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Radiative properties...

    There is a strong interplay between radiation and matter in hot plasmas. Furthermore radiation is traditionnaly used as a diagnostic tool. Finally, is some cases, non-Local Thermodynamical Equilibrium has to be accounted for.

    Radiation structuring the matter : high energy density plasmas, observed in some astrophysical objets or obtained on large laser or Z-pinches facilities, show high temperatures, usually tens to thousands of eV. Radiation plays an important role in the energy budget and thus may influence the hydrodynamic evolution. Depending on the opacity of the medium the reabsorption of the radiation may be neglected and the energy budget reduces to radiative cooling. In other cases, radiation transport (absorption and emission) is so important that the radiation is coupled to hydrodynamics. Radiation is intimaly coupled to the microscopic structure of the matter. Photons emission and absorption result from the various processes of desexcitation or excitation of atomic or molecular states. The computation of the relevant processus relies on appropriate atomic codes, which are developed, for instance in LERMA For molecular properties we rely on databases like, for instance, in this page.

    Matter structuring radiation: Radiation and matter are intimately coupled together. Atomic structure as well as population kinetics are the origin of the emission characteristics. Straigthforwardly, the angular and spectral properties of the emitted radiation are signature of the hydrodynamical structure (temperature, density and velocity maps). Thus computation or analysis of outgoing radiation relies on adapted codes (as for instance described here) for radiation transport / propagation .
       Less known is the effect of internal microscopic state on the reflective properties (albedo) of a solid illuminated by high energy density radiation. Influence of wall albedo in the propagation of radiative shocks has been demonstrated in experiments we have recently performed (Gonzalez et al. 2006, Busquet et al. 2007).

    Radiation as a diagnostics : as a consequence of the previous effect, radiation is one of the most important diagnostic tools which is used in astrophysical and laboratory plasmas to diagnose the physical local conditions of the medium. Temperatures, densities and several other quantities (atomic populations, velocities, magnetic field,...) may be derived from spectroscopic investigations. However, full radiation transfer, eventually coupled to radiation induced (non-LTE) population kinetics, may be required when the medium is not optically thin, (CHOC-SPEC) to compute emerging spectrum. This spectrum synthesis (see for instance SYNSPEC) is the core of the stellar atmospheres models (like TLUSTY)
    Radiation transfer may be also influenced by anisotropic behaviour induced, in particular, by magnetic field.

    LTE versus NLTE : Local Thermodynamical Equilibrium (LTE), when imbalance of upwards and downwards atomic/molecular transitions is negligible, allows simpler derivations of radiative properties (frequency dependent absorption and emission) from thermodynamical law such as the Boltzmann law. However, low densities and high temperatures, together with deviations from Planck radation field, may not enforce LTE. The atomic or molecular local populations of excited states then no more obey the Boltzmann law nor the "action of mass" laws and requires atomic/molecular structure coupled to population kinetics.
      The modeling of anisotropic, dynamic medium, including the coupling of magnetic field, hydrodynamic, radiation and NLTE effects in one of the challenges of the numerical developments of the coming decade.

    References

    [1] D. Mihalas, B.W. Mihalas, Foundations of Radiation Hydrodynamics, Oxford University Press, New York, 1984.

    [2] G.C. Pomraning, The Equations of Radiation Hydrodynamics, Pergamon, Oxford, 1973.

    [3] John I. Castor, Radiation Hydrodynamics, Cambridge University Press, Cambridge, 2004.

    [4] D. Mihalas, Stellar Atmospheres, Freeman, San Francisco, 1978.

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