Radiative properties of stellar plasmas and open challenges

The lifetime of solar-like stars, the envelope structure of more massive stars, and stellar acoustic frequencies largely depend on the radiative properties of the stellar plasma. Up to now, these complex quantities have been estimated only theoretically. The development of the powerful tools of helio- and astero- seismology has made it possible to gain insights on the interiors of stars. Consequently, increased emphasis is now placed on knowledge of the monochromatic opacity coefficients. Here we review how these radiative properties play a role, and where they are most important. We then concentrate specifically on the envelopes of $\beta$ Cephei variable stars. We discuss the dispersion of eight different theoretical estimates of the monochromatic opacity spectrum and the challenges we need to face to check these calculations experimentally.Comment: 6 pages, 5 figures, in press (conference HEDLA 2010

Carburation d'alliage d'aluminium par traitement de surface par laser excimère

Les alliages légers de type aluminium sont très prisés dans l'industrie automobile pour leur légèreté, mais leur mauvais comportement en usure et en frottement les rend inutilisables pour certaines applications. Pour améliorer ce comportement, une solution est de faire une couche très dure en surface, un traitement par laser à excimère de l'alliage en présence d'un gaz contenant du carbone permet une carburation de cet alliage en surface sur quelques microns d'épaisseur. Pour ce traitement on utilise un laser à excimère KrF (λ= 248 nm, σ = 25 ns) dont le faisceau est homogénéisé et focalisé sur l'échantillon à traiter. L'échantillon est placé dans une enceinte à vide remplie d'un gaz carboné (CH4 ou C3H6) à une pression de 1 bar ; pour le traitement l'échantillon peut être déplacé perpendiculairement au faisceau en X et en Y. L'ensemble laser-déplacement est piloté par ordinateur. Ce procédé permet un traitement de la surface localisé aux endroits de fortes sollicitations mécaniques et non polluant

Amélioration de la tenue à la corrosion atmosphérique des matériaux utilisés en connectique. Traitements de surface par laser

L'industrie des composants passifs occupe une place importante dans la filière électronique (télécommunications, automobile, informatique...). Dans ce secteur d'activités, la connectique constitue la part la plus importante. Les contacts électriques sont constitués d'un alliage de cuivre (laiton ou bronze) recouvert de nickel (barrière de diffusion) et d'or. Il existe des défauts d'étanchéité dans les couches d'or et de nickel qui induisent la corrosion des couches sous-jacentes. Afin de modifier la structure du dépôt d'or, des traitements de surface par laser UV ont été entrepris car l'absorption de l'or est meilleure dans l'UV. Ils ont pour objectif de supprimer les porosités de la couche d'or à l'origine des points de corrosion et d'effectuer un lissage de la surface car la rugosité est nuisible à un bon contact électrique

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Supersonic-to-subsonic transition of a radiation wave observed at the LMJ

International audienceWe detail results of an experiment performed at the Laser Mégajoule (LMJ) facilityaimed at studying transition from supersonic radiation front to shock front in a lowdensity CHOBr foam enclosed in a plastic tube driven by thermal emission producedin a laser heated spherical gold cavity. Time resolved 2D hard x-ray radiographyimaging using a Sc source (photon energy at ~ 4.3 keV) is employed to measure thedensity perturbation front position, absorption, curvature and shocked materialcompression (defined as the compressed foam density normalized to its nominal value)from the supersonic to the subsonic regimes of propagation. Between these two regimeswhere compression goes from 1 (limited hydrodynamics) to 4 (strong shock formed), aquick increase of the foam compression is observed at the transition time tHS =6.75±0.75 ns, corresponding to the transient transonic regime (HS means“hydrodynamically separated” and refers to the instant when the shock and the radiationfront physically separate). This time is associated to a foam compression ratio of ~2and a Mach number of the slowing down front below M < 2. Experimental results aresuccessfully compared to 3D hydrodynamics simulations; comparisons never presentedfor that regime in past similar studies to our knowledge. Simulations show that thetransition time tHS is sensitive to the radiation closure of the tube entrance. This closure, which occurs in 3D, affects the amount of x-ray energy coupled from the laser heatedcavity to the CHOBr foam, and consequently the transient transonic regime dynamics

Simultaneous X-ray and XUV absorption measurements in nickel laser-produced plasma close to LTE

International audienceWe present an experiment performed in 2016 at the LULI2000 laser facility in which X-ray and XUV absorption structures of nickel hot plasmas were measured simultaneously. Such experiments may provide stringent tests of the accuracy of plasma atomic-physics codes used to the modeling of plasmas close to local thermodynamic equilibrium. The experimental setup relies on a symmetric heating of the sample foil by two gold hohlraums in order to reduce the spatial gradients. The plasma conditions are characterized by temperatures between 10 and 20 eV and densities of the order of 10$^{−3}$ g/cm $^3$-10$^{−2}$ g/cm$^3$. For the X-ray part, we investigate the 2p-3d and 2p-4d transitions, and for the XUV part, we recorded the $\Delta$n = 0 (n = 3) transitions, which present a high sensitivity to plasma temperature. These latter transitions are of particular interest because, in mid-Z plasmas, they dominate the Planck and Rosseland mean opacities. Measured spectra are compared to calculations performed using the hybrid opacity code SCO-RCG and the Flexible Atomic Code (FAC). The influence of a spectator electron on the calculated spectra is analyzed using the latter code