Smoothing of laser beam intensity fluctuations in low density foam plasmas with the LIL laser

Intensity fluctuations of high-energy laser beams present a danger for efficient compression and ignition of the target in the direct-drive scheme for inertial confinement fusion (ICF). Tem-poral smoothing techniques become operational only after several hundred picoseconds and inhibition of the effect of initial laser intensity perturbations on the ablation pressure remains therefore a crucial problem for target design. This paper presents the first experimental results on the laser imprint reduction in fusion scale plasmas using a low-density foam layer. Experi-ments were conducted with the LIL laser facility using a pulse energy of 12 kJ with millimeter-size plasmas. The configuration used in the experiment reproduces well the initial phase in the direct-drive scheme. The experimental results demonstrate efficient laser energy absorption in the foam, supersonic propagation of the ionization wave, and an efficient smoothing of the laser beam. The development of efficient and robust methods for controlling Rayleigh-Taylor instabili-ties relains an outstanding problem in ICF. The diffusive heat conduction smoothes the laser intensity fluctuations and assures a homogeneous ablation pressure if the distance between the ablation front and the absorption surface is larger than the perturbation wavelength and if th