall protection, target injection, alignment tolerances,
1349 target debris, etc. Moreover, target performance depends on the wavelength and
1350 bandwidth of the laser light used to illuminate the target. Traditionally this
1351 dependence has coupled the choice of direct vs. indirect drive to the choice of laser,
1352 further complicating the scientific issues.
1353 It is important that the laser-target interaction does not produce energetic photons or
1354 electrons that can preheat the fuel and prevent proper compression. A number of
1355 laser-plasma instabilities are known to produce preheat. The product of laser intensity
1356 (power per unit area) and wavelength squared is a measure of the importance of such
1357 instabilities. The instabilities are less important at lower intensities and shorter
1358 wavelengths. Consequently, as explained later in this chapter, solid-state lasers that
1359 typically produce 1-micrometer-wavelength light employ frequency doubling,
1360 tripling, or quadrupling to obtain wavelengths that are more compatible with target
1361 requirements. Krypton fluoride (KrF) lasers intrinsically produce quarter-micron light
1362 and do not require frequency multiplication. Even at shorter wavelengths, important
1363 concerns and uncertainties remain, especially because the targets required for inertial
1364 fusion power production must be larger than targets that have been experimentally
1365 studied. Instabilities are expected to be worse in the larger plasma scale lengths
1366 associated with these larger targets.
1367 The high efficiency of coupling laser energy to the imploding fuel is usually
1368 considered the most important advantage of direct drive. In the case of indirect drive,
1369 a substantial fraction of the laser energy must be used to heat the hohlraum wall.
1370 Typically less than half the laser energy is available as x-rays that actually heat the
1371 ablator. On the other hand, the calculated efficiency of x-ray ablation is usually
1372 somewhat higher than the efficiency of direct ablation—partially offsetting the
1373 hohlraum losses. Nevertheless, the higher coupling efficiency of direct drive is
1374 reflected in the target gain curves (target energy gain vs. laser energy) shown to the
1375 committee. Specifically, for hot-spot ignition, the calculated target gain for direct
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1376 drive at the same drive energy is roughly a factor of 3 higher, or, alternatively, 1.5
1377 times higher at 2/3 of the drive energy. (Higher gain and lower driver energy lead to
1378 improved economics for IFE). If shock ignition (described below) turns out to be
1379 feasible for direct drive but not indirect drive, the difference in gain between direct
1380 and indirect drive for a given driver energy will be more pronounced.
1381 Another potential advantage of direct drive is the chemical simplicity of the target.
1382 Laser direct-drive targets usually contain little high-Z material. In contrast, indirect-
1383 drive targets require a hohlraum made of some high-Z material such as lead. For this
1384 reason the indirect-drive waste stream (from target debris) contains more mass and is
1385 chemically more complex than the direct-drive waste stream. This issue is discussed
1386 more fully in Chapter 3.
1387 Indirect drive also has a number of advantages. For indirect drive, the beams do not
1388 impinge directly on the capsule but rather on the inside of the hohlraum wall (see
1389 Figure 1.4). The radiation produced at any point illuminates nearly half the surface
1390 area of the target. Moreover, the radiation that does not strike the target is absorbed
1391 and re-emitted by the hohlraum wall. Thus, there is a significant smoothing effect
1392 associated with indirect drive. Consequently, beam uniformity, beam energy balance,
1393 and beam alignment requirements are less stringent than they are for direct drive. For
1394 example, for direct drive, a typical beam alignment tolerance might be 20 microns.
1395 The NIF baseline indirect-drive target, however, can tolerate a beam misalignment of
1396 about 80 microns. Furthermore, although the hohlraum complicates the waste stream
1397 from the target, it also provides thermal and mechanical protection for the target as it
1398 is injected into the hot chamber. This protection enables the use of chamber wall
1399 protection schemes (e.g. gas protection) that are not available to direct drive; for
1400 instance, gas in the chamber produces unacceptable heating of bare, direct-drive
1401 targets. Moreover, the smoothing effects of the hohlraum allow greater flexibility in
1402 beam geometry (chamber design) than is the case for direct drive. Specifically, polar
1403 illumination is suitable for indirect drive. It is likely suitable for direct drive as well,
1404 but for direct drive it degrades performance relative to spherical drive.
1405 A final advantage of indirect drive is not a technical advantage at all, but rather a
1406 programmatic advantage. Much of the capsule physics of indirect drive is nearly
1407 independent of the driver. Therefore significant amounts of the information learned
1408 on laser indirect-drive experiments carry over to indirect drive for ion-driven targets.
1409 In regard to interactions with the chamber wall, direct-drive targets and indirect-drive
1410 targets have very different output spectra in terms of the fraction of energy in exhaust
1411 ions compared to the fraction of energy in x-rays. Specifically, for indirect drive a
1412 substantial fraction of the ion energy is converted to x-rays when the ions strike the
1413 hohlraum material. Partly because of the difference in spectra, different wall
1414 protection schemes are usually adopted for the two target options. For example,
1415 magnetic deflection of ions is an option that is being considered for direct drive while
1416 gas or liquid wall protection to absorb x-rays is usually favored for indirect drive.
1417 The issues of output spectra, target debris, chamber options, and target
1418 fabrication costs are discussed more fully in Chapter 3.
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1419 The National Ignition Facility houses the world’s largest operating laser. 5 The NIF
1420 team has selected indirect drive with hot-spot ignition and polar illumination for its
1421 first ignition experiments. Without modification, the NIF could also be used to study
1422 some aspects of direct drive such as the behavior of laser beams in plasmas having
1423 large scale lengths. With modifications to improve beam smoothness, NIF also has
1424 the capability to study polar direct drive with and without shock ignition. 6 Such
1425 modifications are estimated to take four or more years to complete and cost $50-60 M
1426 (including a 25% contingency added by this committee; see Chapter 4). 7
1427 In summary, both direct drive and indirect drive have advantages. The current
1428 uncertainties in target physics are too large to determine which approach is best,
1429 particularly when one includes all the related issues associated with chambers, target
1430 fabrication and injection, wavelength dependence, and so on. This conclusion leads to
1431 recommendation 2-1, below.
