Direct-Drive Measurements of Laser-Imprint--Induced Shock-Velocity Nonuniformities and Laser Imprint Mitigation*

In laser-direct-drive inertial confinement fusion, nonuniformities in the laser drive caused by laser speckle and beam-to-beam intensity variations, as well as mass modulations in the target, can seed the Richtmyer--Meshkov and Rayleigh--Taylor hydrodynamic instabilities and adversely affect the compression of the imploding shell. The physical energy transfer of the laser-intensity modulations to the shock front, called laser imprint, depends strongly on the initial plasma formation. Perturbations in the velocity profile of a laser-ablation driven shock wave seeded by laser imprint were recorded using a 2-D high-resolution velocimeter. The measured results for experiments with one, two, and five overlapping beams incident on target demonstrate a reduction in long-wavelength (\textgreater 25-?m) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint, since they highlight a threefold underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation such as multiphoton ionization are neglected.