Extreme-UV absorption processes in a laser-produced mid-Z plasma: Measurements and theoretical interpretation

A good knowledge of absorption properties of plasmas at temperature of few tens of eV is essential in several domains such as astrophysics and inertial fusion science. For instance the description of stellar envelopes or the analysis of beta-Cephei pulsation requires an accurate determination of the Rosseland absorption coefficient, which strongly depends on the radiative properties of plasmas in the extreme-UV (XUV) range. Contrary to measurements in X-ray range, the literature on the absorption properties of plasmas of mid-Z elements in XUV domain is less abundant. Furthermore the theoretical interpretation of such spectra represents a theoretical challenge since this energy range involves transition arrays from n equal 3 to 3 with an approximately half-open 3d subshell and possibly other open spectator subshells which contain a huge number of lines. The aim of this paper is to describe an experiment recently performed on the LULI 2000 laser facility mostly devoted to measurements of the absorption in the 60 – 180 eV spectral region in a copper plasma at a temperature of 10 to 30 eV and a density of few mg/cm3. The experimental scheme is based on an indirect heating of multilayer thin foils by two gold cavities irradiated by two nanosecond doubled-frequency beams with an energy of several hundreds of J. This scheme allows one to obtain moderate temperature- and density-gradients and ensures conditions close to local thermodynamic equilibrium. The self-emission of cavities in XUV range is tentatively eliminated by the use of a time-dependent detection. A preliminary interpretation of these measurements is proposed. This analysis relies on three different codes: the hybrid code SCO-RCG, the Flexible Atomic Code in detailed or configuration-average mode, and the HULLAC code in level or configuration mode. A partial agreement is obtained between theory and experiment, though the account for temperature gradients is probably necessary to accurately describe the present measurements.