1432 Laser-driven Fast Ignition
1433 In laser-driven fast ignition the target is compressed to high density with a low
1434 implosion velocity and then ignited by a short, high-energy pulse of electrons or ions
1435 induced by a very short, (few picosecond) high-power laser pulse. 8 Fast ignition has
1436 two potential advantages over conventional hot-spot ignition: higher gain, because the
1437 target does not need to be compressed as much, and relaxed symmetry requirements
1438 because ignition does not depend on uniform compression to very high densities. The
1439 fast-ignition concept for inertial confinement fusion was proposed with the
1440 emergence of ultrahigh-intensity, ultra-short pulse lasers using the chirped-pulse-
1441 amplification (CPA) technique. The target compression can be done by a traditional
1442 driver (direct-drive by lasers or ion beams, or indirect drive from X-rays using a
1443 hohlraum driven by nanosecond lasers, ion beams, or a Z-pinch or magnetically
1444 imploded target). The ignition is initiated by a converting a short, high-intensity laser
1445 pulse (the so-called “ignitor pulse”) into an intense electron or ion beam that will
1446 efficiently couple its energy to the compressed fuel.
1447 A number of different schemes for coupling a high-energy, short-pulse laser to a
1448 compressed core have been examined. The “hole-boring” scheme involves two short-
1449 pulse laser beams, one having a ~100-ps duration to create a channel in the coronal
1450 plasma surrounding the imploded dense fuel, through which the high-intensity laser
1451 pulse that generates the energetic electrons or ion beams would propagate. 9 An
5
E. I. Moses “The National Ignition Facility and the Promise of Inertial Fusion Energy”,
Fusion Science and Technology vol 60 pp 11-16 July 2011.
6
J. Quintenz, (NNSA) and Michael Dunne (LLNL), two presentations to the committee on
Feb. 22, 2012, San Diego, CA (see Appendix C).
7
“Polar Drive Ignition Campaign Conceptual Design,” LLNL TR-553311, submitted to
NNSA in April 2012 by LLNL and revised and submitted to NNSA by LLE in September
2012.
8
R. Betti, op. cit.
9
M. Tabak, J. Hammer, M.E. Gilinsky, et al., Phys. Plasmas, Vol. 1, 1994, p. 1626.
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1452 alternative design uses a hollow gold cone inserted in the spherical shell, 10 as
1453 illustrated in Figure 2.2.
1454
1455
1456 FIGURE 2.2. In this fast ignition approach, a hollow, gold cone inserted in the
1457 spherical shell is used to couple energy to the compressed core. SOURCE: H. Azechi,
1458 “Inertial Fusion Energy: Activities and Plans in Japan,” presentation to the
1459 committee, June 15, 2011.
1460 In this scheme, the fuel implosion produces dense plasma at the tip of the cone, while
1461 the hollow cone makes it possible for the short-pulse-ignition laser to be transported
1462 inside the cone without having to propagate through the coronal plasma, and enables
1463 the generation of hot electrons at its tip, very close to the dense plasma. A variant
1464 cone-concept uses a thin foil to generate a proton plasma jet with multi-MeV proton
1465 energies. The protons deliver the energy to the ignition hot spot—with the loss of
1466 efficiency in the conversion of hot electrons into energetic protons balanced by the
1467 ability to focus the protons to a small spot. 11
1468 As is the case for hot-spot ignition, the minimum areal density for ignition at the core
1469 (ρr ~ 0.3 g/cm2 at 10 keV) is set by the 3.5-MeV alpha-particle range in D-T and the
1470 hot-spot disassembly time. This must be matched by the electron-energy deposition
1471 range. This occurs for electron energy in the ~1- to 3-MeV range. The minimum
1472 ignition energy Eig is independent of target size and scales only with the density of the
1473 target; the higher the mass density, the lower the beam energy required for ignition
1474 (about 20 kJ of collimated electron/ion beam energy is required for a ~300 g/cc fuel
1475 assembly). 12
1476 The optimum compressed-fuel configuration for fast ignition is an approximately
1477 uniform-density spherical assembly of high-density DT fuel without a central hot
10
R. Kodama, P.A. Norreys, K. Mima, et al., Nature (London) Vol. 412, 2001, p.798.
11
M.H. Key, “Status of and prospects for the fast ignition inertial fusion concept,” Physics of
Plasmas, Volume 14, Issue 5 (2007).
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, “High-intensity lasers and controlled fusion,”
The European Physics Journal D, Volume 26, Issue 1, pp 73-77 (September 2003).
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1478 spot. High densities can be achieved by imploding thick cryogenic-DT shells with a
1479 low-implosion velocity and low entropy. Such massive cold shells produce a large
1480 and dense DT fuel assembly, leading to high gains and large burn-up fractions.
1481 Experimental investigations of the fast-ignition concept are challenging and involve
1482 extremely high-energy-density physics: ultra-intense lasers (>1019 W cm–2); pressures
1483 in excess of 1 Gbar; magnetic fields in excess of 100 MG; and electric fields in
1484 excess of 1012 V/m. Addressing the sheer complexity and scale of the problem
1485 inherently requires high-energy and high-power laser facilities that are now becoming
1486 available (e.g., OMEGA Extended Performance, NIF-Advanced Radiographic
1487 Capability, etc.) as well as the most advanced theory and computer simulation
1488 capability available.
