The capacity to launch a strong shock wave in a compressed target in the presence of large preplasma has been investigated experimentally and numerically in a planar geometry. The experiment was performed on the LULI 2000 laser facility using one laser beam to compress the target and a second to launch the strong shock simulating the intensity spike in the shock ignition scheme. Thanks to a large set of diagnostics, it has been possible to compare accurately experimental results with 2D numerical simulations. A good agreement has been observed even if a more detailed study of the laser-plasma interaction for the spike is necessary in order to confirm that this scheme is a possible alternative for inertial confinement fusion.

Demonstrating ignition and net energy gain in the near future on MJ-class laser facilities will be a major step towards determining the feasibility of Inertial Fusion Energy (IFE), in Europe as in the United States. The current status of the French Laser MégaJoule (LMJ) programme, from the laser facility construction to the indirectly driven central ignition target design, is presented, as well as validating experimental campaigns, conducted, as part of this programme, on various laser facilities. However, the viability of the IFE approach strongly depends on our ability to address the salient questions related to efficiency of the target design and laser driver performances. In the overall framework of the European HiPER project, two alternative schemes both relying on decoupling target compression and fuel heating—fast ignition (FI) and shock ignition (SI)—are currently considered. After a brief presentation of the HiPER project's objectives, FI and SI target designs are discussed. Theoretical analysis and 2D simulations will help to understand the unresolved key issues of the two schemes. Finally, the on-going European experimental effort to demonstrate their viability on currently operated laser facilities is described.

Abstract. We use the LIL (Ligne d'Intégration Laser) facility to study the coalescence of two planar shocks in an indirectly-driven planar sample of polystyrene. This experiment represents the preliminary stage of the future shock-timing campaign for the Laser Megajoule (LMJ). The main objectives are to validate the experimental concept and to test the numerical simulations. We used a gold spherical hohlraum to convert into X-ray the 351 nm wavelength laser pulse and to initiate the two shocks in the sample. To access time resolved shock velocities and temperature, we used two rear-side diagnostics: a VISAR (Velocity Interferometer System for Any Reflection) working at two different wavelengths and a streaked optical selfemission diagnostic. We observed the coalesced shock, in good agreement with the numerical simulations. We also observed a loss of signal during the first nanoseconds probably due to sample heating from the hohlraum X-ray flux. CONTEXTThe LMJ, like the National Ignition Facility (NIF) in the USA, is designed to achieve thermonuclear ignition in an indirect drive scheme, by imploding a spherical capsule filled with a deuterium-tritium (DT) mixture. The best way to efficiently compress the fuel is quasi-isentropic compression. To approximate this compression, a series of four successive centripetal shocks are sent into the sample [1]. The shocks need to be accurately controlled in timing and strength to coalesce at the same time in the gaseous DT close to the inner surface of the cryogenic DT. The first three shocks allow compressing the capsule quasi-isentropically, keeping the cryogenic DT on a low adiabat. The fourth shock drives the final implosion until the ignition. This scheme requires to adjust the laser power time history entering in the gold hohlraum.In this experiment, we seeked to validate the experimental concept in planar geometry and indirect drive for the first two shocks using a truncated LMJ-like pulse shape and to compare the results with numerical simulations. EXPERIMENTAL SETUPFor this experiment we used the LIL laser facility at 3 (351 nm) with an energy of 10 kJ in 12 ns. A spherical gold hohlraum was used to obtain a radiation temperature similar to the first part of the nominal LMJ-drive. Two planar shocks are sent in a planar sample of polystyrene (PS -C 8 H 8 ) fixed on a

Abstract.A Velocity Interferometer for Any Reflector (VISAR) [1,2] and a Streaked Optical Pyrometer (SOP) [3] were implemented on the "Ligne integration Laser" (LIL) facility. Spatial resolution as good as 10 m in the target plane and velocity resolution as good as 0.1 km/s can be achieved. Several campaigns were performed in 2010 involving various experimental setups and physical processes: Boron EOS, Precompress H2 with special setup of diamond anvil cell and Shock coalescence. This feedback will be of a great help for the Laser Mégajoule facility (LMJ) VISAR design.