On October 15, 1981, John H. Nuckolls this article has occurred because of the received the American Physical Society's dedication and inspiration of John Maxwell Prize for outstanding contribu­ Nuckolls and his colleagues at LLNL, as tions to plasma physics. The citation read well as of hundreds of researchers else­ "For his contributions to the genesis and where in the United States and other na­ progress of inertial confinement fusion. tions. In addition to their work, consistent His insight into the fundamental physics support for the U.S. Inertial Fusion Pro­ issues has served to guide and inspire the gram has been provided by the Depart­ technical evolution of the field." An article ment of Energy and its predecessors and based on his Maxwell lecture to the Soci­ by the Congress of the United States. This ety was published in Physics Today (Sep­ support has led, in 10 years time, to an ef­ tember 1982). Because of the interest that fort with the accomplishments and scope readers of the En ergy and Technol ogy described in this article. It should be fur­ Review could have in John's views on ther noted that, although the thrust of this John's views on inertial confinement fusion, the editors article is directed toward the long-range power-potential of the En ergy and Technology Review have agreed to re­ of the Inertial Fusion Program, current support and print the article in this issue. The work described in nearer-term goals are defense related. Feasibility of Inertial­ Confinement Fusion "We can see no insurmountable roadblocks to th e practi­ temperatures sufficient to cause the hy­ cal achievement of electrical power generated by in ertial­ drogen nuclei to fuse, releasing an confinement fusion ... An ICF reactor may have a rela ­ amount of energy equivalent to that of tively small containment volume [and] its operation, a barrel of oil (see Physics Today, maintenance, and repair will be relatively simple." August 1973, page 46). Those who are skeptical of the inertial-confinement fusion program For further information contact So concluded the chairman of a have a number of objections. They cite John Nuckolls (415) 422-5435. Department of Energy ad hoc commit­ the fact that the magnetic-fusion tee of experts in 1979, after a compre­ machines-tokamaks and mirrors-are Reprinted, with permission, from Physics Today, hensive review of the U.S. inertial­ several years ahead, and they note the 1 September 1982. confinement fusion program. In spite time and effort that will be required to of this positive evaluation, the role of solve the technical problems, princi­ inertial-confinement fusion in the total pally the coupling of the laser beam to . U.s. energy program continues to be a the target, the confinement of the subject of disagreement.2 Before I men­ fusion explosion, and the development tion the issues of contention, let me de­ of a practical driver. scribe inertial-confinement fusion During the past three years, how­ briefly. In a typical reactor scheme, a ever, inertial-confinement fusion has pea-sized target pellet containing hy­ made substantial progress toward solv­ drogen isotopes is projected into a reac­ ing its principal scientific and techno­ tor chamber, where it is suddenly ir­ logical problems. The program has: radiated with an intense beam of light • Achieved efficient beam-target or ions from a "driver" (Fig. 1). As the coupling using short-wavelength lasers, surface of the target blasts away, the • Achieved high-density implosions rocket-like reaction forces implode the with so-called "hohlraum" targets, target's interior to densities and which we will discuss later, 18 DEFENSE PROGRAMS • Improved the design of damage­ In 1980, Congress and the President proof fluid walls for reactors with high strongly endorsed accelerated develop­ power densities, ment of magnetic fusion. Although eco­ • Designed drivers of higher effi­ nomic difficulties in the United States ciency and lower cost, and have postponed this growth, the ur­ • Planned to utilize the physical gency for fusion continues to increase. separation of the reactor and the high­ The Middle East, source of much of our technology components-the driver oil, is unstable. Weaknesses and uncer­ and the target factory-which several tainties are emerging in the coal-fission reactors can share. These developments strategy for energy independence. Fis­ make possible fusion reactors with sion energy is increasingly impeded by minimum cost and radioactive waste. Murphy's Law (what can go wrong, will) and growing public opposition, The Rise of Fusion while coal energy is generating acid In 1942, Hans Bethe, Enrico Fermi, rain and in the 21st century may pro­ Fig. 1 J. Robert Oppenheimer, Edward Teller, duce major global climate changes Inertial-confinement-fusion reactor and others began to develop the phys­ through the CO2 greenhouse effect. concept known as HYLIFE. A shower ics of practical fusion. Later, at Los To contribute to the solution of these of liquid lithium shields the first wall Alamos, fusion research was driven by