1489 Laser-driven Shock Ignition
1490 As in fast ignition, shock ignition separates the compression of the thermonuclear fuel
1491 from the ignition trigger. The ignition process is initiated by a spherically convergent
1492 strong shock (the ignitor shock) launched at the end of the compression pulse. This
1493 late shock collides with the return shock driven by the rising pressure inside the
1494 central hot spot and enhances the hot-spot pressure. 13 Since the ignitor shock is
1495 launched when the imploding shell is still cold, the shock propagation occurs through
1496 a strongly-coupled, dense plasma. If timed correctly, the shock-induced pressure
1497 enhancement triggers the ignition of the central hot spot. In laser direct-drive shock
1498 ignition, the capsule is a thick wetted-foam shell 14, 15 driven at a relatively low
1499 implosion velocity of ~250 km/s. The compression pulse consists of a shaped laser
1500 pulse designed to implode the capsule with low entropy to achieve high volumetric
1501 and areal densities. The fuel mass is typically greater for shock ignition than for hot-
1502 spot ignition. The large mass of fuel leads to high fusion-energy yields and the low
1503 entropy leads to high areal densities and large burn-up fractions. These conditions
1504 lead to high predicted gain. The ignitor shock is required because, at low velocities,
1505 the central hot spot is too cold to reach the ignition condition with the conventional
1506 inertial confinement fusion approach. The ignitor shock can be launched by a spike in
1507 the laser intensity on target or by particle beams incident on the target surface (see
1508 Figure 2.3).
1509
13
R. Betti et al., "Shock Ignition of Thermonuclear Fuel at High Areal Density", Phys. Rev.
Lett. Vol. 98, 2007, p. 155001.
14
Ibid.
15
J. Sethian and S. Obenschain, “Krypton Fluoride Laser Driven Inertial Fusion,”
presentation to the committee, January 29, 2011.
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1510
1511 FIGURE 2.3 Shock ignition power input. SOURCE: J.Sethian and S. Obenschain,
1512 “Krypton Fluoride Laser-Driven Inertial Fusion,” presentation to the committee,
1513 January 29, 2011.
1514 Recent numerical simulations suggest that it may be possible to achieve gains
1515 exceeding 100 at laser energies smaller than 500 kJ. 16 Although the intensity of the
1516 final shock ignition pulse exceeds the threshold for laser-plasma instabilities, there
1517 are grounds to believe that target preheat by fast electrons may not be a problem. 17
1518 Laser Beam-Target Interaction
1519 In order to achieve any of the conditions needed for ignition and thermonuclear burn,
1520 it is essential that the beams interact properly with the target. For example, if too
1521 large a fraction of the beam energy is reflected or refracted away from the target, it is
1522 not possible to achieve high energy gain. Also, as noted above, the beam-target
1523 interaction must not produce a sufficient number of energetic electrons or photons to
1524 preheat the fuel so that it cannot be adequately compressed. For indirect drive, the
1525 beam energy must efficiently convert into x-rays, and for direct drive, the ablation
1526 process must efficiently drive the implosion. Despite extensive theoretical and
1527 experimental work, beam-target interactions are still not fully understood. The beam-
1528 target interaction for ion beams will be discussed in a later section. For laser beams,
1529 effects such as laser-plasma instabilities depend on the size of the plasma. While there
1530 is considerable experimental information at scale sizes that are too small to achieve
1531 ignition and burn, these instabilities are an important concern for both direct drive and
1532 indirect drive for fusion-scale targets, especially because the available experimental
16
A.J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalasek and D.E. Fyfe, “Shock Ignition
Target Design for Inertial Fusion Energy,” Physics of Plasmas, Vol. 17, 2010, p. 042701.
17
A.J. Schmitt, op. cit.
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1533 data is limited. Furthermore, the instabilities become more deleterious with increasing
1534 wavelength and increasing laser intensity. The scaling with wavelength is the reason
1535 that current target experiments are usually performed with frequency-tripled 351 nm
1536 light from solid-state lasers or the 248 nm ultraviolet light from KrF lasers. The
1537 intensity scaling means that laser-plasma instabilities are greater during the brief
1538 shock-ignition pulse than for hot-spot ignition, although hot-spot ignition may be
1539 more vulnerable to the hot electrons produced by laser-plasma instabilities over the
1540 long drive pulse. OMEGA, Nike, and the NIF are valuable national assets that are
1541 continuing to elucidate the unknown features of laser-plasma interactions.
1542 Status of Laser-Driven Target Implosion Research
1543 The NIF laser, commissioned in March 2009, is a unique facility for exploring inertial
1544 fusion energy physics and validating target design and performance. It is the only
1545 facility that may be able to demonstrate laser-driven ignition during the next several
1546 years. It can deliver up to ~1.8 MJ of UV (351 nm) energy with 30-psec timing
1547 precision. The NIF laser has met a 95-percent availability level for requested shots
1548 and more than 300 shots were commissioned through 2012. Critical ignition physics
1549 studies took place during the National Ignition Campaign (NIC) program, which
1550 concluded on September 30, 2012. The goal of this program was to achieve ignition,
1551 to commission targets, and to understand the physics necessary for successful,
1552 reliable ignition. Recent target shots have led to improved symmetry and a measured
1553 yield of 5-9x1014 neutrons at 1.4-1.6 MJ drive energy. To put this in perspective,
1554 alpha particle heating of dense fuel surrounding the hot spot is confirmed at a yield of
1555 ~1016 neutrons and breakeven ignition at ~5.6 x 1017 neutrons on a threshold curve
1556 calculated to be very steep. 18 The NIC made progress in approaching the sphericity,
1557 compression, and velocity needed for ignition. However, the NIC experiments
1558 produced a number of surprising results, particularly regarding a lower-than-expected
1559 implosion velocity. There are also still uncertainties associated with low-mode
1560 asymmetries of the dense fuel and mix.
1561
1562 The Target Panel (see Appendix H) concludes that “Based on its analysis of the gaps
1563 in current understanding of target physics and the remaining disparities between
1564 simulations and experimental results, the panel assesses that ignition using laser
1565 indirect drive is not likely in the next several years (Conclusion 4-2). 19” It also states
1566 that “resolving the present issues and addressing any new challenges that might arise
1567 are likely to push the timetable for ignition to 2013-2014 or beyond.” The report also
1568 concludes that:
1569 • “If ignition is achieved with indirect drive at NIF, then an energy gain of 50-
1570 100 should be possible at a future facility. How high the gain at NIF could be
1571 will be better understood by follow-on experiments once ignition is
1572 demonstrated. At this writing, there are too many unknowns to project a
1573 potential gain (Conclusion 4-3).
