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The theory of target compression by longwave laser emission
324
.foumal of Nuclear Materials
165 (1989) 324-325 North-Holland, Amsterdam
BOOK REVIEW The Theory of Target Compression by Longwuve Laser Emission, Ed. G.V. Slclizlcov (Nova Publishers, Commack, New York); Price: $73.00 (US and Canada), $87.00 (elsewhere)
As progress continues both theoretically and experimentally in inertial fusion and the inertial fusion program moves ever closer to the reality of producing energy, the proper selection of a fusion driver becomes critical. Although most of the effort around the world currently focuses on short wavelength lasers, the argument in favor of the use of long wavelength CO, and HF lasers for driving inertial fusion targets has continued for almost two decades now. The desirable features of fast repetition rate (potenti~ly), cheap optics, and high efficiency have provided the impetus to pursue them as fusion drivers in spite of some difficulties in target performance. Although the long wavelengths ( > 1 micron) do provide better smoothing, illumination symmetry, and potentially better hydrodynamic efficiency, the difficulties of high energy electron generation by collective processes such as resonant absorption and Raman scattering and the decoupling problem of associated fast ion energy loss have resulted in these lasers being regarded as unlikely candidates for inertial fusion. This result has occurred despite (or perhaps because of) the enormous effort which has gone into finding a way to make these lasers have more favorable target coupling. Although one cannot conclude that CO, lasers are impossible for inertial fusion, the enormous difficulty in establishing their viability has resulted in the eli~na~on of the CO, program in the United States. This conclusion was perhaps driven even more by the much more favorable target physics observed at sub-micron wavelengths. Apparently, the same conclusion has not yet been reached in the Soviet Union. The publishing of a monograph on the important physics of the interaction of long wavelength laser light with targets is an important and welcome addition to the field of inertial fusion. This is particularly true because of the lack of publication of much of the data and analysis because of classification issues in the United States program. This monograph is a reprint of a book first published in the Soviet Union in 1986 as a collection of
Science
papers from the Lebedev Physics Institute. Written from a totally theoretical perspective, the book has seven chapters: one introductory chapter, four chapters covering hot electron generation and transport, and two chapters on analytic and numerical hydrodynamic calculations of target performance. It serves an important role for the inertial fusion theorist in the west by giving a fairly comprehensive reference list to the body of Soviet literature on inertial fusion. It is less useful as a reference to western literature, having only a very spotty reference to the existing literature in western journals. As is typical of much of the Soviet plasma physics literature, it relies on a large number of analytic approximations whose validity is difficult to verify or to connect to equivalent work in the western literature. The authors are cautiously optimistic about the future of CO, lasers as drivers and conclude that yields of 100 with 5-10 megajoules input are possible. However, there is no information in this short book which would allow one to draw this conclusion. Basically they argue that the problem of preheat goes away as one scales the laser energy up to the megajoule range. This may, in fact, be the case but how the electron energy spectrum scales with laser is subject to so much uncertainty that this conclusion is, at best, highly speculative. For examples, the simple model of fast electron generation assumes that electron heating can be calculated by a simpb test particle analysis through localized plasma wave fields. This ignores the important feedback between the high energy electrons and the plasma wave field which has been shown to be an essential feature in the hot electron generation process. In addition, the comparison of hydrodynamic calculations with early Helios experiments at Los Alamos do not take into account the measurement of much higher energy electrons present in those experiments which were reported later by Los Alamos and other laboratories. Nevertheless, the various analytic models presented and used in the hydrodynamic calculations should provide a useful baseline for evalua-
Book review
tion of the phenomena which occur in the interaction of long wavelength laser light with plasmas. If long wavelength lasers are ever to be used for inertial fusion in the next century, a long, sustained effort in theory and experiment will be required. This probably will become possible only after inertial fusion has established itself as a viable energy source with continuing need for further optimization or there is a
325
major effort to encourage international cooperation. Monographs like this one could help play a role in encouraging the necessary international cooperation.
