Clusters were produced as a result of argon gas cooling during expansion through a supersonic nozzle. A two-dimensional model was set up in order to calculate gas expansion and partial condensation into clusters. Calculations were validated by experimental measurements using Mach-Zehnder interferometry and Rayleigh scattering, and performed with two types of nozzles ͑Laval and conical nozzles͒. These optical diagnostics together with numerical simulations led to the cluster size and density determination with spatial resolution through the gas and cluster jet. Cluster production was observed to be very sensitive to the nozzle geometry. Homogeneous gas and cluster jets were produced and characterized using conical nozzle geometry, with cluster density about 10 12 per cm 3 . Due to the fast valve-nozzle connecting geometry, shock waves have been observed at the Laval nozzle throat that strongly affected cluster production on the jet axis. Averaged cluster radius was observed to be easily tunable from 180 to 350 Å by varying the upstream gas pressure P 0 from 20 to 60 bars. A different scaling law, versus P 0 , has been observed for this regime of large cluster, compared to Hagena's predictions for the small cluster regime.

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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.

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International audienceExperimental realization of an electron density channel created by a low intensity laser in a helium gas jet is presented. The long (2.5 mm) plasma channel is fully ionized and thus prevents undesirable refraction effects for propagation and guiding of a subsequent high intensity laser pulse. The channel parameters are easily controlled and well suited for laser guiding. The radial plasma expansion and the temperature evolution have been measured and compared to hydrodynamic simulations which show that the plasma expansion is governed by a thermal wave during the laser pulse

This paper presents an analysis of laser-plasma interaction risks of the shock ignition (SI) scheme and experimental results under conditions relevant to the corona of a compressed target. Experiments are performed on the LIL facility at the 10 kJ level, on the LULI 2000 facility with two beams at the kJ level and on the LULI 6-beam facility with 100 J in each beam. Different aspects of the interaction of the SI pulse are studied exploiting either the flexibility of the LULI 6-beam facility to produce a very high intensity pulse or the high energy of the LIL to produce long and hot plasmas. A continuity is found allowing us to draw some conclusions regarding the coupling quality and efficiency of the SI spike pulse. It is shown that the propagation of the SI beams in the underdense plasma present in the corona of inertial confinement fusion targets could strongly modify the initial spot size of the beam through filamentation. Detailed experimental studies of the growth and saturation of backscattering instabilities in these plasmas indicate that significant levels of stimulated scattering reflectivities (larger than 40%) may be reached at least for some time during the SI pulse.

Strong L-shell x-ray emission has been obtained from Kr clusters formed in gas jets and irradiated by 60-500-fs laser pulses. Spectral lines from the F-, Ne- Na-, and Mg-like charge states of Kr have been identified from highly resolved x-ray spectra. Spectral line intensities are used in conjunction with a detailed time-dependent collisional-radiative model to diagnose the electron distribution functions of plasmas formed in various gas jet nozzles with various laser pulse durations. It is shown that L-shell spectra formed by relatively long nanosecond-laser pulses can be well described by a steady-state model without hot electrons when opacity effects are included. In contrast, adequate modeling of L-shell spectra from highly transient and inhomogeneous femtosecond-laser plasmas requires including the influence of hot electrons. It is shown that femtosecond-laser interaction with gas jets from conical nozzles produces plasmas with higher ionization balances than plasmas formed by gas jets from Laval nozzles, in agreement with previous work for femtosecond laser interaction with Ar clusters.