18
E. I. Moses “The National Ignition Facility and the Promise of Inertial Fusion Energy,”
Fusion Science and Technology vol 60 pp 11-16 July 2011.
19
As of its writing in September 2011.
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1574 • “Achieving ignition will validate assumptions underlying theoretical
1575 predictions and simulations. This may allow a better appreciation of the
1576 sensitivities to parameters important to ignition (Conclusion 4-3).
1577 • “NIF has the potential to support the development and further validation of
1578 physics and engineering models relevant to several IFE concepts, from
1579 indirect-drive hohlraum designs to polar direct-drive ICF and shock ignition
1580 (Overarching Conclusion 1).
1581 • “NIF will also be helpful in evaluating indirectly driven, heavy-ion targets. It
1582 will be less helpful in gathering information relevant to current Z-pinch,
1583 heavy-ion direct drive, and heavy-ion advanced target concepts.”
1584
1585 As noted above, the NIC was completed on September 30, 2012. With input from the
1586 ICF laboratories, NNSA produced a report which put forward a “Plan B”
1587 experimental program for FY 2013 and beyond. 20 These issues and tentative plans
1588 were discussed in presentations to the committee. 21
1589 Conclusion 2-1: There has been good technical progress during the past year in
1590 the ignition campaign carried out on the National Ignition Facility. Nevertheless,
1591 ignition has been more difficult than anticipated and has not been achieved in
1592 the National Ignition Campaign that ended on September 30, 2012. The
1593 experiments to date are not fully understood. It will likely take significantly
1594 more than a year to gain a full understanding of the discrepancies between
1595 theory and experiment and to make needed modifications to optimize target
1596 performance.
1597
1598 The NIF is currently a unique tool for addressing these issues. Some could be
1599 addressed with NIF in its present configuration. Others may require modifications
1600 such as improvements in beam smoothness, or ultimately even a different
1601 illumination geometry.
1602
1603 Laser-plasma instabilities (LPI) are present in current NIF indirect-drive experiments
1604 as well as in the most energetic spherical direct drive (SDD) experiments performed
1605 on OMEGA. Robust, high-gain, laser inertial fusion target design must address and
1606 contain the effects of these nonlinear processes, which have an intensity threshold
1607 behavior that in principle makes modeling extrapolation from low gain to high gain
1608 problematic. Both OMEGA (glass laser) and Nike (KrF laser) can test different
1609 ablator materials with respect to laser-plasma instabilities. Following the recent
1610 results from OMEGA experiments, 22 ablators with moderate atomic number (from
1611 carbon to silicon) greatly reduce LPI while preserving good hydrodynamic properties.
1612 OMEGA and Nike can also compare the acceleration of flat foils at the different
1613 wavelengths of 351 nm (OMEGA) and 249 nm (Nike), with different bandwidths or
1614 beam smoothing, to determine whether there is a significant advantage to using the
20
National Nuclear Security Administration, “NNSA’s Path Forward to Achieving Ignition in
the Inertial Confinement Fusion Program: Report to Congress,” December, 2012.
21
J. Quintenz, and M. Dunne, op. cit.
22
V. Smalyuk et al., Phys. Rev. Lett. 165002 (2010).
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2696 The primary conceptual approach to achieving pulsed-power inertial fusion energy,
2697 Magnetized Liner Inertial Fusion (MagLIF), is a direct-drive approach; i.e., fuel
2698 compression and heating is driven directly by magnetic pressure (see Figure 2.4).
2699 This approach offers the potential benefits of a relatively simple cylindrical target
2700 geometry and high efficiency of delivery of driver energy to fuel implosion and
2701 heating. However, there is considerable uncertainty (i.e., technical risk) on all aspects
2702 of this approach due to a paucity of relevant experimental data on target physics and
2703 ignition, and a lack of in-depth design studies on inertial fusion reactors at the
2704 proposed multi-GJ yield and ~0.1 Hz repetition rate called for by the advocates. In
2705 addition to MagLIF, there other promising approaches to pulsed-power fusion energy,
2706 including one called Magnetized Target Fusion. While MagLIF operates on the 100-
2707 ns time scale, is ~1 cm in size and involves open magnetic field lines, MTF operates
2708 on a ~1 microsecond time scale, is tens of cm in size and involves closed (field
2709 reversed) magnetic field lines.
2710 A pulsed-power fusion reactor system would be very different from both laser- and
2711 heavy-ion fusion systems. As such, technological or economic failure modes are
2712 likely to be very different.
2713 Historical Background
2714 The use of < 100-ns-pulse-duration, intense electron beams driven by pulsed-power
2715 generators for inertial confinement fusion was first discussed in the mid-1960s at
2716 Physics International Company as pulsed-power generators capable of hundreds of
2717 kiloamperes and ~10 MeV were being developed there and elsewhere. 112 F.
2718 Winterberg appears to have the earliest full publications on the subject. 113 Sandia
2719 National Laboratories initiated a research program on pulsed-power-driven IFE with
2720 intense electron beams in the early 1970’s. 114 This became the light-ion fusion
2721 program in 1979 when the advantages of intense light ion beams relative to electrons
2722 were recognized and it became possible to produce intense light-ion beams
2723 efficiently. 115 Some progress on the generation of adequately intense light-ion beams
2724 using pulsed-power generators was made by the middle 1990s. 116 However, the
2725 demonstration of efficient coupling of electrical energy into magnetic energy and then
2726 to soft X-rays (through the intermediary of imploding cylindrical wire-array Z-
2727 pinches with hundreds of fine tungsten wires), 117 deflected the pulsed-power-driven
112
F.C. Ford, D. Martin, D. Sloan, and W. Link, Bull. Am. Phys. Soc., Vol. 12, 1967, p. 961.
