The United States is presently deriving most of its energy from petroleum and natural gas. These are rapidly disappearing resources and will have to be replaced in the next twenty-five years by new sources such as fission or fusion nuclear reactors, solar developments and a much expanded use of coal.
Laser developments within the past five years have opened opportunities for photo induced isotope separation that have fostered a field which is characterized by intense, competitive activity and f4ous growth. The tremendous, potential economic impact of the processes that may be developed for U"3 enrichment, estimated to be as high as $80 billion over the next three decades, coupled with the need for expansion of our nuclear power generating capability,have fueled the fires ignited by laser developments. Possible heavy water isolations by an economical laser process could lead to huge savings for the Canadian heavy water reactor plants. Implications of U235 separation in the proliferation of nuclear weapons numbs our hopes for success in this area and stimulates our efforts at the same time. While the overwhelming stimulus fostered by the potential applications of work in this field makes laser isotope separation a concern of every scientist, the conflicting interests involved in these developments have concealed much of the activity as government or corporation secrets. This aspect plus the rapid progress in this field make an up-to-date review difficult at best.
In this paper, we indicate some simple energy balance and economic considerations which bear on the feasibility of laser produced fusion. We present reasons that presently published schemes requiring high compression of homogeneous spheres or shells would be difficult to employ for pure power generation, even given favorable answers to many presently unanswered scientific questions. On the otherhand, extensions of our analysis show that breeding and energy multiplication in a fissile blanket greatly improve the commerical prospects for power generation by laser induced fusion.
Target irradiation experiments with multibeam high power lasers require careful consideration of the system optics. This paper describes some of the current methods of focusing high power laser beams, and measuring the optical quality of the laser.
Isotope separation has been achieved by tunable lasers involving both electronic transitions in atoms and vibrational transitions in molecules. A variety of lasers have been used for spectroscopic studies to identify the isotopic shifts and others for either selective excitation of transitions or for selective photodissociation of molecules. The classes of tunable lasers that are reviewed include dye lasers, semiconductor lasers, tunable gas lasers, spin-flip Raman lasers, parametric oscillators and frequency mixing in nonlinear crystals.
New measurement techniques for the study of combustion processes are currently receiving widespread attention because of their potential utility for combustion modeling. Such modeling promises to offer many benefits for the design of advanced power sources with high efficiency and low pollutant emissions. Here, we very briefly discuss several classes of optical methods for the measurement of combustion system properties. We then describe in more detail the measurement of temperature, density, and composition by Raman scattering, and velocity by laser Doppler velocimetry, as examples of non-perturbing optical diagnostic probes currently under development for combustion measurements purposes.
Rapid switching and interruption of x-ray diffraction from crystals provide interesting possibilities as fast x-ray measurement techniques. This article describes measurements that have been made on an acousto-optic x-ray shuttering device. The shuttering mechanism is provided by acoustic interruption of x-ray transmission through a low dislocation density silicon single crystal oriented for Laue geometry diffraction. A discussion is given of possible methods for the development of subnanosecond x-ray acousto-optic devices.
Large scale programs are underway at major laboratories to study the feasibility of laser-induced fusion. The laser requirements for this investigation are formidable and it is estimated that powers in the range of 100 terrawatts with total energies of 105 to 106 joules may be needed. A major fraction of the effort has been directed toward the development of high-energy short-pulse lasers which can meet these requirements. The parameters of the CO2 laser system have been extensively studied and it appears that the efficiency, energy density, bandwidth, and optical damage limits are compatible with the requirements. The scaling laws are also now well understood. Based upon these results, several large carbon dioxide systems have been developed by the Los Alamos Scientific Laboratory. The first system has been in operation for several years at a power of 0.2 terrawatt with a focused intensity of 5 x 1014 W/cm2. A second system with power and energy outputs increased by an order of magnitude will be operating presently. Succeed-ing systems are being planned and developed to extend this performance to the range of interest for laser fusion energy production.
