Expansion opacity in laboratory conditions

Radiation–matter interaction depends mainly on the state of matter (its density, temperature, etc.), and also on the radiation spectrum. The opacity of thick plasma also depends on plasma velocity—the Doppler effect shifts atomic lines. For the cases when there are many bound–bound transitions, i.e., the plenty of lines contribute to the opacity, the latter is enhanced when the plasma expands with a nonuniform velocity field. It is known as “expansion opacity” in the literature. Existing models are discrepant and predict diverse results in some cases. Here, we present a rigorous derivation of the effect and show that the effect is available for experimental study at modern laser facilities. The plasma created by a Cu target irradiated with an ∼ 100 J nanosecond laser pulse is rich in lines and has enough expansion velocity so that its opacity is increased in the spectral range ∼ 10 2 − 10 3 eV by the order of magnitude. The possible experimental measurement of the effect is briefly discussed.