Riemann's Contribution to Flight and Laser Fusion

The Principles of Laser Fusion Target DesignThe laser fusion targets used todayare built so that the fuel doesnot actually absorb the laser light.Instead, there is a two-stage processof radiation absorption that transfersthe laser energy to the pellet.Initially, the laser light illuminatesthe outside of the pellet; the energymust be absorbed as efficiently aspossible at this point. However, thisenergy is relatively low grade (highentropy) and not suited forcompression of the fuel itself.Therefore, a second layer of thetarget is used to convert this laserenergy into soft X-rays that thenactually implode the fuel.This arrangement solves severalproblems inherent in the laser fusionprocess: It provides a highlyisotropic and uniform deposition ofenergy on the fuel, and it is a highlyefficient converter of electromagneticenergy to the hydrodynamicenergy of compression.The target designs using this intermediatestage of an equilibriumspectrum of X-rays (blackbody radiationtargets, also called holsraumtargets) are classified, but it is publiclyknown that the generation ofan intermediate X-ray spectrum isused in advanced target design. Thisidea was originated outside the nationallaboratories by Dr. FriedwardtWinterberg, and is describedin Fusion, Jan. 1981.This fact solves the mystery of theconstruction of the Shiva targetchamber illumination (see page 25)in which, in spite of the laser fusioncommunity's prolix comments onthe necessity of spherical depositionof energy on the target, uses ahighly anisotropic geometry forlaser illumination of the target. Itseems likely that a cylindrical targetis used to maximize the conversionof the laser light into X-radiationand that then this X-radiation iscontained inside the target shells,heating and compressing the fuelat the center of the pellet.Note that this principle is almostidentical to that used in the constructionof the hydrogen bomb,where the laser driver is replacedby a fission explosion whose radiativeenergy is converted to X-radiationusing a metallically dopedfoam or multifoil configuration.A small part of this informationwas declassified in fall 1980 whenthe Department of Energy publiclyacknowledged the role of soft X-rays and radiation in conversion inthe design of advanced targets forinertial confinement fusion. (This isreported in the April 1981 Fusion.)fuse), a simple calculation shows that for ignition to occuron time scales less than the inertial disassembly time (thetime it takes the pellet of fuel to explode), the density ofthe fuel must be greater than 10 26 particles per cubiccentimeter. That is, at the point that it ignites, the fuelmust be at a density approximately 1,000 times liquiddensity.This means that the driver energy must simultaneouslyprovide the ignition energy for the fusion reaction andcompress the fuel to the high density. The overall efficiencyof the ignition process, then, is the product ofthree efficiencies, the efficiency of the conversion ofelectrical energy into laser light, E,, the efficiency ofconversion of laser energy into pellet energy (basically, afactor of the absorption of light in the pellet), E r , and theefficiency with which the absorbed energy is used forcreation of the conditions for ignition of the fuel, E h . (Seebox on laser fusion targets.)Of most immediate concern is this last efficiency, E h , orthe efficiency of the hydrodynamic processes used toconvert the absorbed laser energy into compression ofthe pellet. The physics of this energy transfer revolvesaround the physics of the shock wave created by theabsorbed radiation. This is the shock wave that performsthe actual compression. To determine the properties ofthis shock compression, Nuckolls and Wood used thefollowing argument:(1) To be ignited, the fuel must be at a density of 600grams per cubic centimeter throughout the entire mass offuel.(2) The most efficient compression of this fuel will occurif the fuel can be maintained in its Fermi degenerate state.24 FUSION October-November 1981