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We present an overview of the interpretation of laser plasma interaction (LPI) experiments carried out on the LIL facility. These multikilojoule experiments have been done using underdense foam targets at 0.351 μm laser light leading to high temperature and large plasmas. We discuss the interpretation using our different numerical tools: hydrodynamics simulations carried out with the code FCI2 to characterize the plasma, linear gain estimates with the postprocessor Piranah and paraxial simulations with the code HERA.

We have designed experiments to study the effect of the laser wavelength (0.527 versus 0.351 μm) on the coupling efficiency in plasma conditions relevant to compression and shock ignition (SI) schemes in different intensity regimes. A difficult issue was to produce interaction conditions that are equivalent for the two wavelengths. This was obtained by using plasma preformed from a solid target with a plasma-preforming beam at the same wavelength as the interaction beam. This produced an almost exponential density profile from vacuum to the critical density of the interaction beam in which all interaction mechanisms are taken into account. The growth and saturation of stimulated Brillouin scattering (SBS) have been measured at the two wavelengths, in backward as well as in near-backward directions. We have found that the SBS intensity threshold is ∼1.5 times higher at 3ω than at 2ω in agreement with the Iλ dependence of the SBS gain. The SBS behaviour is very well reproduced by the linear calculations of the postprocessor PIRANAH, giving us confidence that we have a good control of the relevance of the experimental conditions for the study of the laser wavelength effect on laser-plasma coupling. When SBS reaches the saturation regime, same levels of reflectivity are measured at 2 and 3ω. Numerical simulations were performed with the paraxial code HERA to study the contribution of the fluid mechanisms in the saturation of SBS, showing that pump depletion and interplay with filamentation are likely to be the most important processes in SBS saturation for these conditions. This scenario also applies to the SBS of shock ignition high-intensity beams. [ABSTRACT FROM AUTHOR]

Laser-produced multimaterial jets have been investigated at the Prague Asterix Laser System laser [K. Jungwirth et al., Phys. Plasmas 8, 2495 (2001)]. The method of jet production is based on the laser-plasma ablation process and proved to be easy to set up and robust. The possibility of multimaterial laboratory jet production is demonstrated and complex hydrodynamic flows in the jet body are obtained. Two complementary diagnostics in the optical ray and x-ray ranges provide detailed information about jet characteristics. The latter are in agreement with estimates and two-dimensional radiation hydrodynamic simulation results. The experiment provides a proof of principle that a velocity field could be produced and controlled in the jet body. It opens a possibility of astrophysical jet structure modeling in laboratory. [ABSTRACT FROM AUTHOR]

The interaction of laser driven jets with gas puffs at various pressures is investigated experimentally and is analyzed by means of numerical tools. In the experiment, a combination of two complementary diagnostics allowed to characterize the main structures in the interaction zone. By changing the gas composition and its density, the plasma cooling time can be controlled and one can pass from a quasiadiabatic outflow to a strongly radiation cooling jet. This tuning yields hydrodynamic structures very similar to those seen in astrophysical objects; the bow shock propagating through the gas, the shocked materials, the contact discontinuity, and the Mach disk. From a dimensional analysis, a scaling is made between both systems and shows the study relevance for the jet velocity, the Mach number, the jet-gas density ratio, and the dissipative processes. The use of a two-dimensional radiation hydrodynamic code, confirms the previous analysis and provides detailed structure of the interaction zone and energy repartition between jet and surrounding gases. [ABSTRACT FROM AUTHOR]

A portable (200x100x100 mm[sup 3]), high-luminosity, spherically bent crystal spectrometer was designed for measuring in a wide spectral range of 1.2-19.6 Å very low emissivity x-ray spectra of different clusters heated by 35 fs laser radiation. This spectrometer is associated with a custom design x-ray charge coupled device that features a large sensitive area (24.6x24.6 mm[sup 2]) and a small pixel size (24x24 μm[sup 2]). This apparatus provides simultaneous high spectral (λ/δλ∼1000-5000) and spatial (40-80 μm) resolution. A large (30x10 mm[sup 2]) open aperture mica crystal with R=100 mm is used as the dispersive and focusing element. The large tuneability of the spectrometer makes it possible to record high-resolution spectra of H-like ions of oxygen (CO[sub 2] clusters) in a spectral range of 15-17 Å, Ne-like like ions of Kr in a spectral range of 5-5.7 Å, and He-like spectra of Ar in a spectral range of 3.0-3.4 and 3.7-4.4 Å without any adjustment of the spectrometer setup. Thanks to the high luminosity (high collection efficiency) of the spectrometer, high quality spectra were obtained using only 15 mJ 2000 laser shots. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The generation of long and homogeneous plasmas by multiphoton ionization of low-pressure gases—H2, D2, and N2 at pressures of a few Torr—with a high-power laser at λ=0.53 μm and intensities of the order of 1015 W/cm2 has been studied. The temperature and density of the plasma are measured by Thomson thermal scattering. After the initial full ionization the hydrodynamic density evolution with time is observed. The comparison with a simple model shows that this evolution is mainly due to the ponderomotive force of the laser beam itself. Typical density variations range from 1% to 5% per 100 psec. [ABSTRACT FROM AUTHOR]

Studies the stimulated backscattering reflectivity of a short laser pulse interaction with an optically ionized helium gas. Reflectivity measurement as a function of the gas pressure; Refraction induced during the ionization process; Expulsion of electrons out of the propagation region by the radial ponderomotive force.

Reports on the beating between two colinear Nd-YLF and Nd-YAG lasers in a homogeneous plasma that generates intense relativistic plasma waves associated with a high longitudinal electric field. Coupling of electron waves with ion waves in the regime of modulation instability.

This paper describes the techniques used to create a fully ionized plasma by the multiphoton ionization of gases at pressures of a few Torr to give an initial electron density precision of better than a few percent, necessary for beat-wave experiments. A gas containment vessel has been constructed using high vacuum techniques with a temperature control system to maintain the fill gas pressure constant. Novel alignment techniques are used that do not require an alignment target placed in the vessel. This involves the use of a rotating pencil beam to determine the focusing of the optics. [ABSTRACT FROM AUTHOR]

We propose a new method for studies of laser-induced heating and melting of metallic foils. The method is based on time-integrated measurements of the surface infrared thermal emission. The experimental data are compared with a model where two equations describe the evolution of electron and lattice temperatures and the emissivity is found from the Drude model with the temperature-dependent electron collision frequency. A good agreement between the experimental data and the model is found for the aluminium samples. It is less satisfactory for the copper, but a signature of phase melting can also be pointed out. A multi-pulse laser irradiation study indicates significant changes in the surface emittance, related to preheating, oxidation and/or chemical modification of the copper sample. The proposed method is relatively simple and complementary to the pump–probe technique. [ABSTRACT FROM AUTHOR]