113
F. Winterberg, “The Possibility of Producing Dense Thermonuclear Plasma by an Intense
Field Emission Discharge,” Phys Rev., Vol. 174, 1968, p. 212-220.
114
G. Yonas, J.W. Poukey, and K.R. Prestwich, “Electron Beam Focusing and Application to
Pulsed Fusion, Nuclear Fusion,” Vol. 14, 1974, pp. 731-740.
115
See, for example, J. P. VanDevender, “Inertial Confinement Fusion with Light Ion
Beams,” Plasma Physics and Controlled Fusion, Vol. 28, 1986, pp. 841-855.
116
J.P. Quintnez, T.A. Mehlhorn, et al., “Progress in the Light Ion Driven Inertial
Confinement Fusion Program,” Plasma Physics and Controlled Nuclear Fusion Research,
Vol. 3, 1995, pp. 39-44.
117
T.W.L. Sanford et al., “Improved Symmetry Greatly Increases X-ray Power from Wire-
array Z-pinches,” Phys. Rev. Let., Vol. 77, 1996, 5063-5066.
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2728 inertial fusion community in the direction of radiation-driven (indirect-drive) fuel-
2729 capsule implosions. The even higher potential efficiency of magnetically-driven
2730 (direct-drive) ignition of magnetized fusion fuel—Magnetic Liner Inertial Fusion, and
2731 recent favorable computer simulation results on this concept, have led to MagLIF’s
2732 being a leading candidate for pulsed-power fusion energy. 118
2733 Imploding a magnetized, field-reversed target plasma in a solid or liquid liner by a
2734 pulsed external magnetic field is a 1970’s (or earlier) idea that has been pushed from
2735 the millisecond to the microsecond time scale in the present embodiment, Magnetized
2736 Target Fusion. 119 This approach is very properly described as a hybrid of magnetic
2737 and inertial confinement fusion, since the magnetic field configuration is a closed-
2738 confinement geometry. However, the duration of confinement—should fusion
2739 reactions be ignited—is determined by the inertia of the imploding liner.
2740 Status
2741 The necessary high-efficiency, 0.1−1 pulse-per-second pulsed-power technology is
2742 close to being in-hand and the cost per joule of energy delivered to the fusion target
2743 load is projected to be substantially lower than for all other drivers. Proof of principle
2744 that the necessary driver for a fusion reactor can be built for an acceptable price is
2745 possible within 6 years, according to the advocates. 120
2746 Thus far, target physics for MagLIF has been addressed only through computer
2747 simulations.121 However, current research program plans at Sandia include addressing
2748 many target physics issues using existing facilities as part of the NNSA-sponsored
2749 (single-pulse) ICF program. 122
2750 On the reactor side, the present MagLIF approach as proposed by Sandia involves
2751 extremely high-yield pulses (~10 GJ), at a repetition rate of the order of 1 per 10
2752 seconds (~0.1 Hz). This makes some of the proposed reactor challenges unique, such
2753 as the requirement for power delivery to the fusion fuel by a recyclable transmission
2754 line (RTL; see Figure 2.11). 123,124 There has been some analysis, and some small-
118
M. Cuneo et al., “Pulsed Power IFE: Background, Phased R&D and Roadmap,” Sandia
National Laboratories, presentation to committee on April 1, 2011; M. E. Cuneo et al.,
response from Sandia National Laboratories to the committee, submitted by March, 2011;
S.A. Slutz, M.C. Herrmann, R.A. Vesey et al., “Pulsed-power-driven Cylindrical Implosions
of Laser Pre-heated Fuel Magnetized with an Axial Magnetic Field,” Phys. Plasmas, Vol. 17,
2010, p. 056303.
119
G. Wurden and I. Lindemuth, presentation to the committee, Albuquerque, NM, March 31,
2011.
120
M. Cuneo et al., op. cit.
121
S.A. Slutz et al., op. cit.
122
M. Cuneo et al., op. cit.
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., op. cit., 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.
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2755 scale experiments have been carried out that address how such high yields might be
2756 sustained repetitively in a reactor chamber. 125
2757 Single-pulse tests of Magnetized Target Fusion are being done now with the Shiva
2758 Star facility at the Air Force Research Laboratory at 6 MA. Next generation tests are
2759 proposed that would use explosively driven high-magnetic-field generation to drive
2760 the implosion, but inertial fusion energy would require a high-repetition-rate pulsed-
2761 power driver. Reactor considerations for this concept have not been developed in
2762 detail to our knowledge.
2763 Scientific and Engineering Challenges and Future R&D Priorities for Pulsed-
2764 power Inertial Fusion Energy Applications
2765
2766 Implosion of magnetized plasma inside a conducting cylinder on open field lines to
2767 achieve fusion ignition depends upon magnetic inhibition of radial energy transport
2768 and effective fusion burn before the hot plasma can run out the ends. MagLIF would
2769 achieve this with a ~100 ns implosion time and a few cm of high density plasma
2770 confined by open magnetic field lines. Thus, the major “target physics” challenges that
2771 are to be addressed in the near term on Z are:
2772 1) Demonstrating that the predicted high-efficiency energy transfer from
2773 electrical energy to hot magnetized fusion fuel plasma compressed by
2774 magnetic-field-driven implosion of a cylindrical conducting liner occurs in
2775 experiments. Determining plasma conditions inside the imploding liner is a
2776 major part of this challenge.
2777 2) Demonstrating that the energy-loss rate of the compressed plasma is
2778 considerably reduced relative to an unmagnetized plasma. Understanding how
2779 the magnetic field affects the transport coefficients is a necessary part of this
2780 research in order to be able to validate the design codes.
