POROUS LAYER EFFECTS ON SOFT-X-RAY RADIATION EMITTED FROM A PLASMA GENERATED BY 130-FS LASER-PULSES IRRADIATING A POROUS SILICON TARGET

Differences in the X-ray emission by double-pulse excitation with a po rous Si target and a solid Si target are investigated. The optimum del ay time between the prepulse and the main pulse for obtaining the high est soft X-ray emission on porous Si is longer than that for solid Si. The soft X-ray pulse duration on the porous Si is shorter than that f or the solid Si and is independent of the prepulse intensity. These di fferences can be explained by the ablation of Si clusters with solid d ensity from the porous Si target by the prepulse. A plasma with low av erage density but high local density is thereby excited by the main pu lse.