title: Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments.
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.
authors.affiliation: Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14636, USA. dfroula@lle.rochester.edu
authors.forename: D H
authors.initials: DH
authors.lastname: Froula
authors.forename: I V
authors.initials: IV
authors.lastname: Igumenshchev
authors.forename: D T
authors.initials: DT
authors.lastname: Michel
authors.forename: D H
authors.initials: DH
authors.lastname: Edgell
authors.forename: R
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authors.lastname: Follett
authors.forename: V Yu
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authors.lastname: Glebov
authors.forename: V N
authors.initials: VN
authors.lastname: Goncharov
authors.forename: J
authors.initials: J
authors.lastname: Kwiatkowski
authors.forename: F J
authors.initials: FJ
authors.lastname: Marshall
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authors.initials: PB
authors.lastname: Radha
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authors.initials: W
authors.lastname: Seka
authors.forename: C
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authors.lastname: Sorce
authors.forename: S
authors.initials: S
authors.lastname: Stagnitto
authors.forename: C
authors.initials: C
authors.lastname: Stoeckl
authors.forename: T C
authors.initials: TC
authors.lastname: Sangster