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.

The Laser Integration Line (LIL) is part of the Laser Megajoule (LMJ) project. The LMJ installation, which is of the National Ignition Facility (NIF) class, will deliver 1.8MJ at 0.35μm wavelength on target with 60 quadruplets of elementary beams. The energy on target of LIL is 30kJ corresponding to one quadruplet, it is housed in a separate building and has its own experimental setup. Target diagnostics have been progressively installed in the LIL target area. Energy and imaging diagnostics as well as the broadband and high resolution spectrometers have been fabricated in the frame of a unique industrial contract. Optical pointers are used to align the diagnostics to the target. The supervisory system controlling the diagnostics configuration and data acquisition is now able to manage cooperative and Commissariat à l’ Energie Atomique (CEA) shots. The calibration data base is accessible from the processing network and will very soon include the characterization of all the streaked, gated, and charge coupled device cameras. Only the Raman-Brillouin backscatter spectrometer still requires some final work and operating tests. The first diagnostics results have been obtained at the end of 2004. A gated soft x-ray imager, the Diagnostic de Mesure X (DMX) broadband spectrometer and a pinhole static imager were used in demonstration experiments. The purpose of the first experiment was to observe the closing of a slit etched in a tantalum oxide foam when irradiated by x-rays. The next diagnostics to be activated were the high resolution spectrometers, the gated x-ray imager, the two mirror x-ray imagers, and the scattered energy diagnostic. This was done during the preparation phase of a campaign realized in collaboration with the French Lasers and Plasmas Institute at the end of 2005 in order to study heat conduction in inertial confinement fusion plasmas. The next diagnostic which is now in preparation is a velocity interferometer system for any reflector. This diagnostic is in its fabrication phase and will be operational for diamond equation of state experiments.

Implosion experiments of an inertial confinement fusion (ICF) target on the laser megajoule (LMJ) and the National Ignition Facility require, for certain designs, a precise timing coalescence of four shocks at a specific point of the capsule, which strongly depends on the ablator equation of state. In experiments at the Ligne d'Intégration laser facility, a prototype for the LMJ, coalescence of two shocks was studied in a planar polystyrene (CH) sample in an indirect drive configuration. Shocks were driven by x-ray emission generated in a spherical hohlraum radiatively heated using a 12 ns duration laser pulse temporally shaped to produce two steps in the radiation temperature history that launches these two successive ablation-shock waves. Shock velocity was inferred from a Velocity Interferometer System for Any Reflector (VISAR). Shot performed with 10 ns long truncated laser pulses reduces VISAR blanking, which allows us, for the first time to our knowledge, to observe a photoabsorption-edge induced shock, edge-shock for short, which is a third shock out of the two-step radiation temperature history, coalescing with the second ablation shock. The accurate measurement of this shock with well-controlled x-ray drive should potentially help to constrain the equation of state and opacity of carbon in coronal plasma conditions since the behavior of this shock is very sensitive to both. Moreover, since they can drastically alter the speed of coalesced shocks (in keyhole experiments or ignition designs for ICF), measurements of these edge-shocks may also contribute to improving our ICF design capabilities.

Experimental investigation of stimulated Raman (SRS) and Brillouin (SBS) scattering have been obtained at the Ligne-d'Intégration-Laser facility (LIL, CEA-Cesta, France). The parametric instabilities (LPI) are driven by firing four laser beamlets (one quad) into millimeter size, gas-filled hohlraum targets. A quad delivers energy on target of 15 kJ at 3ω in a 6-ns shaped laser pulse. The quad is focused by means of 3ω gratings and is optically smoothed with a kinoform phase plate and with smoothing by spectral dispersion-like 2 GHz and/or 14 GHz laser bandwidth. Open- and closed-geometry hohlraums have been used, all being filled with 1-atm, neo-pentane (C5H12) gas. For SRS and SBS studies, the light backscattered into the focusing optics is analyzed with spectral and time resolutions. Near-backscattered light at 3ω and transmitted light at 3ω are also monitored in the open geometry case. Depending on the target geometry (plasma length and hydrodynamic evolution of the plasma), it is shown that, at maximum laser intensity about 9 × 1014 W/cm2, Raman reflectivity noticeably increases up to 30% in 4-mm long plasmas while SBS stays below 10%. Consequently, laser transmission through long plasmas drops to about 10% of incident energy. Adding 14 GHz bandwidth to the laser always reduces LPI reflectivities, although this reduction is not dramatic.