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This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 mum of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

Ablation parameters such as velocity, mass, momentum, pressure, and hydrodynamic efficiency have been investigated with plane targets irradiated in the range 3×1011-1015 W cm−2 with 1 nsec pulses and laser wavelengths of 1.06 μm and 0.35 μm. We show that ablation velocity, ablated mass, and momentum are in good agreement with ablation scaling laws deduced from analytical models taking into account inverse bremsstrahlung absorption below the critical density. Nevertheless, processes such as lateral conduction, hot spot, and preheat effects make inaccurate the comparison between ablation pressures, mass ablation rates, or hydrodynamic efficiencies measured for different laser wavelengths. Laser illumination nonuniformities are transmitted to the target in terms of pressure variations. The harmful consequence of a reduced lateral energy flow in 0.35 μm experiments can eclipse the increasing of ablation pressure and hydrodynamic efficiency.

A transmission grating streaked spectrograph (SPARTUVIX) has been developed at the Centre d’Etudes de Limeil-Valenton for measuring the temporal evolution of soft-x-ray spectra in laser-produced plasmas. In this article the ideas behind the instrument are described as well as a description of the actual working diagnostic and its relevant components. This diagnostic has been used to look at the time behavior of x-ray laser emission, and a number of very interesting results have been obtained. A representative sample of these results will also be shown.

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