Target atomic number effect on the duration and conversion efficiency of ultrashort x-ray pulses

High-density plasmas produced by 400 fs, 10(18) W/cm(2) laser pulses are characterized with a transmission grating spectrometer coupled to an x-ray streak camera. Conversion efficiencies and durations of the soft x-ray emission in the 0.1-1.2 keV energy range are analyzed for different target materials (Fe, Cu, Mo, Ta, and Pb). Our results show that the x-ray emission duration is constant for the different materials, with an average value of 4 and 5.7 ps in the 0.8-1.2 keV and 0.2-0.3 keV ranges, respectively. The converstion efficiency is optimal for a Ta target and is about 0.3% for both energy ranges. (C) 1996 American Institute of Physics.