2781 The Magnetized Target Fusion version of items 1) and 2) is to demonstrate at 6 MA
2782 that a sufficiently well confined plasma can be produced to warrant explosively-driven
2783 experiments that have a much higher cost than the pulsed-power experiments.
2784 Diagnostic access to the plasma if it is not generating the predicted number of neutrons
2785 is very limited as in MagLIF, again making the determination of plasma condition
2786 inside the liner a part of this challenge.
125
See J.T. Cook et al., op. cit.; M. Sawan, L. El-Guebaly and P. Wilson, “Three Dimensional
Nuclear Assessment for the Chamber of Z-pinch Power Plant,” Fusion Sci. Technol., Vol. 52,
2007, p. 753; S. B. Rodríguez, V.J. Dandini, V.L. Vigíl and M. Turgeon, “Z-pinch Power
Plant Shock Mitigation Experiments, Modeling and Code Assessment,” Fusion Sci. Technol.,
Vol. 47, 2005, p. 656; S.I. Abdel-Khalik and M. Yoda, “An Overview of Georgia Tech
Studies on the Fluid Dynamics Aspects of Liquid Protection Schemes for Fusion Reactors,”
Fusion Sci. Technol., Vol. 47, 2005, p. 601; S.G. Durbin, M. Yoda and S.I. Abdel-Khalik,
“Flow Conditioning Design in Thick Liquid Protection,” Fusion Sci. and Technol., Vol. 47,
2005, p. 724.
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2787 The biggest early technology challenge for pulsed-power inertial fusion energy is
2788 establishing the technical credibility of the proposed low-repetition-rate (~0.1 Hz), ~10
2789 GJ yield-per-pulse reactor concept. The recyclable transmission line approach for
2790 delivering the current from the pulsed-power system to the fusion-fuel-containing
2791 target must be demonstrated to be technically feasible. Technical issues that must be
2792 addressed for the transmission line include: what material to use, how thick it must be,
2793 and how to recycle it economically; how best to load the assembly in the reactor
2794 chamber (bearing in mind that the fusion-fuel-containing load—possibly requiring
2795 cryogenics—must be attached to it); and how to assure that the assembly makes a
2796 good electrical connection to the pulsed-power system.
2797
2798 Figure 2.11 Recyclable transmission line concept with liquid wall chamber. SOURCE:
2799 M. Cuneo, in a presentation to the committee on April 1, 2011.
2800 Demonstrating the engineering feasibility of a thick-liquid-wall reactor chamber is a
2801 challenge that pulsed-power shares with other possible approaches, particularly heavy-
2802 ion fusion. However, pulsed-power fusion, as most recently proposed, is alone in
2803 requiring compatibility of the reactor chamber with recyclable transmission lines and
2804 with ~10 GJ yield per pulse (the equivalent of 2.5 tons of high explosive). Some
2805 analyses of fatigue and nucleonics limits of possible chamber materials and some
2806 experimental studies relevant to thick liquid wall reactor chambers have been carried
2807 out, 126 but much work is yet to be done here. Design and execution of a
2808 hydrodynamically equivalent experiment that could be conducted in a smaller “scaled”
2809 chamber at a much-reduced energy level should be part of the Phase 1 research
2810 program. This research would benefit heavy-ion fusion as well. If there is no
2811 technically viable solution to the reactor chamber problem at 10 GJ that is also
2812 economically viable, then pulsed-power fusion researchers will have to re-optimize
2813 their system design at a lower energy per pulse and a higher repetition rate than 0.1
2814 Hz. Thus, the technical and economic feasibility of the 10 GJ yield system should be
2815 evaluated as early in Phase 1 as possible.
126
Ibid.
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2816 Given the state of development of Linear Transformer Drivers (LTDs, see Figure 2-
2817 12), 127 the technology challenges associated with the pulsed-power system appear to
2818 be much less daunting than those discussed above. Nevertheless, the technology must
2819 still be demonstrated to be extremely reliable, as there would be hundreds of thousands
2820 of switches and a million capacitors in a pulsed-power reactor driver. 128 Furthermore,
2821 the driver must be demonstrated to be compatible with using recyclable transmission
2822 lines, including their potential failure modes (e.g., sparking due to poor connections).
2823
2824 Figure 2-12a: Pictorial representation of a side section of an annular LTD cavity where
2825 the load now is the coaxial line formed by the inner cylindrical surface of the cavity and
2826 the central (cathode) cylindrical electrode. The red arrows show the current direction in
2827 each conductor. Each unit consists of 2 capacitors charged to ±100 kV, a 200 kV switch
2828 and a portion of the annular ferrite cores that assure that the pulse is delivered to the load
2829 until they saturate. There are many such units in parallel around the annular cavity in
2830 order to produce the desired output current.
2831
2832 Top view of 20 units in parallel in an annular cavity.
127
W. Stygar, “Conceptual Design of Pulsed Power Accelerators for Inertial Fusion Energy,”
presentation to the committee dated April 1, 2011.
128
J.T. Cook et al., op. cit.
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2833 Figure 2-12b: Linear Transformer Driver. SOURCE: Copied with permission of the
2834 first author from: Michael G. Mazarakis, William E. Fowler, Alexander A. Kim,
2835 Vadim A. Sinebryukhov, Sonrisa T. Rogowski, Robin A. Sharpe, Dillon H.
2836 McDaniel, Craig L. Olson, John L. Porter, Kenneth W. Struve, William A. Stygar,
2837 and Joseph R. Woodworth, High current, 0.5-MA, fast, 100-ns, linear transformer
2838 driver experiments, PRST-AB 12, 050401 (2009).
2839
2840 Many of the scientific issues having to do with MagLIF target physics can be
2841 addressed using existing facilities in the next 5 years, and many will be investigated as
2842 part of the NNSA-sponsored (single-pulse) inertial confinement fusion program at
2843 Sandia. It is anticipated that this program will be funded at an estimated level of
2844 $6.8−8.5 M per year combined through 2017. 129 All pulsed power approaches call for
2845 recyclable transmission lines and extremely high-yield pulses at a rep-rate of ~0.1 Hz,
2846 and these requirements make some of the necessary research and development for
2847 pulsed-power IFE unique. The high rep-rate driver technology needed for fusion via
2848 pulsed power is currently receiving development funding at the rate of $1.5-3.3 M per
2849 year 130 and steady progress is being made.
