Simulations of laser imprint for Nova experiments and for ignition capsules Page: 5 of 31
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In direct drive inertial confinement fusion, spatial nonuniformities in the laser intensity
cause modulations in ablation pressure, which in turn generate hydrodynamic perturba-
tions in the target being driven. This process is called "imprint". The favored method of
minimizing imprint is use of laser beam smoothing schemes1-3 which introduce spatial and
temporal incoherence. These create a speckle intensity pattern at any instant of time which
is strongly modulated, but the pattern varies in time in a nearly random manner so that
the modulation smooths out.
Thermal transport between where the laser energy deposits and the ablation front gives
additional smoothing. This thermal smoothing has been called the "cloudy day" effect.4
Modulation of ablation pressure is predicted to be reduced from the modulation in laser
SP/P = -1
where P is the ablation pressure. k is the wavenumber of the transverse intensity modulation,
and Ar is the separation between where the laser energy is deposited and the ablation
front. Note that ICF ignition benefits from a short laser wavelength and a low intensity
(< 1014 W/cm2) foot for the temporal profile of the drive. For such conditions, most of the
light is absorbed at densities significantly below the critical density, so Eqn. 1 should be
convolved over the range of deposition locations. This process is somewhat more complex
to calculate accurately because it takes time for beam smoothing schemes to operate, and
the structure both of the plasma corona where the laser deposition and thermal smoothing
occur, and the hydrodynamic response of the target being driven, change in time.
Imprint has been examined in experiments at several laser facilities5-13 and in
simulations." We have modeled Nova experiments to test our understanding of the physics
of imprint and to verify our ability to predict the results. We will show good agreement
of simulations with experiments at Nova measuring imprint upon CH2 foils at intensities of
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Weber, S.V.; Glendinning, S.G.; Kalantar, D.H.; Key, M.H.; Remington, B.A.; Rothenberg, J.E. et al. Simulations of laser imprint for Nova experiments and for ignition capsules, article, November 8, 1996; California. (https://digital.library.unt.edu/ark:/67531/metadc692918/m1/5/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.