Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments |
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Authors: | Froula D H Igumenshchev I V Michel D T Edgell D H Follett R Glebov V Yu Goncharov V N Kwiatkowski J Marshall F J Radha P B Seka W Sorce C Stagnitto S Stoeckl C Sangster T C |
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Institution: | Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14636, USA. dfroula@lle.rochester.edu |
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Abstract: | A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ~35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8 μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models. |
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