2850 The engineering feasibility challenges of MagLIF should be addressed early in the
2851 program, along with the target physics, to assess viability of pulsed-power fusion. To
2852 do this, new funding would be required starting in 2013 at the level of $8−10 M/yr if a
2853 goal of achieving a Technology Readiness Level of 6 (see Chapter 4) by 2018 is to be
2854 possible for many of the elements of the reactor. 131
2855 Conclusion 2-11: The promise of MagLIF as a high-efficiency approach to
2856 inertial confinement fusion is largely untested, but the program to do so is in
2857 place and is funded by NNSA.
2858 Conclusion 2-12: There has been considerable progress in the development of
2859 efficient pulsed-power drivers of the type needed for inertial confinement fusion
2860 applications, and the funding is in place to continue along that path.
2861 Conclusion 2-13: The physics challenges associated with achieving ignition with
2862 pulsed power are being addressed at present as part of the NNSA-sponsored
2863 (single pulse) inertial confinement fusion program.
2864 Recommendation 2-2: Physics issues associated with the MagLIF concept should
2865 be addressed in single-pulse mode during the next five years so as to determine its
2866 scientific feasibility.
2867 Conclusion 2-14: The major technology issues that would have to be resolved in
2868 order to make a pulsed-power IFE system feasible—the recyclable transmission
2869 line and the ultra-high-yield chamber technology development—are not receiving
2870 any significant attention.
129
M. Cuneo, personal communication to the committee to D. Hammer, date?.
130
Ibid.
131
M. Cuneo et al., op. cit.
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2871 Recommendation 2-3: Technical issues associated with the viability of recyclable
2872 transmission lines and 0.1 Hz, 10-GJ-yield chambers should be addressed with
2873 engineering feasibility studies in the next five years in order to assess the
2874 technical feasibility of MagLIF as an inertial fusion energy system option.
2875 Assuming the necessary milestones are achieved in both target physics and
2876 engineering feasibility, a second phase that would last an additional ~10 years could be
2877 undertaken starting around 2018 to develop the necessary reactor-scale technology and
2878 industrial capacity for a Fusion Test Facility.
2879 Some of the necessary technology infrastructure, specifically the recyclable
2880 transmission line production line, may be close enough to “standard” large-scale
2881 industrial manufacturing that development costs and schedule can be projected with
2882 reasonable confidence without major demonstration projects. The fact that the
2883 cylindrical fusion fuel-containing targets for MagLIF will be inserted into the reactor
2884 chamber as part of the recyclable transmission line assembly is a potential
2885 simplification compared to other IFE approaches, assuming viable engineering
2886 solutions for the line’s fabrication, emplacement, contact and recycling problems are
2887 found.
2888 Magnetized Target Fusion has a 3-year target physics program plan using Shiva Star at
2889 $2.8 M per year, which is to be followed by explosively driven implosion tests in
2890 Nevada at about $100 M per year for 2 years.
2891 Path Forward for Pulsed-power Inertial Fusion Energy
2892 The plan for pulsed-power IFE that follows is based on information provided to the
2893 committee by Sandia National Laboratory.
2894
2895 Near-term (≤ 5 years, initially using NNSA funding)
2896 1) Target Physics: Using existing facilities, validate the magnetically-imploded
2897 cylindrical target concept to the point of achieving scientific breakeven
2898 (fusion energy out = energy delivered to the fuel). This requires developing
2899 tritium-handling capability on Z. Also develop inertial fusion energy target
2900 requirements experimentally and theoretically, which requires validating
2901 computer codes.
2902 2) Pulsed power: Demonstrate the capability of Linear Transformer Driver
2903 pulsed-power technology to deliver the necessary power, energy and rep-rate
2904 with a long operational lifetime and the anticipated high efficiency. Design
2905 the reactor driver.
2906 3) Recyclable Transmission Line: Develop an engineering design of a recyclable
2907 (magnetically insulated) transmission line and demonstrate its engineering
2908 feasibility experimentally at high power (low repetition rate).
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2909 4) Reactor Chamber: Carry out a detailed design study of the presently-favored,
2910 multi-gigajoule, thick liquid wall, low rep-rate (~0.1 Hz) reactor concept;
2911 develop the conceptual design of a credible demonstration power plant in
2912 partnership with industry; initiate necessary technology development R & D.
2913 Design and, if warranted, implement a hydrodynamically equivalent test of the
2914 viability of a thick-liquid-wall chamber to contain repeated 10 GJ yield fusion
2915 explosions. Determine with industrial partners if such a low-rep-rate, high-
2916 yield system is the optimum solution for pulsed power in light of target
2917 physics, recyclable transmission line, and pulsed-power ICF/IFE
2918 developments in phase 1.
2919 5) Industrial infrastructure planning: In partnership with industry, design
2920 production lines and delivery systems needed for recyclable transmission
2921 lines, targets, etc.
2922 6) Next facility design: Determine the necessary new facility for ignition
2923 experiments (defined as fusion alpha-particle heating of the fuel exceeding
2924 energy delivered to the fuel by the driver) and high yield (up to 100 MJ), from
2925 which the fusion burn can be scaled to the ~10 GJ yield per target needed by
2926 the reactor. (See ZFIRE in the pulsed-power IFE roadmap below.)
2927 New funding in the amount of $8−10 M per year is needed to undertake the last 4
2928 engineering development tasks. 132
2929 Medium Term (5-15 years), assumes all milestones in Phase 1 are achieved)
2930 1) Target Physics – Ignition: Achieve ignition in a new, repetitive-pulse-capable
2931 Linear Transformer Driver pulsed-power facility (ZFIRE); fully validate
2932 design codes needed to scale to full reactor yield. This would be an NNSA
2933 facility that can be used for weapon physics and weapon effects testing.
