SIMULTANEOUS MEASUREMENT OF ABSORPTION AND X-RAY-EMISSION FROM PREFORMED PLASMA GENERATED BY ULTRASHORT PREPULSE

The first simultaneous measurement of the reflectivity and the X-ray y ield from the preformed plasma was performed with a fine temporal reso lution between the prepulse and the main ionization pulse. Enhancement of the X-ray emission at the long temporal separation is due to the i ncrease of the absorption of an intense ultrashort laser pulse in a pr eformed plasma with a scale length longer than laser wavelength. The m aximum response of X-ray emission at 22.5 degrees and the weak depende nce on the polarization of input laser light infer that collisional ab sorption with additional resonance absorption in a nonuniform critical density profile is the main absorption process in the preformed plasm a.