Progress on high peak power short pulse glass lasers has been rapid. Prospects for reaching the 1013 - 1014 watt level required for laser fusion appear reasonable. Progress in the control of the onset self-focusing effects offers the hope of the realization in the near future of laser systems which operate at output intensities of P.-1 30 GW/cm4 at overall efficiencies approaching 0.1%. Very high peak power lasers are needed forlaser fusion compared to what exists at present. Present solid state and gas laser systems operate with well controlled beams at power levels of 1011 - 1012 watts per beam. Laser fusion has been estimated to require the symmetric irradiation of targets by pulses of 1013 - 1014 watts for a breakeven experiment(l). Even higher laser powers may be required for a fusion reactor, but here the real problem will be that high high efficiency is also necessary. A first reaction to the gap between present technology and the peak power requirement might be to dismiss the idea out of hand, as simply too difficult to achieve especially when other projected require-ment on a breakeven experiment such as a precise pulse shape and very uniform illumination are considered. This is too simplistic an attitude. The benefits of fusion power if it can be harnessed are so enormous that the question of whether such lasers can be built deserves the closest and most intensive study. This is likely to be money well spent as high peak power lasers will have many other uses even if fusion proves impractical by this route. Examples of this exist now; experiments at NRL on laser and electron beam targets have provided a wealth of atomic physics data on the spectra of highly ionized atoms. These "laboratory astrophysics" experiments are being used to analyze the solar flare data obtained from the Skylab space station.
The rapid development of fields such as laser fusion has necessitated the construction of large, short pulse laser systems. In this paper some of the major design problems of high power glass lasers will be discussed and some possible solution presented.
This paper describes an experimental and theoretical investigation of the efficacy of an axial inhomogeneous magnetic field in increasing the ratio of stored 2nergy to peak small signal amplification in a laser system based on the Pp,- atomic Iodine transition. The experiments were conducted with a photoLaissocia-3/2 tive C3F71-Iodine laser oscillator placed in the fringing field of a solenoid. By studying the variation with magnetic field of the time-integrated dissociative flux at the onset of lasing, energy storage enhancements up to 15 (at a peak field of 20 Kilogauss) were inferred. The results are compared with theoretical calculations of the phenomena which include the hyperfine structure of the transition. The dynamic behavior and extraction efficiency of a master oscillator/power amplifier configuration utilizing this technique for increasing the maximum obtainable output energy per pulse are also discussed.
Pulsed HF chemical laser oscillator energies were scaled from millijoules to several kilojoules over the period 1970 - 1974, reaching q, 10 J with SFE, and transverse discharges, and using electron-beam initiation and elemental F2 above 1000J. This demonstrated scalability to large energy with acceptable electrical efficiency is only one prerequisite for application of this gas laser in fusion; equally important matters are achievement of focusable, % 1 ns pulses, couplable to light-element targets, all from an affordable system. Exploratory MOPA experiments are reported which address control of HF laser beam focusability and pulse duration, using SF6-based experimental oscillator - amplifier sequences and Pockels' cell switching. Simultaneous multiline lasing with 2.6 < X < 3.1 μm and high specific gain and energy density are particularly important factors encountered with HF, where amplifier pumping and lasing occur in a substantially cw temporal relationship, even in < 100 ns bursts. Time-resolved SF6 - HI oscillator spectra contain 27 simultaneous lines from six vibrational bands. An apertured, SF6 -hydrocarbon pin-discharge oscillator generates % 10 mJ of TEMoo radiation, which is amplified to % 1 J in ti 150 ns by a TEA amplifier and propagated tens of meters. A three-stage system coupling these elements through a % 1 ns electrooptic gate to a > 10 J, e-beam energized amplifier is under development.
A variety of refractive and reflective optical focusing configurations for application to laser fusion targeting experiments are discussed with particular attention given to system related constraints. Consideration is given to energy damage thresholds, refractive material availability, space envelope, fabrication requirements, and focused spot uniformity.
Laser fusion experiments involve the focusing of high power laser beams onto fuel pellets. The geometrical intensity is of interest in the cases where the laser is focused to the center of the pellet. Analytic expressions and ray trace methods for evaluating the geometrical intensity are presented.
This paper reviews optical fabrication and measurement techniques for the manufacture of high-energy laser optics. The discussion of fabrication techniques includes continuous polishing of flat surfaces, controlled grinding, and fabrication procedures developed for low absorption metal mirrors and window materials. Characterization techniques of absorption calorimetry, fringes of equal chromatic order, and total integrated scatter are described along with corresponding measurement data.