2934 2) Recyclable Transmission Line Engineering: Demonstrate operation of a
2935 recyclable transmission line at ~ 100 TW and 0.1 Hz (burst mode), with
2936 ignition for one or more “single pulses.”
2937 3) Reactor Chamber: Establish by analysis and demonstrate key technologies
2938 associated with the thick liquid wall IFE reactor chamber needed for ~10 GJ,
2939 0.1 Hz operation (vacuum system, liquid wall recovery, etc.). This technology
2940 may also be beneficial for heavy-ion fusion.
2941 4) Target design and fabrication for inertial fusion energy: Determine
2942 optimized target design and target fabrication requirements for a Fusion Test
2943 Facility and a demonstration power plant.
2944 5) Fusion Test Facility design: With industry, develop an engineering design of
2945 a Fusion Test Facility for pulsed-power fusion, including factories to build
2946 recyclable transmission lines, targets, and other components that must be
132
M. Cuneo et al, op. cit.
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2947 replaced each pulse; tritium breeding and handling systems; all balance of
2948 plant systems. Design must include full resource requirement and safety and
2949 reliability analyses. An economically “competitive” cost of electricity must be
2950 projected or this approach cannot go to the demo stage.
2951 There are two aspects to such a cost, the amortized capital cost of the plant,
2952 which is likely to be estimated to better than a factor of two only at the end of
2953 Phase 2, and the cost of plant operation. In the latter, there is fuel cost,
2954 including operation of the tritium recovery system. Let us assume that is the
2955 same for all of the potential reactors. The dominant additional operating cost
2956 for pulsed-power fusion energy is likely to be manufacturing and recycling the
2957 recyclable transmission lines. At present we don’t know how that will
2958 compare with, for example, the actual costs incurred by laser-driven systems
2959 for replacing optical components or heavy-ion fusion for replacing final
2960 focusing magnets. This kind of operating cost will not be known very well
2961 until the end of Phase 2 for any of the approaches to inertial fusion energy.
2962 Long Term (> 20 years from now) – Build and operate a Fusion Test Facility
2963 Assuming all milestones in the medium-term program are met, a Fusion Test
2964 Facility would be designed to achieve facility breakeven in initial operation (fusion
2965 yield of 100−200 MJ) in repetitive pulse operation but for “bursts” of limited
2966 duration. Upgrades would enable this facility to increase yield to ~2 GJ or more. It is
2967 too early to provide a credible estimate of the cost of a Fusion Test Facility (see
2968 ZFUSE in the Roadmap, below) as the cost of the reactor chamber and recyclable
2969 transmission line factory are likely to be dominant and they will not be established
2970 until the end of Phase 2.
2971 Table 2.3. Elements of a Pulsed-Power Inertial Fusion Energy Program.
2972
Phase 1 Phase 2 Phase 3 Fusion Test
Facility
MagLif Target Physics Target physics - achieve Build and test a Fusion
ignition on a single pulse Test Facility that operates
facility with rep-rate- in burst mode and is
capable pulsed-power capable of achieving
Validate codes
technology breakeven.
LTD Technology
Establish the viability of a Achieve multigigajoule
development
0.1 Hz, 10 GJ yield IFE yield per pulse.
RTL Engineering Studies facility through analysis,
scaled hydrodynamics
Reactor Chamber experiments.
engineering studies
Demonstrate RTL
Infrastructure planning engineering feasibility in
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(targets, etc.) burst mode.
Design an FTF for PP IFE.
2973
2974
2975 A conceptual roadmap for implementing the R&D program for pulsed power inertial
2976 fusion is shown in Figure 2.13 below.
2977
2978 Figure 2.13. Pulsed-power roadmap. SOURCE: M. E. Cuneo, M. C. Herrmann, W. A.
2979 Stygar, A. B. Sefkow, S. A. Slutz, R. A. Vesey, R. E. Nygren, E. M. Waisman, J. P.
2980 VanDevender, M. A. Sweeney, S. B. Hansen, D. B. Sinars, R. D. McBride, J. L.
2981 Porter, M. K. Matzen, B. E. Blue, M. S. Bange, C. Filippone, and F. Venneri, from
2982 the document submitted to the committee in response to the committee’s Second
2983 Request for Input, p. 6, received March 24, 2011.
2984 GENERAL CONCLUSIONS
2985 There are a number of technical approaches, each involving a different combination
2986 of driver, target and chamber that show promise for leading to a viable inertial fusion
2987 energy power plant. These approaches involve three kinds of target: indirect drive,
2988 direct drive, and magnetized target. In addition, the chamber may have a solid or a
2989 thick-liquid first wall that faces the fusion fuel explosion, as discussed in chapter 3.
2990 Substantial progress has been made in the last 10 years in advancing most of the
2991 elements of these approaches, despite erratic funding for some programs.
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2992 Nevertheless, substantial amount of R&D will be required to show that any particular
2993 combination of driver, target and chamber would meet the requirements of a Demo
2994 power plant.
2995 In all cases, the drivers may build upon decades of research in their area. In all
2996 technical approaches there is the need to build a reactor scale driver module for use in
2997 a fusion test facility. The timing for this step is discussed in chapter 4.
2998 As discussed in chapter 4, development of a Fusion Test Facility and the upgrade to a
2999 DEMO plant requires an integrated system engineering approach supported by R&D
3000 at each stage. This statement is true regardless of which driver-target combination is
3001 chosen. It also requires involvement and support from the user community (utilities),
3002 from the facilities engineering community (large engineering firms), and government
3003 (national laboratories) to conduct R&D and risk reduction programs for laser drivers,
3004 target physics, target manufacturing and commissioning, reactors, and balance-of-
3005 plant systems. In addition, work must address licensing and environmental and safety
3006 issues.
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