Optimization of laser parameters for the maximum efficiency in the generation of water-window radiation using a liquid nitrogen jet

A laser-produced plasma is a suitable compact x-ray source that can be of broad band or quasimonochromatic with a proper choice of material and filter. To address the maximum conversion efficiency for an efficient, quasimonochromatic source at 2.88 nm (N VI 1s(2)-1s2p transition) using liquid nitrogen jet for soft x-ray microscopy, the radiation characteristics such as absolute intensity, spectra, and angular distribution have been investigated for different laser pulse durations (picosecond and femtosecond pulses) and laser energies The comparison of conversion efficiencies between picosecond [120 ps full width at half maximum (FWHM)] and femtosecond (40-500 fs FWHM) lasers indicates that the picosecond laser would provide better conversion efficiency, which is 1.6% at 2x10(13) W/cm(2). The investigation shows that the laser intensity for the maximum conversion efficiency scales as I-m proportional to 1/tau(alpha), where alpha=0.9 +/- 0.15. This empirical formula is useful to choose the laser parameters properly for a given pulse width.