Effect of a superthermal electron tail on the yield ratio obtained from DT-targets illuminated with a shaped laser pulse

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Abstract
The possibility of achieving super-high compression of a bare drop of thermonuclear fuel using a shaped laser pulse depends sensitively on the electron distribution produced when the laser is absorbed. Particle simulation results indicate that laser energy absorbed near the critical density by parametric decay instabilities produces a long energetic tail of electrons which can be fit by an exponential of the form exp(−l/2 mv2/αkTe) where α is typically about 12 and Te is the temperature of the background electrons. The yield from a given size pellet is sensitive both to the energy in the tail and to the slope of the tail as determined by α. This paper presents calculations of the optimized yield ratio for various α's and for varying fractions, f, in the tail for a target which is a 60 μg sphere of D-T. With an isotropic source of hot electrons produced in the absorption process, for α = 24, the worst case for this target, somewhat better than breakeven is achieved with 100 kilojoules of input energy and 30% of the laser energy in a superthermal tail. With α = 6, about 5 times breakeven is achieved with an input of 150 kJ and all of the laser energy in a superthermal tail. For comparison, when all of the energy is deposited into an inverse bremsstrahlung spectrum, the same target yields 2.2 megajoules with an input of 46 kilojoules.