Saturation effects inKα absorption spectroscopy of laser-produced plasmas

In laser interaction, absorption of x rays by Kα lines has been shown to be a powerful diagnostic of electronic temperature and integrated density in a dense absorbing plasma. Integrated densities measured in these experiments are lower than expected, especially in implosions where density is high. We present here an experiment and calculations that show that absorption saturates rapidly with increasing thicknesses of the absorber. An aluminum layer of variable thickness is heated by the soft x rays emitted by a samarium layer deposited on the laser-irradiated side of a planar target. We have measured the optical depth of the Kα transitions of ${\mathrm{Al}}^{6+}$ to ${\mathrm{Al}}^{8+}$ ions as a function of the layer thickness. A strong nonlinear behavior is observed as predicted by the calculations. This type of saturation is explained by the high number of superimposed absorbing lines, and it is strongly dependent on the ratio between the linewidth and the instrumental resolution. Such a saturation effect is general for line absorption and does not appear on film when the instrumental broadening is larger than the line profile.