energy problems in the next century, from damaging neutrons, x rays, and the determination and technical leader­ inertial fusion must meet several diffi­ hot plasma from exploding fuel pel­ ship of Teller. In 1952, seven years after cult challenges. The DOE experts' com­ lets. With an average lithium thickness of 50 cm, the neutron damage is suffi­ development of the atomic trigger, seis­ mittee, which was headed by John ciently reduced to increase the life­ mographs worldwide registered the Foster, Jr., vice-president of science and time of the first wall tenfold. The success of the multimegaton inertial­ technology at TRW Inc., and former neutron-induced radioactivity in the confinement device. In the 1950s, the DOD Director of Defense Research and reactor structure is reduced 10- to extraordinary promise of controlled Engineering and Director of Lawrence 100-fold. The average power at the first wall is relatively high, approxi­ fusion-safe, clean, low-cost energy Livermore, noted that " .. many prob­ mately 10 MW/ m2. More than 90% of with inexhaustible fuel accessible to all lems and unknowns remain, but this power is absorbed by the liquid nations-led to research programs on because of the wealth of promising lithium. magnetic-confinement fusion at many scientific laboratories. The 1960s saw ~GraPhite plug the conception and calculation of iner­ ~!I!F . tial approaches to controlled fusion, in­ cluding schemes based on implosions driven by laser and charged-particle beams. The energy crisis stimulated rapid .,...----Laser or heavy growth in both the magnetic and ion beams inertial-confinement fusion programs in the early 1970s. The inertial­ confinement fusion programs at LLNL, the University of Rochester Laboratory for Laser Energies, the Lebedev Insti­ tute in the USSR, and Limeil Labora­ tories in France were joined by the new programs at KMS Fusion Inc., Los Liquid-metal jet array Alamos and Sandia National Labora­ Coolant channels Beam and pellet tories, Naval Research Laboratory, Rutherford Laboratory in England, Graphite reflector Osaka University in Japan, Max Planck Institute at Garching in West Germany, and laboratories elsewhere. Rapid growth was sustained throughout the 1970s, driven by OPEC policies, fis­ Laser or-----­ sion's political and technical difficulties, heavy-ion beams and fusion's scientific progress. 19 approaches, it seems plausible that with The hohlraum target designs are a continued national effort, a prototype classified. experimental IeF reactor will be in op­ Inertial-confinement fusion must eration by the turn of the century."l demonstrate that one can achieve a suf­ What are these problems, unknowns, ficiently high target gain with an af­ and promising approaches? fordable driver. Target gain is the ratio As we have indicated above, the key of the energy yield of the fusion reac­ idea of inertial-confinement fusion is to tion to the driver energy applied to the ignite small-scale, contained fusion ex­ target. Assuming driver and implosion plosions by using high-power beams of efficiencies of 10%, the target gain must energy that implode fuel to superhigh be greater than 100. Such a gain is fea­ densities and thermonuclear tempera­ sible with multimegajoule drivers if tures. For both inertial and magnetic three conditions are satisfied: the beam fusion, burn efficiency-the percentage couples efficiently to the target, the fuel of nuclei that fuse-is proportional to is efficiently compressed to 1000 times the product of density and confinement its liquid density, and a small fraction time. In magnetic fusion, the fuel den­ of the fuel is ignited at the center of the sity is limited by material properties, so target pellet. The remaining fuel must one increases the efficiency of the burn also ignite by propagation from the by extending the duration of confine­ central ignited region and burn ment. In inertial fusion, Newton's laws efficien tl y. and thermal velocity limit the confine­ For practical fusion power, we must ment time, so one tries to compress the develop three technologies to harness fuel to higher densities. Densities up to the high-gain targets: an efficient and 1000 g/cm3 are energetically accessible affordable driver that couples efficiently via isentropic, or constant-entropy, to the target, a practical and economi­ compression because the Fermi cally competitive reactor that incorpo­ energy-the minimum energy of com­ rates solutions to the problems of cyclic pressed matter- is much less than the fatigue and stress from pulsed radia­ thermal energy at ignition. tion, and a practical target factory. We There are two basic target designs will address these scientific and techno­ for inertial-confinement fusion reactors: logical issues . • Direct targets absorb the driver By using high-intensity short­ Fig. 2 beam energy in an outer layer or wavelength laser light, researchers have Absorption of laser light by targets in­ "shell," which acts directly to produce improved the absorption of energy by c reases d r amat ically with shorter the implosion.