Absorption of laser radiation in a laser-produced plasma of Xe – hydrodynamic effects and nonequilibrium ionization

Experiments on measuring absorption of an IR laser radiation in the laser-produced plasma of Xe are described. The absorbed fraction of up to 65% has been obtained when the gas-jet target was illuminated by a wide, defocused beam, whereas it barely reached 8.5% in the case of a sharply focused beam. The phenomenon is explained on the basis of a hypothesis of the plasma hydrodynamic expansion according to which the plasma leaves the illuminated area the faster, the smaller its size. Based on the experimental results, an attempt to estimate plasma parameters (N, T, <Z>) is undertaken, with the mean ion charge, <Z>, being calculated using ionization cross-sections for ions from +7Xe to +14Xe which were obtained by means of a quantum-mechanical numeric simulation especially for the present work. A similarity of the EUV output and the laser energy absorption as functions of the laser beam diameter needs an additional study in a future.