Efficient K x-ray source from femtosecond laser-produced plasmas

We have performed a detailed quantitative study of the intense electron pulse produced by nonlinear absorption during ultrafast laser-solid interaction at near normal incidence. The resulting K x-ray lines have been investigated by time-integrated spectroscopy in the 14 keV range and by Monte Carlo simulations of hot electron energy penetration in Al-SiO2 and Al-CaF2 targets. Calibration of the observed electron fluence and K line intensities was provided by direct monoenergetic electron beam interaction with the same target. Optimum conditions for hot electron production were obtained by setting the prepulse energy fluence close to the target damage threshold. Results indicate that K lines were produced by a distribution function of hot electrons which carry 12% of the incident laser energy with a characteristic temperature of about 8 keV. Spectrally and spatially resolved K emission measurements using a cooled charge-coupled-device detector demonstrate the scaling capabilities of this x-ray source to energies in excess of 6 keV.