Study of Soft-X-Ray Generation by Laser-Heating Solid and Gaseous Ta Plasmas with Subpicosecond Pulses.

Using 100 fsec optical pulses, we are able to couple laser energy into solid Ta before appreciable vaporization takes place and thus have been able to compare the efficiency of x-ray emission in the 17 to 30 nm region from laser-heated solid and gaseous density Ta plasmas. We find that a 0.4% x-ray conversion efficiency from a solid density Ta plasma requires a laser fluence of 200 J/cm sup 2 whereas a gaseous Ta plasma with optimal density profile requires a fluence of only 3 J/cm sup 2. This fact is explained by the short attenuation length and shorter pulse duration of x-rays emitted from the heated solid density plasma.