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PROSPECTS FOR INERTIAL CONFINEMENT SCIENTIFIC FEASIBILITY Research on inertial confinement started some dozen years after research began on magnetic confinement. Consequently, the state of knowledge in inertial confinement is far less advanced than in magnetic confinement. Offsetting this relative disadvantage is the rate of progress achieved in laser technology. In addition, much of the theory associated with inertial confinement relies on well-tested concepts in hydrodynamics, insofar as the concept is closely related to the behavior of dense matter being subjected to intense pressure pulses. Progress in under- standing inertial confinement is being made rapidly now that suitably high-powered laser sources are becoming available and sophisticated diag- nostics are being developed to obtain experimental data on the extreme short time scales (picoseconds to nanoseconds) characteristic of this approach. Although there is still a discrepancy in the degree of understanding extant between the two approaches, magnetic and inertial, it is conceivable that some measure of scientific feasibility will be demonstrated by inertial confinement in the same time frame as may be expected for magnetic confinement. The inertial confinement program differs from the magnetic one in yet another respect, since it serves the dual goals of military and civilian applications. For this reason, portions of the program are classified. We shall have occasion to comment on this matter subsequently. The driver source likely to be available first to workers in the inertial confinement program, with power capabilities approaching what would be needed to initiate an appreciable amount of thermonuclear burn in a target, is the neodymium (Nd) doped glass laser operating at a wave- length of l.06 microns. These have now reached the terawatt (TW) level, and within the next few years facilities at Livermore and the University of Rochester should be able to reach the tens-of-terawatts range. Evi- dence of compression accompanying laser irradiation has already been obtained, but in targets where shock waves probably played the dominant role. These so-called exploding pusher pellets should be relatively free from instabilities, but limited in the density attainable, as confirmed by experimental observations. It will be necessary to show adiabatic compression at the higher power levels of the next generation of lasers in order to ensure that the inertial confinement approach can become feasible. l4
l5 ENGINEERING FEASIBILITY The very low efficiency of glass lasers tend to rule them out as candi- dates for commercial applications. At longer wavelengths, l0.6 microns, gaseous COz lasers are available and their development has shown good promise. Their efficiencies may be adequate for commercial applications; however, it is not at all clear that a long wavelength laser source can serve as an adequate driver since the light is absorbed or reflected in the low-density wing of the plasma blown off from the target. Other laser systems are under development and, as mentioned earlier, electron beams are being explored now; ion beams may be explored in the near future. Whatever the ultimate driver source may be, it will require high conversion efficiencies and pulse repetition rates of l or more per second in order to be of commercial interest. Ultimately, the attractiveness of fusion reactors based on the iner- tial confinement approach may hinge on the ease and cost of pellet fabrication. Assuming a hypothetical example in which the requisite driver energy for ignition is l megajoule, the energy multiplication from thermonuclear burn is l00 and the reactor operates on a cycle of l shot per second, the reactor will produce l00 MW (thermal) and every l0 shots will correspond to an energy yield of l million Btu. If, for the sake of argument, fuel costs are to be kept below $l per million Btu, this implies each pellet must cost less than IOC. Based on information available from unclassified sources, it is some- what doubtful that currently conceived pellet designs and driver sources will lead to commercially attractive pure-fusion power plants. On the other hand, it does appear that a fusion reactor based on the inertial confinement approach may have certain simplifying features, relative to one based on magnetic confinement approaches, which might enhance their attractiveness. It has been speculated, further, that the inertial confinement approach might lead to reactor designs that can be economical at a smaller power rating (~ l00 MWe) than some early design concepts based on Tokamaks (~ l,000 - 2,000 MWe). At the present level of knowl- edge one can attach little confidence to such speculations, nor is there reason to rule out that magnetic confinement schemes might eventually prove attractive in smaller unit sizes. Classified target information does tend to modify conclusions one may draw from information solely in the open literature. At the time of our assessment, another group, under the auspices of the Electric Power Research Institute (EPRI), was also engaged in reviewing the classified domain. Since their observations essentially reflect ours, and since the language of their remarks have met with classification approval, we reproduce them here. "The unpublished laser-fusion target designs suggested by recent LLL and LASL studies offer a very interesting and important possibility of pellet gain markedly exceeding that achievable with the presently published designs. The new design concepts, however, still require a very extensive experimental program. They depend on several aspects of the laser plasma interaction and pellet hydrodynamics which will be studied in
l6 the planned ERDA programs within this decade, with some im- portant results probably achievable by the end of FY 77. "To explore these important possibilities on an optimum schedule appears to require some restructuring of the ERDA program and in particular much increased emphasis on target fabrication. "The proposed targets are more complex and difficult to fabricate, but in compensation they offer an important trade- off in laser characteristics. The economic and technological optimization of a reactor may be altered in a fundamental way by this flexibility in design. "Some possibly important results of these developments are not available to the open engineering and scientific communities because of the classification placed on the work. In any case, characteristics influencing reactor design, such as pellet yields, should be made available as soon as possible for use in unclassified reactor studies. "No unusual problems in a fusion reactor appear to arise from the new target designs, aside from possible difficulties with pellet fabrication and cost. Several of the problems may in fact be alleviated by the expected changes in pellet output. We note however that very high pellet yields, requir- ing large containment vessels and possibly leading to marked variations in thermal output, may lead to difficulties in economics and in compatibility with power grid requirements." Further elaboratorion of technological issues related to inertial con- finement may be found in the next section.