David W. Forslund
Los Alamos National Luborato y
.foumal of Nuclear Materials
165 (1989) 324-325 North-Holland, Amsterdam
BOOK REVIEW The Theory of Target Compression by Longwuve Laser Emission, Ed. G.V. Slclizlcov (Nova Publishers, Commack, New York); Price: $73.00 (US and Canada), $87.00 (elsewhere)
As progress continues both theoretically and experimentally in inertial fusion and the inertial fusion program moves ever closer to the reality of producing energy, the proper selection of a fusion driver becomes critical. Although most of the effort around the world currently focuses on short wavelength lasers, the argument in favor of the use of long wavelength CO, and HF lasers for driving inertial fusion targets has continued for almost two decades now. The desirable features of fast repetition rate (potenti~ly), cheap optics, and high efficiency have provided the impetus to pursue them as fusion drivers in spite of some difficulties in target performance. Although the long wavelengths ( > 1 micron) do provide better smoothing, illumination symmetry, and potentially better hydrodynamic efficiency, the difficulties of high energy electron generation by collective processes such as resonant absorption and Raman scattering and the decoupling problem of associated fast ion energy loss have resulted in these lasers being regarded as unlikely candidates for inertial fusion. This result has occurred despite (or perhaps because of) the enormous effort which has gone into finding a way to make these lasers have more favorable target coupling. Although one cannot conclude that CO, lasers are impossible for inertial fusion, the enormous difficulty in establishing their viability has resulted in the eli~na~on of the CO, program in the United States. This conclusion was perhaps driven even more by the much more favorable target physics observed at sub-micron wavelengths. Apparently, the same conclusion has not yet been reached in the Soviet Union. The publishing of a monograph on the important physics of the interaction of long wavelength laser light with targets is an important and welcome addition to the field of inertial fusion. This is particularly true because of the lack of publication of much of the data and analysis because of classification issues in the United States program. This monograph is a reprint of a book first published in the Soviet Union in 1986 as a collection of
Science
papers from the Lebedev Physics Institute. Written from a totally theoretical perspective, the book has seven chapters: one introductory chapter, four chapters covering hot electron generation and transport, and two chapters on analytic and numerical hydrodynamic calculations of target performance. It serves an important role for the inertial fusion theorist in the west by giving a fairly comprehensive reference list to the body of Soviet literature on inertial fusion. It is less useful as a reference to western literature, having only a very spotty reference to the existing literature in western journals. As is typical of much of the Soviet plasma physics literature, it relies on a large number of analytic approximations whose validity is difficult to verify or to connect to equivalent work in the western literature. The authors are cautiously optimistic about the future of CO, lasers as drivers and conclude that yields of 100 with 5-10 megajoules input are possible. However, there is no information in this short book which would allow one to draw this conclusion. Basically they argue that the problem of preheat goes away as one scales the laser energy up to the megajoule range. This may, in fact, be the case but how the electron energy spectrum scales with laser is subject to so much uncertainty that this conclusion is, at best, highly speculative. For examples, the simple model of fast electron generation assumes that electron heating can be calculated by a simpb test particle analysis through localized plasma wave fields. This ignores the important feedback between the high energy electrons and the plasma wave field which has been shown to be an essential feature in the hot electron generation process. In addition, the comparison of hydrodynamic calculations with early Helios experiments at Los Alamos do not take into account the measurement of much higher energy electrons present in those experiments which were reported later by Los Alamos and other laboratories. Nevertheless, the various analytic models presented and used in the hydrodynamic calculations should provide a useful baseline for evalua-
Book review
tion of the phenomena which occur in the interaction of long wavelength laser light with plasmas. If long wavelength lasers are ever to be used for inertial fusion in the next century, a long, sustained effort in theory and experiment will be required. This probably will become possible only after inertial fusion has established itself as a viable energy source with continuing need for further optimization or there is a
325
major effort to encourage international cooperation. Monographs like this one could help play a role in encouraging the necessary international cooperation.
David W. Forslund
Los Alamos National Luborato y