  Perturbations in the velocity profile of a laser-ablation-driven shock wave
seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been
measured. Direct measurements of these velocity perturbations were recorded
using a two-dimensional high-resolution velocimeter probing plastic material
shocked by a 100-ps picket laser pulse from the OMEGA laser system. The
measured results for experiments with one, two, and five overlapping beams
incident on the target clearly demonstrate a reduction in long-wavelength
($>$25 um) perturbations with an increasing number of overlapping laser beams,
consistent with theoretical expectations. These experimental measurements are
crucial to validate radiation-hydrodynamics simulations of laser imprint for
laser direct drive inertial confinement fusion research since they highlight
the significant (factor of 3) underestimation of the level of seeded
perturbation when the microphysics processes for initial plasma formation, such
as multiphoton ionization are neglected.
