Optical properties of plasmas based on an average-atom model

Optical properties of plasmas, including dielectric constants, indices of refraction, and absorption coefficients, are determined from an average-atom point of view. Linear response of an average atom to a harmonic electric field leads to an average-atom version of the Kubo-Greenwood formula, which is used to calculate the frequency-dependent electric conductivity of the plasma. The frequency-dependent dielectric function is determined from the conductivity using Kramers-Kronig dispersion relations. The index of refraction and absorption coefficient of the plasma are subsequently obtained from the dielectric function. Comparing the present results with the free-electron model helps one understand anomalies observed recently in space and time resolved interferograms of Al plasmas produced by 13.9- and 14.7-nm X-ray lasers.